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Sample records for phase transformation temperature

  1. 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

  2. Phase transformation strengthening of high-temperature superalloys.

    PubMed

    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

    2016-11-22

    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.

  3. Phase transformation strengthening of high-temperature superalloys

    NASA Astrophysics Data System (ADS)

    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.

    2016-11-01

    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.

  4. Low-temperature phase transformation of CZTS thin films

    NASA Astrophysics Data System (ADS)

    Zhao, Wei; Du, Lin-Yuan; Liu, Lin-Lin; Sun, Ya-Li; Liu, Zhi-Wei; Teng, Xiao-Yun; Xie, Juan; Liu, Kuang; Yu, Wei; Fu, Guang-Sheng; Gao, Chao

    2017-04-01

    The low temperature phase transformation in the Cu2ZnSnS4 (CZTS) films was investigated by laser annealing and low temperature thermal annealing. The Raman measurements show that a-high-power laser annealing could cause a red shift of the Raman scattering peaks of the kesterite (KS) structure and promotes the formation of the partially disordered kesterite (PD-KS) structure in the CZTS films, and the low-temperature thermal annealing only shifts the Raman scattering peak of KS phase by several wavenumber to low frequency and the broads Raman peaks in the low frequency region. Moreover, the above two processes were reversible. The Raman analyses of the CZTS samples prepared under different process show that the PD-KS structure tends to be found at low temperatures and low sulfur vapor pressures. Our results reveal that the control of the phase structure in CZTS films is feasible by adjusting the preparation process of the films. Project supported by the Natural Science Foundation for Youth Fund of Hebei Province, China (Grant No. A2016201087), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20131301120003), and the National Natural Science Foundation of China (Grant Nos. 11504078 and 61504054).

  5. Solidification Processing and Phase Transformations in Ordered High Temperature Alloys

    DTIC Science & Technology

    1993-01-20

    transformation from cubic to hexagonal and ends with a transformation, from c2 to Orthorhombic phase. During these stages a complex domain structure is formed...arrangement of rotational domains of the 6 ower symmetry orthorhombic phase. [he Crystal Structure of the Ti2AINb Orthorhombic Phase - The results of a... orthorhombic phase in the Ti-Al-Nb system (Paper #7). The structure is ýmcmn (HgNa or Cd 3Er) with a=0.6089 nm, b=0.9569 nm, and c= 0.4667 rnm. Ti(Nb) fills

  6. Chemically and temperature-induced phase transformations of metal vanadates

    NASA Astrophysics Data System (ADS)

    Patridge, Christopher James

    different individual beta'-Cu xV2O5 nanowires vary widely. Using scanning transmission X-ray microspectroscopy of individual beta'-CuxV2O 5 nanowires, correlations appear to exist between MIT characteristics and the markedly different orbital hybridization of vanadium and oxygen at the O K and V L absorption edges. These comprehensive nanostructure studies hint at the possibility of approaching the incredibly important realm of single-domain measurements which are needed to understand and exploit the intrinsic physical properties of materials. In addition to the bronze MIT studies, the classical MIT material vanadium dioxide, VO2, also shows new properties when scaling down to nanoscale dimensions as well as incorporation of substitutional dopants such as tungsten. X-ray absorption spectroscopy of the dopant local structure suggests an increased symmetry and depairing of V4+-V 4+, which is critical for transition to the lower temperature insulating phase thereby super-cooling the metallic phase to temperatures as low as 254 K. Mechanistic insight and structural changes associated with the intercalation of Li+ are key aspects in understanding and designing useful secondary Li ion batteries. In similarity to the MxV2O 5 studies, another metal vanadate, Ag2VO2PO 4, undergoes phase transformations due to introduction of Li and the vacancy of Ag ions. Employing a comprehensive study on Ag2VO 2PO4 using X-ray absorption spectroscopy, information about chemical state changes and rehybridization of frontier orbitals allows for a more precise understanding of how the material discharges, what, if any, intermediate phases exist during the process, and provides evidence for the posited structural stability at high depths of discharge.

  7. Temperature dependent phase transformation in nano sized magnesium ferrite

    NASA Astrophysics Data System (ADS)

    Sumangala T., P.; Mahender, C.; Venkataramani, N.; Prasad, Shiva

    2015-06-01

    The phase transformation in nanosized stoichiometric magnesium ferrite is being discussed. It was shown by TGA/DSC that there exist two reactions (shown by exothermic peaks) in nano sized magnesium ferrite when synthesized by sol gel combustion synthesis. First one of these reactions resulted in the precipitation of α-Fe2O3 and a resultant spinel. The second reaction resulted in stoichiometric spinel from this two phase system.

  8. 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.

  9. 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.

  10. 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.

  11. Low-temperature solid-state phase transformations in 2H silicon carbide

    NASA Technical Reports Server (NTRS)

    Will, H. A.; Powell, J. A.

    1972-01-01

    Single crystals of 2H SiC were observed to undergo phase transformations at temperatures as low as 400 C. Some 2H crystals transformed to a structure with one-dimensional disorder along the crystal c axis. Others transformed to a faulted cubic/6H structure. The transformation is time and temperature dependent and is greatly enhanced by dislocations. Observations indicate that the transformation takes place by means of a slip process perpendicular to the c axis. Cubic SiC crystals were observed to undergo a solid state transformation above 1400 C.

  12. Interplay between diffusive and displacive phase transformations: time-temperature-transformation diagrams and microstructures.

    PubMed

    Bouville, Mathieu; Ahluwalia, Rajeev

    2006-08-04

    Materials which can undergo extremely fast displacive transformations as well as very slow diffusive transformations are studied using a Ginzburg-Landau framework. This simple model captures the essential physics behind microstructure formation and time-temperature-transformation diagrams in alloys such as steels. It also predicts the formation of mixed microstructures by an interplay between diffusive and displacive mechanisms. The intrinsic volume changes associated with the transformations stabilize mixed microstructures such as martensite-retained austenite (responsible for the existence of a martensite finish temperature) and martensite-pearlite.

  13. Pressure and Temperature effects on the High Pressure Phase Transformation in Zirconium

    SciTech Connect

    Escobedo-Diaz, Juan P.; Cerreta, Ellen K.; Brown, Donald W.; Trujillo, Carl P.; Rigg, Paulo A.; Bronkhorst, Curt A.; Addessio, Francis L.; Lookman, Turab

    2012-06-20

    At high pressure zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase ({alpha}) to the simple hexagonal omega phase ({omega}). Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams. For this reason, the influence of peak shock pressure and temperature on the retention of omega phase in Zr is explored in this study. In situ VISAR measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. This understanding of the role of peak shock stress will be utilized to address physics to be encoded in our present macro-scale models.

  14. In-situ High Temperature Phase Transformations in Ceramics

    DTIC Science & Technology

    2009-07-28

    in high temperature combustion environments leads to accelerated oxidation of Si-based ceramics and composites, such as silicon carbide ( SiC ) fiber...have been achieved in silicon carbide reinforced with continuous, graphite-coated, silicon carbide fibers. The graphite acts as a debonding layer...reinforced SiC ceramic matrix composites (SiCf/ SiC CMCs) and monolithic silicon nitride (Si3N4), and severely limits their application in gas

  15. Phase transformations in amorphous fullerite C60 under high pressure and high temperature

    NASA Astrophysics Data System (ADS)

    Borisova, P. A.; Blanter, M. S.; Brazhkin, V. V.; Somenkov, V. A.; Filonenko, V. P.

    2015-08-01

    First phase transformations of amorphous fullerite C60 at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С60 molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature-pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.

  16. Phase transformation of oxide film in zirconium alloy in high temperature hydrogenated water

    SciTech Connect

    Kim, Taeho; Kim, Jongjin; Choi, Kyoung Joon; Yoo, Seung Chang; Kim, Seung Hyun; Kim, Ji Hyun

    2015-07-23

    The effect of the variation of the dissolved hydrogen concentration on the oxide phase transformation under high-temperature hydrogenated water conditions was investigated using in situ Raman spectroscopy. The Raman spectrum in 50 cm(3)/kg of dissolved hydrogen concentration indicated the formation of monoclinic and tetragonal zirconium oxide at the water-substrate interface. As the dissolved hydrogen concentration decreased to 30 cm(3)/kg, the Raman peaks corresponding to the zirconium oxide phase changed, indicating an oxide phase transformation. And, the results of SEM and TEM analyses were compared with those of in situ analyses obtained for the oxide structure formed on the zirconium alloy.

  17. Phase transformations in the Zn-Al eutectoid alloy after quenching from the high temperature triclinic beta phase

    SciTech Connect

    Sandoval-Jimenez, A.; Torres-Villasenor, G.

    2010-11-15

    Ribbons of the Zn-Al eutectoid alloy obtained by melt-spinning, were heat treated at 350 deg. C during 30 min in a free atmosphere furnace, and then quenched in liquid nitrogen. The temperature correspond to {beta} phase zone, which has a triclinic crystalline structure [1, 2]. Some evidence, obtained by X-ray diffraction, show that the structures present in the just quenched material are both close-packed hexagonal ({eta}-phase) and rhombohedral (R-phase). X-ray diffractograms taken in the same ribbons after annealed 500 h at room temperature, show that the R phase its transform to {alpha} and {eta} phases.

  18. Temperature-dependent mechanical deformation of silicon at the nanoscale: Phase transformation versus defect propagation

    SciTech Connect

    Kiran, M. S. R. N. Tran, T. T.; Smillie, L. A.; Subianto, D.; Williams, J. S.; Bradby, J. E.; Haberl, B.

    2015-05-28

    This study uses high-temperature nanoindentation coupled with in situ electrical measurements to investigate the temperature dependence (25–200 °C) of the phase transformation behavior of diamond cubic (dc) silicon at the nanoscale. Along with in situ indentation and electrical data, ex situ characterizations, such as Raman and cross-sectional transmission electron microscopy, have been used to reveal the indentation-induced deformation mechanisms. We find that phase transformation and defect propagation within the crystal lattice are not mutually exclusive deformation processes at elevated temperature. Both can occur at temperatures up to 150 °C but to different extents, depending on the temperature and loading conditions. For nanoindentation, we observe that phase transformation is dominant below 100 °C but that deformation by twinning along (111) planes dominates at 150 °C and 200 °C. This work, therefore, provides clear insight into the temperature dependent deformation mechanisms in dc-Si at the nanoscale and helps to clarify previous inconsistencies in the literature.

  19. Evidence for the temperature dependence of phase transformation behavior of silicon at nanoscale

    DOE PAGES

    Mangalampalli S. R. N. Kiran; Tran, Tuan; Smillie, Lachlan; ...

    2015-05-27

    This study uses the in-situ high-temperature nanoindentation coupled with electrical measurements to investigate the temperature dependence (25 to 200 C) of the phase transformation behavior of crystalline silicon (dc-Si) at the nanoscale. Along with in-situ indentation and electrical data, ex-situ characterizations such as Raman and cross-sectional transmission electron microscopy (XTEM) have been used to reveal the dominant mode of deformation under the indenter. In contrast to the previous studies, the dominant mode of deformation under the nanoindenter at elevated temperatures is not the dc-Si to metallic phase ( -Sn) transformation. Instead, XTEM images from 150 C indents reveal that themore » dominant mode of deformation is twinning along {111} planes. While the in-situ high-temperature electrical measurements show an increase in the current due to metallic phase formation up to 125 C, it is absent 150 C, revealing that the formation of the metallic phase is negligible in this regime. Thus, this work provides clear insight into the temperature dependent deformation mechanisms in dc-Si at the nanoscale.« less

  20. Evidence for the temperature dependence of phase transformation behavior of silicon at nanoscale

    SciTech Connect

    Mangalampalli S. R. N. Kiran; Tran, Tuan; Smillie, Lachlan; Haberl, Bianca; Subianto, D.; Williams, James S.; Bradby, Jodie E.

    2015-05-27

    This study uses the in-situ high-temperature nanoindentation coupled with electrical measurements to investigate the temperature dependence (25 to 200 C) of the phase transformation behavior of crystalline silicon (dc-Si) at the nanoscale. Along with in-situ indentation and electrical data, ex-situ characterizations such as Raman and cross-sectional transmission electron microscopy (XTEM) have been used to reveal the dominant mode of deformation under the indenter. In contrast to the previous studies, the dominant mode of deformation under the nanoindenter at elevated temperatures is not the dc-Si to metallic phase ( -Sn) transformation. Instead, XTEM images from 150 C indents reveal that the dominant mode of deformation is twinning along {111} planes. While the in-situ high-temperature electrical measurements show an increase in the current due to metallic phase formation up to 125 C, it is absent 150 C, revealing that the formation of the metallic phase is negligible in this regime. Thus, this work provides clear insight into the temperature dependent deformation mechanisms in dc-Si at the nanoscale.

  1. Phase Transformations During Li-Insertion into V2O5 at Elevated Temperature

    NASA Astrophysics Data System (ADS)

    Kalaga, Kaushik; Sayed, Farheen N.; Rodrigues, Marco-Tulio F.; Ajayan, Pulickel M.

    2017-09-01

    Recent interest in developing cathode materials for an elevated temperature operation of Li-ion batteries has motivated researchers to explore the possibility of using layered V2O5 as a potential candidate because of its high capacity and cyclic stability. Despite a wide lithiation voltage window of V2O5 (between 1.0 V and 4.0 V), compositional fluctuations, metal dissolution, and so on contribute to capacity loss at high temperatures. A first discharge of V2O5 to voltages below 2.0 V has been observed to be associated with a series of phase transformations at both room temperature and high temperature and has been characterized here. From structural characterization of harvested electrodes post-first discharge, a new Li-rich phase was observed to be formed at 120°C and the composition was estimated.

  2. Phase Transformations During Li-Insertion into V2O5 at Elevated Temperature

    NASA Astrophysics Data System (ADS)

    Kalaga, Kaushik; Sayed, Farheen N.; Rodrigues, Marco-Tulio F.; Ajayan, Pulickel M.

    2017-06-01

    Recent interest in developing cathode materials for an elevated temperature operation of Li-ion batteries has motivated researchers to explore the possibility of using layered V2O5 as a potential candidate because of its high capacity and cyclic stability. Despite a wide lithiation voltage window of V2O5 (between 1.0 V and 4.0 V), compositional fluctuations, metal dissolution, and so on contribute to capacity loss at high temperatures. A first discharge of V2O5 to voltages below 2.0 V has been observed to be associated with a series of phase transformations at both room temperature and high temperature and has been characterized here. From structural characterization of harvested electrodes post-first discharge, a new Li-rich phase was observed to be formed at 120°C and the composition was estimated.

  3. In-situ Phase Transformation and Deformation of Iron at High Pressure andTemperature

    SciTech Connect

    Miyagi, Lowell; Kunz, Martin; Knight, Jason; Nasiatka, James; Voltolini, Marco; Wenk, Hans-Rudolf

    2008-07-01

    With a membrane based mechanism to allow for pressure change of a sample in aradial diffraction diamond anvil cell (rDAC) and simultaneous infra-red laser heating, itis now possible to investigate texture changes during deformation and phasetransformations over a wide range of temperature-pressure conditions. The device isused to study bcc (alpha), fcc (gamma) and hcp (epislon) iron. In bcc iron, room temperature compression generates a texture characterized by (100) and (111) poles parallel to the compression direction. During the deformation induced phase transformation to hcp iron, a subset of orientations are favored to transform to the hcp structure first and generate a texture of (01-10) at high angles to the compression direction. Upon further deformation, the remaining grains transform, resulting in a texture that obeys the Burgers relationship of (110)bcc // (0001)hcp. This is in contrast to high temperature results that indicate that texture is developed through dominant pyramidal {2-1-12}<2-1-13> and basal (0001)-{2-1-10} slip based on polycrystal plasticity modeling. We also observe that the high temperature fcc phase develops a 110 texture typical for fcc metals deformed in compression.

  4. Crystal structure and phase transformation of BiFeO3 multiferroics on the temperature variation

    NASA Astrophysics Data System (ADS)

    Nuraini, U.; Suasmoro, S.

    2017-04-01

    In this study, multiferroic of BiFeO3 was successfully synthesized using solid state methods. The materials was calcined by variation temperature at 800, 840, 860, and 880°C for 4hours in air atmosphere. Structure analysis and phase transformation were carried out using X-ray diffraction. The results show that not only BiFeO3 phase is formed but also other phases as secondary phases on all variation temperatures. The secondary phases, i.e. Bi25FeO40 and Bi2Fe4O9, have different molar or weight percentage at that variation. BiFeO3 has a perovskite structure and considering as R3c space group, for all secondary phases consist of cubic structure. The results showed that increasing calcination temperaturedecrease the amount of BiFeO3 phases and increase the secondary phases. The highest percentage of BiFeO3 phases (91.69%) is obtained at 800°C.

  5. Machining and Phase Transformation Response of Room-Temperature Austenitic NiTi Shape Memory Alloy

    NASA Astrophysics Data System (ADS)

    Kaynak, Yusuf

    2014-09-01

    This experimental work reports the results of a study addressing tool wear, surface topography, and x-ray diffraction analysis for the finish cutting process of room-temperature austenitic NiTi alloy. Turning operation of NiTi alloy was conducted under dry, minimum quantity lubrication (MQL) and cryogenic cooling conditions at various cutting speeds. Findings revealed that cryogenic machining substantially reduced tool wear and improved surface topography and quality of the finished parts in comparison with the other two approaches. Phase transformation on the surface of work material was not observed after dry and MQL machining, but B19' martensite phase was found on the surface of cryogenically machined samples.

  6. In situ high temperature microscopy study of the surface oxidation and phase transformations in titanium alloys.

    PubMed

    Malinov, S; Sha, W; Voon, C S

    2002-09-01

    Two popular commercial titanium alloys, Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si, were used for in situ high temperature microscopy study. The experiments were performed on an optical microscope equipped with high temperature stage using both normal and florescence lights. Two kinds of experiments were performed, at continuous heating/cooling with different rates and in isothermal conditions at different temperatures. The changes taking place on the sample surface during the experiments were monitored. The morphology of the alpha ==> beta ==> alpha phase transformation was recorded at different heat treatment conditions using the effect of thermal etching. An effect of sample surface oxidation and deoxidation was observed during continuous heating. The appearance and disappearance of ordered titanium oxides Ti3O and Ti2O are discussed based on the phase equilibrium diagram. The kinetics of the surface oxidation was monitored in both isothermal and continuous cooling conditions.

