Annealing effect on the magnetic induced austenite transformation in polycrystalline freestanding Ni-Co-Mn-In films produced by co-sputtering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crouïgneau, G., E-mail: guillaume.crouigneau@neel.cnrs.fr; Univ. Grenoble Alpes, CRETA, F-38000 Grenoble; CNRS, Inst. NEEL, F-38000 Grenoble

2015-01-21

Ni-Co-Mn-In freestanding films, with a magneto-structural transformation at room temperature were successfully produced by co-sputtering and post-annealing methods leading to film composition mastering. For a post-annealing temperature of 700 °C, the phase transformation occurs slightly above room temperature, with a twisted martensitic microstructure phase observed at 300 K by Field Emission Scanning Electron Microscopy. Magnetization measurements on a polycrystalline film showed a phase transformation from a weakly magnetic martensite to a magnetic austenite phase. Moreover, an inverse magnetocaloric effect with an entropy variation of 4 J/kg K under 5 T was also measured. A simple magneto-actuation experiment based on the magnetic induced austenite transformation wasmore » also successfully completed. The possibility to insert such films in microsystems is clearly demonstrated in this work.« less

In situ X-ray diffraction strain-controlled study of Ti–Nb–Zr and Ti–Nb–Ta shape memory alloys: crystal lattice and transformation features

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dubinskiy, S.; National University of Science and Technology “MISIS”, 4, Leninskiy prosp., Moscow 119049; Prokoshkin, S.

2014-02-15

Phase and structure transformations in biomedical Ti–21.8Nb–6.0Zr (TNZ) and Ti–19.7Nb–5.8Ta (TNT) shape memory alloys (at.%) under and without load in the − 150 to 100 °S temperature range are studied in situ using an original tensile module for a low-temperature chamber of an X-ray diffractometer. Alpha″- and beta-phase lattice parameters, the crystallographic resource of recovery strain, phase and structure transformation sequences, and microstress appearance and disappearance are examined, compared and discussed. For both alloys, the crystallographic resource of recovery strain decreases with temperature increase to become 4.5% for TNZ and 2.5% for TNT alloy (at RT). Loading at low temperaturesmore » leads to additional α″-phase formation and reorientation. Heating under load, as compared to strain-free heating, affects the reverse transformation sequence of both alloys in different ways. For TNZ alloy, strain-free heating results in simultaneous ω→β and α″→β transformations, whereas during heating under stress, they are sequential: β + ω→α″ precedes α″→β. For TNT alloy, strain-free heating results in reverse α″→β transformation, whereas during heating under stress, α″→β transformation is preceded by α″-phase reorientation. - Highlights: • Comparative in situ XRD analysis of Ti–Nb–Zr(Ta) shape memory alloys is realized. • Lattice parameters of β- and α″-phases are calculated in the − 150 to + 100 °C range. • The higher the temperature, the lower the α″→β transformation strain. • Loading at low temperatures results in α″-phase formation and reorientation. • Transformation sequences upon heating with and without loading are different.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garlea, Elena; Steiner, M. A.; Calhoun, C. A.

The α-phase transformation kinetics of as-cast U - 8 wt% Mo below the eutectoid temperature have been established by in situ neutron diffraction. α-phase weight fraction data acquired through Rietveld refinement at five different isothermal hold temperatures can be modeled accurately utilizing a simple Johnson-Mehl-Avrami-Kolmogorov impingement-based theory, and the results are validated by a corresponding evolution in the γ-phase lattice parameter during transformation that follows Vegard’s law. Neutron diffraction data is used to produce a detailed Time-Temperature-Transformation diagram that improves upon inconsistencies in the current literature, exhibiting a minimum transformation start time of 40 min at temperatures between 500 °Cmore » and 510 °C. Lastly, the transformation kinetics of U – 8 wt% Mo can vary significantly from as-cast conditions after extensive heat treatments, due to homogenization of the typical dendritic microstructure which possesses non-negligible solute segregation.« less

Phase Transformation Temperatures and Solute Redistribution in a Quaternary Zirconium Alloy

NASA Astrophysics Data System (ADS)

Cochrane, C.; Daymond, M. R.

2018-05-01

This study investigates the phase stability and redistribution of solute during heating and cooling of a quaternary zirconium alloy, Excel (Zr-3.2Sn-0.8Mo-0.8Nb). Time-of-flight neutron diffraction data are analyzed using a novel Vegard's law-based approach to determine the phase fractions and location of substitutional solute atoms in situ during heating from room temperature up to 1050 °C. It is seen that this alloy exhibits direct nucleation of the β Zr phase from martensite during tempering, and stable retention of the β Zr phase to high temperatures, unlike other two-phase zirconium alloys. The transformation strains resulting from the α \\leftrightarrow β transformation are shown to have a direct impact on the development of microstructure and crystallographic texture.

In-Situ Phase Mapping and Direct Observations of Phase Transformations During Arc Welding of 1045 Steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elmer, J; Palmer, T

2005-09-13

In-situ Spatially Resolved X-Ray Diffraction (SRXRD) experiments were performed during gas tungsten arc (GTA) welding of AISI 1045 C-Mn steel. Ferrite ({alpha}) and austenite ({gamma}) phases were identified and quantified in the weld heat-affected zone (HAZ) from the real time x-ray diffraction data. The results were compiled along with weld temperatures calculated using a coupled thermal fluids weld model to create a phase map of the HAZ. This map shows the {alpha} {yields} {gamma} transformation taking place during weld heating and the reverse {gamma} {yields} {alpha} transformation taking place during weld cooling. Superheating is required to complete the {alpha} {yields}more » {gamma} phase transformation, and the amount of superheat above the A3 temperature was shown to vary with distance from the centerline of the weld. Superheat values as high as 250 C above the A3 temperature were observed at heating rates of 80 C/s. The SRXRD experiments also revealed details about the {gamma} phase not observable by conventional techniques, showing that {gamma} is present with two distinct lattice parameters as a result of inhomogeneous distribution of carbon and manganese in the starting pearlitic/ferritic microstructure. During cooling, the reverse {gamma} {yields} {alpha} phase transformation was shown to depend on the HAZ location. In the fine grained region of the HAZ, at distances greater than 2 mm from the fusion line, the {gamma} {yields} {alpha} transformation begins near the A3 temperature and ends near the A1 temperature. In this region of the HAZ where the cooling rates are below 40 C/s, the transformation occurs by nucleation and growth of pearlite. For HAZ locations closer to the fusion line, undercoolings of 200 C or more below the A1 temperature are required to complete the {gamma} {yields} {alpha} transformation. In this region of the HAZ, grain growth coupled with cooling rates in excess of 50 C/s causes the transformation to occur by a bainitic mechanism.« less

Shape-memory alloy micro-actuator

NASA Technical Reports Server (NTRS)

Busch, John D. (Inventor); Johnson, Alfred D. (Inventor)

1991-01-01

A method of producing an integral piece of thermo-sensitive material, which is responsive to a shift in temperature from below to above a phase transformation temperature range to alter the material's condition to a shape-memory condition and move from one position to another. The method is characterized by depositing a thin film of shape-memory material, such as Nickel titanium (Ni-Ti) onto a substrate by vacuum deposition process such that the alloy exhibits an amorphous non-crystalline structure. The coated substrate is then annealed in a vacuum or in the presence of an inert atmosphere at a selected temperature, time and cool down rate to produce an ordered, partially disordered or fully disordered BCC structure such that the alloy undergoes thermoelastic, martinsetic phase transformation in response to alteration in temperature to pass from a martinsetic phase when at a temperature below a phase transformation range and capable of a high level of recoverable strain to a parent austenitic phase in a memory shape when at a temperature above the phase transformation range. Also disclosed are actuator devices employing shape-memory material actuators that deform from a set shape toward an original shape when subjected to a critical temperature level after having been initially deformed from the original shape into the set shape while at a lower temperature. The actuators are mechanically coupled to one or more movable elements such that the temperature-induce deformation of the actuators exerts a force or generates a motion of the mechanical element(s).

Microstructural studies of 35 degrees C copper Ni-Ti orthodontic wire and TEM confirmation of low-temperature martensite transformation.

PubMed

Brantley, William A; Guo, Wenhua; Clark, William A T; Iijima, Masahiro

2008-02-01

Previous temperature-modulated differential scanning calorimetry (TMDSC) study of nickel-titanium orthodontic wires revealed a large exothermic low-temperature peak that was attributed to transformation within martensitic NiTi. The purpose of this study was to use transmission electron microscopy (TEM) to verify this phase transformation in a clinically popular nickel-titanium wire, identify its mechanism and confirm other phase transformations found by TMDSC, and to provide detailed information about the microstructure of this wire. The 35 degrees C Copper nickel-titanium wire (Ormco) with cross-section dimensions of 0.016 in. x 0.022 in. used in the earlier TMDSC investigation was selected. Foils were prepared for TEM analyses by mechanical grinding, polishing, dimpling, ion milling and plasma cleaning. Standard bright-field and dark-field TEM images were obtained, along with convergent-beam electron diffraction patterns. A cryo-stage with the electron microscope (Phillips CM 200) permitted the specimen to be observed at -187, -45, and 50 degrees C, as well as at room temperature. Microstructures were also observed with an optical microscope and a scanning electron microscope. Room temperature microstructures had randomly oriented, elongated grains that were twinned. Electron diffraction patterns confirmed that phase transformations took place over temperature ranges previously found by TMDSC. TEM observations revealed a high dislocation density and fine-scale oxide particles, and that twinning is the mechanism for the low-temperature transformation in martensitic NiTi. TEM confirmed the low-temperature peak and other phase transformations observed by TMDSC, and revealed that twinning in martensite is the mechanism for the low-temperature peak. The high dislocation density and fine-scale oxide particles in the microstructure are the result of the wire manufacturing process.

Sign reversal of transformation entropy change in Co{sub 2}Cr(Ga,Si) shape memory alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Xiao, E-mail: xu@material.tohoku.ac.jp; Omori, Toshihiro; Kainuma, Ryosuke

2015-11-02

In situ X-ray diffraction (XRD) measurements and compression tests were performed on Co{sub 2}Cr(Ga,Si) shape memory alloys. The reentrant martensitic transformation behavior was directly observed during the in situ XRD measurements. The high-temperature parent phase and low-temperature reentrant parent phase were found to have a continuous temperature dependence of lattice parameter, therefore suggesting that they are the same phase in nature. Moreover, compression tests were performed on a parent-phase single crystal sample; an evolution from normal to inverse temperature dependence of critical stress for martensitic transformation was directly observed. Based on the Clausius-Clapeyron analysis, a sign reversal of entropy changemore » can be expected on the same alloy.« less

In situ 3D-X-ray diffraction tracking of individual grains of olivine during high-pressure/ high-temperature phase transitions

NASA Astrophysics Data System (ADS)

Rosa, A. D.; Merkel, S.; Ghosh, S.; Hilairet, N.; Perrillat, J.; Mezouar, N.; Vaughan, G.

2013-12-01

The series of phase transitions between olivine, wadsleyite and ringwoodite play an essential role for large scale dynamical processes in the Earth mantle. Detailed knowledge of the microscopic mechanism at the origin of these high-pressure and high-temperature phase transformations is useful to connect global seismic observations and geodynamics. Indeed, the textures of these phases can be induced either during mantle flow or during the phase transformations and they greatly affect the characteristics of seismic wave propagation. Here, we present a new design of diamond anvil cell experiments to collect three-dimensional diffraction images and track individual grains inside a polycristalline sample at high pressure and high temperature. The instrumentation includes a new resistively heated diamond anvil cell developed at beamline ID27 of the ESRF which provided stable and homogenous temperature condition over more than 24 hours. In our experiments, the pressure is first increased up to 12 GPa at a constant temperature of T = 800 K. The temperature is then further increased to 1300 K to reach the stability field of the high-pressure polymorph. Upon further compression the transformation of olivine to its high-pressure polymorph is successfully monitored. At each pressure-temperature step and while the sample is transforming the crystallographic parameters, the orientations and positions of grains within the sample are tracked in situ using three-dimensional X-ray diffraction. This will provide important information on the micromechanical properties of olivine including orientation statistics, orientation relations between parent and daughter phases, and transformation textures at different stages of the phase transition. This in turn will help in interpreting the geophysical observations. Details of the experimental and analytical approach used in this study will be given.

Influences of W Content on the Phase Transformation Properties and the Associated Stress Change in Thin Film Substrate Combinations Studied by Fabrication and Characterization of Thin Film V1- xW xO2 Materials Libraries.

PubMed

Wang, Xiao; Rogalla, Detlef; Ludwig, Alfred

2018-04-09

The mechanical stress change of VO 2 film substrate combinations during their reversible phase transformation makes them promising for applications in micro/nanoactuators. V 1- x W x O 2 thin film libraries were fabricated by reactive combinatorial cosputtering to investigate the effects of the addition of W on mechanical and other transformation properties. High-throughput characterization methods were used to systematically determine the composition spread, crystalline structure, surface topography, as well as the temperature-dependent phase transformation properties, that is, the hysteresis curves of the resistance and stress change. The study indicates that as x in V 1- x W x O 2 increases from 0.007 to 0.044 the crystalline structure gradually shifts from the VO 2 (M) phase to the VO 2 (R) phase. The transformation temperature decreases by 15 K/at. % and the resistance change is reduced to 1 order of magnitude, accompanied by a wider transition range and a narrower hysteresis with a minimal value of 1.8 K. A V 1- x W x O 2 library deposited on a Si 3 N 4 /SiO 2 -coated Si cantilever array wafer was used to study simultaneously the temperature-dependent stress change σ( T) of films with different W content through the phase transformation. Compared with σ( T) of ∼700 MPa of a VO 2 film, σ( T) in V 1- x W x O 2 films decreases to ∼250 MPa. Meanwhile, σ( T) becomes less abrupt and occurs over a wider temperature range with decreased transformation temperatures.

Phase transformation and magnetic properties of MnAl powders prepared by elemental-doping and salt-assisted ball milling

NASA Astrophysics Data System (ADS)

Qian, Hui-Dong; Si, Ping-Zhan; Choi, Chul-Jin; Park, Jihoon; Cho, Kyung Mox

2018-05-01

The effects of elemental doping of Si and Fe on the ɛ→τ phase transformation and the magnetic properties of MnAl were studied. The magnetic powders of Si- and Fe-doped MnAl were prepared by using induction melting followed by water-quenching, annealing, and salt-assisted ball-milling. The Fe-doped MnAl powders are mainly composed of the L10-structured τ-phase, while the Si-doped MnAl are composed of τ-phase and a small fraction of γ2- and β-phases. A unique thin leaves-like morphology with thickness of several tens of nanometers and diameter size up to 500 nm were observed in the Si-doped MnAl powders. The Fe-doped MnAl powders show irregular shape with much larger dimensions in the range from several to 10 μm. The morphology difference of the samples was ascribed to the variation of the mechanical properties affected by different doping elements. The phase transformation temperatures of the ɛ-phase of the samples were measured. The doping of Fe decreases the onset temperature of the massive phase transformation in MnAl, while the Si-doping increases the massive phase transformation temperature. Both Fe and Si increase the Curie temperature of MnAl. A substantially enhanced coercivity up to 0.45 T and 0.42 T were observed in the ball-milled MnAl powders doped with Si and Fe, respectively.

Development of an interatomic potential for the simulation of defects, plasticity, and phase transformations in titanium

DOE PAGES

Mendelev, M. I.; Underwood, T. L.; Ackland, G. J.

2016-10-17

New interatomic potentials describing defects, plasticity, and high temperature phase transitions for Ti are presented. Fitting the martensitic hcp-bcc phase transformation temperature requires an efficient and accurate method to determine it. We apply a molecular dynamics method based on determination of the melting temperature of competing solid phases, and Gibbs-Helmholtz integration, and a lattice-switch Monte Carlo method: these agree on the hcp-bcc transformation temperatures to within 2 K. We were able to develop embedded atom potentials which give a good fit to either low or high temperature data, but not both. The first developed potential (Ti1) reproduces the hcp-bcc transformationmore » and melting temperatures and is suitable for the simulation of phase transitions and bcc Ti. Two other potentials (Ti2 and Ti3) correctly describe defect properties and can be used to simulate plasticity or radiation damage in hcp Ti. The fact that a single embedded atom method potential cannot describe both low and high temperature phases may be attributed to neglect of electronic degrees of freedom, notably bcc has a much higher electronic entropy. As a result, a temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature.« less

Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films

NASA Astrophysics Data System (ADS)

Milliron, Delia; Dahlman, Clayton; Leblanc, Gabriel; Bergerud, Amy

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. The low-temperature state is insulating and transparent, while the high-temperature state is metallic and IR blocking. Alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, VO2 nanocrystal films have been prepared by solution deposition of V2O3 nanocrystals followed by oxidative annealing. Nanocrystalline VO2 films are electrochemically reduced, inducing changes in their electronic and optical properties. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. Reduction causes an initial transformation to a metallic, IR-colored distorted monoclinic phase. However, an unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase is observed upon further reduction.

Low-temperature phase transformations in 4-cyano-4‧-pentyl-biphenyl (5CB) filled by multiwalled carbon nanotubes

NASA Astrophysics Data System (ADS)

Lebovka, N.; Melnyk, V.; Mamunya, Ye.; Klishevich, G.; Goncharuk, A.; Pivovarova, N.

2013-08-01

The effects of multiwalled carbon nanotubes (NTs) on low-temperature phase transformations in 5CB were studied by means of differential scanning calorimetry (DSC), low-temperature photoluminescence and measurements of electrical conductivity. The concentration of NTs was varied within 0-1 wt% The experimental data, obtained for pure 5CB by DSC and measurements of photoluminescence in the heating mode, evidenced the presence of two crystallization processes at T≈229 K and T≈262 K, which correspond to C1a→C1b, and C1b→C2 phase transformations. Increase of temperature T from 10 K tо 229 K provoked the red shift of photoluminescence spectral band that was explained by flattening of 5CB molecule conformation. Moreover, the photoluminescence data allow to conclude that crystallization at T≈229 K results in conformation transition to non-planar 5CB structure characteristic to ideal crystal. The non-planar conformations were dominating in nematic phase, i.e., at T>297 K. Electrical conductivity data for 5CB-NT composites revealed supplementary anomaly inside the stable crystalline phase C2, identified earlier in the temperature range 229-296.8 K. It can reflect the influence of phase transformation of 5CB in interfacial layers on the transport of charge carriers between NTs.

Anomalous physical properties of Heusler-type Co2Cr (Ga,Si) alloys and thermodynamic study on reentrant martensitic transformation

NASA Astrophysics Data System (ADS)

Xu, Xiao; Nagasako, Makoto; Kataoka, Mitsuo; Umetsu, Rie Y.; Omori, Toshihiro; Kanomata, Takeshi; Kainuma, Ryosuke

2015-03-01

Electronic, magnetic, and thermodynamic properties of Co2Cr(Ga,Si) -based shape-memory alloys, which exhibit reentrant martensitic transformation (RMT) behavior, were studied experimentally. For electric resistivity (ER), an inverse (semiconductor-like) temperature dependence in the parent phase was found, along with anomalous behavior below its Curie temperature. A pseudobinary phase diagram was determined, which gives a "martensite loop" clearly showing the reentrant behavior. Differential scanning calorimetry and specific-heat measurements were used to derive the entropy change Δ S between martensite and parent phases. The temperature dependence of the derived Δ S was analyzed thermodynamically to confirm the appearances of both the RMT and normal martensitic transformation. Detailed studies on the specific heat in martensite and parent phases at low temperatures were also conducted.

a Thermal Conduction Switch Based on Low Hysteresis Nitife Shape Memory Alloy Helical Springs

NASA Astrophysics Data System (ADS)

Krishnan, V. B.; Bewerse, C.; Notardonato, W. U.; Vaidyanathan, R.

2008-03-01

Shape memory alloy (SMA) actuators possess an inherent property of sensing a change in temperature and delivering significant force against external loads through a shape change resulting from a temperature-induced phase transformation. The utilization of a reversible trigonal (R-phase) to cubic phase transformation in NiTiFe SMAs allows for this strain recovery to occur with reduced hysteresis between the forward and reverse transformations. However, the magnitude of the strain recovery associated with the R-phase transformation is lower than that of the monoclinic to cubic phase transformation. The use of helical springs can compensate for this design constraint as they produce significant stroke when compared to straight elements such as thin strips and wires. This work reports on the development and implementation of NiTiFe helical springs in a low-hysteresis thermal conduction switch for advanced spaceport applications associated with NASA's requirements for future lunar and Mars missions. Such a low-hysteresis thermal conduction switch can provide on-demand heat transfer between two reservoirs at different temperatures.

FAST TRACK COMMUNICATION: Field dependence of temperature induced irreversible transformations of magnetic phases in Pr0.5Ca0.5Mn0.975Al0.025O3 crystalline oxide

NASA Astrophysics Data System (ADS)

Lakhani, Archana; Kushwaha, Pallavi; Rawat, R.; Kumar, Kranti; Banerjee, A.; Chaddah, P.

2010-01-01

Glass-like arrest has recently been reported in various magnetic materials. As in structural glasses, the kinetics of a first order transformation is arrested while retaining the higher entropy phase as a non-ergodic state. We show visual mesoscopic evidence of the irreversible transformation of the arrested antiferromagnetic-insulating phase in Pr0.5Ca0.5Mn0.975Al0.025O3 to its equilibrium ferromagnetic-metallic phase with an isothermal increase of magnetic field, similar to its iso-field transformation on warming. The magnetic field dependence of the non-equilibrium to equilibrium transformation temperature is shown to be governed by Le Chatelier's principle.

Prediction and Optimization of Phase Transformation Region After Spot Continual Induction Hardening Process Using Response Surface Method

NASA Astrophysics Data System (ADS)

Qin, Xunpeng; Gao, Kai; Zhu, Zhenhua; Chen, Xuliang; Wang, Zhou

2017-09-01

The spot continual induction hardening (SCIH) process, which is a modified induction hardening, can be assembled to a five-axis cooperating computer numerical control machine tool to strengthen more than one small area or relatively large area on complicated component surface. In this study, a response surface method was presented to optimize phase transformation region after the SCIH process. The effects of five process parameters including feed velocity, input power, gap, curvature and flow rate on temperature, microstructure, microhardness and phase transformation geometry were investigated. Central composition design, a second-order response surface design, was employed to systematically estimate the empirical models of temperature and phase transformation geometry. The analysis results indicated that feed velocity has a dominant effect on the uniformity of microstructure and microhardness, domain size, oxidized track width, phase transformation width and height in the SCIH process while curvature has the largest effect on center temperature in the design space. The optimum operating conditions with 0.817, 0.845 and 0.773 of desirability values are expected to be able to minimize ratio (tempering region) and maximize phase transformation width for concave, flat and convex surface workpieces, respectively. The verification result indicated that the process parameters obtained by the model were reliable.

A comparison of dilatometry and in-situ neutron diffraction in tracking bulk phase transformations in a martensitic stainless steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Christien, F., E-mail: frederic.christien@univ-nantes.fr; Telling, M.T.F.; Department of Materials, University of Oxford, Parks Road, Oxford

2013-08-15

Phase transformations in the 17-4PH martensitic stainless steel have been studied using different in-situ techniques, including dilatometry and high resolution neutron diffraction. Neutron diffraction patterns were quantitatively processed using the Rietveld refinement method, allowing the determination of the temperature-dependence of martensite (α′, bcc) and austenite (γ, fcc) phase fractions and lattice parameters on heating to 1000 °C and then cooling to room temperature. It is demonstrated in this work that dilatometry doesn't permit an accurate determination of the end temperature (Ac3) of the α′ → γ transformation which occurs upon heating to high temperature. The analysis of neutron diffraction datamore » has shown that the respective volumes of the two phases become very close to each other at high temperature, thus making the dilatometric technique almost insensitive in that temperature range. However, there is a very good agreement between neutron diffraction and dilatometry at lower temperature. The martensitic transformation occurring upon cooling has been analysed using the Koistinen–Marburger equation. The thermal expansion coefficients of the two phases have been determined in addition. A comparison of the results obtained in this work with data from literature is presented. - Highlights: • Martensite is still present at very high temperature (> 930 °C) upon heating. • The end of austenitisation cannot be accurately monitored by dilatometry. • The martensite and austenite volumes become similar at high temperature (> ∼ 850 °C)« less

Hydrostatic, uniaxial, and triaxial compression tests on unpoled "Chem-prep" PZT 95/5-2Nb ceramic within temperature range of -55 to 75 degrees C.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zeuch, David Henry; Montgomery, Stephen Tedford; Lee, Moo Yul

Sandia is currently developing a lead-zirconate-titanate ceramic 95/5-2Nb (or PNZT) from chemically prepared ('chem-prep') precursor powders. Previous PNZT ceramic was fabricated from the powders prepared using a 'mixed-oxide' process. The specimens of unpoled PNZT ceramic from batch HF803 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions within the temperature range of -55 to 75 C and pressures to 500 MPa. The objective of this experimental study was to obtain mechanical properties and phase relationships so that the grain-scale modeling effort can develop and test its models and codes using realistic parameters. The stress-strain behavior of 'chem-prep' PNZTmore » under different loading paths was found to be similar to that of 'mixed-oxide' PNZT. The phase transformation from ferroelectric to antiferroelectric occurs in unpoled ceramic with abrupt increase in volumetric strain of about 0.7 % when the maximum compressive stress, regardless of loading paths, equals the hydrostatic pressure at which the transformation otherwise takes place. The stress-volumetric strain relationship of the ceramic undergoing a phase transformation was analyzed quantitatively using a linear regression analysis. The pressure (P{sub T1}{sup H}) required for the onset of phase transformation with respect to temperature is represented by the best-fit line, P{sub T1}{sup H} (MPa) = 227 + 0.76 T (C). We also confirmed that increasing shear stress lowers the mean stress and the volumetric strain required to trigger phase transformation. At the lower bound (-55 C) of the tested temperature range, the phase transformation is permanent and irreversible. However, at the upper bound (75 C), the phase transformation is completely reversible as the stress causing phase transformation is removed.« less

Phase Transformation and Creep Behavior in Ti50Pd30Ni20 High Temperature Shape Memory Alloy in Compression

NASA Technical Reports Server (NTRS)

Kumar, Parikshith K.; Desai, Uri; Monroe, James; Lagoudas, Dimitris C.; Karaman, Ibrahim; Noebe, Ron; Bigelow, Glenn

2010-01-01

The creep behavior and the phase transformation of Ti50Pd30Ni20 High Temperature Shape Memory Alloy (HTSMA) is investigated by standard creep tests and thermomechanical tests. Ingots of the alloy are induction melted, extruded at high temperature, from which cylindrical specimens are cut and surface polished. A custom high temperature test setup is assembled to conduct the thermomechanical tests. Following preliminary monotonic tests, standard creep tests and thermally induced phase transformation tests are conducted on the specimen. The creep test results suggest that over the operating temperatures and stresses of this alloy, the microstructural mechanisms responsible for creep change. At lower stresses and temperatures, the primary creep mechanism is a mixture of dislocation glide and dislocation creep. As the stress and temperature increase, the mechanism shifts to predominantly dislocation creep. If the operational stress or temperature is raised even further, the mechanism shifts to diffusion creep. The thermally induced phase transformation tests show that actuator performance can be affected by rate independent irrecoverable strain (transformation induced plasticity + retained martensite) as well as creep. The rate of heating and cooling can adversely impact the actuators performance. While the rate independent irrecoverable strain is readily apparent early in the actuators life, viscoplastic strain continues to accumulate over the lifespan of the HTSMA. Thus, in order to get full actuation out of the HTSMA, the heating and cooling rates must be sufficiently high enough to avoid creep.

A new phase transformation path from nanodiamond to new-diamond via an intermediate carbon onion

NASA Astrophysics Data System (ADS)

Xiao, J.; Li, J. L.; Liu, P.; Yang, G. W.

2014-11-01

The investigation of carbon allotropes such as graphite, diamond, fullerenes, nanotubes and carbon onions and mechanisms that underlie their mutual phase transformation is a long-standing problem of great fundamental importance. New diamond (n-diamond) is a novel metastable phase of carbon with a face-centered cubic structure; it is called ``new diamond'' because many reflections in its electron diffraction pattern are similar to those of diamond. However, producing n-diamond from raw carbon materials has so far been challenging due to n-diamond's higher formation energy than that of diamond. Here, we, for the first time, demonstrate a new phase transformation path from nanodiamond to n-diamond via an intermediate carbon onion in the unique process of laser ablation in water, and establish that water plays a crucial role in the formation of n-diamond. When a laser irradiates colloidal suspensions of nanodiamonds at ambient pressure and room temperature, nanodiamonds are first transformed into carbon onions serving as an intermediate phase, and sequentially carbon onions are transformed into n-diamonds driven by the laser-induced high temperature and high pressure from the carbon onion as a nanoscaled temperature and pressure cell upon the process of laser irradiation in a liquid. This phase transformation not only provides new insight into the physical mechanism involved, but also offers one suitable opportunity for breaking controllable pathways between n-diamond and carbon allotropes such as diamond and carbon onions.The investigation of carbon allotropes such as graphite, diamond, fullerenes, nanotubes and carbon onions and mechanisms that underlie their mutual phase transformation is a long-standing problem of great fundamental importance. New diamond (n-diamond) is a novel metastable phase of carbon with a face-centered cubic structure; it is called ``new diamond'' because many reflections in its electron diffraction pattern are similar to those of diamond. However, producing n-diamond from raw carbon materials has so far been challenging due to n-diamond's higher formation energy than that of diamond. Here, we, for the first time, demonstrate a new phase transformation path from nanodiamond to n-diamond via an intermediate carbon onion in the unique process of laser ablation in water, and establish that water plays a crucial role in the formation of n-diamond. When a laser irradiates colloidal suspensions of nanodiamonds at ambient pressure and room temperature, nanodiamonds are first transformed into carbon onions serving as an intermediate phase, and sequentially carbon onions are transformed into n-diamonds driven by the laser-induced high temperature and high pressure from the carbon onion as a nanoscaled temperature and pressure cell upon the process of laser irradiation in a liquid. This phase transformation not only provides new insight into the physical mechanism involved, but also offers one suitable opportunity for breaking controllable pathways between n-diamond and carbon allotropes such as diamond and carbon onions. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr05246c

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tan, Y.B.; Yang, L.H.; Duan, J.L.

The kinetics of the β → α phase transformation in the 47Zr–45Ti–5Al–3V (wt.%) alloy with different initial β grain sizes under isothermal conditions was investigated by X-ray diffraction. The results showed that the volume fraction of α phase first increased with increasing aging time, and then reached an equilibrium value. The equilibrium value of α phase decreased with increased aging temperature. At the same aging temperature and time, the volume fraction of α phase in the 47Zr–45Ti–5Al–3V alloy solution-treated at 850 °C was higher than at 1050 °C, and the size of α phase in the 47Zr–45Ti–5Al–3V alloy solution-treated atmore » 850 °C was larger than that at 1050 °C. The kinetics of the β → α phase transformation was modeled under isothermal conditions in the theoretical frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The Avrami index (n) increased with increasing aging temperature, while the reaction rate constant (k) decreased. - Highlights: • The kinetics of the β → α phase transformation in the ZrTiAlV alloy was investigated. • The volume fraction of α phase first increased and then reached an equilibrium value. • The kinetics of the β → α phase transformation was modeled by the JMAK theory. • The n increased with increasing aging temperature, while the k decreased.« less

Irradiation-induced β to α SiC transformation at low temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parish, Chad M.; Koyanagi, Takaaki; Kondo, Sosuke

Here, we observed that β-SiC, neutron irradiated to 9 dpa (displacements per atom) at ≈1440 °C, began transforming to α-SiC, with radiation-induced Frank dislocation loops serving as the apparent nucleation sites. 1440 °C is a far lower temperature than usual β → α phase transformations in SiC. SiC is considered for applications in advanced nuclear systems, as well as for electronic or spintronic applications requiring ion irradiation processing. β-SiC, preferred for nuclear applications, is metastable and undergoes a phase transformation at high temperatures (typically 2000 °C and above). Nuclear reactor concepts are not expected to reach the very high temperaturesmore » for thermal transformation. However, our results indicate incipient β → α phase transformation, in the form of small (~5–10 nm) pockets of α-SiC forming in the β matrix. In service transformation could degrade structural stability and fuel integrity for SiC-based materials operated in this regime. However, engineering this transformation deliberately using ion irradiation could enable new electronic applications.« less

Irradiation-induced β to α SiC transformation at low temperature

DOE PAGES

Parish, Chad M.; Koyanagi, Takaaki; Kondo, Sosuke; ...

2017-04-26

Here, we observed that β-SiC, neutron irradiated to 9 dpa (displacements per atom) at ≈1440 °C, began transforming to α-SiC, with radiation-induced Frank dislocation loops serving as the apparent nucleation sites. 1440 °C is a far lower temperature than usual β → α phase transformations in SiC. SiC is considered for applications in advanced nuclear systems, as well as for electronic or spintronic applications requiring ion irradiation processing. β-SiC, preferred for nuclear applications, is metastable and undergoes a phase transformation at high temperatures (typically 2000 °C and above). Nuclear reactor concepts are not expected to reach the very high temperaturesmore » for thermal transformation. However, our results indicate incipient β → α phase transformation, in the form of small (~5–10 nm) pockets of α-SiC forming in the β matrix. In service transformation could degrade structural stability and fuel integrity for SiC-based materials operated in this regime. However, engineering this transformation deliberately using ion irradiation could enable new electronic applications.« less

Thermally induced phase transformation in multi-phase iron oxide nanoparticles on vacuum annealing

NASA Astrophysics Data System (ADS)

Anupama, A. V.; Keune, W.; Sahoo, B.

2017-10-01

The evolution of magnetic phases in multi-phase iron oxide nanoparticles, synthesized via the transferred arc plasma induced gas phase condensation method, was investigated by X-ray diffraction, vibrating sample magnetometry and 57Fe Mössbauer spectroscopy. The particles are proposed to be consisting of three different iron oxide phases: α-Fe2O3, γ-Fe2O3 and Fe3O4. These nanoparticles were exposed to high temperature (∼935 K) under vacuum (10-3 mbar He pressure), and the thermally induced phase transformations were investigated. The Rietveld refinement of the X-ray diffraction data corroborates the least-squares fitting of the transmission Mössbauer spectra in confirming the presence of Fe3O4, γ-Fe2O3 and α-Fe2O3 phases before the thermal treatment, while only Fe3O4 and α-Fe2O3 phases exist after thermal treatment. On thermal annealing in vacuum, conversion from γ-Fe2O3 to Fe3O4 and α-Fe2O3 was observed. Interestingly, we have observed a phase transformation occurring in the temperature range ∼498 K-538 K, which is strikingly lower than the phase transformation temperature of γ-Fe2O3 to α-Fe2O3 (573-623 K) in air. Combining the results of Rietveld refinement of X-ray diffraction patterns and Mössbauer spectroscopy, we have attributed this phase transformation to the phase conversion of a metastable "defected and strained" d-Fe3O4 phase, present in the as-prepared sample, to the α-Fe2O3 phase. Stabilization of the phases by controlling the phase transformations during the use of different iron-oxide nanoparticles is the key factor to select them for a particular application. Our investigation provides insight into the effect of temperature and chemical nature of the environment, which are the primary factors governing the phase stability, suitability and longevity of the iron oxide nanomaterials prepared by the gas-phase condensation method for various applications.

Surface Characteristics of Machined NiTi Shape Memory Alloy: The Effects of Cryogenic Cooling and Preheating Conditions

NASA Astrophysics Data System (ADS)

Kaynak, Y.; Huang, B.; Karaca, H. E.; Jawahir, I. S.

2017-07-01

This experimental study focuses on the phase state and phase transformation response of the surface and subsurface of machined NiTi alloys. X-ray diffraction (XRD) analysis and differential scanning calorimeter techniques were utilized to measure the phase state and the transformation response of machined specimens, respectively. Specimens were machined under dry machining at ambient temperature, preheated conditions, and cryogenic cooling conditions at various cutting speeds. The findings from this research demonstrate that cryogenic machining substantially alters austenite finish temperature of martensitic NiTi alloy. Austenite finish ( A f) temperature shows more than 25 percent increase resulting from cryogenic machining compared with austenite finish temperature of as-received NiTi. Dry and preheated conditions do not substantially alter austenite finish temperature. XRD analysis shows that distinctive transformation from martensite to austenite occurs during machining process in all three conditions. Complete transformation from martensite to austenite is observed in dry cutting at all selected cutting speeds.

On the Stability of c-BN-Reinforcing Particles in Ceramic Matrix Materials

PubMed Central

Wolfrum, Anne-Kathrin; Michaelis, Alexander; Herrmann, Mathias

2018-01-01

Cubic boron nitride (c-BN) composites produced at high pressures and temperatures are widely used as cutting tool materials. The advent of new, effective pressure-assisted densification methods, such as spark plasma sintering (SPS), has stimulated attempts to produce these composites at low pressures. Under low-pressure conditions, however, transformation of c-BN to the soft hexagonal BN (h-BN) phase can occur, with a strong deterioration in hardness and wear. In the present work, the influence of secondary phases (B2O3, Si3N4, and oxide glasses) on the transformation of c-BN was studied in the temperature range between 1100 °C and 1575 °C. The different heat treated c-BN particles and c-BN composites were analyzed by SEM, X-ray diffraction, and Raman spectroscopy. The transformation mechanism was found to be kinetically controlled solution–diffusion–precipitation. Given a sufficiently low liquid phase viscosity, the transformation could be observed at temperatures as low as 1200 °C for the c-BN–glass composites. In contrast, no transformation was found at temperatures up to 1575 °C when no liquid oxide phase is present in the composite. The results were compared with previous studies concerning the c-BN stability and the c-BN phase diagram. PMID:29414847

Face-centred cubic to body-centred cubic phase transformation under [1 0 0] tensile loading

NASA Astrophysics Data System (ADS)

Xie, Hongxian; Yu, Jiayun; Yu, Tao; Yin, Fuxing

2018-06-01

Molecular dynamics simulation was used to verify a speculation of the existence of a certain face-centred cubic (FCC) to body-centred cubic (BCC) phase transformation pathway. Four FCC metals, Ni, Cu, Au and Ag, were stretched along the [1 0 0] direction at various strain rates and temperatures. Under high strain rate and low temperature, and beyond the elastic limit, the bifurcation of the FCC phase occurred with sudden contraction along one lateral direction and expansion along the other lateral direction. When the lattice constant along the expansion direction converged with that of the stretched direction, the FCC phase transformed into an unstressed BCC phase. By reducing the strain rate or increasing the temperature, dislocation or 'momentum-induced melting' mechanisms began to control the plastic deformation of the FCC metals, respectively.

Method of forming high density oxide pellets by hot pressing at 50$sup 0$ to 100$sup 0$C above the cubic to monoclinic phase transformation temperature

DOEpatents

Pasto, A.E.

1975-08-01

A process for low temperature sintering of rare earth and actinide oxides which have a cubic to monoclinic transformation is described. The process involves hot pressing a powder compact at a temperature just above the transformation temperature. (auth)

The Formation of Martensitic Austenite During Nitridation of Martensitic and Duplex Stainless Steels

NASA Astrophysics Data System (ADS)

Zangiabadi, Amirali; Dalton, John C.; Wang, Danqi; Ernst, Frank; Heuer, Arthur H.

2017-01-01

Isothermal martensite/ferrite-to-austenite phase transformations have been observed after low-temperature nitridation in the martensite and δ-ferrite phases in 15-5 PH (precipitation hardening), 17-7 PH, and 2205 (duplex) stainless steels. These transformations, in the region with nitrogen concentrations of 8 to 16 at. pct, are consistent with the notion that nitrogen is a strong austenite stabilizer and substitutional diffusion is effectively frozen at the paraequilibrium temperatures of our experiments. Our microstructural and diffraction analyses provide conclusive evidence for the martensitic nature of these phase transformations.

Modelling of Strains During SAW Surfacing Taking into Heat of the Weld in Temperature Field Description and Phase Transformations

NASA Astrophysics Data System (ADS)

Winczek, J.; Makles, K.; Gucwa, M.; Gnatowska, R.; Hatala, M.

2017-08-01

In the paper, the model of the thermal and structural strain calculation in a steel element during single-pass SAW surfacing is presented. The temperature field is described analytically assuming a bimodal volumetric model of heat source and a semi-infinite body model of the surfaced (rebuilt) workpiece. The electric arc is treated physically as one heat source. Part of the heat is transferred by the direct impact of the electric arc, while another part of the heat is transferred to the weld by the melted material of the electrode. Kinetics of phase transformations during heating is limited by temperature values at the beginning and at the end of austenitic transformation, while the progress of phase transformations during cooling is determined on the basis of TTT-welding diagramand JMA-K law for diffusive transformations, and K-M law for martensitic transformation. Totalstrains equal to the sum ofthermaland structuralstrainsinduced by phasetransformationsin weldingcycle.

Deformation and Phase Transformation Processes in Polycrystalline NiTi and NiTiHf High Temperature Shape Memory Alloys

NASA Technical Reports Server (NTRS)

Benafan, Othmane

2012-01-01

The deformation and transformation mechanisms of polycrystalline Ni49.9Ti50.1 and Ni50.3Ti29.7Hf20 (in at.%) shape memory alloys were investigated by combined experimental and modeling efforts aided by an in situ neutron diffraction technique at stress and temperature. The thermomechanical response of the low temperature martensite, the high temperature austenite phases, and changes between these two states during thermomechanical cycling were probed and reported. In the cubic austenite phase, stress-induced martensite, deformation twinning and slip processes were observed which helped in constructing a deformation map that contained the limits over which each of the identified mechanisms was dominant. Deformation of the monoclinic martensitic phase was also investigated where the microstructural changes (texture, lattice strains, and phase fractions) during room-temperature deformation and subsequent thermal cycling were compared to the bulk macroscopic response. When cycling between these two phases, the evolution of inelastic strains, along with the shape setting procedures were examined and used for the optimization of the transformation properties as a function of deformation levels and temperatures. Finally, this work was extended to the development of multiaxial capabilities at elevated temperatures for the in situ neutron diffraction measurements of shape memory alloys on the VULCAN Diffractometer at Oak Ridge National Laboratory.

Role of valence electrons in phase transformation kinetics of thallium and its dilute alloys

NASA Technical Reports Server (NTRS)

Ahmed, R.; Ahmed, S.

1991-01-01

The kinetics of the phase transformation of thallium and its dilute alloys were investigated using XRD and calorimetry. Pure thallium exhibits a beta(bcc) to alpha(hcp) phase transformation on cooling at 508 K. With alloying additions, the crystal structure for each phase does not change, although the size of the unit cell increases. The enthalpy and the temperature of phase transformation of each alloy have been determined. The chemical free energy change associated with the phase transformation of each alloy was calculated. The valence electrons make an outstanding contribution to the chemical free energy change required for the phase change.

Scandia-and-Yttria-Stabilized Zirconia for Thermal Barriers

NASA Technical Reports Server (NTRS)

Mess, Derek

2003-01-01

yttria in suitable proportions has shown promise of being a superior thermal- barrier coating (TBC) material, relative to zirconia stabilized with yttria only. More specifically, a range of compositions in the zirconia/scandia/yttria material system has been found to afford increased resistance to deleterious phase transformations at temperatures high enough to cause deterioration of yttria-stabilized zirconia. Yttria-stabilized zirconia TBCs have been applied to metallic substrates in gas turbine and jet engines to protect the substrates against high operating temperatures. These coatings have porous and microcracked structures, which can accommodate strains induced by thermal-expansion mismatch and thermal shock. The longevity of such a coating depends upon yttria as a stabilizing additive that helps to maintain the zirconia in an yttria-rich, socalled non-transformable tetragonal crystallographic phase, thus preventing transformation to the monoclinic phase with an associated deleterious volume change. However, at a temperature greater than about 1,200 C, there is sufficient atomic mobility that the equilibrium, transformable zirconia phase is formed. Upon subsequent cooling, this phase transforms to the monoclinic phase, with an associated volume change that adversely affects the integrity of the coating. Recently, scandia was identified as a stabilizer that could be used instead of, or in addition to, yttria. Of particular interest are scandia-and-yttria-stabilized zirconia (SYSZ) compositions of about 6 mole percent scandia and 1 mole percent yttria, which have been found to exhibit remarkable phase stability at a temperature of 1,400 C in simple aging tests. Unfortunately, scandia is expensive, so that the problem becomes one of determining whether there are compositions with smaller proportions of scandia that afford the required high-temperature stability. In an attempt to solve this problem, experiments were performed on specimens made with reduced proportions of scandia. The criterion used to judge these specimens was whether they retained the non-transformable tetragonal phase after a severe heat treatment of 140 hours at 1,400 C.

Determination of the continuous cooling transformation diagram of a high strength low alloyed steel

NASA Astrophysics Data System (ADS)

Kang, Hun Chul; Park, Bong June; Jang, Ji Hun; Jang, Kwang Soon; Lee, Kyung Jong

2016-11-01

The continuous cooling transformation diagram of a high strength low alloyed steel was determined by a dilatometer and microscopic analysis (OM, SEM) as well as thermodynamic analysis. As expected, Widmanstätten ferrite, bainite and martensite coexisted for most cooling rates, which made it difficult to determine the transformation kinetics of individual phases. However, peaks were clearly observed in the dilatometric {d( {LVDT} )}/{dT} curves. By overlapping the {d( {LVDT} )}/{dT} curves, which were determined using various cooling rates, peaks were separated and the peak rate temperatures, as well as the temperature at the start of transformation (5%) and the end of transformation (95%) of an individual phase, were determined. A SEM analysis was also conducted to identify which phase existed and to quantify the volume fraction of each phase. It was confirmed that the additional {d( {LVDT} )}/{dT} curve analysis described the transformation behavior more precisely than the conventional continuous cooling transformation diagram, as determined by the volume measured from the microstructure analysis.

Transformation of BCC and B2 High Temperature Phases to HCP and Orthorhombic Structures in the Ti-Al-Nb System. Part II: Experimental TEM Study of Microstructures

PubMed Central

Bendersky, L. A.; Boettinger, W. J.

1993-01-01

Possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted by Bendersky, Roytburd, and Boettinger [J. Res. Natl. Inst. Stand. Technol. 98, 561 (1993)] for Ti-Al-Nb alloys cooled from the high temperature BCC/B2 phase field into close-packed orthorhombic or hexagonal phase fields. These predictions were based on structural and symmetry relations between the known phases. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti-25Al-12.5Nb (at%), the path includes the formation of intermediate hexagonal phases, A3 and DO19, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti-25Al-25Nb (at%), the path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing at 700 °C. The structure forms by congruent ordering of the DO19 phase to the O phase, and then reprecipitation of the DO19 phase, possibly by a spinodal mechanism. The thermodynamics underlying the path selection and the two-phase formation are also discussed. PMID:28053488

Atomic disorder, phase transformation, and phase restoration in Co3Sn2

NASA Astrophysics Data System (ADS)

di, L. M.; Zhou, G. F.; Bakker, H.

1993-03-01

The behavior of the intermetallic compound Co3Sn2 upon ball milling was studied by x-ray diffraction, high-field-magnetization measurements, and subsequently by differential scanning calorimetry. It turns out that starting from the stoichiometric-ordered compound, mechanical attrition of Co3Sn2 generates atomic disorder in the early stage of milling. The nonequilibrium phase transformation from the low-temperature phase with orthorhombic structure to the high-temperature phase with a hexagonal structure was observed in the intermediate stage of milling. It was accompanied by the creation of increasing atomic disorder. After long milling periods, the phase transformation was completed and the atomic disordering became saturated. All the physical parameters measured in the present work remained constant during this period. The above outcome was confirmed by comparison with the high-temperature phase thermally induced by quenching. The good agreement of the results obtained by different techniques proves that the ball milling generates well-defined metastable states in Co3Sn2.

In situ Observation of Phase Transformation in MnAl(C) Magnetic Materials

PubMed Central

Si, Ping-Zhan; Qian, Hui-Dong; Choi, Chul-Jin; Park, Jihoon; Han, Sangho; Ge, Hong-Liang; Shinde, Kiran P.

2017-01-01

The phase transformation in two modes, including both displacive and massive growth of τ-phase from ε-MnAl(C), was observed by in situ transmission electron microscopy. The exact temperature range for different phase transformation modes was determined by magnetic measurements. The displacive growth of ε→τ in Mn54Al46 (or Mn54Al46C2.44) occurs at temperatures below 650 K (or 766 K), above which both modes coexist. One-third or less of the ε-phase can be transformed into τ-phase via displacive mode while the remaining two-thirds or more via massive mode. In bulk τ-phase, most τ-nanocrystals formed via displacive mode are distributed in the matrix of large τ-grains that formed via massive mode. The typical massive growth rate of the τ-phase is 8–60 nm/s, while the displacive growth rate is low. A more complete understanding of the ε→τ phase transformations in the MnAl-based magnets was provided in this work, based on which the annealing process for ε→τ was optimized and thus high purity τ-phase with high saturation magnetization was obtained. PMID:28858231

Phase and microstructural development in alumina sol-gel coatings on CoCr alloy.

PubMed

Bae, I J; Standard, O C; Roger, G J; Brazil, D

2004-09-01

Phase transformation of gamma-Al(2)O(3) to alpha-Al(2)O(3) in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above approximately 1200 degrees C. Addition of alpha-Al(2)O(3) seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 degrees C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the alpha-Al(2)O(3) phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of alpha-Al(2)O(3) grains at the expense of the precursor fine-grained gamma-Al(2)O(3) matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was approximately 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.

Phase-shift detection in a Fourier-transform method for temperature sensing using a tapered fiber microknot resonator.

PubMed

Larocque, Hugo; Lu, Ping; Bao, Xiaoyi

2016-04-01

Phase-shift detection in a fast-Fourier-transform (FFT)-based spectrum analysis technique for temperature sensing using a tapered fiber microknot resonator is proposed and demonstrated. Multiple transmission peaks in the FFT spectrum of the device were identified as optical modes having completed different amounts of round trips within the ring structure. Temperature variation induced phase shifts for each set of peaks were characterized, and experimental results show that different peaks have distinct temperature sensitivities reaching values up to -0.542  rad/°C, which is about 10 times greater than that of a regular adiabatic taper Mach-Zehnder interferometer when using similar phase-tracking schemes.

Phase transformation in multiferroic Bi5Ti3FeO15 ceramics by temperature-dependent ellipsometric and Raman spectra: An interband electronic transition evidence

NASA Astrophysics Data System (ADS)

Jiang, P. P.; Duan, Z. H.; Xu, L. P.; Zhang, X. L.; Li, Y. W.; Hu, Z. G.; Chu, J. H.

2014-02-01

Thermal evolution and an intermediate phase between ferroelectric orthorhombic and paraelectric tetragonal phase of multiferroic Bi5Ti3FeO15 ceramic have been investigated by temperature-dependent spectroscopic ellipsometry and Raman scattering. Dielectric functions and interband transitions extracted from the standard critical-point model show two dramatic anomalies in the temperature range of 200-873 K. It was found that the anomalous temperature dependence of electronic transition energies and Raman mode frequencies around 800 K can be ascribed to intermediate phase transformation. Moreover, the disappearance of electronic transition around 3 eV at 590 K is associated with the conductive property.

Thermal and structural alternations in CuAlMnNi shape memory alloy by the effect of different pressure applications

NASA Astrophysics Data System (ADS)

Canbay, Canan Aksu; Polat, Tercan

2017-09-01

In this work the effects of the applied pressure on the characteristic transformation temperatures, the high temperature order-disorder phase transitions, the variation in diffraction peaks and the surface morphology of the CuAlMnNi shape memory alloy was investigated. The evolution of the transformation temperatures was studied by differential scanning calorimetry (DSC) with different heating and cooling rates. The differential thermal analysis measurements were performed to obtain the ordered-disordered phase transformations from room temperature to 900 °C. The characteristic transformation temperatures and the thermodynamic parameters were highly sensitive to variations in the applied pressure and also the applied pressure affected the thermodynamic parameters. The activation energy of the sample according to applied pressure values calculated by Kissinger method. The structural changes of the samples were studied by X-ray diffraction (XRD) measurements and by optical microscope observations at room temperature.

Shock Condition Forensics and Cryptic Phase Transformations from Crystallographic Orientation Relationships in Zircon

NASA Astrophysics Data System (ADS)

Timms, N. E.; Erickson, T. M.; Cavosie, A. J.; Pearce, M. A.; Reddy, S. M.; Zanetti, M.; Tohver, E.; Schmieder, M.; Nemchin, A. A.; Wittmann, A.

2016-08-01

We present an approach to constrain pressure and temperature conditions during impact events involving identification of cryptic histories of phase transformations from orientation relationships in shocked zircon, linked to new P-T phase diagrams.

Effect of yttrium on martensite-austenite phase transformation temperatures and high temperature oxidation kinetics of Ti-Ni-Hf high-temperature shape memory alloys

NASA Astrophysics Data System (ADS)

Kim, Jeoung Han; Kim, Kyong Min; Yeom, Jong Taek; Young, Sung

2016-03-01

The effect of yttrium (< 5.5 at%) on the martensite-austenite phase transformation temperatures, microstructural evolution, and hot workability of Ti-Ni-Hf high-temperature shape memory alloys is investigated. For these purposes, differential scanning calorimetry, hot compression, and thermo-gravimetric tests are conducted. The phase transformation temperatures are not noticeably influenced by the addition of yttrium up to 4.5 at%. Furthermore, the hot workability is not significantly affected by the yttrium addition up to 1.0 at%. However, when the amount of yttrium addition exceeds 1.0 at%, the hot workability deteriorates significantly. In contrast, remarkable improvement in the high temperature oxidation resistance due to the yttrium addition is demonstrated. The total thickness of the oxide layers is substantially thinner in the Y-added specimen. In particular, the thickness of (Ti,Hf) oxide layer is reduced from 200 µm to 120 µm by the addition of 0.3 at% Y.

Effect of Thermal Treatments on Ni-Mn-Ga and Ni-Rich Ni-Ti-Hf/Zr High-Temperature Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Santamarta, Ruben; Evirgen, Alper; Perez-Sierra, Aquilina M.; Pons, Jaume; Cesari, Eduard; Karaman, Ibrahim; Noebe, Ron D.

2015-11-01

Among all the promising high-temperature shape memory alloys (HTSMAs), the Ni-Mn-Ga and the Ni-Ti-Hf/Zr systems exhibit interesting shape memory and superelastic properties that may place them in a good position for potential applications. The present work shows that thermal treatments play a crucial role in controlling the martensitic phase transformation characteristics of both systems, but in different ways. On one hand, the equilibrium phase diagram of the Ni-Mn-Ga family allows selecting compositions with high transformation temperatures and outstanding thermal stability at relatively high temperatures in air, showing no significant changes in the transformation behavior for continuous aging up to ˜5 years at 500 °C. Moreover, the excellent thermal stability correlates with a good thermal cyclic stability and an exceptional oxidation resistance of the parent phase. On the other hand, precipitation processes controlled by thermal treatments are needed to manipulate the transformation temperatures, mechanical properties, and thermal stability of Ni-rich Ni-Ti-Hf/Zr alloys to become HTSMAs. These changes in the functional properties are a consequence of the competition between the mechanical and compositional effects of the precipitates on the martensitic transformation.

Phase-Transformation-Induced Extra Thermal Expansion Behavior of (SrxBa1-x)TiO3/Cu Composite.

PubMed

Sheng, Jie; Wang, Lidong; Li, Shouwei; Yin, Benke; Liu, Xiangli; Fei, Wei-Dong

2016-06-03

The properties of metal matrix composites (MMCs) can be optimized effectively through adjusting the type or the volume fraction of reinforcement. Generally, the coefficient of thermal expansion (CTE) of MMCs can be reduced by increasing the volume fraction of the reinforcement with lower CTE than metal matrix. However, it is great challenge to fabricate low CTE MMCs with low reinforcement volume fraction because of the limitation of reinforcement CTEs. SrxBa1-xTiO3 (SBT) powder presents negative thermal expansion behavior during the phase transformation from tetragonal to cubic phase. Here, we demonstrate that the phase transformation of SBT can be utilized to reduce and design the thermal expansion properties of SBT particle-reinforced Cu (SBT/Cu) composite, and ultralow CTE can be obtained in SBT/Cu composite. The X-ray diffraction analysis on heating indicates that the temperature range of phase transformation is extended greatly, therefore, the low CTE can be achieved within wide temperature range. Landau-Devonshire theory study on the phase transformation behaviors of SBT particles in the composite indicates that thermal mismatch stress significantly affects the Curie temperature of SBT particles and the CTE of the composite. The results given in the present study provide a new approach to design the MMCs with low CTE.

Low temperature synthesis of monolithic transparent Ta2O5 gels from hydrolysis of metal alkoxide

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.

1993-01-01

Tantalum oxide gels in the form of transparent monoliths and powder were prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC2H5)5:C2H50H:H20:HCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide:water:HCl ratio, time of gel formation increased with the alcohol to alkoxide mole ratio. Thermal evolution of the physical and structural changes in the gel was monitored by differential thermal analysis, thermogravimetric analysis, x-ray diffraction, and infrared spectroscopy. On heating to approximately 400 C, the amorphous gel crystallized into the low temperature orthorhombic phase Beta-Ta2O5, which transformed into the high temperature tetragonal phase Alpha-Ta2O5 when further heated to approximately 1450 C. The volume fraction of the crystalline phase increased with the firing temperature. The Alpha-Ta205 converted back into the low temperature phase, Beta-Ta2O5, on slow cooling through the transformation temperature of 1360 C indicating a slow but reversible transformation.

Characterization of Ternary NiTiPt High-Temperature Shape Memory Alloys

NASA Technical Reports Server (NTRS)

Rios, Orlando; Noebe, Ronald; Biles, Tiffany; Garg, Anita; Palczer, Anna; Scheiman, Daniel; Seifert, Hans Jurgen; Kaufman, Michael

2005-01-01

Pt additions substituted for Ni in NiTi alloys are known to increase the transformation temperature of the alloy but only at fairly high Pt levels. However, until now only ternary compositions with a very specific stoichiometry, Ni50-xPtxTi50, have been investigated and then only to very limited extent. In order to learn about this potential high-temperature shape memory alloy system, a series of over twenty alloys along and on either side of a line of constant stoichiometry between NiTi and TiPt were arc melted, homogenized, and characterized in terms of their microstructure, transformation temperatures, and hardness. The resulting microstructures were examined by scanning electron microscopy and the phase compositions quantified by energy dispersive spectroscopy."Stoichiometric" compositions along a line of constant stoichiometry between NiTi to TiPt were essentially single phase but by any deviations from a stoichiometry of (Ni,Pt)50Ti50 resulted in the presence of at least two different intermetallic phases, depending on the overall composition of the alloy. Essentially all alloys, whether single or two-phase, still under went a martensitic transformation. It was found that the transformation temperatures were depressed with initial Pt additions but at levels greater than 10 at.% the transformation temperature increased linearly with Pt content. Also, the transformation temperatures were relatively insensitive to alloy stoichiometry within the range of alloys examined. Finally, the dependence of hardness on Pt content for a series of Ni50-xPtxTi50 alloys showed solution softening at low Pt levels, while hardening was observed in ternary alloys containing more than about 10 at.% Pt. On either side of these "stoichiometric" compositions, hardness was also found to increase significantly.

Matrix Transformation in Boron Containing High-Temperature Co-Re-Cr Alloys

NASA Astrophysics Data System (ADS)

Strunz, Pavel; Mukherji, Debashis; Beran, Přemysl; Gilles, Ralph; Karge, Lukas; Hofmann, Michael; Hoelzel, Markus; Rösler, Joachim; Farkas, Gergely

2018-03-01

An addition of boron largely increases the ductility in polycrystalline high-temperature Co-Re alloys. Therefore, the effect of boron on the alloy structural characteristics is of high importance for the stability of the matrix at operational temperatures. Volume fractions of ɛ (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr2Re3 type) phases were measured at ambient and high temperatures (up to 1500 °C) for a boron-containing Co-17Re-23Cr alloy using neutron diffraction. The matrix phase undergoes an allotropic transformation from ɛ to γ structure at high temperatures, similar to pure cobalt and to the previously investigated, more complex Co-17Re-23Cr-1.2Ta-2.6C alloy. It was determined in this study that the transformation temperature depends on the boron content (0-1000 wt. ppm). Nevertheless, the transformation temperature did not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm of boron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which binds hcp-stabilizing elements (Cr and Re). Moreover, borides were identified in alloys with high boron content.

Cyclic phase change in a cylindrical thermal energy storage capsule

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hasan, M.; Mujumdar, A.S.; Weber, M.E.

1983-12-01

This paper is concerned with a practical melting/freezing problem in conjunction with the more realistic case of a cyclic phase change thermal energy storage device. In this model the phase change medium is encapsulated in long cylindrical tubes, the surface temperature of which is allowed to vary sinusoidally with time about the discrete freezing temperature. Initial temperature of the medium is assumed to be constant at a temperature above or below the freezing/melting temperature. Natural convection in the melt is assumed to be negligible and the variations in the depth of freezing and/or melting in each half cycle is ignored.more » Depending on the half-cycle parameters the problem is simplified to either freezing or melting. The governing one-dimensional heat diffusion equations for both phases are solved by the Finite Integral Transform techniques. The kernels for the transformation are the time-dependent eigen functions separately defined for each phases. This extended transform method can accomodate any time-dependent surface temperature variation. The application of the transform generated a series of coupled, nonlinear first order differential equations, which are solved by Runge Kutta-Verner fifth and sixth order method. Dimensionless solutions of temperature variations in both phases, fusion front position and the fraction solidified (or melted) are displayed graphically to aid in practical calculations. For the special case of a constant surface temperature, comparisons are made between the present results and the existing integral and purely numerical results. The results are found to compare favourably. Results for fractional solidification (or melting and interface position are also compared with the simple Conduction Shape Factor method, after allowing for the time-dependent boundary conditions. Once again the results agree reasonably well.« less

Influence of ignition process on mineral phase transformation in municipal solid waste incineration (MSWI) fly ash: Implications for estimating loss-on-ignition (LOI).

PubMed

Mu, Yue; Saffarzadeh, Amirhomayoun; Shimaoka, Takayuki

2017-01-01

This research focused on the mineral phase transformation under varied ignition conditions with the objective of estimating loss-on-ignition (LOI) parameter in municipal solid waste incineration (MSWI) fly ash residues. LOI is commonly used to measure the volatile species, unburned carbon and moisture in the solid materials. There are criteria for LOI measurement in some research fields, while there is no standard protocol for LOI measurement in MSWI fly ash. Using thermogravimetry technique, the ignition condition candidates were proposed at 440/700/900°C for 1 and 2h. Based on X-ray diffractometry results, obvious mineral phase transformation occurred as a function of ignition temperature variation rather than ignition time. Until 440°C, only some minor phases disappeared comparing with the original state. Significant mineral phase transformations of major phases (Ca- and Cl-based minerals) occurred between 440 and 700°C. The mineral phase transformation and the occurrence of newly-formed phases were determined not only by the ignition condition but also by the content of the co-existing components. Mineral phase components rarely changed when ignition temperature rose from 700 to 900°C. Consequently, in order to prevent critical damages to the original mineralogical composition of fly ash, the lowest ignition temperature (440°C) for 2h was suggested as an ideal measurement condition of LOI in MSWI fly ash. Copyright © 2016 Elsevier Ltd. All rights reserved.

Room-Temperature Deformation and Martensitic Transformation of Two Co-Cr-Based Alloys

NASA Astrophysics Data System (ADS)

Cai, S.; Schaffer, J. E.; Huang, D.; Gao, J.; Ren, Y.

2018-05-01

Deformation of two Co-Cr alloys was studied by in situ synchrotron X-ray diffraction. Both alloys show stress-induced martensite transformation, which is affected by phase stabilities and transformation strains. Crystal structure of WC in Co-20Cr-15W-10Ni is identified. Compared with other phases present, it is elastically isotropic, exhibits high strength, and can elastically withstand strains exceeding 1 pct. Texture change during phase transformation is explained based on the crystal orientation relationship between γ- and ɛ-phases.

Room-Temperature Deformation and Martensitic Transformation of Two Co-Cr-Based Alloys

NASA Astrophysics Data System (ADS)

Cai, S.; Schaffer, J. E.; Huang, D.; Gao, J.; Ren, Y.

2018-07-01

Deformation of two Co-Cr alloys was studied by in situ synchrotron X-ray diffraction. Both alloys show stress-induced martensite transformation, which is affected by phase stabilities and transformation strains. Crystal structure of WC in Co-20Cr-15W-10Ni is identified. Compared with other phases present, it is elastically isotropic, exhibits high strength, and can elastically withstand strains exceeding 1 pct. Texture change during phase transformation is explained based on the crystal orientation relationship between γ- and ɛ-phases.

Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide

DOE PAGES

Whitfield, P. S.; Herron, N.; Guise, W. E.; ...

2016-10-21

Here, we examine the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI 3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q (T c-T) , where T c is the critical temperature and the exponent was close to , as predicted for a tricritical phase transition. We also observed coexistence of the cubic and tetragonal phases over amore » range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Finally, based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI 3 based solar cells.« less

Phase equilibrium modeling for high temperature metallization on GaAs solar cells

NASA Technical Reports Server (NTRS)

Chung, M. A.; Davison, J. E.; Smith, S. R.

1991-01-01

Recent trends in performance specifications and functional requirements have brought about the need for high temperature metallization technology to be developed for survivable DOD space systems and to enhance solar cell reliability. The temperature constitution phase diagrams of selected binary and ternary systems were reviewed to determine the temperature and type of phase transformation present in the alloy systems. Of paramount interest are the liquid-solid and solid-solid transformations. Data are being utilized to aid in the selection of electrical contact materials to gallium arsenide solar cells. Published data on the phase diagrams for binary systems is readily available. However, information for ternary systems is limited. A computer model is being developed which will enable the phase equilibrium predictions for ternary systems where experimental data is lacking.

Solid-solid phase transformation via internal stress-induced virtual melting, significantly below the melting temperature. Application to HMX energetic crystal.

PubMed

Levitas, Valery I; Henson, Bryan F; Smilowitz, Laura B; Asay, Blaine W

2006-05-25

We theoretically predict a new phenomenon, namely, that a solid-solid phase transformation (PT) with a large transformation strain can occur via internal stress-induced virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. We show that the energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking which does not affect the thermodynamics or kinetics of the solid-solid transformation. Thus, virtual melting represents a new mechanism of solid-solid PT, stress relaxation, and loss of coherence at a moving solid-solid interface. It also removes the athermal interface friction and deletes the thermomechanical memory of preceding cycles of the direct-reverse transformation. It is also found that nonhydrostatic compressive internal stresses promote melting in contrast to hydrostatic pressure. Sixteen theoretical predictions are in qualitative and quantitative agreement with experiments conducted on the PTs in the energetic crystal HMX. In particular, (a) the energy of internal stresses is sufficient to reduce the melting temperature from 551 to 430 K for the delta phase during the beta --> delta PT and from 520 to 400 K for the beta phase during the delta --> beta PT; (b) predicted activation energies for direct and reverse PTs coincide with corresponding melting energies of the beta and delta phases and with the experimental values; (c) the temperature dependence of the rate constant is determined by the heat of fusion, for both direct and reverse PTs; results b and c are obtained both for overall kinetics and for interface propagation; (d) considerable nanocracking, homogeneously distributed in the transformed material, accompanies the PT, as predicted by theory; (e) the nanocracking does not change the PT thermodynamics or kinetics appreciably for the first and the second PT beta <--> delta cycles, as predicted by theory; (f) beta <--> delta PTs start at a very small driving force (in contrast to all known solid-solid transformations with large transformation strain), that is, elastic energy and athermal interface friction must be negligible; (g) beta --> alpha and alpha --> beta PTs, which are thermodynamically possible in the temperature range 382.4 < theta < 430 K and below 382.4 K, respectively, do not occur.

Time and metamorphic petrology: Calcite to aragonite experiments

USGS Publications Warehouse

Hacker, B.R.; Kirby, S.H.; Bohlen, S.R.

1992-01-01

Although the equilibrium phase relations of many mineral systems are generally well established, the rates of transformations, particularly in polycrystalline rocks, are not. The results of experiments on the calcite to aragonite transformation in polycrystalline marble are different from those for earlier experiments on powdered and single-crystal calcite. The transformation in the polycrystalline samples occurs by different mechanisms, with a different temperature dependence, and at a markedly slower rate. This work demonstrates the importance of kinetic studies on fully dense polycrystalline aggregates for understanding mineralogic phase changes in nature. Extrapolation of these results to geological time scales suggests that transformation of calcite to aragonite does not occur in the absence of volatiles at temperatures below 200??C. Kinetic hindrance is likely to extend to higher temperatures in more complex transformations.

Optical and magneto-optical studies of martensitic transformation in Ni-Mn-Ga magnetic shape memory alloys

NASA Astrophysics Data System (ADS)

Beran, L.; Cejpek, P.; Kulda, M.; Antos, R.; Holy, V.; Veis, M.; Straka, L.; Heczko, O.

2015-05-01

Optical and magneto-optical properties of single crystal of Ni50.1Mn28.4Ga21.5 magnetic shape memory alloy during its transformation from martensite to austenite phase were systematically studied. Crystal orientation was approximately along {100} planes of parent cubic austenite. X-ray reciprocal mapping confirmed modulated 10 M martensite phase. Temperature depended measurements of saturation magnetization revealed the martensitic transformation at 335 K during heating. Magneto-optical spectroscopy and spectroscopic ellipsometry were measured in the sample temperature range from 297 to 373 K and photon energy range from 1.2 to 6.5 eV. Magneto-optical spectra of polar Kerr rotation as well as the spectra of ellipsometric parameter Ψ exhibited significant changes when crossing the transformation temperature. These changes were assigned to different optical properties of Ni-Mn-Ga in martensite and austenite phases due to modification of electronic structure near the Fermi energy during martensitic transformation.