  7. Effect of calcination temperature on phase transformation of HfO{sub 2} nanoparticles

    SciTech Connect

    Tripathi, S. K.; Kaur, Charanpreet; Kaur, Ramneek; Kaur, Jagdish

    2015-08-28

    Oxides nanomaterials exhibit unique physical, chemical and structural properties and motivated a big research that focus in the integration of these materials for various optoelectronic device applications. In present work, hafnium oxide (HfO{sub 2}) nanoparticles (NPs) have been synthesized using precipitation method. Hafnium tetrachloride and sodium hydroxide has been used as starting precursors. Prepared oxide material has been characterized by X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) and UV-Vis spectroscopy. The phase transformation from amorphous to monoclinic is observed with the increase in calcination temperature from 500 °C and 800 °C. In FTIR spectra, the characteristic bands at ν ∼ 758.53 and 509.57 cm{sup −1} reveals the monoclinic phase of prepared HfO{sub 2} NPs. UV-Vis spectroscopy shows an absorption peak at 204 nm and the bandgap calculated is 6.07 eV.

  8. Pressure, temperature, and electric field dependence of phase transformations in niobium modified 95/5 lead zirconate titanate

    SciTech Connect

    Dong, Wen D.; Carlos Valadez, J.; Gallagher, John A.; Jo, Hwan R.; Lynch, Christopher S.; Sahul, Raffi; Hackenberger, Wes

    2015-06-28

    Ceramic niobium modified 95/5 lead zirconate-lead titanate (PZT) undergoes a pressure induced ferroelectric to antiferroelectric phase transformation accompanied by an elimination of polarization and a volume reduction. Electric field and temperature drive the reverse transformation from the antiferroelectric to ferroelectric phase. The phase transformation was monitored under pressure, temperature, and electric field loading. Pressures and temperatures were varied in discrete steps from 0 MPa to 500 MPa and 25 °C to 125 °C, respectively. Cyclic bipolar electric fields were applied with peak amplitudes of up to 6 MV m{sup −1} at each pressure and temperature combination. The resulting electric displacement–electric field hysteresis loops were open “D” shaped at low pressure, characteristic of soft ferroelectric PZT. Just below the phase transformation pressure, the hysteresis loops took on an “S” shape, which split into a double hysteresis loop just above the phase transformation pressure. Far above the phase transformation pressure, when the applied electric field is insufficient to drive an antiferroelectric to ferroelectric phase transformation, the hysteresis loops collapse to linear dielectric behavior. Phase stability maps were generated from the experimental data at each of the temperature steps and used to form a three dimensional pressure–temperature–electric field phase diagram.

  9. Structural phase transformation of as-prepared Mg–Mn nanoferrites by annealing temperature

    SciTech Connect

    Amer, M.A.; Meaz, T.M.; Attalah, S.S.; Ghoneim, A.I.

    2015-12-15

    As-prepared Mg{sub 0.2}Mn{sub 0.8}Fe{sub 2}O{sub 4} nanoferrite samples by coprecipitation method were annealed at different temperatures T. XRD, TEM, IR, VSM and Mössbauer techniques were used to characterize the samples. This study proved the transformation of the sample structure from cubic spinel to Z-type hexagonal phase at 700 °C. This transformation was assigned to Jahn–Teller effect of the Mn{sup 3+} and Fe{sup 2+} ions and/or atomic disorder in the crystal lattice. All the deduced structural parameters were affected by the transformation process. The infrared and structural parameters showed dependence on T, whereas tetrahedral band position did not. TEM images showed agglomeration of the nanoparticles, where the average particle size ranged 25–55 nm and was a little higher than R. Study on the elastic properties proved the weakening of inter-atomic bonds inside the crystal lattice as T increases. The magnetic properties were affected by the transformation process and showed T dependence, whereas M{sub s} and H{sub c} proved dependence on porosity. Hyperfine interaction parameters, area ratio (B/A) and spontaneous magnetization were discussed as functions of T. - Graphical abstract: The XRD patterns of as-prepared (AP) and annealed Mg{sub 0.2}Mn{sub 0.8}Fe{sub 2}O{sub 4} nanoparticles. - Highlights: • As-prepared Mg–Mn nanoferrites were annealed at different temperatures T. • Sample structure was transformed from cubic to Z-type hexagonal with T. • This transformation was attributed to Jahn–Teller effect and atomic disorder. • The deduced parameters were affected by annealing and transformation process. • The porosity was affected the saturation magnetization and coercivity.

  10. Identification of Defect Prone Peritectic Steel Grades by Analyzing High-Temperature Phase Transformations

    NASA Astrophysics Data System (ADS)

    Presoly, Peter; Pierer, Robert; Bernhard, Christian

    2013-12-01

    Continuous casting of peritectic steels is often difficult and critical; bad surface quality, cracks, and even breakouts may occur. The initial solidification of peritectic steels within the mold leads to formation of surface depressions and uneven shell growth. As commercial steels are always multicomponent alloys, the influence also of the alloying elements besides carbon on the peritectic phase transition needs to be taken into account. Information on the solidification sequence and phase diagrams for initial solidification are lacking especially for new steel grades, like high-alloyed TRIP-steels with high Mn, Si, and particularly high Al contents. Based on a comprehensive method development, the current study shows that differential scanning calorimeter measurements allow a clear prediction if an alloy is peritectic ( i.e., critical to cast). In order to confirm these results, thermo-optical analyses with a high-temperature laser-scanning-confocal-microscope are performed to observe the phase transformations in situ up to the melting point.

  11. Effects of oxygen and/or nitrogen on phase transformations above the monotectoid temperature in NbZr alloys

    NASA Astrophysics Data System (ADS)

    Nakai, K.; Kinoshita, C.; Kitajima, S.

    1981-05-01

    Effects of small amounts of oxygen and/or nitrogen on the NbZr phase diagram and the kinetics of phase transformations have been examined by use of optical microscopy, X-ray diffraction, electron microscopy and electron-probe micro-analysis. Hcp α phase and two bcc phases, βNb, and βZr' are observed near specimen surfaces annealed in vacua at temperatures above the monotectoid temperature and outside the monotectoid loop in Rogers and Atkins' phase diagram. The kinetics of the phase transformation from homogeneous β phase to (βNb + βZr) follows the kinetics of an Ostwald ripening reaction and the reaction rate is reduced by oxygen and/or nitrogen. The conclusion is that oxygen and/or nitrogen have direct effects on the kinetics of phase transformation as well as phase stability in NbZr alloys.

  12. Instantaneous phase shift of annual subsurface temperature cycles derived by the Hilbert-Huang transform

    NASA Astrophysics Data System (ADS)

    Sun, Yang-Yi; Chen, Chieh-Hung; Liu, Jann-Yenq; Wang, Chung-Ho; Chen, Deng-Lung

    2015-03-01

    This study uses the Hilbert-Huang transform to compute the instantaneous (daily) phase shift between temperature signals at the ground surface and at a depth of 5 m. This approach is not restricted to the stationary harmonic surface temperature assumptions invoked by analytical solutions. The annual cycles are extracted from the ground surface temperatures and the shallow subsurface temperatures at 5 m depth recorded at the Hualien (23.98°N, 121.61°E) and Ilan (24.77°N, 121.75°E) meteorology stations of Central Weather Bureau in Taiwan from 1952 to 2008. Significant reductions in the phase shift and increases in the estimated thermal diffusivity from 1980s to 1990s are found and suggest that the recent warming of the Pacific Decadal Oscillation may affect heat transport in the subsurface environment. The marginal spectra of the instantaneous phase shifts and the precipitation intensity records at Hualien and Ilan reveal that precipitation may play a role in the evolution of seasonal variation in shallow subsurface heat transport.

  13. Nanoscale conductive niobium oxides made through low temperature phase transformation for electrocatalyst support

    SciTech Connect

    Huang, K; Li, YF; Yan, LT; Xing, YC

    2014-01-01

    We report an effective approach to synthesize nanoscale Nb2O5 coated on carbon nanotubes (CNTs) and transform it at low temperatures to the conductive form of NbO2. The latter, when used as a Pt electrocatalyst support, shows significant enhancement in catalyst activity and durability in the oxygen reduction reaction (ORR). Direct phase transformation of Nb2O5 to NbO2 often requires temperatures above 1000 degrees C. Here we show that this can be achieved at a much lower temperature (e.g. 700 degrees C) if the niobium oxide is first activated with carbon. Low temperature processing allows retaining nanostructures of the oxide without sintering, keeping its high surface areas needed for being a catalyst support. We further show that Pt supported on the conductive oxides on CNTs has two times higher mass activity for the ORR than on bare CNTs. The electrochemical stability of Pt was also outstanding, with only ca. 5% loss in electrochemical surface areas and insignificant reduction in half-wave potential in ORR after 5000 potential cycles.

  14. Influence of Temperature on Fatigue-Induced Martensitic Phase Transformation in a Metastable CrMnNi-Steel

    NASA Astrophysics Data System (ADS)

    Biermann, Horst; Glage, Alexander; Droste, Matthias

    2016-01-01

    Metastable austenitic steels can exhibit a fatigue-induced martensitic phase transformation during cyclic loading. It is generally agreed that a certain strain amplitude and a threshold of the cumulated plastic strain must be exceeded to trigger martensitic phase transformation under cyclic loading. With respect to monotonic loading, the martensitic phase transformation takes place up to a critical temperature—the so-called M d temperature. The goal of the present investigation is to determine an M d,c temperature which would be the highest temperature at which a fatigue-induced martensitic phase transformation can take place. For this purpose, fatigue tests controlled by the total strain were performed at different temperatures. The material investigated was a high-alloy metastable austenitic steel X3CrMnNi16.7.7 (16.3Cr-7.2Mn-6.6Ni-0.03C-0.09N-1.0Si) produced using the hot pressing technique. The temperatures were set in the range of 283 K (10 °C) ≤ T ≤ 473 K (200 °C). Depending on the temperature and strain amplitude, the onset of the martensitic phase transformation shifted to different values of the cumulated plastic strain, or was inhibited completely. Moreover, it is known that metastable austenitic CrMnNi steels with higher nickel contents can exhibit the deformation-induced twinning effect. Thus, at higher temperatures and strain amplitudes, a transition from the deformation-induced martensitic transformation to deformation-induced twinning takes place. The fatigue-induced martensitic phase transformation was monitored during cyclic loading using a ferrite sensor. The microstructure after the fatigue tests was examined using the back-scattered electrons, the electron channeling contrast imaging and the electron backscatter diffraction techniques to study the temperature-dependent dislocation structures and phase transformations.

  15. Tensile Residual Stress Mitigation Using Low Temperature Phase Transformation Filler Wire in Welded Armor Plates

    SciTech Connect

    Feng, Zhili; Bunn, Jeffrey R; Tzelepis, Demetrios A; Payzant, E Andrew; Yu, Xinghua

    2016-01-01

    Hydrogen induced cracking (HIC) has been a persistent issue in welding of high-strength steels. Mitigating residual stresses is one of the most efficient ways to control HIC. The current study develops a proactive in-process weld residual stress mitigation technique, which manipulates the thermal expansion and contraction sequence in the weldments during welding process. When the steel weld is cooled after welding, martensitic transformation will occur at a temperature below 400 C. Volume expansion in the weld due to the martensitic transformation will reduce tensile stresses in the weld and heat affected zone and in some cases produce compressive residual stresses in the weld. Based on this concept, a customized filler wire which undergoes a martensitic phase transformation during cooling was developed. The new filler wire shows significant improvement in terms of reducing the tendency of HIC in high strength steels. Bulk residual stress mapping using neutron diffraction revealed reduced tensile and compressive residual stresses in the welds made by the new filler wire.

  16. Phase Transformations During the Low-Temperature Nitriding of AISI 2205 Duplex Stainless Steel

    NASA Astrophysics Data System (ADS)

    Yan, Jing; Gu, Tan; Qiu, Shaoyu; Wang, Jun; Xiong, Ji; Fan, Hongyuan

    2015-02-01

    Liquid nitriding of type AISI 2205 duplex stainless steel was conducted at 723 K (450 °C), using one type of novel low-temperature liquid chemical thermo-treatment. The transformation of the nitrided surface microstructure was systematically studied. Experimental results revealed that a nitrided layer formed on the sample surface with the thickness ranging from 3 to 28 μm, depending on nitriding time. After the 2205 duplex stainless steel was subjected to liquid nitriding 723 K (450 °C) for less than 8 hours, the pre-existing ferrite region on the surface transformed into the expanded austenite (S phase) by the infusion of nitrogen atoms, most of which stay in the interstitial sites. Generally, the dominant phase of the nitrided layer was the expanded austenite. When the nitriding time prolonged up to 16 hours, some pre-existing ferrite in expanded austenite was decomposed and ɛ-nitride precipitated subsequently. When the treatment time went up to 40 hours, large amount of ɛ-nitride and CrN precipitates were observed in the pre-existing ferritic region in the expanded austenite. Furthermore, many nitrides precipitated from the pre-austenite region. Acicular nitride was identified by transmission electron microscopy. The thickness of the nitrided layer increased with increasing nitriding time. The growth of the nitrided layer is mainly due to nitrogen diffusion in accordance with the expected parabolic rate law. Liquid nitriding effectively increased the surface hardness of 2205 duplex stainless steel by a factor of 3.

  17. 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.

  18. Low-temperature Phase and Morphology Transformations in Noble Metal Nanocatalysts

    SciTech Connect

    O Malis; C Byard; D Mott; B Wanjala; R Loukrakpam; J Luo; C Zhong

    2011-12-31

    In situ real-time x-ray diffraction was used to study temperature-induced structural changes of 1-5 nm Au, Pt, and AuPt nanocatalysts supported on silicon substrates. Synchrotron-based x-ray diffraction indicates that the as-synthesized Au and Au{sub 64}Pt{sub 36} nanoparticles have a non-crystalline structure, while the Pt nanoparticles have the expected cubic structure. The nanoparticles undergo dramatic structural changes at temperatures as low as 120 C. During low-temperature annealing, the Au and AuPt nanoparticles first melt and then immediately coalesce to form 4-5 nm crystalline structures. The Pt nanoparticles also aggregate but with limited intermediate melting. The detailed mechanisms of nucleation and growth, though, are quite different for the three types of nanoparticles. Most interestingly, solidification of high-density AuPt nanoparticles involves an unusual transient morphological transformation that affects only the surface of the particles. AuPt nanoparticles on silicon undergo partial phase segregation only upon annealing at extremely high temperatures (800 C).

  19. Mechanism of the gamma-beta phase transformation of Mg2SiO4 at high temperature and pressure

    NASA Technical Reports Server (NTRS)

    Rubie, D. C.; Brearley, A. J.

    1990-01-01

    The results of experiments on the phase transformation of Mg2SiO4 olivine at 15 GPa pressure in a multianvil cell are reported. At this pressure and a temperature of 900 C, early formed metastable gamma-spinel transforms partially to the beta-phase. The observed microstructures, which are similar to those in shocked meteorites, show that the gamma-to-beta transformation can occur either by diffusion-controlled growth or by a martensitic mechanism, depending on how far the pressure-temperature conditions deviate from their values at phase equilibrium. The results suggest that the diffusion-controlled mechanism is most likely to operate at the beta/gamma phase boundary in the mantle, but martensitic beta-to-gamma transformation might occur in subduction zones and could reduce the shear strength of the subducting slab.

  20. 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.

  1. Phase Transformation Behavior at Low Temperature in Hydrothermal Treatment of Stable and Unstable Titania Sol

    PubMed

    So; Park; Kim; Moon

    1997-07-15

    Nanosize titania sol was prepared from titanium tetraisopropoxide (TTIP) and conditions for the formation of stable sol were identified. As the H+/TTIP mole ratio decreased and H2O/TTIP mole ratio increased, stable sol was likely to be formed. The size and crystallinity remained unchanged after hydrothermal treatment of the stable sol at between 160 and 240°C. However, hydrothermal treatment of unstable sol produced rod-like particles and crystallinity of particles was changed from anatase to rutile. This difference in phase transformation at low hydrothermal treatment temperature was likely to be caused by the fact that stable sol remained to be stable even after hydrothermal treatment, while unstable sol had a tendency to be aggregated.

  2. Influence of zirconium additions on nitinol shape memory phase stability, transformation temperatures, and thermo-mechanical properties

    NASA Astrophysics Data System (ADS)

    Kornegay, Suzanne M.

    This research focuses on exploring the influence of Zr additions in Ni-rich Nitinol alloys on the phase stability, transformation temperatures, and thermo-mechanical behavior using various microanalysis techniques. The dissertation is divided into three major bodies of work: (1) The microstructural and thermo-mechanical characterization of a 50.3Ni-32.2Ti-17.5Zr (at.%) Zr alloy; (2) The characterization and mechanical behavior of 50.3Ni-48.7Ti-1Zr and 50.3Ni-48.7Ti-1Hf alloys to determine how dilute additions alter the phases, transformation temperatures, and thermo-mechanical properties; and (3) The microstructural evolution and transformation behavior comparison of microstructure and transformation temperature for 50.3Ni-(49.7-X)Ti-XZr alloys, where X is 1,7, or 17.5% Zr aged at either 400°C and 550°C. The major findings of this work include the following: (1) In the dilute limit of 1% Zr, at 400°C aging, a spherical precipitate, denoted as the S-phase, was observed. This is the first report of this phase. Further aging resulted in the secondary precipitation event of the H-phase. Increasing the aging temperature to 550°C, resulted in no evident precipitation of the S- and H-phase precipitates suggestive this temperature is above the solvus boundary for these compositions. (2) For the 7% and 17.5% Zr alloys, aging at 400°C and 550°C resulted in the precipitation of the H-phase. For the lower temperature anneal, this phase required annealing up to 300 hours of aging to be observed for the 17.5% Zr alloy. Upon increasing the aging temperature, the H-phase precipitation was present in both alloys. The transformation behavior and thermo-mechanical properties are linked to the precipitation behavior.

  3. Low temperature phase transformations in the metallic phases of iron and stony-iron meteorites

    NASA Technical Reports Server (NTRS)

    Reuter, K. B.; Williams, D. B.; Goldstein, J. I.

    1988-01-01

    The nickel content and the structure of kamacite and decomposed taenite (clear taenite 1, CT-1; the cloudy zone, CZ; and clear taenite 2, CT-2) in the metallic phases of meteorites were determined using X-ray microanalysis techniques in the AEM. The kamacite near the CT-1 interface was found to contain about 4 wt pct Ni. The CT-1 structure contains 51.4-45.6 wt pct Ni; it is ordered FeNi with the L1(0) superstructure. The CZ structure consists of two phases: a globular phase (ordered FeNi containing 50.9 wt pct Ni) and a surrounding honeycomb martensitic phase containing 11.7 wt pct Ni. The CT-2 was found in all of the iron meteorite groups studied and in the pallasites, but not in the mesosiderites. Based on the preliminary evidence, this region is believed to be ordered Fe3Ni. Possible mechanisms for the decomposition of taenite are discussed.

  4. Low temperature phase transformations in the metallic phases of iron and stony-iron meteorites

    NASA Astrophysics Data System (ADS)

    Reuter, K. B.; Williams, D. B.; Goldstein, J. I.