Deformation of olivine during phase transformation to wadsleyite

NASA Astrophysics Data System (ADS)

Mohiuddin, A.; Girard, J.; Karato, S. I.

2017-12-01

The strength of subducting slabs in the transition zone is critical in controlling the style of mantle convection. However, rheological properties of a subducted slab are elusive: low temperatures of a slab would make slabs strong, but in many regions there is evidence of intense deformation of slabs in the transition zone. One potential cause of intense deformation of subducting slabs is grain size reduction and accompanied microstructural changes during phase transformation of olivine to its higher-pressure polymorphs. There have been no experimental studies to quantify the influence of grain-size evolution. In addition to grain size reduction, distribution of small grains during phase transformation governs the degree of weakening during phase transformation (for e.g. load bearing framework vs. inter-connected layered framework). We conducted laboratory studies on the size and spatial distribution of new grains of wadsleyite after the transformation from olivine. Our results under static conditions show that an interconnected microstructure develops during the initial stage of phase transformation and that the grain size of the interconnected phase (wadsleyite) depends on the temperature at which the phase transformation occurs (smaller grains at lower temperatures). Development of an interconnected microstructure may lead to strain localization in the weaker phase, i.e. the fine-grained interconnected network accommodates most of the strain and therefore weakening of the entire composite. We will test this model through a series of two synchrotron in-situ deformation experiments: (i) Olivine aggregate will be deformed during slow pressure increase from deep upper mantle pressure ( 10 GPa) to transition zone pressure ( 15 GPa) at a given temperature simulating the deformation of a slab penetrating into the transition zone (ii) olivine is partially transformed to wadsleyite in a multi anvil apparatus at Yale and will be deformed within the stability field where olivine and wadsleyite coexist. We will use the Rotational Drickamer Apparatus (RDA) at a synchrotron facility (Argonne National Lab, 6-BM-B beamline, white beam and x-ray radiography) and characterize the stresses acting on olivine and wadsleyite during such simulations. We plan to present our preliminary results.

Temperature and field induced strain measurements in single crystal Gd 5Si 2Ge 2

DOE PAGES

McCall, S. K.; Nersessian, N.; Carman, G. P.; ...

2016-03-29

The first-order magneto-structural transformation that occurs in Gd 5Si 2Ge 2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd 5Si 2.05Ge 1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of –8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of –8500 ppm, +900 ppm and +2300 ppm were measured along the a,more » b and c axes, respectively. Furthermore, using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.« less

Impact of asymmetric martensite and austenite nucleation and growth behavior on the phase stability and hysteresis of freestanding shape-memory nanoparticles

NASA Astrophysics Data System (ADS)

Ko, Won-Seok; Grabowski, Blazej; Neugebauer, Jörg

2018-03-01

Martensitic transformations in nanoscaled shape-memory alloys exhibit characteristic features absent for the bulk counterparts. Detailed understanding is required for applications in micro- and nanoelectromechanical systems, and experimental limitations render atomistic simulation an important complementary approach. Using a recently developed, accurate potential we investigate the phase transformation in freestanding Ni-Ti shape-memory nanoparticles with molecular-dynamics simulations. The results confirm that the decrease in the transformation temperature with decreasing particle size is correlated with an overstabilization of the austenitic surface energy over the martensitic surface energy. However, a detailed atomistic analysis of the nucleation and growth behavior reveals an unexpected difference in the mechanisms determining the austenite finish and martensite start temperature. While the austenite finish temperature is directly affected by a contribution of the surface energy difference, the martensite start temperature is mostly affected by the transformation strain, contrary to general expectations. This insight not only explains the reduced transformation temperature but also the reduced thermal hysteresis in freestanding nanoparticles.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beran, L.; Cejpek, P.; Kulda, M.

Optical and magneto-optical properties of single crystal of Ni{sub 50.1}Mn{sub 28.4}Ga{sub 21.5} magnetic shape memory alloy during its transformation from martensite to austenite phase were systematically studied. Crystal orientation was approximately along (100) planes of parent cubic austenite. X-ray reciprocal mapping confirmed modulated 10 M martensite phase. Temperature depended measurements of saturation magnetization revealed the martensitic transformation at 335 K during heating. Magneto-optical spectroscopy and spectroscopic ellipsometry were measured in the sample temperature range from 297 to 373 K and photon energy range from 1.2 to 6.5 eV. Magneto-optical spectra of polar Kerr rotation as well as the spectra of ellipsometric parameter Ψ exhibitedmore » significant changes when crossing the transformation temperature. These changes were assigned to different optical properties of Ni-Mn-Ga in martensite and austenite phases due to modification of electronic structure near the Fermi energy during martensitic transformation.« less

Phase transformation in δ-Pu alloys at low temperature: An in situ microstructural characterization using X-ray diffraction

NASA Astrophysics Data System (ADS)

Ravat, B.; Platteau, C.; Texier, G.; Oudot, B.; Delaunay, F.

2009-09-01

In order to investigate the martensitic transformation, an isothermal hold at -130 °C for 48 h was performed on a highly homogenized PuGa alloy. The modifications of the microstructure were characterized in situ thanks to a specific tool. This device was developed at the CEA-Valduc to analyze the crystalline structure of plutonium alloys as a function of temperature and more especially at low temperature using X-ray diffraction. The analysis of the recorded diffraction patterns highlighted that the martensitic transformation for this alloy is the result of a direct δ → α' + δ phase transformation. Moreover, a significant Bragg's peaks broadening corresponding to the δ-phase was observed. A microstructural analysis was made to characterize anisotropic microstrain resulting from the stress induced by the unit cell volume difference between the δ and α' phases. The amount of α'-phase evolved was analyzed within the framework of the Avrami theory in order to characterize the nucleation process. The results suggested that the growth mechanism corresponded to a general mechanism where the nucleation sites were in the δ-grain edges and the α'-phase had a plate-like morphology.

Phase transformation in tantalum under extreme laser deformation

DOE PAGES

Lu, C. -H.; Hahn, E. N.; Remington, B. A.; ...

2015-10-19

The structural and mechanical response of metals is intimately connected to phase transformations. For instance, the product of a phase transformation (martensite) is responsible for the extraordinary range of strength and toughness of steel, making it a versatile and important structural material. Although abundant in metals and alloys, the discovery of new phase transformations is not currently a common event and often requires a mix of experimentation, predictive computations, and luck. High-energy pulsed lasers enable the exploration of extreme pressures and temperatures, where such discoveries may lie. The formation of a hexagonal (omega) phase was observed in recovered monocrystalline body-centeredmore » cubic tantalum of four crystallographic orientations subjected to an extreme regime of pressure, temperature, and strain-rate. This was accomplished using high-energy pulsed lasers. The omega phase and twinning were identified by transmission electron microscopy at 70 GPa (determined by a corresponding VISAR experiment). It is proposed that the shear stresses generated by the uniaxial strain state of shock compression play an essential role in the transformation. In conclusion, molecular dynamics simulations show the transformation of small nodules from body-centered cubic to a hexagonal close-packed structure under the same stress state (pressure and shear).« less

Phase Transformation in Tantalum under Extreme Laser Deformation

PubMed Central

Lu, C.-H.; Hahn, E. N.; Remington, B. A.; Maddox, B. R.; Bringa, E. M.; Meyers, M. A.

2015-01-01

The structural and mechanical response of metals is intimately connected to phase transformations. For instance, the product of a phase transformation (martensite) is responsible for the extraordinary range of strength and toughness of steel, making it a versatile and important structural material. Although abundant in metals and alloys, the discovery of new phase transformations is not currently a common event and often requires a mix of experimentation, predictive computations, and luck. High-energy pulsed lasers enable the exploration of extreme pressures and temperatures, where such discoveries may lie. The formation of a hexagonal (omega) phase was observed in recovered monocrystalline body-centered cubic tantalum of four crystallographic orientations subjected to an extreme regime of pressure, temperature, and strain-rate. This was accomplished using high-energy pulsed lasers. The omega phase and twinning were identified by transmission electron microscopy at 70 GPa (determined by a corresponding VISAR experiment). It is proposed that the shear stresses generated by the uniaxial strain state of shock compression play an essential role in the transformation. Molecular dynamics simulations show the transformation of small nodules from body-centered cubic to a hexagonal close-packed structure under the same stress state (pressure and shear). PMID:26478106

Martensitic transformation and phase diagram in ternary Co-V-Ga Heusler alloys

NASA Astrophysics Data System (ADS)

Xu, Xiao; Nagashima, Akihide; Nagasako, Makoto; Omori, Toshihiro; Kanomata, Takeshi; Kainuma, Ryosuke

2017-03-01

We report the martensitic transformation behavior in Co-V-Ga Heusler alloys. Thermoanalysis and thermomagnetization measurements were conducted to observe the martensitic transformation. By using a transmission electron microscope and an in situ X-ray diffractometer, martensitic transformation was found to occur from the L21 Heusler parent phase to the D022 martensite phase. Phase diagrams were determined for two pseudo-binary sections where martensitic transformation was detected. Magnetic properties, including the Curie temperatures and spontaneous magnetization of the parent phase, were also investigated. The magnetic properties showing behaviors different from those of NiMn-based alloys were found.

High pressure–low temperature phase diagram of barium: Simplicity versus complexity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Desgreniers, Serge; Tse, John S., E-mail: John.Tse@usask.ca; State Key Laboratory of Superhard Materials, Jilin University, 130012 Changchun

2015-11-30

Barium holds a distinctive position among all elements studied upon densification. Indeed, it was the first example shown to violate the long-standing notion that high compression of simple metals should preserve or yield close-packed structures. From modest pressure conditions at room temperature, barium transforms at higher pressures from its simple structures to the extraordinarily complex atomic arrangements of the incommensurate and self-hosting Ba-IV phases. By a detailed mapping of the pressure/temperature structures of barium, we demonstrate the existence of another crystalline arrangement of barium, Ba-VI, at low temperature and high pressure. The simple structure of Ba-VI is unlike that ofmore » complex Ba-IV, the phase encountered in a similar pressure range at room temperature. First-principles calculations predict Ba-VI to be stable at high pressure and superconductive. The results illustrate the complexity of the low temperature-high pressure phase diagram of barium and the significant effect of temperature on structural phase transformations.« less

Phase transformation strengthening of high-temperature superalloys

PubMed Central

Smith, T. M.; Esser, B. D.; Antolin, N.; Carlsson, A.; Williams, R. E. A.; Wessman, A.; Hanlon, T.; Fraser, H. L.; Windl, W.; McComb, D. W.; Mills, M. J.

2016-01-01

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new ‘phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ′ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties. PMID:27874007

Measurement of the temperature coefficient of ratio transformers

NASA Technical Reports Server (NTRS)

Briggs, Matthew E.; Gammon, Robert W.; Shaumeyer, J. N.

1993-01-01

We have measured the temperature coefficient of the output of several ratio transformers at ratios near 0.500,000 using an ac bridge and a dual-phase, lock-in amplifier. The two orthogonal output components were each resolved to +/- ppb of the bridge drive signal. The results for three commercial ratio transformers between 20 and 50 C range from 0.5 to 100 ppb/K for the signal component in phase with the bridge drive, and from 4 to 300 ppb/K for the quadrature component.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jana, S.; Devaraj, A.; Kovarik, L.

Transformation kinetics of metastable body-centered cubic γ-UMo phase in U-10 wt.percent Mo alloy during annealing at sub-eutectoid temperatures of 500C and 400C has been determined as a function of time using detailed microstructural characterization by scanning electron microscopy, X-ray diffraction analysis, scanning transmission electron microscopy, and atom probe tomography. Based on the results, we found that the phase transformation is initiated by cellular transformation at both the temperatures, which results in formation of a lamellar microstructure along prior γ-UMo grain boundaries.

The nature of the structural phase transition from the hexagonal (4H) phase to the cubic (3C) phase of silver.

PubMed

Chakraborty, Indrani; Shirodkar, Sharmila N; Gohil, Smita; Waghmare, Umesh V; Ayyub, Pushan

2014-03-19

The phase transition from the hexagonal 4H polytype of silver to the commonly known 3C (fcc) phase was studied in detail using x-ray diffraction, electron microscopy, differential scanning calorimetry and Raman spectroscopy. The phase transition is irreversible and accompanied by extensive microstructural changes and grain growth. Detailed scanning and isothermal calorimetric analysis suggests that it is an autocatalytic transformation. Though the calorimetric data suggest an exothermic first-order phase transition with an onset at 155.6 °C (for a heating rate of 2 K min(-1)) and a latent heat of 312.9 J g(-1), the microstructure and the electrical resistance appear to change gradually from much lower temperatures. The 4H phase shows a Raman active mode at 64.3 cm(-1) (at 4 K) that undergoes mode softening as the 4H → 3C transformation temperature is approached. A first-principles density functional theory calculation shows that the stacking fault energy of 4H-Ag increases monotonically with temperature. That 4H-Ag has a higher density of stacking faults than 3C-Ag, implies the metastability of the former at higher temperatures. Energetically, the 4H phase is intermediate between the hexagonal 2H phase and the 3C ground state, as indicated by the spontaneous transformation of the 2H to the 4H phase at -4 °C. Our data appear to indicate that the 4H-Ag phase is stabilized at reduced dimensions and thermally induced grain growth is probably responsible for triggering the irreversible transformation to cubic Ag.

Alpha – omega and omega – alpha phase transformations in zirconium under hydrostatic pressure: A 3D mesoscale study

DOE PAGES

Yeddu, Hemantha Kumar; Zong, Hongxiang; Lookman, Turab

2015-09-28

Here, a three dimensional (3D) elastoplastic phase-field model is developed for modeling the hydrostatic pressure-induced alpha – omega phase transformation and the reverse phase transformation, i.e. omega – alpha, in zirconium (Zr). Plastic deformation and strain hardening of the material are also considered in the model. The microstructure evolution during both phase transformations is studied. The transformation start pressures at different temperatures are predicted and are plotted as a phase diagram. The effect of phase transformations on the mechanical properties of the material is also studied. The input data corresponding to pure Zr are acquired from experimental studies as wellmore » as by using the CALPHAD method. Our simulations show that three different omega variants form as laths. On release of pressure, reverse phase transformation initiates at lath boundaries. We observe that both phase transformations are martensitic in nature and also occur at the same pressure, i.e. little hysteresis. The transformation start pressures and the kinetics of the transformation predicted by our model are in good agreement with experimental results.« less

Alpha – omega and omega – alpha phase transformations in zirconium under hydrostatic pressure: A 3D mesoscale study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yeddu, Hemantha Kumar; Zong, Hongxiang; Lookman, Turab

Here, a three dimensional (3D) elastoplastic phase-field model is developed for modeling the hydrostatic pressure-induced alpha – omega phase transformation and the reverse phase transformation, i.e. omega – alpha, in zirconium (Zr). Plastic deformation and strain hardening of the material are also considered in the model. The microstructure evolution during both phase transformations is studied. The transformation start pressures at different temperatures are predicted and are plotted as a phase diagram. The effect of phase transformations on the mechanical properties of the material is also studied. The input data corresponding to pure Zr are acquired from experimental studies as wellmore » as by using the CALPHAD method. Our simulations show that three different omega variants form as laths. On release of pressure, reverse phase transformation initiates at lath boundaries. We observe that both phase transformations are martensitic in nature and also occur at the same pressure, i.e. little hysteresis. The transformation start pressures and the kinetics of the transformation predicted by our model are in good agreement with experimental results.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, Pei; Fang, Z. Zak; Koopman, Mark

Hydrogen has been investigated for decades as a temporary alloying element to refine the microstructure of Ti-6Al-4V, and is now being used in a novel powder metallurgy method known as "hydrogen sintering and phase transformation". Pseudo-binary phase diagrams of (Ti-6Al-4V)-xH have been studied and developed, but are not well established due to methodological limitations. In this paper, in situ studies of phase transformations during hydrogenation and dehydrogenation of (Ti-6Al-4V)-xH alloys were conducted using high-energy synchrotron X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The eutectoid phase transformation of β ↔ α + δ was observed in themore » (Ti-6Al-4V)-xH alloy via in situ synchrotron XRD at 211 °C with a hydrogen concentration of 37.5 at.% (measured using TGA-DSC). The relationships of hydrogen composition to partial pressure and temperature were investigated in the temperature range 450-900°C. Based on these results, a partial pseudo-binary phase diagram of (Ti-6Al-4V)-xH is proposed for hydrogen compositions up to 60 at.% in the temperature range 100-900°C. Using the data collected in real time under controlled parameters of temperature, composition and hydrogen partial pressure, this work characterizes relevant phase transformations and microstructural evolution for practical titanium-hydrogen technologies of Ti-6Al-4V.« less

Martensitelike spontaneous relaxor-normal ferroelectric transformation in Pb(Zn1/3Nb2/3)O3-PbLa(ZrTi)O3 system

NASA Astrophysics Data System (ADS)

Deng, Guochu; Ding, Aili; Li, Guorong; Zheng, Xinsen; Cheng, Wenxiu; Qiu, Pingsun; Yin, Qingrui

2005-11-01

The spontaneous relaxor-normal ferroelectric transformation was found in the tetragonal composition of Pb(Zn1/3Nb2/3)O3-PbLa(ZrTi)O3 (0.3PZN-0.7PLZT) complex ABO3 system. The corresponding dielectric permittivities and losses of different compositions located near the morphotrophic phase boundary were analyzed. By reviewing all of the results about this type of transformation in previous references, the electric, compositional, structural, and thermodynamic characteristics of the spontaneous relaxor-normal transformation were proposed. Additionally, the adaptive phase model for martensite transformation proposed by Khachaturyan et al. [Phys. Rev. B 43, 10832 (1991)] was introduced into this ferroelectric transformation to explain the unique transformation pathway and associated features such as the tweedlike domain patterns and the dielectric dispersion under the critical transition temperature. Due to the critical compositions near the MPB, the ferroelectric materials just fulfill the condition, in which the adaptive phases can form in the transformation procedure. The formation of the adaptive phases, which are composed of stress-accommodating twinned domains, makes the system bypass the energy barrier encountered in conventional martensite transformations. The twinned adaptive phase corresponds to the tweedlike domain pattern under a transmission electronic microscope. At lower temperature, these precursor phases transform into the conventional ferroelectric state with macrodomains by the movement of domain walls, which causes a weak dispersion in dielectric permittivity.

Effect of gallium alloying on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary Ni-Mn-Ga alloys

NASA Astrophysics Data System (ADS)

Belosludtseva, E. S.; Kuranova, N. N.; Marchenkova, E. B.; Popov, A. G.; Pushin, V. G.

2016-04-01

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni50Mn50- z Ga z (0 ⩽ z ⩽ 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature-concentration boundaries of the B2 and L21 superstructures in the austenite field, the tetragonal L10 (2 M) martensite, and the 10 M and 14 M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011} B2. Martensite crystals are twinned along one of the 24 24{ {011} }{< {01bar 1} rangle _{B2}} "soft" twinning shear systems, which provides coherent accommodation of the martensitic transformation-induced elastic stresses.

Phase transformation of Ca{sub 4}[Al{sub 6}O{sub 12}]SO{sub 4} and its disordered crystal structure at 1073 K

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kurokawa, Daisuke; R and D Center, Taiheiyo Cement Corporation, Chiba 285-8655; Takeda, Seiya

The phase transformation of Ca{sub 4}[Al{sub 6}O{sub 12}]SO{sub 4} and the crystal structure of its high-temperature phase were investigated by differential thermal analysis, temperature-dependent Raman spectroscopy and high-temperature X-ray powder diffraction (CuKα{sub 1}). We determined the starting temperature of the orthorhombic-to-cubic transformation during heating (=711 K) and that of the reverse transformation during cooling (=742 K). The thermal hysteresis was negative (=−31 K), suggesting the thermoelasticity of the transformation. The space group of the high temperature phase is I4{sup ¯}3m with the unit-cell dimensions of a=0.92426(2) nm and V=0.78955(2) nm{sup 3} (Z=2) at 1073 K. The initial structural model wasmore » derived by the direct methods and further refined by the Rietveld method. The final structural model showed the orientational disordering of SO{sub 4} tetrahedra. The maximum-entropy method-based pattern fitting method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. At around the transformation temperature during heating, the vibrational spectra, corresponding to the Raman-active SO{sub 4} internal stretching mode, showed the continuous and gradual change in the slope of full width at half maximum versus temperature curve. This strongly suggests that the orthorhombic-to-cubic phase transformation would be principally accompanied by the statistical disordering in orientation of the SO{sub 4} tetrahedra, without distinct dynamical reorientation. - Graphical abstract: (Left) Three-dimensional electron-density distributions of the SO{sub 4} tetrahedron with the split-atom model, and (right) a bird's eye view of electron densities on the plane parallel to (111). - Highlights: • Crystal structure of Ca{sub 4}[Al{sub 6}O{sub 12}]SO{sub 4} at 1073 K is determined by powder XRD. • The atom arrangements are represented by the split-atom model. • The MPF method is used to confirm the validity of the model. • The phase transition is accompanied by orientational disordering of SO{sub 4} tetrahedra.« less

Study of the transformation sequence on a high temperature martensitic transformation Ni-Mn-Ga-Co shape memory alloy

NASA Astrophysics Data System (ADS)

Recarte, V.; Pérez-Landazábal, J. I.; Sánchez-Alarcos, V.; Rodríguez-Velamazán, J. A.

2014-11-01

Ni-Mn-Ga alloys show the highest magnetic-field-induced strain among ferromagnetic shape memory alloys. A great effort is being done in this alloy system to increase the application temperature range. In this sense, the addition of small amounts of Cobalt to NiMnGa alloys has been proved to increase the MT temperatures through the increase of the electron per atom relation (e/a). In this work, the analysis of the crystal structure of the present phases and the phase transformations has been performed on a Ni-Mn-Ga-Co alloy by neutron diffraction measurements from 10 K to 673 K. The study has been completed by means of calorimetric and magnetic measurements. On cooling the alloy undergoes a martensitic transformation from a face centered cubic structure to a nonmodulated tetragonal martensite. The appearance of intermartensite transformations can be disregarded in the whole temperature range below the martensitic transformation. However, a jump in the unit-cell volume of the tetragonal martensite has been observed at 325 K. Since this temperature is close to the Curie temperature of the alloy both, the structural and magnetic contributions are taken into account to explain the results.

Experimental dynamic metamorphism of mineral single crystals

USGS Publications Warehouse

Kirby, S.H.; Stern, L.A.

1993-01-01

This paper is a review of some of the rich and varied interactions between non-hydrostatic stress and phase transformations or mineral reactions, drawn mainly from results of experiments done on mineral single crystals in our laboratory or our co-authors. The state of stress and inelastic deformation can enter explicitly into the equilibrium phase relations and kinetics of mineral reactions. Alternatively, phase transformations can have prominent effects on theology and on the nature of inelastic deformation. Our examples represent five types of structural phase changes, each of which is distinguished by particular mechanical effects. In increasing structural complexity, these include: (1) displacive phase transformations involving no bond-breaking, which may produce anomalous brittle behavior. A primary example is the a-?? quartz transition which shows anomalously low fracture strength and tertiary creep behavior near the transition temperature; (2) martensitic-like transformations involving transformation strains dominated by shear deformation. Examples include the orthoenstatite ??? clinoenstatite and w u ??rtzite ??? sphalerite transformations; (3) coherent exsolution or precipitation of a mineral solute from a supersaturated solid-solution, with anisotropy of precipitation and creep rates produced under nonhydrostatic stress. Examples include exsolution of corundum from MgO ?? nAl2O3 spinels and Ca-clinopyroxene from orthopyroxene; (4) order-disorder transformations that are believed to cause anomalous plastic yield strengthening, such as MgO - nAl2O3 spinels; and (5) near-surface devolatilization of hydrous silicate single-crystals that produces a fundamental brittleness thought to be connected with dehydration at microcracks at temperatures well below nominal macroscopic dehydration temperatures. As none of these interactions between single-crystal phase transformations and non-hydrostatic stress is understood in detail, this paper serves as a challenge to field structural geologists to test whether interactions of these types occur in nature, and to theoreticians to reach a deeper understanding of the complex relations between phase transformations, the local state of stress and associated deformation and deformation rates. ?? 1993.

Thermoelastic martensitic transformations in ternary Ni50Mn50- z Ga z alloys

NASA Astrophysics Data System (ADS)

Belosludtseva, E. S.; Kuranova, N. N.; Marchenkova, E. B.; Popov, A. G.; Pushin, V. G.

2016-01-01

We have studied the effect of gallium alloying on the structure, phase composition, and physical properties of ternary alloys of the Ni50Mn50- z Ga z (0 ≤ z ≤ 25 at %) quasi-binary section in a broad temperature range. Dependences of the type of crystalline structure of the high-temperature austenite phase and martensite, as well as the critical temperatures of martensitic transformations on the alloy composition, are determined. A phase diagram of the structural and magnetic transformations is constructed. Concentration boundaries of the existence of tetragonal L10 (2 M) martensite and martensitic phases (10 M and 14 M) with complex multilayer crystalline lattices are found. It is established that the predominant martensite morphology is determined by the hierarchy of packets of thin coherent nano- and submicrocrystalline plates with habit planes close to {011} B2, pairwise twinned along one of 24 equivalent {011}<011> B2 twinning shear systems.

Low-Temperature Criticality of Martensitic Transformations of Cu Nanoprecipitates in α-Fe

NASA Astrophysics Data System (ADS)

Erhart, Paul; Sadigh, Babak

2013-07-01

Nanoprecipitates form during nucleation of multiphase equilibria in phase segregating multicomponent systems. In spite of their ubiquity, their size-dependent physical chemistry, in particular, at the boundary between phases with incompatible topologies, is still rather arcane. Here, we use extensive atomistic simulations to map out the size-temperature phase diagram of Cu nanoprecipitates in α-Fe. The growing precipitates undergo martensitic transformations from the body-centered cubic (bcc) phase to multiply twinned 9R structures. At high temperatures, the transitions exhibit strong first-order character and prominent hysteresis. Upon cooling, the discontinuities become less pronounced and the transitions occur at ever smaller cluster sizes. Below 300 K, the hysteresis vanishes while the transition remains discontinuous with a finite but diminishing latent heat. This unusual size-temperature phase diagram results from the entropy generated by the soft modes of the bcc-Cu phase, which are stabilized through confinement by the α-Fe lattice.

Structure, phase transformations, mechanical characteristics, and cold resistance of low-carbon martensitic steels

NASA Astrophysics Data System (ADS)

Kozvonin, V. A.; Shatsov, A. A.; Ryaposov, I. V.; Zakirova, M. G.; Generalova, K. N.

2016-08-01

Temper-resistant low-carbon Cr-Mn-Ni-Mo-V-Nb steels with concentrations of carbon of 0.15 and 0.27 wt % have been studied. It has been shown that, upon quenching, various morphological types of the α phase can be formed. The structure of the steels is stable in the course of heating below critical temperatures and remains a lath-type structure in the intercritical temperature range. Specific features of structural and phase transformations, as well as the dependence of the mechanical characteristics of the steels, on the tempering temperature have been determined.

Phase-Transformation-Induced Extra Thermal Expansion Behavior of (SrxBa1–x)TiO3/Cu Composite

PubMed Central

Sheng, Jie; Wang, Lidong; Li, Shouwei; Yin, Benke; Liu, Xiangli; Fei, Wei-Dong

2016-01-01

The properties of metal matrix composites (MMCs) can be optimized effectively through adjusting the type or the volume fraction of reinforcement. Generally, the coefficient of thermal expansion (CTE) of MMCs can be reduced by increasing the volume fraction of the reinforcement with lower CTE than metal matrix. However, it is great challenge to fabricate low CTE MMCs with low reinforcement volume fraction because of the limitation of reinforcement CTEs. SrxBa1−xTiO3 (SBT) powder presents negative thermal expansion behavior during the phase transformation from tetragonal to cubic phase. Here, we demonstrate that the phase transformation of SBT can be utilized to reduce and design the thermal expansion properties of SBT particle-reinforced Cu (SBT/Cu) composite, and ultralow CTE can be obtained in SBT/Cu composite. The X-ray diffraction analysis on heating indicates that the temperature range of phase transformation is extended greatly, therefore, the low CTE can be achieved within wide temperature range. Landau-Devonshire theory study on the phase transformation behaviors of SBT particles in the composite indicates that thermal mismatch stress significantly affects the Curie temperature of SBT particles and the CTE of the composite. The results given in the present study provide a new approach to design the MMCs with low CTE. PMID:27255420

Temperature limited heaters using phase transformation of ferromagnetic material

DOEpatents

Vitek, John Michael [Oak Ridge, TN; Brady, Michael Patrick [Oak Ridge, TN

2009-10-06

Systems, methods, and heaters for treating a subsurface formation are described herein. Systems and methods for making heaters are described herein. At least one heater includes a ferromagnetic conductor and an electrical conductor. The electrical conductor is electrically coupled to the ferromagnetic conductor. The heater provides a first amount of heat at a lower temperature. The heater may provide a second reduced amount of heat when the heater reaches a selected temperature, or enters a selected temperature range, at which the ferromagnetic conductor undergoes a phase transformation.

Hidden disorder in the α '→δ transformation of Pu-1.9 at.% Ga

DOE PAGES

Jeffries, J. R.; Manley, M. E.; Wall, M. A.; ...

2012-06-06

Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal (ΔH=TΔS), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at. % Ga alloy, the δ phase is retained as a metastable state at room temperature, but at low temperatures, the δ phase yields to a mixed-phase microstructure of δ- and α'-Pu. The previously measured sources of entropy associated withmore » the α'→δ transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the α'→δ transformation that corroborates the theoretical prediction, and implies that only about 65% of the entropy stabilizing the δ phase is accounted for, leaving a missing entropy of about 0.5 k B/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the δ phase. This hidden disorder represents multiple accessible states per atom within the δ phase of Pu that may not be included in our current understanding of the properties and phase stability of δ-Pu.« less

Low Temperature Phase Transformations in Copper-Quenched Ti-44.5Al-8Nb-2.5V Alloy

PubMed Central

Cao, Shouzhen; Xiao, Shulong; Chen, Yuyong; Xu, Lijuan; Wang, Xiaopeng; Han, Jianchao

2017-01-01

In this study, an easily controlled transformation similar to the β + α → β + α + γ and the analysis of metastable phases in a β solidifying Ti-44.5Al-8Nb-2.5V alloy were investigated. Therefore, a liquid alloy copper-quenching followed by annealing at an application temperature (850 °C) has been carried out. Following quenching, a microstructure composed of several supersaturated phases—the basket-weave β0 (βbv) phase, the plate-like α2 (αp) phase and the stripe-like γ (γs) phase—was obtained. In the annealing processes, phase transformations in the prior βbv and αp phases domain corresponded nicely to the β + α → β + α + γ transformation during solidification. Also, in the annealed γs phase, the kinetics of the phase transformations involving the metastable L12 phase was firstly detected by transmission electron microscopy (TEM). The L12 phase had a lattice structure similar to the γ phase, whereas the composition of the phase was similar to the α2 phase. The formation of the γ pre-twin phase with an anti-phase boundary (APB) was detected in the γs phase of the matrix. The orientation relationships between the γs and precipitated: γ (γp) phase are <101]γs//<114]γp, (101¯)γs//(1¯10)γp and (01¯0)γs//(221¯)γp. PMID:28772561

500 keV Ar2+ ion irradiation induced anatase to brookite phase transformation and ferromagnetism at room temperature in TiO2 thin films

NASA Astrophysics Data System (ADS)

Bharati, B.; Mishra, N. C.; Kanjilal, D.; Rath, Chandana

2018-01-01

In our earlier report, where we have demonstrated ferromagnetic behavior at room temperature (RT) in TiO2 thin films deposited through electron beam evaporation technique followed by annealing either in Ar or O2 atmosphere [Mohanty et al., Journal of Magnetism and Magnetic Materials 355 (2014) 240-245], here we have studied the evolution of structure and magnetic properties after irradiating the TiO2 thin films with 500 keV Ar2+ ions. The pristine film while exhibits anatase phase, the films become amorphous after irradiating at fluence in the range 1 × 1014 to 1 × 1016 ions/cm2. Increasing the fluence up to 5 × 1016 ions/cm2, amorphous to crystalline phase transformation occurs and the structure becomes brookite. Although anatase to rutile phase transformation is usually reported in literatures, anatase to brookite phase transformation is an unusual feature which we have reported here for the first time. Such anatase to brookite phase transformation is accompanied with grain growth without showing any change in film thickness evidenced from Rutherford's Back Scattering (RBS) measurement. From scanning probe micrographs (SPM), roughness is found to be more in amorphous films than in the crystalline ones. Anatase to brookite phase transformation could be realized by considering the importance of intermediate amorphous phase. Because due to amorphous phase, heat deposited by energetic ions are localized as dissipation of heat is less and as a result, the localized region crystallizes in brookite phase followed by grain growth as observed in highest fluence. Further, we have demonstrated ferromagnetic behavior at RT in irradiated films similar to pristine one, irrespective of their phase and crystallinity. Origin for room temperature ferromagnetism (RTFM) is attributed to the presence of oxygen vacancies which is confirmed by carrying out XPS measurement.

Properties and Potential of Two (ni,pt)ti Alloys for Use as High-temperature Actuator Materials

NASA Technical Reports Server (NTRS)

Noebe, Ronald; Gaydosh, Darrell; Padula, Santo, II.; Garg, Anita; Biles, Tiffany; Nathal, Michael

2005-01-01

The microstructure, transformation temperatures, basic tensile properties, shape memory behavior, and work output for two (Ni,Ti)Pt high-temperature shape memory alloys have been characterized. One was a Ni30Pt20Ti50 alloy (referred to as 20Pt) with transformation temperatures above 230 C and the other was a Ni20Pt30Ti50 alloy (30Pt) with transformation temperatures about 530 C. Both materials displayed shape memory behavior and were capable of 100% (no-load) strain recovery for strain levels up to their fracture limit (3-4%) when deformed at room temperature. For the 20Pt alloy, the tensile strength, modulus, and ductility dramatically increased when the material was tested just about the austenite finish (A(sub f)) temperature. For the 30Pt alloy, a similar change in yield behavior at temperatures above the A(sub f) was not observed. In this case the strength of the austentite phase was at best comparable and generally much weaker than the martensite phase. A ductility minimum was also observed just below the A(sub s) temperature in this alloy. As a result of these differences in tensile behavior, the two alloys performed completely different when thermally cycled under constant load. The 20Pt alloy behaved similar to conventional binary NiTi alloys with work output due to the martensite-to-austenite transformation initially increasing with applied stress. The maximum work output measured in the 20Pt alloy was nearly 9 J/cu cm and was limited by the tensile ductility of the material. In contrast, the martensite-to-austenite transformation in the 30Pt alloy was not capable of performing work against any bias load. The reason for this behavior was traced back to its basic mechanical properties, where the yield strength of the austenite phase was similar to or lower than that of the martensite phase, depending on temperature. Hence, the recovery or transformation strain for the 30Pt alloy under load was essentially zero, resulting in zero work output.

Magnetism-Structure Correlations during the epsilon ->tau Transformation in Rapidly-Solidified MnAl Nanostructured Alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jimenez-Villacorta, F; Marion, JL; Oldham, JT

2014-01-21

Magnetic and structural aspects of the annealing-induced transformation of rapidly-solidified Mn55Al45 ribbons from the as-quenched metastable antiferromagnetic (AF) epsilon-phase to the target ferromagnetic (FM) L1(0) tau-phase are investigated. The as-solidified material exhibits a majority hexagonal epsilon-MnAl phase revealing a large exchange bias shift below a magnetic blocking temperature T-B similar to 95 K (H-ex similar to 13 kOe at 10 K), ascribed to the presence of compositional fluctuations in this antiferromagnetic phase. Heat treatment at a relatively low annealing temperature T-anneal approximate to 568 K (295 degrees C) promotes the nucleation of the metastable L1(0) tau-MnAl phase at the expensemore » of the parent epsilon-phase, donating an increasingly hard ferromagnetic character. The onset of the epsilon ->tau transformation occurs at a temperature that is similar to 100 K lower than that reported in the literature, highlighting the benefits of applying rapid solidification for synthesis of the rapidly-solidified parent alloy.« less

Mechanically Induced Graphite-Nanodiamonds-Phase Transformations During High-Energy Ball Milling

NASA Astrophysics Data System (ADS)

El-Eskandarany, M. Sherif

2017-05-01

Due to their unusual mechanical, chemical, physical, optical, and biological properties, nearly spherical-like nanodiamonds have received much attention as desirable advanced nanomaterials for use in a wide spectrum of applications. Although, nanodiamonds can be successfully synthesized by several approaches, applications of high temperature and/or high pressure may restrict the real applications of such strategic nanomaterials. Distinct from the current preparation approaches used for nanodiamonds preparation, here we show a new process for preparing ultrafine nanodiamonds (3-5 nm) embedded in a homogeneous amorphous-carbon matrix. Our process started from high-energy ball milling of commercial graphite powders at ambient temperature under normal atmospheric helium gas pressure. The results have demonstrated graphite-single wall carbon nanotubes-amorphous-carbon-nanodiamonds phase transformations carried out through three subsequent stages of ball milling. Based on XRD and RAMAN analyses, the percentage of nanodiamond phase + C60 (crystalline phase) produced by ball milling was approximately 81%, while the amorphous phase amount was 19%. The pressure generated on the powder together the with temperature increase upon the ball-powder-ball collision is responsible for the phase transformations occurring in graphite powders.

Realization of magnetostructural coupling by modifying structural transitions in MnNiSi-CoNiGe system with a wide Curie-temperature window.

PubMed

Liu, Jun; Gong, Yuanyuan; Xu, Guizhou; Peng, Guo; Shah, Ishfaq Ahmad; Ul Hassan, Najam; Xu, Feng

2016-03-16

The magnetostructural coupling between structural and magnetic transitions leads to magneto-multifunctionalities of phase-transition alloys. Due to the increasing demands of multifunctional applications, to search for the new materials with tunable magnetostructural transformations in a large operating temperature range is important. In this work, we demonstrate that by chemically alloying MnNiSi with CoNiGe, the structural transformation temperature of MnNiSi (1200 K) is remarkably decreased by almost 1000 K. A tunable magnetostructural transformation between the paramagnetic hexagonal and ferromagnetic orthorhombic phase over a wide temperature window from 425 to 125 K is realized in (MnNiSi)1-x(CoNiGe)x system. The magnetic-field-induced magnetostructural transformation is accompanied by the high-performance magnetocaloric effect, proving that MnNiSi-CoNiGe system is a promising candidate for magnetic cooling refrigerant.

Transformation behavior of Ni-Mn-Ga in the low-temperature limit.

PubMed

Pérez-Landazábal, J I; Recarte, V; Sánchez-Alarcos, V; Chernenko, V A; Barandiarán, J M; Lázpita, P; Rodriguez Fernández, J; Righi, L

2012-07-11

The magnetic, magnetocaloric and thermal characteristics have been studied in a Ni(50.3)Mn(20.8)Ga(27.6)V(1.3) ferromagnetic shape memory alloy (FSMA) transforming martensitically at around 40 K. The alloy shows first a transformation from austenite to an intermediate phase and then a partial transformation to an orthorhombic martensite, all the phases being ferromagnetically ordered. The thermomagnetization dependences enabled observation of the magnetocaloric effect in the vicinity of the martensitic transformation (MT). The Debye temperature and the density of states at the Fermi level are equal to θ(D) = (276 ± 4) K and 1.3 states/atom eV , respectively, and scarcely dependent on the magnetic field. The MT exhibited by Ni-Mn-Ga FSMAs at very low temperatures is distinctive in the sense that it is accompanied by a hardly detectable entropy change as a sign of a small driving force. The enhanced stability of the cubic phase and the low driving force of the MT stem from the reduced density of states near the Fermi level.

High-temperature phase transformations. The properties of the phases and their equilibrium under shock loading.

NASA Astrophysics Data System (ADS)

Zaretsky, Eugene

2011-06-01

Introducing the temperature as a variable parameter in shock wave experiments extends essentially the scope of these investigations. The influence of the temperature variations on either high strain rate elastic-plastic response of solids or parameters of the shock-induces phase transformations are not trivial and are not quite clear yet. The technique of VISAR-monitored planar impact experiments with the samples preheated up to 1400 K was developed and used for the studies of the effect of the preheating on the impact response and on the ``dynamic'' phase diagrams of pure metals (U, Ti, Fe, Co, Ag), and ionic compounds (KCl, KBr). The studies show that the increase of the shear strength of the shock-loaded metal with temperature (first reported by Kanel et al. 1996) is typical for pure FCC (Al, Ag, Cu) and some other (Sn, U) metals, and for the ionic crystals. In the metals with BCC lattice (Mo: Duffy and Ahrens 1994, Fe: Zaretsky 2009) such thermal hardening was not found. The abrupt strength anomalies (either yield or spall or both) were observed in a narrow vicinity of the temperature of any, polymorphic, magnetic, or melting, phase transformation. It was found that when a pure element approaches the phase boundary (the line of either first or second order phase transition) the result is a 50-100-% increase of the shear strength of the low-temperature phase. At the same time the presence of a small (~0.5%) amount of impurities may lead to a five-fold decrease of the strength as it takes place in the vicinity of the Curie point of Ni. The same technique being applied to the study of the shear stress relaxation (elastic precursor decay) near the transformation line may be useful for understanding the mechanisms responsible of these anomalies.

Large tensile superelasticity from intermartensitic transformations in Ni49Mn28Ga23 single crystal

NASA Astrophysics Data System (ADS)

Chernenko, V. A.; Villa, E.; Salazar, D.; Barandiaran, J. M.

2016-02-01

A multistep superelastic behavior, with up to a 12% strain, is reported in a <001>P-oriented Ni49Mn28Ga23 single crystal. The observed behavior is produced by intermartensitic transformations during the tensile stress-strain measurements at temperatures between -140 °C and +60 °C. The tensile stress-temperature phase diagram and the stress dependence of the intermartensitic transformation entropies have been obtained. These results provide important input for theoretical modeling of the phase transformations in these alloys and show promising mechanical properties of the classical Ni-Mn-Ga ferromagnetic shape memory alloys.

Reaction process of {alpha} {yields} {gamma} massive transformation in Ti-rich TiAl alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumagai, T.; Abe, E.; Nakamura, M.

1995-08-01

Reaction sequence of the massive transformation from the high-temperature {alpha}-Ti phase to the {gamma}-TiAl phase ({gamma}{sub m}) in a Ti-48at.% Al alloy has been examined in terms of optical and transmission electron microscopes. Both transformed and untransformed regions were macroscopically observed in the sample quenched from the high-temperature {alpha} phase field, when the sample was held there for a extended period of time prior to quenching. The transformed region consists of randomly oriented fine {gamma} single phase grains, in which many thermal anti-phase domains (TAPDs), together with a number of stacking faults were observed. In contrast, the untransformed region comprisesmore » extremely fine lamellae of the {gamma} and {alpha}{sub 2}-Ti{sub 3}Al phases, and the {gamma} plates were found to run through the TAPDs caused by {alpha} {yields} {alpha}{sub 2} ordering. Subsequent aging at 1,273 K causes the microstructure change in the untransformed region from {alpha}{sub 2}/{gamma} lamellae to {gamma}/{gamma} lamellae spontaneously and expands the {gamma}{sub m} region. These observations suggest that the {alpha} {yields} {gamma}{sub m} transformation proceeds through formation of fine {gamma} plates.« less

Nanoscale multiphase phase field approach for stress- and temperature-induced martensitic phase transformations with interfacial stresses at finite strains

NASA Astrophysics Data System (ADS)

Basak, Anup; Levitas, Valery I.

2018-04-01

A thermodynamically consistent, novel multiphase phase field approach for stress- and temperature-induced martensitic phase transformations at finite strains and with interfacial stresses has been developed. The model considers a single order parameter to describe the austenite↔martensitic transformations, and another N order parameters describing N variants and constrained to a plane in an N-dimensional order parameter space. In the free energy model coexistence of three or more phases at a single material point (multiphase junction), and deviation of each variant-variant transformation path from a straight line have been penalized. Some shortcomings of the existing models are resolved. Three different kinematic models (KMs) for the transformation deformation gradient tensors are assumed: (i) In KM-I the transformation deformation gradient tensor is a linear function of the Bain tensors for the variants. (ii) In KM-II the natural logarithms of the transformation deformation gradient is taken as a linear combination of the natural logarithm of the Bain tensors multiplied with the interpolation functions. (iii) In KM-III it is derived using the twinning equation from the crystallographic theory. The instability criteria for all the phase transformations have been derived for all the kinematic models, and their comparative study is presented. A large strain finite element procedure has been developed and used for studying the evolution of some complex microstructures in nanoscale samples under various loading conditions. Also, the stresses within variant-variant boundaries, the sample size effect, effect of penalizing the triple junctions, and twinned microstructures have been studied. The present approach can be extended for studying grain growth, solidifications, para↔ferro electric transformations, and diffusive phase transformations.

Synthesis and characterization of β-Sialon powders from Si, halloysite clay and AlN powders

NASA Astrophysics Data System (ADS)

Yin, Li; Jones, Mark Ian

2017-07-01

Two β-Sialons, with z-values of 1 and 4, respectively, were successfully synthesized by silicothermal reduction and nitridation method under 0.4 MPa nitrogen pressure. The effect of firing temperatures on the phase transformations and morphologies of β-Sialons were analyzed by XRD and SEM. For β-Sialons (z = 1), the product was finally composed of targeted β-Sialon (z = 1) and secondary phase α-Si3N4; for z = 4, β-Sialon (z = 4) was the main phase, and 15R-Sialon and α-Al2O3 co-existed as secondary phases. A higher firing temperature is more beneficial for the phase transformations and crystal growth of β-Sialons, however, the most suitable firing temperature was 1400∘C.

Dilatometric investigation of α(orthorhombic)→β(tetragonal) transformation in U-15 wt.% Cr alloy

NASA Astrophysics Data System (ADS)

Rameshkumar, Santhosh; Raju, Subramanian; Saibaba, Saroja

2018-04-01

The α→β transformation characteristics in U-15wt.% Cr alloy have been investigated by dilatometry at slow heating rates (3-10 K min-1). The starting microstructure of U-15Cr alloy consists of a mixture of metastable βm-U(body centred tetroganal), α-U(orthorhombic) and elemental Cr(bcc) phases. Upon heating, the metastable βmU phase has progressively transformed to equilibrium α-U structure; before, finally undergoing equilibrium α→β transformation with further increase in temperature. The measured α→β transformation temperature, when extrapolated to 0 K min-1 heating rate has been found to be higher than the currently accepted equilibrium phase diagram estimate. This is due to the kinetic difficulty associated with Cr-diffusion in U-15Cr alloy. The kinetics of α→β transformation upon continuous heating has been modeled in terms of a suitable framework for diffusional transformations, and the effective activation energy for overall transformation has been estimated to be in the range 160-180 kJ mol-1.

Shear-driven phase transformation in silicon nanowires

NASA Astrophysics Data System (ADS)

Vincent, L.; Djomani, D.; Fakfakh, M.; Renard, C.; Belier, B.; Bouchier, D.; Patriarche, G.

2018-03-01

We report on an unprecedented formation of allotrope heterostructured Si nanowires by plastic deformation based on applied radial compressive stresses inside a surrounding matrix. Si nanowires with a standard diamond structure (3C) undergo a phase transformation toward the hexagonal 2H-allotrope. The transformation is thermally activated above 500 °C and is clearly driven by a shear-stress relief occurring in parallel shear bands lying on {115} planes. We have studied the influence of temperature and axial orientation of nanowires. The observations are consistent with a martensitic phase transformation, but the finding leads to clear evidence of a different mechanism of deformation-induced phase transformation in Si nanowires with respect to their bulk counterpart. Our process provides a route to study shear-driven phase transformation at the nanoscale in Si.

Shear-driven phase transformation in silicon nanowires.

PubMed

Vincent, L; Djomani, D; Fakfakh, M; Renard, C; Belier, B; Bouchier, D; Patriarche, G

2018-03-23

We report on an unprecedented formation of allotrope heterostructured Si nanowires by plastic deformation based on applied radial compressive stresses inside a surrounding matrix. Si nanowires with a standard diamond structure (3C) undergo a phase transformation toward the hexagonal 2H-allotrope. The transformation is thermally activated above 500 °C and is clearly driven by a shear-stress relief occurring in parallel shear bands lying on {115} planes. We have studied the influence of temperature and axial orientation of nanowires. The observations are consistent with a martensitic phase transformation, but the finding leads to clear evidence of a different mechanism of deformation-induced phase transformation in Si nanowires with respect to their bulk counterpart. Our process provides a route to study shear-driven phase transformation at the nanoscale in Si.

Accelerated formation of cubic phases in phosphatidylethanolamine dispersions.

PubMed Central

Tenchov, B; Koynova, R; Rapp, G

1998-01-01

By means of x-ray diffraction we show that several sodium salts and the disaccharides sucrose and trehalose strongly accelerate the formation of cubic phases in phosphatidylethanolamine (PE) dispersions upon temperature cycling through the lamellar liquid crystalline-inverted hexagonal (Lalpha-HII) phase transition. Ethylene glycol does not have such an effect. The degree of acceleration increases with the solute concentration. Such an acceleration has been observed for dielaidoyl PE (DEPE), dihexadecyl PE, and dipalmitoyl PE. It was investigated in detail for DEPE dispersions. For DEPE (10 wt% of lipid) aqueous dispersions at 1 M solute concentration, 10-50 temperature cycles typically result in complete conversion of the Lalpha phase into cubic phase. Most efficient is temperature cycling executed by laser flash T-jumps. In that case the conversion completes within 10-15 cycles. However, the cubic phases produced by laser T-jumps are less ordered in comparison to the rather regular cubic structures produced by linear, uniform temperature cycling at 10 degrees C/min. Temperature cycles at scan rates of 1-3 degrees C/min also induce the rapid formation of cubic phases. All solutes used induce the formation of Im3m (Q229) cubic phase in 10 wt% DEPE dispersions. The initial Im3m phases appearing during the first temperature cycles have larger lattice parameters that relax to smaller values with continuation of the cycling after the disappearance of the Lalpha phase. A cooperative Im3m --> Pn3m transition takes place at approximately 85 degrees C and transforms the Im3m phase into a mixture of coexisting Pn3m (Q224) and Im3m phases. The Im3m/Pn3m lattice parameter ratio is 1. 28, as could be expected from a representation of the Im3m and Pn3m phases with the primitive and diamond infinite periodic minimal surfaces, respectively. At higher DEPE contents ( approximately 30 wt%), cubic phase formation is hindered after 20-30 temperature cycles. The conversion does not go through, but reaches a stage with coexisting Ia3d (Q230) and Lalpha phases. Upon heating, the Ia3d phase cooperatively transforms into a mixture of, presumably, Im3m and Pn3m phases at about the temperature of the Lalpha-HII transition. This transformation is readily reversible with the temperature. The lattice parameters of the DEPE cubic phases are temperature-insensitive in the Lalpha temperature range and decrease with the temperature in the range of the HII phase. PMID:9675186

Phase transition temperature in the Zr-rich corner of Zr-Nb-Sn-Fe alloys

NASA Astrophysics Data System (ADS)

Canay, M.; Danón, C. A.; Arias, D.

2000-08-01

The influence of small composition changes on the phase transformation temperature of Zr-1Nb-1Sn-0.2(0.7)Fe alloys was studied in the present work, by electrical resistivity measurements and metallographic techniques. For the alloy with 0.2 at.% Fe we have determined Tα↔α+β=741°C and Tα+β↔β=973°C, and for the 0.7 at.% Fe the transformation temperatures were T α↔α+β=712°C and T α+β↔β=961°C. We have verified that the addition of Sn stabilized the β phase.

Phase competition in the growth of SrCoOx/LaAlO3 thin films

NASA Astrophysics Data System (ADS)

Zhang, Jie; Meng, Dechao; Huang, Haoliang; Cai, Honglei; Huang, Qiuping; Wang, Jianlin; Yang, Yuanjun; Zhai, Xiaofang; Fu, Zhengping; Lu, Yalin

2018-02-01

The reversible topotactic phase transformation between brownmillerite SrCoO2.5 to perovskite SrCoO3 has attracted more and more attention for potential applications as solid oxide fuels and electrolysis cells. However, the relatively easy transformation result from small thermal stable energy barriers between the two phases leads to unstable the structures. In the paper, amounts of SrCoO3-δ films have been prepared by pulsed laser deposition at optimized growth conditions with the temperature range of 590-720°C. The X-ray diffraction (XRD) results demonstrated that a phase competition emerged around 650°C. The Gibbs free energies of two phases at high temperature revealed the difference of stability of these two phases under different growth temperature. The optical spectroscopies and X-ray photoelectron spectroscopies were used to verify the electronic structure and chemical state differences between the two phases with distinct crystal structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sicupira, Felipe Lucas; Sandim, Maria José R.; Sandim, Hugo R.Z.

The good performance of supermartensitic stainless steels is strongly dependent on the volume fraction of retained austenite at room temperature. The present work investigates the effect of secondary tempering temperatures on this phase transformation and quantifies the amount of retained austenite by X-ray diffraction and saturation magnetization. The steel samples were tempered for 1 h within a temperature range of 600–800 °C. The microstructure was characterized using scanning electron microscopy and electron backscatter diffraction. Results show that the amount of retained austenite decreased with increasing secondary tempering temperature in both quantification methods. - Highlights: • The phase transformation during secondarymore » tempering temperatures was observed. • Phases were quantified by X-ray diffraction and DC-saturation magnetization. • More retained austenite forms with increasing secondary tempering temperature. • The retained austenite is mainly located at the grain and lath boundaries.« less

Monoclinic to cubic phase transformation and photoluminescence properties in Hf1-xSmxO2 (x = 0-0.12) nanoparticles

NASA Astrophysics Data System (ADS)

Kumar, Sandeep; Rai, S. B.; Rath, Chandana

2018-02-01

Bulk hafnium oxide (HfO2) exhibits the monoclinic phase at room temperature which transforms to tetragonal and cubic phases at 1700 and 2600 °C, respectively, under ambient conditions. For the first time, we observe a monoclinic to stable cubic phase transformation at room temperature in nanoparticles of HfO2 by incorporating Sm3+ ions up to 12 at. %. Although the monoclinic structure is retained at 1 at. % of Sm, a mixed phase of monoclinic and cubic is observed at intermediate Sm concentration (5-11 at. %). Le-Bail profile refinement of X-ray diffraction patterns confirms the monoclinic to cubic phase transformation with increasing Sm3+ ion concentration. While the significant difference in ionic radii of Sm3+ and Hf4+ ion induces strain in the lattice above 9 at. %, a lower valency of Sm produces oxygen vacancy leading to 8-fold coordination with Sm3+ ion and stabilizes the cubic phase at room temperature. Not only the particle size obtained from transmission electron micrograph (TEM) matches well with the size calculated from the Williamson-Hall plot, the lattice spacing estimated from high resolution TEM also confirms the monoclinic and cubic phases in HfO2 and Hf0.88Sm0.12O2, respectively. Apart from phase transformation induced by Sm3+ ions, photoluminescence studies demonstrate an excellent emission in near green and red regions in Hf1-xSmxO2 nanoparticles. A schematic energy band diagram has been proposed based on the excitation and emission processes involved in HfO2 and Hf0.99Sm0.01O2 nanoparticles.

Relaxor ferroelectricity and electric-field-driven structural transformation in the giant lead-free piezoelectric (Ba ,Ca ) (Ti ,Zr ) O3

NASA Astrophysics Data System (ADS)

Brajesh, Kumar; Tanwar, Khagesh; Abebe, Mulualem; Ranjan, Rajeev

2015-12-01

There is great interest in lead-free (B a0.85C a0.15 ) (T i0.90Z r0.10 ) O3 (15/10BCTZ) because of its exceptionally large piezoelectric response [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009), 10.1103/PhysRevLett.103.257602]. In this paper, we have analyzed the nature of: (i) crystallographic phase coexistence at room temperature, (ii) temperature- and field-induced phase transformation to throw light on the atomistic mechanisms associated with the large piezoelectric response of this system. A detailed temperature-dependent dielectric and lattice thermal expansion study proved that the system exhibits a weak dielectric relaxation, characteristic of a relaxor ferroelectric material on the verge of exhibiting a normal ferroelectric-paraelectric transformation. Careful structural analysis revealed that a ferroelectric state at room temperature is composed of three phase coexistences, tetragonal (P 4 m m )+ orthorhombic(Amm 2 )+rhombohedral(R 3 m ) . We also demonstrate that the giant piezoresponse is associated with a significant fraction of the tetragonal phase transforming to rhombohedral. It is argued that the polar nanoregions associated with relaxor ferroelectricity amplify the piezoresponse by providing an additional degree of intrinsic structural inhomogeneity to the system.

Effect of annealing temperature on titania nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Manikandan, K., E-mail: sanjaymani367@gmail.com; Arumugam, S., E-mail: sanjaymani367@gmail.com; Chandrasekaran, G.

2014-04-24

Titania polycrystalline samples are prepared by using sol-gel route hydrolyzing a alkoxide titanium precursor under acidic conditions. The as prepared samples are treated with different calcination temperatures. The anatase phase of titania forms when treated below 600°C, above that temperature the anatase phase tends to transform into the rutile phase of titania. The experimental determination of average grain size, phase formation, lattice parameters and the crystal structures of titania samples at different calcinations is done using X-ray diffraction (XRD). Fourier Transform Infra-red Spectroscopy (FTIR), UV-vis-NIR spectroscopy and Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis X-ray are used to characterizemore » the samples to bring impact on the respective properties.« less

Materials research for passive solar systems: Solid-state phase-change materials

NASA Astrophysics Data System (ADS)

Benson, D. K.; Webb, J. D.; Burrows, R. W.; McFadden, J. D. O.; Christensen, C.

1985-03-01

A set of solid-state phase-change materials is being evaluated for possible use in passive solar thermal energy storage systems. The most promising materials are organic solid solutions of pentaerythritol (C5H12O4), pentaglycerinve (C5H12O3), and neopentyl glycol (C5H12O2). Solid solution mixtures of these compounds can be tailored so that they exhibit solid-to-solid phase transformations at any desired temperature between 25 C and 188 C, and have latent heats of transformation etween 20 and 70 cal/g. Transformation temperatures, specific heats, and latent heats of transformation have been measured for a number of these materials. Limited cyclic experiments suggest that the solid solutions are stable. These phase-change materials exhibit large amounts of undercooling; however, the addition of certain nucleating agents as particulate dispersions in the solid phase-change material greatly reduces this effect. Computer simulations suggest that the use of an optimized solid-state phase-change material in a Trombe wall could provide better performance than a concrete Trombe wall four times thicker and nine times heavier.

Pressure-magnetic field induced phase transformation in Ni{sub 46}Mn{sub 41}In{sub 13} Heusler alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rama Rao, N. V., E-mail: nvrrao@dmrl.drdo.in; Manivel Raja, M.; Pandian, S.

2014-12-14

The effect of hydrostatic pressure and magnetic field on the magnetic properties and phase transformation in Ni{sub 46}Mn{sub 41}In{sub 13} Heusler alloy was investigated. Pressure (P)-magnetic field (H)-temperature (T) phase diagram has been constructed from experimental results. In the P–T contour of the phase diagram, the slope of the austenite-martensite phase boundary line appears positive (dT/dP > 0), while it appears negative (dT/dH < 0) in the H–T contour. The results revealed that pressure and magnetic field have opposite effect on phase stabilization. The combined effect of pressure and magnetic field on martensitic transition has led to two important findings: (i) pressure dependent shiftmore » of austenite start temperature (A{sub s}) is higher when larger field is applied, and (ii) field dependent shift of A{sub s} is lowered when a higher pressure is applied. The pressure and magnetic field dependent shift observed in the martensitic transformation has been explained on the basis of thermodynamic calculations. Curie temperature of the phases was found to increase with pressure at a rate of 0.6 K/kbar.« less

Phenomenology of Polymorphism, III: p, TDiagram and Stability of Piracetam Polymorphs

NASA Astrophysics Data System (ADS)

Céolin, R.; Agafonov, V.; Louër, D.; Dzyabchenko, V. A.; Toscani, S.; Cense, J. M.

1996-02-01

The nootropic drug Piracetam is known to crystallize in three phases. In order to obtain their stability hierarchy from sublimation pressure inequalities, the drawing of a topologicalp,Tdiagram was attempted. For such a purpose and also for quality control, crystallographic and thermodynamic data were required. Powder X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) were used. Molecular energy calculations were performed. Phase I melts at 426 K (ΔfusH(I) = +180 J·g-1). Phase II transforms into Phase I at 399 K (Δ(II→I)H= +24 J·g-1). Phase III transforms into phase I at 392 K (Δ(III→I)H= +28 J·g-1) or melts at 412 K (ΔfusH(III) = +210 J·g-1). Thep,Tdiagram shows that phase I is stable at higher temperature and phase II at lower temperature, like phase III, which is stable under high pressure. At room temperature, phase II is the more stable form, and phase I the less stable one. This agrees with the spontaneous I → II transformation observed at 298 K within a few hours, and with lattice energies, calculated previously. Molecular energy calculations and crystal structure comparison show how intermolecular hydrogen bonds and H-bonded dimers, in phases II and III, may stabilize conformations higher in energy than those of the isolated molecule and of phase I.

Critical temperature transitions in laser-mediated cartilage reshaping

NASA Astrophysics Data System (ADS)

Wong, Brian J.; Milner, Thomas E.; Kim, Hong H.; Telenkov, Sergey A.; Chew, Clifford; Kuo, Timothy C.; Smithies, Derek J.; Sobol, Emil N.; Nelson, J. Stuart

1998-07-01

In this study, we attempted to determine the critical temperature [Tc] at which accelerated stress relaxation occurred during laser mediated cartilage reshaping. During laser irradiation, mechanically deformed cartilage tissue undergoes a temperature dependent phase transformation which results in accelerated stress relaxation. When a critical temperature is attained, cartilage becomes malleable and may be molded into complex new shapes that harden as the tissue cools. Clinically, reshaped cartilage tissue can be used to recreate the underlying cartilaginous framework of structures such as the ear, larynx, trachea, and nose. The principal advantages of using laser radiation for the generation of thermal energy in tissue are precise control of both the space-time temperature distribution and time- dependent thermal denaturation kinetics. Optimization of the reshaping process requires identification of the temperature dependence of this phase transformation and its relationship to observed changes in cartilage optical, mechanical, and thermodynamic properties. Light scattering, infrared radiometry, and modulated differential scanning calorimetry (MDSC) were used to measure temperature dependent changes in the biophysical properties of cartilage tissue during fast (laser mediated) and slow (conventional calorimetric) heating. Our studies using MDSC and laser probe techniques have identified changes in cartilage thermodynamic and optical properties suggestive of a phase transformation occurring near 60 degrees Celsius.

High-temperature phase transformations: The properties of the phases under shock loading

NASA Astrophysics Data System (ADS)

Zaretsky, Eugene

2012-03-01

Introducing the temperature as a variable parameter in shock wave experiments extends essentially the scope of these investigations. The influence of the temperature variations on either high strain rate elastic-plastic response of solids or parameters of the shock induced phase transformations are not trivial and are not quite clear yet. The technique of VISAR-monitored planar impact experiments with the samples preheated up to 1400 K was developed and used for the studies of the effect of the preheating on the impact response and on the "dynamic" phase diagrams of pure metals (U, Ti, Fe, Co, Ag), and ionic compounds (KCl, KBr). The studies show that the increase of the shear strength of the shock-loaded metal with temperature (first reported by Kanel et al. 1996) is typical for pure FCC (Al, Ag, Cu) and some other (Sn, U) metals, and for the ionic crystals. In the metals with BCC lattice (Mo: Duffy and Ahrens 1994, Fe: Zaretsky 2009) such thermal hardening was not observed. It was found that when a pure element approaches the temperature of either a first or second order phase transition the result is a 50-100-% increase of the shear strength of the low-temperature phase. At the same time the presence of a small (~0.5 %) amount of impurities may lead to a five-fold decrease of the strength as it takes place in the vicinity of the Curie point of Ni. Applying the same technique to the study of shear stress relaxation (elastic precursor decay) near the transformation temperature may aid in understanding the mechanisms of these anomalies.

Phase transformation pathways of ultrafast-laser-irradiated Ln2O3 (Ln =Er -Lu )

NASA Astrophysics Data System (ADS)

Rittman, Dylan R.; Tracy, Cameron L.; Chen, Chien-Hung; Solomon, Jonathan M.; Asta, Mark; Mao, Wendy L.; Yalisove, Steven M.; Ewing, Rodney C.

2018-01-01

Ultrafast laser irradiation causes intense electronic excitations in materials, leading to transient high temperatures and pressures. Here, we show that ultrafast laser irradiation drives an irreversible cubic-to-monoclinic phase transformation in Ln2O3 (Ln =Er -Lu ), and explore the mechanism by which the phase transformation occurs. A combination of grazing incidence x-ray diffraction and transmission electron microscopy are used to determine the magnitude and depth-dependence of the phase transformation, respectively. Although all compositions undergo the same transformation, their transformation mechanisms differ. The transformation is pressure-driven for Ln =Tm -Lu , consistent with the material's phase behavior under equilibrium conditions. However, the transformation is thermally driven for Ln =Er , revealing that the nonequilibrium conditions of ultrafast laser irradiation can lead to novel transformation pathways. Ab initio molecular-dynamics simulations are used to examine the atomic-scale effects of electronic excitation, showing the production of oxygen Frenkel pairs and the migration of interstitial oxygen to tetrahedrally coordinated constitutional vacancy sites, the first step in a defect-driven phase transformation.

Phase transformation pathways of ultrafast-laser-irradiated Ln 2 O 3 ( Ln = Er – Lu )

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rittman, Dylan R.; Tracy, Cameron L.; Chen, Chien-Hung

Ultrafast laser irradiation causes intense electronic excitations in materials, leading to transient high temperatures and pressures. Here, we show that ultrafast laser irradiation drives an irreversible cubic-to-monoclinic phase transformation in Ln 2O 3 ( Ln = Er – Lu ) , and explore the mechanism by which the phase transformation occurs. A combination of grazing incidence x-ray diffraction and transmission electron microscopy are used to determine the magnitude and depth-dependence of the phase transformation, respectively. Although all compositions undergo the same transformation, their transformation mechanisms differ. The transformation is pressure-driven for Ln = Tm – Lu , consistent with themore » material's phase behavior under equilibrium conditions. However, the transformation is thermally driven for Ln = Er , revealing that the nonequilibrium conditions of ultrafast laser irradiation can lead to novel transformation pathways. Ab initio molecular-dynamics simulations are used to examine the atomic-scale effects of electronic excitation, showing the production of oxygen Frenkel pairs and the migration of interstitial oxygen to tetrahedrally coordinated constitutional vacancy sites, the first step in a defect-driven phase transformation.« less

Phase transformation pathways of ultrafast-laser-irradiated Ln 2 O 3 ( Ln = Er – Lu )

DOE PAGES

Rittman, Dylan R.; Tracy, Cameron L.; Chen, Chien-Hung; ...

2018-01-10

Ultrafast laser irradiation causes intense electronic excitations in materials, leading to transient high temperatures and pressures. Here, we show that ultrafast laser irradiation drives an irreversible cubic-to-monoclinic phase transformation in Ln 2O 3 ( Ln = Er – Lu ) , and explore the mechanism by which the phase transformation occurs. A combination of grazing incidence x-ray diffraction and transmission electron microscopy are used to determine the magnitude and depth-dependence of the phase transformation, respectively. Although all compositions undergo the same transformation, their transformation mechanisms differ. The transformation is pressure-driven for Ln = Tm – Lu , consistent with themore » material's phase behavior under equilibrium conditions. However, the transformation is thermally driven for Ln = Er , revealing that the nonequilibrium conditions of ultrafast laser irradiation can lead to novel transformation pathways. Ab initio molecular-dynamics simulations are used to examine the atomic-scale effects of electronic excitation, showing the production of oxygen Frenkel pairs and the migration of interstitial oxygen to tetrahedrally coordinated constitutional vacancy sites, the first step in a defect-driven phase transformation.« less

X-ray diffraction study of low-temperature phase transformations in nickel-titanium orthodontic wires.

PubMed

Iijima, M; Brantley, W A; Guo, W H; Clark, W A T; Yuasa, T; Mizoguchi, I

2008-11-01

Employ conventional X-ray diffraction (XRD) to analyze three clinically important nickel-titanium orthodontic wire alloys over a range of temperatures between 25 and -110 degrees C, for comparison with previous results from temperature-modulated differential scanning calorimetry (TMDSC) studies. The archwires selected were 35 degrees C Copper Ni-Ti (Ormco), Neo Sentalloy (GAC International), and Nitinol SE (3M Unitek). Neo Sentalloy, which exhibits superelastic behavior, is marketed as having shape memory in the oral environment, and Nitinol SE and 35 degrees C Copper Ni-Ti also exhibit superelastic behavior. All archwires had dimensions of 0.016in.x0.022in. (0.41 mm x 0.56 mm). Straight segments cut with a water-cooled diamond saw were placed side-by-side to yield a 1 cm x 1cm test sample of each wire product for XRD analysis (Rint-Ultima(+), Rigaku) over a 2theta range from 30 degrees to 130 degrees and at successive temperatures of 25, -110, -60, -20, 0 and 25 degrees C. The phases revealed by XRD at the different analysis temperatures were in good agreement with those found in previous TMDSC studies of transformations in these alloys, in particular verifying the presence of R-phase at 25 degrees C. Precise comparisons are not possible because of the approximate nature of the transformation temperatures determined by TMDSC and the preferred crystallographic orientation present in the wires. New XRD peaks appear to result from low-temperature transformation in martensite, which a recent transmission electron microscopy (TEM) study has shown to arise from twinning. While XRD is a useful technique to study phases in nickel-titanium orthodontic wires and their transformations as a function of temperature, optimum insight is obtained when XRD analyses are combined with complementary TMDSC and TEM study of the wires.