    1988-03-01

    The nickel content and the structure of kamacite and decomposed taenite (clear taenite 1, CT-1; the cloudy zone, CZ; and clear taenite 2, CT-2) in the metallic phases of meteorites were determined using X-ray microanalysis techniques in the AEM. The kamacite near the CT-1 interface was found to contain about 4 wt pct Ni. The CT-1 structure contains 51.4-45.6 wt pct Ni; it is ordered FeNi with the L1(0) superstructure. The CZ structure consists of two phases: a globular phase (ordered FeNi containing 50.9 wt pct Ni) and a surrounding honeycomb martensitic phase containing 11.7 wt pct Ni. The CT-2 was found in all of the iron meteorite groups studied and in the pallasites, but not in the mesosiderites. Based on the preliminary evidence, this region is believed to be ordered Fe3Ni. Possible mechanisms for the decomposition of taenite are discussed.

  5. [Study on the Temperature Dependent Phase Transformation of Raman Spectra for Cyclobutanol].

    PubMed

    Zhang, Huan-jun; Cheng, Xue-rui; Ren, Yu-fen; Zhu, Xiang; Yuan, Chao-sheng

    2016-02-01

    Cyclobutanol (C₄H₈O) is one of the four-membered ring type molecules, which usually adopts a non-planar equilibrium conformation, and the substituent group OH can adopt two positions relative to the puckered ring, the axial or the equatorial, giving rise to an additional degree of freedom and various molecular conformations. Additionally, temperature is one important thermodynamic parameter that greatly influents the structure and induces the possibility of conformational change or crystal change. As a consequence, there may be a number of phase transitions and molecular conformations for cyclobutanol under different temperature. In this paper, Raman and infrared spectroscopic technique were applied to investigate the vibration modes of cyclobutanol. The results indicate that the main component of the liquid cyclobutanol is equatorial-trans (Eq-t) conformer with a few Eq-g conformers at ambient condition. Then differential scanning calorimetry (DSC) and low temperature Raman spectroscopic were applied to study the phase transition of cyclobutanol during the cooling and heating process. It is observed that the Raman spectra and the intensities of these bands are not significantly changed during the cooling process. The results indicate that there is sill no presence of solidification especially cooling to 140K, which indicates that the cyclobutanol still remains the liquid state and supercooled state is observed during the cooling process. And this supercooled liquid is one metastable state, not in thermodynamic equilibrium. Further cooling to 138 K, the super-cooling liquid cyclobutanol will transform into the glassy state, accompanied with a small change of entropy. During the heating process, as the temperature is raised to 180 K, the Raman peaks became sharper and some new characteristic peaks appeared abruptly and a discontinuous change was observed in bandwidths versus temperature. And these new signatures can be maintained upon to 220 K, and then will

  6. 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.

  7. 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.

  8. 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.

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

    PubMed

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

    2017-02-18

    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, (10 1 ¯ )γs//( 1 ¯ 10)γp and (0 1 ¯ 0)γs//(22 1 ¯ )γp.

  10. Solidification processing and phase transformations in ordered high temperature alloys. Final report, 30 March 1990-30 September 1992

    SciTech Connect

    Boettinger, W.J.; Bendersky, L.A.; Kattner, U.R.

    1993-01-20

    Useful high temperature alloys generally have microstructures consisting of more than one phase. Multiphase microstructures are necessary to develop acceptable toughness and creep strength in high temperature intermetallic alloy matrices. The optimum microstructures must be developed by a careful selection of processing path that includes both solidification and solid state heat treatment. Research has been conducted on the rapid solidification of selected intermetallic alloys and on the phase transformation paths that occur during cooling, primarily in the Ti-Al-Nb system. This report describes research performed in the Metallurgy Division at NIST under DARPA order 7469 between 1/1/89 and 12/31/92. Various research tasks were completed and the results have been published or have been submitted for publication.... Intermetallics, Ti-Al-Nb Alloys, Phase Diagrams, Phase Transformations, Ti-Al-Ta Alloys, MoSi2 Alloys.

  11. Effect of calcination temperature on phase transformation and crystallite size of copper oxide (CuO) powders

    NASA Astrophysics Data System (ADS)

    Ratnawulan, Fauzi, Ahmad; AE, Sukma Hayati

    2017-08-01

    Copper oxide powder was prepared from Copper iron from South Solok, Indonesia. The samples was dried and calcined for an hour at temperatures of 145°C, 300°C,850°C, 1000°C. Phase transformation and crystallite size of the calcined powders have been investigated as a function of calcination temperature by room-temperature X-ray diffraction (XRD). It was seen that the tenorite, CuO was successfully obtained. With increasing calcining temperature, CuO transformed from malachite Cu2(CO3)(OH)2 to tenorite phase (CuO) and crystallite size of prepared samples increased from 36 nm to 76 nm.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. X-ray diffraction study of the t-to-m phase transformation in 12-mol%-ceria-doped zirconia at low temperatures

    SciTech Connect

    Zhu, H.Y. . Dept. of Materials Science and Engineering)

    1994-09-01

    The t-to-m phase transformation in a 12-mol%-ceria-doped zirconia to subzero temperatures was studied in situ by XRD with a cryogenic sample stage. It was found that the t-to-m transformation continuously proceeded to 30 vol% as the temperature decreased to 125 K, when a burstlike transformation suddenly occurred with a transformation to 75 vol%. A preferred orientation of the ([bar 1]11)[sub m] plane parallel to the surface during the preburst transformation and XRD line broadening of both m and t phases after the burst were observed. The preburst transformation, which has not been revealed by previous dilatometry studies, is explained as surface martensitic transformation. This surface transformation occurs at higher temperature than the burstlike transformation due to less matrix constraint and the higher probability of nucleation.

  17. Research on change of phase transformation temperatures and electrical resistance triggered by heat treatment of alloy from Cu-Mn system

    NASA Astrophysics Data System (ADS)

    Karakaya, N.; Aldirmaz, E.

    2016-05-01

    This paper is aimed at studying influence of various heat treatments on transformation temperatures and electrical resistance properties of alloys from binary Cu-Mn system. It was noticed that with an increase in sample's grain size, transformation temperatures also increased. The activation energies of samples were calculated according to Kissinger and Augis-Bennett. Thermogravimetric and differential thermal analysis measurements were used to investigate phase transformations and kinetic parameters. The electrical values of resistance of alloy were investigated at different temperatures. The resistance as a function of quenching temperature showed a decrease. Depending on quenching techniques, Cu-Mn alloy can display different product phases such as parent phase and precipitation.

  18. Phase transformation and long-term service of high-temperature martensitic chromium steels

    NASA Astrophysics Data System (ADS)

    Kalashnikov, I. S.; Tarasenko, L.; Acselrad, O.; Pereira, L. C.; Shalkevich, A.; Soboleva, G.

    2000-02-01

    Martensitic high Cr (10 - 16%) steels alloyed with Ni (Co), Mo, W, V, and N are widely used in constructions subjected to cyclic loads at temperatures up to 600 degrees Celsius, in general after quenching from 1100 - 1150 degrees Celsius followed by tempering at 650 - 690 degrees Celsius. Due to long term service exposure at high temperatures, different microstructural changes take place, such as second-phases precipitation, formation of low-angle grain boundaries, as well as internal damage caused by cyclic loads and creep. Specific phase diagrams are presented that can be used to define time periods for reliable operation of parts with given composition, based on the time required for the appearance of second phase particles known to be detrimental to mechanical strength and performance. Restoring thermal treatments to be applied after long time exposure at service conditions, aiming at increasing service life, are also presented and discussed. The combined use of the diagrams and the restoring treatment ensures prediction of a reliable service-life period for components made of these steels.

  19. Phase transformation and long-term service of high-temperature martensitic chromium steels

    NASA Astrophysics Data System (ADS)

    Kalashnikov, I. S.; Tarasenko, L.; Acselrad, O.; Pereira, L. C.; Shalkevich, A.; Soboleva, G.

    2001-02-01

    Martensitic high Cr (10 - 16%) steels alloyed with Ni (Co), Mo, W, V, and N are widely used in constructions subjected to cyclic loads at temperatures up to 600 degrees Celsius, in general after quenching from 1100 - 1150 degrees Celsius followed by tempering at 650 - 690 degrees Celsius. Due to long term service exposure at high temperatures, different microstructural changes take place, such as second-phases precipitation, formation of low-angle grain boundaries, as well as internal damage caused by cyclic loads and creep. Specific phase diagrams are presented that can be used to define time periods for reliable operation of parts with given composition, based on the time required for the appearance of second phase particles known to be detrimental to mechanical strength and performance. Restoring thermal treatments to be applied after long time exposure at service conditions, aiming at increasing service life, are also presented and discussed. The combined use of the diagrams and the restoring treatment ensures prediction of a reliable service-life period for components made of these steels.

  20. The effects of salinity and temperature on phase transformation of copper-laden sludge.

    PubMed

    Hsieh, Ching-Hong; Shih, Kaimin; Hu, Ching-Yao; Lo, Shang-Lien; Li, Nien-Hsun; Cheng, Yi-Ting

    2013-01-15

    To stabilize the copper and aluminum ions in simulated sludge, a series of sintering processes were conducted to transform Cu/Al precipitation into spinel structure, CuAl(2)O(4). The results indicated that the large amount of salt content in the simulated sludge would hinder the formation of crystalline CuAl(2)O(4) generated from the incorporation of CuO and Al(2)O(3), even after the sintering process at 1200 °C. Opposite to the amorphous CuAl(2)O(4), the crystalline CuAl(2)O(4) can be formed in the sintering process at 700-1100 °C for 3 h with the desalinating procedure. According to the theory of free energy, the experimental data and references, the best formation temperature of CuAl(2)O(4) was determined at 900-1000 °C. As the temperature rose to 1200 °C, CuAlO(2) was formed with the dissociation of CuAl(2)O(4). The XPS analysis also showed that the binding energy of copper species in the simulated sludge was switched from 933.8 eV for Cu(II) to 932.8 eV for Cu(I) with the variation of temperature. In this system, the leaching concentration of copper and aluminum ions from sintered simulated sludge was decreased with ascending temperature and reached the lowest level at 1000 °C. Furthermore, the descending tendency coincided with the formation tendency of spinel structure and the diminishing of copper oxide.

  1. Temperature induced phase transformations on the Li2MoO4 system studied by Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Saraiva, G. D.; Paraguassu, W.; Freire, P. T. C.; Ramiro de Castro, A. J.; de Sousa, F. F.; Mendes Filho, J.

    2017-07-01

    The present research reports results of lattice dynamics calculations and temperature-dependent Raman scattering study of the dilithium molybdate system, Li2MoO4, in the 25-600 °C temperature range. The effects of the temperature duly produced gradual changes, associated with the disorder and anharmonic effects followed by thermodynamic instability, leading the structure to a phase transformation. Calorimetric measurements up to 1000 °C corroborated that the crystal structure experienced gradual modifications, characterized by changes in the DSC base line. These stated thermal events as well as the changes in the profile of the Raman spectra, suggested that a phase transformation is connected with tilting and/or rotations of the MoO4 tetrahedron leading to a disorder in the MoO4 sites. The observation of one exothermic peak on the DSC curve at about 702 °C is related with the melting in the sample. The experimental results were discussed based on the mode assignments performed by using lattice dynamics calculations from where we predicted both wavenumbers and atomic displacements.

  2. Equations of State and Phase Transformation of Depleted Uranium DU-238 by High Pressure-Temperature Diffraction Studies

    SciTech Connect

    Zhao,Y.; Zhang, J.; Brown, D.; Korzekwa, D.; Hixson, R.

    2007-01-01

    We have conducted in situ high-pressure diffraction experiments on depleted uranium (DU-238) at pressures up to 8.5 GPa and temperatures up to 1123 K. From the pressure (P)-volume (V)-temperature (T) measurements, thermoelastic parameters were derived for a-uranium based on a modified high-T Birch-Murnaghan equation of state and a thermal-pressure approach. With the pressure derivative of the bulk modulus K0' fixed at 4.0, we obtained ambient bulk modulus K0=117(2) GPa, temperature derivative of bulk modulus at constant pressure ({partial_derivative}K/{partial_derivative}T)P=-3.4(4)x10-2 GPa/K and at constant volume ({partial_derivative}K/{partial_derivative}T)v=-1.1(6)x10-2 GPa/K, volumetric thermal expansivity aT=a+bT, with a=1.2({+-}0.4)x10-5 K-1 and b=8.0({+-}0.7)x10-8 K-2, and the pressure derivative of thermal expansion ({partial_derivative}a/{partial_derivative}P)T=-2.5(5)x10-6 GPa-1 K-1. Within the experimental errors, the ambient bulk modulus and volumetric thermal expansion derived from this work are in good agreement with previous experimental results, whereas all other thermoelastic parameters represent the first determinations for the a phase of uranium. We also studied the a- phase transformation and obtained a phase boundary described by T (in K)=1032+7.4P (in GPa). Although the -phase uranium cannot be pressure quenched to ambient conditions, it was observed to be stable upon cooling from 1123 to 300 K at pressures of 7-8 GPa. These observations indicate that pressure, in addition to the commonly utilized alloying techniques, provides an alternative route for stabilizing the {gamma}-uranium at room temperature.

  3. 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

  4. Solid solution strengthening and phase transformation in high-temperature annealed Si80Ge20 alloy

    NASA Astrophysics Data System (ADS)

    Chiang, Tun-Yuan; Wen, Hua-Chiang; Chou, Wu-Ching; Tsai, Chien-Huang

    2014-03-01

    This investigation demonstrates the temperature-dependent mechanical properties of Si80Ge20 alloy films via a nanoindenter in the indentation depth of 100 nm. The roughly equal root mean square roughness (Rrms) values and repeatable load-displacement (P-δ) curves for the samples ensure the mechanical performances mainly contributed from the influences of annealing temperatures. The hardness (H) values of samples increase with the temperatures of an initial annealing in the range from RT to 900 °C, and, conversely, decrease for annealing temperatures over 900 °C. Accordingly, both E/H and hf/hmax values, exhibiting an inverse tendency in the above temperature range, hints that the solid solution strengthening effect and the softening phenomenon occur for the initial-annealing and over-annealing stages, respectively. In addition, grazing incidence X-ray diffraction (GIXRD) analysis demonstrates the lattice expansion and the broadened peak that attribute to the solid solution strengthening of samples and the segregation of Ge, respectively. Through observing the value of the (200) lattice spacing of 5.624 Å for a 900 °C-annealed sample by transmission electron microscopy (TEM) analysis, it is verified that the segregation of Ge is responsible for the decreased hardness for the 1000 °C-annealed sample.

  5. 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

  6. Phase transformation of lithium tungsten bronzes, Li {sub x}WO{sub 3}, at room temperature ambient conditions

    SciTech Connect

    Dey, K.R.; Ruescher, C.H. . E-mail: C.ruescher@mineralogie.uni-hannover.de; Gesing, Th.M.; Hussain, A.

    2007-04-12

    Samples of Li {sub x}WO{sub 3} with x = 0.05-0.7 were synthesized at 700 deg. C for 7 days using appropriate amounts of Li{sub 2}WO{sub 4}, WO{sub 3} and WO{sub 2} in evacuated sealed silica tubes. The products reveal different phases of perovskite tungsten bronze (PTB). An interesting phenomenon observed for the PTB phases is the gradual change in colours when they are exposed at room temperature ambient conditions (in air). This effect has been investigated using X-ray powder diffraction, infrared absorption and optical reflectivity methods for the powdered samples before and after 30 and 90 days in air. The spectra of the samples with x = 0.25-0.5 are dominated by a peak with maximum around 16,000 cm{sup -1} in the Kubelka Munk spectra which is related to the cubic Li {sub x}WO{sub 3} phase. The peak intensity increases with increasing x. After 30 days of exposure in air this peak disappeared for x < 0.5 samples due to a diffusion of Li from Li {sub x}WO{sub 3}. X-ray and IR data show a gradual transformation into the lower symmetric phases (PTB{sub cubic} {sup {yields}} PTB{sub tetragonal} {sup {yields}} PTB{sub orthorhombic} {sup {yields}} PTB{sub monoclinic}). The results suggest that Li is attracted by O{sub 2} to the surface forming Li{sub 2}O which further reacts with H{sub 2}O and CO{sub 2} in air. The in air altered samples regain their original colour when reheated at 500 deg. C in vacuum.

  7. Low-temperature solid state synthesis and in situ phase transformation to prepare nearly pure cBN.

    PubMed

    Lian, Gang; Zhang, Xiao; Tan, Miao; Zhang, Shunjie; Cui, Deliang; Wang, Qilong

    2011-07-14

    Cubic boron nitride (cBN) is synthesized by a low-temperature solid state synthesis and in situ phase transformation route with NH(4)BF(4), B, NaBH(4) and KBH(4) as the boron sources and NaN(3) as the nitrogen source. Furthermore, two new strategies are developed, i.e., applying pressure on the reactants during the reaction process and introducing the structural induction effect. These results reveal that the relative contents of cBN are greatly increased by applying these new strategies. Finally, almost pure cBN (∼90%) crystals are obtained by reacting NH(4)BF(4) and NaN(3) at 250 °C and 450 MPa for 24 h, with NaF as the structural induction material. The heterogeneous nucleation mechanism can commendably illuminate the structure induction effect of NaF with face center cubic structure. In addition, the induction effect results in the cBN nanocrystals presenting obvious oriented growth of {111} planes.

  8. Stability of crystalline solids—II: Application to temperature-induced martensitic phase transformations in a bi-atomic crystal

    NASA Astrophysics Data System (ADS)

    Elliott, Ryan S.; Shaw, John A.; Triantafyllidis, Nicolas

    2006-01-01

    This paper applies the stability theory of crystalline solids presented in the companion paper (Part I) to the study of martensitic transformations found in shape memory alloys (SMA's). The focus here is on temperature-induced martensitic transformations of bi-atomic crystals under stress-free loading conditions. A set of temperature-dependent atomic potentials and a multilattice description are employed to derive the energy density of a prototypical SMA ( B2 cubic austenite crystal). The bifurcation and stability behavior are then investigated with respect to two stability criteria (Cauchy-Born (CB) and phonon). Using a 4-lattice description five different equilibrium crystal structures are predicted: B2 cubic, L10 tetragonal, B19 orthorhombic, Cmmm orthorhombic, and B19' monoclinic. For our chosen model only the B2 and B19 equilibrium paths have stable segments which satisfy both the CB- and phonon-stability criteria. These stable segments overlap in temperature indicating the possibility of a hysteretic temperature-induced proper martensitic transformation. The B2 and B19 crystal structures are common in SMA's and therefore the simulated jump in the deformation gradient at a temperature for which both crystals are stable is compared to experimental values for NiTi, AuCd, and CuAlNi. Good agreement is found for the two SMA's which have cubic to orthorhombic transformations (AuCd and CuAlNi).