Structural and spectroscopic characterization of irreversible phase changes in rapidly heated precursors of europium-doped titania nanoparticles

NASA Astrophysics Data System (ADS)

Gunawidjaja, Ray; Anderson, Benjamin R.; Eilers, Hergen

2018-02-01

We observe temperature-dependent phase changes in a precursor of europium-doped titania (p-Eu:TiO2) that is prepared via precipitation and is laser-heated to temperatures between 473 K and 1246 K within sub-second heating durations. The phase changes are characterized using X-ray diffraction and site-selective photoluminescence spectroscopy. We find that upon heating, the initially amorphous p-Eu:TiO2 first transforms into the anatase phase and then into a mixed anatase/rutile phase. These phase transformations change the local environment of the dopant Eu3+ ions resulting in modifications to the Eu3+ ions spectroscopic properties, with the modifications occurring for calcination temperatures above approximately 573 K following sub-second durations. These results demonstrate the temperature sensing ability of p-Eu:TiO2 nanoparticles for use in sub-second heating events. Moreover, at 573 K this temperature is lower than other host materials that we have evaluated (i.e., La2O3, ZrO2 and Y2O3).

Pressure Induced Phase Transformations of Silica Polymorphs and Glasses

NASA Astrophysics Data System (ADS)

Cagin, Tahir; Demiralp, Ersan; Goddard, William A., III

1998-03-01

Silica, SiO_2, is one of the most widely studied substance, and it has some complex and unusual properties. We have used a recently developed 2-body interaction force field (E. Demiralp, T. Cagin, W.A. Goddard, III, unpublished.) to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are α-quartz to stishovite, coesite to stishovite and fused glass to stishovite-like dense, a dominantly six-coordinated glassy phase. Molecular dynamics simulations are performed under the constant loading rates ranging from 0.1 GPa/ps to 2.0 GPa/ps, pressures upto 100 GPa and at temperatures 300, 500, 700 and 900 K. We observe the crystal to crystal transformations to occur reconstructively, whereas it occurs in a smooth and displacive manner from glass to a stishovite-like phase confirming earlier conjectures. (E.M. Stolper and T.J. Ahrens, Geophys. Res. Let.) 14, 1231 (1987). To elucidate the shock loading experiments, we studied the dependence of transition pressure on the loading rate and the temperature. To assess the hysterisis effect we also studied the unloading behavior of each transformation.

Phase transformations of siderite ore by the thermomagnetic analysis data

NASA Astrophysics Data System (ADS)

Ponomar, V. P.; Dudchenko, N. O.; Brik, A. B.

2017-02-01

Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650 °C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was 320 °C. Saturation magnetization of obtained samples increases up to 20 Am2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite.

Phase transitions in samarium at high pressures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, W.Y.; Lin, T.H.; Dunn, K.J.

1987-01-15

The electrical behavior of Sm was studied for pressures up to 43 GPa and temperatures from 430 down to 2 K. The two Neel temperatures at ambient pressure are found to move toward each other as the pressure increases and finally merge into one at the dhcp phase. At room temperature, we found that Sm transforms to a new phase, presumably fcc, at about 12 GPa. The phase line between the dhcp and the new phase appears to tie with the cusp of the bcc phase line.

Structural characterization and electrical conductivity of the Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} solid series

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cao, Yong; Duan, Nanqi; Yan, Dong, E-mail: yand@hust.edu.cn

Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} (x=0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.99) is prepared by using a solid reaction route, and single phase is achieved. Structural and phase transformation of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} have been characterized by high temperature X-ray diffraction. The lattice parameters a, b, c decrease and γ increases with increasing x, at both room and high temperature. The phase transformation temperature increases linearly with increasing x for Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ}. The electrical conductivity of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} is measured in wet air. A clear relationship between the structural, phase transformation andmore » electrical conductivity of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} is built, which will provide a guideline to tailor the electrical conductivity. - Graphical abstract: Structural and phase transformation of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} have been characterized by high temperature X-ray diffraction, as well as the conductivity of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} in wet air. A clear relationship between the structural, phase transformation and electrical conductivity of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} is built. - Highlights: • Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} with various Sm contents was prepared. • Structure, phase transformation and electrical conductivity of Ca{sub 0.01}La{sub 0.99−x}Sm{sub x}NbO{sub 4−δ} were characterized. • A relationship between the structure, phase transformation and electrical conductivity was well established.« less

Distribution trends and influence of 4d transition metal elements (Ru, Rh and Pd) doping on mechanical properties and martensitic transformation temperature of B2-ZrCu phase

NASA Astrophysics Data System (ADS)

Guo, Fuda; Zhan, Yongzhong

2017-12-01

The prediction for distribution trends and effect of three 4d transition metal elements (Ru, Rh and Pd) on mechanical properties and martensitic transformation temperature of B2-ZrCu phase were investigated by first-principles calculations. The convex surface of formation energy suggests that the alloying elements prefer to occupy the Cu sites in B2-ZrCu phase and the dopants studied in present are able to strengthen the phase stability. The calculated results of substitutional formation energy suggest that the distribution trend of dopants in B2-ZrCu phase is Ru > Rh > Pd below the dopant concentration 9 at. %, and the distribution trend is Rh > Pd > Ru from 9 at. % to 12.5 at. %. The elastic constants and mechanical properties including bulk modulus and shear modulus were calculated and discussed. The brittleness/ductility characteristic was investigated using the B/G ratio, Poisson's ratio v and Cauchy pressure Cp. The martensitic transformation temperature (Ms) and melting point (Tm) were predicted by using two cubic elastic moduli (C‧ and C44). The prediction results suggest that only the Ms of Zr8Cu7Pd is higher than the parent. The martensitic transformation temperatures of other compounds decrease with the addition of 4d transition metal dopants. Finally, the electronic structures and electron density different were discussed to reveal the bonding characteristics.

New transformations between crystalline and amorphous ice

NASA Technical Reports Server (NTRS)

Hemley, R. J.; Chen, L. C.; Mao, H. K.

1989-01-01

High-pressure optical and spectroscopic techniques were used to obtain directly the ice I(h) - hda-ice transformation in a diamond-anvil cell, and the stability of the amorphous form is examined as functions of pressure and temperature. It is demonstrated that hda-ice transforms abruptly at 4 GPa and 77 K to a crystalline phase close in structure to orientationally disordered ice-VII and to a more highly ordered, ice-VIII-like structure at higher temperatures. This is the first time that an amorphous solid is observed to convert to a crystalline solid at low temperatures by compression alone. Phase transitions of this type may be relevant on icy planetary satellites, and there may also be implications for the high-pressure behavior of silica.

Generalization of soft phonon modes

NASA Astrophysics Data System (ADS)

Rudin, Sven P.

2018-04-01

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. Here, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system with N atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, PVM0, represents the 3 N -dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, PVM0 serves as a generalization of soft phonon modes. At low temperatures, PVM0 reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case PVM0 culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, PVM0 can be equally well calculated on either side of the structural phase transition. Two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the β phase of uranium, the higher-symmetry structure of which stabilizes with temperature.

Reverse Shape Memory Effect Related to α → γ Transformation in a Fe-Mn-Al-Ni Shape Memory Alloy

NASA Astrophysics Data System (ADS)

Peng, Huabei; Huang, Pan; Zhou, Tiannan; Wang, Shanling; Wen, Yuhua

2017-05-01

In this study, we investigated the shape memory behavior and phase transformations of solution-treated Fe43.61Mn34.74Al13.38Ni8.27 alloy between room temperature and 1173 K (900 °C). This alloy exhibits the reverse shape memory effect resulting from the phase transformation of α (bcc) → γ (fcc) between 673 K and 1073 K (400 °C and 800 °C) in addition to the shape memory effect resulting from the martensitic reverse transformation of γ' (fcc) → α (bcc) below 673 K (400 °C). There is a high density of hairpin-shaped dislocations in the α phase undergoing the martensitic reverse transformation of γ' → α. The lath γ phase, which preferentially nucleates and grows in the reversed α phase, has the same crystal orientation with the reverse-transformed γ' martensite. However, the vermiculate γ phase, which is precipitated in the α phase between lath γ phase, has different crystal orientations. The lath γ phase is beneficial to attaining better reverse shape memory effect than the vermiculate γ phase.

Electron-electron correlations in Raman spectra of VO2

NASA Astrophysics Data System (ADS)

Goncharuk, I. N.; Ilinskiy, A. V.; Kvashenkina, O. E.; Shadrin, E. B.

2013-01-01

It has been shown that, in single crystals and films of a strongly correlated material, namely, vanadium dioxide, upon a thermally stimulated phase transition from the low-temperature monoclinic phase to the high-temperature tetragonal phase, the narrow-line Raman spectrum of the insulating (monoclinic) phase transforms into the broad-band Raman spectrum, which contains two peaks at 500 and 5000 cm-1 with widths of 400 and 3500 cm-1, respectively. It has been found that, as the temperature of the monoclinic phase approaches the structural phase transition temperature (340 K), the line profile of soft-mode phonons at a frequency of 149 cm-1 with A g symmetry and the line profile of phonons at a frequency of 201 cm-1 with A g symmetry acquire an asymmetric shape with a Fano antiresonance that is characteristic of the interaction of a single phonon vibration with a continuum of strongly correlated electrons. It has been demonstrated that the thermal transformation of peaks in the Raman spectra of the VO2 metallic phase is in quantitative agreement with the theory of Raman scattering in strongly correlated materials.

Phase Transformation and Aging Behavior of Al0.5CoCrFeNiSi0.2 High-Entropy Alloy

NASA Astrophysics Data System (ADS)

Zhang, C.; Wu, G. F.; Dai, P. Q.

2015-05-01

An Al0.5CoCrFeNiSi0.2 high-entropy alloy was prepared by vacuum arc melting. The alloy was aged from 700 to 1100 °C. The effects of aging on the phase transformation and mechanical performances were explored. The as-cast alloy showed a dendritic (DR) microstructure. The DR region was an Fe,Cr-rich FCC phase, while the interdendritic (ID) region was a spinodal structure composed of Fe,Cr-rich BCC (A2) and Ni,Al-rich BCC (B2) phases. At aging temperatures between 700 and 900 °C, the Fe,Cr-rich BCC (A2) phase in the ID region transformed into σ and Fe,Cr-rich FCC phases. Meanwhile, some Ni,Al-rich FCC phase particles precipitated from the DR region. During aging at 1100 °C, the DR microstructure disappeared, and a microstructure composed of Fe,Cr-rich FCC and Ni,Al-rich BCC (B2) phases both possessing a lamellar shape was developed. The alloy exhibited evident hardening and lower tensile strain when the aging temperature was lower than 1000 °C, which was mainly attributed to the generation of the σ phase in the ID region. However, a contrasting behavior was observed when the aging temperature was higher than 1000 °C, which was attributed to the redissolution of the σ phase and the microstructure coarsening.

Potential High-Temperature Shape-Memory-Alloy Actuator Material Identified

NASA Technical Reports Server (NTRS)

Noebe, Ronald D.; Gaydosh, Darrell J.; Biles, Tiffany A.; Garg, Anita

2005-01-01

Shape-memory alloys are unique "smart materials" that can be used in a wide variety of adaptive or "intelligent" components. Because of a martensitic solid-state phase transformation in these materials, they can display rather unusual mechanical properties including shape-memory behavior. This phenomenon occurs when the material is deformed at low temperatures (below the martensite finish temperature, Mf) and then heated through the martensite-to-austenite phase transformation. As the material is heated to the austenite finish temperature Af, it is able to recover its predeformed shape. If a bias is applied to the material as it tries to recover its original shape, work can be extracted from the shape-memory alloy as it transforms. Therefore, shape-memory alloys are being considered for compact solid-state actuation devices to replace hydraulic, pneumatic, or motor-driven systems.

Phase Transformation Study in Nb-Mo Microalloyed Steels Using Dilatometry and EBSD Quantification

NASA Astrophysics Data System (ADS)

Isasti, Nerea; Jorge-Badiola, Denis; Taheri, Mitra L.; Uranga, Pello

2013-08-01

A complete microstructural characterization and phase transformation analysis has been performed for several Nb and Nb-Mo microalloyed low-carbon steels using electron backscattered diffraction (EBSD) and dilatometry tests. Compression thermomechanical schedules were designed resulting in the undeformed and deformed austenite structures before final transformation. The effects of microalloying additions and accumulated deformation were analyzed after CCT diagram development and microstructural quantification. The resulting microstructures ranged from polygonal ferrite and pearlite at slow cooling ranges, to a combination of quasipolygonal ferrite and granular ferrite for intermediate cooling rates, and finally, to bainitic ferrite with martensite for fast cooling rates. The addition of Mo promotes a shift in the CCT diagrams to lower transformation start temperatures. When the amount of Nb is increased, CCT diagrams show little variations for transformations from the undeformed austenite and higher initial transformation temperatures in the transformations from the deformed austenite. This different behavior is due to the effect of niobium on strain accumulation in austenite and its subsequent acceleration of transformation kinetics. This article shows the complex interactions between chemical composition, deformation, and the phases formed, as well as their effect on microstructural unit sizes and homogeneity.

Volume Change During Intermartensitic Transformations in Ni-Mn-Ga Alloy

NASA Astrophysics Data System (ADS)

Polyakov, P. I.; Slyusarev, V. V.; Kokorin, V. V.; Konoplyuk, S. M.; Semenova, Yu. S.; Khovaylo, V. V.

2014-09-01

Characteristics of phase transitions in Ni54Mn24Ga22 alloy were studied at different hydrostatic pressures to shed light on the nature and mechanisms of intermartensitic transformations. The temperature dependence of resistivity of the alloy was used to find characteristic temperatures of martensitic and intermartensitic transformations as a function of hydrostatic pressure. The latent heat of these transformations was determined by differential scanning calorimetry. The transformation volume effects were calculated using Clausius-Clapeyron equation. They make up 0.082% for martensitic and 0.024% for intermartensitic transformations.

Reduction of the allotropic transition temperature in nanocrystalline zirconium: Predicted by modified equation of state (MEOS) method and molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Salati, Amin; Mokhtari, Esmail; Panjepour, Masoud; Aryanpour, Gholamreza

2013-04-01

The temperature at which polymorphic phase transformation occurs in nanocrystalline (NC) materials is different from that of coarse-grained specimens. This anomaly has been related to the role of grain boundary component in these materials and can be predicted by a dilated crystal model. In this study, based on this model, a modified equation of state (MEOS) method (instead of equation of state, EOS, method) is used to calculate the total Gibbs free energy of each phase (β-Zr or α-Zr) in NC Zr. Thereupon, the change in the total Gibbs free energy for β-Zr to α-Zr phase transformation (ΔGβ→α) via the grain size is calculated by this method. Similar to polymorphic transformation in other NC materials (Fe, Nb, Co, TiO2, Al2O3 and ZnS), it is found that the estimated transformation temperature in NC Zr (β→α) is reduced with decreasing grain size. Finally, a molecular dynamics (MD) simulation is employed to confirm the theoretical results.

Self-Organization of Amorphous Carbon Nanocapsules into Diamond Nanocrystals Driven by Self-Nanoscopic Excessive Pressure under Moderate Electron Irradiation without External Heating.

PubMed

Wang, Chengbing; Ling, San; Yang, Jin; Rao, Dewei; Guo, Zhiguang

2018-01-01

Phase transformation between carbon allotropes usually requires high pressures and high temperatures. Thus, the development of low-temperature phase transition approaches between carbon allotropes is highly desired. Herein, novel amorphous carbon nanocapsules are successfully synthesized by pulsed plasma glow discharge. These nanocapsules are comprised of highly strained carbon clusters encapsulated in a fullerene-like carbon matrix, with the formers serving as nucleation sites. These nucleation sites favored the formation of a diamond unit cell driven by the self-nanoscopic local excessive pressure, thereby significantly decreasing the temperature required for its transformation into a diamond nanocrystal. Under moderate electron beam irradiation (10-20 A cm -2 ) without external heating, self-organization of the energetic carbon clusters into diamond nanocrystals is achieved, whereas the surrounding fullerene-like carbon matrix remains nearly unchanged. Molecular dynamics simulations demonstrate that the defective rings as the active sites dominate the phase transition of amorphous carbon to diamond nanocrystal. The findings may open a promising route to realize phase transformation between carbon allotropes at a lower temperature. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Phase field modeling of tetragonal to monoclinic phase transformation in zirconia

NASA Astrophysics Data System (ADS)

Mamivand, Mahmood

Zirconia based ceramics are strong, hard, inert, and smooth, with low thermal conductivity and good biocompatibility. Such properties made zirconia ceramics an ideal material for different applications form thermal barrier coatings (TBCs) to biomedicine applications like femoral implants and dental bridges. However, this unusual versatility of excellent properties would be mediated by the metastable tetragonal (or cubic) transformation to the stable monoclinic phase after a certain exposure at service temperatures. This transformation from tetragonal to monoclinic, known as LTD (low temperature degradation) in biomedical application, proceeds by propagation of martensite, which corresponds to transformation twinning. As such, tetragonal to monoclinic transformation is highly sensitive to mechanical and chemomechanical stresses. It is known in fact that this transformation is the source of the fracture toughening in stabilized zirconia as it occurs at the stress concentration regions ahead of the crack tip. This dissertation is an attempt to provide a kinetic-based model for tetragonal to monoclinic transformation in zirconia. We used the phase field technique to capture the temporal and spatial evolution of monoclinic phase. In addition to morphological patterns, we were able to calculate the developed internal stresses during tetragonal to monoclinic transformation. The model was started form the two dimensional single crystal then was expanded to the two dimensional polycrystalline and finally to the three dimensional single crystal. The model is able to predict the most physical properties associated with tetragonal to monoclinic transformation in zirconia including: morphological patterns, transformation toughening, shape memory effect, pseudoelasticity, surface uplift, and variants impingement. The model was benched marked with several experimental works. The good agreements between simulation results and experimental data, make the model a reliable tool for predicting tetragonal to monoclinic transformation in the cases we lack experimental observations.

Study of thermomechanical treatment on mechanical-induced phase transformation of NiTi and TiNiCu wires.

PubMed

Seyyed Aghamiri, S M; Nili Ahmadabadi, M; Shahmir, H; Naghdi, F; Raygan, Sh

2013-05-01

The nickel-titanium shape memory alloys have been used in orthodontic application due to their unique properties like superelasticity and biocompatibility. The phase transformation behavior of these alloys can be changed by alloying elements and thermomechanical processing conditions. In this study, two types of NiTi and TiNiCu wires of 0.4mm diameter were produced via thermomechanical treatments with final step of 20% cold drawing followed by annealing at different temperatures of 300 and 400 °C for varying times of 10, 30 and 60 min. The processed wires were characterized by oral cavity configuration three point bending (OCTPB) test at 37 °C to specify the mechanical transformation features. Also, differential scanning calorimetry (DSC) was used to analyze the thermal transformation temperatures of selected wires. The results showed the thermomechanical treatment at 300 °C for 30 min was the suitable process in terms of superelasticity and transformation temperatures for orthodontic application. Copyright © 2013 Elsevier Ltd. All rights reserved.

Transformation of BCC and B2 High Temperature Phases to HCP and Orthorhombic Structures in the Ti-Al-Nb System. Part I: Microstructural Predictions Based on a Subgroup Relation Between Phases

PubMed Central

Bendersky, L. A.; Roytburd, A.; Boettinger, W. J.

1993-01-01

Possible paths for the constant composition coherent transformation of BCC or B2 high temperature phases to low temperature HCP or Orthorhombic phases in the Ti-Al-Nb system are analyzed using a sequence of ciystallographic structural relationships developed from subgroup symmetry relations. Symmetry elements lost in each step of the sequence determine the possibilities for variants of the low symmetry phase and domains that can be present in the microstructure. The orientation of interdomain interfaces is determined by requiring the existence of a strain-free interface between the domains. Polydomain structures are also determined that minimize elastic energy. Microstructural predictions are made for comparison to experimental results given by Benderslcy and Boettinger [J. Res. Natl. Inst. Stand. Technol. 98, 585 (1993)]. PMID:28053487

Structural transition in lanthanum gallate and transformation of the fine structure of the EPR spectrum of a Gd3+ impurity center

NASA Astrophysics Data System (ADS)

Vazhenin, V. A.; Guseva, V. B.; Fokin, A. V.; Potapov, A. P.; Artyomov, M. Yu.

2011-04-01

Abrupt changes in resonance positions, hysteretic temperature behavior, and coexistence of phases, which indicate a first-order phase transition, have been revealed from measurements of temperature dependences of the EPR spectra of Gd3+ and Mn4+ centers in the vicinity of the structural transition of lanthanum gallate. The transformation of monoclinic Gd3+ centers into trigonal Gd3+ centers upon the phase transition has been used to estimate the adequacy of two approximations of the superposition model for parameters of the zero-field splitting of the ground state.

THERMODYNAMICS AND KINETICS OF PHASE TRANSFORMATIONS IN PLUTONIUM ALLOYS - PART I

DOE Office of Scientific and Technical Information (OSTI.GOV)

Turchi, P A; Kaufman, L; Liu, Z

2004-08-18

In this report we investigate order, stability, and phase transformations for a series of actinide-based alloys. The statics and kinetics of precipitation and ordering in this class of alloys are modeled with a scheme that couples fundamental information on the alloy energetics obtained from experimental and assessed thermo-chemical data to the CALPHAD approach commonly used in industry for designing alloys with engineering specificity with the help of the Thermo-Calc software application. The CALPHAD approach is applied to the study of the equilibrium thermodynamic properties of Pu-based alloys, Pu-X, where X=Al, Fe, Ga. The assessment of the equilibrium phase diagrams inmore » the whole range of alloy composition has been performed with the PARROT module of the Thermo-Calc application software. Predictions are made on the low temperature and Pu-rich side of the phase diagrams of Pu-Ga and Pu-Al for which controversy has been noted in the past. The validity of the assessed thermo-chemical database will be discussed by comparing predicted heats of transformation for pure Pu with measured values from differential scanning calorimetry analysis. An overall picture for the stability properties of Pu-Ga and Pu-Al that reconciles the results of past studies carried out on these alloys is proposed. Results on phase stability in the ternary Fe-Ga-Pu and Al-Fe-Pu alloys are discussed. The information collected in this study is then used to model metastability, long-term stability and aging for this class of alloys by coupling Thermo-Calc with DICTRA, a series of modules that allow the analysis of DIffusion Controlled TRAnsformations. Kinetics information is then summarized in so-called TTT (temperature-time-transformations) diagrams for the most relevant phases of actinide alloys. Specifically, results are presented on kinetics of phase transformations associated with the eutectoid-phase decomposition reaction occurring at low temperature, and with the martensitic transformation that takes place at low Ga content in Pu-Ga alloys. Finally, after a summary of the most salient results, suggestions are made for further studies at the micro- and mesoscales.« less

Phase transformations at interfaces: Observations from atomistic modeling

DOE PAGES

Frolov, T.; Asta, M.; Mishin, Y.

2016-10-01

Here, we review the recent progress in theoretical understanding and atomistic computer simulations of phase transformations in materials interfaces, focusing on grain boundaries (GBs) in metallic systems. Recently developed simulation approaches enable the search and structural characterization of GB phases in single-component metals and binary alloys, calculation of thermodynamic properties of individual GB phases, and modeling of the effect of the GB phase transformations on GB kinetics. Atomistic simulations demonstrate that the GB transformations can be induced by varying the temperature, loading the GB with point defects, or varying the amount of solute segregation. The atomic-level understanding obtained from suchmore » simulations can provide input for further development of thermodynamics theories and continuous models of interface phase transformations while simultaneously serving as a testing ground for validation of theories and models. They can also help interpret and guide experimental work in this field.« less

The application of rational approximation in the calculation of a temperature field with a non-linear surface heat-transfer coefficient during quenching for 42CrMo steel cylinder

NASA Astrophysics Data System (ADS)

Cheng, Heming; Huang, Xieqing; Fan, Jiang; Wang, Honggang

1999-10-01

The calculation of a temperature field has a great influence upon the analysis of thermal stresses and stains during quenching. In this paper, a 42CrMo steel cylinder was used an example for investigation. From the TTT diagram of the 42CrMo steel, the CCT diagram was simulated by mathematical transformation, and the volume fraction of phase constituents was calculated. The thermal physical properties were treated as functions of temperature and the volume fraction of phase constituents. The rational approximation was applied to the finite element method. The temperature field with phase transformation and non-linear surface heat-transfer coefficients was calculated using this technique, which can effectively avoid oscillationin the numerical solution for a small time step. The experimental results of the temperature field calculation coincide with the numerical solutions.

Generalized thermoelastic problem of an infinite body with a spherical cavity under dual-phase-lags

NASA Astrophysics Data System (ADS)

Karmakar, R.; Sur, A.; Kanoria, M.

2016-07-01

The aim of the present contribution is the determination of the thermoelastic temperatures, stress, displacement, and strain in an infinite isotropic elastic body with a spherical cavity in the context of the mechanism of the two-temperature generalized thermoelasticity theory (2TT). The two-temperature Lord-Shulman (2TLS) model and two-temperature dual-phase-lag (2TDP) model of thermoelasticity are combined into a unified formulation with unified parameters. The medium is assumed to be initially quiescent. The basic equations are written in the form of a vector matrix differential equation in the Laplace transform domain, which is then solved by the state-space approach. The expressions for the conductive temperature and elongation are obtained at small times. The numerical inversion of the transformed solutions is carried out by using the Fourier-series expansion technique. A comparative study is performed for the thermoelastic stresses, conductive temperature, thermodynamic temperature, displacement, and elongation computed by using the Lord-Shulman and dual-phase-lag models.

Change in generally accepted regularity of phase transformations of quartzite

NASA Astrophysics Data System (ADS)

Kukartsev, V. A.; Kukartsev, V. V.; Chzhan, E. A.; Tynchenko, V. S.; Stupina, A. A.

2018-05-01

The subject of this research is phasic transformations of quartzites that are under temperature treatment to remove moisture. This technology is used in enterprises operating melting furnaces. The studies have shown that using a temperature regime consisting in heating to 800° C and holding for 2 hours, after cooling, quartzite changes its color and appears a shift in the angle of the interplanar distances of the crystal lattice by 6.6% in it. The use of a temperature treatment regime consisting in heating to 200° C and holding for 4 hours does not reveal such changes. With subsequent exposure to these samples of the temperature regime corresponding to the sintering process of the liner, the following is established. In a sample pretreated with a temperature of 800° C, at a temperature of 1550° C, a tridymite phase appears. In the sample of a 200° C pretreated with temperature, a phase of cristobalite appears without tridymite.

Study on stress-strain response of multi-phase TRIP steel under cyclic loading

NASA Astrophysics Data System (ADS)

Dan, W. J.; Hu, Z. G.; Zhang, W. G.; Li, S. H.; Lin, Z. Q.

2013-12-01

The stress-strain response of multi-phase TRIP590 sheet steel is studied in cyclic loading condition at room temperature based on a cyclic phase transformation model and a multi-phase mixed kinematic hardening model. The cyclic martensite transformation model is proposed based on the shear-band intersection, where the repeat number, strain amplitude and cyclic frequency are used to control the phase transformation process. The multi-phase mixed kinematic hardening model is developed based on the non-linear kinematic hardening rule of per-phase. The parameters of transformation model are identified with the relationship between the austenite volume fraction and the repeat number. The parameters in Kinematic hardening model are confirmed by the experimental hysteresis loops in different strain amplitude conditions. The responses of hysteresis loop and stress amplitude are evaluated by tension-compression data.

Effect of thermal cycling on the R-phase and martensitic transformations in a Ti-rich NiTi alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pelosin, V.; Riviere, A.

1998-04-01

The effect of thermal cycling on transformation temperature was studied on a Ti-rich NiTi alloy. The study was carried out by determining the electrical resistance, the internal friction, and the elastic modulus vs temperature. This study shows that the martensite microstructure is modified by the successive cycling transformation. In addition, the authors established that both the martensite internal friction and the transition peak are sensitive to the transient effect (the vibration frequency lies around 300 Hz). But the major results concern the behavior associated with the R phase occurrence and its evolution. They have stated that the premartensitic phase becomesmore » stable following the diminishment of the beginning of the martensite formation (M{sub s}). Interrupted cooling has also shown that, contrary to the martensite, the R phase exhibits no hysteretic behavior.« less

Nanoindentation hardness and atomic force microscope imaging studies of pressure-quenched zirconium metal

NASA Astrophysics Data System (ADS)

Catledge, Shane A.; Spencer, Philemon T.; Vohra, Yogesh K.

2000-11-01

We have carried out mechanical property measurements on zirconium metal compressed in a diamond anvil cell to 19 GPa at room temperature with subsequent quenching to room pressure. The irreversible transformation from the ambient hexagonal-close-packed phase to the simple hexagonal ω phase (AlB2 structure) is confirmed by synchrotron energy dispersive x-ray diffraction followed by nanoindentation of the pressure-quenched sample. We document an 80% increase in hardness as a consequence of the pressure-induced transformation to the ω phase at room temperature. This is a large increase for a metallic phase transformation and can be attributed to the presence of sp2-hybrid bonds forming graphite-like nets in the (0001) plane of the AlB2 structure. Atomic force microscopy of the indents shows that a plastic deformation of 2 μm in depth was achieved with a force of 200 mN.

Phase study of titanium dioxide nanoparticle prepared via sol-gel process

NASA Astrophysics Data System (ADS)

Oladeji Araoyinbo, Alaba; Bakri Abdullah, Mohd Mustafa Al; Salleh, Mohd Arif Anuar Mohd; Aziz, Nurul Nadia Abdul; Iskandar Azmi, Azwan

2018-03-01

In this study, titanium dioxide nanoparticles have been prepared via sol-gel process using titanium tetraisopropoxide as a precursor with hydrochloric acid as a catalyst, and ethanol with deionized water as solvents. The value of pH used is set to 3, 7 and 8. The sols obtained were dried at 100 °C for 1 hr and calcined at 350, 550, and 750 °C for 3 hrs to observe the phase transformation of titanium dioxide nanoparticle. The samples were characterized by x-ray diffraction and field emission scanning electron microscope. The morphology analysis is obtained from field emission scanning electron microscope. The phase transformation was investigated by x-ray diffraction. It was found that the pH of the solution affect the agglomeration of titanium dioxide particle. The x-ray diffraction pattern of titanium dioxide shows the anatase phase most abundant at temperature of 350 °C. At temperature of 550 °C the anatase and rutile phase were present. At temperature of 750 °C the rutile phase was the most abundant for pH 3, 7 and 8. It was confirmed that at higher temperature the rutile phase which is the stable phase are mostly present.

Crystallographic and magnetic structure of HAVAR under high-pressure using diamond anvil cell (DAC)

NASA Astrophysics Data System (ADS)

Halevy, Itzhak; Haroush, Shlomo; Eisen, Yosef; Silberman, Ido; Moreno, Dany; Hen, Amir; Winterrose, Mike L.; Ghose, Sanjit; Chen, Zhiqiang

2010-04-01

Annealed (H1) and cold-rolled (H2) HAVAR has been studied using high-pressure synchrotron X-ray diffraction. A structural phase transformation was discovered at ˜13 GPa at ambient temperature, transforming from m - 3 m (S.G. 225) to P 63/m m c (S.G. 194) symmetry. The transition was not reversible on pressure release. The low-pressure cubic phase was found to be more compressible than the high-pressure hexagonal phase. Conventional Mössbauer and NFS shows that the HAVAR is not magnetic at room temperature and no splitting is observed. The SQUID indicates a huge difference in the temperature dependence of the magnetic susceptibility between the cold Rolled HAVAR compared to the annealed HAVAR.

Study of thermal stability of Cu{sub 2}Se thermoelectric material

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bohra, Anil, E-mail: anilbohra786@gmail.com; Bhatt, Ranu; Bhattacharya, Shovit

2016-05-23

Sustainability of thermoelectric parameter in operating temperature range is a key consideration factor for fabricating thermoelectric generator or cooler. In present work, we have studied the stability of thermoelectric parameter of Cu{sub 2}Se within the temperature range of 50-800°C. Temperature dependent Seebeck coefficients and electrical resistivity measurement are performed under three continuous thermal cycles. X-ray diffraction pattern shows the presence of mixed cubic-monoclinic Cu{sub 2}Se phase in bare pellet which transforms to pure α-Cu{sub 2}Se phase with repeating thermal cycle. Significant enhancement in Seebeck coefficient and electrical resistivity is observed which may be attributed to (i) Se loss observed inmore » EDS and (ii) the phase transformation from mixed cubic-monoclinic structure to pure monoclinic α-Cu{sub 2}Se phase.« less

Estimation of Transformation Temperatures in Ti-Ni-Pd Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Narayana, P. L.; Kim, Seong-Woong; Hong, Jae-Keun; Reddy, N. S.; Yeom, Jong-Taek

2018-03-01

The present study focused on estimating the complex nonlinear relationship between the composition and phase transformation temperatures of Ti-Ni-Pd shape memory alloys by artificial neural networks (ANN). The ANN models were developed by using the experimental data of Ti-Ni-Pd alloys. It was found that the predictions are in good agreement with the trained and unseen test data of existing alloys. The developed model was able to simulate new virtual alloys to quantitatively estimate the effect of Ti, Ni, and Pd on transformation temperatures. The transformation temperature behavior of these virtual alloys is validated by conducting new experiments on the Ti-rich thin film that was deposited using multi target sputtering equipment. The transformation behavior of the film was measured by varying the composition with the help of aging treatment. The predicted trend of transformational temperatures was explained with the help of experimental results.

Characterization of a Diamond Ground Y-TZP and Reversion of the Tetragonal to Monoclinic Transformation.

PubMed

Candido, L M; Fais, Lmg; Ferreira, E B; Antonio, S G; Pinelli, Lap

To characterize the surface of an yttria-stabilized zirconia (Y-TZP) ceramic after diamond grinding in terms of its crystalline phase, morphology, mean roughness (Ra), and wettability as well as to determine a thermal treatment to reverse the resulting tetragonal to monoclinic (t-m) transformation. Y-TZP specimens were distributed into different groups according to the actions (or no action) of grinding and irrigation. Grinding was accomplished using a diamond stone at a low speed. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy, goniometry, and profilometry. In situ high-temperature XRD was used to determine an annealing temperature to reverse the t-m transformation. Ra was submitted to the Kruskal-Wallis test, followed by the Dunn test (α=0.05). The volume fraction of the monoclinic phase and contact angle were submitted to one-way analysis of variance, followed by the Tukey test (α=0.05). Monoclinic zirconia was observed on the surface of samples after dry and wet grinding with a diamond stone. The volume fraction of the monoclinic phase was smaller on the dry ground samples (3.6%±0.3%) than on the wet ground samples (5.6%±0.3%). High-temperature XRD showed reversion of the t-m phase transformation, which started at 700°C and completed at 800°C in a conventional oven. Grinding with a diamond stone partially transformed the crystalline phase on the surface of a Y-TZP ceramic from tetragonal to monoclinic zirconia while simultaneously increasing the surface roughness and wettability. The t-m transformation could be reversed by heat treatment at 800°C or 900°C for 60 minutes or 1000°C for 30 minutes.

On the synthesis of AlPO4-21 molecular sieve by vapor phase transport method and its phase transformation to AlPO4-15 molecular sieve

NASA Astrophysics Data System (ADS)

Shao, Hui; Chen, Jingjing; Chen, Xia; Leng, Yixin; Zhong, Jing

2015-04-01

An experimental design was applied to the synthesis of AlPO4-21 molecular sieve (AWO structure) by vapor phase transport (VPT) method, using tetramethylguanidine (TMG) as the template. In this study, the effects of crystallization time, crystallization temperature, phosphor content, template content and water content in the synthesis gel were investigated. The materials obtained were characterized by X-ray diffraction, scanning electron microscopy and fourier transform infrared spectroscopy (FT-IR). Microstructural analysis of the crystal growth in vapor synthetic conditions revealed a revised crystal growth route from zeolite AlPO4-21 to AlPO4-15 in the presence of the TMG. Homogenous hexagonal prism AlPO4-21 crystals with size of 7 × 3 μm were synthesized at a lower temperature (120 °C), which were completely different from the typical tabular parallelogram crystallization microstructure of AlPO4-21 phase. The crystals were transformed into AlPO4-21 phase with higher crystallization temperature, longer crystallization time, higher P2O5/Al2O3 ratio and higher TMG/Al2O3 ratio.

Direct Laser Writing of δ- to α-Phase Transformation in Formamidinium Lead Iodide

PubMed Central

2017-01-01

Organolead halide perovskites are increasingly considered for applications well beyond photovoltaics, for example, as the active regions within photonic devices. Herein, we report the direct laser writing (DLW: 458 nm cw-laser) of the formamidinium lead iodide (FAPbI3) yellow δ-phase into its high-temperature luminescent black α-phase, a remarkably easy and scalable approach that takes advantage of the material’s susceptibility to transition under ambient conditions. Through the DLW of α-FAPbI3 tracks on δ-FAPbI3 single-crystal surfaces, the controlled and rapid microfabrication of highly luminescent structures exhibiting long-term phase stability is detailed, offering an avenue toward the prototyping of complex perovskite-based optical devices. The dynamics and kinetics of laser-induced δ- to α-phase transformations are investigated in situ by Raman microprobe analysis, as a function of irradiation power, time, temperature, and atmospheric conditions, revealing an interesting connection between oxygen intercalation at the surface and the δ- to α-phase transformation dynamics, an insight that will find application within the wider context of FAPbI3 thermal phase relations. PMID:28763617

Effect of V-Nd co-doping on phase transformation and grain growth process of TiO2

NASA Astrophysics Data System (ADS)

Khatun, Nasima; Amin, Ruhul; Anita, Sen, Somaditya

2018-05-01

The pure and V-Nd co-doped TiO2 samples are prepared by the modified sol-gel process. The phase formation is confirmed by XRD spectrum. Phase transformation is delayed in V-Nd co-doped TiO2 (TVN) samples compared to pure TiO2. The particle size is comparatively small in TVN samples at both the temperature 450 °C and 900 °C. Hence the effect of Nd doping is dominated over V doping in both phase transformation and grain growth process of TiO2.

Evaluation of phase transformation in ferromagnetic shape memory Fe-Pd alloy by magnetic Barkhausen noise

NASA Astrophysics Data System (ADS)

Furuya, Yasubumi; Tamoto, Shizuka; Kubota, Takeshi; Okazaki, Teiko; Hagood, Nesbitt W.; Spearing, S. Mark

2002-07-01

The possibility to detect the phase transformation with martensites by heating or cooling as well as stress-loading in ferromagnetic shape memory Fe-30at percent Pd alloy thin foil by using magnetic Markhausen noise sensor was studied. MBHN is caused by the irregular interactions between magnetic domain and thermally activated martensite twins during magnetization. In general, the envelope of the MBHN voltage versus time signals in Fe-29at percent Pd ribbon showed two peaks during magnetization, where secondary peak at intermediate state of magnetization process decreased with increasing temperature, while the MBHN envelopes in pure iron did not change with increasing temperature. The variety of MBHN due to the phase transformation was apt to arise at higher frequency part of spectrum during intermediate state of magnetization process and it decreased with disappearance of martensite twins. Besides, MBHN increased monotonically with increasing loading stress and then, it decreased with unloading, however MBHN showed large hysteresis between loading and unloading passes. Based on the experimental results from MBHN measurements for both thermoelastic and stress-induced martensite phase transformations in Fe-30at percent Pd ribbon samples, MBHN method seems a useful technique to non-destructive evaluation of martensite phase transformation of ferromagnetic shape memory alloy.

Small-angle neutron scattering study of magnetic ordering and inhomogeneity across the martensitic phase transformation in Ni 50–xCo xMn₄₀Sn₁₀ alloys

DOE PAGES

Bhatti, Kanwal Preet; El-Khatib, S.; Srivastava, Vijay; ...

2012-04-27

The Heusler-derived multiferroic alloy Ni 50–xCo xMn₄₀Sn₁₀ has recently been shown to exhibit, at just above room temperature, a highly reversible martensitic phase transformation with an unusually large magnetization change. In this work the nature of the magnetic ordering above and below this transformation has been studied in detail in the critical composition range x = 6–8 via temperature-dependent (5–600 K) magnetometry and small-angle neutron scattering (SANS). We observe fairly typical paramagnetic to long-range-ordered ferromagnetic phase transitions on cooling to 420–430 K, with the expected critical spin fluctuations, followed by first-order martensitic phase transformations to a nonferromagnetic state below 360–390more » K. The static magnetization reveals complex magnetism in this low-temperature nonferromagnetic phase, including a Langevin-like field dependence, distinct spin freezing near 60 K, and significant exchange bias effects, consistent with superparamagnetic blocking of ferromagnetic clusters of nanoscopic dimensions. We demonstrate that these spin clusters, whose existence has been hypothesized in a variety of martensitic alloys exhibiting competition between ferromagnetic and antiferromagnetic exchange interactions, can be directly observed by SANS. The scattering data are consistent with a liquidlike spatial distribution of interacting magnetic clusters with a mean center-to-center spacing of 12 nm. Considering the behavior of the superparmagnetism, cooling-field and temperature-dependent exchange bias, and magnetic SANS, we discuss in detail the physical form and origin of these spin clusters, their intercluster interactions, the nature of the ground-state magnetic ordering in the martensitic phase, and the implications for our understanding of such alloy systems.« less

Properties of TiNi intermetallic compound industrially produced by combustion synthesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaieda, Yoshinari

Most TiNi shape memory intermetallic compounds are conventionally produced by the process including high frequency induction vacuum melting and casting. A gravity segregation occurs in a cast TiNi ingot because of the big difference in the specific gravity between Ti and Ni. It is difficult to control accurately the phase transformation temperature of TiNi shape memory intermetallic compound produced by the conventional process, because the martensitic transformation temperature shifts by 10K due to the change in 0.1 % of Ni content. Homogeneous TiNi intermetallic compound is produced by the industrial process including combustion synthesis method, which is a newly developedmore » manufacturing process. In the new process, phase transformation temperatures of TiNi can be controlled accurately by controlling the ratio of Ti and Ni elemental starting powders. The chemical component, the impurities and the phase transformation temperatures of the TiNi products industrially produced by the process are revealed. These properties are vitally important when combustion synthesis method is applied to an industrial mass production process for producing TiNi shape memory intermetallic compounds. TiNi shape memory products are industrially and commercially produced today the industrial process including combustion synthesis. The total production weight in a year is 30 tins in 1994.« less

Generalization of soft phonon modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rudin, Sven P.

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. In this paper, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system withmore » $N$ atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, $${\\mathrm{PVM}}_{0}$$, represents the $3N$-dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, $${\\mathrm{PVM}}_{0}$$ serves as a generalization of soft phonon modes. At low temperatures, $${\\mathrm{PVM}}_{0}$$ reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case $${\\mathrm{PVM}}_{0}$$ culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, $${\\mathrm{PVM}}_{0}$$ can be equally well calculated on either side of the structural phase transition. Finally, two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the $${\\beta}$$ phase of uranium, the higher-symmetry structure of which stabilizes with temperature.« less

Generalization of soft phonon modes

DOE PAGES

Rudin, Sven P.

2018-04-27

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. In this paper, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system withmore » $N$ atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, $${\\mathrm{PVM}}_{0}$$, represents the $3N$-dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, $${\\mathrm{PVM}}_{0}$$ serves as a generalization of soft phonon modes. At low temperatures, $${\\mathrm{PVM}}_{0}$$ reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case $${\\mathrm{PVM}}_{0}$$ culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, $${\\mathrm{PVM}}_{0}$$ can be equally well calculated on either side of the structural phase transition. Finally, two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the $${\\beta}$$ phase of uranium, the higher-symmetry structure of which stabilizes with temperature.« less

Role of Exposure Atmospheres on Particle Coarsening and Phase Transformation of LiAlO 2

DOE PAGES

Heo, Su Jeong; Hu, Boxun; Uddin, Md Aman; ...

2017-05-05

The phase transformation and particle coarsening of lithium aluminate (α-LiAlO 2) in electrolyte are the major causes of degradation affecting the performance and the lifetime of the molten carbonate fuel cell (MCFC). The stability of LiAlO 2 has been studied in Li 2CO 3-Na 2CO 3 electrolyte under accelerated conditions in reducing and oxidizing gas atmospheres at temperatures of 650 and 750 for up to 500 hours. X-ray diffraction analyses show that the progressive transformation of α-LiAlO 2 to γ-LiAlO 2 phase proceeds with increasing temperature in lower P CO2 and lower P O2 environments. Spherical LiAlO 2 particles weremore » transformed to coarsened pyramid-shape particles in 4% H 2-3% H 2O-N 2 and 100% N 2 (~10 ppm P O2 ) atmospheres. Under CO 2-rich atmospheres (4% H 2-30% CO 2-N 2 and 70% air-30% CO 2), both phase and particle size remained unchanged at 650 and 750ºC. The selected area electron diffraction (SAED) pattern analysis indicated that the large pyramidal shape particles (~30 μm) were γ-LiAlO 2 phase. Experimental observations and related simulation results pertaining to particle coarsening and phase transformation behavior of LiAlO 2 are presented.« less

Controlling Fundamentals in High-Energy High-Rate Pulsed Power Materials Processing of Powdered Tungsten, Titanium Aluminides, and Copper-Graphite Composites

DTIC Science & Technology

1990-10-01

phase systems such as tungsten; plastic flow of a minor low -temperature phase in a two phase non-interacting system such as tungsten- copper ; and...consolidation heat treatment. The de- the wetting of graphite by copper or during consolidation by this tech- tailed phase transformation evolution other...The driving potential for this solid state phase transformation is the free surface energy associated with the total powder particle surface area in the

Study on the Microstructure and Liquid Phase Formation in a Semisolid Gray Cast Iron

NASA Astrophysics Data System (ADS)

Benati, Davi Munhoz; Ito, Kazuhiro; Kohama, Kazuyuki; Yamamoto, Hajime; Zoqui, Eugenio José

2017-10-01

The development of high-quality semisolid raw materials requires an understanding of the phase transformations that occur as the material is heated up to the semisolid state, i.e., its melting behavior. The microstructure of the material plays a very important role during semisolid processing as it determines the flow behavior of the material when it is formed, making a thorough understanding of the microstructural evolution essential. In this study, the phase transformations and microstructural evolution in Fe2.5C1.5Si gray cast iron specially designed for thixoforming processes as it was heated to the semisolid state were observed using in situ high-temperature confocal laser scanning microscopy. At room temperature, the alloy has a matrix of pearlite and ferrite with fine interdendritic type D flake graphite. During heating, the main transformations observed were graphite precipitation inside the grains and at the austenite grain boundaries; graphite flakes and graphite precipitates growing and becoming coarser with the increasing temperature; and the beginning of melting at around 1413 K to 1423 K (1140 °C to 1150 °C). Melting begins with the eutectic phase ( i.e., the carbon-rich phase) and continues with the primary phase (primary austenite), which is consumed as the temperature increases. Melting of the eutectic phase composed by coarsened interdendritic graphite flakes produced a semi-continuous liquid network homogeneously surrounding and wetting the dendrites of the solid phase, causing grains to detach from each other and producing the intended solid globules immersed in liquid.

Molecular dynamics study of dual-phase microstructure of Titanium and Zirconium metals during the quenching process

NASA Astrophysics Data System (ADS)

Miyazaki, Narumasa; Sato, Kazunori; Shibutani, Yoji

Dual-phase (DP) transformation, which is composed of felite- and/or martensite- multicomponent microstructural phases, is one of the most effective tools to product functional alloys. To obtain this DP structure such as DP steels and other materials, we usually apply thermal processes such as quenching, tempering and annealing. As the transformation dynamics of DP microstructure depends on conditions of temperature, annealing time, and quenching rate, physical properties of materials are able to be tuned by controlling microstructure type, size, their interfaces and so on. In this study, to understand the behavior of DP transformation and to control physical properties of materials by tuning DP microstructures, we analyze the atomistic dynamics of DP transformation during the quenching process and the detail of DP microstructures by using the molecular dynamics simulations. As target metals of DP transformation, we focus on group 4 transition metals, such as Ti and Zr described by EAM interatomic potentials. For Ti and Zr models we perform molecular dynamics simulations by assuming melt-quenching process from 3000 K to 0 K under the isothermal-isobaric ensemble. During the process for each material, we observe liquid to HCP like transition around the melting temperature, and continuously HCP-BCC like transition around martensitic transformation temperature. Furthermore, we clearly distinguish DP microstructure for each quenched model.

Experiment and Modeling of Simultaneous Creep, Plasticity and Transformation of High Temperature Shape Memory Alloys During Cyclic Actuation

NASA Technical Reports Server (NTRS)

Kumar, Parikshith K.; Desai, Uri; Chatzigeorgiou, George; Lagoudas, Dimitris C.; Monroe, James; Karaman, Ibrahim; Noebe, Ron; Bigelow, Glen

2010-01-01

The present work is focused on studying the cycling actuation behavior of HTSMAs undergoing simultaneous creep and transformation. For the thermomechanical testing, a high temperature test setup was assembled on a MTS frame with the capability to test up to temperatures of 600 C. Constant stress thermal cycling tests were conducted to establish the actuation characteristics and the phase diagram for the chosen HTSMA. Additionally, creep tests were conducted at constant stress levels at different test temperatures to characterize the creep behavior of the alloy over the operational range. A thermodynamic constitutive model is developed and extended to take into account a) the effect of multiple thermal cycling on the generation of plastic strains due to transformation (TRIP strains) and b) both primary and secondary creep effects. The model calibration is based on the test results. The creep tests and the uniaxial tests are used to identify the viscoplastic behavior of the material. The parameters for the SMA properties, regarding the transformation and transformation induced plastic strain evolutions, are obtained from the material phase diagram and the thermomechanical tests. The model is validated by predicting the material behavior at different thermomechanical test conditions.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qu, Wentao; Sun, Xuguang; Yuan, Bifei

The microstructures, phase transformations and shape memory properties of Ti-30Zr-xNb (x = 5, 7, 9, 13 at.%) alloys were investigated. The X-ray diffraction and transmission electron microscopy observations showed that the Ti-30Zr-5Nb, Ti-30Zr-7/9Nb and Ti-30Zr-13Nb alloys were composed of the hcp α′-martensite, orthorhombic α″-martensite and β phases, respectively. The results indicated the enhanced β-stabilizing effect of Nb in Ti-30Zr-xNb alloys than that in Ti-Nb alloys due to the high content of Zr. The differential scanning calorimetry test indicated that the Ti-30Zr-5Nb alloy displayed a reversible transformation with a high martensitic transformation start temperature of 776 K and a reverse martensiticmore » transformation start temperature (A{sub s}) of 790 K. For the Ti-30Zr-7Nb and Ti-30Zr-9Nb alloys, the martensitic transformation temperatures decreased with the increasing Nb content. Moreover, an ω phase transformation occurred in the both alloys upon heating at a temperature lower than the corresponding A{sub s}, which is prompted by more addition of Nb. Although the critical stress in tension of the three martensitic alloys decreased with increasing Nb content, the Ti-30Zr-9Nb alloy showed a critical stress of as high as 300 MPa. Among all the alloys, the Ti-30Zr-9Nb alloy exhibited the maximum shape memory effect of 1.61%, due to the lowest critical stress for the martensite reorientation. - Highlights: •Ti-30Zr-5Nb alloy is composed of hcp α′-martensite with the M{sub s} of 776 K. •Ti-30Zr-7Nb and Ti-30Zr-9Nb alloys are predominated by orthorhombic α″-martensite. •Ti-30Zr-13Nb alloy consists of a single β phase due to the β-stabilizing effect of Nb. •The martensitic transformation temperatures decrease with increasing Nb content. •Ti-30Zr-9Nb alloy shows the maximum shape memory effect of 1.61%.« less

Temperature variations at nano-scale level in phase transformed nanocrystalline NiTi shape memory alloys adjacent to graphene layers.

PubMed

Amini, Abbas; Cheng, Chun; Naebe, Minoo; Church, Jeffrey S; Hameed, Nishar; Asgari, Alireza; Will, Frank

2013-07-21

The detection and control of the temperature variation at the nano-scale level of thermo-mechanical materials during a compression process have been challenging issues. In this paper, an empirical method is proposed to predict the temperature at the nano-scale level during the solid-state phase transition phenomenon in NiTi shape memory alloys. Isothermal data was used as a reference to determine the temperature change at different loading rates. The temperature of the phase transformed zone underneath the tip increased by ∼3 to 40 °C as the loading rate increased. The temperature approached a constant with further increase in indentation depth. A few layers of graphene were used to enhance the cooling process at different loading rates. Due to the presence of graphene layers the temperature beneath the tip decreased by a further ∼3 to 10 °C depending on the loading rate. Compared with highly polished NiTi, deeper indentation depths were also observed during the solid-state phase transition, especially at the rate dependent zones. Larger superelastic deformations confirmed that the latent heat transfer through the deposited graphene layers allowed a larger phase transition volume and, therefore, more stress relaxation and penetration depth.

Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments.

PubMed

Pereira, E S J; Peixoto, I F C; Viana, A C D; Oliveira, I I; Gonzalez, B M; Buono, V T L; Bahia, M G A

2012-05-01

To compare physical and mechanical properties of one conventional and one thermomechanically treated nickel-titanium (NiTi) wire used to manufacture rotary endodontic instruments. Two NiTi wires 1.0 mm in diameter were characterized; one of them, C-wire (CW), was processed in the conventional manner, and the other, termed M-Wire (MW), received an additional heat treatment according to the manufacturer. Chemical composition was determined by energy-dispersive X-ray spectroscopy, phase constitution by XRD and the transformation temperatures by DSC. Tensile loading/unloading tests and Vickers microhardness measurements were performed to assess the mechanical behaviour. Data were analysed using analysis of variance (α = 0.05). The two wires showed approximately the same chemical composition, close to the 1 : 1 atomic ratio, and the β-phase was the predominant phase present. B19' martensite and the R-phase were found in MW, in agreement with the higher transformation temperatures found in this wire compared with CW, whose transformation temperatures were below room temperature. Average Vickers microhardness values were similar for MW and CW (P = 0.91). The stress at the transformation plateau in the tensile load-unload curves was lower and more uniform in the M-Wire, which also showed the smallest stress hysteresis and apparent elastic modulus. The M-Wire had physical and mechanical properties that can render endodontic instruments more flexible and fatigue resistant than those made with conventionally processed NiTi wires. © 2011 International Endodontic Journal.

Kinetics of Phase Transition from Lamellar to Hexagonally Packed Cylinders for a Triblock Copolymer in a Selective Solvent

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu,Y.; Li, M.; Bansil, R.

2007-01-01

We examined the kinetics of the transformation from the lamellar (LAM) to the hexagonally packed cylinder (HEX) phase for the triblock copolymer, polystyrene-b-poly (ethylene-co-butylene)-b-polystyrene (SEBS) in dibutyl phthalate (DBP), a selective solvent for polystyrene (PS), using time-resolved small-angle X-ray scattering (SAXS). We observe the HEX phase with the EB block in the cores at a lower temperature than that observed for the LAM phase due to the solvent selectivity of DBP for the PS block. Analysis of the SAXS data for a deep temperature quench well below the LAM-HEX transition shows that the transformation occurs in a one-step process. Wemore » calculate the scattering using a geometric model of rippled layers with adjacent layers totally out of phase during the transformation. The agreement of the calculations with the data further supports the continuous transformation mechanism from the LAM to HEX for a deep quench. In contrast, for a shallow quench close to the order-order transition, we find agreement with a two-step nucleation and growth mechanism.« less

Microstructure–property relationships in a high-strength 51Ni–29Ti–20Hf shape memory alloy

DOE PAGES

Coughlin, D. R.; Casalena, L.; Yang, F.; ...

2015-09-18

NiTiHf alloys exhibit remarkable shape memory and pseudoelastic properties that are of fundamental interest to a growing number of industries. In this study, differential scanning calorimetry and isothermal compression tests have revealed that the 51Ni–29Ti–20Hf alloy has useful shape memory properties that include a wide range of transformation temperatures as well as highly stable pseudoelastic behavior. These properties are governed by short-term aging conditions, which may be tailored to control transformation temperatures while giving rise to exceptionally high austenite yield strengths which aid transformation stability. The yield strength of the austenite phase can reach 2.1 GPa by aging for 3hrsmore » at 500°C, while aging for 3hrs at 700°C produced an alloy with an austenite finish temperature (A f ) of 146°C. High-resolution scanning transmission electron microscopy has revealed a new precipitate phase, H-phase, under the homogenized and extruded condition and the aged 3 hrs at 500°C condition, but only the previously identified H-phase precipitate was observed after aging at temperatures of 600°C and 700°C for 3 hrs. Finally, dislocation analysis indicated that plastic deformation of the austenite phase occurred by <100> type slip, similar to that observed in binary NiTi.« less

X-ray diffraction studies of phase transformations in heavy-metal fluoride glasses

NASA Technical Reports Server (NTRS)

Bansal, N. P.; Doremus, R. H.

1985-01-01

Powder X-ray diffraction and differential scanning calorimetry studies of the crystallization properties of five ZrF4-based glass compositions have indicated that the crystalline phase in Zr-Ba-La-Pb fluoride glass is beta-BaZrF6; no such identification of crystal phases was obtainable, however, for the other glasses. Reversible polymorphic phase transformations occur in Zr-Ba-La-Li and Zr-Ba-La-Na fluoride glasses, upon heating to higher temperatures.

Possible existence of two amorphous phases of d-mannitol related by a first-order transition

NASA Astrophysics Data System (ADS)

Zhu, Men; Wang, Jun-Qiang; Perepezko, John H.; Yu, Lian

2015-06-01

We report that the common polyalcohol d-mannitol may have two amorphous phases related by a first-order transition. Slightly above its glass transition temperature Tg (284 K), the supercooled liquid (SCL) of d-mannitol transforms to a low-energy, apparently amorphous phase with stronger hydrogen bonds. The enthalpy of this so-called Phase X is approximately halfway between those of the known amorphous and crystalline phases, a position low for glass aging and high for crystal polymorphs. Similar to the SCL, Phase X is transparent with broad X-ray diffraction and Raman scattering; upon temperature cycling, it exhibits a glass-transition-like change of heat capacity. On fast heating, Phase X transforms back to the SCL near Tg + 50 K, enabling a determination of their equilibrium temperature. The presence of d-sorbitol as a plasticizer enables observation of a first-order transition from the SCL to Phase X entirely in the liquid state (liquid-liquid transition). The transition from d-mannitol's SCL to Phase X has intriguing similarities with the formation of the glacial phase of triphenyl phosphite (TPP) and the conversion from high-density to low-density amorphous ice, both studied intensely in the context of polyamorphism. All three processes occur near Tg with substantial enthalpy decrease toward the crystalline phases; the processes in water and d-mannitol both strengthen the hydrogen bonds. In contrast to TPP, d-mannitol's Phase X forms more rapidly and can transform back to the SCL. These features make d-mannitol a valuable new model for understanding polyamorphism.

Microstructural stability of fine-grained fully lamellar XD TiAl alloys by step aging

NASA Astrophysics Data System (ADS)

Zhu, Hanliang; Maruyama, K.; Seo, D. Y.; Au, P.

2005-05-01

XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8 vol pct TiB2) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of α 2 to γ, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in α 2 volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the α 2 lamellae become finer due to dissolution, whereas most of the γ lamellae coarsen. The dissolution of α 2 involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of γ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in α 2/ γ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.

High-Temperature Phase Change Materials (PCM) Candidates for Thermal Energy Storage (TES) Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gomez, J. C.

2011-09-01

It is clearly understood that lower overall costs are a key factor to make renewable energy technologies competitive with traditional energy sources. Energy storage technology is one path to increase the value and reduce the cost of all renewable energy supplies. Concentrating solar power (CSP) technologies have the ability to dispatch electrical output to match peak demand periods by employing thermal energy storage (TES). Energy storage technologies require efficient materials with high energy density. Latent heat TES systems using phase change material (PCM) are useful because of their ability to charge and discharge a large amount of heat from amore » small mass at constant temperature during a phase transformation like melting-solidification. PCM technology relies on the energy absorption/liberation of the latent heat during a physical transformation. The main objective of this report is to provide an assessment of molten salts and metallic alloys proposed as candidate PCMs for TES applications, particularly in solar parabolic trough electrical power plants at a temperature range from 300..deg..C to 500..deg.. C. The physical properties most relevant for PCMs service were reviewed from the candidate selection list. Some of the PCM candidates were characterized for: chemical stability with some container materials; phase change transformation temperatures; and latent heats.« less

Effect of different annealing condition on the structural and magnetic properties of Mn2NiGa Heusler alloys

NASA Astrophysics Data System (ADS)

Vagadia, Megha; Hester, James; Nigam, A. K.

2018-04-01

We studied the effect of different annealing conditions on structural and magnetic properties of Mn2NiGa Heusler alloys. Reitveld refinement of neutron diffraction pattern at RT confirms the tetragonal structure with cubic phase for I-W quenched alloy whereas Le Bail fitting trials performed on neutron diffraction pattern collected for other three alloys confirm 7M monoclinic structure with cubic phase. It is found that starting and finish temperatures associated with martensite and austenite phase transformation depends strongly on the cooling rate corresponding to different cooling techniques. Slow furnace cooled sample possesses the highest martensite start temperature above room temperature ˜ 326K which decreases to ˜ 198K for ice -water quenched sample. Variation in the drop in the magnetization around MS obtained upon warming from martensite to austenite phase under ZFC cycle suggests that change in the cooling condition strongly affects the magnetization in the low temperature martensite phase. Present results suggest that by varying the cooling rate, martensite transformation as well as the martensite structure can be tuned.

Stability of Ni-bsed bulk metallic glasses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tokarz, Michelle L; Speakman, Scott A; Porter, Wallace D

Several ternary (Ni{sub x}Nb{sub y}Sn{sub z}) refractory alloy glasses (RAGs) were studied at elevated temperatures in order to assess the stability of the amorphous state, i.e. devitrification, and to identify subsequent phase transformations in these materials. differential scanning calorimetry (DSC) experiments indicated a complex phase transformation sequence with several distinct crystallization and melting events being recorded above the glass transition temperature, T{sub g}. Below T{sub g} the RAG samples were studied with an in situ environmental X-ray furnace facility, which allowed step-wise isothermal ramping experiments commencing at a temperature below the reduced temperature of T/T{sub g} {approx} 0.80. Distinct crystallinemore » phases were observed when T/T{sub g} {approx} 0.84 for ternary RAG alloys, while similar experiments on Zr-based Vit 106 glass alloys did not reveal any apparent phase separation until T/T{sub g} {approx} 0.96. The phase separation kinetics followed an Arrhenius type of relationship with Ni{sub 3}Sn, and Nb{sub 2}O{sub 5} being the principle crystalline precipitates.« less

Analytical electron microscope study of the omega phase transformation in a zirconium-niobium alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zaluzec, N. J.

1979-01-01

The study of the as-quenched omega phase morphology shows that the domain size of Zr-15% Nb is on the order of 30 A. No alignment of omega domains along <222>..beta.. directions was observed and samples having undergone thermal cycling in thin foil form, did not develop a long-period structure of alternating ..beta.. and ..omega.. phases below the omega transformation temperature. (FS)

Glass ceramic toughened with tetragonal zirconia

DOEpatents

Keefer, K.D.

1984-02-10

A phase transformation-toughened glass ceramic and a process for making it are disclosed. A mixture of particulate network-forming oxide, network-modifying oxide, and zirconium oxide is heated to yield a homogeneous melt, and this melt is then heat treated to precipitate an appreciable quantity of tetragonal zirconia, which is retained at ambient temperature to form a phase transformation-toughened glass ceramic. Nuclearing agents and stabilizing agents may be added to the mixture to facilitate processing and improve the ceramic's properties. Preferably, the mixture is first melted at a temperature from 1200 to 1700/sup 0/C and is then heat-treated at a temperature within the range of 800 to 1200/sup 0/C in order to precipitate tetragonal ZrO/sub 2/. The composition, as well as the length and temperature of the heat treatment, must be carefully controlled to prevent solution of the precipitated tetragonal zirconia and subsequent conversion to the monoclinic phase.

Glass ceramic toughened with tetragonal zirconia

DOEpatents

Keefer, Keith D.; Michalske, Terry A.

1986-01-01

A phase transformation-toughened glass ceramic and a process for making it are disclosed. A mixture of particulate network-forming oxide, network-modifying oxide, and zirconium oxide is heated to yield a homogeneous melt, and this melt is then heat-treated to precipitate an appreciable quantity of tetragonal zirconia, which is retained at ambient temperature to form a phase transformation-toughened glass ceramic. Nucleating agents and stabilizing agents may be added to the mixture to facilitate processing and improve the ceramic's properties. Preferably, the mixture is first melted at a temperature from 1200.degree. to 1700.degree. C. and is then heat-treated at a temperature within the range of 800.degree. to 1200.degree. C. in order to precipitate tetragonal ZrO.sub.2. The composition, as well as the length and temperature of the heat-treatment, must be carefully controlled to prevent solution of the precipitated tetragonal zirconia and subsequent conversion to the monoclinic phase.

Kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.; Drummond, Charles H., III

1992-01-01

The kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8 have been investigated. Phase pure hexacelsian was prepared by heat treatment of glass flakes at 990 C for 10 h. Bulk hexacelsian was isothermally heat treated at 1026, 1050, 1100, 1152, and 1200 C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting a diffusionless, one-dimensional transformation mechanism. From the temperature dependence of k, the activation energy for this reaction was evaluated to be 527 plus or minus 50 kJ/mole (126 plus or minus 12 kcal/mole). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.

Kinetics of hexacelsian-to-celsian phase transformation in SrAl2Si2O8

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.; Drummond, Charles H., III

1993-01-01

The kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8 have been investigated. Phase pure hexacelsian was prepared by heat treatment of glass flakes at 990 C for 10 h. Bulk hexacelsian was isothermally heat treated at 1026, 1050, 1100, 1152, and 1200 C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting a diffusionless, one-dimensional transformation mechanism. From the temperature dependence of k, the activation energy for this reaction was evaluated to be 527 plus or minus 50 kJ/mole (126 plus or minus 12 kcal/mole). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.

Magnetic Properties and Phase Diagram of Ni50Mn_{50-x}Ga_{x/2}In_{x/2} Magnetic Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Xu, Xiao; Yoshida, Yasuki; Omori, Toshihiro; Kanomata, Takeshi; Kainuma, Ryosuke

2016-12-01

Ni50Mn50- x Ga x/2In x/2 magnetic shape memory alloys were systematically prepared, and the magnetic properties as well as the phase diagram, including atomic ordering, martensitic and magnetic transitions, were investigated. The B2- L21 order-disorder transformation showed a parabolic-like curve against the Ga+In composition. The martensitic transformation temperature was found to decrease with increasing Ga+In composition and to slightly bend downwards below the Curie temperature of the parent phase. Spontaneous magnetization was investigated for both parent and martensite alloys. The magnetism of martensite phase was found to show glassy magnetic behaviors by thermomagnetization and AC susceptibility measurements.

Phase transformation upon cooling path in Ca2SiO4: Possible geological implication

NASA Astrophysics Data System (ADS)

Chang, Yun-Ting; Kung, Jennifer; Hsu, Han

2016-04-01

At the contact metamorphism zone two different Ca2SiO4 phases can be found; calcio-olivine (γ phase) and larnite (β phase). In-situ experiments illustrated the existence of five various polymorphs in Ca2SiO4, i.e., α, α'H, α'L, β and γ. The path of phase transformation and the transformation temperatures are shown as follows. γ → α'L(700° C) → α'H(1100° C) → α (1450° C) α'L → β (680° C) → γ (500° C) Experiments showed that the phase transitions at lower temperature is not reversible and seemed to be complicated; β phase is only stable from 500° C to 680° C upon cooling. To understand the possible mechanism of the β phase being metastable at room temperature, atmosphere condition, we were motivated to investigate the route of phase transition in Ca2SiO4 in different thermal process. Powder samples were synthesized by the solid-state reaction. Pure reagent oxides CaCO3 and SiO2 were mixed in 2:1 stoichiometric mole. Two control factors were designated in the experiments; the sintering temperature of starting materials and the cooling path. The sintering temperature was set within the range of stable phase field of α'L phase (˜900° C) and α'H phase (1300° C). The cooling process was designed in three different routes: 1) the quenched procedure from sintering temperature with rate of 900° C/min and 1300° C/min, 2) the furnace cooling procedure, 3) set a slow cooling rate (0.265 ° C/min). The products were examined for the crystal structure by X-ray powder diffraction. First-principle calculation was also applied to investigate the thermodynamic properties of α'H, β and γ phases. A major finding in this study showed that the γ phase presented in the final product when the sintering temperature was set at the stable field of α'H phase; on the other hand, the β phase would present when the sintering temperature was set within the field of α'L phase. It was noted that the existing phase in the product would be modified by the cooling procedures. Our calculation indicates the enthalpy of beta phase was slightly higher than that of the gamma phase at zero pressure, verifying the metastable β phase observed in the natural. In this meeting we present the detailed experimental results and discuss the potential implication for the thermal history of geological setting using the phase transition path upon cooling of Ca2SiO4.

Phase transformation and sustained load crack growth in ZrO[sub 2] + 3 mol% Y[sub 2]O[sub 3]: Experiments and kinetic modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yin, H.; Gao, M.; Wei, R.P.

1995-01-01

To better understand environmentally assisted crack growth (SCG) in yttria stabilized zirconia, experimental studies were undertaken to characterize the kinetics of crack growth and the associated stress/moisture induced phase transformation in ZrO[sub 2] + 3 mol% Y[sub 2]O[sub 3] (3Y-TZP) in water, dry nitrogen and toluene from 3 to 70 C. The results showed that crack growth in water depended strongly on stress intensity factor (K[sub 1]) and temperature (T) and involved the transformation of a thin layer of material near the crack tip from the tetragonal (t) to the monoclinic (m) phase. These results, combined with literature data onmore » moisture-induced phase transformation, suggested that crack growth enhancement by water is controlled by the rate of this transformation and reflects the environmental cracking susceptibility of the transformed m-phase. A model was developed to link subcritical crack growth (SCG) rate to the kinetics of t [yields] m phase transformation. The SCG rate is expressed as an exponential function of stress-free activation energy, a stress-dependent contribution in terms of the mode 1 stress intensity factor K[sub I] and actuation volume, and temperature. The stress-free activation energies for water and the inert environments were determined to be 82 [+-] 3 and 169 [+-] 4 kJ/mol, respectively, at the 95% confidence level, and the corresponding activation volumes were 14 and 35 unit cells. The decreases in activation energy and activation volume may be attributed to a change in surface energy by water.« less

ε- and β-LiVOPO4: Phase Transformation and Electrochemistry.

PubMed

Zhou, Hui; Shi, Yong; Xin, Fengxia; Omenya, Fredrick; Whittingham, M Stanley

2017-08-30

ε- and β-LiVOPO 4 were synthesized from the same precursor at different temperatures in an air atmosphere. ε-LiVOPO 4 is obtained at 400 and 700 °C. The 700 °C sample has better purity and crystallinity, but the 400 °C sample has a little better electrochemical performance due to its smaller particle size and the conducting carbon residue in the sample. β-LiVOPO 4 is formed between the above two temperatures, which gives slightly lower capacity than that of the ε-LiVOPO 4 sample, indicating higher kinetics of the lithium reaction for the ε phase than those of the β one. The phase transformation from ε to β then back reversibly to ε was also observed by ex situ X-ray diffraction. This thermal study verifies that ε-LiVOPO 4 is the more stable phase for LiVOPO 4 ; however, reaction kinetics control the phases formed at lower temperatures.

Relationships Between the Phase Transformation Kinetics, Texture Evolution, and Microstructure Development in a 304L Stainless Steel Under Biaxial Loading Conditions: Synchrotron X-ray and Electron Backscatter Diffraction Studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cakmak, Ercan; Choo, Hahn; Kang, Jun-Yun

2015-02-11

The relationships between the martensitic phase transformation kinetics, texture evolution, and the microstructure development in the parent austenite phase were studied for a 304L stainless steel that exhibits the transformation-induced plasticity effect under biaxial loading conditions at ambient temperature. The applied loading paths included: pure torsion, simultaneous biaxial torsion/tension, simultaneous biaxial torsion/compression, and stepwise loading of tension followed by torsion (i.e., first loading by uniaxial tension and then by pure torsion in sequence). Synchrotron X-ray and electron backscatter diffraction techniques were used to measure the evolution of the phase fractions, textures, and microstructures as a function of the applied strains.more » The influence of loading character and path on the changes in martensitic phase transformation kinetics is discussed in the context of (1) texture-transformation relationship and the preferred transformation of grains belonging to certain texture components over the others, (2) effects of axial strains on shear band evolutions, and (3) volume changes associated with martensitic transformation.« less

Raman spectroscopic study of calcite III to aragonite transformation under high pressure and high temperature

NASA Astrophysics Data System (ADS)

Liu, Chuanjiang; Zheng, Haifei; Wang, Duojun

2017-10-01

In our study, a series of Raman experiments on the phase transition of calcite at high pressure and high temperature were investigated using a hydrothermal diamond anvil cell and Raman spectroscopy technique. It was found that calcite I transformed to calcite II and calcite III at pressures of 1.62 and 2.12 GPa and room temperature. With increasing temperature, the phase transition of calcite III to aragonite occurred. Aragonite was retained upon slowly cooling of the system, indicating that the transition of calcite III to aragonite was irreversible. Based on the available data, the phase boundary between calcite III and aragonite was determined by the following relation: P(GPa) = 0.013 × T(°C) + 1.22 (100°C ≤ T ≤ 170°C). It showed that the transition pressure linearly rose with increasing temperature. A better understanding of the stability of calcite III and aragonite is of great importance to further explore the thermodynamic behavior of carbonates and carbon cycling in the mantle.

Temperature Dependence of the Magnetization of the Ni52Mn24Ga24 Alloy in Various Structural States

NASA Astrophysics Data System (ADS)

Musabirov, I. I.; Sharipov, I. Z.; Mulyukov, R. R.

2015-10-01

are presented of a study of the temperature dependence of the magnetization σ(Т) of the polycrystalline Ni52Mn24Ga24 alloy in various structural states: in the initial coarse-grained state, after severe plastic deformation by high pressure torsion, and after stepped annealing of the deformed specimen at temperatures from 200 to 700°С for 30 min. As a study of the σ(Т) curve shows, in an alloy possessing a coarse-grained initial structure, a martensitic phase transition and a magnetic phase transition are observed in the room temperature interval. The martensitic transformation takes place in the ferromagnetic state of the alloy. This transformation is accompanied by an abrupt lowering of the magnetization of the material, associated with a lowering of the symmetry of the crystalline lattice and a high value of the magnetocrystalline anisotropy constant of the alloy in the martensitic phase. It is shown that as a result of plastic deformation there takes place a destruction of ferromagnetic order and a suppression of the martensitic transformation. Consecutive annealing after deformation leads to a gradual recovery of ferromagnetic order and growth of the magnetization of the material. Recovery of the martensitic transformation begins to be manifested only after annealing of the alloy at a temperature of 500°C, when the mean grain size in the recrystallized structure reaches a value around 1 μm.

Structural, magnetothermal, and magnetotransport properties of single crystal terbium silicon germanide and spontaneous generation of voltage in single crystal gadolinium silicon germanide and gadolinium

NASA Astrophysics Data System (ADS)

Zou, Min

A systematic study of single crystalline Tb5Si2.2Ge1.8, including magnetic field induced crystallographic and magnetic phase transformations, magnetocaloric effect, ferromagnetic short-range correlations, electrical resistivity, magnetoresistance, and spontaneous generation of voltage (SGV) has been presented. A study of SGV in single crystalline Gd5Si2Ge2 and Gd has also been included. The metamagnetic-like transitions and giant magnetocaloric effect were observed with the magnetic field applied parallel to the a- and c-axes, but not the b-axis in a Tb5Si 2.2Ge1.8 single crystal. The in-situ x-ray powder diffraction study indicates that these metamagnetic-like transitions are coupled to a crystallographic phase transformation occurring via strong magnetoelastic interactions. The magnetocrystalline anisotropy plays an important role in this system. Magnetic fields less than 40 kOe can not drive either the magnetic or the crystallographic phase transition to completion for Tb5Si2.2Ge1.8 powder due to the strong single ion anisotropy of Tb. Magnetic field dependencies of the critical temperatures of magnetic phase transitions of Tb5Si2.2Ge1.8 are highly anisotropic for both the main magnetic ordering process occurring around 120 K and a spin reorientation transition at ~70 K. Magnetic-field-induced phase transitions occur with the magnetic field applied isothermally along the a-and b-axes (but not along the c-axis) between 1.8 and 70 K in fields below 70 kOe. Strongly anisotropic thermal irreversibility is observed in the Griffiths phase regime between 120 and 200 K with applied fields ranging from 10 to 1000 Oe. Our data: (1) show that the magnetic and structural phase transitions around 120 K are narrowly decoupled; (2) uncover the anisotropy of ferromagnetic short-range order in the Griffiths phase; and (3) reveal some unusual magnetic domain effects in the long-range ordered state of the Tb5Si2.2Ge1.8 compound. The temperature-magnetic field phase diagrams with field applied along the three major crystallographic directions have been constructed. The positive colossal magnetoresistance (CMR) with a magnitude of ~150% was observed with the magnetic field applied parallel to the a-axis, but not the b- and c-axes in Tb5Si 2.2Ge1.8 single crystals. The electrical resistivity shows a low-temperature high-resistivity behavior (i.e. the resistivity at low temperature is higher after the transformation to the low temperature phase than the resistivity of the phase before the transition) along the a-axis, contrary to those along the b- and c-axes. The positive CMR effect originates from an intrinsic crystallographic phase coexistence state frozen below the Curie Temperature (TC). The differences in the temperature dependencies of electrical resistivities and longitudinal magnetoresistance along the a-axis and those along the b- and c-axes can be explained by the geometry of the phase boundaries at low temperatures, and the inability of the external magnetic field to induce the crystallographic phase transformation along the b- and c-axes. Temperature-induced SGVs were observed along all three principal crystallographic axes of Tb5Si2.2Ge1.8, but not in Gd. Field-induced SGVs were observed with magnetic fields less than 40 kOe applied along the a-axis of Tb5Si2.2Ge1.8, and the c-axis of Gd. The absence of the temperature induced SGV in Gd indicates the key role first-order phase transformations play in the appearance of the effect when temperature varies. The anisotropy of magnetic field induced SGV in Tb5Si2.2Ge1.8 and the existence of field induced SGV in Gd, highlight the importance of the magnetocaloric effect in bringing about the SGV. In single crystal and polycrystalline Gd5Si 2Ge2 during the coupled magneto-structural transformations, reversible and repeatable SGV responses of the materials to the temperature and magnetic field have been observed. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal, and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic field changes.

Phase transformation changes in thermocycled nickel-titanium orthodontic wires.

PubMed

Berzins, David W; Roberts, Howard W

2010-07-01

In the oral environment, orthodontic wires will be subject to thermal fluctuations. The purpose of this study was to investigate the effect of thermocycling on nickel-titanium (NiTi) wire phase transformations. Straight segments from single 27 and 35 degrees C copper NiTi (Ormco), Sentalloy (GAC), and Nitinol Heat Activated (3M Unitek) archwires were sectioned into 5mm segments (n=20). A control group consisted of five randomly selected non-thermocycled segments. The remaining segments were thermocycled between 5 and 55 degrees C with five randomly selected segments analyzed with differential scanning calorimetry (DSC; -100<-->150 degrees C at 10 degrees C/min) after 1000, 5000, and 10,000 cycles. Thermal peaks were evaluated with results analyzed via ANOVA (alpha=0.05). Nitinol HA and Sentalloy did not demonstrate qualitative or quantitative phase transformation behavior differences. Significant differences were observed in some of the copper NiTi transformation temperatures, as well as the heating enthalpy with the 27 degrees C copper NiTi wires (p<0.05). Qualitatively, with increased thermocycling the extent of R-phase in the heating peaks decreased in the 35 degrees C copper NiTi, and an austenite to martensite peak shoulder developed during cooling in the 27 degrees C copper NiTi. Repeated temperature fluctuations may contribute to qualitative and quantitative phase transformation changes in some NiTi wires. Copyright 2010 Academy of Dental Materials. All rights reserved.

Transformation process for production of ultrahigh carbon steels and new alloys

DOEpatents

Strum, M.J.; Goldberg, A.; Sherby, O.D.; Landingham, R.L.

1995-08-29

Ultrahigh carbon steels with superplastic properties are produced by heating a steel containing ferrite and carbide phases to a soaking temperature approximately 50 C above the A{sub 1} transformation temperature, soaking the steel above the A{sub 1} temperature for a sufficient time that the major portion of the carbides dissolve into the austenite matrix, and then cooling the steel in a controlled manner within predetermined limits of cooling rate or transformation temperature, to obtain a steel having substantially spheroidal carbides. New alloy compositions contain aluminum and solute additions which promote the formation of a fine grain size and improve the resistance of the carbides to coarsening at the forming temperature. 9 figs.

Transformation process for production of ultrahigh carbon steels and new alloys

DOEpatents

Strum, Michael J.; Goldberg, Alfred; Sherby, Oleg D.; Landingham, Richard L.

1995-01-01

Ultrahigh carbon steels with superplastic properties are produced by heating a steel containing ferrite and carbide phases to a soaking temperature approximately 50.degree. C. above the A.sub.1 transformation temperature, soaking the steel above the A.sub.1 temperature for a sufficient time that the major portion of the carbides dissolve into the austenite matrix, and then cooling the steel in a controlled manner within predetermined limits of cooling rate or transformation temperature, to obtain a steel having substantially spheroidal carbides. New alloy compositions contain aluminum and solute additions which promote the formation of a fine grain size and improve the resistance of the carbides to coarsening at the forming temperature.

Mathematical model of phase transformations and elastoplastic stress in the water spray quenching of steel bars

NASA Astrophysics Data System (ADS)

Nagasaka, Y.; Brimacombe, J. K.; Hawbolt, E. B.; Samarasekera, I. V.; Hernandez-Morales, B.; Chidiac, S. E.

1993-04-01

A mathematical model, based on the finite-element technique and incorporating thermo-elasto-plastic behavior during the water spray quenching of steel, has been developed. In the model, the kinetics of diffusion-dependent phase transformation and martensitic transformation have been coupled with the transient heat flow to predict the microstructural evolution of the steel. Furthermore, an elasto-plastic constitutive relation has been applied to calculate internal stresses resulting from phase changes as well as temperature variation. The computer code has been verified for internal consistency with previously published results for pure iron bars. The model has been applied to the water spray quenching of two grades of steel bars, 1035 carbon and nickel-chromium alloyed steel; the calculated temperature, hardness, distortion, and residual stresses in the bars agreed well with experimental measurements. The results show that the phase changes occurring during this process affect the internal stresses significantly and must be included in the thermomechanical model.

Temperature-modulated DSC provides new insight about nickel-titanium wire transformations.

PubMed

Brantley, William A; Iijima, Masahiro; Grentzer, Thomas H

2003-10-01

Differential scanning calorimetry (DSC) is a well-known method for investigating phase transformations in nickel-titanium orthodontic wires; the microstructural phases and phase transformations in these wires have central importance for their clinical performance. The purpose of this study was to use the more recently developed technique of temperature-modulated DSC (TMDSC) to gain insight into transformations in 3 nickel-titanium orthodontic wires: Neo Sentalloy (GAC International, Islandia, NY), 35 degrees C Copper Ni-Ti (Ormco, Glendora, Calif) and Nitinol SE (3M Unitek, Monrovia, Calif). In the oral environment, the first 2 superelastic wires have shape memory, and the third wire has superelastic behavior but not shape memory. All wires had cross-section dimensions of 0.016 x 0.022 in. Archwires in the as-received condition and after bending 135 degrees were cut into 5 or 6 segments for test specimens. TMDSC analyses (Model 2910 DSC, TA Instruments, Wilmington, Del) were conducted between -125 degrees C and 100 degrees C, using a linear heating and cooling rate of 2 degrees C per min, an oscillation amplitude of 0.318 degrees C with a period of 60 seconds, and helium as the purge gas. For all 3 wire alloys, strong low-temperature martensitic transformations, resolved on the nonreversing heat-flow curves, were not present on the reversing heat-flow curves, and bending appeared to increase the enthalpy change for these peaks in some cases. For Neo Sentalloy, TMDSC showed that transformation between martensitic and austenitic nickel-titanium, suggested as occurring directly in the forward and reverse directions by conventional DSC, was instead a 2-step process involving the R-phase. Two-step transformations in the forward and reverse directions were also found for 35 degrees C Copper Ni-Ti and Nitinol SE. The TMDSC results show that structural transformations in these wires are complex. Some possible clinical implications of these observations are discussed.

Nucleation and Growth of Tetrataenite (FeNi) in Meteorites

NASA Astrophysics Data System (ADS)

Goldstein, J. I.; Williams, D. B.; Zhang, J.

1992-07-01

The mineral tetrataenite (ordered FeNi) has been observed in chondrites, stony irons, and iron meteorites (1). FeNi is an equilibrium phase in the Fe-Ni phase diagram (Figure 1) and orders to tetrataenite at ~320 degrees C (2). The phase forms at temperatures at or below the eutectoid temperature (~400 degrees C) where taenite (gamma) transforms to kamacite (alpha) plus FeNi (gamma"). An understanding of the formation of tetrataenite can lead to a new method for determining cooling rates at low temperatures (<400 degrees C) for all types of meteorites. In a recent study of plessite in iron meteorites (3), two transformation sequences for the formation of tetrataenite were observed. In either sequence, during the cooling process, the taenite (gamma) phase initially undergoes a diffusionless transformation to a martensite (alpha, bcc) phase without a composition change. The martensite then decomposes either above or below the eutectoid temperature (~400 degrees C) during cooling or upon subsequent reheating. During martensite decomposition above the eutectoid, the taenite (gamma) phase nucleates by the reaction alpha(sub)2 ---> alpha + gamma and grows under volume diffusion control. The Ni composition of the taenite increases continuously following the equilibrium gamma/alpha + gamma boundary while the Ni composition of the kamacite matrix decreases following the alpha/alpha + gamma phase boundary (2), see Figure 1. Below the eutectoid temperature, the precipitate composition follows the equilibrium gamma"/alpha + gamma" boundary and reaches ~52 wt% Ni, the composition of FeNi, gamma". The kamacite (alpha) matrix composition approaches ~4 to 5 wt% Ni. The ordering transformation starts at ~320 degrees C forming the tetrataenite phase. During martensite decomposition below the eutectoid temperature, FeNi should form directly by the reaction alpha2 --> alpha + gamma" (FeNi). If this transformation sequence occurs, then the composition of kamacite and tetrataenite should also be given by the alpha/alpha + gamma" and gamma"/alpha + gamma" boundaries of the Fe-Ni phase diagram (Figure 1). However, the Ni content of kamacite and tetrataenite in black plessite, which forms below 400 degrees C, is ~10 wt% in kamacite and ~57 to 60 wt% in tetrataenite, much higher than the values given by the equilibrium phase diagram (3). It has been observed experimentally (4) that the Ni composition of the gamma phase formed by martensite decomposition below 400 degrees C lies along a metastable extension of the high temperature gamma/alpha + gamma phase boundary, Figure 2. Therefore, the FeNi phase formed by alpha(sub)2 decomposition below 400 degrees C has a non-equilibrium Ni content, >50 to 56 wt%. The growth or thickening of the FeNi phase occurs by some combination of interface and diffusion control (3). References: (1) Clarke R. S. and Scott E. R. D. (1980) Amer. Mineral. 65, 624-630. (2) Reuter K. B., Williams D. B., and Goldstein J. I. (1989) Met. Trans. 20A, 719-725. (3) Zhang J., Williams D. B. and Goldstein J. I. (1992) Submitted to Geochim. Cosmochim. Acta. (4) Zhang J., Williams L). B. and Goldstein J. I. (1992) Submitted to Met. Trans. Figure 1, which in the hard copy appears here, is an Fe-Ni phase diagram (2). Figure 2, which in the hard copy appears here, shows measured FeNi composition from heat-treated alloys (4).

Effect of point defects and disorder on structural phase transitions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Toulouse, J.

1997-06-01

Since the beginning in 1986, the object of this project has been Structural Phase Transitions (SPT) in real as opposed to ideal materials. The first stage of the study has been centered around the role of Point Defects in SPT`s. Our intent was to use the previous knowledge we had acquired in the study of point defects in non-transforming insulators and apply it to the study of point defects in insulators undergoing phase transitions. In non-transforming insulators, point defects, in low concentrations, marginally affect the bulk properties of the host. It is nevertheless possible by resonance or relaxation methods tomore » study the point defects themselves via their local motion. In transforming solids, however, close to a phase transition, atomic motions become correlated over very large distances; there, even point defects far removed from one another can undergo correlated motions which may strongly affect the transition behavior of the host. Near a structural transition, the elastic properties win be most strongly affected so as to either raise or decrease the transition temperature, prevent the transition from taking place altogether, or simply modify its nature and the microstructure or domain structure of the resulting phase. One of the well known practical examples is calcium-stabilized zirconia in which the high temperature cubic phase is stabilized at room temperature with greatly improved mechanical properties.« less

Effect of structural phase transformation in FeGaO{sub 3} on its magnetic and ferroelectric properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lone, A. G., E-mail: agl221986@gmail.com; Bhowmik, R. N.

2015-06-24

We investigate the structural phase transformation from orthorhombic to rhombohedral structure in FeGaO{sub 3} by adopting a combined effect of mechanical alloying/milling and solid state sintering techniques. The structural phase formation of the FeGaO{sub 3} compound has been characterized by X-ray diffraction pattern. Mechanical milling played a significant role on the stabilization of rhombohedral phase in FeGaO{sub 3}, where as high temperature sintering stabilized the system in orthorhombic phase. A considerable difference has been observed in magnetic and ferroelectric properties of the system in two phases. The system in rhombohedral (R-3c) phase exhibited better ferromagnetic and of ferroelectric properties atmore » room temperature in comparison to orthorhombic (Pc2{sub 1}n) phase. The rhombohedral phase appears to be good for developing metal doped hematite system for spintronics applications and in that process mechanical milling played an important role.« less

Effect of Cr-N codoping on structural phase transition, Raman modes, and optical properties of TiO2 nanoparticles

NASA Astrophysics Data System (ADS)

Hassnain Jaffari, G.; Tahir, Adnan; Ali, Naveed Zafar; Ali, Awais; Qurashi, Umar S.

2018-04-01

Noncompensated cation-anion codoping in TiO2 nanoparticles has been achieved by a chemical synthesis route. Significant reduction in the optical bandgap and enhancement in the absorption of visible light have been observed. Structural phase transformation has been tracked in detail as a function of doping and heat treatment temperature. Anatase to rutile phase transition temperature for doped samples was higher in comparison to the pure TiO2 nanoparticles. Nitrogen and chromium addition increases the phase transformation barrier, where the effect of the former dopant is of more significance. The Raman results showed an increase in the oxygen content with higher post annealing temperatures. With Cr incorporation, the peak associated with the Eg mode has been found to shift towards a higher wave number, while with nitrogen incorporation, the shift was towards a lower wave number. A decrease in reflectance with N co-doping for all samples, irrespective of phase and annealing temperatures, has been observed. In compositions with nitrogen of the same content, bandgap reduction was higher in the rutile phase in comparison to the anatase phase. In general, overall results revealed that with a higher loading fraction of ammonia, the N content increases, while Cr addition prevents nitrogen loss even up to high post annealing temperatures, i.e., 850 °C.

Effect of lattice-mismatch-induced strains on coupled diffusive and displacive phase transformations

NASA Astrophysics Data System (ADS)

Bouville, Mathieu; Ahluwalia, Rajeev

2007-02-01

Materials which can undergo slow diffusive transformations as well as fast displacive transformations are studied using the phase-field method. The model captures the essential features of the time-temperature-transformation (TTT) diagrams, continuous cooling transformation (CCT) diagrams, and microstructure formation of these alloys. In some material systems there can exist an intrinsic volume change associated with these transformations. We show that these coherency strains can stabilize mixed microstructures (such as retained austenite-martensite and pearlite-martensite mixtures) by an interplay between diffusive and displacive mechanisms, which can alter TTT and CCT diagrams. Depending on the conditions there can be competitive or cooperative nucleation of the two kinds of phases. The model also shows that small differences in volume changes can have noticeable effects on the early stages of martensite formation and on the resulting microstructures.

Transformation behavior of the γU(Zr,Nb) phase under continuous cooling conditions

NASA Astrophysics Data System (ADS)

Komar Varela, C. L.; Gribaudo, L. M.; González, R. O.; Aricó, S. F.

2014-10-01

The selected alloy for designing a high-density monolithic-type nuclear fuel with U-Zr-Nb alloy as meat and Zry-4 as cladding, has to remain in the γU(Zr,Nb) phase during the whole fabrication process. Therefore, it is necessary to define a range of concentrations in which the γU(Zr,Nb) phase does not decompose under the process conditions. In this work, several U alloys with concentrations between 28.2-66.9 at.% Zr and 0-13.3 at.% Nb were fabricated to study the possible transformations of the γU(Zr,Nb) phase under different continuous cooling conditions. The results of the electrical resistivity vs temperature experiments are presented. For a cooling rate of 4 °C/min a linear regression was determined by fitting the starting decomposition temperature as a function of Nb concentration. Under these conditions, a concentration of 45.3 at.% Nb would be enough to avoid any transformation of the γU(Zr,Nb) phase. In experiments that involve higher cooling conditions, it has been determined that this concentration can be halved.

Influence of Pressure on Physical Property of Ammonia Borane and its Re-hydrogenation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Jiuhua

The project systematically studied the high pressure behavior of ammonia borane and its derivative lithium amidoborane. Phase transitions in these materials are investigated in the pressure range up to 20 GPa and temperature range from 80 K to 400K. A number of new phase transitions are discovered in this pressure and temperature range including a second order transformation at 5 GPa and a first order transformation at 12 GPa at room temperature, and four new transitions at high pressure and low temperatures. The Clapeyron slopes for both pressure-induce tetragonal (I4mm) phase to orthorhombic (Cmc21) phase and temperature-induce tetragonal (I4mm) phasemore » to orthorhombic (Pmn21) phase are determined to be positive, indicating these phase transitions are exothermic. This result demonstrates that the high pressure orthorhombic phase of ammonia borane has lower enthalpy than that of tetragonal phase at ambient conditions. If we assume decomposition from the orthorhombic phase yields the same products as that from the tetragonal phase, the decomposition of the orthorhombic phase will be less exothermic. Therefore rehydrogenation from the decomposed product into the orthorhombic phase at high pressure may become easier. The project also studied the influences of nanoconfinement on the phase transitions. Comparative study using Raman spectroscopy indicates that the temperature induced I4mm to Pmn21 transition is suppressed from 217 K to 195 K when the sample is confined in SBA15 (7-9 nm pore size). When the pore size is reduced from 7-9 nm to 3-4 nm, this transition is totally suppressed in the temperature down to 80 K. A similar influence of the nanoconfiement on pressure induced phase transitions is also observed using Raman spectroscopy. The phase boundary between the I4mm phase and high pressure Cmc21 phase at ambient temperature shifts from 0.9 GPa to 0.5 GPa; and that between the Cmc21 phase and higher pressure P21 phase shifts from 10.2 GPa to 9.7 GPa.« less

Possible existence of two amorphous phases of D-mannitol related by a first-order transition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Men; Yu, Lian, E-mail: lian.yu@wisc.edu; Wang, Jun-Qiang

2015-06-28

We report that the common polyalcohol D-mannitol may have two amorphous phases related by a first-order transition. Slightly above its glass transition temperature T{sub g} (284 K), the supercooled liquid (SCL) of D-mannitol transforms to a low-energy, apparently amorphous phase with stronger hydrogen bonds. The enthalpy of this so-called Phase X is approximately halfway between those of the known amorphous and crystalline phases, a position low for glass aging and high for crystal polymorphs. Similar to the SCL, Phase X is transparent with broad X-ray diffraction and Raman scattering; upon temperature cycling, it exhibits a glass-transition-like change of heat capacity.more » On fast heating, Phase X transforms back to the SCL near T{sub g} + 50 K, enabling a determination of their equilibrium temperature. The presence of D-sorbitol as a plasticizer enables observation of a first-order transition from the SCL to Phase X entirely in the liquid state (liquid-liquid transition). The transition from D-mannitol’s SCL to Phase X has intriguing similarities with the formation of the glacial phase of triphenyl phosphite (TPP) and the conversion from high-density to low-density amorphous ice, both studied intensely in the context of polyamorphism. All three processes occur near T{sub g} with substantial enthalpy decrease toward the crystalline phases; the processes in water and D-mannitol both strengthen the hydrogen bonds. In contrast to TPP, D-mannitol’s Phase X forms more rapidly and can transform back to the SCL. These features make D-mannitol a valuable new model for understanding polyamorphism.« less

Low temperature structural and transport studies of La{sub 0.175}Pr{sub 0.45}Ca{sub 0.375}MnO{sub 3-δ}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sharma, Shivani; Shahee, Aga; Singh, Kiran

2016-05-23

The temperature (T) dependent x-ray diffraction (XRD) and resistivity measurements of La{sub 0.175}Pr{sub 0.45}Ca{sub 0.375}MnO{sub 3-δ} (LPCMO) have been performed down to 2 K to understand the structural and transport properties. From room temperature down to 220 K, LPCMO exists in orthorhombic phase with Pnma structure and at T~220 K, it transforms to charge ordered (CO) monoclinic phase with P2{sub 1}/m structure and remains as it is down to 2 K. The CO phase is evident from the occurrence of weak but well defined superlattice peaks in the XRD pattern. This structural transformation is of first order in nature asmore » evident from the phase coexistence across the transition region. These results thus clearly illustrate that LPCMO undergoes a first order structural phase transition from charge disordered orthorhombic phase to CO monoclinic phase at ~220 K, consistent with temperature dependent resistivity results. Our structural analysis of T dependent XRD data using Rietveld refinement infers that below 220 K, LPCMO forms commensurate CO monoclinic P2{sub 1}/m structure with four times structural modulation.« less

High-temperature ultrasonic characterization of the mechanical and microstructural behavior of a fibrous composite with a magnesium lithium aluminum silicate glass-ceramic matrix

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cutard, T.; Huger, M.; Fargeot, D.

The mechanical behavior and the microstructural modifications of a SiC-fiber-reinforced magnesium lithium aluminum silicate glass-ceramic (SiC/MASL) have been characterized by ultrasonic measurement of uniaxial Young`s modulus at high temperature. Under vacuum, long isothermal agings in the 750--1,000 C temperature range have shown matrix modifications in terms of crystallization of residual glassy phases, and of phase transformations in the Li{sub 2}O-Al{sub 2}O{sub 3}-SiO{sub 2} system. In air, long isothermal agings performed under the same conditions have led to the same matrix transformations but in competition with oxidation mechanisms of the carbon fiber-matrix interphase. All of these matrix and/or interface transformations havemore » been confirmed by X-ray diffraction analysis, scanning electron microscopy, scanning acoustic microscopy, and microindentation tests.« less

Transformation from insulating p-type to semiconducting n-type conduction in CaCu3Ti4O12-related Na(Cu5/2Ti1/2)Ti4O12 ceramics

NASA Astrophysics Data System (ADS)

Li, Ming; Sinclair, Derek C.

2013-07-01

A double doping mechanism of Na+ + 1/2 Ti4+ → Ca2+ + 1/2 Cu2+ on the general formula Ca1-xNax(Cu3-x/2Tix/2)Ti4O12 has been used to prepare a series of isostructural CaCu3Ti4O12 (CCTO)-type perovskites. A complete solid solution exists for 0 ≤ x ≤ 1 and all compositions exhibit incipient ferroelectric behaviour with higher than expected intrinsic relative permittivity. Although CCTO ceramics typically exhibit n-type semiconductivity (room temperature, RT, resistivity of ˜10-100 Ω cm), Na(Cu5/2Ti1/2)Ti4O12 (NCTO) ceramics sintered at 950 °C consist of two insulating bulk phases (RT resistivity > 1 GΩ cm), one p-type and the other n-type. With increasing sintering temperature/period, the p-type phase transforms into the n-type phase. During the transformation, the resistivity and activation energy for electrical conduction (Ea ˜ 1.0 eV) of the p-type phase remain unchanged, whereas the n-type phase becomes increasingly conductive with Ea decreasing from ˜ 0.71 to 0.11 eV with increasing sintering temperature. These changes are attributed to small variations in stoichiometry that occur during high temperature ceramic processing with oxygen-loss playing a crucial role.

Phase stability, porosity distribution and microstructural evolution of amorphous Al{sub 50}Ti{sub 50} powders consolidated by electrical resistance sintering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Urban, P., E-mail: purban@us.es; Montes, J. M.; Cintas, J.

2015-03-30

The effect of intensity and duration of the electrical resistance sintering process on the phase stability, porosity distribution and microstructural evolution of Al{sub 50}Ti{sub 50} amorphous powders is studied. The phase transformations during the consolidation process were determined by X-ray diffraction. The porosity distribution was observed by optical and scanning electron microscopy. The amorphous phase is partially transformed to the crystalline phase during the sintering process, and formation of AlTi and AlTi{sub 3} intermetallic compounds occurs for temperatures higher than 300 °C. Finally, it is observed that the compacts core have lower porosity and a higher tendency to the amorphous-crystallinemore » phase transformation than the periphery.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heo, Su Jeong; Hu, Boxun; Uddin, Md Aman

The phase transformation and particle coarsening of lithium aluminate (α-LiAlO 2) in electrolyte are the major causes of degradation affecting the performance and the lifetime of the molten carbonate fuel cell (MCFC). The stability of LiAlO 2 has been studied in Li 2CO 3-Na 2CO 3 electrolyte under accelerated conditions in reducing and oxidizing gas atmospheres at temperatures of 650 and 750 for up to 500 hours. X-ray diffraction analyses show that the progressive transformation of α-LiAlO 2 to γ-LiAlO 2 phase proceeds with increasing temperature in lower P CO2 and lower P O2 environments. Spherical LiAlO 2 particles weremore » transformed to coarsened pyramid-shape particles in 4% H 2-3% H 2O-N 2 and 100% N 2 (~10 ppm P O2 ) atmospheres. Under CO 2-rich atmospheres (4% H 2-30% CO 2-N 2 and 70% air-30% CO 2), both phase and particle size remained unchanged at 650 and 750ºC. The selected area electron diffraction (SAED) pattern analysis indicated that the large pyramidal shape particles (~30 μm) were γ-LiAlO 2 phase. Experimental observations and related simulation results pertaining to particle coarsening and phase transformation behavior of LiAlO 2 are presented.« less

High pressure-high temperature phase diagram of an energetic crystal: Dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50)

DOE PAGES

Dreger, Z. A.; Breshike, C. J.; Gupta, Y. M.

2017-05-08

Raman spectroscopy was used to examine the high pressure-high temperature structural and chemical stability of an insensitive, high-performance energetic crystal – dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50). The phase diagram was determined over 8 GPa and (293-760) K. Under isobaric heating, the melting/decomposition of TKX-50 is preceded by a transformation to two consecutive high-temperature intermediates; a lower-temperature intermediate – diammonium 5,5’-bistetrazole-1,1'-diolate, and a higher-temperature intermediate – dihydroxylammonium 5,5'-bistetrazolate and/or diammonium 5,5'-bistetrazolate. Pressure strongly increases the transition temperatures for these transformations and subsequent decomposition. As a result, significant increase in the chemical stability of TKX-50 and intermediates with pressure was attributed to a suppressionmore » of hydrogen-transfer.« less

Influence of Ni on Martensitic Phase Transformations in NiTi Shape Memory Alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frenzel, J.; George, Easo P; Dlouhy, A.

High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on M{sub S}, M{sub F}, A{sub S}, A{sub F} and T{sub 0}, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalizedmore » on the basis of the crystallographic data of Prokoshkin et al. 2004 and the theory of Ball and James. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained.« less

Crystal structure transformation in potassium acrylate

NASA Astrophysics Data System (ADS)

Pai Verneker, V. R.; Vasanthakumari, R.

1983-10-01

Potassium acrylate undergoes a reversible phase transformation around 335°K with an activation energy of 133 kcal/mole. Differential scanning calorimetry and high temperature X-ray powder diffraction techniques have been used to probe this phenomenon.

High Temperature Elastic Properties of Reduced Activation Ferritic-Martensitic (RAFM) Steel Using Impulse Excitation Technique

NASA Astrophysics Data System (ADS)

Tripathy, Haraprasanna; Raju, Subramanian; Hajra, Raj Narayan; Saibaba, Saroja

2018-03-01

The polycrystalline elastic constants of an indigenous variant of 9Cr-1W-based reduced activation ferritic-martensitic (RAFM) steel have been determined as a function of temperature from 298 K to 1323 K (25 °C to 1000 °C), using impulse excitation technique (IET). The three elastic constants namely, Young's modulus E, shear modulus G, and bulk modulus B, exhibited significant softening with increasing temperature, in a pronounced non-linear fashion. In addition, clearly marked discontinuities in their temperature variations are noticed in the region, where ferrite + carbides → austenite phase transformation occurred upon heating. Further, the incidence of austenite → martensite transformation upon cooling has also been marked by a step-like jump in both elastic E and shear moduli G. The martensite start M s and M f finish temperatures estimated from this study are, M s = 652 K (379 °C) and M f =580 K (307 °C). Similarly, the measured ferrite + carbide → austenite transformation onset ( Ac 1) and completion ( Ac 3) temperatures are found to be 1126 K and 1143 K (853 °C and 870 °C), respectively. The Poisson ratio μ exhibited distinct discontinuities at phase transformation temperatures; but however, is found to vary in the range 0.27 to 0.29. The room temperature estimates of E, G, and μ for normalized and tempered microstructure are found to be 219 GPa, 86.65 GPa, and 0.27, respectively. For the metastable austenite phase, the corresponding values are: 197 GPa, 76.5 GPa, and 0.29, respectively. The measured elastic properties as well as their temperature dependencies are found to be in good accord with reported estimates for other 9Cr-based ferritic-martensitic steel grades. Estimates of θ D el , the elastic Debye temperature and γ G, the thermal Grüneisen parameter obtained from measured bulk elastic properties are found to be θ D el = 465 K (192 °C) and γ G = 1.57.

The Effect of AOT and Octanoic Acid on the Formation of Stable Water-in-diesel Microemulsion

NASA Astrophysics Data System (ADS)

Zhang, Yue; Misran, Misni Bin; Wang, Zhicheng; Zhang, Yu

2017-05-01

Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and octanoic acid (OA) were used as surfactants to prepare water-in-diesel microemulsion. The effect of mixed surfactants ratio on the phase behavior of water-in-diesel microemulsion was investigated. The R0-T plot phase diagrams for the diesel/AOT and OA/water system with different surfactant ratios were constructed at 30-80 °C. The results indicate that the largest single phase region could be obtained when OA to AOT molar ratio was 1. The temperature had a significant influence on phase transformation behavior. The single phase separated into two immiscible phases with the increase of temperature when R0 value was above 10. Compared with applying AOT alone, mixing AOT with appropriate amount of OA is benefit to form smaller nanosized W/O droplets. The determination of particle size was performed to verify the phase transformation behavior, and the results were consistent with the phase diagrams.

Prediction of Austenite Formation Temperatures Using Artificial Neural Networks

NASA Astrophysics Data System (ADS)

Schulze, P.; Schmidl, E.; Grund, T.; Lampke, T.

2016-03-01

For the modeling and design of heat treatments, in consideration of the development/ transformation of the microstructure, different material data depending on the chemical composition, the respective microstructure/phases and the temperature are necessary. Material data are, e.g. the thermal conductivity, heat capacity, thermal expansion and transformation data etc. The quality of thermal simulations strongly depends on the accuracy of the material data. For many materials, the required data - in particular for different microstructures and temperatures - are rare in the literature. In addition, a different chemical composition within the permitted limits of the considered steel alloy cannot be predicted. A solution for this problem is provided by the calculation of material data using Artificial Neural Networks (ANN). In the present study, the start and finish temperatures of the transformation from the bcc lattice to the fcc lattice structure of hypoeutectoid steels are calculated using an Artificial Neural Network. An appropriate database containing different transformation temperatures (austenite formation temperatures) to train the ANN is selected from the literature. In order to find a suitable feedforward network, the network topologies as well as the activation functions of the hidden layers are varied and subsequently evaluated in terms of the prediction accuracy. The transformation temperatures calculated by the ANN exhibit a very good compliance compared to the experimental data. The results show that the prediction performance is even higher compared to classical empirical equations such as Andrews or Brandis. Therefore, it can be assumed that the presented ANN is a convenient tool to distinguish between bcc and fcc phases in hypoeutectoid steels.

Structural-Phase Transformations of CuZn Alloy Under Thermal-Impact Cycling

NASA Astrophysics Data System (ADS)

Potekaev, A. I.; Chaplygina, A. A.; Kulagina, V. V.; Chaplygin, P. A.; Starostenkov, M. D.; Grinkevich, L. S.

2017-02-01

Using the Monte Carlo method, special features of structural - phase transformations in β-brass are investigated during thermal impact using thermal cycling as an example (a number of successive order - disorder and disorder - order phase transitions in the course of several heating - cooling cycles). It is shown that a unique hysteresis is observed after every heating and cooling cycle, whose presence indicates irreversibility of the processes, which suggests a difference in the structural - phase states both in the heating and cooling stages. A conclusion is drawn that the structural - phase transformations in the heating and cooling stages occur within different temperature intervals, where the thermodynamic stimuli of one or the other structural - phase state are low. This is also demonstrated both in the plots of configurational energy, long- and short-range order parameter, atomic structure variations, and structural - phase state distributions. Simultaneously, there coexist ordered and disordered phases and a certain collection of superstructure domains. This implies the presence of low - stability states in the vicinity of the order - disorder phase transition. The results of investigations demonstrate that the structural - phase transitions within two successive heating and cooling cycles at the same temperature are different in both stages. These changes, though not revolutionary, occur in every cycle and decrease with the increasing cycle number. In fact, the system undergoes training with a tendency towards a certain sequence of structural - phase states.

Design and development of NiTi-based precipitation-strengthened high-temperature shape memory alloys for actuator applications

NASA Astrophysics Data System (ADS)

Hsu, Derek Hsen Dai

As a vital constituent in the field of smart materials and structures, shape memory alloys (SMAs) are becoming ever-more important due to their wide range of commercial and industrial applications such as aircraft couplings, orthodontic wires, and eyeglasses frames. However, two major obstacles preventing SMAs from fulfilling their potential as excellent actuator materials are: 1) the lack of commercially-viable SMAs that operate at elevated temperatures, and 2) the degradation of mechanical properties and shape memory behavior due to thermal cyclic fatigue. This research utilized a thermodynamically-driven systems design approach to optimize the desired properties by controlling the microstructure and processing of high-temperature SMAs (HTSMAs). To tackle the two aforementioned problems with HTSMAs, the introduction of Ni2TiAl coherent nanoprecipitates in a Ni-Ti-Zr/Hf HTSMA matrix is hypothesized to strengthen the martensite phase while simultaneously increasing the transformation temperature. Differential scanning calorimetry (DSC) was used to determine the transformation temperatures and thermal cyclic stability of each alloy. Also, microstructural characterization was performed using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Lastly, compression testing was used to assess the mechanical behavior of the alloys. From the investigation of the first set of Ni48.5Ti31.5-X Zr20AlX (X = 0, 1, 2, 3) prototype alloys, Al addition was found to decrease the transformation temperatures, decrease the thermal cyclic stability, but also increase the strength due to the nucleation and growth of embrittling NiTi2 and NiTiZr Laves phases. However, the anticipated Heusler phase precipitation did not occur. The next study focused on Ni50Ti30-XHf20Al X (X = 0, 1, 2, 3, 4, 5) prototype alloys which replaced Zr with Hf to avoid the formation of brittle Laves phases. Heusler precipitation was successfully demonstrated in the aged 4 and 5% Al alloys, but no transformation was detected. Finally, the last investigation explored the potential of high transformation temperatures in Ni50Ti25-XHf25AlX and Ni50Ti20-XHf30AlX (X = 0, 1, 2, 3, 4, 5) prototype alloys. The final design was narrowed down to a Ni 50Ti20Hf25Al5 alloy aged at 800°C that is expected to exhibit high transformation temperatures while concurrently strengthened by Heusler nanoprecipitates.

[Study of the phase transformation of TiO2 with in-situ XRD in different gas].

PubMed

Ma, Li-Jing; Guo, Lie-Jin

2011-04-01

TiO2 sample was prepared by sol-gel method from chloride titanium. The phase transformation of the prepared TiO2 sample was studied by in-situ XRD and normal XRD in different gas. The experimental results showed that the phase transformation temperatures of TiO2 were different under in-situ or normal XRD in different kinds of gas. The transformation of amorphous TiO2 to anatase was controlled by kinetics before 500 degrees C. In-situ XRD showed that the growth of anatase was inhibited, but the transformation of anatase to rutile was accelerated under inactive nitrogen in contrast to air. Also better crystal was obtained under hydrogen than in argon. These all showed that external oxygen might accelerate the growth of TiO2, but reduced gas might partly counteract the negative influence of lack of external oxygen. The mechanism of phase transformation of TiO2 was studied by in-situ XRD in order to control the structure in situ.

Ab Initio Simulations of Temperature Dependent Phase Stability and Martensitic Transitions in NiTi

NASA Technical Reports Server (NTRS)

Haskins, Justin B.; Thompson, Alexander E.; Lawson, John W.

2016-01-01

For NiTi based alloys, the shape memory effect is governed by a transition from a low-temperature martensite phase to a high-temperature austenite phase. Despite considerable experimental and computational work, basic questions regarding the stability of the phases and the martensitic phase transition remain unclear even for the simple case of binary, equiatomic NiTi. We perform ab initio molecular dynamics simulations to describe the temperature-dependent behavior of NiTi and resolve several of these outstanding issues. Structural correlation functions and finite temperature phonon spectra are evaluated to determine phase stability. In particular, we show that finite temperature, entropic effects stabilize the experimentally observed martensite (B19') and austenite (B2) phases while destabilizing the theoretically predicted (B33) phase. Free energy computations based on ab initio thermodynamic integration confirm these results and permit estimates of the transition temperature between the phases. In addition to the martensitic phase transition, we predict a new transition between the B33 and B19' phases. The role of defects in suppressing these phase transformations is discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, J. P.; Wang, Y. D.; Hao, Y. L.

Two main explanations exist for the deformation mechanisms in Ti-Nb-based gum metals, i.e. the formation of reversible nanodisturbance and reversible stress-induced martensitic transformation. In this work, we used the in situ synchrotron-based high-energy X-ray diffuse-scattering technique to reveal the existence of a specific deformation mechanism, i.e. deformation-induced spatially confined martensitic transformations, in Ti-24Nb-4Zr-8Sn-0.10O single crystals with cubic 13 parent phase, which explains well some anomalous mechanical properties of the alloy such as low elastic modulus and nonlinear superelasticity. Two kinds of nanosized martensites with different crystal structures were found during uniaxial tensile loading along the [11 0](beta) axis at roommore » temperature and 190 K, respectively. The detailed changes in the martensitic phase transformation characteristics and the transformation kinetics were experimentally observed at different temperatures. The domain switch from non-modulated martensite to a modulated one occurred at 190 K, with its physical origin attributed to the heterogeneity of local phonon softening depending on temperature and inhomogeneous composition in the parent phase. An in-depth understanding of the formation of stress-induced spatially confined nanosized martensites with a large gradient in chemical composition may benefit designs of high-strength and high-ductility alloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.« less

Stresses and pressures at the quartz-to-coesite phase transformation in shear deformation experiments

NASA Astrophysics Data System (ADS)

Richter, B.; Stünitz, H.; Heilbronner, R.

2016-11-01

Coesite was found in quartz aggregates, experimentally deformed at confining pressures of 1.0-1.5 GPa and temperatures between 600°C and 900°C. The confining pressure (Pc) and, in most cases, the mean stress (σm) of the experiments were below those of the quartz-to-coesite phase transformation. Yet coesite formed when the maximum principal stress (σ1) was within the P-T range of the coesite stability field. In one sample, the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz. It is inferred that this sample started to deform with σ1 above the quartz-to-coesite phase transformation and, with ongoing deformation, σ1 decreased to values in the quartz stability field due to strain weakening. In all cases, σ1 triggered the quartz-to-coesite reaction as well as the reverse reaction, suggesting that σ1 is the critical parameter for the quartz-to-coesite transformation—not Pc or σm. With progressive deformation, the coesite laths rotated toward the shear plane as more rigid particles with the sense of shear. In case of back reaction, new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite. The experiments cover different degrees of pressure "overstepping," different temperatures, and different experimental durations at P and T, and deformation always enhances the reaction kinetics. The observation that σ1 is critical for a pressure-dependent phase transformation (also for reversals) poses questions for the thermodynamic treatment of such phase transformations.

Microstructure Evolution and Mechanical Behavior of a Hot-Rolled High-Manganese Dual-Phase Transformation-Induced Plasticity/Twinning-Induced Plasticity Steel

NASA Astrophysics Data System (ADS)

Fu, Liming; Shan, Mokun; Zhang, Daoda; Wang, Huanrong; Wang, Wei; Shan, Aidang

2017-05-01

The microstructures and deformation behavior were studied in a high-temperature annealed high-manganese dual-phase (28 vol pct δ-ferrite and 72 vol pct γ-austenite) transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steel. The results showed that the steel exhibits a special Lüders-like yielding phenomenon at room temperature (RT) and 348 K (75 °C), while it shows continuous yielding at 423 K, 573 K and 673 K (150 °C, 300 °C and 400 °C) deformation. A significant TRIP effect takes place during Lüders-like deformation at RT and 348 K (75 °C) temperatures. Semiquantitative analysis of the TRIP effect on the Lüders-like yield phenomenon proves that a softening effect of the strain energy consumption of strain-induced transformation is mainly responsible for this Lüders-like phenomenon. The TWIP mechanism dominates the 423 K (150 °C) deformation process, while the dislocation glide controls the plasticity at 573 K (300 °C) deformation. The delta-ferrite, as a hard phase in annealed dual-phase steel, greatly affects the mechanical stability of austenite due to the heterogeneous strain distribution between the two phases during deformation. A delta-ferrite-aided TRIP effect, i.e., martensite transformation induced by localized strain concentration of the hard delta-ferrite, is proposed to explain this kind of Lüders-like phenomenon. Moreover, the tensile curve at RT exhibits an upward curved behavior in the middle deformation stage, which is principally attributed to the deformation twinning of austenite retained after Lüders-like deformation. The combination of the TRIP effect during Lüders-like deformation and the subsequent TWIP effect greatly enhances the ductility in this annealed high-manganese dual-phase TRIP/TWIP steel.

In situ analysis of phase transformation in sol-gel cogelified nanopowder mixture of Al 2O 3 and TiO 2 using synchrotron X-ray radiation diffraction experiments

NASA Astrophysics Data System (ADS)

Jianu, A.; Stanciu, L.; Groza, J. R.; Lathe, Ch.; Burkel, E.

2003-01-01

Aluminium titanate (Al 2TiO 5) has been selected for study due to its high melting point and thermal shock resistance. In situ analysis of phase transformation and of transformation kinetics of sol-gel powder mixture of alumina and titania cogelified samples was performed using high-temperature synchrotron radiation X-ray diffraction experiments. The high reactivity and molecular mixing of sol-gel cogelified precursor powders contributed to the evolution of the reaction. The stability of the TiO 2-tetragonal structure (anatase) increases due to Al 2O 3 presence. The temperature of the aluminium titanate endothermic reaction decreases when heating rate increases. The results obtained by in situ analysis have been used to establish the sintering parameters in order to obtain fully transformed, dense aluminium titanate bulk ceramics.

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

PubMed Central

Birkner, Nancy; Navrotsky, Alexandra

2014-01-01

Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn2O3) to hausmannite (Mn3O4) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn2O3 and Mn3O4) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings. PMID:24733903

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration.

PubMed

Birkner, Nancy; Navrotsky, Alexandra

2014-04-29

Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn2O3) to hausmannite (Mn3O4) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn2O3 and Mn3O4) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings.

Nano-scale phase transformation in Ti-implanted austenitic 301 stainless steel.

PubMed

Gustiono, Dwi; Sakaguchi, Norihito; Shibayama, Tamaki; Kinoshita, Hiroshi; Takahashi, Heishichiro

2003-01-01

Phase-transformation behaviours were investigated for austenitic 301 stainless steel during implantation at room temperature with 300 keV Ti ions to fluences of 8 x 10(19) to approximately 3 x 10(21) ions m(-2) by means of transmission electron microscopy. The cross-sectional specimen was prepared using a focused ion beam. Plan observation of the implanted specimen showed that phase transformation from gamma-phase to alpha-phase was induced by implantation to a fluence of 3 x 10(20) Ti ions m(-2). The nucleation of the irradiation (implantation)-induced phase increased with the increase of the dose. The orientation relationship between the gamma matrix and the induced alpha martensitic phase was identified as (011)alpha//(111)gamma and [11-1]alpha//[10-1], close to the Kurdjumov-Sachs relationship. Cross-sectional observation after implantation to a fluence of 5 x 10(20) ions m(-2) showed that phase transformation mostly nucleated near the surface and occurred in the higher the concentration gradient of the implanted ion, i.e. a higher stress concentration takes place and this stress introduced by the implanted ions acts as a driving force for the transformation.

Ikaite pseudomorphs in the Zaire deep-sea fan: An intermediate between calcite and porous calcite

NASA Astrophysics Data System (ADS)

Jansen, J. H. F.; Woensdregt, C. F.; Kooistra, M. J.; van der Gaast, S. J.

1987-03-01

Translucent brown aggregates of calcium-carbonate crystals have been found in cores from the Zaire deep-sea fan (west equatorial Africa). The aggregates are well preserved but very friable. Upon storage they become yellowish white and cloudy and release water. Chemical, mineralogical (XRD), petrographical, crystal-morphological, and stable-isotope data demonstrate that the crystals have passed through three phases: (1) an authigenic carbonate phase, probably calcium carbonate, which is represented by the external habit of the present crystals; (2) a translucent brown ikaite phase (CaCO3·6H2O), unstable at temperatures above 5 °C; and (3) a phase consisting of calcite microcrystals that are poorly cemented and form a porous mass within the crystal form of the morphologically unchanged first phase. The transformation from the first phase into ikaite was probably a kinetic replacement. The transformation from ikaite into the third phase occurred because of storage at room temperature. The presence of ikaite is indicative of a low-temperature, anaerobic, organic-carbon-rich marine environment. Ikaite is probably the precursor of a great number of porous calcite pseudomorphs, and possibly also of many marine authigenic microcrystalline carbonate nodules.

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

PubMed

Hong, Woong-Ki; Park, Jong Bae; Yoon, Jongwon; Kim, Bong-Joong; Sohn, Jung Inn; Lee, Young Boo; Bae, Tae-Sung; Chang, Sung-Jin; Huh, Yun Suk; Son, Byoungchul; Stach, Eric A; Lee, Takhee; Welland, Mark E

2013-04-10

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

Structural transformation study of TiO2 nanoparticles annealing at different temperatures and the photodegradation process of eosin-Y

NASA Astrophysics Data System (ADS)

Pandi, P.; Gopinathan, C.

2018-04-01

Hydrothermal method was used to prepare TiO2 nanoparticles with annealing temperature at 500 °C-700 °C. The mixture of anatase-rutile phase was investigated by powerful tool of X-ray diffraction (XRD). The structural parameters of anatase and rutile mixture phaseTiO2 nanoparticles were calculated from the Rietveld refinement. The transformation rate of rutile was increased linearly with an annealing temperature of 500 °C-700 °C. The spherical morphology of the anatase and rutile mixed phase were obtained by scanning electron microscope and transmission electron microscope. The spherical particle of the anatase and rutile TiO2 shows with great aggregation with different size and within the range of few tens nm. The EDAX study revealed the presence of titanium and oxygen. The best photocatalytic activity was identified as the 87.04% of anatase and 12.96% of rutile mixer phase of TiO2. Various factors could be involved for a better photocatalytic activity.

Raising the superconducting Tc of gallium: In situ characterization of the transformation of α -Ga into β -Ga

NASA Astrophysics Data System (ADS)

Campanini, D.; Diao, Z.; Rydh, A.

2018-05-01

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable β -Ga with a critical temperature Tc=6.04 (5 ) K , while stable α -Ga has a much lower Tc<1.2 K . Here we use a membrane-based nanocalorimeter to initiate the transition from α -Ga to β -Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of α -Ga. After such treatment, the liquid supercools down to 232 K , where β -Ga solidifies. We find that β -Ga is a strong-coupling type-I superconductor with Δ (0 ) /kBTc=2.00 (5 ) and a Sommerfeld coefficient γn=1.53 (4 ) mJ /molK2 , 2.55 times higher than that in the α phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

Dynamic Diffraction Studies on the Crystallization, Phase Transformation, and Activation Energies in Anodized Titania Nanotubes.

PubMed

Albetran, Hani; Vega, Victor; Prida, Victor M; Low, It-Meng

2018-02-23

The influence of calcination time on the phase transformation and crystallization kinetics of anodized titania nanotube arrays was studied using in-situ isothermal and non-isothermal synchrotron radiation diffraction from room temperature to 900 °C. Anatase first crystallized at 400 °C, while rutile crystallized at 550 °C. Isothermal heating of the anodized titania nanotubes by an increase in the calcination time at 400, 450, 500, 550, 600, and 650 °C resulted in a slight reduction in anatase abundance, but an increase in the abundance of rutile because of an anatase-to-rutile transformation. The Avrami equation was used to model the titania crystallization mechanism and the Arrhenius equation was used to estimate the activation energies of the titania phase transformation. Activation energies of 22 (10) kJ/mol for the titanium-to-anatase transformation, and 207 (17) kJ/mol for the anatase-to-rutile transformation were estimated.

Phase transformations in cast duplex stainless steels

NASA Astrophysics Data System (ADS)

Kim, Yoon-Jun

Duplex stainless steels (DSS) constitute both ferrite and austenite as a matrix. Such a microstructure confers a high corrosion resistance with favorable mechanical properties. However, intermetallic phases such as sigma (sigma) and chi (chi) can also form during casting or high-temperature processing and can degrade the properties of the DSS. This research was initiated to develop time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams of two types of cast duplex stainless steels, CD3MN (Fe-22Cr-5Ni-Mo-N) and CD3MWCuN (Fe-25Cr-7Ni-Mo-W-Cu-N), in order to understand the time and temperature ranges for intermetallic phase formation. The alloys were heat treated isothermally or under controlled cooling conditions and then characterized using conventional metallographic methods that included tint etching, and also using electron microscopy (SEM, TEM) and wavelength dispersive spectroscopy (WDS). The kinetics of intermetallic-phase (sigma + chi) formation were analyzed using the Johnson-Mehl-Avrami (JMA) equation in the case of isothermal transformations and a modified form of this equation in the case of continuous cooling transformations. The rate of intermetallic-phase formation was found to be much faster in CD3MWCuN than CD3MN due mainly to differences in the major alloying contents such as Cr, Ni and Mo. To examine in more detail the effects of these elements of the phase stabilities, a series of eight steel castings was designed with the Cr, Ni and Mo contents systematically varied with respect to the nominal composition of CD3MN. The effects of varying the contents of alloying additions on the formation of intermetallic phases were also studied computationally using the commercial thermodynamic software package, Thermo-Calc. In general, a was stabilized with increasing Cr addition and chi by increasing Mo addition. However, a delicate balance among Ni and other minor elements such as N and Si also exists. Phase equilibria in DSS can be affected by local composition fluctuations in the cast alloy. This may cause discrepancy between thermodynamic prediction and experimental observation.

Effect of temperature in the selective reduction process of limonite nickel ore

NASA Astrophysics Data System (ADS)

Mayangsari, W.; Febriana, Eni; Prasetyo, A. B.

2018-05-01

Temperature is the main factor for the reduction process that influence to reduction degree, phase and morphology transformation. In order to determine these effects which is caused by reduction temperature, this study was conducted. Limoniticnickel ore was prepared by drying and size reduction. A part of prepared limonitewas characterized with XRF to determine the chemical composition. The other part was mixed with reducing agent and CaSO4 to produce pellet. A series of selective reduction processes were conducted to the pellet by using graphite crucible in the muffle furnace carbolite at 800° - 1100°C for 60 minutes. Reduced ore characterized by using XRD and SEM analysis. Based on the result study, weight loss and reduction degree increase as temperature raised along with CaSO4 addition. Moreover, it caused decomposition and transformation to the metallic phase of kamacite and iron up to 7.51% and 41.44% respectively in the reduction process at 1100°C for 60 minutes. Furthermore, particle size growth as metallic phase content increased.

Relationships Between the Phase Transformation Kinetics, Texture Evolution, and Microstructure Development in a 304L Stainless Steel Under Biaxial Loading Conditions: Synchrotron X-ray and Electron Backscatter Diffraction Studies

DOE PAGES

Cakmak, Ercan; Choo, Hahn; Kang, Jun-Yun; ...

2015-02-11

Here we report that the relationships between the martensitic phase transformation kinetics, texture evolution, and the microstructure development in the parent austenite phase were studied for a 304L stainless steel that exhibits the transformation-induced plasticity effect under biaxial loading conditions at ambient temperature. The applied loading paths included: pure torsion, simultaneous biaxial torsion/tension, simultaneous biaxial torsion/compression, and stepwise loading of tension followed by torsion (i.e., first loading by uniaxial tension and then by pure torsion in sequence). Synchrotron X-ray and electron backscatter diffraction techniques were used to measure the evolution of the phase fractions, textures, and microstructures as a functionmore » of the applied strains. The influence of loading character and path on the changes in martensitic phase transformation kinetics is discussed in the context of (1) texture-transformation relationship and the preferred transformation of grains belonging to certain texture components over the others, (2) effects of axial strains on shear band evolutions, and (3) volume changes associated with martensitic transformation.« less

Transformation Stasis Phenomenon of Bainite Formation in Low-Carbon, Multicomponent Alloyed Steel

NASA Astrophysics Data System (ADS)

Lan, Liangyun; Kong, Xiangwei

2017-11-01

The transformation stasis phenomenon of bainite formation in low-carbon steel was detected using a high-resolution dilatometer. The phenomenon occurred at different stages for different isothermal temperatures. In combination with microstructural observation, the calculated overall activation energy of transformation and interface migration velocity shed new light on the cause of formation of the stasis phenomenon. The temporary stasis formed at the initial stage of phase transformation for high isothermal temperature was attributed to the drag effect of substitutional atoms, which leads to low-interface migration velocity and large overall activation energy.

Synthesis of zirconium dioxide by ultrasound assisted precipitation: effect of calcination temperature.

PubMed

Prasad, Krishnamurthy; Pinjari, D V; Pandit, A B; Mhaske, S T

2011-09-01

Nanostructured zirconium dioxide was synthesized from zirconyl nitrate using both conventional and ultrasound assisted precipitation in alkaline medium. The synthesized samples were calcinated at temperatures ranging from 400°C to 900°C in steps of 100°C. The ZrO(2) specimens were characterized using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The thermal characteristics of the samples were studied via Differential Scanning Calorimetry-Thermo-Gravimetry Analysis (DSC-TGA). The influence of the calcination temperature on the phase transformation process from monoclinic to tetragonal to cubic zirconia and its consequent effect on the crystallite size and % crystallinity of the synthesized ZrO(2) was studied and interpreted. It was observed that the ultrasound assisted technique helped to hasten to the phase transformation and also at some point resulted in phase stabilization of the synthesized zirconia. Copyright © 2011 Elsevier B.V. All rights reserved.

H-Phase Precipitation and Martensitic Transformation in Ni-rich Ni-Ti-Hf and Ni-Ti-Zr High-Temperature Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Evirgen, A.; Pons, J.; Karaman, I.; Santamarta, R.; Noebe, R. D.

2018-03-01

The distributions of H-phase precipitates in Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys formed by aging treatments at 500 and 550 °C or slow furnace cooling and their effects on the thermal martensitic transformation have been investigated by TEM and calorimetry. The comparative study clearly reveals faster precipitate-coarsening kinetics in the NiTiZr alloy than in NiTiHf. For precipitates of a similar size of 10-20 nm in both alloys, the martensite plates in Ni50.3Ti29.7Zr20 have larger widths and span a higher number of precipitates compared with the Ni50.3Ti29.7Hf20 alloy. However, for large H-phase particles with hundreds of nm in length, no significant differences in the martensitic microstructures of both alloy systems have been observed. The martensitic transformation temperatures of Ni50.3Ti29.7Hf20 are 80-90 °C higher than those of Ni50.3Ti29.7Zr20 in the precipitate-free state and in the presence of large particles of hundreds on nm in length, but this difference is reduced to only 10-20 °C in samples with small H-phase precipitates. The changes in the transformation temperatures are consistent with the differences in the precipitate distributions between the two alloy systems observed by TEM.

T-p phase diagrams and the barocaloric effect in materials with successive phase transitions

NASA Astrophysics Data System (ADS)

Gorev, M. V.; Bogdanov, E. V.; Flerov, I. N.

2017-09-01

An analysis of the extensive and intensive barocaloric effect (BCE) at successive structural phase transitions in some complex fluorides and oxyfluorides was performed. The high sensitivity of these compounds to a change in the chemical pressure allows one to vary the succession and parameters of the transformations (temperature, entropy, baric coefficient) over a wide range and obtain optimal values of the BCE. A comparison of different types of schematic T-p phase diagrams with the complicated T( p) dependences observed experimentally shows that in some ranges of temperature and pressure the BCE in compounds undergoing successive transformations can be increased due to a summation of caloric effects associated with distinct phase transitions. The maximum values of the extensive and intensive BCE in complex fluorides and oxyfluorides can be realized at rather low pressure (0.1-0.3 GPa). In a narrow temperature range around the triple points conversion from conventional BCE to inverse BCE is observed, which is followed by a gigantic change of both \\vertΔ S_BCE\\vert and \\vertΔ T_AD\\vert .

REVIEWS OF TOPICAL PROBLEMS: Order-disorder transformations and phase equilibria in strongly nonstoichiometric compounds

NASA Astrophysics Data System (ADS)

Gusev, Aleksandr I.

2000-01-01

Data on order-disorder phase transformations in strongly nonstoichiometric carbides and nitrides MXy (X=C, N) of Group IV and V transition metals at temperatures below 1300-1400 K are reviewed. The order-parameter functional method as applied to atomic and vacancy ordering in strongly nonstoichiometric MXy compounds and to phase equilibrium calculations for M-X systems is discussed. Phase diagram calculations for the Ti-C, Zr-C, Hf-C, V-C, Nb-C, Ta-C, Ti-N, and Ti-B-C systems (with the inclusion of the ordering of nonstoichiometric carbides and nitrides) and those for pseudobinary carbide M(1)C-M(2)C systems are presented. Heat capacity, electrical resistivity and magnetic susceptibility changes at reversible order-disorder phase transformations in nonstoichiometric carbides are considered.

Tunable resistivity due to kinetic arrest of antiferro-ferromagnetic transition in FeRh0.46Pd0.54

NASA Astrophysics Data System (ADS)

Saha, Pampi; Rawat, R.

2018-05-01

We show a large negative magnetoresistance (MR) of ≈10% near room temperature in FeRh0.46Pd0.54, which increases to more than 60% at low temperatures. The magnitude of resistivity and, hence, MR depend on the history of the sample in HT (magnetic field-temperature) space, e.g., resistivity at 5 K changes by more than 70% with thermal cycling. These results are explained due to slow kinetics of the transformation from austenite antiferromagnetic (AF) to martensite ferromagnetic (FM) state with the decrease in temperature. As a result, AF to FM transformation remains incomplete on experimental time scales and non-ergodic AF phase co-exists with a low temperature equilibrium FM phase. In the present system, the kinetics of the transition is shown to dominate up to 150 K, which is significantly high in comparison to other kinetically arrested systems.

An Investigation into the Polymorphism and Crystallization of Levetiracetam and the Stability of its Solid Form.

PubMed

Xu, Kailin; Xiong, Xinnuo; Guo, Liuqi; Wang, Lili; Li, Shanshan; Tang, Peixiao; Yan, Jin; Wu, Di; Li, Hui

2015-12-01

Levetiracetam (LEV) crystals were prepared using different solvents at different temperatures. The LEV crystals were systematically characterized by X-ray powder diffraction (XRPD) and morphological analysis. The results indicated that many kinds of crystal habits exist in a solid form of LEV. To investigate the effects of LEV concentration, crystallization temperature, and crystallization type on crystallization and solid phase transformation of LEV, multiple methods were performed for LEV aqueous solution to determine if a new solid form exists in solid-state LEV. However, XRPD data demonstrate that the LEV solid forms possess same spatial arrangements that are similar to the original solid form. This result indicates that the LEV concentration, crystallization temperature, and crystallization type in aqueous solution have no influence on the crystallization and solid phase transformation of LEV. Moreover, crystallization by sublimation, melt cooling, and quench cooling, as well as mechanical effect, did not result in the formation of new LEV solid state. During melt cooling, the transformation of solid form LEV is a direct process from melting amorphous phase to the original LEV crystal phase, and the conversion rate is very quick. In addition, stability investigation manifested that LEV solid state is very stable under various conditions. © 2015 Wiley Periodicals, Inc. and the American Pharmacists Association.