  9. 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

  10. 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)

  11. Study of the {beta}-{alpha} phase transformations of a Ti-64 sheet induced from a high-temperature {beta} state and a high-temperature {alpha} + {beta} state

    SciTech Connect

    Moustahfid, H.; Gey, N.; Humbert, M.; Philippe, M.J.

    1997-01-01

    The room-temperature {alpha} textures of Ti-64 sheets, inherited from the {beta}-{alpha} phase transformation of high-temperature {beta} textures of the material in the {beta} and {alpha} + {beta} fields, respectively, have been studied. The corresponding high-temperature {beta} textures were also determined by a method developed in their laboratory. The knowledge of the high-temperature {beta} textures allows discussions of the variant selections through transformation modeling. As a result, a variant selection occurs in the presence of the stable {alpha} grains of the {alpha} + {beta} field.

  12. 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.

  13. Phase transformations upon doping in WO3

    NASA Astrophysics Data System (ADS)

    Wang, Wennie; Janotti, Anderson; Van de Walle, Chris G.

    2017-06-01

    High levels of doping in WO3 have been experimentally observed to lead to structural transformation towards higher symmetry phases. We explore the structural phase diagram with charge doping through first-principles methods based on hybrid density functional theory, as a function of doping the room-temperature monoclinic phase transitions to the orthorhombic, tetragonal, and finally cubic phase. Based on a decomposition of energies into electronic and strain contributions, we attribute the transformation to a gain in energy resulting from a lowering of the conduction band on an absolute energy scale.

  14. A Study on Structure/Phase Transformation of TiO2 nanorods at Various Annealing Temperatures

    NASA Astrophysics Data System (ADS)

    Wahyuningsih, S.; Ramelan, A. H.; Saputri, L. N. M. Z.

    2017-07-01

    Synthesis of TiO2 nanorods was conducted through mechanochemical of ball milling at speed of 175 rpm for 5 hours and strong base 12 M NaOH reaction by hydrothermal at 150°C overnight on variation annealing at 400, 500, and 600°C. Material characterization were performed by X-Ray Diffraction (XRD), Transmission Electron Microscoe (SEM), and Surface Area Analyzer (SAA). Strong base reaction by hydrothermal showed the presence of anatase, brookite, and rutile phase for annealing up to 400°C. The diffraction pattern of annealing at 500 and 600°C contain peaks of both brookite and rutile phase. Morphology transformation of TiO2 to form nanorods TiO2 was showed by rod-shape from TEM micrographs and increase surface area into 111.9 m2/g.

  15. Detection of normal transitions in a hybrid single-phase Bi2223 high temperature superconducting transformer by using the active power method and a magnetic flux detection coil

    NASA Astrophysics Data System (ADS)

    Nanato, N.; Nakamura, S.; Tanaka, S.

    2017-07-01

    The authors have been developing a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer used in the AC current source with rated current of over 500A. Its primary coil is a copper coil and secondary coil is a Bi2223 HTS coil. In this paper, the authors propose a new detection method of normal transitions in the secondary coil by using the active power method and a magnetic flux detection coil attached on the inside of the secondary coil. In the proposed method, the normal transitions are detected by measuring active power dissipated in the secondary coil, and induced voltage of the magnetic flux detection coil by a primary and leakage flux of the secondary coil enables to calculate the active power dissipated in only the secondary coil. As experimental results for a hybrid single-phase Bi2223 HTS transformer, it was found that the proposed method enabled to detect the normal transitions in its secondary superconducting coil.

  16. Principles of Phase Transformations

    DTIC Science & Technology

    1945-05-12

    weight per cent. Thereafter the lattice changes to a cubic structure with the same car- bon content (0.2fo) as the prior slightly tetragonal...exists below which the stable structure is tetragonal, above which the cubic structure is stable. When the car- bon content is 0,8 weight per cent, this...tetragonallty Is gradually lost upon aging at room temperature, is rapidly lost upon heating to^lOO0^ the martensite acquiring a cubic structure . Two

  17. Reduction of the C54-TiSi2 phase transformation temperature using refractory metal ion implantation

    NASA Astrophysics Data System (ADS)

    Mann, R. W.; Miles, G. L.; Knotts, T. A.; Rakowski, D. W.; Clevenger, L. A.; Harper, J. M. E.; D'Heurle, F. M.; Cabral, C., Jr.

    1995-12-01

    We report that the ion implantation of a small dose of Mo into a silicon substrate before the deposition of a thin film of Ti lowers the temperature required to form the commercially important low resistivity C54-TiSi2 phase by 100-150 °C. A lesser improvement is obtained with W implantation. In addition, a sharp reduction in the dependence of C54 formation on the geometrical size of the silicided structure is observed. The enhancement in C54 formation observed with the ion implantation of Mo is not explained by ion mixing of the Ti/Si interface or implant-induced damage. Rather, it is attributed to an enhanced nucleation of C54-TiSi2 out of the precursor high resistance C49-TiSi2 phase.

  18. Phase transformations in engineering materials

    SciTech Connect

    Bourke, M.A.M.; Lawson, A.C.; Dunand, D.C.; Priesmeyer, H.G.

    1996-06-01

    Phase transformations in engineering materials are inevitably related to mechanical behavior and are often precursors to residual stress and distortion. Neutron scattering in general is a valuable tool for studying their effects, and pulsed neutrons are of special value, because of the inherently comprehensive crystallographic coverage they provide in each measurement. At the Manuel Lujan neutron scattering center several different research programs have addressed the relationships between phase transformation/mechanical behavior and residual strains. Three disparate examples are presented; (1) stress induced transformation in a NiTi shape memory alloy, (2) cryogenically induced transformation in a quenched 5180 steel, and (3) time resolved evolution of strain induced martensite in 304 stainless steel. In each case a brief description of the principle result will be discussed in the context of using neutrons for the measurement.

  19. Phase Transformations in Confined Nanosystems

    SciTech Connect

    Shield, Jeffrey E.; Belashchenko, Kirill

    2014-04-29

    This project discovered that non-equilibrium structures, including chemically ordered structures not observed in bulk systems, form in isolated nanoscale systems. Further, a generalized model was developed that effectively explained the suppression of equilibrium phase transformations. This thermodynamic model considered the free energy decrease associated with the phase transformation was less than the increase in energy associated with the formation of an interphase interface, therefore inhibiting the phase transformation. A critical diameter exists where the system transitions to bulk behavior, and a generalized equation was formulated that successfully predicted this transition in the Fe-Au system. This provided and explains a new route to novel structures not possible in bulk systems. The structural characterization was accomplished using transmission electron microscopy in collaboration with Matthew Kramer of Ames Laboratory. The PI and graduate student visited Ames Laboratory several times a year to conduct the experiments.

  20. Thermochromic VO2 thin films: solution-based processing, improved optical properties, and lowered phase transformation temperature.

    PubMed

    Zhang, Zongtao; Gao, Yanfeng; Chen, Zhang; Du, Jing; Cao, Chuanxiang; Kang, Litao; Luo, Hongjie

    2010-07-06

    This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO(2) thin film, the integral values of visible transmittance (T(int)) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (DeltaT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T(int) values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the DeltaT at 2000 nm remained high. These results represent the best data for VO(2) to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO(2) will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 microm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart

  1. Phase Transformation in Cast Superaustenitic Stainless Steels

    SciTech Connect

    Lee Phillips, Nathaniel Steven

    2006-01-01

    Superaustenitic stainless steels constitute a group of Fe-based alloys that are compositionally balanced to have a purely austenitic matrix and exhibit favorable pitting and crevice corrosion resistant properties and mechanical strength. However, intermetallic precipitates such as sigma and Laves can form during casting or exposure to high-temperature processing, which degrade the corrosion and mechanical properties of the material. The goal of this study was to accurately characterize the solid-solid phase transformations seen in cast superaustenitic stainless steels. Heat treatments were performed to understand the time and temperature ranges for intermetallic phase formations in alloys CN3MN and CK3MCuN. Microstructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy and wavelength dispersive spectroscopy (EDS, WDS). The equilibrium microstructures, composed primarily of sigma and Laves within purely austenitic matrices, showed slow transformation kinetics. Factors that determine the extent of transformation, including diffusion, nucleation, and growth, are discussed.

  2. In situ variable temperature X-ray diffraction studies on the transformations of nano-precursors to La-Ni-O phases

    SciTech Connect

    Weng Xiaole; Knowles, Jonathan C.; Abrahams, Isaac; Wu Zhongbiao; Darr, Jawwad A.

    2011-07-15

    In situ variable temperature XRD (VT-XRD) measurements on the transformation of nano-precursors to La-Ni-O phases are presented. Experimental results showed that LaNiO{sub 3} and La{sub 2}NiO{sub 4} phases were formed at ca. 700 deg. C via the reaction of La{sub 2}O{sub 3} and NiO (from the initial nano-precursors), where a relatively low temperature of 700 deg. C was found for the synthesis of La{sub 2}NiO{sub 4}. The formation of La{sub 3}Ni{sub 2}O{sub 7} at higher temperature (up to 1150 deg.C) appeared to proceed through a further reaction of La{sub 2}NiO{sub 4} with unreacted NiO, whilst the formation of La{sub 4}Ni{sub 3}O{sub 10} (at 1075 deg. C) proceeded via a further decomposition of LaNiO{sub 3}. Although phase pure La{sub 3}Ni{sub 2}O{sub 7} and La{sub 4}Ni{sub 3}O{sub 10} were not directly obtained under the processing conditions herein, the results of this study allow for a better understanding of formation pathways, particularly for the higher order La-Ni-O phases. - Graphical abstract: In situ variable temperature XRD showing the phase formation pathway of La{sub n+1}Ni{sub n}O{sub 3n+1} at evaluated temperatures. Highlights: > In situ VT-XRD was utilized to study the pathways for La{sub n+1}Ni{sub n}O{sub 3n+1} formations. > LaNiO{sub 3} and La{sub 2}NiO{sub 4} phases were formed via the reaction of La{sub 2}O{sub 3} and NiO, respectively. > La{sub 3}Ni{sub 2}O{sub 7} phase was formed via further reaction of La{sub 2}NiO{sub 4} phase with unreacted NiO. > La{sub 4}Ni{sub 3}O{sub 10} phase was formed via further decomposition of LaNiO{sub 3} phase.

  3. The Kinetics of Phase Transformation in Welds

    SciTech Connect

    Elmer, J W; Wong, J; Palmer, T

    2002-02-06

    The fundamentals of welding-induced phase transformations in metals and alloys are being investigated using a combination of advanced synchrotron based experimental methods and modem computational science tools. In-situ experimental methods have been developed using a spatially resolved x-ray probe to enable direct observations of phase transformations under the real non- isothermal conditions experienced during welding. These experimental techniques represent a major step forward in the understanding of phase transformations that occur during welding, and are now being used to aid in the development of models to predict microstructural evolution under the severe temperature gradients, high peak temperatures and rapid thermal fluctuations characteristic of welds. Titanium alloys, stainless steels and plain carbon steels are currently under investigation, and the phase transformation data being obtained here cannot be predicted or measured using conventional metallurgical approaches. Two principal synchrotron-based techniques have been developed and refined for in-situ investigations of phase transformation dynamics in the heat-affected zone (HAZ) and fusion zone (FZ) of welds: Spatially Resolved X-Ray Diffraction (SRXRD) and Time Resolved X-Ray Diffraction (TRXRD). Both techniques provide real-time observations of phases that exist during welding, and both have been developed at the Stanford Synchrotron Radiation Laboratory (SSRL) using a high flux wiggler beam line. The SRXRD technique enables direct observations of the phases existing in the HAZ of quasi-stationary moving arc welds, and is used to map the HAZ phases by sequentially jogging the weld with respect to the x-ray beam while taking x-ray diffraction (XRD) patterns at each new location. These spatially resolved XRD patterns are collected in linear traverses perpendicular to the direction of weld travel. The XRD data contained in multiple traverses is later compiled to produce an areal map of the phases

  4. Effect of thermocycling on the temperatures of phase transformations, structure, and properties of the equiatomic alloy Ti50.0Ni50.0

    NASA Astrophysics Data System (ADS)

    Churakova, A. A.; Gunderov, D. V.

    2016-01-01

    This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2- B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti50Ni50 in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation ( M s, M f) decrease by 2-3 K, and the temperatures of the reverse transformation ( A s, A f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.

  5. Bio-inspired synthesis of titania with polyamine induced morphology and phase transformation at room-temperature: insight into the role of the protonated amino group.

    PubMed

    Yan, Yong; Hao, Bo; Wang, Xiaobo; Chen, Ge

    2013-09-14

    Poly(allylamine hydrochloride) (PAAH), a mimic of biopolyamines, was used to induce the mineralization of titania at room-temperature, hollow spheres with a mixed phase (anatase and rutile) were obtained, and the fine anatase and rutile nanocrystals were observed mixing at the sub-10 nm scale on the hollow spheres. The structural information about the precipitated titania gained by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy revealed a distinct dependence of the polymorph and morphology of the titania precipitates on the molecular structure of the polyamines and titanium precursors. Moreover, we have observed the phase transformation from anatase to rutile in the formation process of hollow spheres, and it was suggested that the protonated amino groups on PAAH played key roles in the transformation of both polymorphs and morphologies of titania. Additionally, poly(diallyldimethylammonium chloride) (PDDA), a long chain quaternary ammonium polymer, was used as another catalytic template for the synthesis of titania, which also led to a hollow structure with a mixture of anatase and TiO2-B.

  6. 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

  7. Sustained arc temperature: better marker for phase transformation of carbon black to multiwalled carbon nanotubes in arc discharge method

    NASA Astrophysics Data System (ADS)

    Arora, Neha; Sharma, N. N.

    2016-10-01

    The present study investigates the role of temperature in the formation of multiwalled carbon nanotubes from carbon black using arc discharge technique. Carbon black has been used as precursor to synthesize carbon nanotubes in argon atmosphere. The arc current has been varied from 25 to 40 A in order to study the morphological changes in carbon black as it gets converted to multi walled carbon nanotubes (MWCNTs). We observed formation of MWCNTs at an arc current of 25 A; however the recorded temperature data suggested correlation of sustained arc temperature with the nanotube formation rather than the magnitude of current in its absoluteness. Interesting to note is that reported current magnitude in published literature are very high (>40 A) for conversion of carbon black but the present investigation shows that it is possible to convert the carbon black to MWCNTs even at lower current values in case the arc temperature is stabilized and sustained for longer period. Detailed investigations suggested that a sustained stable critical temperature of 1400 °C-1600 °C is essential for the growth of nanotubes and an unstable arc causing temperature fluctuation from critical temperature value yields very low or no MWCNTs.

  8. Coal transformation under high-temperature catagenesis

    SciTech Connect

    Melenevsky, V.N.; Sokol, E.V.; Fomin, A.N.

    2006-07-01

    In this paper we consider products of natural pyrolysis of lignite, which resulted from the high-temperature spontaneous combustion of spoil heaps of the Chelyabinsk coal basin. These products were studied by pyrolysis, element and petrographic analyses, chromatomass spectrometry, and X-ray diffraction method. We have established that under reducing conditions, the degree of pyrogenic coal transformation and the composition of pyrolysis products vary greatly, from graphite-like phases to bitumens, and depend on the temperature and degree of the system openness.

  9. Phase Transformations in Cast Duplex Stainless Steels

    SciTech Connect

    Kim, Yoon-Jun

    2004-01-01

    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 σ and χ 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 (σ + χ) formation were analyzed using the Johnson-Mehl-Avrami (MA) 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, σ was stabilized with increasing Cr addition and χ 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

  10. Crystal Level Continuum Modeling of Phase Transformations: The (alpha) <--> (epsilon) Transformation in Iron

    SciTech Connect

    Barton, N R; Benson, D J; Becker, R; Bykov, Y; Caplan, M

    2004-10-18

    We present a crystal level model for thermo-mechanical deformation with phase transformation capabilities. The model is formulated to allow for large pressures (on the order of the elastic moduli) and makes use of a multiplicative decomposition of the deformation gradient. Elastic and thermal lattice distortions are combined into a single lattice stretch to allow the model to be used in conjunction with general equation of state relationships. Phase transformations change the mass fractions of the material constituents. The driving force for phase transformations includes terms arising from mechanical work, from the temperature dependent chemical free energy change on transformation, and from interaction energy among the constituents. Deformation results from both these phase transformations and elasto-viscoplastic deformation of the constituents themselves. Simulation results are given for the {alpha} to {epsilon} phase transformation in iron. Results include simulations of shock induced transformation in single crystals and of compression of polycrystals. Results are compared to available experimental data.

  11. Identification, structural characterization and transformations of the high-temperature Zn9-δSb7 phase in the Zn-Sb system.

    PubMed

    He, Allan; Svitlyk, Volodymyr; Chernyshov, Dmitry; Mozharivskyj, Yurij

    2015-12-28

    The Zn9-δSb7 phase has been identified via high-temperature powder diffraction studies. Zn9-δSb7 adopts two modifications: an α form stable between 514 °C and 539 °C and a Zn-poorer β form stable from 539 °C till its melting temperature of 581 °C. The Zn9-δSb7 structure was solved from the powder data using the simulated annealing approach. Both modifications adopt the same hexagonal structure (P6/mmm) but with slightly different lattice parameters. The α-to-β transformation is abrupt and first-order in nature. The Zn atoms occupy the tetrahedral holes created by Sb atoms. The ideal Zn9Sb7 composition can be explained by its tendency to adopt a charge balance configuration. Out of 7 Sb atoms, 3 Sb atoms form dimers (Sb(2-) ions) and 4 Sb atoms are isolated (Sb(3-) ions), which require 9 Zn(2+) cations for charge neutrality.

  12. Phase Transformations and Microstructural Evolution: Part II

    SciTech Connect

    Clarke, Amy Jean

    2015-10-30

    The activities of the Phase Transformations Committee of the Materials Processing & Manufacturing Division (MPMD) of The Minerals, Metals & Materials Society (TMS) are oriented toward understanding the fundamental aspects of phase transformations. Emphasis is placed on the thermodynamic driving forces for phase transformations, the kinetics of nucleation and growth, interfacial structures and energies, transformation crystallography, surface reliefs, and, above all, the atomic mechanisms of phase transformations. Phase transformations and microstructural evolution are directly linked to materials processing, properties, and performance. In this issue, aspects of liquid–solid and solid-state phase transformations and microstructural evolution are highlighted. Many papers in this issue are highlighted by this paper, giving a brief summary of what they bring to the scientific community.