Embedded Shape Memory Alloy Particles for the Self-Sensing of Fatigue Crack Growth in an Aluminum Alloy

NASA Astrophysics Data System (ADS)

Leser, William Paul

Future aerospace vehicles will be built using novel materials for mission conditions that are difficult to replicate in a laboratory. Structural health monitoring and condition-based maintenance will be critical to ensure the reliability of such vehicles. A multi-functional aluminum alloy containing embedded shape memory alloy (SMA) particles to detect fatigue crack growth is proposed. The regions of intensified strain near the tip of a growing fatigue crack cause the SMA particles to undergo a solid-to-solid phase transformation from austenite to martensite, releasing a detectable and identifiable acoustic emission (AE) signal that can be used to locate the crack in the affected component. This study investigates the AE response of two SMA systems, Ni-Ti, and Co-Ni-Al. Tensile (Ni-Ti) and compressive (Co-Ni-Al) tests were conducted to study the strain-induced transformation response in both of the alloy systems. It was found that the critical stress for transformation in both SMA systems was easily identified by a burst of AE activity during both transformation and reverse transformation. AE signals from these experiments were collected for use as training data for a Bayesian classifier to be used to identify transformation signals in a Al7050 matrix with embedded SMA particles. The Al/SMA composite was made by vacuum hot pressing SMA powder between aluminum plates. The effect of hot pressing temperature and subsequent heat treatments (solutionizing and peak aging) on the SMA particles was studied. It was found that, at the temperatures required, Co-Ni-Al developed a second phase that restricted the transformation from austenite to martensite, thus rendering it ineffective as a candidate for the embedded particles. Conversely, Ni-Ti did survive the embedding process and it was found that the solutionizing heat treatment applied after hot pressing was the main driver in determining the final transformation temperatures for the Ni-Ti particles. The effect of hot pressing on the transformation temperatures was negated upon solutionizing and peak aging occurred at a sufficiently low temperature to as not affect the properties of the Ni-Ti. Strain-induced transformation was confirmed in the Ni-Ti particles by digital image correlation (DIC) using an environmental scanning electron microscope (ESEM). Specimens were fatigue pre-cracked until a crack was produced and observed to be approaching a particle that could be monitored on the surface, at which point it was put into the ESEM for DIC under tensile loading. Acoustic emission activity was observed during this experiment. In order to distinguish AE signals arising due to phase transformation in the particles from those due to crack extension in the matrix, a Bayesian classifier was constructed based on frequency parameters calculated using the Hilbert-Huang transform (HHT). Using this classifier, AE signals consistent with those arising from phase transformation in bulk Ni-Ti were identified during phase transformation in the particles as observed with DIC. In addition to tensile crack growth in the ESEM, a fatigue crack was grown through a specimen with particles interspersed along the specimen center line. Several low amplitude AE events were observed as the crack grew through the aluminum. As the fatigue crack passed through the line of particles AE events increased dramatically in rate of occurance and amplitude. Amplitudes were 6-10 times higher as the crack passed near the particles. These AE events were also shown to be consistent with Ni-Ti phase transformation. A successful proof-of-concept was demonstrated for an aluminum alloy with embedded particles that emit an identifiable and repeatable AE signal in the presence of a fatigue crack, allowing for quick diagnosis of fatigue crack damage in this material.

Comparison of vapor formation of water at the solid/water interface to colloidal solutions using optically excited gold nanostructures.

PubMed

Baral, Susil; Green, Andrew J; Livshits, Maksim Y; Govorov, Alexander O; Richardson, Hugh H

2014-02-25

The phase transformation properties of liquid water to vapor is characterized by optical excitation of the lithographically fabricated single gold nanowrenches and contrasted to the phase transformation properties of gold nanoparticles located and optically excited in a bulk solution system [two and three dimensions]. The 532 nm continuous wave excitation of a single gold nanowrench results in superheating of the water to the spinodal decomposition temperature of 580 ± 20 K with bubble formation below the spinodal decomposition temperature being a rare event. Between the spinodal decomposition temperature and the boiling point liquid water is trapped into a metastable state because a barrier to vapor nucleation exists that must be overcome before the thermodynamically stable state is realized. The phase transformation for an optically heated single gold nanowrench is different from the phase transformation of optically excited colloidal gold nanoparticles solution where collective heating effects dominates and leads to the boiling of the solution exactly at the boiling point. In the solution case, the optically excited ensemble of nanoparticles collectively raises the ambient temperature of water to the boiling point where liquid is converted into vapor. The striking difference in the boiling properties of the single gold nanowrench and the nanoparticle solution system can be explained in terms of the vapor-nucleation mechanism, the volume of the overheated liquid, and the collective heating effect. The interpretation of the observed regimes of heating and vaporization is consistent with our theoretical modeling. In particular, we explain with our theory why the boiling with the collective heating in a solution requires 3 orders of magnitude less intensity compared to the case of optically driven single nanowrench.

Effects of Rolling and Cooling Conditions on Microstructure of Umbrella-Bone Steel

NASA Astrophysics Data System (ADS)

Wu, Yan-Xin; Fu, Jian-Xun; Zhang, Hua; Xu, Jie; Zhai, Qi-Jie

2017-10-01

The effects of deformation temperature and cooling rate on the micro-structure evolution of umbrella-bone steel was investigated using a Gleeble thermal-mechanical testing machine and dynamic continuous cooling transformation (CCT) curves. The results show that fast cooling which lowers the starting temperature of ferrite transformation leads to finer ferrite grains and more pearlite. Low temperature deformation enhances the hardening effect of austenite and reduces hardenability, allowing a wider range of cooling rates and thus avoiding martensite transformation after deformation. According to the phase transformation rules, the ultimate tensile strength and reduction in area of the wire rod formed in the optimized industrial trial are 636 MPa and 73.6 %, respectively, showing excellent strength and plasticity.

In-situ monitoring of ? phase transformation in Ti-6Al-6V-2Sn using laser ultrasonics

NASA Astrophysics Data System (ADS)

Hinterlechner, Irina; Barriobero-Vila, Pere; Reitinger, Bernhard; Fromherz, Thomas; Requena, Guillermo; Burgholzer, Peter

2018-04-01

Titanium is of great interest for metal processing industries due to its superior material properties, but it is also quite expensive. Therefore, a detailed knowledge of ? phase transformation and consequential the distribution of ? and ? phase in titanium alloys is crucial for their material properties and as a consequence for further processing steps. Measuring the ultrasonic velocity and attenuation by laser ultrasonics technology (LUS) as a non-destructive and non-contact technique, it is possible to qualitatively monitor in-situ the phase transformation during heating the sample from room temperature up to ?. We validate LUS methodology against high energy X-ray diffraction as well as against conventional metallurgic measurements and get excellent agreement between the results of these methods.

Proceedings from the Conference on Critical Issues in the Development of High Temperature Structural Materials Held in Kona, Hawaii on March 7-14, 1993,

DTIC Science & Technology

1993-01-14

composite has not been established and may restrict the use temperature; and/or lifetime. Precipitation of second phase particles in a master alloy (XD...intermetallic alloys with both ordered and disordered precipitate phases including precipitates based upon carbide or nitride phases which involve an...to identify likely pathways or to eliminate others i.e., define a transformation hierarchy. The crystallographic symmetries of the precipitate phase

Deformation behavior of austenitic stainless steel at deep cryogenic temperatures

NASA Astrophysics Data System (ADS)

Han, Wentuo; Liu, Yuchen; Wan, Farong; Liu, Pingping; Yi, Xiaoou; Zhan, Qian; Morrall, Daniel; Ohnuki, Somei

2018-06-01

The nonmagnetic austenite steels are the jacket materials for low-temperature superconductors of fusion reactors. The present work provides evidences that austenites transform to magnetic martensite when deformation with a high-strain is imposed at 77 K and 4.2 K. The 4.2 K test is characterized by serrated yielding that is related to the specific motion of dislocations and phase transformations. The in-situ transmission electron microscope (TEM) observations in nanoscale reveal that austenites achieve deformation by twinning under low-strain conditions at deep cryogenic temperatures. The generations of twins, martensitic transformations, and serrated yielding are in order of increasing difficulty.

Kinetic parameters and structural variations in Cu-Al-Mn and Cu-Al-Mn-Mg shape memory alloys

NASA Astrophysics Data System (ADS)

Canbay, Canan Aksu

2017-02-01

In this work polycrystalline Cu-Al-Mn and Cu-Al-Mn-Mg SMAs were fabricated by arc melting. The thermal analysis was made to determine the characteristic transformation temperatures of the samples and kinetic parameters. Also the effect of Mg on transformation temperatures and kinetic parameters detected. The structural analysis was made to designate the diffraction planes of martensite phase at room temperature and this was supported by optical measurement observations.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wiengmoon, A., E-mail: ampornw@nu.ac.th

The effects of Si on microstructure and phase transformation at elevated temperature of ferritic 31wt.%Cr-1.1wt.%C white cast irons with up to 3wt.%Si have been studied. Applications of these irons include parts requiring heat resistance at elevated temperature. The irons were produced by sand casting. The microstructure in as-cast condition and after being subjected to high temperature (700 to 1000 °C) was investigated by light microscopy, X-ray diffraction, and electron microscopy. The results revealed that the as-cast microstructure consisted mainly of primary ferrite dendrites and eutectic (ferrite + M{sub 7}C{sub 3}). Si promotes M{sub 7}C{sub 3}-to-M{sub 23}C{sub 6} transformation in themore » irons subjected to transformation at elevated temperature, but no sigma phase was found. The extent of M{sub 7}C{sub 3}-to-M{sub 23}C{sub 6} transformation increases proportional to the increasing transformation temperature, holding time and Si content in the irons. For the iron with 1.0wt.%Si content after holding at elevated temperatures, martensite was also found, which could be attributed to carbon accretion effects in eutectic ferrite. Si was incorporated in M{sub 23}C{sub 6} such that M{sub 23}C{sub 6} containing Si can show darker contrast under SEM-BEI as compared to M{sub 7}C{sub 3}; this is the opposite to what has been observed for the cases of typical M{sub 23}C{sub 6} and M{sub 23}C{sub 6} containing Mo or W. The results obtained are important to understand the change in properties of ferritic, high chromium irons containing Si subjected to elevated temperature.« less

Nature of metastable amorphous-to-crystalline reversible phase transformations in GaSb

NASA Astrophysics Data System (ADS)

Kalkan, B.; Edwards, T. G.; Raoux, S.; Sen, S.

2013-08-01

The structural, thermodynamic, and kinetic aspects of the transformations between the metastable amorphous and crystalline phases of GaSb are investigated as a function of pressure at ambient temperature using synchrotron x-ray diffraction experiments in a diamond anvil cell. The results are consistent with the hypothesis that the pressure induced crystallization of amorphous GaSb into the β-Sn crystal structure near ˜5 GPa is possibly a manifestation of an underlying polyamorphic phase transition between a semiconducting, low density and a metallic, high density amorphous (LDA and HDA, respectively) phases. In this scenario, the large differences in the thermal crystallization kinetics between amorphous GaSb deposited in thin film form by sputtering and that prepared by laser melt quenching may be related to the relative location of the glass transition temperature of the latter in the pressure-temperature (P-T) space with respect to the location of the critical point that terminate the LDA ↔ HDA transition. The amorphous → β-Sn phase transition is found to be hysteretically reversible as the β-Sn phase undergoes decompressive amorphization near ˜2 GPa due to the lattice instabilities that give rise to density fluctuations in the crystal upon decompression.

Effect of Co Addition on the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

NASA Astrophysics Data System (ADS)

Maji, Bikas C.; Krishnan, Madangopal; Sujata, M.; Gouthama; Ray, Ranjit K.

2013-01-01

The effect of Co addition has been studied in Fe-30Mn-6Si- xCo ( x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (γ) up to 5 pct Co and beyond that becomes two-phase comprising γ and off-stoichiometric (Fe,Co)5Mn3Si2 intermetallic π-phases. The forward γ-ɛ martensite transformation start temperature ( M S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M S possibly on account of two-phase microstructure. Unlike M S, the ɛ-γ reverse transformation start temperature ( A S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ɛ martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)5Mn3Si2 precipitates within the two-phase microstructure and due to higher amount of stress induced ɛ martensite.

Photoinduced electro-optics measurements of biosilica transformation to cristobalite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fuchs, Ido; Aluma, Yaniv; Ilan, Micha

2015-03-15

In this paper we studied the photoinduced electro optics effects in the thermal transformation process of biosilica to cristobalite, at a relatively low temperature and ambient pressure. This process was characterized by a variety of standards techniques with emphasis on linear electro optic effect measurements. Overall we demonstrated that photoinduced electro optics measurements are very sensitive to the transformation from amorphous structure of silica in the natural sponge samples to laminar string morphology of cristobalite. With this technique we could probe the change in the samples chirality from achiral bio silica to chiral cristobalite structure. Furthermore it is shown thatmore » natural biosilica have photoinduced linear electro optics respond indicating the chiral natural of biosilica. - Graphical abstract: The phase transformation of biosilica from marine sponges to Cristobalite under thermal treatment was investigated using photoinduced electro optics measurements. The figure shows the changes of the electro-optic coefficient of cristobalite and biosilica. - Highlights: • We examine phase transformation of biosilica. • We report transition from amorphous biosilica to crystalline Cristobalite. • Biosilica transformation to Cristobalite at temperature of 850 °C. • Biosilica transformation is studied with photoinduced measurements. • We examine changes in the photoinduced linear electro optics properties.« less

Factors Affecting the Strength and Toughness of Ultra-Low Carbon Steel Weld Metal

DTIC Science & Technology

1999-12-01

ferrite or martensite due to its strength and toughness, respectively. Ferrite with non-aligned second phase is associated with ferrite completely...with interphase carbides and pearlite. It forms at high temperatures and slow cooling rates than ferrite with secondary phase or martensite . It is...therefore termed a diffusionless transformation . In low carbon steels, the transformation occurs from fccy (austenite) to beta’ ( martensite ),

Coupled gamma/alpha phase transformations in low-carbon steels

NASA Astrophysics Data System (ADS)

Mizutani, Yasushi

Since steels have been the most prevalently utilized materials for many years, the desire for steels with low alloying components with a well-balanced combination of high strength and toughness is increasing. Low carbon steels consisting of bainitic microstructures are ideally suited to meeting such technological and economic requirements. Thus it is extremely important to fully clarify the mechanism of bainite formation in order to produce this type of engineering steel by optimized alloy and process design. This research focuses on understanding the mechanism of coupled displacive/diffusional gamma/alpha transformation in low-carbon steels including bainitic and martensitic transformation, and establishing a more comprehensive and physically rational computational model for predictive control of coupled gamma/alpha transformation phenomena. Models for coupled gamma/alpha phase transformation proposed in this study are based on a mechanistic and unified theory and the following assumptions: (1) The energy dissipation due to interface motion can be linearly combined with the energy dissipation due to carbon diffusion. (2) The carbon concentrations at the interface in both gamma and alpha phases are constrained by an interface solute trapping law. (3) Interface motion during nucleation is also governed by the carbon diffusion field velocity. (4) The response function of glissile interface motion can be expressed in the form of thermally activated dislocation glide. In contrast to the conventional semi-empirical models of the previous literature, the computational model proposed in this study is demonstrated to successfully provide a comprehensive and quantitative prediction of the effects of temperature, composition, microstructure, and the interactions among them. This includes the effects of substitutional solutes, morphology of the parent gamma phase, density of nucleation sites, temperature dependent variation of flow stress of matrix, and dynamic recovery of forest dislocations on the kinetics of coupled gamma/alpha phase transformation.

Synthesis, Characterization, and Self-Controlled Orthorhombic to Tetragonal Polymorphic Transformation in BaTiO3 Nanoparticles

NASA Astrophysics Data System (ADS)

Ram, S.; Jana, A.; Kundu, T. K.

The phase formation and thermal-induced phase transformation are studied in BaTiO3 nanoparticles. 2 h of heating a polymer precursor at 550°C in air formed a single phase BaTiO3 of 15 nm average crystallite size D. The X-ray diffraction peaks are analyzed assuming a Pnma orthorhombic (o) crystal structure of lattice parameters a = 0.6435 nm, b = 0.5306 nm, and c = 0.8854 nm. The lattice volume V = 0.3023 nm3, with z = 4 formula units, yields a density ρ = 5.124 g/cm3. This is a new polymorph in comparison to well-known Pm3m tetragonal (t) structure, V = 0.0644 nm3 or ρ = 6.016 g/cm3 (z = 1). An o ↦ t transformation appears on heating at temperature as high as 650°C in air. A proposed model explains the transformation above a certain D value in terms of the Gibbs free energy. Unless heating above 750°C, the two phases coexist in a composite structure (D≤27 nm), with as much residual o-phase trace as ~28 vol%. As a function of temperature both the phases decrease in the V values up to 0.2975 and 0.0643 nm3 at 750°C respectively (0.0650 nm3 at 650°C). This is an important parameter for designing useful ferroelectric and other properties in a hybrid composite structure.

Study of microwave drying of wet materials based on one-dimensional two-phase model

NASA Astrophysics Data System (ADS)

Salomatov, Vl V.; Karelin, V. A.

2017-11-01

Currently, microwave is one of the most interesting ways to conduct drying of dielectric materials, in particular coal. In this paper, two processes were considered - heating and drying. The temperature field of the coal semi-mass in the heating mode is found analytically strictly with the use of integral transformations. The drying process is formulated as a nonlinear Stephen problem with a moving boundary of the liquid-vapor phase transformation. The temperature distribution, speed and drying time in this mode are determined approximately analytically. Parametric analysis of the influence of the material and boundary conditions on the dynamics of warming up and drying is revealed.

Modeling of phase transformations of Ti6Al4 V during laser metal deposition

NASA Astrophysics Data System (ADS)

Suárez, A.; Tobar, M. J.; Yáñez, A.; Pérez, I.; Sampedro, J.; Amigó, V.; Candel, J. J.

The low density, excellent high temperature mechanical properties and good corrosion resistance of titanium and its alloys have led to a diversified range of successful applications. As a consequence, there is a demand of increasing the capabilities of processing such alloys. The laser cladding technique allows direct metal deposition with an excellent metallurgical bond and a pore free fine grained microstructure. A nonlinear transient thermo-metallurgical model was developed to study the technique with titanium alloys to get a better understanding of the thermal and metallurgical underlying aspects. The calculated temperatures and phase transformations are compared with experimental tests.

Initial stages of ion beam-induced phase transformations in Gd2O3 and Lu2O3

NASA Astrophysics Data System (ADS)

Chen, Chien-Hung; Tracy, Cameron L.; Wang, Chenxu; Lang, Maik; Ewing, Rodney C.

2018-02-01

The atomic-scale evolution of lanthanide sesquioxides Gd2O3 and Lu2O3 irradiated with 1 MeV Kr ions at room temperature and 120 K, up to fluences of 1 × 1016 ions/cm2 (˜20 dpa), has been characterized by in situ transmission electron microscopy. At room temperature, both oxides exhibited high radiation tolerance. Irradiation did not cause any observable structural change in either material, likely due to the mobility of irradiation-induced point defects, causing efficient defect annihilation. For Gd2O3, having the larger cation ionic radius of the two materials, an irradiation-induced stacking fault structure appeared at low fluences in the low temperature irradiation. As compared with the cubic-to-monoclinic phase transformations known to result from higher energy (˜GeV) ion irradiation, Kr ions of lower energies (˜MeV) yield much lower rates of damage accumulation and thus less extensive structural modification. At a fluence of 2.5 × 1015 ions/cm2, only the initial stages of the cubic-to-monoclinic (C to B) phase transformation process, consisting of the formation and aggregation of defects, have been observed.

Potential High-Temperature Shape-Memory Alloys Identified in the Ti(Ni,Pt) System

NASA Technical Reports Server (NTRS)

Noebe, Ronald D.; Biles, Tiffany A.; Garg, Anita; Nathal, Michael V.

2004-01-01

"Shape memory" is a unique property of certain alloys that, when deformed (within certain strain limits) at low temperatures, will remember and recover to their original predeformed shape upon heating. It occurs when an alloy is deformed in the low-temperature martensitic phase and is then heated above its transformation temperature back to an austenitic state. As the material passes through this solid-state phase transformation on heating, it also recovers its original shape. This behavior is widely exploited, near room temperature, in commercially available NiTi alloys for connectors, couplings, valves, actuators, stents, and other medical and dental devices. In addition, there are limitless applications in the aerospace, automotive, chemical processing, and many other industries for materials that exhibit this type of shape-memory behavior at higher temperatures. But for high temperatures, there are currently no commercial shape-memory alloys. Although there are significant challenges to the development of high-temperature shape-memory alloys, at the NASA Glenn Research Center we have identified a series of alloy compositions in the Ti-Ni-Pt system that show great promise as potential high-temperature shape-memory materials.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ari-Gur, Pnina; Garlea, Vasile O.; Cao, Huibo

In this study, Heusler alloys of Ni-Mn-Ga compositions demonstrate ferromagnetic shape memory effect in the martensitic state. The transformation temperature and the chemical order depend strongly on the composition. In the current work, the structure and chemical order of the martensitic phase of Ni 1.91Mn 1.29Ga 0.8 were studied using neutron diffraction; the diffraction pattern was refined using the FullProf software. It was determined that the structural transition occurs around 330 K. At room temperature, 300 K, which is below the martensite transformation temperature, all the Bragg reflections can be described by a monoclinic lattice with a symmetry of spacemore » group P 1 2/m 1 and lattice constants of a = 4.23047(7) [Å], b = 5.58333(6) [Å], c = 21.0179(2) [Å], beta = 90.328(1). The chemical order is of critical importance in these alloys, and it was previously studied at 363 K. Analysis of the neutron diffraction in the monoclinic phase shows that the chemical order is maintained during the martensitic transformation.« less

Mechanically controlling the reversible phase transformation from zinc blende to wurtzite in AlN

DOE PAGES

Li, Zhen; Yadav, Satyesh; Chen, Youxing; ...

2017-04-10

III–V and other binary octet semiconductors often take two phase forms—wurtzite (wz) and zinc blende (zb) crystal structures—with distinct functional performance at room temperature. Here, we investigate how to control the synthesized phase structure to either wz or zb phase by tuning the interfacial strain by taking AlN as a representative III–V compound. Furthermore, by applying in situ mechanical tests at atomic scale in a transmission electron microscope, we observed the reversible phase transformation from zb to wz, and characterized the transition path—the collective glide of Shockley partials on every two {111} planes of the zb AlN.

Structural transformation in antiferroelectric PbZrO3-relaxor ferroelectric Pb(Ni1/3Nb2/3)O3 solid solution system

NASA Astrophysics Data System (ADS)

Wirunchit, S.; Vittayakorn, N.

2008-07-01

The solid solution between the antiferroelectric (AFE) PbZrO3 (PZ) and the relaxor ferroelectric (FE) Pb(Ni1/3Nb2/3)O3 (PNN) was synthesized by the columbite precursor method. The crystal structure, phase transformations, and dielectric and thermal properties of (1-x )PZ-xPNN where x =0.00-0.30 were investigated. With these data, the FE phase diagram between PZ and PNN has been established. The crystal structure data obtained from X-ray diffraction indicate that the solid solution PZ-PNN, where x =0.00-0.30, successively transforms from orthorhombic to rhombohedral symmetry with an increase in the PNN concentration. The AFE phase→FE phase transition occurs in compositions of 0.00⩽x⩽0.08. The AFE →FE phase transition shifts to lower temperatures with higher compositions of x. The FE phase temperature range width increases with increased PNN. Apparently the replacement of the Zr4+ ion by Ni2+/Nb5+ ions decreases the driving force for an antiparallel shift of Pb2+ ions because they interrupt the translational symmetry and facilitates the appearance of a rhombohedral FE phase when the amount of PNN is higher than 8mol%.

Theoretical modelling of residual and transformational stresses in SMA composites

NASA Astrophysics Data System (ADS)

Berman, J. B.; White, S. R.

1996-12-01

SMA composites are a class of smart materials in which shape memory alloy (SMA) actuators are embedded in a polymer matrix composite. The difference in thermal expansion between the SMA and the host material leads to residual stresses during processing. Similarly, the SMA transformations from martensite to austenite, or the reverse, also generate stresses. These stresses acting in combination can lead to SMA/epoxy interfacial debonding or microcracking of the composite phase. In this study the residual and transformational stresses are investigated for a nitinol wire embedded in a graphite/epoxy composite. A three-phase micromechanical model is developed. The nitinol wire is assumed to behave as a thermoelastic material. Nitinol austenitic and martensitic transformations are modelled using linear piecewise interpolation of experimental data. The interphase is modelled as a thermoelastic polymer. A transversely isotropic thermoelastic composite is used for the outer phase. Stress-free conditions are assumed immediately before cool down from the cure temperature. The effect of nitinol, coating and composite properties on residual and transformational stresses are evaluated. Fiber architectures favoring the axial direction decrease the magnitude of all residual stresses. A decrease in stresses at the composite/coating interface is also predicted through the use of thick, compliant coatings. Reducing the recovery strain and moving the transformation to higher temperatures were found to be most effective in reducing residual stresses.

Martensitic Transformation in a β-Type Mg-Sc Alloy

NASA Astrophysics Data System (ADS)

Ogawa, Yukiko; Ando, Daisuke; Sutou, Yuji; Somekawa, Hidetoshi; Koike, Junichi

2018-03-01

Recently, we found that a Mg-Sc alloy with a bcc (β) phase exhibits superelasticity and a shape memory effect at low temperature. In this work, we examined the stress-induced and thermally induced martensitic transformation of the β-type Mg-Sc alloy and investigated the crystal structure of the thermally induced martensite phase based on in situ X-ray diffraction (XRD) measurements. The lattice constants of the martensite phase were calculated to be a = 0.3285 nm, b = 0.5544 nm, and c = 0.5223 nm when we assumed that the martensite phase has an orthorhombic structure (Cmcm). Based on the lattice correspondence between a bcc and an orthorhombic structures such as that in the case of β-Ti shape memory alloys, we estimated the transformation strain of the β Mg-Sc alloy. As a result, the transformation strains along the 001, 011, and 111 directions in the β phase were calculated to be + 5.7, + 8.8, and + 3.3%, respectively.

High pressure study of Pu(0.92)Am(0.08) binary alloy.

PubMed

Klosek, V; Griveau, J C; Faure, P; Genestier, C; Baclet, N; Wastin, F

2008-07-09

The phase transitions (by means of x-ray diffraction) and electrical resistivity of a Pu(0.92)Am(0.08) binary alloy were determined under pressure (up to 2 GPa). The evolution of atomic volume with pressure gives detailed information concerning the degree of localization of 5f electronic states and their delocalization process. A quasi-linear V = f(P) dependence reflects subtle modifications of the electronic structure when P increases. The electrical resistivity measurements reveal the very high stability of the δ phase for pressures less than 0.7 GPa, since no martensitic-like transformation occurs at low temperature. Remarkable electronic behaviours have also been observed. Finally, resistivity curves have shown the temperature dependence of the phase transformations together with unexpected kinetic effects.

Equation of state, phase stability, and phase transformations of uranium-6 wt. % niobium under high pressure and temperature

NASA Astrophysics Data System (ADS)

Zhang, Jianzhong; Vogel, Sven; Brown, Donald; Clausen, Bjorn; Hackenberg, Robert

2018-05-01

In-situ time-of-flight neutron diffraction experiments were conducted on the uranium-niobium alloy with 6 wt. % Nb (U-6Nb) at pressures up to 4.7 GPa and temperatures up to 1073 K. Upon static compression at room temperature, the monoclinic structure of U-6Nb (α″ U-6Nb) remains stable up to the highest experimental pressure. Based on the pressure-volume measurements at room temperature, the least-squares fit using the finite-strain equation of state (EOS) yields an isothermal bulk modulus of B0 = 127 ± 2 GPa for the α″-phase of U-6Nb. The calculated zero-pressure bulk sound speed from this EOS is 2.706 ± 0.022 km/s, which is in good agreement with the linear extrapolation of the previous Hugoniot data above 12 GPa for α″ U-6Nb, indicating that the dynamic response under those shock-loading conditions is consistent with the stabilization of the initial monoclinic phase of U-6Nb. Upon heating at ambient and high pressures, the metastable α″ U-6Nb exhibits complex transformation paths leading to the diffusional phase decomposition, which are sensitive to applied pressure, stress state, and temperature-time path. These findings provide new insight into the behavior of atypical systems such as U-Nb and suggest that the different U-Nb phases are separated by rather small energies and hence highly sensitive to compositional, thermal, and mechanical perturbations.

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni2MnGa Compound.

PubMed

Zelený, Martin; Straka, Ladislav; Sozinov, Alexei; Heczko, Oleg

2018-05-08

In order to explain the formation of low-temperature phases in stoichiometric Ni 2 MnGa magnetic shape memory alloy, we investigate the phase transformation paths from cubic austenite with Heusler structure to low-symmetry martensitic structures. We used ab initio calculations combined with the generalized solid state nudged elastic band method to determine the minimum energy path and corresponding changes in crystal lattice. The four-, five-, and seven-layered modulated phases of martensite (4O, 10M, and 14M) are built as the relaxed nanotwinned non-modulated (NM) phase. Despite having a total energy larger than the other martensitic phases, the 10M phase will spontaneously form at 0 K, because there is no energy barrier on the path and the energy decreases with a large negative slope. Moreover, a similar negative slope in the beginning of path is found also for the transformation to the 6M premartensite, which appears as a local minimum on the path leading further to 10M martensite. Transformation paths to other structures exhibit more or less significant barriers in the beginning hindering such a transformation from austenite. These findings correspond to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure.

Pressure-induced structural phase transformation and superconducting properties of titanium mononitride

NASA Astrophysics Data System (ADS)

Li, Qian; Guo, Yanan; Zhang, Miao; Ge, Xinlei

2018-03-01

In this work, we have systematically performed the first-principles structure search on titanium mononitride (TiN) within Crystal Structure AnaLYsis by Particle Swarm Optimization (CALYPSO) methodology at high pressures. Here, we have confirmed a phase transition from cubic rock-salt (fcc) phase to CsCl (bcc) phase of TiN at ∼348 GPa. Further simulations reveal that the bcc phase is dynamically stable, and could be synthesized experimentally in principle. The calculated elastic anisotropy decreases with the phase transformation from fcc to bcc structure under high pressures, and the material changes from ductile to brittle simultaneously. Moreover, we found that both structures are superconductive with the superconducting critical temperature of 2-12 K.

Smart vortex generator transformed by change in ambient temperature and aerodynamic force

NASA Astrophysics Data System (ADS)

Ikeda, Tadashige; Masuda, Shinya; Ueda, Tetsuhiko

2007-04-01

A Smart Vortex Generator (SVG) concept has been proposed, where the SVG is autonomously transformed between an upright vortex-generating position in take-off and landing and a flat drag-reducing position in a cruise. This SVG is made of a Shape Memory Alloy (SMA), which is in the austenite phase and memorizes the upright position at high temperatures of the take-off and landing. At low temperatures during ascent the SVG is transformed into a martensite phase, and it lies flat against a base structure due to external or/and internal forces. In this paper, we examine whether the SVG can be transformed into the drag-reducing position by an aerodynamic force. To this end, numerical simulations are carried out with a simple line element model. The aerodynamic force applied on the SVG is calculated by a commercial CFD program. Result reveals that this SVG can be transformed from the upright vortex-generating position into the drag-reducing position by just an airplane climbing, and vice versa, if the SMA applied to the SVG has the two-way shape memory effect. If the SMA has the one-way shape memory effect, it is necessary to reduce the stiffness of the SVG or/and use a counter spring.

Latent heat contribution to the direct magnetocaloric effect in Ni-Mn-Ga shape memory alloys with coupled martensitic and magnetic transformations

NASA Astrophysics Data System (ADS)

Caballero-Flores, R.; Sánchez-Alarcos, V.; Recarte, V.; Pérez-Landazábal, J. I.; Gómez-Polo, C.

2016-05-01

We report the direct magnetocaloric response of materials that present a second-order phase transition in the temperature range where a first-order structural transition also occurs. In particular, the influence of the latent heat on the field-induced adiabatic temperature change has been analyzed in a Ni-Mn-Ga alloy with coupled martensitic and magnetic transformations. It is found that discrepancies around 20% arise depending on whether the latent heat is taken into account or not. From the observed results, a general expression for the indirect determination of the adiabatic temperature change, that takes into account the contributions of both the martensitic and magnetic transformations, is proposed and experimentally confirmed. The observed key role of the latent heat allows us to understand why materials with first-order transformations do not present adiabatic temperature changes as higher as those which would correspond to materials undergoing second-order transformations with similar isothermal entropy change.

Design of Fatigue Resistant Heusler-strengthened PdTi-based Shape Memory Alloys for Biomedical Applications

NASA Astrophysics Data System (ADS)

Frankel, Dana J.

The development of non-surgical transcatheter aortic valve implantation (TAVI) techniques, which utilize collapsible artificial heart valves with shape memory alloy (SMA)-based frames, pushes performance requirements for biomedical SMAs beyond those for well-established vascular stent applications. Fatigue life for these devices must extend into the ultra-high cycle fatigue (UHCF) regime (>600M cycles) with zero probability of failure predicted at applied strain levels. High rates of Ni-hypersensitivity raise biocompatibility concerns, driving the development of low-Ni and Ni-free SMAs. This work focuses on the development of biocompatible, precipitation-strengthened, fatigue-resistant PdTi-based SMAs for biomedical applications. Functional and structural fatigue are both manifestations of cyclic instability resulting in accumulation of slip and eventual structural damage. While functional fatigue is easily experimentally evaluated, structural fatigue is more difficult to measure without the proper equipment. Therefore, in this work a theoretical approach using a model well validated in steels is utilized to investigate structural fatigue behavior in NiTi in the UHCF regime, while low cycle functional fatigue is evaluated in order to monitor the core phenomena of the cyclic instability. Results from fatigue simulations modeling crack nucleation at non-metallic inclusions in commercial NiTi underscore the importance of increasing yield strength for UHCF performance. Controlled precipitation of nanoscale, low-misfit, L21 Heusler aluminides can provide effective strengthening. Phase relations, precipitation kinetics, transformation temperature, transformation strain, cyclic stability, and mechanical properties are characterized in both Ni-free (Pd,Fe)(Ti,Al) and low-Ni high-strength "hybrid" (Pd,Ni)(Ti,Zr,Al) systems. Atom probe tomography is employed to measure phase compositions and particle sizes used to calibrate LSW models for coarsening kinetics and Gibbs-Thompson models for composition trajectories for systems under evolving unstable equilibrium. Mechanical and thermal cyclic stability are investigated using compression testing and differential scanning calorimetry. Mechanical properties are characterized using room temperature and high temperature Vickers microhardness as well as nanoindentation. A superelastic Ni-free (Pd,Fe)(Ti,Al) alloy with near-ambient transformation temperatures, low hysteresis, a highly stable cyclic response, and reversible transformation strains of 3.2% was designed. Due to Pd softening, the addition of Zr is considered to improve strength in a low-Ni "hybrid" (Pd,Ni)(Ti,Zr,Al) alloy. Aging studies at 600°C result in unusually fast coarsening kinetics, while low-temperature aging studies at 500-530°C reveal the presence of a Zr-rich phase in association with the matrix and Heusler phase. A strengthening study on a nontransforming hybrid prototype shows lower than expected precipitation strengthening at 600°C but significant strengthening when aged at 500°C due to the Zr-rich phase. Transformation temperatures, transformation strain, and cyclic stability are characterized in a set of transforming hybrid prototypes.

Heating- and pressure-induced transformations in amorphous and hexagonal ice: A computer simulation study using the TIP4P/2005 model

NASA Astrophysics Data System (ADS)

Engstler, Justin; Giovambattista, Nicolas

2017-08-01

We characterize the phase behavior of glassy water by performing extensive out-of-equilibrium molecular dynamics simulations using the TIP4P/2005 water model. Specifically, we study (i) the pressure-induced transformations between low-density (LDA) and high-density amorphous ice (HDA), (ii) the pressure-induced amorphization (PIA) of hexagonal ice (Ih), (iii) the heating-induced LDA-to-HDA transformation at high pressures, (iv) the heating-induced HDA-to-LDA transformation at low and negative pressures, (v) the glass transition temperatures of LDA and HDA as a function of pressure, and (vi) the limit of stability of LDA upon isobaric heating and isothermal decompression (at negative pressures). These transformations are studied systematically, over a wide range of temperatures and pressures, allowing us to construct a P-T phase diagram for glassy TIP4P/2005 water. Our results are in qualitative agreement with experimental observations and with the P-T phase diagram obtained for glassy ST2 water that exhibits a liquid-liquid phase transition and critical point. We also discuss the mechanism for PIA of ice Ih and show that this is a two-step process where first, the hydrogen-bond network (HBN) is distorted and then the HBN abruptly collapses. Remarkably, the collapse of the HB in ice Ih occurs when the average molecular orientations order, a measure of the tetrahedrality of the HBN, is of the same order as in LDA, suggesting a common mechanism for the LDA-to-HDA and Ih-to-HDA transformations.

Heating- and pressure-induced transformations in amorphous and hexagonal ice: A computer simulation study using the TIP4P/2005 model.

PubMed

Engstler, Justin; Giovambattista, Nicolas

2017-08-21

We characterize the phase behavior of glassy water by performing extensive out-of-equilibrium molecular dynamics simulations using the TIP4P/2005 water model. Specifically, we study (i) the pressure-induced transformations between low-density (LDA) and high-density amorphous ice (HDA), (ii) the pressure-induced amorphization (PIA) of hexagonal ice (I h ), (iii) the heating-induced LDA-to-HDA transformation at high pressures, (iv) the heating-induced HDA-to-LDA transformation at low and negative pressures, (v) the glass transition temperatures of LDA and HDA as a function of pressure, and (vi) the limit of stability of LDA upon isobaric heating and isothermal decompression (at negative pressures). These transformations are studied systematically, over a wide range of temperatures and pressures, allowing us to construct a P-T phase diagram for glassy TIP4P/2005 water. Our results are in qualitative agreement with experimental observations and with the P-T phase diagram obtained for glassy ST2 water that exhibits a liquid-liquid phase transition and critical point. We also discuss the mechanism for PIA of ice I h and show that this is a two-step process where first, the hydrogen-bond network (HBN) is distorted and then the HBN abruptly collapses. Remarkably, the collapse of the HB in ice I h occurs when the average molecular orientations order, a measure of the tetrahedrality of the HBN, is of the same order as in LDA, suggesting a common mechanism for the LDA-to-HDA and I h -to-HDA transformations.

Experimental Investigation on the Mechanical Instability of Superelastic NiTi Shape Memory Alloy

NASA Astrophysics Data System (ADS)

Xiao, Yao; Zeng, Pan; Lei, Liping

2016-09-01

In this paper, primary attention is paid to the mechanical instability of superelastic NiTi shape memory alloy (SMA) during localized forward transformation at different temperatures. By inhibiting the localized phase transformation, we can obtain the up-down-up mechanical response of NiTi SMA, which is closely related to the intrinsic material softening during localized martensitic transformation. Furthermore, the material parameters of the up-down-up stress-strain curve are extracted, in such a way that this database can be utilized for simulation and validation of the theoretical analysis. It is found that during forward transformation, the upper yield stress, lower yield stress, Maxwell stress, and nucleation stress of NiTi SMA exhibit linear dependence on temperature. The relation between nucleation stress and temperature can be explained by the famous Clausius-Clapeyron equation, while the relation between upper/lower yield stress and temperature lacks theoretical study, which needs further investigation.

Effects of Dopant on Depoling Temperature in Modified BiScO3 - PbTiO3

NASA Technical Reports Server (NTRS)

Kowalski, Benjamin; Sehirlioglu, Alp

2014-01-01

In recent years there has been a renewed interest for high temperature piezoelectrics for both terrestrial and aerospace applications. These applications are limited in part by the operating temperature, which is usually taken as one half of the Curie temperature (Tc), and is 200C for one of the most widely used commercial piezoelectrics, Pb(Zr,Ti)O3 (PZT). In an effort to increase Tc, subsequent research into high temperature Bi(BB)O3 PbTiO3 piezoelectrics led to the discovery of the morphotropic phase boundary (MPB) in the high-Tc BiScO3 PbTiO3 (BS-PT) system with a Tc of 460C and a d33 of 460 pmV. The Tc marks the ferroelectric to paraelectric phase transformation and while, in general, a phase transformation leads to thermal depoling in piezoelectrics with low or moderate Tcs, for high Tc piezoelectrics thermally assisted dipole rotation can lead to randomization of domains at temperatures below Tc. It becomes necessary to determine the depoling temperature (Td) which dictates the actual working temperature range. By doping for Sc and Ti the Td can be shifted while maintaining similar electromechanical properties as a function of temperature. The effect of this B-site doping on depoling temperature has been explored through the characterization of microstructure and weakhigh field measurements.

Low temperature synthesis of nano alpha-alumina powder by two-step hydrolysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Ting; Guo, Xiaode, E-mail: guoxiaodenj@sina.com; Zhang, Xiang

Highlights: • The nano α-Al{sub 2}O{sub 3} with good dispersion was prepared by two-step hydrolysis. • α-Al{sub 2}O{sub 3} powders were added as seed particles in the hydrolysis. • This article indicated that the glucose could impel the γ-Al{sub 2}O{sub 3} transformed to α-Al{sub 2}O{sub 3} directly. • This article indicated that the addictive of α-Al{sub 2}O{sub 3} seed could improve the phase transformation rate of γ-Al{sub 2}O{sub 3} to α-Al{sub 2}O{sub 3}. • In this article, the pure α-Al{sub 2}O{sub 3} could be obtained by calcining at 1000 °C for 1.5 h. - Abstract: The ultral fine alpha-alumina powdermore » has been successfully synthesized via two-step hydrolysis of aluminum isopropoxide. Glucose and polyvinyl pyrrolidone were used as surfactants during the appropriate processing step. The alpha-alumina powder was used as seed particles. Several synthesis parameters, such as the amount of seeds, surfactants, and calcination temperature, were studied by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Thermogravimetry-differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that glucose greatly lower the phase transformation temperature of alpha-alumina by impelling the gamma-alumina transformed to alpha-alumina directly, and the seed could improve the phase transformation rate of alpha-alumina, the polyvinylpyrrolidone have an effect on preventing excessive grain growth and agglomeration of alpha-alumina powder. Comparatively well dispersed alpha-alumina powder with particle size less than 50 nm can be synthesized through this method after calcinations at 1000 °C for 2 h.« less

Phase formation during the carbothermic reduction of eudialyte concentrate

NASA Astrophysics Data System (ADS)

Krasikov, S. A.; Upolovnikova, A. G.; Sitnikova, O. A.; Ponomarenko, A. A.; Agafonov, S. N.; Zhidovinova, S. V.; Maiorov, D. V.

2013-07-01

The phase transformations of eudialyte concentrate during the carbothermic reduction in the temperature range 25-2000°C are studied by thermodynamic simulation, differential thermal analysis, and X-ray diffraction. As the temperature increases to 1500°C, the following phases are found to form sequentially: iron and manganese carbides, free iron, niobium carbide, iron silicides, silicon and titanium carbides, and free silicon. Strontium, yttrium, and uranium in the temperature range under study are not reduced and are retained in an oxide form, and insignificant reduction of zirconium oxides with the formation of carbide ZrC is possible only at temperatures above 1500°C.

Thermal Aging Behavior of Axial Suspension Plasma-Sprayed Yttria-Stabilized Zirconia (YSZ) Thermal Barrier Coatings

NASA Astrophysics Data System (ADS)

Zhao, Yuexing; Wang, Liang; Yang, Jiasheng; Li, Dachuan; Zhong, Xinghua; Zhao, Huayu; Shao, Fang; Tao, Shunyan

2015-02-01

7.5YSZ thermal barrier coatings (TBCs) were deposited onto the stainless steel substrates using axial suspension plasma spraying (ASPS). Free-standing coatings were isothermally aged in air from 1200 to 1600 °C for 24 h and at 1550 °C for 20 to 100 h, respectively. Thermal aging behavior such as phase composition, microstructure evolutions, grain growth, and mechanical properties for thermal-aged coatings were investigated. Results show that the as-sprayed metastable tetragonal (t'-ZrO2) phase decomposes into equilibrium tetragonal (t-ZrO2) and cubic (c-ZrO2) phases during high-temperature exposures. Upon further cooling, the c-ZrO2 may be retained or transform into another metastable tetragonal (t″-ZrO2) phase, and tetragonal → monoclinic phase transformation occurred after 1550 °C/40 h aging treatment. The coating exhibits a unique structure with segmentation cracks and micro/nano-size grains, and the grains grow gradually with increasing aging temperature and time. In addition, the hardness ( H) and Young's modulus ( E) significantly increased as a function of temperature due to healing of pores or cracks and grain growth of the coating. And a nonmonotonic variation is found in the coatings thermal aged at a constant temperature (1550 °C) with prolonged time, this is a synergetic effect of coating sintering and m-ZrO2 phase formation.

Effects of Stoichiometry on Transformation Temperatures and Actuator-Type Performance of NiTiPd and NiTiPdX High-Temperature Shape Memory Alloys

NASA Technical Reports Server (NTRS)

Bigelow, Glen S.; Gaydosh, Darrell; Garg, Anita; Padula, Santo A., II; Noebe, Ronald D.

2007-01-01

High-temperature shape memory NiTiPd and NiTiPdX (X=Au, Pt, Hf) alloys were produced with titanium equivalent (Ti+Hf) compositions of 50.5, 50.0, 49.5, and 49.0 at.%. Thermo-mechanical testing in compression was used to evaluate the transformation temperatures, transformation strain, work output, and permanent deformation behavior of each alloy to study the effects of quaternary alloying and stoichiometry on high-temperature shape memory alloy behavior. Microstructural evaluation showed the presence of second phases for all alloy compositions. No load transformation temperatures in the stoichiometric alloys were relatively unchanged by Au and Pt substitutions, while the substitution of Hf for Ti causes a drop in transformation temperatures. The NiTiPd, NiTiPdAu and NiTiPdHf alloys exhibited transformation temperatures that were highest in the Ti-rich compositions, slightly lower at stoichiometry, and significantly reduced when the Ti equivalent composition was less than 50 at.%. For the NiTiPdPt alloy, transformation temperatures were highest for the Ti-rich compositions, lowest at stoichiometry, and slightly higher in the Ni-rich composition. When thermally cycled under constant stresses of up to 300 MPa, all of the alloys had transformation strains, and therefore work outputs, which increased with increasing stress. In each series of alloys, the transformation strain and thus work output was highest for stoichiometric or Ti-rich compositions while permanent strain associated with the constant-load thermal cycling was lowest for alloys with Ni-equivalent-rich compositions. Based on these results, basic rules for optimizing the composition of NiTiPd alloys for actuator performance will be discussed.

Fabrication and characterization of Si3N4 ceramics without additives by high pressure hot pressing

NASA Technical Reports Server (NTRS)

Shimada, M.; Tanaka, A.; Yamada, T.; Koizumi, M.

1984-01-01

High pressure hot-pressing of Si3N4 without additives was performed using various kinds of Si3N4 powder as starting materials, and the relation between densification and alpha-beta phase transformation was studied. The temperature dependences of Vickers microhardness and fracture toughness were also examined. Densification of Si3N4 was divided into three stages, and it was found that densification and phase transformation of Si3N4 under pressure were closely associated. The results of the temperature dependence of Vickers microhardness indicated that the high-temperature hardness was strongly influenced not only by the density and microstructure of sintered body but also by the purity of starting powder. The fracture toughness values of Si3N4 bodies without additives were 3.29-4.39 MN/m to the 3/2 power and independent of temperature up to 1400 C.

Automation of the temperature elevation test in transformers with insulating oil.

PubMed

Vicente, José Manuel Esteves; Rezek, Angelo José Junqueira; de Almeida, Antonio Tadeu Lyrio; Guimarães, Carlos Alberto Mohallem

2008-01-01

The automation of the temperature elevation test is outlined here for both the oil temperature elevation and the determination of the winding temperature elevation. While automating this test it is necessary to use four thermometers, one three-phase wattmeter, a motorized voltage variator and a Kelvin bridge to measure the resistance. All the equipments must communicate with a microcomputer, which will have the test program implemented. The system to be outlined here was initially implemented in the laboratory and, due to the good results achieved, is already in use in some transformer manufacturing plants.

Relaxation dynamics and transformation kinetics of deeply supercooled water: Temperature, pressure, doping, and proton/deuteron isotope effects.

PubMed

Lemke, Sonja; Handle, Philip H; Plaga, Lucie J; Stern, Josef N; Seidl, Markus; Fuentes-Landete, Violeta; Amann-Winkel, Katrin; Köster, Karsten W; Gainaru, Catalin; Loerting, Thomas; Böhmer, Roland

2017-07-21

Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states.

Relaxation dynamics and transformation kinetics of deeply supercooled water: Temperature, pressure, doping, and proton/deuteron isotope effects

NASA Astrophysics Data System (ADS)

Lemke, Sonja; Handle, Philip H.; Plaga, Lucie J.; Stern, Josef N.; Seidl, Markus; Fuentes-Landete, Violeta; Amann-Winkel, Katrin; Köster, Karsten W.; Gainaru, Catalin; Loerting, Thomas; Böhmer, Roland

2017-07-01

Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states.

Phase transitions in mixed gas hydrates: experimental observations versus calculated data.

PubMed

Schicks, Judith M; Naumann, Rudolf; Erzinger, Jörg; Hester, Keith C; Koh, Carolyn A; Sloan, E Dendy

2006-06-15

This paper presents the phase behavior of multicomponent gas hydrate systems formed from primarily methane with small amounts of ethane and propane. Experimental conditions were typically in a pressure range between 1 and 6 MPa, and the temperature range was between 260 and 290 K. These multicomponent systems have been investigated using a variety of techniques including microscopic observations, Raman spectroscopy, and X-ray diffraction. These techniques, used in combination, allowed for measurement of the hydrate structure and composition, while observing the morphology of the hydrate crystals measured. The hydrate formed immediately below the three-phase line (V-L --> V-L-H) and contained crystals that were both light and dark in appearance. The light crystals, which visually were a single solid phase, showed a spectroscopic indication for the presence of occluded free gas in the hydrate. In contrast, the dark crystals were measured to be structure II (sII) without the presence of these occluded phases. Along with hydrate measurements near the decomposition line, an unexpected transformation process was visually observed at P-T-conditions in the stability field of the hydrates. Larger crystallites transformed into a foamy solid upon cooling over this transition line (between 5 and 10 K below the decomposition temperature). Below the transition line, a mixture of sI and sII was detected. This is the first time that these multicomponent systems have been investigated at these pressure and temperature conditions using both visual and spectroscopic techniques. These techniques enabled us to observe and measure the unexpected transformation process showing coexistence of different gas hydrate phases.

Parameters of Models of Structural Transformations in Alloy Steel Under Welding Thermal Cycle

NASA Astrophysics Data System (ADS)

Kurkin, A. S.; Makarov, E. L.; Kurkin, A. B.; Rubtsov, D. E.; Rubtsov, M. E.

2017-05-01

A mathematical model of structural transformations in an alloy steel under the thermal cycle of multipass welding is suggested for computer implementation. The minimum necessary set of parameters for describing the transformations under heating and cooling is determined. Ferritic-pearlitic, bainitic and martensitic transformations under cooling of a steel are considered. A method for deriving the necessary temperature and time parameters of the model from the chemical composition of the steel is described. Published data are used to derive regression models of the temperature ranges and parameters of transformation kinetics in alloy steels. It is shown that the disadvantages of the active visual methods of analysis of the final phase composition of steels are responsible for inaccuracy and mismatch of published data. The hardness of a specimen, which correlates with some other mechanical properties of the material, is chosen as the most objective and reproducible criterion of the final phase composition. The models developed are checked by a comparative analysis of computational results and experimental data on the hardness of 140 alloy steels after cooling at various rates.

Magnetic and spontaneous Barkhausen noise techniques used in investigation of a martensitic transformation

NASA Astrophysics Data System (ADS)

Capò Sànchez, J.; Huallpa, E.; Farina, P.; Padovese, L. R.; Goldenstein, H.

2011-10-01

Magnetic Barkhausen noise (MBN) was used to characterize the progress of austenite to martensite phase transformation while cooling steel specimens, using a conventional Barkhausen noise emission setup stimulated by an alternating magnetic field. The phase transformation was also followed by electrical resistivity measurements and by optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473 K and cooled to liquid nitrogen temperature presented a clear change near 225 K during cooling, corresponding to the MS (martensite start) temperature, as confirmed by resistivity measurements. Analysis of the resulting signals suggested a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. Spontaneous magnetic noise emission measurements were registered in situ while cooling an initially austenitic sample in liquid nitrogen, showing that local microstructural changes, corresponding to an avalanche or "burst" phenomena, could be detected. This spontaneous magnetic emission (SME) can thus be considered a new experimental tool for the study of martensite transformations in ferrous alloys, at the same level as acoustic emission.

Dynamic Diffraction Studies on the Crystallization, Phase Transformation, and Activation Energies in Anodized Titania Nanotubes

PubMed Central

Albetran, Hani; Vega, Victor

2018-01-01

The influence of calcination time on the phase transformation and crystallization kinetics of anodized titania nanotube arrays was studied using in-situ isothermal and non-isothermal synchrotron radiation diffraction from room temperature to 900 °C. Anatase first crystallized at 400 °C, while rutile crystallized at 550 °C. Isothermal heating of the anodized titania nanotubes by an increase in the calcination time at 400, 450, 500, 550, 600, and 650 °C resulted in a slight reduction in anatase abundance, but an increase in the abundance of rutile because of an anatase-to-rutile transformation. The Avrami equation was used to model the titania crystallization mechanism and the Arrhenius equation was used to estimate the activation energies of the titania phase transformation. Activation energies of 22 (10) kJ/mol for the titanium-to-anatase transformation, and 207 (17) kJ/mol for the anatase-to-rutile transformation were estimated. PMID:29473854

Crystallization Kinetics of Barium and Strontium Aluminosilicate Glasses of Feldspar Composition

NASA Technical Reports Server (NTRS)

Hyatt, Mark J.; Bansal, Narottam P.

1994-01-01

Crystallization kinetics of BaO.Al2O3.2SiO2 (BAS) and SrO.Al2O3.2SiO2 (SAS) glasses in bulk and powder forms have been studied by non-isothermal differential scanning calorimetry (DSC). The crystal growth activation energies were evaluated to be 473 and 451 kJ/mol for bulk samples and 560 and 534 kJ/mol for powder specimens in BAS and SAS glasses, respectively. Development of crystalline phases on thermal treatments of glasses at various temperatures has been followed by powder x-ray diffraction. Powder samples crystallized at lower temperatures than the bulk and the crystallization temperature was lower for SAS glass than BAS. Crystallization in both glasses appeared to be surface nucleated. The high temperature phase hexacelsian, MAl2Si2O8 (M = Ba or Sr), crystallized first by nucleating preferentially on the glass surface. Also, monoclinic celsian does not nucleate directly in the glass, but is formed at higher temperatures from the transformation of the metastable hexagonal phase. In SAS the transformation to monoclinic celsian occurred rapidly after 1 h at 1100 C. In contrast, in BAS this transformation is sluggish and difficult and did not go to completion even after 10 h heat treatment at 1400 C. The crystal growth morphologies in the glasses have been observed by optical microscopy. Some of the physical properties of the two glasses are also reported.

Neutron diffraction study of the martensitic transformation and chemical order in Heusler alloy Ni 1.91Mn 1.29Ga 0.8

DOE PAGES

Ari-Gur, Pnina; Garlea, Vasile O.; Cao, Huibo; ...

2015-11-05

In this study, Heusler alloys of Ni-Mn-Ga compositions demonstrate ferromagnetic shape memory effect in the martensitic state. The transformation temperature and the chemical order depend strongly on the composition. In the current work, the structure and chemical order of the martensitic phase of Ni 1.91Mn 1.29Ga 0.8 were studied using neutron diffraction; the diffraction pattern was refined using the FullProf software. It was determined that the structural transition occurs around 330 K. At room temperature, 300 K, which is below the martensite transformation temperature, all the Bragg reflections can be described by a monoclinic lattice with a symmetry of spacemore » group P 1 2/m 1 and lattice constants of a = 4.23047(7) [Å], b = 5.58333(6) [Å], c = 21.0179(2) [Å], beta = 90.328(1). The chemical order is of critical importance in these alloys, and it was previously studied at 363 K. Analysis of the neutron diffraction in the monoclinic phase shows that the chemical order is maintained during the martensitic transformation.« less

Nanotwin and phase transformation in tetragonal Pb(Fe1/2Nb1/2)1-xTixO3 single crystal

NASA Astrophysics Data System (ADS)

Tu, C.-S.; Tseng, C.-T.; Chien, R. R.; Schmidt, V. Hugo; Hsieh, C.-M.

2008-09-01

This work is a study of phase transformation in (001)-cut Pb(Fe1/2Nb1/2)1-xTixO3 (x =48%) single crystals by means of dielectric permittivity, domain structure, and in situ x-ray diffraction. A first-order T(TNT)-C(TNT) phase transition was observed at the Curie temperature TC≅518 K upon zero-field heating. T, TNT, and C are tetragonal, tetragonal nanotwin, and cubic phases, respectively. T(TNT) and C(TNT) indicate that minor TNT domains reside in the T and C matrices. Nanotwins, which can cause broad diffraction peak, remain above TC≅518 K and give an average microscopic cubic symmetry in the polarizing microscopy. Colossal dielectric permittivity (>104) was observed above room temperature with strong frequency dispersion. This study suggests that nanotwins can play an important role in relaxor ferroelectric crystals while phase transition takes place. The Fe ion is a potential candidate as a B-site dopant for enhancing dielectric permittivity.

Structural transformations in Ge{sub 2}Sb{sub 2}Te{sub 5} under high pressure and temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mio, A. M.; Privitera, S., E-mail: stefania.privitera@imm.cnr.it; D'Arrigo, G.

2015-08-14

The structural transformations occurring in Ge{sub 2}Sb{sub 2}Te{sub 5} films heated at temperature up to 400 °C, and under hydrostatic pressure up to 12 GPa, have been investigated through in-situ X ray diffraction measurements. The adopted experimental conditions are close to those experienced by the phase change material during the SET (crystallization)/RESET (amorphization) processes in a nonvolatile memory device. The compression enhances the thermal stability of the amorphous phase, which remains stable up to 180 °C at 8 GPa and to 230 °C at 12 GPa. The structure of the crystalline phases is also modified, with the formation of a CsCl-type structure instead of rock-salt andmore » of a GeS-type structure at the temperature at which usually the trigonal stable phase is formed. Overall, the stability of the stable phase appears to be more affected by the compression. We argue that the presence of weak bonds associated to the van der Waals gaps is a determining factor for the observed reduced stability.« less

Electric Field-Induced Large Strain in Ni/Sb-co Doped (Bi0.5Na0.5) TiO3-Based Lead-Free Ceramics

NASA Astrophysics Data System (ADS)

Li, Liangliang; Hao, Jigong; Xu, Zhijun; Li, Wei; Chu, Ruiqing

2018-02-01

Lead-free piezoelectric ceramics (Bi0.5Na0.5)0.935Ba0.065Ti1- x (Ni0.5Sb0.5) x O3 (BNBT6.5- xNS) have been fabricated using conventional solid sintering technique. The effect of (Ni, Sb) doping on the phase structure and electrical properties of BNBT6.5 ceramics were systematically investigated. Results show that the addition of (Ni, Sb) destroyed the ferroelectric long-range order of BNBT6.5 and shifted the ferroelectric-relaxor transition temperature ( T F-R) down to room temperature. Thus, this process induced an ergodic relaxor phase at zero field in samples with x = 0.005. Under the electric field, the ergodic relaxor phase could reversibly transform to ferroelectric phase, which promotes the strain response with peak value of 0.38% (at 80 kV/cm, corresponding to d 33 * = 479 pm/V) at x = 0.005. Temperature-dependent measurements of both polarization and strain confirmed that the large strain originated from a reversible field-induced ergodic relaxor to ferroelectric phase transformation. The proposed material exhibits potential for nonlinear actuators.

Potential energy landscape of the apparent first-order phase transition between low-density and high-density amorphous ice.

PubMed

Giovambattista, Nicolas; Sciortino, Francesco; Starr, Francis W; Poole, Peter H

2016-12-14

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics to describe supercooled liquids and glasses. Here we use the PEL formalism and computer simulations to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) at different temperatures. We employ the ST2 water model for which the LDA-HDA transformations are remarkably sharp, similar to what is observed in experiments, and reminiscent of a first-order phase transition. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that low-density liquid (LDL) configurations are located in the same megabasin as LDA, and that high-density liquid (HDL) configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid is different from the path followed by the amorphous solid. At higher pressure, we also study the liquid-to-ice-VII first-order phase transition, and find that the behavior of the PEL properties across this transition is qualitatively similar to the changes found during the LDA-HDA transformation. This similarity supports the interpretation that the LDA-HDA transformation is a first-order phase transition between out-of-equilibrium states. Finally, we compare the PEL properties explored during the LDA-HDA transformations in ST2 water with those reported previously for SPC/E water, for which the LDA-HDA transformations are rather smooth. This comparison illuminates the previous work showing that, at accessible computer times scales, a liquid-liquid phase transition occurs in the case of ST2 water, but not for SPC/E water.

Potential energy landscape of the apparent first-order phase transition between low-density and high-density amorphous ice

NASA Astrophysics Data System (ADS)

Giovambattista, Nicolas; Sciortino, Francesco; Starr, Francis W.; Poole, Peter H.

2016-12-01

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics to describe supercooled liquids and glasses. Here we use the PEL formalism and computer simulations to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) at different temperatures. We employ the ST2 water model for which the LDA-HDA transformations are remarkably sharp, similar to what is observed in experiments, and reminiscent of a first-order phase transition. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that low-density liquid (LDL) configurations are located in the same megabasin as LDA, and that high-density liquid (HDL) configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid is different from the path followed by the amorphous solid. At higher pressure, we also study the liquid-to-ice-VII first-order phase transition, and find that the behavior of the PEL properties across this transition is qualitatively similar to the changes found during the LDA-HDA transformation. This similarity supports the interpretation that the LDA-HDA transformation is a first-order phase transition between out-of-equilibrium states. Finally, we compare the PEL properties explored during the LDA-HDA transformations in ST2 water with those reported previously for SPC/E water, for which the LDA-HDA transformations are rather smooth. This comparison illuminates the previous work showing that, at accessible computer times scales, a liquid-liquid phase transition occurs in the case of ST2 water, but not for SPC/E water.

Transformation temperatures of martensite in beta phase nickel aluminide

NASA Technical Reports Server (NTRS)

Smialek, J. L.; Hehemann, R. F.

1972-01-01

Resistivity and thermal arrest measurements determined that the compositional dependence of Ms (martensite state) temperatures for NiAl martensite was linear between 60 and 69 atomic percent nickel, with Ms = 124 Ni - 7410 K. Resistivity and surface relief experiments indicated the presence of thermoelastic martensite for selected alloys. Some aspects of the transformation were studied by hot stage microscopy and related to the behavior observed for alloys exhibiting the shape-memory effect.

Transformation temperatures of martensite in beta-phase nickel aluminide.

NASA Technical Reports Server (NTRS)

Smialek, J. L.; Hehemann, R. F.

1973-01-01

Resistivity and thermal arrest measurements determined that the compositional dependence of M sub s temperatures for NiAl martensite was linear between 60 and 69 at. % Ni, with M sub s = (124 Ni - 7410)K. Resistivity and surface relief experiments for selected alloys indicated the presence of thermoelastic martensite. Some aspects of the transformation were studied by hot-stage microscopy and related to the behavior observed for alloys exhibiting the shape-memory effect.

Structure and thermal expansion of Lu 2O 3 and Yb 2O 3 up to the melting points

DOE PAGES

Pavlik, Alfred; Ushakov, Sergey V.; Navrotsky, Alexandra; ...

2017-08-24

Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. We studied the structure and thermal expansion of Lu 2O 3 and Yb 2O 3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb 2O 3 and Lumore » 2O 33 with temperature is linear within experimental error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10 -6 K -1 and (7.7 ± 0.6) · 10 -6 K -1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb 2O 3 phase transformation to hexagonal phase prior to melting.« less

Structure and thermal expansion of Lu 2O 3 and Yb 2O 3 up to the melting points

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pavlik, Alfred; Ushakov, Sergey V.; Navrotsky, Alexandra

Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. We studied the structure and thermal expansion of Lu 2O 3 and Yb 2O 3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb 2O 3 and Lumore » 2O 33 with temperature is linear within experimental error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10 -6 K -1 and (7.7 ± 0.6) · 10 -6 K -1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb 2O 3 phase transformation to hexagonal phase prior to melting.« less

Micromechanics of composites with shape memory alloy fibers in uniform thermal fields

NASA Technical Reports Server (NTRS)

Birman, Victor; Saravanos, Dimitris A.; Hopkins, Dale A.

1995-01-01

Analytical procedures are developed for a composite system consisting of shape memory alloy fibers within an elastic matrix subject to uniform temperature fluctuations. Micromechanics for the calculation of the equivalent properties of the composite are presented by extending the multi-cell model to incorporate shape memory alloy fibers. A three phase concentric cylinder model is developed for the analysis of local stresses which includes the fiber, the matrix, and the surrounding homogenized composite. The solution addresses the complexities induced by the nonlinear dependence of the in-situ martensite fraction of the fibers to the local stresses and temperature, and the local stresses developed from interactions between the fibers and matrix during the martensitic and reverse phase transformations. Results are presented for a nitinol/epoxy composite. The applications illustrate the response of the composite in isothermal longitudinal loading and unloading, and in temperature induced actuation. The local stresses developed in the composite under various stages of the martensitic and reverse phase transformation are also shown.

High-coercivity, thermally stable and low unblocking temperature magnetic phase: Implications for Archeomagnetic studies

NASA Astrophysics Data System (ADS)

Hartmann, G. A.; Gallet, Y.; Trindade, R. I.; Genevey, A.; Berquo, T. S.; Neumann, R.; Le Goff, M.

2013-05-01

The thermoremanent magnetization in baked clay archeological materials provide very useful information on the time evolution of the Earth's magnetic field over the past few millennia. In these materials, a thermally stable magnetic phase characterized by high coercivities (>400 mT) and low unblocking temperatures (~200 degrees Celsius) has recently been recognized in European bricks, tiles, kilns and hearth samples. Both the identification and the origin of this phase remain, however, poorly constrained. The very same high-coercivity, thermally stable, low unblocking temperature (HCSLT) magnetic phase has been identified in Brazilian bricks fragments dated of the past five centuries. We report here a large set of measurements on a selected collection of samples showing variable contributions of the HCSLT phase. These measurements include low-field magnetic susceptibility vs. temperature curves, hysteresis loops, isothermal remanent magnetization (IRM) acquisition, thermal demagnetization of the three-axis IRM, first order reversal curves (FORC), low-temperature magnetization experiments (remanent magnetization curves and alternating current susceptibility), Mössbauer spectroscopy and X-ray diffraction. Results show the coexistence of low-coercivity magnetic minerals (magnetite and titanomagnetite) and high-coercivity minerals (hematite, HCSLT phase and, in some cases, goethite). We note that the HCSLT magnetic phase is always found in association with hematite. We further observe that the Mössbauer spectroscopy, X-ray diffraction spectra, and the FORC diagrams are also very similar to results previously obtained from annealed clays in which nontronite or iron-rich montmorillonite was transformed into Al-substituted hematite by heating. The HCSLT magnetic phase is thus confidently identified as being hematite with Al substitution. Moreover, considering the abundance of montmorillonite in clay mining settings, we suggest that the widespread occurrence of HCSLT in archeological materials predominantly originates from the transformation of iron-rich montmorillonite during the manufacturing (heating) process.

Thermal properties and phase transition in the fluoride, (NH{sub 4}){sub 3}SnF{sub 7}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kartashev, A.V.; Astafijev Krasnoyarsk State Pedagogical University, 660049 Krasnoyarsk; Gorev, M.V.

2016-05-15

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH{sub 4}){sub 3}SnF{sub 7} for a wide range of temperatures and pressures. Large entropy (δS{sub 0}=22 J/mol K) and elastic deformation (δ(ΔV/V){sub 0}=0.89%) jumps have proven that the Pa-3↔Pm-3m phase transition is a strong first order structural transformation. A total entropy change of ΔS{sub 0}=32.5 J/mol K is characteristic for the order–disorder phase transition, and is equal to the sum of entropy changes in the related material, (NH{sub 4}){sub 3}TiF{sub 7}, undergoing transformation between the two cubic phases through the intermediate phases. Hydrostatic pressure decreases the stability of the highmore » temperature Pm-3m phase in (NH{sub 4}){sub 3}SnF{sub 7}, contrary to (NH{sub 4}){sub 3}TiF{sub 7}, characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH{sub 4}){sub 3}SnF{sub 7} was observed. - Graphical abstract: Strong first order structural transformation Pa-3↔Pm-3m in (NH{sub 4}){sub 3}SnF{sub 7} is associated with very large total entropy change of ΔS{sub 0}=32.5 J/mol K characteristic for the ordering processes and equal to the sum of entropy changes in the related (NH{sub 4}){sub 3}TiF{sub 7} undergoing transformation between the same two cubic phases through the intermediate phases. - Highlights: • (NH{sub 4}){sub 3}SnF{sub 7} undergoes strong first order Pa-3↔Pm-3m phase transition. • Anomalous behaviour of ΔC{sub p} and ΔV/V exists far below phase transition temperature. • Structural distortions are accompanied by huge total entropy change ΔS≈Rln50. • High pressure strongly increases the stability of Pa-3 phase in (NH{sub 4}){sub 3}SnF{sub 7}. • Entropy of the Pa-3↔Pm-3m phase transition does not depend on pressure.« less

Fundamental Studies of Phase Transformations and Mechanical Properties in the Heat Affected Zone of 10 wt% Nickel Steel

NASA Astrophysics Data System (ADS)

Barrick, Erin J.

United States naval applications require the use of steels with high strength and resistance to fracture at low temperatures to provide good ballistic properties. In recent years, 10 wt% Ni steel has been developed with strength and toughness values exceeding those of steels currently used, and is now being considered as a candidate material to replace existing high-strength, low alloy steels. This steel has excellent toughness from the mechanically induced transformation of interlath austenite films to martensite. These austenite films are formed via a carefully developed quenching, lamellarizing, and tempering heat treatment. However, before 10 wt% Ni steel can be implemented for full-scale applications, the effects of the rapid heating and cooling rates associated with welding thermal cycles on phase transformations and mechanical properties must be understood. In this research, a fundamental understanding of phase transformations and mechanical properties in the heat-affected zone of fusion welds in 10 wt% Ni steel was developed through heating and cooling rate dilatometry experiments, gas tungsten arc welding, and simulation of gas metal arc welding. First, an investigation into the effects of heating and cooling rate on the phase transformations in 10 wt% Ni steel was performed. The Ac1 and Ac3 temperatures during heating were determined as a function of heating rate, and sluggish transformation during fast heating rates manifested itself as a high Ac3 temperature of 1050°C as opposed to a temperature of 850°C at slow heating rates. A continuous cooling transformation diagram produced for 10 wt% Ni steel reveals that martensite will form over a very wide range of cooling rates, which reflects a very high hardenability of this alloy. This is significant because the range of cooling rates for which the diagram was constructed over easily covers the range associated with fusion welding, so there would not be the need for precise control over the weld processing conditions. The microstructures observed in a single pass gas tungsten arc weld were rationalized with the observations from the heating and cooling rate experiments. The microhardness of gas tungsten arc weld is highest in the intercritical heat affected zone, which is unexpected based on the usual behavior of quench and tempered steels. The hardness of the heat affected zone is always higher than the base metal which is a promising outcome. Having understood the overall effects of heating and cooling on the phase transformations in 10 wt% Ni steel, the microstructure and mechanical property evolution through the heat affected zone was investigated. A Gleeble 3500 thermo-mechanical simulator was used to replicate microstructures observed in the gas-tungsten arc weld, and the microstructural factors influencing the strength and toughness in the simulated heat affected zone samples were correlated to mechanical property results. The strength is the highest in the intercritical heat-affected zone, mostly attributed to microstructural refinement. With increasing peak temperature of the thermal cycle, the volume fraction of retained austenite decreases. The local atom probe tomography results suggest this is due to the destabilization of the austenite brought on by the diffusion of Ni out of the austenite. There is a local low toughness region in the intercritical heat-affected zone, corresponding to a low retained austenite content. However, the retained austenite is similarly low in higher peak temperature regions but the toughness is high. This suggests that while 10 wt% Ni steel is a TRIP-assisted steel and thus obtains high toughness from the plasticity-induced martensite to austenite transformation, the toughness of the steel is also based on other microstructural factors. Overall, the results presented in this work have established, for the first time, the effects of rapid heating and cooling on the phase transformations and mechanical properties in 10 wt% Ni steel, and have started to identify the microstructural features influencing the strength and toughness of this alloy.

Phase Stability and Transformations in Vanadium Oxide Nanocrystals

NASA Astrophysics Data System (ADS)

Bergerud, Amy Jo

Vanadium oxides are both fascinating and complex, due in part to the many compounds and phases that can be stabilized as well as the phase transformations which occur between them. The metal to insulator transitions (MITs) that take place in vanadium oxides are particularly interesting for both fundamental and applied study as they can be induced by a variety of stimuli ( i.e., temperature, pressure, doping) and utilized in many applications (i.e., smart windows, sensors, phase change memory). Nanocrystals also tend to demonstrate interesting phase behavior, due in part to the enhanced influence of surface energy on material thermodynamics. Vanadium oxide nanocrystals are thus expected to demonstrate very interesting properties in regard to phase stability and phase transformations, although synthesizing vanadium oxides in nanocrystal form remains a challenge. Vanadium sesquioxide (V2O3) is an example of a material that undergoes a MIT. For decades, the low temperature monoclinic phase and high temperature corundum phase were the only known crystal structures of V2O3. However, in 2011, a new metastable polymorph of V2O3 was reported with a cubic, bixbyite crystal structure. In Chapter 2, a colloidal route to bixbyite V2O 3 nanocrystals is presented. In addition to being one of the first reported observations of the bixbyite phase in V2O3, it is also one of the first successful colloidal syntheses of any of the vanadium oxides. The nanocrystals possess a flower-like morphology, the size and shape of which are dependent on synthesis time and temperature, respectively. An aminolysis reaction mechanism is determined from Fourier transform infrared spectroscopy data and the bixbyite crystal structure is confirmed by Rietveld refinement of X-ray diffraction (XRD) data. Phase stability is assessed in both air and inert environments, confirming the metastable nature of the material. Upon heating in an inert atmosphere above 700°C, the nanocrystals irreversibly transform to the bulk stable corundum phase of V2O3 with concurrent particle coarsening. This, in combination with the enhanced stability of the nanocrystals over bulk, suggests that the bixbyite phase may be stabilized due to surface energy effects, a well-known phenomenon in nanocrystal research. In Chapter 3, the reversible incorporation of oxygen in bixbyite V 2O3 is reported, which can be controlled by varying temperature and oxygen partial pressure. Based on XRD and thermogravimetric analysis, it is found that oxygen occupies interstitial sites in the bixbyite lattice. Two oxygen atoms per unit cell can be incorporated rapidly and with minimal changes to the structure while the addition of three or more oxygen atoms destabilizes the structure, resulting in a phase change that can be reversed upon oxygen removal. Density functional theory (DFT) supports the reversible occupation of interstitial sites in bixbyite by oxygen and the 1.1 eV barrier to oxygen diffusion predicted by DFT matches the activation energy of the oxidation process derived from observations by in situ XRD. The observed rapid oxidation kinetics are thus facilitated by short diffusion paths through the bixbyite nanocrystals. Due to the exceptionally low temperatures of oxidation and reduction, this material, made from earth-abundant atoms, is proposed for use in oxygen storage applications, where oxygen is reversibly stored and released. Further oxidation of bixbyite V2O3 under controlled oxygen partial pressure can lead to the formation of nanocrystalline vanadium dioxide (VO2), a material that is studied for its MIT that occurs at 68 C in the bulk. This transformation is accompanied by a change in crystal structure, from monoclinic to rutile phase, and a change in optical properties, from infrared transparent to infrared blocking. Because of this, VO2 is promising for thermochromic smart window applications, where optical properties vary with temperature. Recently, alternative stimuli have been utilized to trigger MITs in VO2, including electrochemical gating. Rather than inducing the expected monoclinic to rutile phase transition as originally proposed, electrochemical gating of the insulating phase was recently shown to induce oxygen vacancy formation in VO2, thereby inducing metallization, while the characteristic V-V dimerization of the monoclinic phase was retained. In Chapter 4, the preparation and electrochemical reduction of VO2 nanocrystal films is presented. The nanocrystalline morphology allows for the study of transformations under conditions that enhance the gating effect by creating a large VO2-electrolyte interfacial area and by reducing the path length for diffusion. The resulting transitions are observed optically, from insulator to metal to insulator and back, with in situ visible-near infrared spectroelectrochemistry and correlated with structural changes monitored by Raman and X-ray absorption spectroscopies. The never-before-seen transition to an insulating phase under progressive electrochemical reduction is attributed to an oxygen defect induced phase transition to a new phase. This is likely enabled by the nanocrystalline nature of the sample, which may enhance the kinetics of oxygen diffusion, support a higher degree of lattice expansion-induced strain, or simply alter the thermodynamics of the system.

Transformation of multiwall carbon nanotubes to onions with layers cross-linked by sp3 bonds under high pressure and shear deformation

NASA Astrophysics Data System (ADS)

Pankov, A. M.; Bredikhina, A. S.; Kulnitskiy, B. A.; Perezhogin, I. A.; Skryleva, E. A.; Parkhomenko, Yu. N.; Popov, M. Yu.; Blank, V. D.

2017-08-01

A pressure-induced phase transition of multiwall carbon nanotubes (MWNT) to a new structure at room temperature is studied using a shear diamond anvil cell, X-ray photoelectron spectra (XPS), transmission electron microscope (TEM) and Raman procedures. We observe a cardinal pressure-induced change in the nanoparticles shape from multi-shell tubes to multi-shell spheres. MWNT transforms to onions with layers cross-linked by sp3 bonds under the 45-65 GPa compressive stress combined with shear deformation at room temperature. TEM and XPS results show that about 40% of the carbon atoms in the new phase are sp3-bounded.

Low temperature structural transformation in T[Ni(CN){sub 4}].xpyz with x=1,2; T=Mn,Co,Ni,Zn,Cd; pyz=pyrazine

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rodriguez-Hernandez, J.; Instituto de Ciencia y Tecnologia de Materiales, Universidad de La Habana; Lemus-Santana, A.A.

2010-01-15

The materials under study are pillared solids T[Ni(CN){sub 4}].xpyz with one and two (x=1,2) pyrazine (pyz) molecules and where T=Mn, Co, Ni, Zn, Cd. Stimulated by their structural features and potential role as prototype of porous solids for hydrogen storage, the structural stability under cryogenic conditions for this series of pillared solids was studied. At low temperature, in the 100-200 K range, the occurrence of a reversible structural transformation was found. For T=Mn, Co, Zn, Cd, with x=2, the structural transformation was observed to occur around 185 K, and the low temperature phase crystallizes with a monoclinic unit cell (spacemore » group Pc). This structure change results from certain charge redistribution on cooling within the involved ligands. For T=Ni with x=1, both the low and high temperature phases crystallize with unit cells of tetragonal symmetry, within the same space group but with a different unit cell volume. In this case the structure change is observed around 120 K. Above that temperature the rotational states for the pyrazine molecule are thermally excited and all the pyrazine molecules in the structure become equivalent. Under this condition the material structure is described using a smaller structural unit. The structural study using X-ray powder diffraction data was complemented with calorimetric and Raman spectroscopy measurements. For the low temperature phases the crystal structures were solved from Patterson methods and then refined using the Rietveld method. - Graphical abstract: Low temperature ordered structure for pyrazine in T[Ni(CN){sub 4}].pyz.« less

Reducing the Schottky barrier between few-layer MoTe2 and gold

NASA Astrophysics Data System (ADS)

Qi, Dianyu; Wang, Qixing; Han, Cheng; Jiang, Jizhou; Zheng, Yujie; Chen, Wei; Zhang, Wenjing; Thye Shen Wee, Andrew

2017-12-01

Schottky barriers greatly influence the performance of optoelectronic devices. Schottky barriers can be reduced by harnessing the polymorphism of 2D metal transition dichalcogenides, since both semiconducting and metallic phases exist. However, high energy, high temperature or chemicals are normally required for phase transformation, or the processes are complex. In this work, stable low-resistance contacts between few layer MoTe2 flakes and gold electrodes are achieved by a simple thermal annealing treatment at low temperature (200-400 °C). The resulting Schottky barrier height of the annealed MoTe2/Au interface is low (~23 meV). A new Raman A g mode of the 1T‧ metallic phase of MoTe2 on gold electrode is observed, indicating that the low-resistance contact is due to the phase transition of 2H-MoTe2. The gold substrate plays an important role in the transformation, and a higher gold surface roughness increases the transformation rate. With this method, the mobility and ON-state current of the MoTe2 transistor increase by ~3-4 orders of magnitude, the photocurrent of vertically stacked graphene/MoTe2/Au device increases ~300%, and the response time decreases by ~20%.

Influence of the chemical composition of rapidly quenched amorphous alloys (Ni, Fe, Cr)-B-Si on its crystallization process

NASA Astrophysics Data System (ADS)

Elmanov, G.; Dzhumaev, P.; Ivanitskaya, E.; Skrytnyi, V.; Ruslanov, A.

2016-04-01

This paper presents results of research of the structure and phase transformations during the multistage crystallization of the metallic glasses with the compositions Ni71,5Cr6,8Fe2,7B11,9Si7,1 and Ni63,4Cr7,4Fe4,3Mn0,8B15,6Si8,5 labeled as AWS BNi-2 according to American Welding Society. Differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX) were used as experimental research methods. The influence of the alloys chemical composition (boron, manganese and iron) on the temperatures and the exothermic heat effects of phase transformations, as well as on the phase composition of alloys at three stages of crystallization was analyzed. We present a thermodynamic explanation of the observed heat effects. It has been shown that manganese has the main influence on the phase transformations temperatures and heat effects in these two alloys. It is also assumed that at the final crystallization stage simultaneously with the formation of phases Ni3B and β1-Ni3Si should occur the nucleation of borides of CrB type with high Cr and low Si content.

Phase change thermal energy storage material

DOEpatents

Benson, David K.; Burrows, Richard W.

1987-01-01

A thermal energy storge composition is disclosed. The composition comprises a non-chloride hydrate having a phase change transition temperature in the range of 70.degree.-95.degree. F. and a latent heat of transformation of at least about 35 calories/gram.

The Effect of Secondary Phases and Birefringence on Visible Light Transmission in Translucent alpha-Sialon Ceramics

DTIC Science & Technology

2016-07-06

lenses / High Temperature Tubes and Glass a-Si3N4 6 ≈2.07 77.1 Oxygen Barrier / Passivation / Dielectric Layer in Semiconductor Devices and...1987). 77A. Rosenflanz and I.-W. Chen, "Kinetics of Phase Transformations in SiAlON Ceramics: Effects of Cation Size, Composition and Temperature ," J... oxygen content, neither smelled of ammonia anymore, suggesting that oxidation of the powders at room temperature had become kinetically limited. For

DOE Office of Scientific and Technical Information (OSTI.GOV)

X Li; Y Mao; H Ma

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2-3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at highmore » temperature (120 C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.« less

Magneto-optical spectroscopy of ferromagnetic shape-memory Ni-Mn-Ga alloy

NASA Astrophysics Data System (ADS)

Veis, M.; Beran, L.; Zahradnik, M.; Antos, R.; Straka, L.; Kopecek, J.; Fekete, L.; Heczko, O.

2014-05-01

Magneto-optical properties of single crystal of Ni50.1Mn28.4Ga21.5 magnetic shape memory alloy in martensite and austenite phase were systematically studied. Crystal orientation was approximately along {100} planes of parent cubic austenite. At room temperature, the sample was in modulated 10M martensite phase and transformed to cubic austenite at 323 K. Spectral dependence of polar magneto-optical Kerr effect was obtained by generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.2 to 4 eV, and from room temperature to temperature above the Curie point. The Kerr rotation spectra exhibit prominent features typical for complexes containing Mn atoms. Significant spectral changes during transformation to austenite can be explained by different optical properties caused by changes in density of states near the Fermi energy.

The Measurement and Interpretation of Transformation Temperatures in Nitinol

NASA Astrophysics Data System (ADS)

Duerig, T. W.; Pelton, A. R.; Bhattacharya, K.

2017-12-01

A previous paper (Duerig and Bhattacharya in Shap Mem Superelasticity 1:153-161, 2015) introduced several engineering considerations surrounding the R-phase in Nitinol and highlighted a common, if not pervasive, misconception regarding the use of the term Af by the medical device industry. This paper brings additional data to bear on the issue and proposes more accurate terminology. Moreover, a variety of tools are used to establish the forward and reverse stress-temperature phase diagrams for a superelastic wire typical of that used in medical devices. Once established, the two most common methods of measuring transformation temperatures, Differential Scanning Calorimetry and Bend Free Recovery, are tested against the observed behavior. Light is also shed upon the origin of the Clausius-Clapeyron ratio (d σ/d T), the triple point, and why such large variations are reported in superelastic alloys.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luňáček, J., E-mail: jiri.lunacek@vsb.cz

The present paper is devoted to detailed study of the magnetically separable sorbents based on a cerium dioxide/iron oxide composite annealed at temperatures T{sub a} = 773 K, 873 K, and 973 K. The X-ray diffraction and high resolution transmission electron microscopy are used to determine the phase composition and microstructure morphology. Mössbauer spectroscopy at room (300 K) and low (5 K) temperatures has contributed to more exact identification of iron oxides and their transformations Fe{sub 3}O{sub 4} → γ-Fe{sub 2}O{sub 3} (ε-Fe{sub 2}O{sub 3}) → α-Fe{sub 2}O{sub 3} in dependence on calcination temperature. Different iron oxide phase compositions andmore » grain size distributions influence the magnetic characteristics determined from the room- and low-temperature hysteresis loop measurements. The results are supported by zero-field-cooled and field-cooled magnetization measurements allowing a quantitative estimation of the grain size distribution and its effect on the iron oxide transformations. - Highlights: •Magnetically separable sorbents based on a CeO{sub 2}/Fe{sub 2}O{sub 3} composite were investigated. •Microstructure of sorbents was determined by XRD, TEM and Mössbauer spectroscopy. •Magnetic properties were studied by hysteresis loops at room- and low-temperatures. •Phase transitions of iron oxides with increasing annealing temperature are observed.« less

Evaluation of stress-induced martensite phase in ferromagnetic shape memory alloy Fe-30.2at%Pd by non-destructive Barkhausen noise

NASA Astrophysics Data System (ADS)

Furuya, Yasubumi; Okazaki, Teiko; Ueno, Takasi; Spearing, Mark; Wutting, Manfred

2005-05-01

Barkhausen noise (BHN) method seems a useful tecnique to non-destructive evaluation of martensite phase transformation of ferromagnetic shape memory alloy, which is used as the filler of our proposing "Smart Composite Board". The concept of design for "Smart Composite Board" which can combine the non-destructive magnetic inspection and shape recovery function in the material itself was formerly proposed. In the present study, we survey the possibility of Barkhausen noise (BHN) method to detect the transformation of microscopic martensite phase caused by stress-loading in Fe-30.2at%Pd thin foil, which has a stable austenite phase (fcc structure) at room temperature. The BHN voltage was measured at loading stress up to 100 MPa in temperature range of 300K to 373K. Stress-induced martensite twin was observed by laser microscope above loading stress of 25 MPa. A phase transformation caused by loading stress were analyzed also by X-ray diffraction. The signals of BHN are analyzed by the time of magnetization and the noise frequency. BHN caused by grain boundaries appears in the lower frequency range (1kHz-3kHz) and BHN by martensite twin in the higher frequency range (8kHz-10kHz). The envelope of the BHN voltage as a function of time of magnetization shows a peak due to austenite phase at weak magnetic field. The BHN envelope due to martensite twins creates additional two peaks at intermediate magnetic field. BHN method turns out to be a powerful technique for non-destructive evaluation of the phase transformation of ferromagnetic shape memory alloy.

In situ optical microscopy of the martensitic phase transformation of lithium

NASA Astrophysics Data System (ADS)

Krystian, M.; Pichl, W.

2000-12-01

The phase transformation of lithium was investigated by in situ optical microscopy in a helium cryostat. The martensite microstructure is composed of irregular segments which grow in rapid bursts from many nuclei to a final size of 10 to 20 μm and then are immobilized. A major part of the segments is arranged in groups of parallel lamellas. A theoretical consideration of lattice compatibility predicts the existence of an almost perfectly coherent habit-plane interface between bcc and 9R in lithium. Therefore, the irregular microstructure is interpreted by the presence of the disordered polytype phase. Comparison with an earlier investigation in comparably impure lithium indicates a strong influence of impurities on the transformation mechanism. The connections between the low-temperature phase diagram, the geometrical compatibility condition, and the martensite microstructure are discussed.

Size-Controlled AgI/Ag Heteronanowires in Highly Ordered Alumina Membranes: Superionic Phase Stabilization and Conductivity.

PubMed

Zhang, Hemin; Tsuchiya, Takashi; Liang, Changhao; Terabe, Kazuya

2015-08-12

Nanoscaled ionic conductors are crucial for future nanodevices. A well-known ionic conductor, AgI, exhibited conductivity greater than 1 Ω(-1) cm(-1) in α-phase and transformed into poorly conducting β-/γ-phase below 147 °C, thereby limiting applications. Here, we report that transition temperatures both from the β-/γ- to α-phase (Tc↑) and the α- to β-/γ-phase (Tc↓) are tuned by AgI/Ag heteronanowires embedded in anodic aluminum oxide (AAO) membranes with 10-30 nm pores. Tc↑ and Tc↓ shift to correspondingly higher and lower temperature as pore size decreases, generating a progressively enlarged thermal hysteresis. Tc↑ and Tc↓ specifically achieve 185 and 52 °C in 10 nm pores, and the final survived conductivity reaches ∼8.3 × 10(-3) Ω(-1) cm(-1) at room temperature. Moreover, the low-temperature stabilizing α-phase (down to 21 °C, the lowest in state of the art temperatures) is reproducible and survives further thermal cycling. The low-temperature phase stabilization and enhancement conductivity reported here suggest promising applications in silver-ion-based future nanodevices.

Characterization of a New Phase and Its Effect on the Work Characteristics of a Near-Stoichiometric Ni30Pt20Ti50 High-Temperature Shape Memory Alloy (HTSMA)

NASA Technical Reports Server (NTRS)

Garg, A.; Gaydosh, D.; Noebe, R.D.; Padula II, Santo; Bigelow, G.S.; Kaufman, M.; Kovarik, L.; Mills, M.J.; Diercks, D.; McMurray, S.

2008-01-01

A new phase observed in a nominal Ni30Pt20Ti50 (at.%) high temperature shape memory alloy has been characterized using transmission electron microscopy and 3-D atom probe tomography. This phase forms homogeneously in the B2 austenite matrix by a nucleation and growth mechanism and results in a concomitant increase in the martensitic transformation temperature of the base alloy. Although the structure of this phase typically contains a high density of faults making characterization difficult, it appears to be trigonal (-3m point group) with a(sub o) approx. 1.28 nm and c(sub o) approx. 1.4 nm. Precipitation of this phase increases the microhardness of the alloy substantially over that of the solution treated and quenched single-phase material. The effect of precipitation strengthening on the work characteristics of the alloy has been explored through load-biased strain-temperature testing in the solution-treated condition and after aging at 500 C for times ranging from 1 to 256 hours. Work output was found to increase in the aged alloy as a result of an increase in transformation strain, but was not very sensitive to aging time. The amount of permanent deformation that occurred during thermal cycling under load was small but increased with increasing aging time and stress. Nevertheless, the dimensional stability of the alloy at short aging times (1-4 hours) was still very good making it a potentially useful material for high-temperature actuator applications.

Dehydration and dehydroxylation of C-S-H phases synthesized on silicon wafers

NASA Astrophysics Data System (ADS)

Giraudo, Nicolas; Bergdolt, Samuel; Laye, Fabrice; Krolla, Peter; Lahann, Joerg; Thissen, Peter

2018-03-01

In this work, the synthesis of specific ultrathin Calcium-Silicate-Hydrate (C-S-H) phases on silicon wafers and their transformation into C-S phases is achieved. Specific mineral phases are identified, and the synthesis is successful controlled. Samples are investigated by means of Fourier Transform Infrared (FTIR) spectroscopy and X-ray Diffraction (XRD) and the results are analyzed based on first-principles calculations. When C-S-H phases are transformed into C-S phases, only a few reflexes are detected on XRD, and the coherent scattering domains decrease with the increment of the temperature and time of exposure. This behavior is explained by the Ca/Si changes, which are identified by changes in the FTIR spectra. A thermodynamic analysis is performed with the help of first-principles calculations to underline the influence of the calcium-to-silicon (Ca/Si) ratio in the process of dehydroxylation. To increase the Ca/Si ratio water is partially substituted by methanol at the synthesis. This is observed in the FTIR spectra and is confirmed by lower temperatures of dehydroxylation. The catalytic nature of calcium towards the dehydroxylation is confirmed. The core of this work lies in the preparation of a model, which perfection makes possible to model reactivity, stability and mechanical properties using first-principles calculations, and is the starting point for the synthesis of many others.

Experimental heat treatment of silcrete implies analogical reasoning in the Middle Stone Age.

PubMed

Wadley, Lyn; Prinsloo, Linda C

2014-05-01

Siliceous rocks that were not heated to high temperatures during their geological formation display improved knapping qualities when they are subjected to controlled heating. Experimental heat treatment of South African silcrete, using open fires of the kind used during the Middle Stone Age, shows that the process needed careful management, notwithstanding recent arguments to the contrary. Silcrete blocks fractured when heated on the surface of open fires or on coal beds, but were heated without mishap when buried in sand below a fire. Three silcrete samples, a control, a block heated underground with maximum temperature between 400 and 500 °C and a block heated in an open fire with maximum temperature between 700 and 800 °C, were analysed with X-ray powder diffraction (XRD), X-ray fluorescence (XRF), optical microscopy, and both Fourier transform infrared (FTIR) and Raman spectroscopy. The results show that the volume expansion during the thermally induced α- to β-quartz phase transformation and the volume contraction during cooling play a major role in the heat treatment of silcrete. Rapid heating or cooling through the phase transformation at 573 °C will cause fracture of the silcrete. Successful heat treatment requires controlling surface fire temperatures in order to obtain the appropriate underground temperatures to stay below the quartz inversion temperature. Heat treatment of rocks is a transformative technology that requires skilled use of fire. This process involves analogical reasoning, which is an attribute of complex cognition. Copyright © 2014 Elsevier Ltd. All rights reserved.

Quench-age method for the fabrication of niobium-aluminum superconductors

DOEpatents

Pickus, Milton R.; Ciardella, Robert L.

1978-01-01

A flexible Nb.sub.3 Al superconducting wire is fabricated from a niobium-aluminum composite wire by heating to form a solid solution which is retained at room temperature as a metastable solid solution by quenching. The metastable solid solution is then transformed to the stable superconducting A-15 phase by low temperature aging. The transformation induced by aging can be controlled to yield either a multifilamentary or a solid A-15 core surrounded by ductile niobium.

Stable Eutectoid Transformation in Nodular Cast Iron: Modeling and Validation

NASA Astrophysics Data System (ADS)

Carazo, Fernando D.; Dardati, Patricia M.; Celentano, Diego J.; Godoy, Luis A.

2017-01-01

This paper presents a new microstructural model of the stable eutectoid transformation in a spheroidal cast iron. The model takes into account the nucleation and growth of ferrite grains and the growth of graphite spheroids. Different laws are assumed for the growth of both phases during and below the intercritical stable eutectoid. At a microstructural level, the initial conditions for the phase transformations are obtained from the microstructural simulation of solidification of the material, which considers the divorced eutectic and the subsequent growth of graphite spheroids up to the initiation of the stable eutectoid transformation. The temperature field is obtained by solving the energy equation by means of finite elements. The microstructural (phase change) and macrostructural (energy balance) models are coupled by a sequential multiscale procedure. Experimental validation of the model is achieved by comparison with measured values of fractions and radius of 2D view of ferrite grains. Agreement with such experiments indicates that the present model is capable of predicting ferrite phase fraction and grain size with reasonable accuracy.

Phase transitions of amorphous solid acetone in confined geometry investigated by reflection absorption infrared spectroscopy.

PubMed

Shin, Sunghwan; Kang, Hani; Kim, Jun Soo; Kang, Heon

2014-11-26

We investigated the phase transformations of amorphous solid acetone under confined geometry by preparing acetone films trapped in amorphous solid water (ASW) or CCl4. Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) were used to monitor the phase changes of the acetone sample with increasing temperature. An acetone film trapped in ASW shows an abrupt change in the RAIRS features of the acetone vibrational bands during heating from 80 to 100 K, which indicates the transformation of amorphous solid acetone to a molecularly aligned crystalline phase. Further heating of the sample to 140 K produces an isotropic solid phase, and eventually a fluid phase near 157 K, at which the acetone sample is probably trapped in a pressurized, superheated condition inside the ASW matrix. Inside a CCl4 matrix, amorphous solid acetone crystallizes into a different, isotropic structure at ca. 90 K. We propose that the molecularly aligned crystalline phase formed in ASW is created by heterogeneous nucleation at the acetone-water interface, with resultant crystal growth, whereas the isotropic crystalline phase in CCl4 is formed by homogeneous crystal growth starting from the bulk region of the acetone sample.

RH-temperature phase diagrams of hydrate forming deliquescent crystalline ingredients.

PubMed

Allan, Matthew; Mauer, Lisa J

2017-12-01

Several common deliquescent crystalline food ingredients (including glucose and citric acid) are capable of forming crystal hydrate structures. The propensity of such crystals to hydrate/dehydrate or deliquesce is dependent on the environmental temperature and relative humidity (RH). As an anhydrous crystal converts to a crystal hydrate, water molecules internalize into the crystal structure resulting in different physical properties. Deliquescence is a solid-to-solution phase transformation. RH-temperature phase diagrams of the food ingredients alpha-d-glucose and citric acid, along with sodium sulfate, were produced using established and newly developed methods. Each phase diagram included hydrate and anhydrate deliquescence boundaries, the anhydrate-hydrate phase boundary, and the peritectic temperature (above which the hydrate was no longer stable). This is the first report of RH-temperature phase diagrams of glucose and citric acid, information which is beneficial for selecting storage and processing conditions to promote or avoid hydrate formation or loss and/or deliquescence. Copyright © 2017 Elsevier Ltd. All rights reserved.

Numerical Simulation of the Freeze-Thaw Behavior of Mortar Containing Deicing Salt Solution

PubMed Central

Esmaeeli, Hadi S.; Farnam, Yaghoob; Bentz, Dale P.; Zavattieri, Pablo D.; Weiss, Jason

2016-01-01

This paper presents a one-dimensional finite difference model that is developed to describe the freeze-thaw behavior of an air-entrained mortar containing deicing salt solution. A phenomenological model is used to predict the temperature and the heat flow for mortar specimens during cooling and heating. Phase transformations associated with the freezing/melting of water/ice or transition of the eutectic solution from liquid to solid are included in this phenomenological model. The lever rule is used to calculate the quantity of solution that undergoes the phase transformation, thereby simulating the energy released/absorbed during phase transformation. Undercooling and pore size effects are considered in the numerical model. To investigate the effect of pore size distribution, this distribution is considered using the Gibbs-Thomson equation in a saturated mortar specimen. For an air-entrained mortar, the impact of considering pore size (and curvature) on freezing was relatively insignificant; however the impact of pore size is much more significant during melting. The fluid inside pores smaller than 5 nm (i.e., gel pores) has a relatively small contribution in the macroscopic freeze-thaw behavior of mortar specimens within the temperature range used in this study (i.e., +24 °C to −35 °C), and can therefore be neglected for the macroscopic freeze-thaw simulations. A heat sink term is utilized to simulate the heat dissipation during phase transformations. Data from experiments performed using a low-temperature longitudinal guarded comparative calorimeter (LGCC) on mortar specimens fully saturated with various concentration NaCl solutions or partially saturated with water is compared to the numerical results and a promising agreement is generally obtained. PMID:28082830

Numerical Simulation of the Freeze-Thaw Behavior of Mortar Containing Deicing Salt Solution.

PubMed

Esmaeeli, Hadi S; Farnam, Yaghoob; Bentz, Dale P; Zavattieri, Pablo D; Weiss, Jason

2017-02-01

This paper presents a one-dimensional finite difference model that is developed to describe the freeze-thaw behavior of an air-entrained mortar containing deicing salt solution. A phenomenological model is used to predict the temperature and the heat flow for mortar specimens during cooling and heating. Phase transformations associated with the freezing/melting of water/ice or transition of the eutectic solution from liquid to solid are included in this phenomenological model. The lever rule is used to calculate the quantity of solution that undergoes the phase transformation, thereby simulating the energy released/absorbed during phase transformation. Undercooling and pore size effects are considered in the numerical model. To investigate the effect of pore size distribution, this distribution is considered using the Gibbs-Thomson equation in a saturated mortar specimen. For an air-entrained mortar, the impact of considering pore size (and curvature) on freezing was relatively insignificant; however the impact of pore size is much more significant during melting. The fluid inside pores smaller than 5 nm (i.e., gel pores) has a relatively small contribution in the macroscopic freeze-thaw behavior of mortar specimens within the temperature range used in this study (i.e., +24 °C to -35 °C), and can therefore be neglected for the macroscopic freeze-thaw simulations. A heat sink term is utilized to simulate the heat dissipation during phase transformations. Data from experiments performed using a low-temperature longitudinal guarded comparative calorimeter (LGCC) on mortar specimens fully saturated with various concentration NaCl solutions or partially saturated with water is compared to the numerical results and a promising agreement is generally obtained.

Thermal stability and phase transformation in fully indium oxide (InO{sub 1.5}) stabilized zirconia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Piva, R.H., E-mail: honorato.piva@ua.pt; Piva, D.H

2017-01-15

Indium oxide (InO{sub 1.5}) stabilized zirconia (InSZ) is an attractive material as electrolyte, or electrode, in solid oxide fuel cells (SOFCs), and as corrosion resistant top coat in thermal barrier coatings. However, little is known about the phase stability of cubic InSZ at temperatures that simulate the conditions in an operating SOFC or turbine. This article provides an investigation of the phase stability and phase transformations in cubic InSZ after heat treatments at 800, 1000, and 1200 °C for periods up to 2000 h. The results revealed that cubic InSZ is not stable during annealing at 1000 and 1200 °C,more » owing to a fast destabilization of the initial cubic phase to tetragonal, and eventually to monoclinic (c → t → m). The c → t → m transition in InSZ is intimately associated with the indium volatilization. On the other hand, cubic InSZ remained stable for 2000 h at 800 °C, although the partial formation of the tetragonal phase was observed along with a 0.25% contraction in the unit cell volume of the cubic phase, caused by short-range ordering. These results demonstrate that technological applications of cubic InSZ are restricted to temperatures at which the volatilization of the InO{sub 1.5} stabilizer does not occur. - Highlights: •Phase stability of fully InO{sub 1.5} stabilized zirconia (cubic InSZ) was evaluated. •Cubic InSZ is instable at temperatures ≥ 1000 °C, owing to the cubic-to-tetragonal-to-monoclinic destabilization. •Cubic InSZ undergoes the cubic-to-tetragonal transformation at ~ 800 °C. •Owing to the low phase stability, applications of cubic InSZ in TBCs or SOFCs are restricted.« less

Two-phase transformation of lepidocrocite to maghemite

NASA Astrophysics Data System (ADS)

Dekkers, M. J.; Gapeev, A. K.; Gendler, T. S.; Gribov, S. K.; Shcherbakov, V. P.

2003-04-01

A detailed investigation of CRM acquired at different stages of the transformation lepidocrocite -> maghemite -> hematite is carried out. Apparently, at least two-stage lepidocrocite maghemite transformation was revealed from: a) the two-peak Ms(T) curve; b) the observation of constricted hysteresis loops appearing after annealing fresh lepidocrocite samples at elevated temperatures; c) continuous monitoring (for 500 hrs) of CRM acquisition at elevated temperatures. For the latter two sets of CRM acquisition experiments at 12 temperatures from 175C to 550C in the presence of 0.1 mT magnetic field were performed: 1) with fine dispersed natural lepidocrocite grains in a kaolin matrix (about 1 volume % of lepidocrocite), 2) for lepidocrocite peaces 3x3x3 mm in size. In both cases the CRM was detected already at 175C after 1 day of annealing. Note that this temperature is lower than the temperature of the TGA peak of the lepidocrocite -> maghemite transformation. Mossbauer spectra obtained from the peaces after annealing at 225C during 6 and 14 hours, respectively, revealed significantly different patterns. Unexpectadly, fine dispersed maghemite grains formed due the lepidocrocite dehydration in the first peace (6 hrs of annealing) occurred to be more ordered than those of from the second peace. The samples are subjected to the X-ray analysis in an attempt to clarify the observed difference. The observed phenomena can be explained by the two-phase conception of the transformation lepidocrocite -> maghemite. First the precipitation of small superparamagnetic particles of maghemite takes place growing with time. Second, these grains coalesce with each other resulting in appearance of the antiphase boundaries decreasing the susceptibility, slowing down the process of CRM acquisition and generating the constricted hysteresis loops. The work is supported by INTAS 99-1273.

In-situ determination of austenite and martensite formation in 13Cr6Ni2Mo supermartensitic stainless steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bojack, A., E-mail: a.bojack@tudelft.nl; Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft; Zhao, L.

2012-09-15

In-situ analysis of the phase transformations in a 13Cr6Ni2Mo supermartensitic stainless steel (X2CrNiMoV13-5-2) was carried out using a thermo-magnetic technique, dilatometry and high temperature X-ray diffractometry (HT-XRD). A combination of the results obtained by the three applied techniques gives a valuable insight in the phase transformations during the austenitization treatment, including subsequent cooling, of the 13Cr6Ni2Mo supermartensitic stainless steel, where the magnetic technique offers a high accuracy in monitoring the austenite fraction. It was found by dilatometry that the austenite formation during heating takes place in two stages, most likely caused by partitioning of Ni into austenite. The in-situ evolutionmore » of the austenite fraction is monitored by high-temperature XRD and dilatometry. The progress of martensite formation during cooling was described with a Koistinen-Marburger relation for the results obtained from the magnetic and dilatometer experiments. Enhanced martensite formation at the sample surface was detected by X-ray diffraction, which is assumed to be due to relaxation of transformation stresses at the sample surface. Due to the high alloy content and high thermodynamic stability of austenite at room temperature, 4 vol.% of austenite was found to be stable at room temperature after the austenitization treatment. - Highlights: Black-Right-Pointing-Pointer We in-situ analyzed phase transformations and fractions of a 13Cr6Ni2Mo SMSS. Black-Right-Pointing-Pointer Higher accuracy of the austenite fraction was obtained from magnetic technique. Black-Right-Pointing-Pointer Austenite formation during heating takes place in two stages. Black-Right-Pointing-Pointer Enhanced martensite formation at the sample surface detected by X-ray diffraction.« less

Construction of continuous cooling transformation (CCT) diagram using Gleeble for coarse grained heat affected zone of SA106 grade B steel

NASA Astrophysics Data System (ADS)

Vimalan, G.; Muthupandi, V.; Ravichandran, G.

2018-05-01

A continuous cooling transformation diagram is constructed for simulated coarse grain heat affected zone (CGHAZ) of SA106 grade B carbon steel. Samples are heated to a peak temperature of 1200°C in the Gleeble thermo mechanical simulator and then cooled at different cooling rates varying from 0.1°C/s to 100°C/s. Microstructure of the specimens simulated at different cooling rates were characterised by optical microscopy and hardness was assessed by Vicker's hardness test and micro-hardness test. Transformation temperatures and the corresponding phase fields were identified from dilatometric curves and the same could be confirmed by correlating with the microstructures at room temperature. These data were used to construct the CCT diagram. Phase fields were found to have ferrite, pearlite, bainite and martensite or their combinations. With the help of this CCT diagram it is possible to predict the microstructure and hardness of coarse grain HAZ experiencing different cooling rates. The constructed CCT diagram becomes an important tool in evaluating the weldability of SA106 grade B carbon steel.

A Shape Memory Alloy Based Cryogenic Thermal Conduction Switch

NASA Technical Reports Server (NTRS)

Notardonato, W. U.; Krishnan, V. B.; Singh, J. D.; Woodruff, T. R.; Vaidyanathan, R.

2005-01-01

Shape memory alloys (SMAs) can produce large strains when deformed (e.g., up to 8%). Heating results in a phase transformation and associated recovery of all the accumulated strain. This strain recovery can occur against large forces, resulting in their use as actuators. Thus an SMA element can integrate both sensory and actuation functions, by inherently sensing a change in temperature and actuating by undergoing a shape change as a result of a temperature-induced phase transformation. Two aspects of our work on cryogenic SMAs are addressed here. First - a shape memory alloy based cryogenic thermal conduction switch for operation between dewars of liquid methane and liquid oxygen in a common bulkhead arrangement is discussed. Such a switch integrates the sensor element and the actuator element and can be used to create a variable thermal sink to other cryogenic tanks for liquefaction, densification, and zero boil-off systems for advanced spaceport applications. Second - fabrication via arc-melting and subsequent materials testing of SMAs with cryogenic transformation temperatures for use in the aforementioned switch is discussed.

New method for the transformation of solar radiation energy into electric power for energy feeding of the space vehicles

NASA Astrophysics Data System (ADS)

Ludanov, K. I.

The author proposes a new method for the transformation of solar radiation energy into electric power, which is alternative for photo-transformation. Ukrpatents's positive decisions are obtained for the method and for the installation for its realization. The method includes two phases: concentration of solar radiation by paraboloid mirrors with high potential heat obtaining in the helio receiver and the next heat transformation into electric power in the framework of the thermal cycle "high temperature electrolytic steam decomposition on the components (H2 and O2) + electrochemical generation by the way of the water recombination from H2 and O2 in the low temperature fuel cell". The new method gives the double superiority in comparison with the photo-transformation.

Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments.

PubMed

Shen, Ya; Zhou, Hui-Min; Zheng, Yu-Feng; Campbell, Les; Peng, Bin; Haapasalo, Markus

2011-11-01

To improve the fracture resistance of nickel-titanium (NiTi) files, manufacturers have introduced new alloys and developed new manufacturing processes for the fabrication of NiTi files. This study aimed to examine the phase transformation behavior and microstructure of NiTi instruments from a novel controlled memory NiTi wire (CM wire). Instruments of EndoSequence (ES), ProFile (PF), ProFile Vortex (Vortex), Twisted Files (TF), Typhoon (TYP), and Typhoon™ CM (TYP CM), all size 25/.04, were examined by differential scanning calorimetry (DSC) and x-ray diffraction (XRD). Microstructures of etched instruments were observed by optical microscopy and scanning electron microscopy with x-ray energy-dispersive spectrometric (EDS) analyses. The DSC analyses showed that each segment of the TYP CM and Vortex instruments had an austenite transformation completion or austenite-finish (A(f)) temperature exceeding 37°C, whereas the NiTi instruments made from conventional superelastic NiTi wire (ES, PF, and TYP) and TF had A(f) temperatures substantially below mouth temperature. The higher A(f) temperature of TYP CM instruments was consistent with a mixture of austenite and martensite structure, which was observed at room temperature with XRD. All NiTi instruments had room temperature martensite microstructures consisting of colonies of lenticular features with substantial twinning. EDS analysis indicated that the precipitates in all NiTi instruments were titanium-rich, with an approximate composition of Ti(2)Ni. The TYP CM and Vortex instruments with heat treatment contribute to increase austenite transformation temperature. The CM instrument has significant changes in the phase transformation behavior, compared with conventional superelastic NiTi instruments. Copyright © 2011 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.

Quantitative X-ray Diffraction (QXRD) analysis for revealing thermal transformations of red mud.

PubMed

Liao, Chang-Zhong; Zeng, Lingmin; Shih, Kaimin

2015-07-01

Red mud is a worldwide environmental problem, and many authorities are trying to find an economic solution for its beneficial application or/and safe disposal. Ceramic production is one of the potential waste-to-resource strategies for using red mud as a raw material. Before implementing such a strategy, an unambiguous understanding of the reaction behavior of red mud under thermal conditions is essential. In this study, the phase compositions and transformation processes were revealed for the Pingguo red mud (PRM) heat-treated at different sintering temperatures. Hematite, perovskite, andradite, cancrinite, kaolinite, diaspore, gibbsite and calcite phases were observed in the samples. However, unlike those red mud samples from the other regions, no TiO2 (rutile or anatase) or quartz were observed. Titanium was found to exist mainly in perovskite and andradite while the iron mainly existed in hematite and andradite. A new silico-ferrite of calcium and aluminum (SFCA) phase was found in samples treated at temperatures above 1100°C, and two possible formation pathways for SFCA were suggested. This is the first SFCA phase to be reported in thermally treated red mud, and this finding may turn PRM waste into a material resource for the iron-making industry. Titanium was found to be enriched in the perovskite phase after 1200°C thermal treatment, and this observation indicated a potential strategy for the recovery of titanium from PRM. In addition to noting these various resource recovery opportunities, this is also the first study to quantitatively summarize the reaction details of PRM phase transformations at various temperatures. Copyright © 2015 Elsevier Ltd. All rights reserved.

On the Fracture Toughness and Stable Crack Growth in Shape Memory Alloys Under Combined Thermomechanical Loading

NASA Astrophysics Data System (ADS)

Jape, Sameer Sanjay

Advanced multifunctional materials such as shape memory alloys (SMAs) offer unprecedented improvement over conventional materials when utilized as high power output solid-state actuators in a plethora of engineering applications, viz. aerospace, automotive, oil and gas exploration, etc., replacing complex multi-component assemblies with compact single-piece adaptive components. These potential applications stem from the material's ability to produce large recoverable actuation strains when subjected to combined thermomechanical loads, via a diffusionless solid-to-solid phase transition between high-temperature cubic austenite and low-temperature monoclinic martensite crystalline phases. To ensure reliable design, functioning and durability of SMA-based actuators, it is imperative to develop a thorough scientific knowledge base and understanding about their fracture properties i.e. crack-initiation and growth during thermal actuation, vis-a-vis the phase transformation metrics (i.e. transformation strains, hysteresis, and temperatures, critical stresses for phase transformation, etc.) and microstructural features (grain size, precipitates, and texture). Systematic experimental and analytical investigation of SMA fracture response based on known theories and methodologies is posed with significant challenges due to the inherent complexity in SMA thermomechanical constitutive response arising out of the shape memory and pseudoelastic effects, martensite detwinning and variant reorientation, thermomechanical coupling, and transformation induced plasticity (TRIP). In this study, a numerical analysis is presented that addresses the fundamental need to study fracture in SMAs in the presence of aforementioned complexities. Finite element modeling with an energetics based fracture toughness criterion and SMA thermomechanical behavior with nonlinearities from thermomechanical coupling and TRIP was conducted. A specific analysis of a prototype boundary value fracture problem yielded results similar to those obtained experimentally, viz. stable crack growth with transformation toughening, dependence of failure cycle on bias load and catastrophic failure during cooling, and are explained using classical fracture mechanics theories. Influence of TRIP as a monotonically accumulating irrecoverable plastic strain on the crack-tip mechanical fields in case of stationary and advancing cracks is also investigated using the same computational tools. Thermomechanical coupling in shape memory alloys, which is an important factor when utilized as solid-state actuators manifests itself through the generation and absorption of latent of transformation and leads to non-uniform temperature distribution. The effect of this coupling vis-a-vis the mechanics of static and advancing cracks is also analyzed using the energetics based approach.

Communication: Molecular-level insights into asymmetric triblock copolymers: Network and phase development

NASA Astrophysics Data System (ADS)

Tallury, Syamal S.; Mineart, Kenneth P.; Woloszczuk, Sebastian; Williams, David N.; Thompson, Russell B.; Pasquinelli, Melissa A.; Banaszak, Michal; Spontak, Richard J.

2014-09-01

Molecularly asymmetric triblock copolymers progressively grown from a parent diblock copolymer can be used to elucidate the phase and property transformation from diblock to network-forming triblock copolymer. In this study, we use several theoretical formalisms and simulation methods to examine the molecular-level characteristics accompanying this transformation, and show that reported macroscopic-level transitions correspond to the onset of an equilibrium network. Midblock conformational fractions and copolymer morphologies are provided as functions of copolymer composition and temperature.

Influence of temperature on the rhombic shape of paracetamol molecular crystals

NASA Astrophysics Data System (ADS)

Belyaev, A. P.; Rubets, V. P.; Antipov, V. V.

2017-04-01

The method of differential scanning ellipsometry has been used to study the influence of heating on the rhombic shape of paracetamol molecular crystals. Rhombic molecular paracetamol crystals have been synthesized in vacuum from the vapor phase of paracetamol as a result of complex transformation, which includes a second-order transition that gives rise to a pretransition phase. It has been found that these crystals contain monoclinic nuclei, which favor the form I-to-form II polymorphic transformation during heating.

First interactions between hydrogen and stress-induced reverse transformation of Ni-Ti superelastic alloy

NASA Astrophysics Data System (ADS)

Yokoyama, Ken'ichi; Hashimoto, Tatsuki; Sakai, Jun'ichi

2017-11-01

The first dynamic interactions between hydrogen and the stress-induced reverse transformation have been investigated by performing an unloading test on a Ni-Ti superelastic alloy subjected to hydrogen charging under a constant applied strain in the elastic deformation region of the martensite phase. Upon unloading the specimen, charged with a small amount of hydrogen, no change in the behaviour of the stress-induced reverse transformation is observed in the stress-strain curve, although the behaviour of the stress-induced martensite transformation changes. With increasing amount of hydrogen charging, the critical stress for the reverse transformation markedly decreases. Eventually, for a larger amount of hydrogen charging, the reverse transformation does not occur, i.e. there is no recovery of the superelastic strain. The residual martensite phase on the side surface of the unloaded specimen is confirmed by X-ray diffraction. Upon training before the unloading test, the properties of the reverse transformation slightly recover after ageing in air at room temperature. The present study indicates that to change the behaviour of the reverse transformation a larger amount of hydrogen than that for the martensite transformation is necessary. In addition, it is likely that a substantial amount of hydrogen in solid solution more strongly suppresses the reverse transformation than hydrogen trapped at defects, thereby stabilising the martensite phase.

Factors that determine the level of the yield strength and the return of the yield-point elongation in low-alloy ferrite-martensite steels

NASA Astrophysics Data System (ADS)

Fonstein, N.; Kapustin, M.; Pottore, N.; Gupta, I.; Yakubovsky, O.

2007-09-01

The results of laboratory investigations of dual-phase steels with different contents of carbon and alloying elements after the controlled cooling from the two-phase field and the final low-temperature tempering are presented. It is shown that the ratio of the yield strength to the tensile strength of dual-phase steels, just as the return of the yield-point elongation, depends on the volume fraction of martensite, temperature of the martensite transformation of the austenite component, quenching stresses, concentration of carbon in ferrite, and the temperature of the final tempering.

Temperature-mediated phase transformation, pore geometry and pore hysteresis transformation of borohydride derived in-born porous zirconium hydroxide nanopowders

PubMed Central

Nayak, Nadiya B.; Nayak, Bibhuti B.

2016-01-01

Development of in-born porous nature of zirconium hydroxide nanopowders through a facile hydrogen (H2) gas-bubbles assisted borohydride synthesis route using sodium borohydride (NaBH4) and novel information on the temperature-mediated phase transformation, pore geometry as well as pore hysteresis transformation of in-born porous zirconium hydroxide nanopowders with the help of X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) isotherm and Transmission Electron Microscopy (TEM) images are the main theme of this research work. Without any surfactants or pore forming agents, the borohydride derived amorphous nature of porous powders was stable up to 500 °C and then the seed crystals start to develop within the loose amorphous matrix and trapping the inter-particulate voids, which led to develop the porous nature of tetragonal zirconium oxide at 600 °C and further sustain this porous nature as well as tetragonal phase of zirconium oxide up to 800 °C. The novel hydrogen (H2) gas-bubbles assisted borohydride synthesis route led to develop thermally stable porous zirconium hydroxide/oxide nanopowders with an adequate pore size, pore volume, and surface area and thus these porous materials are further suggested for promising use in different areas of applications. PMID:27198738

In situ observation of quasimelting of diamond and reversible graphite-diamond phase transformations.

PubMed

Huang, J Y

2007-08-01

Because of technique difficulties in achieving the extreme high-pressure and high-temperature (HPHT) simultaneously, direct observation of the structures of carbon at extreme HPHT conditions has not been possible. Banhart and Ajayan discovered remarkably that carbon onions can act as nanoscopic pressure cells to generate high pressures. By heating carbon onions to approximately 700 degrees C and under electron beam irradiation, the graphite-to-diamond transformation was observed in situ by transmission electron microscopy (TEM). However, the highest achievable temperature in a TEM heating holder is less than 1000 degrees C. Here we report that, by using carbon nanotubes as heaters and carbon onions as high-pressure cells, temperatures higher than 2000 degrees C and pressures higher than 40 GPa were achieved simultaneously in carbon onions. At such HPHT conditions and facilitated by electron beam irradiation, the diamond formed in the carbon onion cores frequently changed its shape, size, orientation, and internal structure and moved like a fluid, implying that it was in a quasimelting state. The fluctuation between the solid phase of diamond and the fluid/amorphous phase of diamond-like carbon, and the changes of the shape, size, and orientation of the solid diamond, were attributed to the dynamic crystallization of diamond crystal from the quasimolten state and the dynamic graphite-diamond phase transformations. Our discovery offers unprecedented opportunities to studying the nanostructures of carbon at extreme conditions in situ and at an atomic scale.

Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

NASA Astrophysics Data System (ADS)

Singh, Anar; Kaifeng, Dong; Chen, Jing-Sheng

2018-03-01

Epitaxial BiFeO3 thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La0.67Sr0.33MnO3 buffered SrTiO3 (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (MA) of BiFeO3 thin film initially transforms to a tetragonal phase (T1) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T2) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO3 films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO3, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La0.67Sr0.33MnO3. This phase of BiFeO3 can be utilized for the ferroelectric control of magnetism at the interface of BiFeO3 and La0.67Sr0.33MnO3.

Multiscale Characterization of Nickel Titanium Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Gall, Keith

Shape memory alloys were characterized by a variety of methods to investigate the relationship between microstructural phase transformation, macroscale deformation due to mechanical loading, material geometry, and initial material state. The major portion of the work is application of digital image correlation at several length scales to SMAs under mechanical loading. In addition, the connection between electrical resistance, stress, and strain was studied in NiTi wires. Finally, a new processing method was investigated to develop porous NiTi samples, which can be examined under DIC in future work. The phase transformation temperatures of a Nickel-Titanium based shape memory alloy (SMA) were initially evaluated under stress-free conditions by the differential scanning calorimetric (DSC) technique. Results show that the phase transformation temperature is significantly higher for transition from de-twinned martensite to austenite than from twinned martensite or R phase to austenite. To further examine transformation temperatures as a function of initial state a tensile test apparatus with in-situ electrical resistance (ER) measurements was used to evaluate the transformation properties of SMAs at a variety of stress levels and initial compositions. The results show that stress has a significant influence on the transformation of detwinned martensite, but a small influence on R phase and twinned martensite transformations. Electrical resistance changes linearly with strain during the transformations from both kinds of martensite to austenite. The linearity between ER and strain during the transformation from de-twinned martensite to austenite is not affected by the stress, facilitating application to control algorithms. A revised phase diagram is drawn to express these results. To better understand the nature of the local and global strain fields that accompany phase transformation in shape memory alloys (SMAs), here we use high resolution imaging together with image correlation processing at several length scales. The Digital Image Correlation (DIC) method uses digital images captured during material deformation to generate displacement and strain field maps of the specimen surface. Both 5x optical magnification and low magnification provide details of localized strain behavior during the stress induced phase transformation in polycrystalline Nickel-Titanium SMA samples. Tension bars with (and without) machined geometric defects are tested with (and without) paint speckle pattern to investigate the response near pore-like defects. Results from the standard tensile bars (no defect) show a recoverable transformation propagate across the sample (from both ends towards center) that is observed as localization in the DIC calculated strain field. Biaxial strain measurements from the DIC method also provide data to calculate a Poisson Ratio as a function of transformation progress. Specimens with a circular (0.5 mm dia) defect exhibit similar strain-localization behaviors, but the stress concentration causes early material transformation near the defect. Analysis of the magnified images illustrates strain field localization due to the underlying polycrystalline microstructure of the NiTi specimen. Last, a study presents the development of new processing techniques for porous SMA materials. Porous SMAs are potential candidates in a variety of applications where micro-macrochannels might improve thermal response of mechanical actuators or promote bone ingrowth for biomedical implant devices. Recent methods in powder metallurgy have shown that adding small amounts of Niobium improves densification of sintered NiTi alloys. New results here show how porous NiTiNb microstructures are processed using temporary steel wire space holder. The wires (or layered 2-D meshes) are electrochemically dissolved to leave a complex network of pores throughout a dense NiTiNb alloy. The processing method presented here allows better control of pore geometry and arrangement when compared to existing techniques in NiTiNb powder metallurgy.

Tuning Phase Composition of TiO2 by Sn(4+) Doping for Efficient Photocatalytic Hydrogen Generation.

PubMed

Wang, Fenglong; Ho, Jie Hui; Jiang, Yijiao; Amal, Rose

2015-11-04

The anatase-rutile mixed-phase photocatalysts have attracted extensive research interest because of the superior activity compared to their single phase counterparts. In this study, doping of Sn(4+) ions into the lattice of TiO2 facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn(4+) ions entered into the lattice of TiO2 by substituting some of the Ti(4+) ions and distributed evenly in the matrix of TiO2. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn(4+) ions into the TiO2 exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixed-phase TiO2 catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase-rutile mixtures.

Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off.

PubMed

Li, Zhiming; Pradeep, Konda Gokuldoss; Deng, Yun; Raabe, Dierk; Tasan, Cemal Cem

2016-06-09

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off

NASA Astrophysics Data System (ADS)

Li, Zhiming; Pradeep, Konda Gokuldoss; Deng, Yun; Raabe, Dierk; Tasan, Cemal Cem

2016-06-01

Metals have been mankind’s most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys

PubMed Central

2016-01-01

We report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy versus temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’. PMID:27402934

Phase transformation of GaAs at high pressures and temperatures

NASA Astrophysics Data System (ADS)

Ono, Shigeaki; Kikegawa, Takumi

2018-02-01

The high-pressure behavior of gallium arsenide, GaAs, has been investigated using an in-situ X-ray powder diffraction technique in a diamond anvil cell combined with a resistance heating method, at pressures and temperatures up to 25 GPa and 1000 K respectively. The pressure-induced phase transition from a zincblende to an orthorhombic (Cmcm) structure was observed. This transition occurred at 17.3 GPa and at room temperature, where a negative temperature dependence for this transition was confirmed. The transition boundary was determined to be P (GPa) = 18.0 - 0.0025 × T (K).

Phase Transformations and Formation of Ultra-Fine Microstructure During Hydrogen Sintering and Phase Transformation (HSPT) Processing of Ti-6Al-4V

NASA Astrophysics Data System (ADS)

Sun, Pei; Fang, Zhigang Zak; Koopman, Mark; Xia, Yang; Paramore, James; Ravi Chandran, K. S.; Ren, Yang; Lu, Jun

2015-12-01

The hydrogen sintering and phase transformation (HSPT) process is a novel powder metallurgy method for producing Ti alloys, particularly the Ti-6Al-4V alloy, with ultra-fine microstructure in the as-sintered state. The ultra-fine microstructure is obtained as a direct result of the use of H2 gas during sintering. The refinement of the microstructure during HSPT is similar to that of thermal hydrogen processing (THP) of bulk Ti alloys. For both THP and HSPT of Ti-6Al-4V alloy, the mechanisms of the grain refinement depend on the phase equilibria and phase transformations in the presence of hydrogen, which are surprisingly still not well established to date and are still subjected to research and debate. In recent work by the present authors, a pseudo-binary phase diagram of (Ti-6Al-4V)-H has been determined by using in situ synchrotron XRD and TGA/DSC techniques. Aided by this phase diagram, the current paper focuses on the series of phase transformations during sintering and cooling of Ti-6Al-4V in a hydrogen atmosphere and the mechanisms for the formation of the ultra-fine microstructures obtained. Using experimental techniques, including in situ synchrotron XRD, SEM, EBSD, and TEM, the microstructural refinement was found to be the result of (1) the precipitation of ultra-fine α/α2 within coarse β grains during an isothermal hold at intermediate temperatures, and (2) the eutectoid transformation of β → α + δ at approximately 473 K (200 °C).

Phase states and thermomorphologic, thermotropic, and magnetomorphologic properties of lyotropic mesophases: Sodium lauryl sulphate-water-1-decanol liquid-crystalline system

NASA Astrophysics Data System (ADS)

Özden, Pınar; Nesrullajev, Arif; Oktik, Şener

2010-12-01

Phase states in sodium lauryl sulphate-water-1-decanol lyotropic liquid-crystalline system have been investigated for different temperature ranges. The dependence of triangle phase diagram types, phase boundaries, and sequence of lyotropic mesophases vs temperature has been found. The thermomorphologic, thermotropic, and magnetomorphologic properties of hexagonal E, lamellar D, nematic-calamitic NC , nematic-discotic ND , and biaxial nematic Nbx mesophases have been studied in detail. Dynamics of transformations of magnetically induced textures has been investigated. Peculiarities of typical and magnetically induced textures have been investigated in detail. Triangle phase diagrams of sodium lauryl sulphate-water-1-decanol lyotropic liquid-crystalline system for different temperatures and typical and magnetically induced textures of E, D, NC , ND , and Nbx mesophases are presented.

The Influence of Mo, Cr and B Alloying on Phase Transformation and Mechanical Properties in Nb Added High Strength Dual Phase Steels

NASA Astrophysics Data System (ADS)

Girina, O.; Fonstein, N.; Yakubovsky, O.; Panahi, D.; Bhattacharya, D.; Jansto, S.

The influence of Nb, Mo, Cr and B on phase transformations and mechanical properties are studied in a 0.15C-2.0Mn-0.3Si-0.020Ti dual phase steel separately and in combination. The formation and decomposition of austenite together with recrystallization of ferrite are evaluated by dilatometry and constructed CCT-diagrams in laboratory processed cold rolled material cooled after full austenitization and from intercritical temperature range. The effect of alloying elements on formation of austenite through their effect on initial hot rolled structure is taken into account. The interpretation of phase transformations during heating and cooling is supported by metallography. The effect of alloying elements on mechanical properties and structure are evaluated by annealing simulations. It has been shown that mechanical properties are strongly influenced by alloying additions such as Nb, Mo, Cr and B through their effect on ferrite formation during continuous cooling and corresponding enrichment of remaining austenite by carbon. Depending on combined effect of these alloying elements, different phase transformations can be promoted during cooling. This allows controlling of final microstructural constituents and mechanical properties.

Hydroxyapatite: Vibrational spectra and monoclinic to hexagonal phase transition

NASA Astrophysics Data System (ADS)

Slepko, Alexander; Demkov, Alexander A.

2015-02-01

Fundamental studies of biomaterials are necessary to deepen our understanding of their degradation and to develop cure for related illnesses. Biomineral hydroxyapatite Ca10(PO4)6(OH)2 is the main mineral constituent of mammal bone, and its synthetic analogues are used in biomedical applications. The mineral can be found in either hexagonal or monoclinic form. The transformation between these two phases is poorly understood, but knowing its mechanism may be critical to reversing processes in bone related to aging. Using density functional theory, we investigate the mechanisms of the phase transformation and estimate the transition temperature to be 680 K in fair agreement with the experimental temperature of 470 K. We also report the heat capacity of hydroxyapatite and a peculiarity in its phonon dispersion that might allow for non-destructive measurements of the crystal composition with applications in preventive medical screening for bone mineral loss.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice.

PubMed

Lin, Chuanlong; Yong, Xue; Tse, John S; Smith, Jesse S; Sinogeikin, Stanislav V; Kenney-Benson, Curtis; Shen, Guoyin

2017-09-29

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ∼1  Pa, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice

NASA Astrophysics Data System (ADS)

Lin, Chuanlong; Yong, Xue; Tse, John S.; Smith, Jesse S.; Sinogeikin, Stanislav V.; Kenney-Benson, Curtis; Shen, Guoyin

2017-09-01

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ˜1 Pa , to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Chuanlong; Yong, Xue; Tse, John S.

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transitionmore » to low-density amorphous ice at 96 K and ~ 1 Pa , to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cai, S.; Schaffer, J. E.; Ren, Y.

Room temperature deformation of a Ni46.7Ti42.8Nb10.5 alloy was studied by in-situ synchrotron X-ray diffraction. Compared to binary NiTi alloy, the Nb dissolved in the matrix significantly increased the onset stress for Stress-Induced Martensite Transformation (SIMT). The secondary phase, effectively a Nb-nanowire dispersion in a NiTi-Nb matrix, increased the elastic stiffness of the bulk material, reduced the strain anisotropy in austenite families by loading sharing during SIMT, and increased the stress hysteresis by resisting reverse phase transformation during unloading. The stress hysteresis can be controlled over a wide range by changing the heat treatment temperature through its influences on the residualmore » stress-strain state of the Nb-nanowire dispersion.« less

Kinetics of Cr/Mo-rich precipitates formation for 25Cr-6.9Ni-3.8Mo-0.3N super duplex stainless steel

NASA Astrophysics Data System (ADS)

Byun, Sang-Ho; Kang, Namhyun; Lee, Tae-Ho; Ahn, Sang-Kon; Lee, Hae Woo; Chang, Woong-Seong; Cho, Kyung-Mox

2012-04-01

The amount and composition of Cr-rich (σ) and Mo-rich (χ) precipitates in super duplex stainless steels was analyzed. An isothermal heat treatment was conducted at temperatures ranging from 700 °C to 1000 °C for up to 10 days. A time-temperature transformation (TTT) diagram was constructed for the mixture of σ and χ phases. The mixture of the σ and χ phases exhibited the fastest rate of formation at approximately 900 °C. Minor phases, such as Cr2N, M23C6, and M7C3, were also detected using a transmission electron microscopy (TEM). Also, a continuous cooling transformation (CCT) diagram was constructed for the mixture of σ and χ phases using the Johnson-Mehl-Avrami equation. Compared with the known CCT diagram of the σ phase, this study revealed faster kinetics with an order of magnitude difference and a new CCT diagram was also developed for a mixture of σ and χ phases. The calculated fraction of σ and χ phases obtained at a cooling speed of 0.5 °C/s was in good agreement with the experimental data.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stan, Camelia V.; Dutta, Rajkrishna; White, Claire E.

Lead fluoride, PbF 2, was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters displayed highly anomalous trends between 10-22 GPa with enhanced compressibility along the a direction and reduced or even negative compressibility along b and c. Theoretical calculations of valence electron densities at 22 GPa showed that α-PbFmore » 2 underwent a pressure-induced isosymmetric phase transition to a postcotunnite Co 2Si structure and also revealed the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition smoothly occurring over 10-22 GPa rather than abruptly as previously suggested. The characteristic values for the cell constants a/c and (a+c)/b, which are used to distinguish among cotunnite-, Co 2Si-, and Ni 2In-type phases, require modification based on our results. An equation of state fit yields a bulk modulus, K 0, of 72(3) GPa for the cotunnite-type, and an ambient-pressure volume, V 0, of 182(2)Å 3, and K 0=81(4)GPa for the Co 2Si-type phase when fixing the pressure derivative of the bulk modulus, K 0'=4. Upon heating above 1200 K at pressures at or above 25.9 GPa, PbF 2 partially transformed to the hexagonal Ni 2In-type phase but wholly or partially reverted back to Co 2Si-type phase upon temperature quench. From 43-65 GPa, nearly complete transformation to the Ni 2In-type PbF 2 was observed at high temperature, but the material partially transformed back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of PbF 2 is distinct from that of the alkaline earth fluorides with similar ionic radii. These results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.« less

High-pressure polymorphism of Pb F 2 to 75 GPa

DOE PAGES

Stan, Camelia V.; Dutta, Rajkrishna; White, Claire E.; ...

2016-07-06

Lead fluoride, PbF 2, was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters displayed highly anomalous trends between 10-22 GPa with enhanced compressibility along the a direction and reduced or even negative compressibility along b and c. Theoretical calculations of valence electron densities at 22 GPa showed that α-PbFmore » 2 underwent a pressure-induced isosymmetric phase transition to a postcotunnite Co 2Si structure and also revealed the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition smoothly occurring over 10-22 GPa rather than abruptly as previously suggested. The characteristic values for the cell constants a/c and (a+c)/b, which are used to distinguish among cotunnite-, Co 2Si-, and Ni 2In-type phases, require modification based on our results. An equation of state fit yields a bulk modulus, K 0, of 72(3) GPa for the cotunnite-type, and an ambient-pressure volume, V 0, of 182(2)Å 3, and K 0=81(4)GPa for the Co 2Si-type phase when fixing the pressure derivative of the bulk modulus, K 0'=4. Upon heating above 1200 K at pressures at or above 25.9 GPa, PbF 2 partially transformed to the hexagonal Ni 2In-type phase but wholly or partially reverted back to Co 2Si-type phase upon temperature quench. From 43-65 GPa, nearly complete transformation to the Ni 2In-type PbF 2 was observed at high temperature, but the material partially transformed back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of PbF 2 is distinct from that of the alkaline earth fluorides with similar ionic radii. These results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.« less

High-pressure polymorphism of Pb F2 to 75 GPa

NASA Astrophysics Data System (ADS)

Stan, Camelia V.; Dutta, Rajkrishna; White, Claire E.; Prakapenka, Vitali; Duffy, Thomas S.

2016-07-01

Lead fluoride, Pb F2 , was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters displayed highly anomalous trends between 10-22 GPa with enhanced compressibility along the a direction and reduced or even negative compressibility along b and c . Theoretical calculations of valence electron densities at 22 GPa showed that α -Pb F2 underwent a pressure-induced isosymmetric phase transition to a postcotunnite C o2Si structure and also revealed the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition smoothly occurring over 10-22 GPa rather than abruptly as previously suggested. The characteristic values for the cell constants a /c and (a +c )/b , which are used to distinguish among cotunnite-, C o2Si -, and N i2In -type phases, require modification based on our results. An equation of state fit yields a bulk modulus, K0, of 72(3) GPa for the cotunnite-type, and an ambient-pressure volume, V0, of 182 (2 ) Å3 , and K0=81 (4 ) GPa for the C o2Si -type phase when fixing the pressure derivative of the bulk modulus, K0 '=4 . Upon heating above 1200 K at pressures at or above 25.9 GPa, Pb F2 partially transformed to the hexagonal N i2In -type phase but wholly or partially reverted back to C o2Si -type phase upon temperature quench. From 43-65 GPa, nearly complete transformation to the N i2In -type Pb F2 was observed at high temperature, but the material partially transformed back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of Pb F2 is distinct from that of the alkaline earth fluorides with similar ionic radii. Our results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.

Effect of atomic order on the martensitic and magnetic transformations in Ni-Mn-Ga ferromagnetic shape memory alloys.

PubMed

Sánchez-Alarcos, V; Pérez-Landazábal, J I; Recarte, V; Rodríguez-Velamazán, J A; Chernenko, V A

2010-04-28

The influence of long-range L2(1) atomic order on the martensitic and magnetic transformations of Ni-Mn-Ga shape memory alloys has been investigated. In order to correlate the structural and magnetic transformation temperatures with the atomic order, calorimetric, magnetic and neutron diffraction measurements have been performed on polycrystalline and single-crystalline alloys subjected to different thermal treatments. It is found that both transformation temperatures increase with increasing atomic order, showing exactly the same linear dependence on the degree of L2(1) atomic order. A quantitative correlation between atomic order and transformation temperatures has been established, from which the effect of atomic order on the relative stability between the structural phases has been quantified. On the other hand, the kinetics of the post-quench ordering process taking place in these alloys has been studied. It is shown that the activation energy of the ordering process agrees quite well with the activation energy of the Mn self-diffusion process.

Low temperature nickel titanium iron shape memory alloys: Actuator engineering and investigation of deformation mechanisms using in situ neutron diffraction at Los Alamos National Laboratory

NASA Astrophysics Data System (ADS)

Krishnan, Vinu B.