  13. Phase Transformations and Microstructural Evolution: Part I

    SciTech Connect

    Clarke, Amy Jean

    2015-08-29

    The activities of the Phase Transformations Committee of the Materials Processing & Manufacturing Division (MPMD) of The Minerals, Metals & Materials Society (TMS) are oriented toward understanding the fundamental aspects of phase transformations. Emphasis is placed on the thermodynamic driving forces for phase transformations, the kinetics of nucleation and growth, interfacial structures and energies, transformation crystallography, surface reliefs, and, above all, the atomic mechanisms of phase transformations. Phase transformations and microstructural evolution are directly linked to materials processing, properties, and performance, including in extreme environments, of structural metal alloys. In this paper, aspects of phase transformations and microstructural evolution are highlighted from the atomic to the microscopic scale for ferrous and non-ferrous alloys. Many papers from this issue are highlighted with small summaries of their scientific achievements given.

  14. Phase Transformations and Microstructural Evolution: Part II

    DOE PAGES

    Clarke, Amy Jean

    2015-10-30

    The activities of the Phase Transformations Committee of the Materials Processing & Manufacturing Division (MPMD) of The Minerals, Metals & Materials Society (TMS) are oriented toward understanding the fundamental aspects of phase transformations. Emphasis is placed on the thermodynamic driving forces for phase transformations, the kinetics of nucleation and growth, interfacial structures and energies, transformation crystallography, surface reliefs, and, above all, the atomic mechanisms of phase transformations. Phase transformations and microstructural evolution are directly linked to materials processing, properties, and performance. In this issue, aspects of liquid–solid and solid-state phase transformations and microstructural evolution are highlighted. Many papers in thismore » issue are highlighted by this paper, giving a brief summary of what they bring to the scientific community.« less

  15. Phase Transformations and Microstructural Evolution: Part I

    DOE PAGES

    Clarke, Amy Jean

    2015-08-29

    The activities of the Phase Transformations Committee of the Materials Processing & Manufacturing Division (MPMD) of The Minerals, Metals & Materials Society (TMS) are oriented toward understanding the fundamental aspects of phase transformations. Emphasis is placed on the thermodynamic driving forces for phase transformations, the kinetics of nucleation and growth, interfacial structures and energies, transformation crystallography, surface reliefs, and, above all, the atomic mechanisms of phase transformations. Phase transformations and microstructural evolution are directly linked to materials processing, properties, and performance, including in extreme environments, of structural metal alloys. In this paper, aspects of phase transformations and microstructural evolution aremore » highlighted from the atomic to the microscopic scale for ferrous and non-ferrous alloys. Many papers from this issue are highlighted with small summaries of their scientific achievements given.« less

  16. Field dependence of temperature induced irreversible transformations of magnetic phases in Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) crystalline oxide.

    PubMed

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

    2010-01-27

    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 Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) 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.

  17. 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.

  18. Phase transformations coupled to deformation processes

    NASA Astrophysics Data System (ADS)

    Lookman, Turab

    2013-06-01

    Phase transformation processes have a substantial impact on the inelastic and damage response of materials. Yet, our understanding of how different loading conditions affect volume fractions of transformed phases, microstructure and transformation pathways is very much in its infancy. With an emphasis on distilling single crystal physics that can, in principle, be incorporated into higher length scale models, I will discuss how recent atomistic simulations on Ti are beginning to provide insights into transformation pathways and the interplay of phase transformations and deformation processes. These simulations are complemented by shock experiments on Zr, Ti together with characterization studies at the Advanced Photon Source.

  19. High-temperature superconducting transformer evaluation

    SciTech Connect

    DeSteese, J.G.; Dagle, J.E.; Dirks, J.A.

    1995-04-01

    The advancing development of high-temperature superconducting (HTS) materials is encouraging the evaluation of many practical applications. This paper summarizes a study that examined the future potential of HTS power transformers in the 30-MVA to 1000-MVA capacity range. Transformer performance was characterized on the basis of potentially achievable HTS materials capabilities and dominant transformer design parameters. Life-cycle costs were estimated and compared with those of conventional transformers to evaluate the economic viability and market potential of HTS designs. HTS transformers are projected to have both capital and energy cost advantages attributable to their ability to be intrinsically smaller and lighter than conventional transformers of comparable capacity.

  20. Thermodynamics of entropy-driven phase transformations.

    PubMed

    Radosz, A; Ostasiewicz, K; Magnuszewski, P; Damczyk, J; Radosiński, Ł; Kusmartsev, F V; Samson, J H; Mituś, A C; Pawlik, G

    2006-02-01

    Thermodynamic properties of one-dimensional lattice models exhibiting entropy-driven phase transformations are discussed in quantum and classical regimes. Motivated by the multistability of compounds exhibiting photoinduced phase transitions, we consider systems with asymmetric, double, and triple well on-site potential. One finds that among a variety of regimes, quantum versus classical, discrete versus continuum, a key feature is asymmetry distinguished as a "shift" type and "shape" type in limiting cases. The behavior of the specific heat indicates one phase transformation in a "shift" type and a sequence of two phase transformations in "shape"-type systems. Future analysis in higher dimensions should allow us to identify which of these entropy-driven phase transformations would evolve into phase transitions of the first order.

  1. Phase transformations in xerogels of mullite composition

    NASA Technical Reports Server (NTRS)

    Hyatt, Mark J.; Bansal, Narottam P.

    1988-01-01

    Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al2O3-2SiO2). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminum nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment, in these two types of sol-gel derived mullite precursor powders have been characterized by DTA, TGA, X-ray diffraction, SEM and infrared spectroscopy. Monophasic xerogel transforms to an Al-Si spinel from an amorphous structure at approximately 980 C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at approximately 1360 C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (alpha-Al2O3, cristobalite) even after heating at high temperatures for long times (1600 C, 6 hr). The diphasic powder could be sintered to high density but not the monophasic powder in spite of its molecular level homogeneity.

  2. Phase transformations in xerogels of mullite composition

    NASA Technical Reports Server (NTRS)

    Hyatt, Mark J.; Bansal, Narottam P.

    1990-01-01

    Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al203-2Si02). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminum nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment, in these two types of sol-gel derived mullite precursor powders have been characterized by DTA, TGA, X-ray diffraction, SEM and infrared spectroscopy. Monophasic xerogel transforms to an Al-Si spinel from an amorphous structure at approximately 980 C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at approximately 1360 C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (alpha-Al203, cristobalite) even after heating at high temperatures for long times (1600 C, 6 hr). The diphasic powder could be sintered to high density but not the monophasic powder in spite of its molecular level homogeneity.

  3. Phase transformations at interfaces: Observations from atomistic modeling

    SciTech Connect

    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 such 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.

  4. 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

  5. Phase transformation analysis of varied nickel-titanium orthodontic wires.

    PubMed

    Ren, Chao-chao; Bai, Yu-xing; Wang, Hong-mei; Zheng, Yu-feng; Li, Song

    2008-10-20

    The shape memory effect of nickel-titanium (NiTi) archwires is largely determined by the phase transition temperature. It is associated with a reversible transformation from martensite to austenite. The aim of this study was to characterize austenite, martensite and R phase temperatures as well as transition temperature ranges of the commonly used clinical NiTi orthodontic arch wires selected from several manufacturers. Differential scanning calorimetry (DSC) method was used to study the phase transformation temperatures and the phase transition processes of 9 commonly used clinical NiTi alloys (types: 0.40 mm (0.016 inch), 0.40 mm x 0.56 mm (0.016 inch x 0.022 inch)). The austenite finish temperatures (Af) of 0.40 mm Smart, Ormco and 3M NiTi wires were lower than the room temperature, and no phase transformation was detected during oral temperature. Therefore, we predicted that these types of NiTi did not possess shape memory property. For 0.40 mm and 0.40 mm x 0.56 mm Youyan I NiTi wires, no phase transformation was detected during the scanning temperature range, suggesting that these two types of wires did not possess shape memory either. The Af of 0.40 mm x 0.56 mm Smart, L&H, Youyan II Ni-Ti wires were close to the oral temperature and presented as martensitic-austenitic structures at room temperature, suggesting the NiTi wires listed above have good shape memory effect. Although the 0.40 mm x 0.56 mm Damon CuNiTi wire showed martensitic-austenitic structures at oral temperature, its Af was much higher than the oral temperature. It means that transformation from martensite to austenite for this type of NiTi only finishes when oral temperature is above normal. The phase transformation temperatures and transformation behavior varied among different commonly used NiTi orthodontic arch wires, leading to variability in shape memory effect.

  6. Phase Equilibria, Phase Diagrams and Phase Transformations - 2nd Edition

    NASA Astrophysics Data System (ADS)

    Hillert, Mats

    2006-03-01

    Computational tools allow material scientists to model and analyze increasingly complicated systems to appreciate material behavior. Accurate use and interpretation however, requires a strong understanding of the thermodynamic principles that underpin phase equilibrium, transformation and state. This fully revised and updated edition covers the fundamentals of thermodynamics, with a view to modern computer applications. The theoretical basis of chemical equilibria and chemical changes is covered with an emphasis on the properties of phase diagrams. Starting with the basic principles, discussion moves to systems involving multiple phases. New chapters cover irreversible thermodynamics, extremum principles, and the thermodynamics of surfaces and interfaces. Theoretical descriptions of equilibrium conditions, the state of systems at equilibrium and the changes as equilibrium is reached, are all demonstrated graphically. With illustrative examples - many computer calculated - and worked examples, this textbook is an valuable resource for advanced undergraduates and graduate students in materials science and engineering. Fully revised and updated edition covering the fundamentals of thermodynamics with a view to modern computer applications such as Thermo-Calc Emphasis is placed on phase diagrams, the key application of thermodynamics Contains numerous illustrative examples, many computer-calculated and some for real systems, and worked examples to help demonstrate the principles

  7. Transformation of the semi-transparent into the metallic phase of zirconium nitride compounds by implantation at controlled temperature: the evolution of the optical properties

    NASA Astrophysics Data System (ADS)

    Camelio, S.; Girardeau, T.; Pichon, L.; Straboni, A.; Fayoux, C.; Guérin, Ph

    2000-09-01

    Zirconium nitrides reveal interesting optical and electrical properties, which strongly depend on the nitrogen stoichiometry. When preparing these compounds using dual-ion-beam sputtering, very contrasting optical film properties can be obtained: a stable ZrN metallic phase with gold-like colour and a Zr3N4 metastable phase with an insulating semi-transparent behaviour. In this paper, optical properties (complex index of refraction and colorimetric data) of these two zirconium nitride phases are presented. A colorimetric comparison between the colour of gold and that of ZrN is also reported. The Zr3N4 phase is thermodynamically unstable: this metastable character permits us to foresee a transition to the stable phase ZrN {on providing an} energy supply. Compared with other ways of providing energy, ion implantation is well suited because of its directional aspect. This work deals with the optical property evolution of Zr3N4 phase irradiated at 600 °C by 340 keV Ar2+ ions with different fluences. Ellipsometric measurements reveal a gradual evolution of the optical properties, starting from an insulating behaviour (Zr3N4 sample) and ending with a conductor-like behaviour. It is shown that a fluence of 5×1016 ions cm-2 is sufficient to transform the semi-transparent Zr3N4 film into metallic film. A study of the electrical properties of the most highly implanted samples and of ZrN films is also reported.

  8. Reversible metal-hydride phase transformation in epitaxial films

    NASA Astrophysics Data System (ADS)

    Roytburd, Alexander L.; Boyerinas, Brad M.; Bruck, Hugh A.

    2015-03-01

    Metal-hydride phase transformations in solids commonly proceed with hysteresis. The extrinsic component of hysteresis is the result of the dissipation of energy of internal stress due to plastic deformation and fracture. It can be mitigated on the nanoscale, where plastic deformation and fracture are suppressed and the transformation proceeds through formation and evolution of coherent phases. However, the phase coherency introduces intrinsic thermodynamic hysteresis, preventing reversible transformation. In this paper, it is shown that thermodynamic hysteresis of coherent metal-hydride transformation can be eliminated in epitaxial film due to substrate constraint. Film-substrate interaction leads to formation of heterophase polydomain nanostructure with variable phase fraction which can change reversibly by varying temperature in a closed system or chemical potential in an open system.

  9. Reversible metal-hydride phase transformation in epitaxial films.

    PubMed

    Roytburd, Alexander L; Boyerinas, Brad M; Bruck, Hugh A

    2015-03-11

    Metal-hydride phase transformations in solids commonly proceed with hysteresis. The extrinsic component of hysteresis is the result of the dissipation of energy of internal stress due to plastic deformation and fracture. It can be mitigated on the nanoscale, where plastic deformation and fracture are suppressed and the transformation proceeds through formation and evolution of coherent phases. However, the phase coherency introduces intrinsic thermodynamic hysteresis, preventing reversible transformation. In this paper, it is shown that thermodynamic hysteresis of coherent metal-hydride transformation can be eliminated in epitaxial film due to substrate constraint. Film-substrate interaction leads to formation of heterophase polydomain nanostructure with variable phase fraction which can change reversibly by varying temperature in a closed system or chemical potential in an open system.

  10. Fiber phase transformation and matrix plastic flow in a room temperature tensile strained NiTi shape memory alloy fiber reinforced 6082 aluminum matrix composite

    SciTech Connect

    Armstrong, W.D.; Lorentzen, T.

    1997-05-01

    Macroscopic tensile and in-situ neutron diffraction measurements are reported from a 20.5 volume percent, 50.7 at% Ni-Ti fiber reinforced 6082-T6 aluminum matrix composite subjected to a room temperature, 4% tensile elongation. The austenite B2(110) diffraction intensity was essentially stable until approximately 0.9% strain, beyond which, the austenite B2(110) diffraction intensity strongly decreased with increasing tensile strain. The martensite M(001) diffraction intensity strongly increased from a zero intensity intercept at approximately 2.3% strain to the conclusion of tensile straining. This report concludes that the initial decrease in austenite B2(11) diffraction intensity locates the initiation of stress induced transformations in the NiTi reinforcement, furthermore this feature corresponds with an elevated yield point region in the macroscopic tensile results. Therefore, it appears that the elevated yield point region is caused by a temporary inhibition of fiber stress induced transformations.

  11. Transformation of the corundum structure upon high-temperature reduction

    NASA Astrophysics Data System (ADS)

    Dan'ko, A. Ja.; Rom, M. A.; Sidelnikova, N. S.; Nizhankovskiy, S. V.; Budnikov, A. T.; Grin', L. A.; Kaltaev, Kh. Sh-O.

    2008-12-01

    This paper reports on the results of investigations into the transformation of the corundum structure upon reducing annealing at high temperatures in the range from 1700 to 2050°C. It is established that the reduction results in the transformation of corundum into new phases with a lower oxygen content, including the phase with a spinel structure. These structures are assumed to be stabilized by anion vacancies. A model of the crystal structure of the spinel phase is proposed. This model provides an adequate description of the compound obtained in the experiment.

  12. Comprehensive School Transformation, Phase III. Program Evaluation.

    ERIC Educational Resources Information Center

    Prine, Donald; Wilkinson, David

    The Iowa Educational Excellence Act and its amendments provide for four types of Phase III programs: performance-based pay, supplemental pay, a combination of both, or comprehensive school transformation. In 1993, the Des Moines Phase III program, which originated as a combination of performance-based pay and supplemental pay, changed to a…

  13. Atomic Structure and Phase Transformations in Pu Alloys

    SciTech Connect

    Schwartz, A J; Cynn, H; Blobaum, K M; Wall, M A; Moore, K T; Evans, W J; Farber, D L; Jeffries, J R; Massalski, T B

    2008-04-28

    Plutonium and plutonium-based alloys containing Al or Ga exhibit numerous phases with crystal structures ranging from simple monoclinic to face-centered cubic. Only recently, however, has there been increased convergence in the actinides community on the details of the equilibrium form of the phase diagrams. Practically speaking, while the phase diagrams that represent the stability of the fcc {delta}-phase field at room temperature are generally applicable, it is also recognized that Pu and its alloys are never truly in thermodynamic equilibrium because of self-irradiation effects, primarily from the alpha decay of Pu isotopes. This article covers past and current research on several properties of Pu and Pu-(Al or Ga) alloys and their connections to the crystal structure and the microstructure. We review the consequences of radioactive decay, the recent advances in understanding the electronic structure, the current research on phase transformations and their relations to phase diagrams and phase stability, the nature of the isothermal martensitic {delta} {yields} {alpha}{prime} transformation, and the pressure-induced transformations in the {delta}-phase alloys. New data are also presented on the structures and phase transformations observed in these materials following the application of pressure, including the formation of transition phases.

  14. Phase transformation and stabilization of a high strength austenite

    NASA Technical Reports Server (NTRS)

    Jin, S.; Huang, D.

    1976-01-01

    An investigation of the phase transformation and the austenite stabilization in a high strength austenite has been made. An Fe-29Ni-4.3Ti austenite age-hardened by gamma-prime (Ni3Ti) precipitates showed a further increase of strength after martensitic and reverse martensitic phase transformations. The stability of ausaged austenite as well as ausaged and transformation-strengthened austenite was improved significantly through an isothermal treatment at 500 C. The Ms temperature of the strengthened austenite was restored to nearly that of annealed austenite while the austenite was hardened to R(C) 41 through precipitation and phase transformations. The observed austenite stabilization is attributed to the formation of GP zones or short-range order of less than about 10A in size.

  15. Phase Transformation in Tantalum under Extreme Laser Deformation.

    PubMed

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

  16. Phase transformation in tantalum under extreme laser deformation

    SciTech Connect

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

  17. 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

  18. 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

  19. Phase Transformation in Tantalum under Extreme Laser Deformation

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    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).

  20. Phase transformation of zirconia ceramics by hydrothermal degradation.

    PubMed

    Kawai, Yohei; Uo, Motohiro; Wang, Yongming; Kono, Sayaka; Ohnuki, Somei; Watari, Fumio

    2011-01-01

    Zirconia has found wide application in dentistry because of its high mechanical strength and superior esthetic properties. However, zirconia degradation caused by phase transformation occurring in a hydrothermal environment is of concern. In the present study, phase transformation and microstructure of tetragonal zirconia polycrystal partially stabilized with yttrium oxide (Y-TZP) and alumina-toughened zirconia (ATZ) sintered at different temperatures were estimated. On grazing angle X-ray diffraction analysis, ATZ showed less phase transformation to the monoclinic phase during hydrothermal treatment and this transformation appeared to occur within a few micrometers below the surface. At a higher sintering temperature the monoclinic phase content of ATZ was found to be lesser than that of Y-TZP, indicating that the alumina in ATZ was effective in suppressing hydrothermal degradation. Examination by transmission electron microscopy and studying of electron backscatter diffraction patterns indicated that grain growth in ATZ was slightly suppressed compared with that in Y-TZP at higher sintering temperatures. The present study demonstrated the effect of adding alumina to zirconia for suppressing hydrothermal degradation and studied the effect of this addition on grain growth in zirconia.

  1. 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.

  2. Magnetostructural phase transformations in Tb 1-x Mn 2

    DOE PAGES

    Zou, Junding; Paudyal, Durga; Liu, Jing; ...

    2015-01-16

    Magnetism and phase transformations in non-stoichiometric Tb1-xMn2 (x = 0.056, 0.039) have been studied as functions of temperature and magnetic field using magnetization, heat capacity, and X-ray powder diffraction measurements. Lowering the temperature, the compounds sequentially order ferrimagnetically and antiferromagnetically, and finally, exhibit spin reorientation transitions. Moreover, these structural distortions from room temperature cubic to low temperature rhombohedral structures occur at TN, and are accompanied by large volume changes reaching ~-1.27% and -1.42%, respectively. First principles electronic structure calculations confirm the phase transformation from the ferrimagnetic cubic structure to the antiferromagnetic rhombohedral structure in TbMn2.

  3. A diffuse interface approach to phase transformation via virtual melting

    NASA Astrophysics Data System (ADS)

    Momeni, Kasra

    This work represents development of the first phase field models and detailed study solid-solid transformations via intermediate melting within nanometer size interface. Such phase transformations can occur in different materials, including HMX energetic crystals, PbTiO3 nanowires, complex pharmaceutical substances, electronic and geological materials, as well as colloidal, and superhard materials. A thermodynamically consistent phase field model for three phases is developed using two polar order parameters. It includes the effect of energy and width of solid-solid and solid-melt interfaces, interaction between two solid-melt interfaces, temperature, mechanics, and interface stresses. The derived thermodynamic potential satisfies all the equilibrium and stability conditions for homogeneous phases. The HMX energetic crystal is used as the model material and numerical simulations are performed using COMSOL and Cystorm high performance computing facility. Depending on parameters, the intermediate melt may appear and disappear by continuous or discontinuous barrierless disordering or via critical nucleus due to thermal fluctuations. The intermediate melt may appear during heating and persist during cooling at temperatures well below what it follows from sharp-interface approach. For some parameters when intermediate melt is expected, it does not form, producing an intermediate melt free gap. Elastic energy promotes barrierless intermediate melt formation in terms of an increasing degree of disordering, interface velocity, and width of intermediate melt. Drastic reduction (by a factor of 16) of the energy of the critical nuclei of the intermediate melt within the solid-solid interface caused by mechanics is captured. Interfacial stresses surprisingly increase nucleation temperature for the intermediate melt. Interfacial stresses alter the kinetics of phase transformation, resulting in formation of new interfacial phases and drifting of a thermally activated spontaneous

  4. Structural control of Fe-based alloys through diffusional solid/solid phase transformations in a high magnetic field

    PubMed Central

    Ohtsuka, Hideyuki

    2008-01-01

    A magnetic field has a remarkable influence on solid/solid phase transformations and it can be used to control the structure and function of materials during phase transformations. The effects of magnetic fields on diffusional solid/solid phase transformations, mainly from austenite to ferrite, in Fe-based alloys are reviewed. The effects of magnetic fields on the transformation temperature and phase diagram are explained thermodynamically, and the transformation behavior and transformed structures in magnetic fields are discussed. PMID:27877922

  5. Structural control of Fe-based alloys through diffusional solid/solid phase transformations in a high magnetic field.

    PubMed

    Ohtsuka, Hideyuki

    2008-01-01

    A magnetic field has a remarkable influence on solid/solid phase transformations and it can be used to control the structure and function of materials during phase transformations. The effects of magnetic fields on diffusional solid/solid phase transformations, mainly from austenite to ferrite, in Fe-based alloys are reviewed. The effects of magnetic fields on the transformation temperature and phase diagram are explained thermodynamically, and the transformation behavior and transformed structures in magnetic fields are discussed.

  6. Structure and phase transformations in uranium metal

    NASA Astrophysics Data System (ADS)

    Axe, J. D.; Gruebel, G.; Lander, G. H.

    1994-10-01

    In common with other elemental actinides, metallic uranium exists in several allotropic forms, differing from one another by complex but subtle atomic rearrangements. This article reviews progress in understanding the successive phase transformations from the perspective of soft-mode instabilities.

  7. Structure and phase transformations in uranium metal

    SciTech Connect

    Axe, J.D.; Gruebel, G.; Lander, G.H.

    1993-12-01

    In common with other elemental actinides, metallic uranium exists in several allotropic forms, differing from one another by complex but subtle atomic rearrangements. This article reviews progress in understanding the successive phase transformations from the perspective of soft-mode instabilities.

  8. Free energy functionals for efficient phase field crystal modeling of structural phase transformations.

    PubMed

    Greenwood, Michael; Provatas, Nikolas; Rottler, Jörg

    2010-07-23

    The phase field crystal (PFC) method is a promising technique for modeling materials with atomic resolution on mesoscopic time scales. While numerically more efficient than classical density functional theory (CDFT), its single mode free energy limits the complexity of structural transformations that can be simulated. We introduce a new PFC model inspired by CDFT, which uses a systematic construction of two-particle correlation functions that allows for a broad class of structural transformations. Our approach considers planar spacings, lattice symmetries, planar atomic densities, and atomic vibrational amplitudes in the unit cell, and parameterizes temperature and anisotropic surface energies. The power of our approach is demonstrated by two examples of structural phase transformations.

  9. Phase transformations in multiferroic Bi{sub 1−x}La{sub x}Fe{sub 1−y}Ti{sub y}O{sub 3} ceramics probed by temperature dependent Raman scattering

    SciTech Connect

    Xu, L. P.; Zhang, X. L.; Zhang, J. Z.; Hu, Z. G. Chu, J. H.; Zhang, L. L.; Yu, J.

    2014-10-28

    Optical phonons and phase transitions of Bi{sub 1−x}La{sub x}Fe{sub 1−y}Ti{sub y}O{sub 3} (BLFTO, 0.02 ≤ x ≤ 0.12, 0.01 ≤ y ≤ 0.08) ceramics have been investigated by Raman scattering in the temperature range from 80 to 680 K. Four phase transitions around 140, 205, 570, and 640 K can be observed. The Raman modes are sensitive to the spin reorientation around 140 and 205 K, owing to the strong magnon-phonon coupling. The transformation around 570 K is a structural transition from rhombohedral to orthorhombic phase due to an external pressure induced by the chemical substitution. The anomalies of the phonon frequencies near Néel temperature T{sub N} have been discussed in the light of the multiferroicity. Moreover, it was found that the structural transition temperature and T{sub N} of BLFTO ceramics decrease towards room temperature with increasing doping composition as a result of size mismatch between substitution and host cations.

  10. 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.

  11. Study on phase transformation pathways of iron sulfide minerals

    NASA Astrophysics Data System (ADS)

    Lin, M. Y.; Chen, Y. H.

    2016-12-01

    The iron sulfide minerals, including mackinawite, greigite, pyrite etc., are critical role in global sulfur cycle, recorders of geomagnetic fields and paleoenvironment in sedimentary rocks. However, due to lots of factors like oxygen pressure, pH, concentration and temperature etc., the transformation process mechanism and pathway were still unknown. Co-precipitation method was used to synthesize precursor (amorphous FeS) and pH value, time, temperature were adjusted to observe transformation process. For ex-situ X-ray diffraction (XRD) results of iron sulfide growth, it was observed the initially appearing phase was mackinawite at acidic and neutral conditions, but mackinawite along with magnetite occurred at the alkaline condition. While the reaction time was increased from 1 hour to 3 days and 20 days, it displayed the transformation phase from mackinawite to greigite, finally marcasite and pyrite were appeared in acidic condition. It showed that the transformation rate in the acidic environment is faster than that in neutral and alkaline conditions. For in-situ X-ray diffraction (XRD) results, it had the same phase transition pathway as mackinawite transformed into greigite at 80oC, and then pyrite appeared at 100oC in the acidic medium. However, it always kept the same phases in neutral and alkaline conditions during 3 hours at 80oC and 100oC, which indicated that mackinawite was very unstable and performed visible transformation sequence under acidic conditions. It concluded that the reaction time, pH value, and temperature indeed influenced the Fe-S formation pathway.

  12. 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.

  13. 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.

  14. 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.

  15. The HCP To BCC Phase Transformation in Ti Characterized by Nanosecond Electron Microscopy

    SciTech Connect

    Campbell, G; LaGrange, T; King, W; Colvin, J; Ziegler, A; Browning, N; Kleinschmidt, H; Bostanjoglo, O

    2005-06-21

    The general class of martensitic phase transformations occurs by a rapid lattice-distortive mechanism, where kinetics and morphology of the transformation are dominated by the strain energy. Since transformation is diffusionless, phase fronts propagate through a crystal with great speed that can approach the speed of sound. We have observed a particular example of this class of phase transformation, the hexagonal close packed (HCP) to body centered cubic (BCC) transformation in titanium that is driven by a rapid increase in temperature. We have used a novel nanosecond electron microscope (the dynamic transmission electron microscope, DTEM) to acquire diffraction and imaging information on the transformation, which is driven in-situ by nanosecond laser irradiation. Using nanosecond exposure times that are possible in the DTEM, data can be collected about the transient events in these fast transformations. We have identified the phase transformation with diffraction patterns and correlated the time of the phase transformation with calculated conditions in the sample.

  16. Effect of phase transformations on microstructures in deep mantle materials

    NASA Astrophysics Data System (ADS)

    Merkel, Sébastien; Langrand, Christopher; Rosa, Angelika; Hilairet, Nadège

    2017-04-01

    árik, Vaughan, Merkel, Reliability of Multigrain Indexing for Orthorhombic Polycrystals above 1 Mbar: Application to MgSiO3-Post-Perovskite, J Appl Cryst 50, in press (2017) Rosa, Hilairet, Ghosh, Garbarino, Jacobs, Perrillat, Vaughan, Merkel, In situ monitoring of phase transformation microstructures at Earth's mantle pressure and temperature using multi-grain XRD, J Appl Cryst 48, 1346-1354 (2015) Rosa, Hilairet, Ghosh, Perrillat, Garbarino, Merkel, Evolution of grain sizes and orientations during phase transitions in hydrous Mg2SiO4, J Geophys Res 121, 7161-7176 (2016)

  17. Phase transformations in some hafnium-tantalum-titanium-zirconium alloys

    SciTech Connect

    Ohriner, E.K.; Kapoor, D.

    1997-11-01

    Phase transformations in hafnium alloys are of interest as a means of achieving a material which exhibits flow softening and high localized strains during deformation at high strain rates. Hafnium transforms from a body-centered-cubic beta phase to a hexagonal alpha phase upon cooling below 1749{degrees}C. Hafnium-based alloys containing up to 17.5% Ti, up to 17.5% Ta, and up to 7.3% Zr by weight were button-arc melted and, in some cases, hot extruded to obtain a refined grain size. A number of alloys were shown to have beta solvus temperatures in the range of 1100 to 1300{degrees}C and showed evidence of a shear transformation upon water quenching. The Vickers microhardness of the quenched materials are typically above 350 HV as compared to 300 HV or less for materials with an alpha plus beta structure. Quenching dilatometry indicates a martensite start temperature of about 750{degrees}C for the Hf-7.5 Ta-10 Ti-1 Zr alloy and 800{degrees}C or more for the Hf-7.5 Ta-7.5 Ti-1 Zr alloy. Tensile tests at 1 s{sup {minus}1} strain rate show a constant ultimate tensile strength for temperatures up to 600{degrees}C for the above two alloys and a rapid decrease in strength with a further increase in temperature.

  18. Phase transformation in AFM silicon tips.

    PubMed

    Kopycinska-Müller, M; Barth, M; Küttner, M; Köhler, B

    2017-09-01

    We confirmed the occurrence of phase transformations in an atomic force microscopy silicon tip during loading and unloading experiments performed on a polycrystalline Ti sample. The influence of the phase transformations on the effective mechanical and electrical properties of the tip was observed with the help of load-unload curves measured simultaneously for the tip-sample contact stiffness k (*) and the effective electrical resistance of the system R eff. We used the atomic force acoustic microscopy (AFAM) method to determine the values of k (*). To measure the changes in R eff, we combined a high voltage source/measure unit with the existing AFAM system. The data obtained showed that the phase transformation from Si-I to Si-II is preceded by other structural changes such as formation of distorted diamond structures and formation of Si-III. This conclusion was reached after observing a small hysteretic behavior in the load-unload stiffness curve accompanied by only very small changes in the resistance of the tip-sample system occurring on the unloading. The coinciding of a sudden increase in the values of the contact stiffness with a decrease in the resistance of the system indicated that the formation of metallic Si-II occurred in the subsequent measurements. The interpretation of our results found confirmation in the results of molecular dynamics and atomistic simulations performed for silicon under nanoindentation experiments.

  19. Phase transformation in AFM silicon tips

    NASA Astrophysics Data System (ADS)

    Kopycinska-Müller, M.; Barth, M.; Küttner, M.; Köhler, B.

    2017-09-01

    We confirmed the occurrence of phase transformations in an atomic force microscopy silicon tip during loading and unloading experiments performed on a polycrystalline Ti sample. The influence of the phase transformations on the effective mechanical and electrical properties of the tip was observed with the help of load-unload curves measured simultaneously for the tip-sample contact stiffness k * and the effective electrical resistance of the system R eff. We used the atomic force acoustic microscopy (AFAM) method to determine the values of k *. To measure the changes in R eff, we combined a high voltage source/measure unit with the existing AFAM system. The data obtained showed that the phase transformation from Si-I to Si-II is preceded by other structural changes such as formation of distorted diamond structures and formation of Si-III. This conclusion was reached after observing a small hysteretic behavior in the load-unload stiffness curve accompanied by only very small changes in the resistance of the tip-sample system occurring on the unloading. The coinciding of a sudden increase in the values of the contact stiffness with a decrease in the resistance of the system indicated that the formation of metallic Si-II occurred in the subsequent measurements. The interpretation of our results found confirmation in the results of molecular dynamics and atomistic simulations performed for silicon under nanoindentation experiments.

  20. 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.

  1. Microstructural transformations and kinetics of high-temperature heterogeneous gasless reactions by high-speed x-ray phase-contrast imaging

    SciTech Connect

    Reeves, Robert V.; Son, Steven F.; White, Jeremiah D. E.; Mukasyan, Alexander S.; Dufresne, Eric M.; Fezzaa, Kamel; Varma, Arvind

    2009-12-01

    Heterogeneous gasless reactive systems, including high-energy density metal-nonmetal compositions, have seen increasing study due to their various applications. However, owing to their high reaction temperature, short reaction time, and small scale of heterogeneity, investigation of their reaction mechanisms and kinetics is very difficult. In this study, microstructural changes and the kinetics of product layer growth in the W-Si system was investigated using a high-speed x-ray phase-contrast imaging technique. Using the Advanced Photon Source of Argonne National Laboratory, this method allowed direct imaging of irreversible reactions in the W-Si reactive system at frame rates up to 36 000 frames per second with 4 {mu}s exposure and spatial resolution of 10 {mu}m. Details of the Si melt and reactions between W and Si, that are unable to be viewed with visible-light imaging, were revealed. These include processes such as the initiation of nucleated melting and other physical phenomena that provide insight into the mixing of reactants and subsequent reaction. Through the use of this imaging technique and future optimization in the imaging process, a model for accurately identifying kinetics of chemical reactions, both spatially and temporally, is also proposed.

  2. Materials Science and Technology, Volume 5, Phase Transformations in Materials

    NASA Astrophysics Data System (ADS)

    Haasen, Peter

    1996-12-01

    This volume covers phase transformations, a general phenomenon central to understanding the behavior of materials and to creating high-performance materials. From the Contents: Pelton: Thermodynamics and Phase Diagrams of Materials. Murch: Diffusion in Crystalline Solids. Binder: Statistical Theories of Phase Transitions/Spinodal Decomposition. Wagner/Kampmann: Homogeneous Second Phase Precipitation. Purdy: Transformations Involving Interfacial Diffusion. Delaey: Diffusionless Transformations. Ruoff: High Pressure Phase Transformations. Pitsch/Inden: Atomic Ordering. Müller- Krumbhaar/Kurz: Solidification.

  3. High-temperature transformations of Cu-rich hydrotalcites

    NASA Astrophysics Data System (ADS)

    Kannan, S.; Rives, V.; Knözinger, H.

    2004-01-01

    Cu M(II)Al ternary hydrotalcites ( M(II) = Ni, Co and Mg) with a (Cu+ M(II))/Al atomic ratio of 3.0 and Cu/ M(II) atomic ratio of 5.0 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction of the as-synthesized samples showed a pattern characteristic of hydrotalcite-like (HT-like) structure (JCPDS: 41-1428). Thermal analyses of these samples showed four stages of weight loss/heat change when recorded in nitrogen. Analysis of the evolved gases characterized the nature of these transformations. The thermoanalytical effects differed significantly especially for the high-temperature transformations, when the treatment was performed in oxygen. In situ powder X-ray diffraction of the samples was carried out to elucidate the phase evolution of these compounds. Surprisingly formation of CuO was noted at temperatures around 200°C well below the destruction of the layered network. The nature of the resulting phases varied with both the nature of the co-bivalent metal ion and the heating atmosphere. FT-IR spectroscopy confirmed the retention of carbonate ions at higher temperatures (above 700°C), although the concentration of carbonate anion (most likely unidentate) varied with the calcination temperature. The crystallinity of CuO increased significantly above 600°C, probably through dissociation of copper oxycarbonate. Significant differences in the thermal transformation temperatures (for the third and the fourth transformations) of these samples containing different co-bivalent metal ions were not observed. This suggests that an association of the co-bivalent metal ions and/or trivalent metal ion in this phase is unlikely. A plausible thermal evolution scheme of these hydrotalcites is proposed.

  4. 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.

  5. Polymorphic Phase Transformation in the 3-Bromo- trans-cinnamic Acid System

    NASA Astrophysics Data System (ADS)

    Ahn, Shinbyoung; Harris, Kenneth D. M.; Kariuki, Benson M.; Zin, Dimple M. S.

    2001-01-01

    3-Bromo-trans-cinnamic acid (3-BrCA) exists as two crystalline polymorphic forms (designated as β and γ phases). A polymorphic phase transformation from the γ phase to the β phase has been investigated using ex situ powder X-ray diffraction, in situ high-temperature optical microscopy, and differential scanning calorimetry. The transformation occurs at an observable rate at temperatures above about 100°C. A reverse transformation on subsequently cooling the β phase is not observed. Thermodynamic aspects of the polymorphic 3-BrCA system are discussed, together with kinetic aspects of the transformation from the γ phase to the β phase. The structural properties of the β phase (reported previously) and the γ phase (determined in this work from single-crystal X-ray diffraction data) are in accord with the α/β/γ structural classification of trans-cinnamic acid derivatives.

  6. Phase Transformations upon Doping in Tungsten Trioxide

    NASA Astrophysics Data System (ADS)

    Wang, Wennie; Janotti, Anderson; van de Walle, Chris G.