Shape memory alloys are incorporated as actuator elements due to their inherent ability to sense a change in temperature and actuate against external loads by undergoing a shape change as a result of a temperature-induced phase transformation. The cubic so-called austenite to the trigonal so-called R-phase transformation in NiTiFe shape memory alloys offers a practical temperature range for actuator operation at low temperatures, as it exhibits a narrow temperature-hysteresis with a desirable fatigue response. Overall, this work is an investigation of selected science and engineering aspects of low temperature NiTiFe shape memory alloys. The scientific study was performed using in situ neutron diffraction measurements at the newly developed low temperature loading capability on the Spectrometer for Materials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory and encompasses three aspects of the behavior of Ni46.8Ti50Fe3.2 at 92 K (the lowest steady state temperature attainable with the capability). First, in order to study deformation mechanisms in the R-phase in NiTiFe, measurements were performed at a constant temperature of 92 K under external loading. Second, with the objective of examining NiTiFe in one-time, high-stroke, actuator applications (such as in safety valves), a NiTiFe sample was strained to approximately 5% (the R-phase was transformed to B19' phase in the process) at 92 K and subsequently heated to full strain recovery under a load. Third, with the objective of examining NiTiFe in cyclic, low-stroke, actuator applications (such as in cryogenic thermal switches), a NiTiFe sample was strained to 1% at 92 K and subsequently heated to full strain recovery under load. Neutron diffraction spectra were recorded at selected time and stress intervals during these experiments. The spectra were subsequently used to obtain quantitative information related to the phase-specific strain, texture and phase fraction evolution using the Rietveld technique. The mechanical characterization of NiTiFe alloys using the cryogenic capability at SMARTS provided considerable insight into the mechanisms of phase transformation and twinning at cryogenic temperatures. Both mechanisms contribute to shape memory and pseudoelasticity phenomena. Three phases (R, B19' and B33 phases) were found to coexist at 92 K in the unloaded condition (nominal holding stress of 8 MPa). For the first time the elastic modulus of R-phase was reported from neutron diffraction experiments. Furthermore, for the first time a base-centered orthorhombic (B33) martensitic phase was identified experimentally in a NiTi-based shape memory alloy. The orthorhombic B33 phase has been theoretically predicted in NiTi from density function theory (DFT) calculations but hitherto has never been observed experimentally. The orthorhombic B33 phase was observed while observing shifting of a peak (identified to be {021}B33) between the {111}R and {100}B19' peaks in the diffraction spectra collected during loading. Given the existing ambiguity in the published literature as to whether the trigonal R-phase belongs to the P3 or P3¯ space groups, Rietveld analyses were separately carried out incorporating the symmetries associated with both space groups and the impact of this choice evaluated. The constrained recovery of the B19' phase to the R-phase recorded approximately 4% strain recovery between 150 K and 170 K, with half of that recovery occurring between 160 K and 162 K. Additionally, the aforementioned research methodology developed for Ni46.8Ti50Fe3.2 shape memory alloys was applied to experiments performed on a new high temperature Ni 29.5Ti50.5Pd20 shape memory alloys. The engineering aspect focused on the development of (i) a NiTiFe based thermal conduction switch that minimized the heat gradient across the shape memory actuator element, (ii) a NiTiFe based thermal conduction switch that incorporated the actuator element in the form of helical springs, and (iii) a NiTi based release mechanism. Patents are being filed for all the three shape memory actuators developed as a part of this work. This work was supported by grants from SRI, NASA (NAG3-2751) and NSF (CAREER DMR-0239512) to UCF. Additionally, this work benefited from the use of the Lujan Center at the Los Alamos Neutron Science Center, funded by the United States Department of Energy, Office of Basic Energy Sciences, under Contract No. W-7405-ENG-36.

Anion Exchange in II-VI Semiconducting Nanostructures via Atomic Templating.

PubMed

Agarwal, Rahul; Krook, Nadia M; Ren, Ming-Liang; Tan, Liang Z; Liu, Wenjing; Rappe, Andrew M; Agarwal, Ritesh

2018-03-14

Controlled chemical transformation of nanostructures is a promising technique to obtain precisely designed novel materials, which are difficult to synthesize otherwise. We report high-temperature vapor-phase anion-exchange reactions to chemically transform II-VI semiconductor nanostructures (100-300 nm length scale) while retaining the single crystallinity, crystal structure, morphology, and even defect distribution of the parent material via atomic templating. The concept of atomic templating is employed to obtain kinetically controlled, thermodynamically metastable structural phases such as zincblende CdSe and CdS from zincblende CdTe upon complete chemical replacement of Te with Se or S. The underlying transformation mechanisms are explained through first-principles density functional theory calculations. Atomic templating is a unique path to independently tune materials' phase and composition at the nanoscale, allowing the synthesis of novel materials.

Study of the phase transformations and equation of state of magnesium by synchrotron x-ray diffraction

NASA Astrophysics Data System (ADS)

Errandonea, Daniel; Meng, Yue; Häusermann, Daniel; Uchida, Takeyuki

2003-03-01

We studied the phase behaviour and the P - V - T equation of state of Mg by in situ energy-dispersive x-ray diffraction in a multi-anvil apparatus in the pressure-temperature range up to 18.6 GPa and 1527 K. At high temperatures, an hcp to dhcp transition was found above 9.6 GPa, which differs from the hcp to bcc transformation predicted by theoretical calculations. At room temperature, the hcp phase remains stable within the pressure range of this study with an axial ratio, c/a, close to the ideal. The melting of Mg was determined at 2.2, 10 and 12 GPa; the detected melting temperatures are in good agreement with previous diamond anvil cell results. The P - V - T equation of state determined based on the data of this study gives B0 = (36.8 ± 3) GPa, B0 ' = 4.3 ± 0.4, alpha0 = 25 × 10-6 K-1, partialalpha/partialT = (2.3 ± 0.2) × 10-7 K-2 and partialB0,T /partialT = (-2.08 ± 0.09) × 10-2 GPa K-1.

Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni)2 Ni-MH Electrode Alloys: Part II. Solid-State Transformation of the Interdendritic B2 Phase

NASA Astrophysics Data System (ADS)

Bendersky, L. A.; Wang, K.; Boettinger, W. J.; Newbury, D. E.; Young, K.; Chao, B.

2010-08-01

Solidification microstructure of multicomponent (Zr,Ti)-Ni-(V,Cr,Mn,Co) alloys intended for use as negative electrodes in Ni-metal hydride (Ni-MH) batteries was studied in Part I of this series of articles. Part II of the series examines the complex internal structure of the interdendritic grains formed by solid-state transformation and believed to play an important role in the electrochemical charge/discharge characteristics of the overall alloy composition. By studying one alloy, Zr21Ti12.5V10Cr5.5Mn5.1Co5.0Ni40.2Al0.5Sn0.3, it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-type, and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni-type intermetallics, and consequently lead to a multivariant structure. The major orientation relationship for the orthorhombic Zr7Ni10 type is [011]Zr7Ni10//[001]B2; (100)Zr7Ni10//(100)B2. The orientation relationship for the tetragonal Zr9Ni11 type is [001]Zr9Ni11//[001]B2; (130)Zr9Ni11//(100)B2. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.

Fractional Order Two-Temperature Dual-Phase-Lag Thermoelasticity with Variable Thermal Conductivity

PubMed Central

Mallik, Sadek Hossain; Kanoria, M.

2014-01-01

A new theory of two-temperature generalized thermoelasticity is constructed in the context of a new consideration of dual-phase-lag heat conduction with fractional orders. The theory is then adopted to study thermoelastic interaction in an isotropic homogenous semi-infinite generalized thermoelastic solids with variable thermal conductivity whose boundary is subjected to thermal and mechanical loading. The basic equations of the problem have been written in the form of a vector-matrix differential equation in the Laplace transform domain, which is then solved by using a state space approach. The inversion of Laplace transforms is computed numerically using the method of Fourier series expansion technique. The numerical estimates of the quantities of physical interest are obtained and depicted graphically. Some comparisons of the thermophysical quantities are shown in figures to study the effects of the variable thermal conductivity, temperature discrepancy, and the fractional order parameter. PMID:27419210

Temperature- and composition-dependent hydrogen diffusivity in palladium from statistically-averaged molecular dynamics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhou, Xiaowang; Heo, Tae Wook; Wood, Brandon C.

Solid-state hydrogen storage materials undergo complex phase transformations whose kinetics is often limited by hydrogen diffusion. Among metal hydrides, palladium hydride undergoes a diffusional phase transformation upon hydrogen uptake, during which the hydrogen diffusivity varies with hydrogen composition and temperature. Here we perform robust statistically-averaged molecular dynamics simulations to obtain a well-converged analytical expression for hydrogen diffusivity in bulk palladium that is valid throughout all stages of the reaction. Our studies confirm significant dependence of the diffusivity on composition and temperature that elucidate key trends in the available experimental measurements. Whereas at low hydrogen compositions, a single process dominates, atmore » high hydrogen compositions, diffusion is found to exhibit behavior consistent with multiple hopping barriers. Further analysis, supported by nudged elastic band computations, suggests that the multi-barrier diffusion can be interpreted as two distinct mechanisms corresponding to hydrogen-rich and hydrogen-poor local environments.« less

Temperature- and composition-dependent hydrogen diffusivity in palladium from statistically-averaged molecular dynamics

DOE PAGES

Zhou, Xiaowang; Heo, Tae Wook; Wood, Brandon C.; ...

2018-03-09

Solid-state hydrogen storage materials undergo complex phase transformations whose kinetics is often limited by hydrogen diffusion. Among metal hydrides, palladium hydride undergoes a diffusional phase transformation upon hydrogen uptake, during which the hydrogen diffusivity varies with hydrogen composition and temperature. Here we perform robust statistically-averaged molecular dynamics simulations to obtain a well-converged analytical expression for hydrogen diffusivity in bulk palladium that is valid throughout all stages of the reaction. Our studies confirm significant dependence of the diffusivity on composition and temperature that elucidate key trends in the available experimental measurements. Whereas at low hydrogen compositions, a single process dominates, atmore » high hydrogen compositions, diffusion is found to exhibit behavior consistent with multiple hopping barriers. Further analysis, supported by nudged elastic band computations, suggests that the multi-barrier diffusion can be interpreted as two distinct mechanisms corresponding to hydrogen-rich and hydrogen-poor local environments.« less

Magneto-optical spectroscopy of ferromagnetic shape-memory Ni-Mn-Ga alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Veis, M., E-mail: veis@karlov.mff.cuni.cz; Beran, L.; Zahradnik, M.

2014-05-07

Magneto-optical properties of single crystal of Ni{sub 50.1}Mn{sub 28.4}Ga{sub 21.5} magnetic shape memory alloy in martensite and austenite phase were systematically studied. Crystal orientation was approximately along (100) planes of parent cubic austenite. At room temperature, the sample was in modulated 10M martensite phase and transformed to cubic austenite at 323 K. Spectral dependence of polar magneto-optical Kerr effect was obtained by generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.2 to 4 eV, and from room temperature to temperature above the Curie point. The Kerr rotation spectra exhibit prominent features typical for complexes containing Mn atoms. Significantmore » spectral changes during transformation to austenite can be explained by different optical properties caused by changes in density of states near the Fermi energy.« less

Magnetic and conventional shape memory behavior of Mn-Ni-Sn and Mn-Ni-Sn(Fe) alloys

NASA Astrophysics Data System (ADS)

Turabi, A. S.; Lázpita, P.; Sasmaz, M.; Karaca, H. E.; Chernenko, V. A.

2016-05-01

Magnetic and conventional shape memory properties of Mn49Ni42Sn9(at.%) and Mn49Ni39Sn9Fe3(at.%) polycrystalline alloys exhibiting martensitic transformation from ferromagnetic austenite into weakly magnetic martensite are characterized under compressive stress and magnetic field. Magnetization difference between transforming phases drastically increases, while transformation temperature decreases with the addition of Fe. Both Mn49Ni42Sn9 and Mn49Ni39Sn9Fe3 alloys show remarkable superelastic and shape memory properties with recoverable strain of 4% and 3.5% under compression at room temperature, respectively. These characteristics can be counted as extraordinary among the polycrystalline NiMn-based magnetic shape memory alloys. Critical stress for phase transformation was increased by 34 MPa in Mn49Ni39Sn9Fe3 and 21 MPa in Mn49Ni42Sn9 at 9 T, which can be qualitatively understood in terms of thermodynamic Clausius-Clapeyron relationships and in the framework of the suggested physical concept of a volume magnetostress.

Shock temperatures in silica glass - Implications for modes of shock-induced deformation, phase transformation, and melting with pressure

NASA Technical Reports Server (NTRS)

Schmitt, Douglas R.; Ahrens, Thomas J.

1989-01-01

Observations of shock-induced radiative thermal emissions are used to determine the gray body temperatures and emittances of silica glass under shock compression between 10 and 30 GPa. The results suggest that fused quartz deforms heterogeneously in this shock pressure range. It is shown that the 10-16 GPa range coincides with the permanent densification region, while the 16-30 GPa range coincides with the inferred mixed phase region along the silica glass Hugoniot. Low emittances in the mixed phase region are thought to represent the melting temperature of the high-pressure phase, stishovite. Also, consideration is given to the effects of pressure on melting relations for the system SiO2-Mg2SiO4.

Synthesis of TiO2 Nanoparticle and its phase Transition

NASA Astrophysics Data System (ADS)

Mangrola, M. H.; Joshi, V. G.; Parmar, B. H.

2011-12-01

Here we report the synthesis of titanium dioxide (TiO2) nanoparticles and study of its phase transition from anataze to rutile. Titanium dioxide (TiO2) nanoparticles have been prepared by hydrolysis of Titanium isopropoxide an aqueous solution with constant value of pH 2 and peptizing the resultant suspension gel(white-Blue) and calcinate gel at different temperature. Structures of synthetic samples of TiO2 have been examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). The anatase-rutile transition has been a popular topic due to its interest to scientific and engineering fields. . Here we have seen that the 100 °C calcinate powder consist of anatase fine crystalline phase with a particle size 14 to 15 nm. The prepared TiO2 nanoparticles have uniform size and morphology, and the phase transformation kinetics of obtained material was studied by interpretation of the X-ray diffraction patterns peaks. The phase transform occurred from anatase to rutile at calcinate temperature up to 600 °C. A very fine network texture made from uniform nanoparticles was revealed by scanning electron microscopy (SEM) analyses.

Assessment of Shape Memory Alloys - From Atoms To Actuators - Via In Situ Neutron Diffraction

NASA Technical Reports Server (NTRS)

Benafan, Othmane

2014-01-01

As shape memory alloys (SMAs) become an established actuator technology, it is important to identify the fundamental mechanisms responsible for their performance by understanding microstructure performance relationships from processing to final form. Yet, microstructural examination of SMAs at stress and temperature is often a challenge since structural changes occur with stress and temperature and microstructures cannot be preserved through quenching or after stress removal, as would be the case for conventional materials. One solution to this dilemma is in situ neutron diffraction, which has been applied to the investigation of SMAs and has offered a unique approach to reveal the fundamental micromechanics and microstructural aspects of bulk SMAs in a non-destructive setting. Through this technique, it is possible to directly correlate the micromechanical responses (e.g., internal residual stresses, lattice strains), microstructural evolutions (e.g., texture, defects) and phase transformation properties (e.g., phase fractions, kinetics) to the macroscopic actuator behavior. In this work, in situ neutron diffraction was systematically employed to evaluate the deformation and transformation behavior of SMAs under typical actuator conditions. Austenite and martensite phases, yield behavior, variant selection and transformation temperatures were characterized for a polycrystalline NiTi (49.9 at. Ni). As the alloy transforms under thermomechanical loading, the measured textures and lattice plane-level variations were directly related to the cyclic actuation-strain characteristics and the dimensional instability (strain ratcheting) commonly observed in this alloy. The effect of training on the shape memory characteristics of the alloy and the development of two-way shape memory effect (TWSME) were also assessed. The final conversion from a material to a useful actuator, typically termed shape setting, was also investigated in situ during constrained heatingcooling and subsequent shape recovery experiments. Neutron diffraction techniques are also being applied to the investigation of novel high temperature SMAs with the objective of designing alloys with better stability, higher transition temperatures and ultimately superior durability.

Phonon Mode Transformation across the Orthohombic-Tetragonal Phase Transition in a Lead-Iodide Perovskite CH3NH3PbI3: a Terahertz Time-Domain Spectroscopy Approach

NASA Astrophysics Data System (ADS)

Chia, Elbert E. M.; La-O-Vorakiat, Chan; Kadro, Jeannette; Salim, Teddy; Zhao, Daming; Ahmed, Towfiq; Lam, Yeng Ming; Zhu, Jian-Xin; Marcus, Rudolph; Michel-Beyerle, Maria-Elisabeth

Using terahertz time-domain spectroscopy (THz-TDS), we study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH3NH3PbI3 thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. King Mongkut's University of Technology Thonburi (Grant No. SCI58-003), Singapore MOE Tier 1 (RG13/12, RG123/14), ONR, ARO, NTU Biophysics Center, LANL LDRD, LANL CINT.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries.

PubMed

Zhu, YinBo; Wang, FengChao; Wu, HengAn

2016-08-07

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

Evolution of rapidly solidified NiAlCu(B) alloy microstructure.

PubMed

Czeppe, Tomasz; Ochin, Patrick

2006-10-01

This study concerned phase transformations observed after rapid solidification and annealing at 500, 700 and 800 degrees C in 56.3 Ni-39.9 Al-3.8 Cu-0.06 B (E1) and 59.8 Ni-36.0 Al-4.3 Cu-0.06 B (E2) alloys (composition in at.%). Injection casting led to a homogeneous structure of very small, one-phase grains (2-4 microm in size). In both alloys, the phase observed at room temperature was martensite of L1(0) structure. The process of the formation of the Ni(5)Al(3) phase by atomic reordering proceeded at 285-394 degrees C in the case of E1 alloy and 450-550 degrees C in the case of E2 alloy. Further decomposition into NiAl (beta) and Ni(3)Al (gamma') phases, the microstructure and crystallography of the phases depended on the path of transformations, proceeding in the investigated case through the transformation of martensite crystallographic variants. This preserved precise crystallographic orientation between the subsequent phases, very stable plate-like morphology and very small beta + gamma' grains after annealing at 800 degrees C.

Defect-induced local variation of crystal phase transition temperature in metal-halide perovskites.

PubMed

Dobrovolsky, Alexander; Merdasa, Aboma; Unger, Eva L; Yartsev, Arkady; Scheblykin, Ivan G

2017-06-26

Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals.Understanding crystal phase transition in materials is of fundamental importance. Using luminescence spectroscopy and super-resolution imaging, Dobrovolsky et al. study the transition from the tetragonal to orthorhombic crystal phase in methylammonium lead triiodide nanowires at low temperature.

New structural transformations in congruent ferroelectric LiNbO3 fibres evidenced by Raman spectroscopy

NASA Astrophysics Data System (ADS)

Noiret, I.; Lefebvre, J.; Schamps, J.; Delattre, F.; Brenier, A.; Ferriol, M.

2000-03-01

Temperature dependent Stokes and anti-Stokes Raman-scattering experiments have been performed to study the ferroelectric phase of congruent LiNbO3 fibres in the external and internal mode regions. Mode splittings and changes in the slope of frequency-temperature plots at 590 and 790 K show the occurrence of two structural transformations at these temperatures. The anisotropy of the correlation time associated with the width of the central component and anomalies observed in previous neutron investigations are related to a migration process of the lithium atoms along the hexagonal axis and along the pseudo-cubic axis of the highly distorted related perovskite structure. The observed transformations are tentatively assigned to long-range correlated rearrangements in the intrinsic defect structure of the crystal.

Experimental investigation on local mechanical response of superelastic NiTi shape memory alloy

NASA Astrophysics Data System (ADS)

Xiao, Yao; Zeng, Pan; Lei, Liping

2016-01-01

In this paper, primary attention is paid to the local mechanical response of NiTi shape memory alloy (SMA) under uniaxial tension. With the help of in situ digital image correlation, sets of experiments are conducted to measure the local strain field at various thermomechanical conditions. Two types of mechanical responses of NiTi SMA are identified. The residual strain localization phenomena are observed, which can be attributed to the localized phase transformation (PT) and we affirm that most of the irreversibility is accumulated simultaneously during PT. It is found that temperature and PT play important roles in inducing delocalization of the reverse transformation. We conclude that forward transformation has more influence on the transition of mechanical response in NiTi SMA than reverse transformation in terms of the critical transition temperature for inducing delocalized reverse transformation.

Analysis of features of stainless steels in dissimilar welded joints in chloride inducted corrosion

NASA Astrophysics Data System (ADS)

Topolska, S.; Łabanowski, J.

2017-08-01

Stainless steels of femtic-austenitic microstructure that means the duplex Cr-Ni-Mo steels, in comparison with austenitic steel includes less expensive nickel and has much better mechanical properties with good formability and corrosion resistance, even in environments containing chloride ions. Similar share of high chromium ferrite and austenite, which is characterized by high ductility, determines that the duplex steels have good crack resistance at temperatures up to approximately -40°C. The steels containing approximately 22% Cr, 5% Ni, 3% Mo and 0.2% N crystallizes as a solid solution δ, partially transforming from the temperature of about 1200°C to 850°C into the phase α. The stable structure of considered steels, at temperatures above 850°C, is ferrite, and at lower temperatures the mixture of phase γ+α +σ. The two-phase structure α+γ the duplex steel obtains after hyperquenching at the temperature of stability of the mixture of α+γ phases, and the share of the phases depends on the hyper quenching attributes. Hyperquenching in water, with a temperature close to 1200°C, ensures the instance in the microstructure of the steel a large share of ferrite and a small share of the high chromium austenite. This causes the increase of strength properties and reducing the plasticity of the steel and its resistance ability to cracking and corrosion. Slower cooling from the mentioned temperature, for example in the air, enables the partial transformation of the a phase into the γ one (α → γ) and increasing the share of austenite in the steel structure. It leads to improvement of plasticity properties. In the paper are presented the results of investigations of heteronymous welded joints of duplex steel and austenitic one. The results include the relation between the chemical composition of steels and their weldability.

Hydrogen storage and phase transformations in Mg-Pd nanoparticles

NASA Astrophysics Data System (ADS)

Callini, E.; Pasquini, L.; Rude, L. H.; Nielsen, T. K.; Jensen, T. R.; Bonetti, E.

2010-10-01

Microstructure refinement and synergic coupling among different phases are currently explored strategies to improve the hydrogen storage properties of traditional materials. In this work, we apply a combination of these methods and synthesize Mg-Pd composite nanoparticles by inert gas condensation of Mg vapors followed by vacuum evaporation of Pd clusters. Irreversible formation of the Mg6Pd intermetallic phase takes place upon vacuum annealing, resulting in Mg/Mg6Pd composite nanoparticles. Their hydrogen storage properties are investigated and connected to the undergoing phase transformations by gas-volumetric techniques and in situ synchrotron radiation powder x-ray diffraction. Mg6Pd transforms reversibly into different Mg-Pd intermetallic compounds upon hydrogen absorption, depending on temperature and pressure. In particular, at 573 K and 1 MPa hydrogen pressure, the metal-hydride transition leads to the formation of Mg3Pd and Mg5Pd2 phases. By increasing the pressure to 5 MPa, the Pd-richer MgPd intermetallic is obtained. Upon hydrogen desorption, the Mg6Pd phase is reversibly recovered. These phase transformations result in a specific hydrogen storage capacity associated with Mg-Pd intermetallics, which attain the maximum value of 3.96 wt % for MgPd and influence both the thermodynamics and kinetics of hydrogen sorption in the composite nanoparticles.

Ultra-fine grained microstructure of metastable beta Ti-15Mo alloy and its effects on the phase transformations

NASA Astrophysics Data System (ADS)

Václavová, K.; Stráský, J.; Zháňal, P.; Veselý, J.; Polyakova, V.; Semenova, I.; Janeček, M.

2017-05-01

Processing of metastable titanium alloys by severe plastic deformation provides an opportunity to achieve exceptional grain refinement, to enhance the strength and to affect phase transformations occurring during thermal treatment. The main aim of this study is to investigate the microstructure of ultra-fine grained (UFG) material and effect of microstructural changes on phase transformations in metastable β-Ti alloy Ti-15Mo. Metastable β-Ti alloys are currently the most studied Ti-based materials with prospective use in medicine. Ti-15Mo alloy after solution treatment contains metastable β-phase. Metastable ω-phase and stable α-phase particles are formed upon annealing,. Solution treated Ti-15Mo alloy was deformed by high pressure torsion (HPT) at room temperature. Severely deformed structure after HPT with grain size of ~200 nm was studied by transmission electron microscopy. In-situ electrical resistance measurements showed significant changes in undergoing phase transformations when compared to coarse-grained (CG) material. Scanning electron microscopy revealed heterogeneous precipitation of α-particles at grain boundaries (GB). Due to the high density of GBs in UFG structure, these precipitates are very fine and equiaxed. The study demonstrates that SPD is capable of enhancing mechanical properties due to grain refinement and via affecting precipitation processes in metastable β-Ti alloys.

Behavior of Sn atoms in GeSn thin films during thermal annealing: Ex-situ and in-situ observations

NASA Astrophysics Data System (ADS)

Takase, Ryohei; Ishimaru, Manabu; Uchida, Noriyuki; Maeda, Tatsuro; Sato, Kazuhisa; Lieten, Ruben R.; Locquet, Jean-Pierre

2016-12-01

Thermally induced crystallization processes for amorphous GeSn thin films with Sn concentrations beyond the solubility limit of the bulk crystal Ge-Sn binary system have been examined by X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, and (scanning) transmission electron microscopy. We paid special attention to the behavior of Sn before and after recrystallization. In the as-deposited specimens, Sn atoms were homogeneously distributed in an amorphous matrix. Prior to crystallization, an amorphous-to-amorphous phase transformation associated with the rearrangement of Sn atoms was observed during heat treatment; this transformation is reversible with respect to temperature. Remarkable recrystallization occurred at temperatures above 400 °C, and Sn atoms were ejected from the crystallized GeSn matrix. The segregation of Sn became more pronounced with increasing annealing temperature, and the ejected Sn existed as a liquid phase. It was found that the molten Sn remains as a supercooled liquid below the eutectic temperature of the Ge-Sn binary system during the cooling process, and finally, β-Sn precipitates were formed at ambient temperature.

High field 27Al MAS NMR and TPD studies of active sites in ethanol dehydration using thermally treated transitional aluminas as catalysts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, Jian Zhi; Xu, Suochang; Kwak, Ja Hun

High field quantitative 27Al MAS NMR and temperature programmed desorption (TPD) of ethanol are used to study the surface and phase transformation of gamma-Al2O3 during calcination in the temperature range of 500 to 1300 degrees C. Following ethanol adsorption, ethylene is generated during TPD with a desorption temperature > 200 degrees C. With increasing calcination temperature prior to TPD, the amount of ethylene produced decreases monotonically. Significantly, 27Al MAS NMR reveals that the amount of penta-coordinate Al3+ ions (Lewis acid sites) also decreases with increasing calcination temperature. In fact, a strong correlation between the amount of penta-coordinate Al3+ ions andmore » the amount of strongly adsorbed ethanol molecules (i.e., the ones that convert to ethylene during TPD) is obtained. This result indicates that the penta-coordinate aluminum sites are the catalytic active sites on alumina surfaces during ethanol dehydration reaction across the entire course of gamma- to alpha-Al2O3 phase transformations.« less

Phase Transformation, Surface Morphology and Dielectric Property of P(VDF-HFP)/MgCl2·6H2O Nanocomposites

NASA Astrophysics Data System (ADS)

Yuennan, J.; Sukwisute, P.; Boripet, B.; Muensit, N.

2017-09-01

Nanocomposite piezoelectric films based on the blend of poly(vinylidenefluoride-hexafluoropropylene) (PVDF-HFP) and magnesium chloride hexahydrate (MgCl2•6H2O) have been investigated in this work. The films incorporated with 0.5 wt% MgCl2•6H2O were prepared using a solution casting technique and uniaxially stretched at various ratios from 2 to 6 times in order to characterize phase transformation, surface morphology and dielectric behaviour. The piezoelectric β phase transformation and crystallinity of the stretched films were identified by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). A scanning electron microscopy (SEM) was conducted to observe the surface microstructure and porosity. The frequency dependence of dielectric properties was also measured by LCR meter at room temperature. The stretched films show the larger the stretching ratio, the greater the microdefects appearance. This leads to a decrease of dielectric constant with stretching ratio. Nevertheless, the P(VDF-HFP) nanocomposites with stretching ratio of 4 times display a higher β phase fraction of 90% than the unstretched films. Thus, this result points out that the β phase transformation of the composite films can be enhanced by mechanically stretching process.

Elucidation of structure and nature of the PdO-Pd transformation using in situ PDF and XAS techniques.

PubMed

Keating, Jonathan; Sankar, Gopinathan; Hyde, Timothy I; Kohara, Shinji; Ohara, Koji

2013-06-14

The PdO-Pd phase transformation in a 4 wt% Pd/Al2O3 catalyst has been investigated using in situ X-ray absorption spectroscopy (XAS) and in situ X-ray total scattering (also known as high-energy X-ray diffraction) techniques. Both the partial and total pair distribution functions (PDF) from these respective techniques have been analysed in depth. New information from PDF analysis of total scattering data has been garnered using the differential PDF (d-PDF) approach where only correlations orginating from PdO and metallic Pd are extracted. This method circumvents problems encountered in characerising the catalytically active components due to the diffuse scattering from the disordered γ-Al2O3 support phase. Quantitative analysis of the palladium components within the catalyst allowed for the phase composition to be established at various temperatures. Above 850 °C it was found that PdO had converted to metallic Pd, however, the extent of reduction was of the order ca. 70% Pd metal and 30% PdO. Complementary in situ XANES and EXAFS were performed, with heating to high temperature and subsequent cooling in air, and the results of the analyses support the observations, that residual PdO is detected at elevated temperatures. Hysteresis in the transformation upon cooling is confirmed from XAS studies where reoxidation occurs below 680 °C.

Beamline Electrostatic Levitator (BESL) for in-situ High Energy K-Ray Diffraction Studies of Levitated Solids and Liquids at High Temperature

NASA Technical Reports Server (NTRS)

Gangopadhyay, A. K.; Lee, G. W.; Kelton, K. F.; Rogers, J. R.; Goldman, A. I.; Robinson, D. S.; Rathz, T. J.; Hyers, R. W.

2005-01-01

Determinations of the phase formation sequence, the crystal structures and the thermodynamic properties of materials at high temperatures are difficult because of contamination from the sample container and environment. Containerless processing techniques, such as electrostatic (ESL), electromagnetic (EML), aerodynamic, and acoustic levitation, are most suitable these studies. An adaptation of ESL for in-situ structural studies of a wide range of materials, including metals, semiconductors, insulators using high energy (125 keV) synchrotron x-rays is described here. This beamline ESL (BESL) allows the in-situ determination of the atomic structures of equilibrium solid and liquid phases, including undercooled liquids, as well as real-time studies of solid-solid and liquid-solid phase transformations. The use of image plate (MAR345) or GE-Angio detectors enables fast (30 ms - 1s) acquisition of complete diffraction patterns over a wide q-range (4 - 140/mm). The wide temperature range (300 - 2500 K), containerless processing under high vacuum (10(exp -7) - 10(exp -8) torr), and fast data acquisition, make BESL particularly suitable for phase diagram studies of high temperature materials. An additional, critically important, feature of BESL is the ability to also make simultaneous measurement of a host of thermo-physical properties, including the specific heat, enthalpy of transformation, solidus and liquidus temperatures, density, viscosity, and surface tension; all on the same sample and simultaneous with the structural measurements.

A high-stability non-contact dilatometer for low-amplitude temperature-modulated measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luckabauer, Martin; Sprengel, Wolfgang; Würschum, Roland

2016-07-15

Temperature modulated thermophysical measurements can deliver valuable insights into the phase transformation behavior of many different materials. While especially for non-metallic systems at low temperatures numerous powerful methods exist, no high-temperature device suitable for modulated measurements of bulk metallic alloy samples is available for routine use. In this work a dilatometer for temperature modulated isothermal and non-isothermal measurements in the temperature range from room temperature to 1300 K is presented. The length measuring system is based on a two-beam Michelson laser interferometer with an incremental resolution of 20 pm. The non-contact measurement principle allows for resolving sinusoidal length change signalsmore » with amplitudes in the sub-500 nm range and physically decouples the length measuring system from the temperature modulation and heating control. To demonstrate the low-amplitude capabilities, results for the thermal expansion of nickel for two different modulation frequencies are presented. These results prove that the novel method can be used to routinely resolve length-change signals of metallic samples with temperature amplitudes well below 1 K. This high resolution in combination with the non-contact measurement principle significantly extends the application range of modulated dilatometry towards high-stability phase transformation measurements on complex alloys.« less

One-pot solvothermal synthesis of dual-phase titanate/titania Nanoparticles and their adsorption and photocatalytic Performances

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheng, Yu Hua; Gong, Dangguo; Tang, Yuxin

2014-06-01

Dual phase titanate/titania nanoparticles undergo phase transformation gradually with the increase of solvothermal synthesis temperature from 100 °C to 200 °C, and eventually are fully transformed into anatase TiO{sub 2}. The crystal structure change results in the changes of optical absorption, sensitizer/dopant formation and surface area of the materials which finally affect the overall dye removal ability. Reactions under dark and light have been conducted to distinguish the contributions of surface adsorption from photocatalytic degradation. The sample synthesized at 160 °C (S160) shows the best performances for both adsorption under dark and photocatalytic degradation of methylene blue (MB) under visiblemore » light irradiation. The adsorption mechanism for S160 is determined as monolayer adsorption based on the adsorption isotherm test under dark condition, and an impressive adsorption capacity of 162.19 mg/g is achieved. For the photocatalytic application, this sample at 0.1 g/L loading is also able to degrade 20 ppm MB within 6 hours under the visible light (>420 nm) condition. - Graphical abstract: The effect of solvothermal synthesis temperature on the formation and dye removal performance of dual phase titanate/titania nanoparticles was unveiled and optimized. - Highlights: • Low temperature one-pot solvothermal synthesis of dual-phase photocatalysts. • Correlation of the synthesis temperature is made with the phase composition. • Adsorption isotherm, kinetics, photocatalytic degradation were studied. • Synthesis at 160 °C yields the best material for adsorption of MB in dark. • The same sample also shows the best visible light degradation of MB.« less

Microfabricated therapeutic actuators

DOEpatents

Lee, Abraham P.; Northrup, M. Allen; Ciarlo, Dino R.; Krulevitch, Peter A.; Benett, William J.

1999-01-01

Microfabricated therapeutic actuators are fabricated using a shape memory polymer (SMP), a polyurethane-based material that undergoes a phase transformation at a specified temperature (Tg). At a temperature above temperature Tg material is soft and can be easily reshaped into another configuration. As the temperature is lowered below temperature Tg the new shape is fixed and locked in as long as the material stays below temperature Tg. Upon reheating the material to a temperature above Tg, the material will return to its original shape. By the use of such SMP material, SMP microtubing can be used as a release actuator for the delivery of embolic coils through catheters into aneurysms, for example. The microtubing can be manufactured in various sizes and the phase change temperature Tg is determinate for an intended temperature target and intended use.

Microfabricated therapeutic actuators

DOEpatents

Lee, A.P.; Northrup, M.A.; Ciarlo, D.R.; Krulevitch, P.A.; Benett, W.J.

1999-06-15

Microfabricated therapeutic actuators are fabricated using a shape memory polymer (SMP), a polyurethane-based material that undergoes a phase transformation at a specified temperature (Tg). At a temperature above temperature Tg material is soft and can be easily reshaped into another configuration. As the temperature is lowered below temperature Tg the new shape is fixed and locked in as long as the material stays below temperature Tg. Upon reheating the material to a temperature above Tg, the material will return to its original shape. By the use of such SMP material, SMP microtubing can be used as a release actuator for the delivery of embolic coils through catheters into aneurysms, for example. The microtubing can be manufactured in various sizes and the phase change temperature Tg is determinate for an intended temperature target and intended use. 8 figs.

Release mechanism utilizing shape memory polymer material

DOEpatents

Lee, Abraham P.; Northrup, M. Allen; Ciarlo, Dino R.; Krulevitch, Peter A.; Benett, William J.

2000-01-01

Microfabricated therapeutic actuators are fabricated using a shape memory polymer (SMP), a polyurethane-based material that undergoes a phase transformation at a specified temperature (Tg). At a temperature above temperature Tg material is soft and can be easily reshaped into another configuration. As the temperature is lowered below temperature Tg the new shape is fixed and locked in as long as the material stays below temperature Tg. Upon reheating the material to a temperature above Tg, the material will return to its original shape. By the use of such SMP material, SMP microtubing can be used as a release actuator for the delivery of embolic coils through catheters into aneurysms, for example. The microtubing can be manufactured in various sizes and the phase change temperature Tg is determinate for an intended temperature target and intended use.

Studies on Se75Te25-x In x chalcogenide glasses; a material for phase change memory

NASA Astrophysics Data System (ADS)

Srivastava, Archana; Tiwari, S. N.; Alvi, M. A.; Khan, Shamshad A.

2018-01-01

This research paper describes the non-isothermal crystallization during phase transformation in Se75Te25-x In x glasses synthesized by melt quenching method. For crystallization studies in these glasses, non-isothermal differential scanning calorimetry (DSC) measurements was done at constant heating rates of 5, 10, 15, 20 and 25 K min-1 in air atmosphere. The glass transition temperature (T g), on-set crystallization temperature (T c), peak crystallization temperature (T p) and melting temperatures (T m) were derived by DSC thermograms. Using various thermal parameters the activation energy of glass transition and crystallization were determined by using Kissinger, Moynihan and Ozawa approaches and found to be in good agreement. The value of the activation energy of glass transition (ΔE t) was found to be minimum for Se75Te19In6 alloys confirming its maximum probability of transition in a metastable state. Thermal stability parameters of Se75Te25-x In x were determined and found to be increased with indium content. High resolution x-ray diffraction and field emission scanning electron microscopy studies were employed for the study of phase transformation in Se75Te25-x In x glasses. The outcome of these studies shows that the investigated materials may be suitable for phase change memory devices.

Pretransitional phenomena and pinning in liquid-crystalline blue phases

NASA Astrophysics Data System (ADS)

Demikhov, E.; Stegemeyer, H.; Tsukruk, V.

1992-10-01

Blue phases (BP's) in liquid-crystalline systems of high chirality exhibiting a short cholesteric temperature interval are investigated. In a BP I supercooled with respect to the cholesteric phase, the orientation of the cubic lattice with the (1,1,0) wave vector perpendicular to the substrate is spontaneously turned to a [200] orientation within small areas of several tenths of micrometers in diameter. A pinning of BP I lattice temperature waves is observed on the [200] orientational inhomogeneities. The pinning effect explains the observed saturation of the BP I lattice constant on decreasing temperature and its dependence on the cooling rate observed in supercooled region. A different type of cubic blue phase, BP S (``S'' represents supercooled), is observed transforming reversibly from the supercooled BP I but metastable with respect to the cholesteric phase. The BP S has two scales of order: a long-range orientational blue-phase-like order and a short-range positional smecticlike order.

Phase change studies in Se85In15-xZnx chalcogenide thin films

NASA Astrophysics Data System (ADS)

Srivastava, Archana; Tiwari, S. N.; Alvi, M. A.; Khan, Shamshad A.

2018-03-01

This research work describes the phase change studies in Se85In15-xZnx thin films at various annealing temperatures. Glassy samples of Se85In15-xZnx were synthesized by the melt quenching method and thin films of thickness 400 nm were prepared by the vacuum evaporation technique on a glass/Si wafer substrate. The glass transition temperature (Tg) and the on-set crystallization temperature (Tc) of the prepared alloys were evaluated by non-isothermal differential scanning calorimetry studies. Thin films were annealed at three temperatures 330 K, 340 K, and 350 K (which are in between Tg and Tc of the synthesized samples) in a vacuum furnace for 2 h. High resolution X-ray diffraction studies demonstrate that the as-prepared films are amorphous in nature whereas the annealed films are of crystalline/polycrystalline in nature. Field emission scanning electron microscopy studies of thin films (as-deposited and crystallized) confirm the phase transformation in Se85In15-xZnx thin films. Optical band gaps were calculated from the Tauc's extrapolation procedure and were found to be enhanced with the Zn concentration and decrease with the increasing annealing temperature. Various optical parameters were evaluated for as-prepared and annealed Se85In15-xZnx thin films. The changes in optical parameters with annealing temperature were described on the basis of structural relaxation as well as changes in defect states and density of localized states during amorphous to crystalline phase transformation in Se85In15-xZnx thin films.

Absolute determination of the gelling point of gelatin under quasi-thermodynamic equilibrium.

PubMed

Bellini, Franco; Alberini, Ivana; Ferreyra, María G; Rintoul, Ignacio

2015-05-01

Thermodynamic studies on phase transformation of biopolymers in solution are useful to understand their nature and to evaluate their technological potentials. Thermodynamic studies should be conducted avoiding time-related phenomena. This condition is not easily achieved in hydrophilic biopolymers. In this contribution, the simultaneous effects of pH, salt concentration, and cooling rate (Cr) on the folding from random coil to triple helical collagen-like structures of gelatin were systematically studied. The phase transformation temperature at the absolute invariant condition of Cr = 0 °C/min (T(T)Cr=0) ) is introduced as a conceptual parameter to study phase transformations in biopolymers under quasi-thermodynamic equilibrium and avoiding interferences coming from time-related phenomena. Experimental phase diagrams obtained at different Cr are presented. The T(T)(Cr=0) compared with pH and TT(Cr=0) compared with [NaCl] diagram allowed to explore the transformation process at Cr = 0 °C/min. The results were explained by electrostatic interactions between the biopolymers and its solvation milieu. © 2015 Institute of Food Technologists®

Real-Time Observation of Order-Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites.

PubMed

Yang, Bin; Ming, Wenmei; Du, Mao-Hua; Keum, Jong K; Puretzky, Alexander A; Rouleau, Christopher M; Huang, Jinsong; Geohegan, David B; Wang, Xiaoping; Xiao, Kai

2018-05-01

A fundamental understanding of the interplay between the microscopic structure and macroscopic optoelectronic properties of organic-inorganic hybrid perovskite materials is essential to design new materials and improve device performance. However, how exactly the organic cations affect the structural phase transition and optoelectronic properties of the materials is not well understood. Here, real-time, in situ temperature-dependent neutron/X-ray diffraction and photoluminescence (PL) measurements reveal a transformation of the organic cation CH 3 NH 3 + from order to disorder with increasing temperature in CH 3 NH 3 PbBr 3 perovskites. The molecular-level order-to-disorder transformation of CH 3 NH 3 + not only leads to an anomalous increase in PL intensity, but also results in a multidomain to single-domain structural transition. This discovery establishes the important role that organic cation ordering has in dictating structural order and anomalous optoelectronic phenomenon in hybrid perovskites. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The α-γ-ɛ triple point and phase boundaries of iron under shock compression

NASA Astrophysics Data System (ADS)

Li, Jun; Wu, Qiang; Xue, Tao; Geng, Huayun; Yu, Jidong; Jin, Ke; Li, Jiabo; Tan, Ye; Xi, Feng

2017-07-01

The phase transition of iron under shock compression has attracted much attention in recent decades because of its importance in fields such as condensed matter physics, geophysics, and metallurgy. At room temperature, the transition of iron from the α-phase (bcc) to the ɛ-phase (hpc) occurs at a stress of 13 GPa. At high temperature, a triple point followed by transformation to the γ-phase (fcc) is expected. However, the details of the high-temperature phase transitions of iron are still under debate. Here, we investigate the phase-transition behavior of polycrystalline iron under compression from room temperature to 820 K. The results show that the shock-induced phase transition is determined unequivocally from the measured three-wave-structure profiles, which clearly consist of an elastic wave, a plastic wave, and a phase-transition wave. The phase transition is temperature-dependent, with an average rate Δσtr/ΔT of -6.91 MPa/K below 700 K and -34.7 MPa/K at higher temperatures. The shock α-ɛ and α-γ phase boundaries intersect at 10.6 ± 0.53 GPa and 763 K, which agrees with the α-ɛ-γ triple point from early shock wave experiments and recent laser-heated diamond-anvil cell resistivity and in situ X-ray diffraction data but disagrees with the shock pressure-temperature phase diagram reported in 2009 by Zaretsky [J. Appl. Phys. 106, 023510 (2009)].

Kinetic boundaries and phase transformations of ice i at high pressure.

PubMed

Wang, Yu; Zhang, Huichao; Yang, Xue; Jiang, Shuqing; Goncharov, Alexander F

2018-01-28

Raman spectroscopy in diamond anvil cells has been employed to study phase boundaries and transformation kinetics of H 2 O ice at high pressures up to 16 GPa and temperatures down to 15 K. Ice i formed at nearly isobaric cooling of liquid water transforms on compression to high-density amorphous (HDA) ice at 1.1-3 GPa at 15-100 K and then crystallizes in ice vii with the frozen-in disorder (ice vii') which remains stable up to 14.1 GPa at 80 K and 15.9 GPa at 100 K. Unexpectedly, on decompression of ice vii', it transforms to ice viii in its domain of metastability, and then it relaxes into low-density amorphous (LDA) ice on a subsequent pressure release and warming up. On compression of ice i at 150-170 K, ice ix is crystallized and no HDA ice is found; further compression of ice ix results in the sequential phase transitions to stable ices vi and viii. Cooling ice i to 210 K at 0.3 GPa transforms it to a stable ice ii. Our extensive investigations provide previously missing information on the phase diagram of water, especially on the kinetic paths that result in formation of phases which otherwise are not accessible; these results are keys for understanding the phase relations including the formation of metastable phases. Our observations inform on the ice modifications that can occur naturally in planetary environments and are not accessible for direct observations.

Kinetic boundaries and phase transformations of ice i at high pressure

NASA Astrophysics Data System (ADS)

Wang, Yu; Zhang, Huichao; Yang, Xue; Jiang, Shuqing; Goncharov, Alexander F.

2018-01-01

Raman spectroscopy in diamond anvil cells has been employed to study phase boundaries and transformation kinetics of H2O ice at high pressures up to 16 GPa and temperatures down to 15 K. Ice i formed at nearly isobaric cooling of liquid water transforms on compression to high-density amorphous (HDA) ice at 1.1-3 GPa at 15-100 K and then crystallizes in ice vii with the frozen-in disorder (ice vii') which remains stable up to 14.1 GPa at 80 K and 15.9 GPa at 100 K. Unexpectedly, on decompression of ice vii', it transforms to ice viii in its domain of metastability, and then it relaxes into low-density amorphous (LDA) ice on a subsequent pressure release and warming up. On compression of ice i at 150-170 K, ice ix is crystallized and no HDA ice is found; further compression of ice ix results in the sequential phase transitions to stable ices vi and viii. Cooling ice i to 210 K at 0.3 GPa transforms it to a stable ice ii. Our extensive investigations provide previously missing information on the phase diagram of water, especially on the kinetic paths that result in formation of phases which otherwise are not accessible; these results are keys for understanding the phase relations including the formation of metastable phases. Our observations inform on the ice modifications that can occur naturally in planetary environments and are not accessible for direct observations.

Length scale effects and multiscale modeling of thermally induced phase transformation kinetics in NiTi SMA

NASA Astrophysics Data System (ADS)

Frantziskonis, George N.; Gur, Sourav

2017-06-01

Thermally induced phase transformation in NiTi shape memory alloys (SMAs) shows strong size and shape, collectively termed length scale effects, at the nano to micrometer scales, and that has important implications for the design and use of devices and structures at such scales. This paper, based on a recently developed multiscale model that utilizes molecular dynamics (MDs) simulations at small scales and MD-verified phase field (PhF) simulations at larger scales, reports results on specific length scale effects, i.e. length scale effects in martensite phase fraction (MPF) evolution, transformation temperatures (martensite and austenite start and finish) and in the thermally cyclic transformation between austenitic and martensitic phase. The multiscale study identifies saturation points for length scale effects and studies, for the first time, the length scale effect on the kinetics (i.e. developed internal strains) in the B19‧ phase during phase transformation. The major part of the work addresses small scale single crystals in specific orientations. However, the multiscale method is used in a unique and novel way to indirectly study length scale and grain size effects on evolution kinetics in polycrystalline NiTi, and to compare the simulation results to experiments. The interplay of the grain size and the length scale effect on the thermally induced MPF evolution is also shown in this present study. Finally, the multiscale coupling results are employed to improve phenomenological material models for NiTi SMA.

First-principles study on influence of molybdenum on acicular ferrite formation on TiC particles in microallyed steels

NASA Astrophysics Data System (ADS)

Hua, Guomin; Li, Changsheng; Cheng, Xiaonong; Zhao, Xinluo; Feng, Quan; Li, Zhijie; Li, Dongyang; Szpunar, Jerzy A.

2018-01-01

In this study, influences of molybdenum on acicular ferrite formation on precipitated TiC particles are investigated from thermodynamic and kinetic respects. In thermodynamics, Segregation of Mo towards austenite/TiC interface releases the interfacial energy and induces phase transformation from austenite to acicular ferrite on the precipitated TiC particles. The Phase transformation can be achieved by displacive deformation along uniaxial Bain path. In addition, the segregation of Mo atom will also lead to the enhanced stability of ferrite in comparison with austenite no matter at low temperature or at high temperature. In kinetics, the Mo solute in acicular ferrite can effectively suppress the diffusion of carbon atoms, which ensures that orientation relationship between acicular ferrite and austenitized matrix can be satisfied during the diffusionless phase transformation. In contrast to ineffectiveness of TiC particles, the alloying Mo element can facilitate the formation of acicular ferrite on precipitated TiC particles, which is attributed to the above thermodynamic and kinetic reasons. Furthermore, Interfacial toughness and ductility of as-formed acicular ferrite/TiC interface can be improved simultaneously by segregation of Mo atom.

Anomalous behaviour of thermodynamic properties at successive phase transitions in (NH4)3GeF7

NASA Astrophysics Data System (ADS)

Bogdanov, Evgeniy V.; Kartashev, Andrey V.; Pogoreltsev, Evgeniy I.; Gorev, Mikhail V.; Laptash, Natalia M.; Flerov, Igor N.

2017-12-01

Heat capacity, thermal dilatation, susceptibility to hydrostatic pressure and dielectric properties associated with succession of three phase transitions below room temperature in double fluoride salt (NH4)3GeF7 were studied. A possible transformation into the parent Pm-3m cubic phase was not observed up to the decomposition of compound. Nonferroelectric nature of structural distortions was confirmed. The DTA under pressure studies revealed a high temperature stability of two phases: P4/mbm and Pbam. The entropies of the phase transitions agree well with the model of structural distortions. Analysis of the thermal properties associated with the individual phase transitions in the framework of thermodynamic equations has shown a high reliability of the data obtained.

Joining of parts via magnetic heating of metal aluminum powders

DOEpatents

Baker, Ian

2013-05-21

A method of joining at least two parts includes steps of dispersing a joining material comprising a multi-phase magnetic metal-aluminum powder at an interface between the at least two parts to be joined and applying an alternating magnetic field (AMF). The AMF has a magnetic field strength and frequency suitable for inducing magnetic hysteresis losses in the metal-aluminum powder and is applied for a period that raises temperature of the metal-aluminum powder to an exothermic transformation temperature. At the exothermic transformation temperature, the metal-aluminum powder melts and resolidifies as a metal aluminide solid having a non-magnetic configuration.

Rapid Solidification and Phase Transformations in Additive Manufactured Materials

DOE PAGES

Asle Zaeem, Mohsen; Clarke, Amy Jean

2016-01-14

Within the past few years, additive manufacturing (AM) has emerged as a promising manufacturing technique to enable the production of complex engineering structures with high efficiency and accuracy. Among the important factors establishing AM as a sustainable manufacturing process is the ability to control the microstructures and properties of AM products. In most AM processes, such as laser sintering (LS), laser melting (LM), and laser metal deposition (LMD), rapid solidification and high-temperature phase transformations play primary roles in determining nano- and microstructures, and consequently the mechanical and other properties of AM products. This topic of JOM is dedicated to summarizingmore » the current research efforts in the area of rapid solidification and phase transformations in additively manufactured materials. Finally, a brief summary follows below of 10 journal articles in this topic.« less

Thermal, spectroscopic and structural characterization of isostructural phase transition in 4-hydroxybenzaldehyde

NASA Astrophysics Data System (ADS)

Panicker, Lata

2018-05-01

Polycrystalline samples of 4-hydroxybenzaldehyde (4-HOBAL) were investigated using differential scanning calorimeter (DSC), Raman spectroscopy and X-ray powder diffraction. The DSC data indicated that 4-HOBAL on heating undergoes a polymorphic transformation from polymorph I to polymorph II. The polymorph II formed remains metastable at ambient condition and transforms to polymorph I when annealed at ambient temperature for more than seven days. The structural information of polymorphs I and II obtained using its X-ray powder diffraction patterns indicated that 4-HOBAL undergoes an isostructural phase transition from polymorph I (monoclinic, P21/c) to polymorph II (monoclinic, P21/c). Raman data suggest that this structural change is associated with some change in its molecular interactions. Thus, in 4-HOBAL the polymorphic phase transformation (II to I) even though energetically favoured is kinetically hindered.

Comparative study on microstructure and martensitic transformation of aged Ni-rich NiTi and NiTiCo shape memory alloys

NASA Astrophysics Data System (ADS)

El-Bagoury, Nader

2016-05-01

In this article the influence of aging heat treatment conditions of 250, 350, 450 and 550 °C for 3 h on the microstructure, martensitic transformation temperatures and mechanical properties of Ni51Ti49Co0 and Ni47 Ti49Co4 shape memory alloys was investigated. This comparative study was carried out using X-ray diffraction analysis, scanning electron microscope, energy dispersive spectrometer, differential scanning calorimeter and Vickers hardness tester. The results show that the microstructure of both aged alloys contains martensite phase and Ti2Ni in addition to some other precipitates. The martensitic transformation temperature was increased steadily by increasing the ageing temperature and lowering the value of valence electron number (ev/a) and concentration. Moreover, the hardness measurements were gradually increased at first by increasing the aging temperature from 250 to 350 °C. Further elevating in aging temperature to 450 and 550 °C decreases the hardness value.

Calibration of the Geostationary Imaging Fourier Transform Spectrometer (GIFTS)

NASA Technical Reports Server (NTRS)

Best, F. A.; Revercomb, H. E.; Bingham, G. E.; Knuteson, R. O.; Tobin, D. C.; LaPorte, D. D.; Smith, W. L.

2001-01-01

The NASA New Millennium Program's Geostationary Imaging Fourier Transform Spectrometer (GIFTS) requires highly accurate radiometric and spectral calibration in order to carry out its mission to provide water vapor, wind, temperature, and trace gas profiling from geostationary orbit. A calibration concept has been developed for the GIFTS Phase A instrument design. The in-flight calibration is performed using views of two on-board blackbody sources along with cold space. A radiometric calibration uncertainty analysis has been developed and used to show that the expected performance for GIFTS exceeds its top level requirement to measure brightness temperature to better than 1 K. For the Phase A GIFTS design, the spectral calibration is established by the highly stable diode laser used as the reference for interferogram sampling, and verified with comparisons to atmospheric calculations.

Phase-Transformation Ductilization of Brittle High-Entropy Alloys via Metastability Engineering

DOE PAGES

Huang, Hailong; Wu, Yuan; He, Junyang; ...

2017-06-07

High-entropy alloys (HEAs) in which interesting physical, chemical, and structural properties are being continuously revealed have recently attracted extensive attention. Body-centered cubic (bcc) HEAs, particularly those based on refractory elements are promising for high-temperature application but generally fail by early cracking with limited plasticity at room temperature, which limits their malleability and widespread uses. In this paper, the “metastability-engineering” strategy is exploited in brittle bcc HEAs via tailoring the stability of the constituent phases, and transformation-induced ductility and work-hardening capability are successfully achieved. Finally, this not only sheds new insights on the development of HEAs with excellent combination of strengthmore » and ductility, but also has great implications on overcoming the long-standing strength–ductility tradeoff of metallic materials in general.« less

Phase-Transformation Ductilization of Brittle High-Entropy Alloys via Metastability Engineering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, Hailong; Wu, Yuan; He, Junyang

High-entropy alloys (HEAs) in which interesting physical, chemical, and structural properties are being continuously revealed have recently attracted extensive attention. Body-centered cubic (bcc) HEAs, particularly those based on refractory elements are promising for high-temperature application but generally fail by early cracking with limited plasticity at room temperature, which limits their malleability and widespread uses. In this paper, the “metastability-engineering” strategy is exploited in brittle bcc HEAs via tailoring the stability of the constituent phases, and transformation-induced ductility and work-hardening capability are successfully achieved. Finally, this not only sheds new insights on the development of HEAs with excellent combination of strengthmore » and ductility, but also has great implications on overcoming the long-standing strength–ductility tradeoff of metallic materials in general.« less

Magnetically Controlled Shape Memory Behaviour—Materials and Applications

NASA Astrophysics Data System (ADS)

Gandy, A. P.; Sheikh, A.; Neumann, K.; Neumann, K.-U.; Pooley, D.; Ziebeck, K. R. A.

2008-06-01

For most metals a microscopic change in shape occurs above the elastic limit by the irreversible creation and movement of dislocations. However a large number of metallic systems undergo structural, martensitic, phase transformations which are diffusionless, displacive first order transitions from a high-temperature phase to one of lower symmetry below a certain temperature TM. These transitions which have been studied for more than a century are of vital importance because of their key role in producing shape memory phenomena enabling the system to reverse large deformations in the martensitic phase by heating into the austenite phase. In addition to a change in shape (displacement) the effect can also produce a force or a combination of both. Materials having this unique property are increasing being used in medical applications—scoliosis correction, arterial clips, stents, orthodontic wire, orthopaedic implants etc. The structural phase transition essential for shape memory behaviour is usually activated by a change in temperature or applied stress. However for many applications such as for actuators the transformation is not sufficiently rapid. Poor energy conversion also limits the applicability of many shape memory alloys. In medicine a change of temperature or pressure is often inappropriate and new ferromagnetic materials are being considered in which the phenomena can be controlled by an applied magnetic field at constant temperature. In order to achieve this, it is important to optimise three fundamental parameters. These are the saturation magnetisation σs, the Curie temperature Tc and the martensitic temperature TM. Here, σs is important because the magnetic pressure driving the twin boundary motion is 2σsH. Furthermore the material must be in the martensitic state at the operating temperature which should be at or above room temperature. This may be achieved by alloying or controlling the stoichiometry. Recently new intermetallic compounds based on the ferromagnetic prototype Ni2MnGa have been discovered which offer the possibility of controlling the structural phase transition by a magnetic field, hence opening up new possible applications particularly in the field of medicine. The properties of these new materials will be presented and their suitability for applications discussed.

Crystal grain growth at the α -uranium phase transformation in praseodymium

NASA Astrophysics Data System (ADS)

Cunningham, Nicholas C.; Velisavljevic, Nenad; Vohra, Yogesh K.

2005-01-01

Structural phase transformations under pressure are examined in praseodymium metal for the range 0-40GPa at ambient temperature. Pressure was generated with a diamond-anvil cell, and data were collected using high-resolution synchrotron x-ray diffraction and the image plate technique. The structural sequence double hexagonal close packed (dhcp)→face centered cubic (fcc)→distorted-fcc (d-fcc)→ α -uranium (α-U) is observed with increasing pressure. Rietveld refinement of all crystallographic phases provided confirmation of the hR24 structure for the d-fcc phase while the previously reported monoclinic phase between the d-fcc and the α-U phase was not confirmed. We observe dramatic crystal grain growth during the volume collapse concurrent with the symmetry-lowering transition to the α-U structure. No preferred orientation axis is observed, and the formation process for these large grains is expected to be via a nucleation and growth mechanism. An analogous effect in rare earth metal cerium suggests that the grain growth during transformation to the α-U structure is a common occurrence in f -electron metals at high pressures.

Soft x-ray resonant diffraction study of magnetic structure in magnetoelectric Y-type hexaferrite

NASA Astrophysics Data System (ADS)

Ueda, H.; Tanaka, Y.; Wakabayashi, Y.; Kimura, T.

2018-05-01

The effect of magnetic field on the magnetic structure associated with magnetoelectric properties in a Y-type hexaferrite, Ba1.3Sr0.7CoZnFe11AlO22, was investigated by utilizing the soft x-ray resonant diffraction technique. In this hexaferrite, the so-called alternating longitudinal conical phase is stabilized at room temperature and zero magnetic field. Below room temperature, however, this phase is transformed into the so-called transverse conical phase by applying an in-plane magnetic field (≈ 0.3 T). The transverse conical phase persists even after removing the magnetic field. The magnetoelectricity, which is magnetically-induced electric polarization, observed in the hexaferrite is discussed in terms of the temperature-dependent magnetic structure at zero field.

Anisotropic Negative Thermal Expansion Behavior of the As-Fabricated Ti-Rich and Equiatomic Ti-Ni Alloys Induced by Preferential Grain Orientation

NASA Astrophysics Data System (ADS)

Zhao, Zhong-Xun; Ma, Xiao; Cao, Shan-Shan; Ke, Chang-Bo; Zhang, Xin-Ping

2017-12-01

The present study focuses on the anisotropic negative thermal expansion (NTE) behaviors of Ti-rich (Ti54Ni46) and equiatomic Ti-Ni (Ti50Ni50) alloys fabricated by vacuum arc melting and without subsequent plastic deformation. Both alloys exhibit NTE responses in vertical and horizontal directions, and the total strains and CTEs of the NTE stage along the two mutually perpendicular measuring directions are obviously different, indicating obvious anisotropic NTE behavior of the alloys. Besides, the numerical differences between the starting temperature of NTE and austenitic transformation and between the finishing temperature of NTE and austenitic transformation are very small, which indicates that an apparent relationship exists between the NTE behavior and the phase transformation. The microstructure in the vertical cross sections shows obviously preferential orientation characteristics: Ti2Ni phases of both alloys grow along the vertical direction, and B19' martensite of Ti50Ni50 alloy has distinct preferential orientation, which results from a large temperature gradient between the top and the bottom of the button ingots during solidification. The microstructure with preferential orientation induces the anisotropic NTE behavior of the samples.

Anisotropic Negative Thermal Expansion Behavior of the As-Fabricated Ti-Rich and Equiatomic Ti-Ni Alloys Induced by Preferential Grain Orientation

NASA Astrophysics Data System (ADS)

Zhao, Zhong-Xun; Ma, Xiao; Cao, Shan-Shan; Ke, Chang-Bo; Zhang, Xin-Ping

2018-03-01

The present study focuses on the anisotropic negative thermal expansion (NTE) behaviors of Ti-rich (Ti54Ni46) and equiatomic Ti-Ni (Ti50Ni50) alloys fabricated by vacuum arc melting and without subsequent plastic deformation. Both alloys exhibit NTE responses in vertical and horizontal directions, and the total strains and CTEs of the NTE stage along the two mutually perpendicular measuring directions are obviously different, indicating obvious anisotropic NTE behavior of the alloys. Besides, the numerical differences between the starting temperature of NTE and austenitic transformation and between the finishing temperature of NTE and austenitic transformation are very small, which indicates that an apparent relationship exists between the NTE behavior and the phase transformation. The microstructure in the vertical cross sections shows obviously preferential orientation characteristics: Ti2Ni phases of both alloys grow along the vertical direction, and B19' martensite of Ti50Ni50 alloy has distinct preferential orientation, which results from a large temperature gradient between the top and the bottom of the button ingots during solidification. The microstructure with preferential orientation induces the anisotropic NTE behavior of the samples.

The Influence of Annealing Temperature and Time on the Formation of δ-Phase in Additively-Manufactured Inconel 625

NASA Astrophysics Data System (ADS)

Stoudt, M. R.; Lass, E. A.; Ng, D. S.; Williams, M. E.; Zhang, F.; Campbell, C. E.; Lindwall, G.; Levine, L. E.

2018-07-01

This research evaluated the kinetics of δ-phase growth in laser powder bed additively-manufactured (AM) Inconel 625 during post-build stress-relief heat treatments. The temperatures ranged between 650 °C and 1050 °C, and the times from 0.25 to 168 hours. The presence of δ-phase was verified for each temperature/time combination through multiple techniques. A conventional time-temperature-transformation diagram was constructed from the time-temperature data. Comparison to the growth in wrought IN625 with a similar nominal composition revealed that δ-phase formation occurred at least two orders of magnitude faster in the AM IN625. The results of this study also revealed that the segregated microstructure in the as-built condition has a strong influence on the kinetics of δ-phase formation in AM IN625 as compared to a homogenized material. Since control of the δ-phase growth is essential for reliable prediction of the performance of IN625 components in service, avoiding heat treatments that promote the formation of δ-phase in AM components that are not homogenized is highly recommended. This will be particularly true at elevated temperatures where the microstructural stability and the consistency of mechanical properties are more likely to be affected by the presence of δ-phase.

The Influence of Annealing Temperature and Time on the Formation of δ-Phase in Additively-Manufactured Inconel 625

NASA Astrophysics Data System (ADS)

Stoudt, M. R.; Lass, E. A.; Ng, D. S.; Williams, M. E.; Zhang, F.; Campbell, C. E.; Lindwall, G.; Levine, L. E.

2018-05-01

This research evaluated the kinetics of δ-phase growth in laser powder bed additively-manufactured (AM) Inconel 625 during post-build stress-relief heat treatments. The temperatures ranged between 650 °C and 1050 °C, and the times from 0.25 to 168 hours. The presence of δ-phase was verified for each temperature/time combination through multiple techniques. A conventional time-temperature-transformation diagram was constructed from the time-temperature data. Comparison to the growth in wrought IN625 with a similar nominal composition revealed that δ-phase formation occurred at least two orders of magnitude faster in the AM IN625. The results of this study also revealed that the segregated microstructure in the as-built condition has a strong influence on the kinetics of δ-phase formation in AM IN625 as compared to a homogenized material. Since control of the δ-phase growth is essential for reliable prediction of the performance of IN625 components in service, avoiding heat treatments that promote the formation of δ-phase in AM components that are not homogenized is highly recommended. This will be particularly true at elevated temperatures where the microstructural stability and the consistency of mechanical properties are more likely to be affected by the presence of δ-phase.

Annealing effects on the structural and magnetic properties of off-stoichiometric Fe-Mn-Ga ferromagnetic shape memory alloys

DOE PAGES

Chen, Yan; Bei, Hongbin; Dela Cruz, Clarina R; ...

2016-05-07

Annealing plays an important role in modifying structures and properties of ferromagnetic shape memory alloys (FSMAs). The annealing effect on the structures and magnetic properties of off-stoichiometric Fe 45Mn 26Ga 29 FSMA has been investigated at different elevated temperatures. Rietveld refinements of neutron diffraction patterns display that the formation of the γ phase in Fe 45Mn 26Ga 29 annealed at 1073 K increases the martensitic transformation temperature and reduces the thermal hysteresis in comparison to the homogenized sample. The phase segregation of a Fe-rich cubic phase and a Ga-rich cubic phase occurs at the annealing temperature of 773 K. Themore » atomic occupancies of the alloys are determined thanks to the neutron's capability of differentiating transition metals. The annealing effects at different temperatures introduce a different magnetic characteristic that is associated with distinctive structural changes in the crystal.« less

High-temperature fcc phase of Pr:  Negative thermal expansion and intermediate valence state

NASA Astrophysics Data System (ADS)

Kuznetsov, A. Yu.; Dmitriev, V. P.; Bandilet, O. I.; Weber, H.-P.

2003-08-01

A high-temperature angle-dispersive synchrotron radiation diffraction study has revealed the double hexagonal-close-packed-to-face-centered-cubic (dhcp-to-fcc) transformation in the Pr metal occurring martensitically between 575 and 1035 K. The high-temperature fcc phase shows a negative thermal expansion in the range 600 800 K, attributed to the 4f-electron delocalization. A phenomenological theory is developed, which explains consistently the observed effect in terms of the mean valence variation of the metal as a function of temperature; it also predicts the existence of an isostructural phase transition and of a critical end point of a gas-liquid type in compressed Pr. The analysis of published data on P-T variation of conductivity of Pr supports this prediction.

Phase change in CoTi2 induced by MeV electron irradiation

NASA Astrophysics Data System (ADS)

Zensho, Akihiro; Sato, Kazuhisa; Yasuda, Hidehiro; Mori, Hirotaro

2018-07-01

The phase change induced by MeV electron irradiation in the intermetallic compound E93-CoTi2 was investigated using high-voltage electron microscopy. Under MeV electron irradiation, CoTi2 was first transformed into an amorphous phase and, with continued irradiation, crystallite formation in the amorphous phase (i.e. formation of crystallites of a solid-solution phase within the amorphous phase) was induced. The critical temperature for amorphisation was around 250 K. The total dose (dpa) required for crystallite formation (i.e. that required for partial crystallisation) was high (i.e. 27-80 dpa) and, even after prolonged irradiation, the amorphous phase was retained in the irradiated sample. Such partial crystallisation behaviour of amorphous Co33Ti67 was clearly different from the crystallisation behaviour (i.e. amorphous-to-solid solution, polymorphous transformation) of amorphous Cr67Ti33 reported in the literature. A possible cause of the difference is discussed.

Investigation of the Nd-rich phases in the Nd-Fe-B system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tang, W.; Zhou, S.; Wang, R.