    Tungsten trioxide (WO3) is an emerging semiconductor material, with a growing number of applications in Li-ion batteries, photocatalysis, gas sensors and electrochromic devices. As an electrochromic material, WO3 turns from transparent to blue upon doping with monovalent species. Due to it having an empty A-site in the ABO3 perovskite structure, high doping concentrations are possible through intercalation. Tungsten trioxide has been experimentally shown to transform from the ground-state monoclinic symmetry to cubic symmetry with increasing monovalent doping. We use first-principles calculations to understand this transformation. Our calculations show that the addition of electrons to the conduction band is a primary driver of the phase transformation. We quantify the energetics and structural aspects of this transformation using density functional theory, allowing us to elucidate the mechanism. Comparison with experiment, role of the dopant species, and implications of structural changes for device applications will be discussed. This work is supported by the DOE and NSF GRFP.

  7. 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

  8. Domain walls in ω-phase transformations

    NASA Astrophysics Data System (ADS)

    Sanati, Mahdi; Saxena, Avadh

    1998-11-01

    The β-phase (body-centered cubic: b.c.c.) to ω-phase transformation in certain elements (e.g. Zr) and alloys (e.g. ZrNb) is induced either by quenching or application of pressure. The ω-phase is a metastable state and usually coexists with the β-matrix in the form of small particles. To study the formation of domain walls in these materials we have extended the Landau model of Cook for the ω-phase transition by including a spatial gradient (Ginzburg) term of the scalar order parameter. In general, the Landau free energy is an asymmetric double-well potential. From the variational derivative of the total free energy we obtain a static equilibrium condition. By solving this equation for different physical parameters and boundary conditions, we obtained different quasi-one-dimensional soliton-like solutions. These solutions correspond to three different types of domain walls between the ω-phase and the β-matrix. In addition, we obtained soliton lattice (domain wall array) solutions, calculated their formation energy and the asymptotic interaction between the solitons.

  9. Phase amplitude conformal symmetry in Fourier transforms

    NASA Astrophysics Data System (ADS)

    Kuwata, S.

    2015-04-01

    For the Fourier transform ℑ : L2(R) → L2(R) of a complex-valued even or odd function ψ, it is found that the amplitude invariance |ℑψ| = |ψ| leads to a phase invariance or inversion as arg(ℑψ) = ±argψ + θ (θ = constant). The converse holds unless arg ψ = constant. The condition |ψ| = |ℑψ| is required in dealing with, for example, the minimum uncertainty relation between position and momentum. Without the evenness or oddness of ψ, |ℑψ| = |ψ| does not necessarily imply arg(ℑψ) = ±argψ + θ, nor is the converse.

  10. Transition temperature of martensitic transformations in hafnia and zirconia

    NASA Astrophysics Data System (ADS)

    Luo, Xuhui; Demkov, A. A.

    2008-03-01

    Transition metal oxides find applications in ceramics, catalysis and semiconductor technology. In particular, hafnium dioxide or hafnia will succeed silica as a gate dielectric in advanced transistors. However, thermodynamic properties of thin hafnia films are not well understood, despite their technological importance. We use density functional theory to investigate the tetragonal to monoclinic phase transition in hafnia and zirconia. We find that unlike the case of the cubic to tetragonal transition, this phase transition is not driven by a soft mode. We use transition state theory to identify the minimum energy path (MEP) employing first principle calculations for hafnia and zirconia, sow that both transformations are martensitic, and obtain the transition barriers. Martensitic transformations include both the internal coordinate transformation and deformation of the cell lattice vectors (``strain and shuffle''), therefore the potential energy surface and MEP are function not only of the internal atomic coordinates but also of the unit cell lattice vectors. Considering the simplest case of uniform strain the transition temperatures we then relate the barrier height to the transition temperature. As a self-consistency check, assuming the equality of thermodynamics potentials of the tetragonal and monoclinic phases during the transition, and using the difference in the internal energy calculated from first principles we estimate the entropy change associated with the transition which is found in good agreement with that calculated form the phonon spectra.

  11. Microstructural formation in Ti alloys: In-situ characterization of phase transformation kinetics

    NASA Astrophysics Data System (ADS)

    Aeby-Gautier, E.; Bruneseaux, F.; da Costa Teixeira, J.; Appolaire, B.; Geandier, G.; Denis, S.

    2007-02-01

    The prediction of microstructure during processing needs to characterize the phase transformation occurring during the thermal treatments and their kinetics. In-situ high-energy synchrotron x-ray diffraction experiments performed during temperature variations allow the characterization of the phase evolution. For some transformation conditions, the continuous recording of diffraction diagrams evidences clearly intermediate phases. The quantitative analysis of the diffraction diagrams gives the transformation kinetics of each phase as well as their cell parameters. Transformation kinetics obtained by this method are compared to results obtained by electrical resistivity.

  12. 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.

  13. Phase transformation and growth of hygroscopic aerosols

    SciTech Connect

    Tang, I.N.

    1995-09-01

    Ambient aerosols frequently contain large portions of hygroscopic inorganic salts such as chlorides, nitrates, and sulfates in either pure or mixed forms. Such inorganic salt aerosols exhibit the properties of deliquescence and efflorescence in air. The phase transformation from a solid particle to a saline droplet usually occurs spontaneously when the relative humidity of the atmosphere reaches a level specific to the chemical composition of the aerosol particle. Conversely, when the relative humidity decreases and becomes low enough, the saline droplet will evaporate and suddenly crystallize, expelling all its water content. The phase transformation and growth of aerosols play an important role in many atmospheric processes affecting air quality, visibility degradation, and climate changes. In this chapter, an exposition of the underlying thermodynamic principles is given, and recent advances in experimental methods utilizing single-particle levitation are discussed. In addition, pertinent and available thermodynamic data, which are needed for predicting the deliquescence properties of single and multi-component aerosols, are compiled. This chapter is useful to research scientists who are either interested in pursuing further studies of aerosol thermodynamics, or required to model the dynamic behavior of hygroscopic aerosols in a humid environment.

  14. 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

  15. Rapid Solidification and Phase Transformations in Additive Manufactured Materials

    SciTech Connect

    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 summarizing 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.

  16. Novel microporous zirconium silicate (K{sub 2}ZrSi{sub 3}O{sub 9}.2H{sub 2}O) from high temperature phase transformation

    SciTech Connect

    Ferreira, Artur; Lin Zhi; Soares, Maria R.; Rocha, Joao

    2010-12-15

    A new microporous zirconosilicate K{sub 2}ZrSi{sub 3}O{sub 9}.2H{sub 2}O (AV-15) has been prepared by high-temperature phase transformation at 910 {sup o}C. Its structure has been determined ab initio from powder X-ray diffraction data. The unit cell is orthorhombic, space group C222{sub 1} (no. 20), Z=4 with cell dimensions: a=8.105(3), b=10.684(5), c=12.030(5) A, V=1041.76(7) A{sup 3}. The framework connection of AV-15 is essentially the same as the previously reported sodium stannosilicate AV-10 while the locations of potassium and water molecules in the former are quite different from those of the sodium and water molecules in AV-10. In AV-10 sodium and water molecules form a sinucoidal chain, while potassium and water molecules build up a linear chain in AV-15. The water molecules in AV-15 are lost on heating with a typical zeolitic behaviour. SEM shows that the particle sizes and habits of AV-15 and parent umbite material are the same. The {sup 29}Si MAS NMR spectrum of AV-15 displays two resonances at ca. -89.4 and -90.1 ppm in a 1:2 intensity ratio. Thermogravimetry analysis confirms the existence of water in this material. -- Graphical abstract: A new microporous zirconosilicate has been prepared by high temperature phase transformation at 910 {sup o}C. Its structure has been determined ab initio from powder X-ray diffraction data. The water molecules in this material are lost below 125 {sup o}C in a way typical of zeolites and molecular sieves. Display Omitted

  17. Phase Change Fabrics Control Temperature

    NASA Technical Reports Server (NTRS)

    2009-01-01

    Originally featured in Spinoff in 1997, Outlast Technologies Inc. (formerly Gateway Technologies Inc.) has built its entire product line on microencapsulated phase change materials, developed in Small Business Innovation Research (SBIR) contracts with Johnson Space Center after initial development for the U.S. Air Force. The Boulder, Colorado-based company acquired the exclusive patent rights and now integrates these materials into textiles or onto finished apparel, providing temperature regulation in bedding materials and a full line of apparel for both ordinary and extreme conditions.

  18. Nanoscale thermodynamic study on phase transformation in the nanocrystalline Sm2Co17 alloy.

    PubMed

    Xu, Wenwu; Song, Xiaoyan; Lu, Nianduan; Seyring, Martin; Rettenmayr, Markus

    2009-11-01

    The characteristics of phase transformation in nanocrystalline alloys were studied both theoretically and experimentally from the viewpoint of thermodynamics. With a developed thermodynamic model, the dependence of phase stability and phase transformation tendency on the temperature and the nanograin size were calculated for the nanocrystalline Sm(2)Co(17) alloy. It is thermodynamically predicted that the critical grain size for the phase transformation between hexagonal and rhombohedral nanocrystalline Sm(2)Co(17) phases increases with increasing temperature. When the grain size is reduced to below 30 nm, the hexagonal Sm(2)Co(17) phase can stay stable at room temperature, which is a stable phase only at temperatures above 1520 K in the conventional polycrystalline alloys. A series of experiments were performed to investigate the correlation between the phase constitution and the grain structure in the nanocrystalline Sm(2)Co(17) alloy with different grain size levels. The experimental results agree well with the thermodynamic predictions of the grain-size dependence of the room-temperature phase stability. It is proposed that at a given temperature the thermodynamic properties, as well as the phase stability and phase transformation behavior of the nanocrystalline alloys, are modulated by the variation of nanograin size, i.e. the grain size effects on the structure and energy state of the nanograin boundaries.

  19. Phase transformation and growth of hygroscopic aerosols

    SciTech Connect

    Tang, I.N.

    1999-11-01

    Ambient aerosols play an important role in many atmospheric processes affecting air quality, visibility degradation, and climatic changes as well. Both natural and anthropogenic sources contribute to the formation of ambient aerosols, which are composed mostly of sulfates, nitrates, and chlorides in either pure or mixed forms. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. For pure inorganic salt particles with diameter larger than 0.1 micron, the phase transformation from a solid particle to a saline droplet occurs only when the relative humidity in the surrounding atmosphere reaches a certain critical level corresponding to the water activity of the saturated solution. The droplet size or mass in equilibrium with relative humidity can be calculated in a straightforward manner from thermodynamic considerations. For aqueous droplets 0.1 micron or smaller, the surface curvature effect on vapor pressure becomes important and the Kelvin equation must be used.

  20. Phase transformations in ion-irradiated silicides

    NASA Technical Reports Server (NTRS)

    Hewett, C. A.; Lau, S. S.; Suni, I.; Hung, L. S.

    1985-01-01

    The present investigation has three objectives. The first is concerned with the phase transformation of CoSi2 under ion implantation and the subsequent crystallization characteristics during annealing, taking into account epitaxial and nonepitaxial recrystallization behavior. The second objective is related to a study of the general trend of implantation-induced damage and crystallization behavior for a number of commonly used silicides. The last objective involves a comparison of the recrystallization behavior of cosputtered refractory silicides with that of the ion-implanted silicides. It was found that epitaxial regrowth of ion-irradiated CoSi2 occurred for samples with an epitaxial seed left at the Si/CoSi2 interface. A structural investigation of CoSi2 involving transmission electron microscopy (TEM) showed that after high-dose implantation CoSi2 is amorphous.

  1. 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

  2. Effect of the Temperature and Duration of Aging on Structural and Phase Transformations in Steel 08Kh18N10T

    NASA Astrophysics Data System (ADS)

    Ovchinnikov, V. V.; Kangezova, E. A.

    2016-11-01

    The influence of long holds (500 - 10,000 h) at a temperature of 450 - 750°C on the structure and properties of steel 08Kh18N10T used for furnace coil pipes in devices for hydrofining of diesel fuel is studied. Metallographic (light and electron microscopy) and x-ray diffraction analyses are performed; mechanical properties of the steel are determined.

  3. 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.

  4. Phase transformation of NiTi alloys during vacuum sintering

    NASA Astrophysics Data System (ADS)

    Wang, Jun; Hu, Kuang

    2017-05-01

    The aim of this study is to ascertain the Phase transformation of NiTi alloys during vacuum sintering. NiTi shape memory alloys (SMA) of atomic ratio 1:1 were prepared through press forming and vacuum sintering with the mixture of Ni and Ti powders. Different samples were prepared by changing the sintering time and the sintering temperature. Phase and porosity of the samples were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that in the process of sintering NiTi2 and Ni3Ti phases are formed firstly and then transform into NiTi phase. The quantity of NiTi2 and Ni3Ti phases gradually decreased but not eliminate completely with increase of sintering time. The porosity of specimen sintering at 900°C decreases slightly with increase of sintering time. With increase of sintering time the porosity of specimen sintering at 1050°C decreased firstly and then increased because of generation rich titanium liquid in the process of sintering.

  5. Stress and phase transformation phenomena in oxide films

    SciTech Connect

    Exarhos, G.J.; Hess, N.J.

    1992-04-01

    In situ optical methods are reviewed for characterization of phase transformation processes and evaluation of residual stress in solution- deposited metastable oxide films. Such low density films most often are deposited as disordered phases making them prone to crystallization and attendant densification when subjected to increased temperature and/or applied pressure. Inherent stress imparted during film deposition and its evolution during the transformation are evaluated from phonon frequency shifts seen in Raman spectra (TiO{sub 2}) or from changes in the laser-induced fluorescence emission spectra for films containing rare earth (Sm{sup +3}:Y{sub 3}Al{sub 5}O{sub 12}) or transition metal (Cr{sup +3}:Al{sub 2}O{sub 3}) dopants. The data in combination with measured increases in line intensities intrinsic to the evolving phase are used to follow crystallization processes in thin films. In general, film deposition parameters are found to influence the crystallite ingrowth kinetics and the magnitude of stress and stress relaxation in the film during the transformation. The utility of these methods to probe crystallization phenomena in oxide films will be addressed.

  6. Special phase transformation and crystal growth pathways observed in nanoparticles†

    PubMed Central

    Gilbert, Benjamin; Zhang, Hengzhong; Huang, Feng; Finnegan, Michael P; Waychunas, Glenn A; Banfield, Jillian F

    2003-01-01

    Phase transformation and crystal growth in nanoparticles may happen via mechanisms distinct from those in bulk materials. We combine experimental studies of as-synthesized and hydrothermally coarsened titania (TiO2) and zinc sulfide (ZnS) with thermodynamic analysis, kinetic modeling and molecular dynamics (MD) simulations. The samples were characterized by transmission electron microscopy, X-ray diffraction, synchrotron X-ray absorption and scattering, and UV-vis spectroscopy. At low temperatures, phase transformation in titania nanoparticles occurs predominantly via interface nucleation at particle–particle contacts. Coarsening and crystal growth of titania nanoparticles can be described using the Smoluchowski equation. Oriented attachment-based crystal growth was common in both hydrothermal solutions and under dry conditions. MD simulations predict large structural perturbations within very fine particles, and are consistent with experimental results showing that ligand binding and change in aggregation state can cause phase transformation without particle coarsening. Such phenomena affect surface reactivity, thus may have important roles in geochemical cycling.

  7. X-ray diffraction study of the phase transformations in NiTi shape memory alloy

    SciTech Connect

    Uchil, J.; Fernandes, F.M. Braz . E-mail: kkmahesh@rediffmail.com

    2007-03-15

    The phase transformations occurring in heat-treated NiTi shape memory alloys have been studied through the analysis of variation in integrated peak area (integrated intensity) with temperature, under the XRD peak profiles in the transformation temperature range. For this purpose, integrated peak area under the prominent peak corresponding to (110) plane of the austenitic phase has been chosen. The results so obtained are compared with those got from the DSC method. The XRD method is found to be more sensitive.

  8. High temperature measurements of martensitic transformations using digital holography.

    PubMed

    Thiesing, Benjamin P; Mann, Christopher J; Dryepondt, Sebastien

    2013-07-01

    During thermal cycling of nickel-aluminum-platinum (NiAlPt) and single crystal iron-chromium-nickel (FeCrNi) alloys, the structural changes associated with the martensite to austenite phase transformation were measured using dual-wavelength digital holography. Real-time in situ measurements reveal the formation of striations within the NiAlPt alloy at 70°C and the FeCrNi alloy at 520°C. The results demonstrate that digital holography is an effective technique for acquiring noncontact, high precision information of the surface evolution of alloys at high temperatures.

  9. High Temperature Measurements Of Martensitic transformations Using Digital Holography

    SciTech Connect

    Thiesing, Benjamin; Mann, Christopher J; Dryepondt, Sebastien N

    2013-01-01

    During thermal cycling of nickel-aluminum-platinum (NiAlPt) and single crystal Fe-15Cr-15Ni alloys, the structural changes associated with the martensite to austenite phase transformation were measured using dual-wavelength digital holography. Real-time in-situ measurements reveal the formation of striations within the NiPtAl alloy at 70 C and the FeCrNi alloy at 520 C. The results demonstrate that digital holography is an effective technique for acquiring non-contact, high precision information of the surface evolution of alloys at high temperatures.

  10. Phase transformations and vibrational properties of coronene under pressure

    NASA Astrophysics Data System (ADS)

    Zhao, Xiao-Miao; Zhang, Jiang; Berlie, Adam; Qin, Zhen-Xing; Huang, Qiao-Wei; Jiang, Shan; Zhang, Jian-Bo; Tang, Ling-Yun; Liu, Jing; Zhang, Chao; Zhong, Guo-Hua; Lin, Hai-Qing; Chen, Xiao-Jia

    2013-10-01

    Both the vibrational and structural properties of coronene have been investigated upon compression up to 30.5 GPa at room temperature by a combination of Raman scattering and synchrotron x-ray diffraction measurements. The spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa where the high-pressure phases are identified as monoclinic and orthorhombic crystal structures with space groups of P2/m and Pmmm, respectively. A kink in the slope of the cell parameters as a function of pressure is associated with the disappearance of several internal Raman modes, which suggests the existence of structural distortions or reorganizations at approximately 6.0 GPa. Above 17.1 GPa, almost no evidence of crystallinity can be observed, indicating a possible transformation of coronene into an amorphous phase.