1988-11-15

The crystal structures and the compositions of the Nd-rich phases in the Nd-Fe-B system have been investigated by means of transmission electron microscopy, x-ray diffraction, and Auger spectroscopy techniques. It has been observed that there are two kinds of Nd-rich phases with different structures and compositions. Most of the Nd-rich phases will undergo a phase transformation when the powder is sintered at high temperatures. This phase transformation is accompanied by the introduction of oxygen into the alloy which will change from a ternary system into a quaternary one. Both of the two Nd-rich phases are stable phases in this latermore » system. With the aid of the EDX and the Auger spectroscope, the compositions of the dhcp and the fcc Nd-rich phases have been determined. The Nd contents of the two phases are about 90 and 70 at. %, respectively, with the later phase containing about 15 at. % of oxygen.« less

Intermediate phases in [111]- and [001]-oriented PbMg1/3Nb2/3O3-29PbTiO3 single crystals

NASA Astrophysics Data System (ADS)

Kamzina, L. S.

2017-09-01

Phase transformations in [111]- and [001]-oriented PbMg1/3Nb2/3O3-29PbTiO3 single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral ( R)—tetragonal ( T)—cubic ( C) phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, M a ) is induced after preliminary polarization at room temperature and the R- M a - T- C phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the M a - T- C transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.

Mössbauer study of iron minerals transformations by Fuchsiella ferrireducens

NASA Astrophysics Data System (ADS)

Gracheva, M. A.; Chistyakova, N. I.; Antonova, A. V.; Rusakov, V. S.; Zhilina, T. N.; Zavarzina, D. G.

2017-11-01

Biogenic transformations of iron-containing minerals synthesized ferrihydrite, magnetite and hydrothermal siderite by anaerobic alkaliphilic bacterium Fuchsiella ferrireducens (strain Z-7101T) were studied by 57Fe Mössbauer spectroscopy. Mössbauer investigations of solid phase samples obtained after microbial transformation were carried out at room temperature and at 82 K. It was found that all tested minerals transformed during bacterial growth. In the presence of synthesized ferrihydrite, added as an electron acceptor, a mixture of large (more than 100 nm) and small (˜5 nm) particles of magnetically ordered phase and siderite was formed. Synthesized magnetite that contains both Fe3+ and Fe2+ forms could serve as electron acceptor as well as an electron donor for F.ferrireducens growth. As a result of its biotransformation, no siderite formation was observed while small particles of magnetite were formed. In the case of the addition of siderite as an electron donor formation of a small amount of a new phase containing Fe2+ caused by recrystallization of siderite during bacterial growth was detected.

Heavy ion irradiation-induced microstructural evolution in pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} at room temperature and 723 K

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Q.R.; Zhang, J., E-mail: zhangjian@xmu.edu.cn; Dong, X.N.

Polycrystalline pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated with 600 keV Kr{sup 3+} at room temperature and 723 K to a fluence of 4×10{sup 15} ions/cm{sup 2}, corresponding to an average ballistic damage dose of 10 displacements per atom in the peak damage region. Irradiation-induced microstructural evolution was examined by grazing incidence X-ray diffraction, and cross-sectional transmission electron microscopy. Incomplete amorphization was observed in the sample irradiated at room temperature due to the formation of nano-crystal which has the identical structure of pyrochlore, and the formation of nano-crystal is attributed to the mechanism of epitaxial recrystallization. However, an orderedmore » pyrochlore phase to a swelling disordered fluorite phase transformation is occurred for the Lu{sub 2}Ti{sub 2}O{sub 7} sample irradiated at 723 K, which is due to the disordering of metal cations and anion vacancies. - Graphical Abstract: Polycrystalline pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated with 600 keV Kr{sup 3+} to a fluence of 4×10{sup 15} ions/cm{sup 2} at room temperature and 723 K, Incomplete amorphization was observed in the sample irradiated at room temperature due to the formation of nano-crystal. However, an ordered pyrochlore phase to a swelling disordered fluorite phase transformation is occurred for the Lu{sub 2}Ti{sub 2}O{sub 7} sample irradiated at 723 K, which is due to the disordering of metal cations and anion vacancies. - Highlights: Pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated by heavy ions at RT and 723 K. At RT irradiation, ~75% of amorphization was achieved. The nano-crystals were formed in the damage layer at RT irradiation. The formed nano-crystals enhanced the radiation tolerance of Lu{sub 2}Ti{sub 2}O{sub 7}. A pyrochlore to fluorite phase transformation was observed at 723 K irradiation.« less

Pressure-dependence of the phase transitions and thermal expansion in zirconium and hafnium pyrovanadate

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gallington, Leighanne C.; Hester, Brett R.; Kaplan, Benjamin S.

Low or negative thermal expansion (NTE) has been previously observed in members of the ZrP{sub 2}O{sub 7} family at temperatures higher than their order-disorder phase transitions. The thermoelastic properties and phase behavior of the low temperature superstructure and high temperature negative thermal expansion phases of ZrV{sub 2}O{sub 7} and HfV{sub 2}O{sub 7} were explored via in situ variable temperature/pressure powder x-ray diffraction measurements. The phase transition temperatures of ZrV{sub 2}O{sub 7} and HfV{sub 2}O{sub 7} exhibited a very strong dependence on pressure (∼700 K GPa), with moderate compression suppressing the formation of their NTE phases below 513 K. Compression alsomore » reduced the magnitude of the coefficients of thermal expansion in both the positive and negative thermal expansion phases. Additionally, the high temperature NTE phase of ZrV{sub 2}O{sub 7} was found to be twice as stiff as the low temperature positive thermal expansion superstructure (24 and 12 GPa respectively). - Graphical abstract: The temperature at which ZrV{sub 2}O{sub 7} transforms to a phase displaying negative thermal expansion is strongly pressure dependent. The high temperature form of ZrV{sub 2}O{sub 7} is elastically stiffer than the low temperature form. - Highlights: • The order-disorder phase transition temperatures in ZrV{sub 2}O{sub 7} and HfV{sub 2}O{sub 7} are strongly pressure dependent (∼700 K.GPa). • The high temperature (disordered) phase of ZrV{sub 2}O{sub 7} is much stiffer than the ambient temperature (ordered) phase. • Compression reduces the magnitude of the negative thermal expansion in the high temperature phase of ZrV{sub 2}O{sub 7}.« less

High-pressure transformation in the cobalt spinel ferrites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blasco, J., E-mail: jbc@posta.unizar.es; Subías, G.; García, J.

2015-01-15

We report high pressure angle-dispersive x-ray diffraction measurements on Co{sub x}Fe{sub 3−x}O{sub 4} (x=1, 1.5, 1.75) spinels at room temperature up to 34 GPa. The three samples show a similar structural phase transformation from the cubic spinel structure to an analogous post-spinel phase at around 20 GPa. Spinel and post-spinel phases coexist in a wide pressure range (∼20–25 GPa) and the transformation is irreversible. The equation of state of the three cubic spinel ferrites was determined and our results agree with the data obtained in related oxide spinels showing the role of the pressure-transmitting medium for the accurate determination ofmore » the equation of state. Measurements releasing pressure revealed that the post-spinel phase is stable down to 4 GPa when it decomposes yielding a new phase with poor crystallinity. Later compression does not recover either the spinel or the post-spinel phases. This phase transformation induced by pressure explains the irreversible lost of the ferrimagnetic behavior reported in these spinels. - Graphical abstract: Pressure dependence of the unit cell volume per formula unit for Co{sub 1.5}Fe{sub 1.5}O{sub 4} spinel. Circles and squares stand for spinel and postspinel phases, respectively. Dark (open) symbols: determination upon compression (decompression). - Highlights: • The pressure induces similar phase transformation in Co{sub 3−x}Fe{sub x}O{sub 4} spinels (1≤x≤2). • The postspinel phases decompose after releasing pressure. • The irreversibility of this phase transformation explains the disappearance of magnetism in these spinels after applying pressure. • Accurate equation of state can be obtained up to 10 GPa using an alcohol mixture as pressure transmitting medium. • The equation of state suggests similar elastic properties for these spinels in this composition range.« less

Prediction of precipitate evolution and martensite transformation in Ti-Ni-Cu shape memory alloys by computational thermodynamics

NASA Astrophysics Data System (ADS)

Povoden-Karadeniz, A.; Cirstea, D. C.; Kozeschnik, E.

2016-04-01

Ti-50Ni to Ti-55Ni (at.%) can be termed as the pioneer of shape memory alloys (SMA). Intermetallic precipitates play an important role for strengthening. Their influence on the start temperature of the martensitic transformation is a crucial property for the shape memory effect. Efforts for increasing the martensite start temperature include replacement of a part of Ni atoms by Cu. The influence of Cu-addition to Ti-Ni SMA on T0- temperatures and the character of the austenite-martensite transformation is evaluated using a new thermodynamic database for the Ti-Ni-system extended by Cu. Trends of precipitation of intermetallic phases are simulated by combining the assessed thermodynamics of the Ti-Ni-Cu system with assessed diffusion mobility data and kinetic models, as implemented in the solid-state transformation software MatCalc and are presented in the form of time-temperature-precipitation diagrams. Thermodynamic equilibrium considerations, complemented by predictive thermo-kinetic precipitation simulation, facilitates SMA alloy design and definition of optimized aging conditions.

Heat storage in alloy transformations

NASA Technical Reports Server (NTRS)

Birchenall, C. E.; Gueceri, S. I.

1980-01-01

The theory of eutectic transformation was examined to find guidelines to the best material combinations to examine. The heats of transformation were measured calorimetrically, and the volume changes of expanding solid mixtures and homogeneous liquid solutions, especially during the transformation between the two states at fixed temperature, were measured by changes in X-ray absorption. Heat flow models appropriate to storage in phase change materials were developed along with efficient calculating procedures so that the relative importance of the problems associated with energy storage density, heat conduction, and similar properties could be assessed.

Phase transformation and tribological properties of Ag-MoO3 contained NiCrAlY based composite coatings fabricated by laser cladding

NASA Astrophysics Data System (ADS)

Wang, Lingqian; Zhou, Jiansong; Xin, Benbin; Yu, Youjun; Ren, Shufang; Li, Zhen

2017-08-01

Ag-MoO3 contained NiCrAlY based composite coating was successfully prepared on GH4169 stainless steel substrate by high energy ball milling and laser cladding. The microstructure and phase transformation were investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction spectrum (XRD). The tribological behavior and mechanism from room temperature to 800 °C were investigated. Results showed that MoO3 in the composite powders transformed to Mo2C reinforcement under the high energy density of laser, and a series of opposite transformation occurred during friction process. The coating showed the lowest friction coefficient and low wear rate at 600 °C and 800 °C due to the generation of Ag2MoO4 during tribo-chemical reactions and the formation of lubrication glaze on the worn surface. Ag made effective lubrication when the temperature rose up to 200 °C. The coating displayed a relatively high friction coefficient (about 0.51) at 400 °C, because though MoO3 (oxidation products of Mo2C) and Ag2MoO4 were detected on the worn surface, they could not realize effective lubrication at this temperature. Abrasive wear, adhesive wear and plastic deformation contributed to the increased friction and wear.

Effects of sintering temperature on the pyrochlore phase in PZT nanotubes and their transformation to the perovskite phase by coating with PbO multilayers.

PubMed

Han, Jin Kyu; Choi, Yong Chan; Jeon, Do Hyen; Lee, Min Ku; Bu, Sang Don

2014-11-01

We report the phase evolution of Pb(Zr0.52Ti0.48)O3 nanotubes (PZT-NTs), from the pyrochlore to perovskite phase, with an outer diameter of about 420 nm and a wall thickness of about 10 nm. The PZT-NTs were fabricated in pores of porous anodic alumina membrane (PAM) using a spin coating of PZT sol-gel solution and subsequent annealing at 500-700 degrees C in oxygen gas. The pyrochlore phase was found to be formed at 500 degrees C, and also found not to be transformed into the perovskite phase, even though annealing was performed at higher temperatures to 700 degrees C. Elementary distribution analysis of PZT-NTs embedded in PAM reveal that Pb diffusion from nanotubes into pore walls of PAM is one of the main reasons. By employing firstly an additional PbO coating on the pyrochlore nanotubes and then subsequent annealing at 700 degrees C, we have successfully achieved an almost pure perovskite phase in nanotubes. These results suggest that PbO acts as a Pb-compensation agent in the Pb- deficient PZT-NTs. Moreover, our method can be used in the synthesis of all metal-oxide materials, including volatile elements.

Thermal stability of simple tetragonal and hexagonal diamond germanium

DOE PAGES

Huston, Larissa Q.; Johnson, Brett C.; Haberl, Bianca; ...

2017-11-07

Here, exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 °C for st12-Ge and 200 to 550 °C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursormore » materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 ± 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 ± 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.« less

Thermal stability of simple tetragonal and hexagonal diamond germanium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huston, Larissa Q.; Johnson, Brett C.; Haberl, Bianca

Here, exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 °C for st12-Ge and 200 to 550 °C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursormore » materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 ± 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 ± 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.« less

Atomistic simulation of cubic and tetragonal phases of U-Mo alloy: Structure and thermodynamic properties

NASA Astrophysics Data System (ADS)

Starikov, S. V.; Kolotova, L. N.; Kuksin, A. Yu.; Smirnova, D. E.; Tseplyaev, V. I.

2018-02-01

We studied structure and thermodynamic properties of cubic and tetragonal phases of pure uranium and U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. The main attention was paid to the metastable γ0 -phase that is formed in U-Mo alloys at low temperature. Structure of γ0 -phase is similar to body-centered tetragonal (bct) lattice with displacement of a central atom in the basic cell along [ 001 ] direction. Such displacements have opposite orientations for part of the neighbouring basic cells. In this case, such ordering of the displacements can be designated as antiferro-displacement. Formation of such complex structure may be interpreted through forming of short U-U bonds. At heating, the tetragonal structure transforms into cubic γs -phase, still showing ordering of central atom displacements. With rise in temperature, γs -phase transforms to γ-phase with a quasi body-centered cubic (q-bcc) lattice. The local positions of uranium atoms in γ-phase correspond to γs -phase, however, orientations of the central atom displacements become disordered. Transition from γ0 to γ can be considered as antiferro-to paraelastic transition of order-disorder type. This approach to the structure description of uranium alloy allows to explain a number of unusual features found in the experiments: anisotropy of lattice at low temperature; remarkably high self-diffusion mobility in γ-phase; decreasing of electrical resistivity at heating for some alloys. In addition, important part of this work is the development of new interatomic potential for U-Mo system made with taking into account details of studied structures.

A phase-field approach to nonequilibrium phase transformations in elastic solids via an intermediate phase (melt) allowing for interface stresses.

PubMed

Momeni, Kasra; Levitas, Valery I

2016-04-28

A phase-field approach for phase transformations (PTs) between three different phases at nonequilibrium temperatures is developed. It includes advanced mechanics, thermodynamically consistent interfacial stresses, and interface interactions. A thermodynamic Landau-Ginzburg potential developed in terms of polar order parameters satisfies the desired instability and equilibrium conditions for homogeneous phases. The interfacial stresses were introduced with some terms from large-strain formulation even though the small-strain assumption was utilized. The developed model is applied to study the PTs between two solid phases via a highly disordered intermediate phase (IP) or an intermediate melt (IM) hundreds of degrees below the melting temperature. In particular, the β ↔ δ PTs in HMX energetic crystals via IM are analyzed. The effects of various parameters (temperature, ratios of widths and energies of solid-solid (SS) to solid-melt (SM) interfaces, elastic energy, and interfacial stresses) on the formation, stability, and structure of the IM within a propagating SS interface are studied. Interfacial and elastic stresses within a SS interphase and their relaxation and redistribution with the appearance of a partial or complete IM are analyzed. The energy and structure of the critical nucleus (CN) of the IM are studied as well. In particular, the interfacial stresses increase the aspect-ratio of the CN. Although including elastic energy can drastically reduce the energy of the CN of the IM, the activation energy of the CN of the IM within the SS interface increases when interfacial tension is taken into account. The developed thermodynamic potential can also be modified to model other multiphase physical phenomena, such as multi-variant martensitic PTs, grain boundary and surface-induced pre-melting and PTs, as well as developing phase diagrams for IPs.

Lithium ion conduction in sol-gel synthesized LiZr2(PO4)3 polymorphs

NASA Astrophysics Data System (ADS)

Kumar, Milind; Yadav, Arun Kumar; Anita, Sen, Somaditya; Kumar, Sunil

2018-04-01

Safety issue associated with the high flammability and volatility of organic electrolytes used in commercial rechargeable lithium ion batteries has led to significant attention to ceramic-based solid electrolytes. In the present study, lithium ion conduction in two polymorphs of LiZr2(PO4)3 synthesized via the sol-gel route has been investigated. Rietveld refinement of room temperature X-ray diffraction data of LiZr2(PO4)3 powders calcined at 900 °C and 1300 °C confirmed these to be the monoclinic phase with P21/n structure and rhombohedral phase with R3¯c structure, respectively. Increase in calcination temperature and resultant phase transformation improved the room temperature conductivity from 2.27×10-6 ohm-1m-1 for the monoclinic phase to 1.41×10-4 ohm-1m-1 for rhombohedral phase. Temperature dependence of conductivity was modeled using Arrhenius law and activation energy of ˜ 0.59 eV (for monoclinic phase) and ˜0.50 eV (for rhombohedral phase) were obtained.

Effect of 1.0% Ni on high-temperature impression creep and hardness of recycled aluminium alloy with high Fe content

NASA Astrophysics Data System (ADS)

Faisal, M.; Mazni, Noor; Prasada Rao, A. K.

2018-03-01

Reported work focusses on the effect of 1.0% Ni addition on the microstructure, high- temperature impression creep and thereby the hardness of recycled Al-alloy containing >2wt% Fe, obtained from automotive scrap. Present studies have shown that the addition of 1.0% Ni have supress the formation of α-phase (Al5FeSi) by supressing the peritectic transformation of β-phase (Al8Fe2Si). Such suppression is found to improve the hardness and high-temperature impression creep of the recycled aluminium alloy.

REPLY: Reply to Comment on 'Study of the phase transformations and equation of state of magnesium by synchrotron x-ray diffraction'

NASA Astrophysics Data System (ADS)

Errandonea, Daniel

2004-12-01

This reply aims to clarify some of the arguments presented in a previous publication (Errandonea et al 2003 J. Phys.: Condens. Matter 15 1277), which have been criticized in the preceding comment by Olijnyk. The article in question reported the existence of a new high-pressure and high-temperature dhcp phase in magnesium and presented strong evidence that invites one to re-study the up-to-now-established room temperature structural sequence of magnesium.

Ferroelasticity in the LnNbO/sub 4/-type rare earth niobates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brixner, L.H.; Whitney, J.F.; Zumsteg, F.C.

1977-01-01

The previously reported phase transitions for the isostructural rare earth niobates between 500/sup 0/C and 850/sup 0/C correspond to a point group transformation 4/mF2/m, which is purely ferroelastic. The correct room temperature point group for all LnNbO/sub 4/ compounds is 2/m. Crystal growth and domain wall behavior is discussed for LaNbO/sub 4/. The high temperature phase transition is described for YbNbO/sub 4/.

Exploration of a Metastable Normal Spinel Phase Diagram for the Quaternary Li–Ni–Mn–Co–O System

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kan, Wang Hay; Huq, Ashfia; Manthiram, Arumugam

2016-02-27

In an attempt to enlarge the normal spinel phase diagram for the quaternary Li-Ni-Mn-Co-O system, the transformation at moderate temperatures (150-210 °C) of layered Li 0.5(Ni 1-y-zMn yCo z)O 2 (Rmore » $$\\bar{3}$$m), which were obtained by an ambient-temperature extraction of lithium from Li 0.5(Ni 1-y-zMn yCo z)O 2, into normal spinel-like (Fd$$\\bar{3}$$m) Li(Ni 1-y-zMn yCo z) 2O 4 has been investigated. The phase-conversion mechanism has been studied by joint time-of-flight (TOF) neutron and X-ray diffractions, thermogravimetric analysis, and bond valence sum map. The ionic diffusion of lithium (3a, 6c) and nickel (3a, 3b) ions has been quantified as a function of temperature. The investigated spinel phases are metastable, and they are subject to change into rock-salt phases at higher temperatures. The phases have been characterized as cathodes in lithium-ion cells. Finally, the study may serve as a strategic model to access other metastable phases by low-temperature synthesis approaches.« less

The influence of tempering process for DP lateritic steel in hardness and microstructure behavior

NASA Astrophysics Data System (ADS)

Hasbi, Muhammad Yunan; Saefudin, Romijarso, Toni Bambang

2018-05-01

In this study, the influence of tempering temperature on dual phase (DP) steel lateritic has been examined. Lateritic is chosen because of its excellence as austenite stabilizer in the formation of martensite and also increase the weldability due to nickel content. The hardness and microstructure behavior of steels were the main focus of this research. One of the goals was to obtain the combination of high strength and ductile materials for automotive application. The specimens used in this study were low carbon steel made by the hot-rolled process and followed by the initial heating process with various temperature (760 °C, 800 °C, 840 °C) continued with rapid cooling. The specimens also conducted by secondary heating with tempering process at 450 °C in an hour with very slow cooling. The experimental results showed that correlation between temperatures with hardness properties of materials. The hardness of the specimens increases as temperature increases. It was because austenite phase has a sufficient time and temperature to form, therefore the amount of transformed austenite becomes martensite was greater. The highest hardness reached by T = 840 °C was 46.98 HRC, it was about 153% from as cast (18.54 HRC). Decreasing in hardness value when the specimen was tempering at 450 °C indicated that martensite phase has been transformed into tempered martensite.

Rayleigh analysis of domain dynamics across temperature induced polymorphic phase transitions in lead-free piezoceramics (1‑x)(BaTi0.88Sn0.12)–x(Ba0.7Ca0.3)TiO3

NASA Astrophysics Data System (ADS)

Abebe, Mulualem; Brajesh, Kumar; Singh Malhotra, Jaskaran; Ranjan, Rajeev

2018-05-01

We carried out a Rayleigh analysis of the dielectric permittivity of a lead-free piezoceramic system (1‑x)(BaTi0.88Sn0.12)–x(Ba0.7Ca0.3)TiO3 across the composition and temperature induced polymorphic phase transformations to determine the trend in the reversible and irreversible domain wall motion across the composition and temperature induced structural changes. Experiments were carried out on three representative compositions x  =  0.10, 0.2, and 0.25 exhibiting rhombohedral, orthorhombic, and tetragonal phases at room temperature. While confirming that the irreversible Rayleigh parameter is large in the orthorhombic phase, we discuss a correspondence between the reduction in the coercive field and the corresponding increase in the irreversible Rayleigh parameter. We also show how the proximity of the Curie point to the polymorphic phase boundary greatly undermines this correspondence.

New High Pressure Phase of CaCO3: Implication for the Deep Diamond Formation

NASA Astrophysics Data System (ADS)

Mao, Z.; Li, X.; Zhang, Z.; Lin, J. F.; Ni, H.; Prakapenka, V.

2017-12-01

Surface carbon can be transported to the Earth's deep interior through sinking subduction slabs. Carbonates, including CaCO3, MgCO3 and MgCa(CO3)2, are important carbon carriers for the deep carbon cycle. Experimental studies on the phase stability of carbonates with coexisting mantle minerals at relevant pressure and temperature conditions are thus important for understanding the deep carbon cycle. In particular, recent petrological studies have revealed the evidence for the transportation of CaCO3 to the depth at least of the top lower mantle by analyzing the diamond inclusions. Yet the phase stability of CaCO3 at relevant pressure and temperature conditions of the top lower mantle is still unclear. Previous single-crystal study has shown that CaCO3 transforms from the CaCO3-III structure to CaCO3-VI at 15 GPa and 300 K. The CaCO3-VI is stable at least up to 40 GPa at 300 K. At high temperatures, CaCO3 in the aragonite structure will directly transform into the post-aragonite structure at 40 GPa. However, a recent theoretical study predicted a new phase of CaCO3 with a space group of P21/c between 32 and 48 GPa which is different from previous experimental results. In this study, we have investigated the phase stability of CaCO3 at high pressure-temperature conditions using synchrotron X-ray diffraction in laser-heated diamond anvil cells. We report the discovery of a new phase of CaCO3 at relevant pressure-temperature conditions of the top lower mantle which is consistent with previous theoretical predictions. This new phase is an important carrier for the transportation of carbon to the Earth's lower mantle and crucial for growing deep diamonds in the region.

Laterally structured ripple and square phases with one and two dimensional thickness modulations in a model bilayer system.

PubMed

Debnath, Ananya; Thakkar, Foram M; Maiti, Prabal K; Kumaran, V; Ayappa, K G

2014-10-14

Molecular dynamics simulations of bilayers in a surfactant/co-surfactant/water system with explicit solvent molecules show formation of topologically distinct gel phases depending upon the bilayer composition. At low temperatures, the bilayers transform from the tilted gel phase, Lβ', to the one dimensional (1D) rippled, Pβ' phase as the surfactant concentration is increased. More interestingly, we observe a two dimensional (2D) square phase at higher surfactant concentration which, upon heating, transforms to the gel Lβ' phase. The thickness modulations in the 1D rippled and square phases are asymmetric in two surfactant leaflets and the bilayer thickness varies by a factor of ∼2 between maximum and minimum. The 1D ripple consists of a thinner interdigitated region of smaller extent alternating with a thicker non-interdigitated region. The 2D ripple phase is made up of two superimposed square lattices of maximum and minimum thicknesses with molecules of high tilt forming a square lattice translated from the lattice formed with the thickness minima. Using Voronoi diagrams we analyze the intricate interplay between the area-per-head-group, height modulations and chain tilt for the different ripple symmetries. Our simulations indicate that composition plays an important role in controlling the formation of low temperature gel phase symmetries and rippling accommodates the increased area-per-head-group of the surfactant molecules.

Analysis of reverse martensitic transformation of prehardened 16XCH steel

NASA Astrophysics Data System (ADS)

Muravyev, Vasily; Frolov, Alexey; Lonchakov, Sergey; Bakhmatov, Pavel

2015-10-01

In the paper the structural evolution of previously tempered 16XCH steel is investigated. The influence of temperature and time conditions of heating on temperature of austenization is revealed and the influence of structural changes on steel properties is defined. The analysis of the obtained results showed an increase of plasticity at the initial stage of reverse martensitic transformation and an increase of plasticity at increased durability. It is experimentally found that reverse transformation of packet and lath martensite into the initial phase (holding for a fraction of a second, temperature 400-450°C) leads to a sharp, more than 2-fold, reduction of strength and increase of plasticity. The effect of increased plasticity under reverse martensitic transformation conditions is observed. The structure of packet and lath martensite is more fine-grained in comparison with initial quenching; the durability and plasticity are much higher. Despite the derived results, the revealed effects of increased plasticity and strength require further exploration to increase the reliability of constructions made of low-alloyed steels.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, YinBo; Wang, FengChao, E-mail: wangfc@ustc.edu.cn; Wu, HengAn

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates thatmore » the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the “buckling failure” of the square-ice-nanotube columns, which is dominated by the lateral pressure.« less

Magnetotransport properties of microstructured AlCu2Mn Heusler alloy thin films in the amorphous and crystalline phase

NASA Astrophysics Data System (ADS)

Barzola-Quiquia, José; Stiller, Markus; Esquinazi, Pablo D.; Quispe-Marcatoma, Justiniano; Häussler, Peter

2018-06-01

We have studied the resistance, magnetoresistance and Hall effect of AlCu2Mn Heusler alloy thin films prepared by flash evaporation on substrates cooled at 4He liquid temperature. The as-prepared samples were amorphous and were annealed stepwise to induce the transformation to the crystalline phase. The amorphous phase is metastable up to above room temperature and the transition to the crystalline phase was observed by means of resistance measurements. Using transmission electron microscopy, we have determined the structure factor S (K) and the pair correlation function g (r) , both results indicate that amorphous AlCu2Mn is an electronic stabilized phase. The X-ray diffraction of the crystallized film shows peaks corresponding to the well ordered L21 phase. The resistance shows a negative temperature coefficient in both phases. The magnetoresistance (MR) is negative in both phases, yet larger in the crystalline state compared to the amorphous one. The magnetic properties were studied further by anomalous Hall effect measurements, which were present in both phases. In the amorphous state, the anomalous Hall effect disappears at temperatures below 175 K and is present up to above room temperature in the case of crystalline AlCu2Mn.

Transformation and Precipitation Kinetics in 30Cr10Ni Duplex Stainless Steel

NASA Astrophysics Data System (ADS)

Fazarinc, Matevz; Terčelj, Milan; Bombač, David; Kugler, Goran

2010-09-01

To improve the microstructure during casting, hot forming, and heat treatment of 30Cr10Ni duplex stainless steel, accurate data on the precipitation and transformation processes at high temperatures are needed. In this article, the precipitation and transformation processes at various aging times in the temperature range 873 K to 1573 K (600 °C to 1300 °C) were studied. The 30Cr10Ni ferrous alloy contains a relatively large amount of Cr, Ni, and C, which results in a complex microstructure. In addition to the ferrite, austenite, and sigma phase, the M23C6 and MC carbides were also observed in the microstructure. The precipitation of the sigma phase was observed after just 3 minutes of aging, and after 30 minutes of aging at approximately 1053 K (780 °C), its fraction exceeded 40 pct. An intensive austenite-to-ferrite transformation was observed above 1423 K (1150 °C). Optical microscopy, energy-dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD), as well as micro-indentation hardness, hardness, impact toughness, and tensile tests, were carried out to evaluate the obtained microstructures of aged samples.

New ordered metastable phases between the gel and subgel phases in hydrated phospholipids.

PubMed Central

Tenchov, B; Koynova, R; Rapp, G

2001-01-01

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase. PMID:11259300

FAST TRACK COMMUNICATION Temperature-driven phase transformation in self-assembled diphenylalanine peptide nanotubes

NASA Astrophysics Data System (ADS)

Heredia, A.; Bdikin, I.; Kopyl, S.; Mishina, E.; Semin, S.; Sigov, A.; German, K.; Bystrov, V.; Gracio, J.; Kholkin, A. L.

2010-11-01

Diphenylalanine (FF) peptide nanotubes (PNTs) represent a unique class of self-assembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity and chemical stability. In addition, strong piezoelectric activity has recently been observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show a gradual decrease in polarization with increasing temperature accompanied by an irreversible phase transition into another crystalline phase at about 140-150 °C. The results are corroborated by the molecular dynamic simulations predicting an order-disorder phase transition into a centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighbouring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to the high coercive field in FF PNTs.

Raman and infrared spectroscopic investigations of a ferroelastic phase transition in B a2ZnTe O6 double perovskite

NASA Astrophysics Data System (ADS)

Moreira, Roberto L.; Lobo, Ricardo P. S. M.; Ramos, Sérgio L. L. M.; Sebastian, Mailadil T.; Matinaga, Franklin M.; Righi, Ariete; Dias, Anderson

2018-05-01

The low-temperature vibrational properties of B a2ZnTe O6 double-perovskite ceramics obtained by the solid-state route were investigated by Raman scattering and Fourier-transform infrared reflectivity. We found that this material undergoes a reversible ferroelastic phase transition at around 140 K, well compatible with a recently proposed rhombohedral-to-monoclinic structural change that would occur below 165 K. Complementary calorimetric measurements showed that the phase transition has a first-order character, with an entropy jump compatible with a displacive mechanism. The vibrational spectra show clearly the splitting of the doubly degenerate E modes into nondegenerate representations of the low-symmetry phase. In particular, the lowest-frequency Raman mode presents soft-mode behavior and splits below the critical temperature, confirming the in-plane ferroelastic deformation in the low-temperature phase.

Shock induced polymorphic transition in quartz, carbon, and boron nitride

NASA Technical Reports Server (NTRS)

Tan, Hua; Ahrens, Thomas J.

1990-01-01

The model proposed by Ahrens (1988) to explain the mechanism of the polymorphism in silicates is revised, and the revised model is applied to the quartz/stishovite, graphite/diamond, and graphite-boron nitride (g-BN) phase transformations. In this model, a key assumption is that transformation to a high-density amorphous or possibly liquid phase which rapidly crystallized to the high-pressure phase is triggered by the high temperatures in the shear band and upon crossing the metastable extension of a melting curve. Good agreement between the calcualted results and published data is obtained. The present theory predicts the standard entropy for cubic BN to be 0.4-0.5 J/g K.

Microstructure and Shape Memory Behavior of Ti-Nb Shape Memory Alloy Thin Film

NASA Astrophysics Data System (ADS)

Meng, X. L.; Sun, B.; Sun, J. Y.; Gao, Z. Y.; Cai, W.; Zhao, L. C.

2017-09-01

Ti-Nb shape memory alloy (SMA) thin film is a promising candidate applied as microactuator in biomedical field. In this study, the microstructure and shape memory behavior of Ti-Nb SMA thin films in different heat treatment conditions have been investigated. Fine ω phases embedded in the β phase matrix suppress the martensitic transformation of the films. As a result, the as-deposited and most of the annealed films consist of the β and α″ dual phases. The annealed Ti-Nb thin film shows excellent superelasticity effect when deformed above the reverse martensitic transformation temperature, that is 3.5% total recovery strain can be obtained when 4% pre-strain is loaded.

Improved phase stability of formamidinium lead triiodide perovskite by strain relaxation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, Xiaojia; Wu, Congcong; Jha, Shikhar K.

2016-10-18

Though formamidinium lead triiodide (FAPbI 3) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI 3-MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement inmore » the stability of α-FAPbI 3. As a result, solar cells fabricated using FAPbI 3-MABr demonstrated significantly enhanced stability under the humid air.« less

Control of temperature and aqueous Mg2+/Ca2+ ratio on the (trans-)formation of ikaite

NASA Astrophysics Data System (ADS)

Purgstaller, B.; Dietzel, M.; Baldermann, A.; Mavromatis, V.

2017-11-01

The calcium carbonate hexahydrate mineral ikaite (CaCO3 ṡ 6 H2O) has been documented in aquatic environments at near-freezing temperatures. An increase of the prevailing temperature in the depositional environment, results in the transformation of natural ikaite into less soluble calcium carbonate phases occasionally leaving calcite pseudomorphs in the sediments, which are considered as an indicator for primary cold water temperatures. Detailed understanding on the physicochemical parameters controlling ikaite (trans-)formation however, such as temperature and reactive solution chemical composition, are still under debate. In order to study the formation of ikaite, we conducted precipitation experiments under controlled physicochemical conditions (pH = 8.3 ± 0.1; T = 6, 12, and 18 ± 0.1 °C) at defined aqueous molar Mg/Ca ratios. The transformation of ikaite into anhydrous calcium carbonate polymorphs was investigated in solution and at air exposure. The obtained results reveal the formation of ikaite at temperatures up to 12 °C, whereas Mg-rich amorphous calcium carbonate precipitated at 18 °C. In contact with the reactive solution ikaite transformed into aragonite at aqueous molar Mg2+/Ca2+ ratios of ≥14. In contrast, ikaite separated from the Mg-rich solution and exposed to air transformed in all cases into calcite/vaterite. The herein obtained temperature limit of ≤12 for ikaite formation is significantly higher than formerly expected and most probably caused by (i) the high saturation degree of the solution with respect to ikaite and (ii) the slow dehydration of the aqueous Ca2+ ion at low temperatures. This result questions the suitability of calcite pseudomorphs (i.e. glendonites) as a proxy for near-freezing temperatures. Moreover, our findings show that the CaCO3 polymorph formed from ikaite is strongly controlled by the physicochemical conditions, such as aqueous molar Mg2+/Ca2+ ratio of the reactive fluid and H2O availability throughout the transformation process.

Effect of Nano CeO2 Addition on the Microstructure and Properties of a Cu-Al-Ni Shape Memory Alloy

NASA Astrophysics Data System (ADS)

Pandey, Abhishek; Jain, Ashish Kumar; Hussain, Shahadat; Sampath, V.; Dasgupta, Rupa

2016-08-01

This article deals with the effect of adding nano CeO2 to act as a grain pinner/refiner to a known Cu-Al-Ni shape memory alloy. Elements were taken in a predefined ratio to prepare 300 g alloy per batch and melted in an induction furnace. Casting was followed by homogenization at 1173 K (900 °C) and rolling to make sheets of 0.5-mm thickness. Further, samples were characterized for microstructure using optical and electron microscope, hardness, and different phase studies by X-ray and transformation temperatures by differential scanning calorimetry. X-ray peak broadenings and changes were investigated to estimate the crystallite size, lattice strain, and phase changes due to different processing steps. A nearly uniform distribution of CeO2 and better martensitic structure were observed with increasing CeO2. The addition of CeO2 also shows a visible effect on the transformation temperature and phase formation.

Phase transformations of 4,4'-biphenyldicarboxylic acid on Cu(001)

NASA Astrophysics Data System (ADS)

Schwarz, Daniel; van Gastel, Raoul; Zandvliet, Harold J. W.; Poelsema, Bene

2012-06-01

The growth and structure of 4,4'-biphenyldicarboxylic-acid (BDA) on Cu(001) at temperatures between 300 and 400 K was studied by low energy electron microscopy and μ-LEED. First, the adsorbed BDA molecules form a disordered dilute phase. Once this phase reaches a sufficiently high density, a crystalline phase nucleates, in which the molecules form a hydrogen-bonded two-dimensional (2D) supramolecular c(8×8) network. By a careful analysis of the bright-field image intensity, we can measure the density in the dilute phase, which is up to 30% of that in the crystalline phase. From the respective equilibrium densities at different temperatures, we determine the 2D phase diagram and extract a cohesive energy of 0.35 eV. We also analyze the island decay behavior and estimate the BDA molecule diffusion constants. Steps are found to be highly transparent for diffusing BDA molecules. In the temperature range of 362-400 K, we find chemical diffusion constants between 850-1700nm2s-1.

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

DOE PAGES

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth; ...

2018-03-30

Here, we demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing.more » A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U 2MoSi 2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.« less

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth

Here, we demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing.more » A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U 2MoSi 2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.« less

Austenite-martensite transformation in electrodeposited Fe70Pd30 NWs: a step towards making bio-nano-actuators tested on in vivo systems

NASA Astrophysics Data System (ADS)

Zuzek Rozman, K.; Pecko, D.; Trafela, S.; Samardzija, Z.; Spreitzer, M.; Jaglicic, Z.; Nadrah, P.; Zorko, M.; Bele, M.; Tisler, T.; Pintar, A.; Sturm, S.; Kostevsek, N.

2018-03-01

Fe69±3Pd31±3 nanowires (NWs) with lengths of a few microns and diameters of 200 nm were synthesized via template-assisted pulsed electrodeposition into alumina-based templates. The as-deposited Fe69±3Pd31±3 NWs exhibited α-Fe (bcc-solid solution of Fe, Pd) nanocrystalline structure as seen from the x-ray diffraction (XRD), that got confirmed by transmission electron microscopy (TEM) with some larger grains up 50 nm observed. Annealing of the as-deposited Fe69±3Pd31±3 NWs at 1173 K/45 min was followed by quenching in ice water and resulted in a transformation to the fcc crystal structure (XRD) with grain sizes up to 200 nm (TEM). To induce the austenite-to-martensite, i.e., fcc-to-fct phase transformation the fcc Fe69±3Pd31±3 NWs were cooled to 73 K. The XRD showed the disappearance of the (200) fcc reflection (at room temperature) and the appearance of the (200) fct reflection (at 73 K), confirming the fcc-to-fct transformation took place. The magnetic measurements revealed that the fcc Fe69±3Pd31±3 NWs measured at low temperatures (50 K) had a larger coercivity than at room temperature, which suggests the fct phase was present in the undercooled state, exhibiting a larger magnetocrystalline anisotropy than the fcc phase present at room temperature. As part of our interest in magnetic-shape-memory actuators, the as-deposited Fe69±3Pd31±3 NWs were tested for toxicity on zebrafish. In vivo tests showed no acute lethal or sub-lethal effects, which implies that the Fe69±3Pd31±3 NWs have the potential to be used as nano-actuators in biomedical applications.

Potential Energy Landscape of the Liquid-Liquid Phase Transition in Water and the transformation between Low-Density and High-Density Amorphous Ice

NASA Astrophysics Data System (ADS)

Giovambattista, N.; Sciortino, F.; Starr, F. W.; Poole, P. H.

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics for describing supercooled liquids and glasses. We use the PEL formalism and computer simulations to study the transformation between low-density (LDL) and high-density liquid (HDL) water, and between low-density (LDA) and high-density amorphous ice (HDA). We employ the ST2 water model that exhibits a LDL-HDL first-order phase transition and a sharp LDA-HDA transformation, as observed in experiments. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that LDL configurations are located in the same megabasin as LDA, and that HDL configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid and the amorphous ice differ. We also study the liquid-to-ice-VII first-order phase transition. The PEL properties across this transition are qualitatively similar to the changes found during the LDA-HDA transformation, supporting the interpretation that the LDA-HDA transformation is a first-order-like phase transition between out-of-equilibrium states.

Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination

NASA Astrophysics Data System (ADS)

Ćakιr, Aslι; Righi, Lara; Albertini, Franca; Acet, Mehmet; Farle, Michael; Aktürk, Selçuk

2013-11-01

Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50-xGax in the composition range 12≤x≤25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7M→L10, 5M →7M, and 5M→7M→L10 with decreasing temperature. The L10 non-modulated structure is most stable at low temperature.

Adjusting alloy compositions for selected properties in temperature limited heaters

DOEpatents

Brady; Michael Patrick , Horton, Jr.; Joseph Arno , Vitek; John Michael

2010-03-23

Heaters for treating a subsurface formation are described herein. Such heaters can be obtained by using the systems and methods described herein. The heater includes a heater section including iron, cobalt, and carbon. The heater section has a Curie temperature less than a phase transformation temperature. The Curie temperature is at least 740.degree. C. The heater section provides, when time varying current is applied to the heater section, an electrical resistance.

Phase transformations in an ascending adiabatic mixed-phase cloud volume

NASA Astrophysics Data System (ADS)

Pinsky, M.; Khain, A.; Korolev, A.

2015-04-01

Regimes of liquid-ice coexistence that may form in an adiabatic parcel ascending at constant velocity at freezing temperatures are investigated. Four zones with different microphysical structures succeeding one another along the vertical direction have been established. On the basis of a novel balance equation, analytical expressions are derived to determine the conditions specific for each of these zones. In particular, the necessary and sufficient conditions for formation of liquid water phase within an ascending parcel containing only ice particles are determined. The results are compared to findings reported in earlier studies. The role of the Wegener-Bergeron-Findeisen mechanism in the phase transformation is analyzed. The dependence of the phase relaxation time on height in the four zones is investigated on the basis of a novel analytical expression. The results obtained in the study can be instrumental for analysis and interpretation of observed mixed-phase clouds.

In situ probing of doping- and stress-mediated phase transitions in a single-crystalline VO2 nanobeam by spatially resolved Raman spectroscopy

NASA Astrophysics Data System (ADS)

Chang, Sung-Jin; Park, Jong Bae; Lee, Gaehang; Kim, Hae Jin; Lee, Jin-Bae; Bae, Tae-Sung; Han, Young-Kyu; Park, Tae Jung; Huh, Yun Suk; Hong, Woong-Ki

2014-06-01

We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO2 nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO2 nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO2 nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under air. The triclinic T phase of the VO2 nanobeam upon heating under hydrogen gas transformed to the R phase and this R phase was stabilized even at room temperature in air after sample cooling. In particular, after the VO2 nanobeam with the R phase was annealed at approximately 250 °C in air, it exhibited the monoclinic M1 phase (not the T phase) at room temperature during heating and cooling cycles. These results were attributed to the interplay between hydrogen doping and stress associated with nanobeam-substrate interactions. Our study has important implications for engineering metal-insulator transition properties and developing functional devices based on VO2 nanostructures through doping and stress.We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO2 nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO2 nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO2 nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under air. The triclinic T phase of the VO2 nanobeam upon heating under hydrogen gas transformed to the R phase and this R phase was stabilized even at room temperature in air after sample cooling. In particular, after the VO2 nanobeam with the R phase was annealed at approximately 250 °C in air, it exhibited the monoclinic M1 phase (not the T phase) at room temperature during heating and cooling cycles. These results were attributed to the interplay between hydrogen doping and stress associated with nanobeam-substrate interactions. Our study has important implications for engineering metal-insulator transition properties and developing functional devices based on VO2 nanostructures through doping and stress. Electronic supplementary information (ESI) available: Illustration, photograph, Raman data, and EDX spectra. See DOI: 10.1039/c4nr01118j

Shape memory behavior of single and polycrystalline nickel rich nickel titanium alloys

NASA Astrophysics Data System (ADS)

Kaya, Irfan

NiTi is the most commonly used shape memory alloy (SMA) and has been widely used for bio-medical, electrical and mechanical applications. Nickel rich NiTi shape memory alloys are coming into prominence due to their distinct superelasticity and shape memory properties as compared to near equi-atomic NiTi shape memory alloys. Besides, their lower density and higher work output than steels makes these alloys an excellent candidate for aerospace and automotive industry. Shape memory properties and phase transformation behavior of high Ni-rich Ni54Ti46 (at.%) polycrystals and Ni-rich Ni 51Ti49 (at.%) single-crystals are determined. Their properties are sensitive to heat treatments that affect the phase transformation behavior of these alloys. Phase transformation properties and microstructure were investigated in aged Ni54Ti46 alloys with differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) to reveal the precipitation characteristics and R-phase formation. It was found that Ni54Ti46 has the ability to exhibit perfect superelasticity under high stress levels (~2 GPa) with 4% total strain after 550°C-3h aging. Stress independent R-phase transformation was found to be responsible for the change in shape memory behavior with stress. The shape memory responses of [001], [011] and [111] oriented Ni 51Ti49 single-crystals alloy were reported under compression to reveal the orientation dependence of their shape memory behavior. It has been found that transformation strain, temperatures and hysteresis, Classius-Clapeyron slopes, critical stress for plastic deformation are highly orientation dependent. The effects of precipitation formation and compressive loading at selected temperatures on the two-way shape memory effect (TWSME) properties of a [111]- oriented Ni51Ti49 shape memory alloy were revealed. Additionally, aligned Ni4Ti3 precipitates were formed in a single crystal of Ni51Ti49 alloy by aging under applied compression stress along the [111] direction. Formation of a single family of Ni4Ti3 precipitates were exhibited significant TWSME without any training or deformation. When the homogenized and aged specimens were loaded in martensite, positive TWSME was observed. After loading at high temperature in austenite, the homogenized specimen did not show TWSME while the aged specimen revealed negative TWSME.

Microdomain Formation, Oxidation, and Cation Ordering in LaCa 2Fe 3O 8+y

DOE PAGES

Price, Patrick M.; Browning, Nigel D.; Butt, Darryl P.

2015-03-23

The compound LaCa 2Fe 3O 8+y, also known as the Grenier phase, is known to undergo an order-disorder transformation (ODT) at high temperatures. Oxidation has been observed when the compound is cooled in air after the ODT. In this study, we have synthesized the Grenier compound in air using traditional solid state reactions and investigated the structure and composition before and after the ODT. Thermal analysis showed that the material undergoes an order-disorder transformation in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscopy (STEM) suggest that the Grenier phase hasmore » preferential segregation of Ca and La on the two crystallographic A-sites before the ODT, but a random distribution above the ODT temperature. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La-rich regions within microdomains rather than at microdomain boundaries.« less

Effect of heat treatment on properties of HfO2 film deposited by ion-beam sputtering

NASA Astrophysics Data System (ADS)

Liu, Huasong; Jiang, Yugang; Wang, Lishuan; Li, Shida; Yang, Xiao; Jiang, Chenghui; Liu, Dandan; Ji, Yiqin; Zhang, Feng; Chen, Deying

2017-11-01

The effects of atmosphere heat treatment on optical, stress, and microstructure properties of an HfO2 film deposited by ion-beam sputtering were systematically researched. The relationships among annealing temperature and refractive index, extinction coefficient, physical thickness, forbidden-band width, tape trailer width, Urbach energy, crystal phase structure, and stress were assessed. The results showed that 400 °C is the transformation point, and the microstructure of the HfO2 film changed from an amorphous into mixed-phase structure. Multistage phonons appeared on the HfO2 film, and the trends of the refractive index, extinction coefficient, forbidden-band width change, and Urbach energy shifted from decrease to increase. With the elevation of the annealing temperature, the film thickness increased monotonously, the compressive stress gradually turned to tensile stress, and the transformation temperature point for the stress was between 200 °C and 300 °C. Therefore, the change in the stress is the primary cause for the shifts in thin-film thickness.

Thermal expansion characteristics of Fe-9Cr-0.12C-0.56Mn-0.24V-1.38W-0.06Ta (wt.%) reduced activation ferritic-martensitic steel

NASA Astrophysics Data System (ADS)

Subramanian, Raju; Tripathy, Haraprasanna; Rai, Arun Kumar; Hajra, Raj Narayan; Saibaba, Saroja; Jayakumar, Tammana; Rajendra Kumar, Ellappan

2015-04-01

The lattice and bulk thermal expansion behavior of an Indian version of reduced activation ferritic-martensitic (INRAFM) steel has been quantified using high temperature X-ray diffraction and dilatometry. The lattice parameter of tempered α-ferrite phase exhibited a smooth quadratic increase with temperature, while that of γ-austenite remained fairly linear up to 1273 K. The results suggest that α-ferrite + Carbides → γ-austenite transformation occurs upon continuous heating in the temperature range, 1146 ⩽ T ⩽ 1173 K. Further, this transformation is found to be accompanied by a reduction in average atomic volume. The mean linear thermal expansion coefficients of tempered α-ferrite and γ-austenite phases are estimated to be about 1.48 × 10-5 and 2.4 × 10-5 K-1 respectively. The magnetic contribution to relative thermal dilatation (Δl/l298)mag is found to be small and negative, as compared to phonon contribution.

Relaxor-ferroelectric crossover in (B i1 /2K1 /2)Ti O3 : Origin of the spontaneous phase transition and the effect of an applied external field

NASA Astrophysics Data System (ADS)

Hagiwara, Manabu; Ehara, Yoshitaka; Novak, Nikola; Khansur, Neamul H.; Ayrikyan, Azatuhi; Webber, Kyle G.; Fujihara, Shinobu

2017-07-01

The temperature evolution of polar order in an A -site complex perovskite (B i1 /2K1 /2)Ti O3 (BKT) has been investigated by measurements of dielectric permittivity, depolarization current, and stress-stain curves at elevated temperatures. Upon cooling from high temperatures, BKT first enters a relaxor state and then spontaneously transforms into a ferroelectric state. The analyses of temperature and frequency dependence of permittivity have revealed that polar nanoregions of the relaxor phase appear at temperatures higher than 560°C, and also that their freezing at 296°C triggers the spontaneous relaxor-ferroelectric transition. We discuss the key factors determining the development of long-range polar order in A -site complex perovskites through a comparison with the relaxor (B i1 /2N a1 /2)Ti O3 . We also show that application of biasing electric fields and compressive stresses to BKT favors its ferroelectric phase, resulting in a significant shift of the relaxor-ferroelectric transition temperature towards higher temperatures. Based on the obtained results, electric field-temperature and stress-temperature phase diagrams are firstly determined for BKT.

Ferrite Formation Dynamics and Microstructure Due to Inclusion Engineering in Low-Alloy Steels by Ti2O3 and TiN Addition

NASA Astrophysics Data System (ADS)

Mu, Wangzhong; Shibata, Hiroyuki; Hedström, Peter; Jönsson, Pär Göran; Nakajima, Keiji

2016-08-01

The dynamics of intragranular ferrite (IGF) formation in inclusion engineered steels with either Ti2O3 or TiN addition were investigated using in situ high temperature confocal laser scanning microscopy. Furthermore, the chemical composition of the inclusions and the final microstructure after continuous cooling transformation was investigated using electron probe microanalysis and electron backscatter diffraction, respectively. It was found that there is a significant effect of the chemical composition of the inclusions, the cooling rate, and the prior austenite grain size on the phase fractions and the starting temperatures of IGF and grain boundary ferrite (GBF). The fraction of IGF is larger in the steel with Ti2O3 addition compared to the steel with TiN addition after the same thermal cycle has been imposed. The reason for this difference is the higher potency of the TiO x phase as nucleation sites for IGF formation compared to the TiN phase, which was supported by calculations using classical nucleation theory. The IGF fraction increases with increasing prior austenite grain size, while the fraction of IGF in both steels was the highest for the intermediate cooling rate of 70 °C/min, since competing phase transformations were avoided, the structure of the IGF was though refined with increasing cooling rate. Finally, regarding the starting temperatures of IGF and GBF, they decrease with increasing cooling rate and the starting temperature of GBF decreases with increasing grain size, while the starting temperature of IGF remains constant irrespective of grain size.

Evolution of phase transformation and magnetic properties with Fe content in Ni55-x Fe x Mn20Ga25 Heusler alloys

NASA Astrophysics Data System (ADS)

Zhang, Yuanlei; Li, Zhe; He, Xijia; Huang, Yinsheng; Xu, Kun; Jing, Chao

2018-02-01

A series of Ni55-x Fe x Mn20Ga25 (0  ⩽  x  ⩽  5) Heusler alloys was prepared to investigate their phase transitions and magnetic properties. At room temperature, these alloys present various crystal structures, and the unit cell volume enlarges with increase of Fe content in both austenite and martensite. Multiple magneto-structural transformations were observed in the parent alloy (x  =  0). In the process of cooling, it undergoes martensitic transformation (MT) from L21-type paramagnetic austenite to L10-type ferromagnetic martensite, accompanying an intermartensitic transformation (IMT, 7M  →  L10). By establishing a detailed phase diagram, we found that both MT and IMT shift to lower temperature simultaneously, while the ferromagnetic (FM) transition of austenite moves to higher temperature as Fe increases. With the further increase of Fe content beyond a critical value, both the IMT and the FM transitions split off from MT, and the former follows with the transforming sequence of 7M  →  5M. Based on the experimental data, some key magnetic parameters have been obtained in this system. The calculated magnetocrystalline anisotropy constant ({{K}1} ) of martensite quickly increases as Fe increases, and then it almost reaches a saturated value (~5.5  ×  105 J m-3) for the alloys with x  >  3. However, the spontaneous magnetic moment ({μs} ) attains a peak value of about 4.2 μ B/f.u. in the alloy with x  =  4, which is not consistent with the linear increasing of effective magnetic moment ({μef f} ). Further magnetic measurements with hydrostatic pressure indicate that such a discrepancy could be ascribed to the competition between the magnetic exchange interaction and the volume change of unit cell governed by the dopant Fe content.

Thermokinetic Modeling of Phase Transformation in the Laser Powder Deposition Process

NASA Astrophysics Data System (ADS)

Foroozmehr, Ehsan; Kovacevic, Radovan

2009-08-01

A finite element model coupled with a thermokinetic model is developed to predict the phase transformation of the laser deposition of AISI 4140 on a substrate with the same material. Four different deposition patterns, long-bead, short-bead, spiral-in, and spiral-out, are used to cover a similar area. Using a finite element model, the temperature history of the laser powder deposition (LPD) process is determined. The martensite transformation as well as martensite tempering is considered to calculate the final fraction of martensite, ferrite, cementite, ɛ-carbide, and retained austenite. Comparing the surface hardness topography of different patterns reveals that path planning is a critical parameter in laser surface modification. The predicted results are in a close agreement with the experimental results.

Effects of Palladium Content, Quaternary Alloying, and Thermomechanical Processing on the Behavior of Ni-Ti-Pd Shape Memory Alloys for Actuator Applications

NASA Technical Reports Server (NTRS)

Bigelow, Glen

2008-01-01

The need for compact, solid-state actuation systems for use in the aerospace, automotive, and other transportation industries is currently driving research in high-temperature shape memory alloys (HTSMA) having transformation temperatures above 100 C. One of the basic high temperature systems under investigation to fill this need is NiTiPd. Prior work on this alloy system has focused on phase transformations and respective temperatures, no-load shape memory behavior (strain recovery), and tensile behavior for selected alloys. In addition, a few tests have been done to determine the effect of boron additions and thermomechanical treatment on the aforementioned properties. The main properties that affect the performance of a solid state actuator, namely work output, transformation strain, and permanent deformation during thermal cycling under load have mainly been neglected. There is also no consistent data representing the mechanical behavior of this alloy system over a broad range of compositions. For this thesis, ternary NiTiPd alloys containing 15 to 46 at.% palladium were processed and the transformation temperatures, basic tensile properties, and work characteristics determined. However, testing reveals that at higher levels of alloying addition, the benefit of increased transformation temperature begins to be offset by lowered work output and permanent deformation or "walking" of the alloy during thermal cycling under load. In response to this dilemma, NiTiPd alloys have been further alloyed with gold, platinum, and hafnium additions to solid solution strengthen the martensite and parent austenite phases in order to improve the thermomechanical behavior of these materials. The tensile properties, work behavior, and dimensional stability during repeated thermal cycling under load for the ternary and quaternary alloys were compared and discussed. In addition, the benefits of more advanced thermomechanical processing or training on the dimensional stability of these alloys during repeated actuation were investigated. Finally, the effect of quaternary alloying on the thermal stability of NiTiPdX alloys is determined via thermal cycling of the materials to increasing temperatures under load. It was found that solid solution additions of platinum and gold resulted in about a 30 C increase in upper use temperature compared to the baseline NiTiPd alloy, providing an added measure of over-temperature protection.

Highly Porous NiTi with Isotropic Pore Morphology Fabricated by Self-Propagated High-Temperature Synthesis

NASA Astrophysics Data System (ADS)

Hosseini, S. A.; Alizadeh, M.; Ghasemi, A.; Meshkot, M. A.

2013-02-01

Highly porous NiTi with isotropic pore morphology has been successfully produced by self-propagating high-temperature synthesis of elemental Ni/Ti metallic powders. The effects of adding urea and NaCl as temporary pore fillers were investigated on pore morphology, microstructure, chemical composition, and the phase transformation temperatures of specimens. These parameters were studied by optical microscopy, scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC). Highly porous specimens were obtained with up to 83% total porosity and pore sizes between 300 and 500 μm in diameter. Results show pore characteristics were improved from anisotropic to isotropic and pore morphology was changed from channel-like to irregular by adding pore filler powders. Furthermore, the highly porous specimens produced when using urea as a space holder, were of more uniform composition in comparison to NaCl. DSC results showed that a two-step martensitic phase transformation takes place during the cooling cycles and the austenite finish temperature ( A f) is close to human body temperature. Compression test results reveal that the compressive strength of highly porous NiTi is about 155 MPa and recoverable strain about 6% in superelasticity regime.

Alteration behavior of mineral structure and hazardous elements during combustion of coal from a power plant at Huainan, Anhui, China.

PubMed

Tang, Quan; Sheng, Wanqi; Li, Liyuan; Zheng, Liugen; Miao, Chunhui; Sun, Ruoyu

2018-08-01

The alteration behavior of minerals and hazardous elements during simulated combustion (100-1200 °C) of a raw coal collected from a power plant were studied. Thermogravimetric analysis indicated that there were mainly four alteration stages during coal combustion. The transformation behavior of mineral phases of raw coal, which were detected by X-ray polycrystalline diffraction (XRD) technique, mainly relied on the combustion temperature. A series of changes were derived from the intensities of mineral (e.g. clays) diffraction peaks when temperature surpassed 600 °C. Mineral phases tended to be simple and collapsed to amorphous glass when temperature reached up to 1200 °C. The characteristics of functional groups for raw coal and high-temperature (1200 °C) ash studied by Fourier transform infrared spectroscopy (FTIR) were in accordance with the result obtained from XRD analysis. The volatilization ratios of Co, Cr, Ni and V increased consistently with the increase of combustion temperature, suggesting these elements were gradually released from the organic matter and inorganic minerals of coal. Copyright © 2018 Elsevier Ltd. All rights reserved.

Transformation twinning of Ni-Mn-Ga characterized with temperature-controlled atomic force microscopy.

PubMed

Reinhold, Matthew; Watson, Chad; Knowlton, William B; Müllner, Peter

2010-06-01

The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25 degrees C and in the austenite phase at 55 degrees C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature (55 degrees C), the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25 degrees C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.

Transformation twinning of Ni–Mn–Ga characterized with temperature-controlled atomic force microscopy

PubMed Central

Reinhold, Matthew; Watson, Chad; Knowlton, William B.; Müllner, Peter

2010-01-01

The magnetomechanical properties of ferromagnetic shape memory alloy Ni–Mn–Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni–Mn–Ga single crystal. Experiments were performed in the martensite phase at 25 °C and in the austenite phase at 55 °C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature (55 °C), the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25 °C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys. PMID:20589105

Inverse Thermal Analysis of Titanium GTA Welds Using Multiple Constraints

NASA Astrophysics Data System (ADS)

Lambrakos, S. G.; Shabaev, A.; Huang, L.

2015-06-01

Inverse thermal analysis of titanium gas-tungsten-arc welds using multiple constraint conditions is presented. This analysis employs a methodology that is in terms of numerical-analytical basis functions for inverse thermal analysis of steady-state energy deposition in plate structures. The results of this type of analysis provide parametric representations of weld temperature histories that can be adopted as input data to various types of computational procedures, such as those for prediction of solid-state phase transformations. In addition, these temperature histories can be used to construct parametric function representations for inverse thermal analysis of welds corresponding to other process parameters or welding processes whose process conditions are within similar regimes. The present study applies an inverse thermal analysis procedure that provides for the inclusion of constraint conditions associated with both solidification and phase transformation boundaries.

Phase Transformations and Metallization of Magnesium Oxide at High Pressure and Temperature

NASA Astrophysics Data System (ADS)

McWilliams, R. Stewart; Spaulding, Dylan K.; Eggert, Jon H.; Celliers, Peter M.; Hicks, Damien G.; Smith, Raymond F.; Collins, Gilbert W.; Jeanloz, Raymond

2012-12-01

Magnesium oxide (MgO) is representative of the rocky materials comprising the mantles of terrestrial planets, such that its properties at high temperatures and pressures reflect the nature of planetary interiors. Shock-compression experiments on MgO to pressures of 1.4 terapascals (TPa) reveal a sequence of two phase transformations: from B1 (sodium chloride) to B2 (cesium chloride) crystal structures above 0.36 TPa, and from electrically insulating solid to metallic liquid above 0.60 TPa. The transitions exhibit large latent heats that are likely to affect the structure and evolution of super-Earths. Together with data on other oxide liquids, we conclude that magmas deep inside terrestrial planets can be electrically conductive, enabling magnetic field-producing dynamo action within oxide-rich regions and blurring the distinction between planetary mantles and cores.

Thermal effects of carbonated hydroxyapatite modified by glycine and albumin

NASA Astrophysics Data System (ADS)

Gerk, S. A.; Golovanova, O. A.; Kuimova, M. V.

2017-01-01

In this work calcium phosphate powders were obtained by precipitation method from simulated solutions of synovial fluid containing glycine and albumin. X-ray diffraction and IR spectroscopy determined that all samples are single-phase and are presented by carbonate containing hydroxyapatite (CHA). The thermograms of solid phases of CHA were obtained and analyzed; five stages of transformation in the temperature range of 25-1000°C were marked. It is shown that in this temperature range dehydration, decarboxylation and thermal degradation of amino acid and protein connected to the surface of solid phase occur. The tendency of temperature lowering of the decomposition of powders synthesized from a medium containing organic substances was determined. Results demonstrate a direct dependence between the concentration of the amino acid in a model solution and its content in the solid phase.

Formation of Minor Phases in a Nickel-Based Disk Superalloy

NASA Technical Reports Server (NTRS)

Gabb, T. P.; Garg, A.; Miller, D. R.; Sudbrack, C. K.; Hull, D. R.; Johnson, D.; Rogers, R. B.; Gayda, J.; Semiatin, S. L.

2012-01-01

The minor phases of powder metallurgy disk superalloy LSHR were studied. Samples were consistently heat treated at three different temperatures for long times to approximate equilibrium. Additional heat treatments were also performed for shorter times, to then assess non-equilibrium conditions. Minor phases including MC carbides, M23C6 carbides, M3B2 borides, and sigma were identified. Their transformation temperatures, lattice parameters, compositions, average sizes and total area fractions were determined, and compared to estimates of an existing phase prediction software package. Parameters measured at equilibrium sometimes agreed reasonably well with software model estimates, with potential for further improvements. Results for shorter times representing non-equilibrium indicated significant potential for further extension of the software to such conditions, which are more commonly observed during heat treatments and service at high temperatures for disk applications.

Measuring of temperatures of phase objects using a point-diffraction interferometer plate made with the thermocavitation process

NASA Astrophysics Data System (ADS)

Aguilar, Juan C.; Berriel-Valdos, L. R.; Aguilar, J. Felix; Mejia-Romero, S.

An optical system formed by four point-diffraction interferometers is used for measuring the refractive index distribution of a phase object. The phase of the object is assumed enough smooth to be computed in terms of the Radon Transform and it is processed with a tomographic iterative algorithm. Then, the associated refractive index distribution is calculated. To recovery the phase from the inteferograms we use the Kreis method, which is useful for interferograms having only few fringes. As an application of our technique, the temperature distribution of a candle flame is retrieved, this was made with the aid of the Gladstone-Dale equation. We also describe the process of manufacturing the point-diffraction interferometer (PDI) plates. These were made by means of the thermocavitation process. The obtained three dimensional distribution of temperature is presented.

Crystal structure and phase transformations of calcium yttrium orthophosphate, Ca 3Y(PO 4) 3

NASA Astrophysics Data System (ADS)

Fukuda, Koichiro; Iwata, Tomoyuki; Niwa, Takahiro

2006-11-01

Crystal structure and phase transformations of calcium yttrium orthophosphate Ca 3Y(PO 4) 3 were investigated by X-ray powder diffraction, selected-area electron diffraction, transmission electron microscopy and optical microscopy. The high-temperature phase is isostructural with eulytite, cubic (space group I4¯3d) with a=0.983320(5) nm, V=0.950790(8) nm 3, Z=4 and D x=3.45 Mg m -3. The crystal structure was refined with a split-atom model, in which the oxygen atoms are distributed over two partially occupied sites. Below the stable temperature range of eulytite, the crystal underwent a martensitic transformation, which is accompanied by the formation of platelike surface reliefs. The inverted crystal is triclinic (space group P1) with a=1.5726(1) nm, b=0.84267(9) nm, c=0.81244(8) nm, α=109.739(4)°, β=90.119(5)°, γ=89.908(7)°, V=1.0134(1) nm 3, Z=4 and D x=3.24 Mg m -3. The crystal grains were composed of pseudo-merohedral twins. The adjacent twin domains were related by the pseudo-symmetry mirror planes parallel to {101¯} with the composition surface {101¯}. When the eulytite was cooled relatively slowly from the stable temperature range, the decomposition reaction of Ca 3Y(PO 4) 3→ β-Ca 3(PO 4) 2+YPO 4 occurred.

Phase transformations and equation of state of praseodymium metal to 103 GPa

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chesnut, Gary N.; Vohra, Yogesh K.

2000-08-01

Pressure-induced structural phase transformations in a trivalent rare-earth metal praseodymium (Pr) were studied at room temperature in a diamond anvil cell to 103 GPa by energy dispersive x-ray diffraction using a synchrotron source. Our x-ray diffraction studies document the following crystal structure sequence: dhcp{yields}fcc{yields}distorted fcc(hR24 type){yields}monoclinic(C2/m){yields}{alpha}-uranium with increasing pressure. We measure a 16.7% volume collapse at the transition to the {alpha}-uranium phase at 20 GPa. The high-pressure {alpha}-uranium phase in Pr was found to be stable to the highest pressure of 103 GPa, which corresponds to a volume compression V/V{sub 0}=0.407. (c) 2000 The American Physical Society.

Phase Transformation Behavior of Medium Manganese Steels with 3 Wt Pct Aluminum and 3 Wt Pct Silicon During Intercritical Annealing

NASA Astrophysics Data System (ADS)

Sun, Binhan; Fazeli, Fateh; Scott, Colin; Yue, Stephen

2016-10-01

Medium manganese steels alloyed with sufficient aluminum and silicon amounts contain high fractions of retained austenite adjustable to various transformation-induced plasticity/twinning-induced plasticity effects, in addition to a reduced density suitable for lightweight vehicle body-in-white assemblies. Two hot rolled medium manganese steels containing 3 wt pct aluminum and 3 wt pct silicon were subjected to different annealing treatments in the present study. The evolution of the microstructure in terms of austenite transformation upon reheating and the subsequent austenite decomposition during quenching was investigated. Manganese content of the steels prevailed the microstructural response. The microstructure of the leaner alloy with 7 wt pct Mn (7Mn) was substantially influenced by the annealing temperature, including the variation of phase constituents, the morphology and composition of intercritical austenite, the Ms temperature and the retained austenite fraction. In contrast, the richer variant 10 wt pct Mn steel (10Mn) exhibited a substantially stable ferrite-austenite duplex phase microstructure containing a fixed amount of retained austenite which was found to be independent of the variations of intercritical annealing temperature. Austenite formation from hot band ferrite-pearlite/bainite mixtures was very rapid during annealing at 1273 K (1000 °C), regardless of Mn contents. Austenite growth was believed to be controlled at early stages by carbon diffusion following pearlite/bainite dissolution. The redistribution of Mn in ferrite and particularly in austenite at later stages was too subtle to result in a measureable change in austenite fraction. Further, the hot band microstructure of both steels contained a large fraction of coarse-grained δ-ferrite, which remained almost unchanged during intercritical annealing. A recently developed thermodynamic database was evaluated using the experimental data. The new database achieved a better agreement with the experimental results for the 7Mn steel compared with the existing commercial TCFE database; however, some discrepancy in the predicted phase fractions and compositions still existed. The phase transformation behavior of the two steels during annealing and its implication on the design of high aluminum-silicon medium manganese steels were discussed in detail.