  11. Three-Dimensional Numerical Model Considering Phase Transformation in Friction Stir Welding of Steel

    NASA Astrophysics Data System (ADS)

    Cho, Hoon-Hwe; Kim, Dong-Wan; Hong, Sung-Tae; Jeong, Yong-Ha; Lee, Keunho; Cho, Yi-Gil; Kang, Suk Hoon; Han, Heung Nam

    2015-12-01

    A three-dimensional (3D) thermo-mechanical model is developed considering the phase transformation occurring during the friction stir welding (FSW) of steel, and the simulated result is compared with both the measured temperature distribution during FSW and the microstructural changes after FSW. The austenite grain size (AGS) decreases significantly because of the frictional heat and severe plastic deformation generated during FSW, and the decreased AGS accelerates the diffusional phase transformation during FSW. The ferrite phase, one of the diffusional phases, is developed mainly in mild steel, whereas the bainite phase transformation occurs significantly in high-strength steel with large hardenability. Additionally, transformation-induced heat is observed mainly in the stir zone during FSW. The measured temperature distribution and phase fraction agree fairly well with the predicted data.

  12. Phase-field modeling of shock-induced α- γ phase transformation of RDX

    NASA Astrophysics Data System (ADS)

    Rahul, -; de, Suvranu

    2015-06-01

    A thermodynamically consistent continuum phase field model has been developed to investigate the role of shock-induced α- γ phase transition in the sensitivity of RDX. Dislocations and phase transformations are distinguished and modeled within a crystal plasticity framework. The Landau potential is derived for the finite elastic deformation analysis. The response of the shock loaded RDX crystal is obtained by solving the continuum momentum equation along with phase evolution equation using a Helmholtz free energy functional, which consists of elastic potential energy and local interfacial energy that follows from the Cahn-Hilliard formalism. We observe that the orientations for which there is a resolved shear stress along the slip direction, the material absorbs large shear strain through plastic deformation, allowing it to be less sensitive as less mechanical work is available for temperature rise. Therefore, plastic slip should be associated with greater shear relaxation and, hence, decreased sensitivity. For elastic orientations, large shear stress arises from steric hindrance that may provides much more mechanical work to increase the temperature and hence more sensitive to detonation. Our simulations suggest that the α- γ phase transformation in RDX may be associated with the increased temperature rise and hence the shock sensitivity. The authors gratefully acknowledge the support of this work through Office of Naval Research (ONR) Grants N000140810462 and N000141210527 with Dr. Clifford Bedford as the cognizant Program Manager.

  13. Enhanced densification of white cast iron powders by cyclic phase transformations under stress

    NASA Astrophysics Data System (ADS)

    Ruano, Oscar A.; Wadsworth, Jeffrey; Sherby, Oleg D.

    1982-03-01

    It is shown that densification of white cast iron powders under stress can be enhanced by multiple phase transformations through thermal cycling. This enhancement occurs by accelerated creep flow during phase changes (transformation superplasticity). The approximate stress range where transformation-assisted densification can occur is shown to be between 1.7 MPa (250 psi) and 34.5 MPa (5000 psi). Below 1.7 MPa insufficient strain occurs during phase transformation to cause significant densification even after many transformation cycles. Above 34.5 MPa, densification occurs principally by normal slip creep. Transformation warm pressing of white cast iron powders leads to dense compacts at low pressures and short times. In addition, because the transformation temperature is low, the ultrafine structures existing in the original powders are retained in the densified compacts.

  14. Phase transformations and thermodynamics of aluminum-based metallic glasses

    NASA Astrophysics Data System (ADS)

    Gao, Changhua (Michael)

    This thesis examines the thermodynamics and associated kinetics and phase transformations of the glass forming Al-Ni-Gd and Al-Fe-Gd systems. In order to fully understand the unique glass forming ability (GFA) of Al-based metallic glasses, the ternary Al-Fe-Gd and Al-Ni-Gd systems in their Al-rich corners were examined experimentally to assist in a thermodynamic assessment. The solid-state phase equilibria are determined using XRD and TEM-EDS techniques. While this work basically confirms the solid-state equilibria in Al-Fe-Gd reported previously, the ternary phase in Al-Ni-Gd system has been identified to be Al15Ni3Gd2 rather than Al16Ni 3Gd reported in the literature. DTA analysis of 24 alloys in the Al-Fe-Gd system and 42 alloys in the Al-Ni-Gd system have yielded critical temperatures pertaining to the solid-liquid transition. Based on these data and information from the literature, a self-consistent thermodynamic database for these systems has been developed using the CALPHAD technique. Parameters describing the Gibbs free energy for various phases of the Al-Gd, Al-Fe-Gd and Al-Ni-Gd systems are manually optimized in this study. Once constructed, the database is used to calculate driving forces for nucleation of crystalline phases which can qualitatively explain the phase formation sequence during crystallization at low temperatures. It was also confirmed that alloy compositions with the lowest Gibbs free energy difference between the equilibrium state and undercooled liquid state exhibit better GFA than other chemistries. Based on 250°C isothermal devitrification phase transformations of 17 Al-Ni-Gd alloys, a phase formation sequence map is constructed. Fcc-Al nanocrystals are formed first in most of the alloys studied, but eutectic crystallization of a metastable phase and fcc-Al is also observed. Addition of Al or Ni promotes fcc-Al phase formation, while increasing Gd suppresses it. The continuous heating DSC scans revealed that crystallization in Al

  15. Fundamental Development on Utilizing the R-phase Transformation in NiTi Shape Memory Alloys

    NASA Astrophysics Data System (ADS)

    Wang, Xiebin; Kustov, Sergey; Verlinden, Bert; Van Humbeeck, Jan

    2015-06-01

    In near equiatomic NiTi alloys, the reversible thermoelastic transformation between B2-structured austenite phase and the R-phase is attracting increasing interest for practical applications. However, the following two issues limit the widespread utilization of the R-phase transformation: (1) there is no effective approach to control the R-phase transformation temperatures; (2) it is not easy to largely separate the temperature domain of the R-phase and the B19' martensite phase transformation, especially in the presence of an external force. This article reviews concisely the work of the present authors on solving the above two problems. The effect of grain size on the aging microstructure and related transformation behavior is first discussed. Inspired by these findings, an approach to solve the above two problems has been developed by introducing nanoscaled Ni4Ti3 precipitates in the samples with micron-sized grains. The performance of alloys associated with the R-phase transformation, which shows controllable transformation temperatures, is summarized.

  16. Effect of Bonding Temperature on Phase Transformation of Diffusion-Bonded Joints of Duplex Stainless Steel and Ti-6Al-4V Using Nickel and Copper as Composite Intermediate Metals

    NASA Astrophysics Data System (ADS)

    Kundu, Sukumar; Thirunavukarasu, Gopinath; Chatterjee, Subrata; Mishra, Brajendra

    2015-12-01

    In the present study, the effect of bonding temperature on phase transformation of diffusion-bonded joints of duplex stainless steel (DSS) and Ti-6Al-4V (Ti64) using simultaneously both nickel (Ni) and copper (Cu) interlayers was investigated in the temperature range of 1148 K to 1223 K (875 °C to 950 °C) insteps of 25 K (25 °C) for 60 minutes under 4 MPa uniaxial pressure in vacuum. Interfaces were characterized by scanning electron microscopy and interdiffusion of the chemical species across the diffusion interfaces were witnessed by electron probe microanalysis. At 1148 K (875 °C), layer-wise Cu4Ti, Cu2Ti, Cu4Ti3, CuTi, and CuTi2 phases were observed at the Cu-Ti64 interface; however, DSS-Ni and Ni-Cu interfaces were free from any intermetallic. At 1173 K and 1198 K (900 °C and 925 °C), Cu interlayer could not restrict the diffusion of atoms from Ti64 to Ni, and vice versa; and Ni-Ti-based intermetallics were formed at the Ni-Cu interface and throughout the Cu zone as well; however, at 1223 K (950 °C), both Ni and Cu interlayers could not inhibit the diffusion of atoms from Ti64 to DSS, and vice versa. The maximum shear strength of ~377 MPa was obtained for the diffusion couple processed at 1148 K (875 °C) and strength of the bonded joints gradually decreased with the increasing bonding temperature due to the widening of brittle intermetallics at the diffusion zone. Fracture path indicated that failure took place through the Cu4Ti intermetallic at the Cu-Ti64 interface when bonding was processed at 1148 K (875 °C). When bonding was processed at 1173 K and 1198 K (900 °C and 925 °C), fracture took place through the Ni3Ti intermetallic at the Ni-(Ni + Cu + Ti64 diffusion reaction) interface; however, at 1223 K (950 °C), fracture morphology indicated the brittle nature and the fracture took place apparently through the σ phase at the DSS-(DSS + Ni + Cu + Ti64 diffusion reaction) interface.

  17. High temperature co-axial winding transformers

    NASA Technical Reports Server (NTRS)

    Divan, Deepakraj M.; Novotny, Donald W.

    1993-01-01

    The analysis and design of co-axial winding transformers is presented. The design equations are derived and the different design approaches are discussed. One of the most important features of co-axial winding transformers is the fact that the leakage inductance is well controlled and can be made low. This is not the case in conventional winding transformers. In addition, the power density of co-axial winding transformers is higher than conventional ones. Hence, using co-axial winding transformers in a certain converter topology improves the power density of the converter. The design methodology used in meeting the proposed specifications of the co-axial winding transformer specifications are presented and discussed. The final transformer design was constructed in the lab. Co-axial winding transformers proved to be a good choice for high power density and high frequency applications. They have a more predictable performance compared with conventional transformers. In addition, the leakage inductance of the transformer can be controlled easily to suit a specific application. For space applications, one major concern is the extraction of heat from power apparatus to prevent excessive heating and hence damaging of these units. Because of the vacuum environment, the only way to extract heat is by using a cold plate. One advantage of co-axial winding transformers is that the surface area available to extract heat from is very large compared to conventional transformers. This stems from the unique structure of the co-axial transformer where the whole core surface area is exposed and can be utilized for cooling effectively. This is a crucial issue here since most of the losses are core losses.

  18. Phase Transformation in Silica-Coated FePt Nanoparticles

    NASA Astrophysics Data System (ADS)

    Colak, Levent; Hadjipanayis, George

    2009-03-01

    The A1 to L10 phase transformation has been examined in silica-coated FePt particles. The nanoparticles were synthesized by reduction of platinum acetylacetonate (Pt (acac) 2) followed by thermal decomposition of iron pentacarbonyl (Fe(CO)5) in the presence of oleic acid (OA) and oleyl amine (OY) as surfactants at low temperature ^[1]. The monodispersed FePt nanoparticles, with a size of 5.8 nm were then coated with silica (SiO2) shells ^[2] . The thickness of the silica shell could be controlled between 7.5-25 nm. The coated particles were subjected to thermal processing at 800 C for various amounts of times. No significant sintering was observed up to 2 hours of annealing for the shell thickness of 15.0 nm. In some silica-coated samples an increase in the particle size was observed after annealing. Selected Area Diffraction analysis and magnetic measurements showed the development of ordered L10 structure. Coercivity values up to 15 kOe at 7K are obtained. The phase transformation is currently being examined in other samples annealed at different times and temperatures and the results will be reported.1.Levent Colak and George C. Hadjipanayis, Nanotechnology 19 (2008) 235703.2.M. Aslam, L. Fu, S. Li, Vinayak P. Dravid, Journal of Colloid and Interface Science 290 (2005) 444--449.

  19. Nanoscale heat transfer and phase transformation surrounding intensely heated nanoparticles

    NASA Astrophysics Data System (ADS)

    Sasikumar, Kiran

    Over the last decade there has been significant ongoing research to use nanoparticles for hyperthermia-based destruction of cancer cells. In this regard, the investigation of highly non-equilibrium thermal systems created by ultrafast laser excitation is a particularly challenging and important aspect of nanoscale heat transfer. It has been observed experimentally that noble metal nanoparticles, illuminated by radiation at the plasmon resonance wavelength, can act as localized heat sources at nanometer-length scales. Achieving biological response by delivering heat via nanoscale heat sources has also been demonstrated. However, an understanding of the thermal transport at these scales and associated phase transformations is lacking. A striking observation made in several laser-heating experiments is that embedded metal nanoparticles heated to extreme temperatures may even melt without an associated boiling of the surrounding fluid. This unusual phase stability is not well understood and designing experiments to understand the physics of this phenomenon is a challenging task. In this thesis, we will resort to molecular dynamics (MD) simulations, which offer a powerful tool to investigate this phenomenon, without assumptions underlying continuum-level model formulations. We present the results from a series of steady state and transient non-equilibrium MD simulations performed on an intensely heated nanoparticle immersed in a model liquid. For small nanoparticles (1-10 nm in diameter) we observe a stable liquid phase near the nanoparticle surface, which can be at a temperature well above the boiling point. Furthermore, we report the existence of a critical nanoparticle size (4 nm in diameter) below which we do not observe formation of vapor even when local fluid temperatures exceed the critical temperature. Instead, we report the existence of a stable fluid region with a density much larger than that of the vapor phase. We explain this stability in terms of the

  20. Phase-field-crystal methodology for modeling of structural transformations.

    PubMed

    Greenwood, Michael; Rottler, Jörg; Provatas, Nikolas

    2011-03-01

    We introduce and characterize free-energy functionals for modeling of solids with different crystallographic symmetries within the phase-field-crystal methodology. The excess free energy responsible for the emergence of periodic phases is inspired by classical density-functional theory, but uses only a minimal description for the modes of the direct correlation function to preserve computational efficiency. We provide a detailed prescription for controlling the crystal structure and introduce parameters for changing temperature and surface energies, so that phase transformations between body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal-close-packed (hcp), and simple-cubic (sc) lattices can be studied. To illustrate the versatility of our free-energy functional, we compute the phase diagram for fcc-bcc-liquid coexistence in the temperature-density plane. We also demonstrate that our model can be extended to include hcp symmetry by dynamically simulating hcp-liquid coexistence from a seeded crystal nucleus. We further quantify the dependence of the elastic constants on the model control parameters in two and three dimensions, showing how the degree of elastic anisotropy can be tuned from the shape of the direct correlation functions.

  1. Fundamental Mechanisms Driving the Amorphous to Crystalline Phase Transformation

    SciTech Connect

    Reed, B W; Browning, N D; Santala, M K; LaGrange, T; Gilmer, G H; Masiel, D J; Campbell, G H; Raoux, S; Topuria, T; Meister, S; Cui, Y

    2011-01-04

    -stabilized metastable rock salt structure. Each transformation takes {approx}10-100 ns, and the cycle can be driven repeatedly a very large number of times with a nanosecond laser such as the DTEM's sample drive laser. These materials are widely used in optical storage devices such as rewritable CDs and DVDs, and they are also applied in a novel solid state memory technology - phase change memory (PCM). PCM has the potential to produce nonvolatile memory systems with high speed, extreme density, and very low power requirements. For PCM applications several materials properties are of great importance: the resistivities of both phases, the crystallization temperature, the melting point, the crystallization speed, reversibility (number of phase-transformation cycles without degradation) and stability against crystallization at elevated temperature. For a viable technology, all these properties need to have good scaling behavior, as dimensions of the memory cells will shrink with every generation. In this LDRD project, we used the unique single-shot nanosecond in situ experimentation capabilities of the DTEM to watch these transformations in GST on the time and length scales most relevant for device applications. Interpretation of the results was performed in conjunction with atomistic and finite-element computations. Samples were provided by collaborators at IBM and Stanford University. We observed, and measured the kinetics of, the amorphous-crystalline and melting-solidification transitions in uniform thin-film samples. Above a certain threshold, the crystal nucleation rate was found to be enormously high (with many nuclei appearing per cubic {micro}m even after nanosecond-scale incubation times), in agreement with atomistic simulation and consistent with an extremely low nucleation barrier. We developed data reduction techniques based on principal component analysis (PCA), revealing the complex, multi-dimensional evolution of the material while suppressing noise and irrelevant

  2. Laser induced structural phase transformation of cobalt oxides nanostructures.

    PubMed

    Ravindra, A V; Behera, B C; Padhan, P

    2014-07-01

    Face-centered-cubic (fcc) and hexagonal-close-packed (hcp) phases of cobalt monoxide (CoO) nanostructures are prepared using thermolysis route at the same reaction temperature 296 degrees C with synthetic approach conditions. These nanostructures show mixture of nearly spherical and nanoflake morphologies. The structural phases of these nanostructures transform to spinel-Co3O4 by application of heat or Raman excitation laser beam power. The absorbance spectra of fcc and hcp-CoO and Co3O4 nanostructures yield significantly higher values of band gap which can be explained by electron confinement. Such results provide new opportunities for optimizing and enhancing the properties and performance of cobalt oxide nanomaterial.

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

    SciTech Connect

    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} {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.

  4. 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.

  5. Phase and structural transformations in magnetorheological suspensions

    NASA Astrophysics Data System (ADS)

    Iskakova, L. Yu.; Romanchuk, A. P.; Zubarev, A. Yu.

    2006-07-01

    Particle condensation in magnetorheological suspensions (MRS) under external magnetic field is studied theoretically. It is shown that the bulk condensation of particles into dense phases is preceded by the formation of fairly long chain aggregates. Phase transition occurs as a condensation of such chains due to their magnetic interaction. In thin layers of MRS, placed into the normal magnetic field, scenario of the phase transition differs essentially from that in infinite volumes of these systems. Equilibrium state of the system after the phase transition corresponds to the formation of ensemble of discrete domains of the dense phase rather than to separation into two massive phases as it takes place in infinite media.

  6. Ultrasonic velocity measurements during phase transformations in steels using laser ultrasonics

    NASA Astrophysics Data System (ADS)

    Dubois, Marc; Moreau, André; Bussière, Jean F.

    2001-06-01

    This article presents accurate laser-ultrasonic measurements of longitudinal velocity in the 500-1000 °C temperature range in carbon steel samples for different conditions known to affect phase transformations such as cooling rate, carbon concentration, and rolling. Measurements were performed during continuous heating and cooling at rates varying between 0.1 and 20 °C/s. Carbon concentrations ranged from 0.0% to 0.72%. Hot-rolled and cold-rolled samples were measured. For the hot-rolled samples, a reproducible hysteresis was observed in the dependence of the ultrasonic velocity versus temperature of samples having a significant carbon concentration. This hysteresis is attributed to the combined effects of the phase transformation and of the ferromagnetic-paramagnetic transition. In particular, the rate of change of velocity with temperature during heating suddenly diminishes at the Curie temperature, and the velocity behavior during cooling shows clearly the start and end of phase transformations, even allowing discrimination between ferrite and pearlite nucleations. For the cold-rolled samples, significant drops in ultrasonic velocity were observed at the transformation temperatures when the samples were heated for the first time. However, the magnitude of these drops decreased for subsequent heating. These drops are attributed to irreversible crystallographic orientation changes caused by phase transformations. This effect was modeled using the Kurdjumov-Sach transformation relationship. The model calculation agrees well with the experimental data.

  7. TIME-TEMPERATURE-TRANSFORMATION (TTT) DIAGRAMS FOR FUTURE WASTE COMPOSITIONS

    SciTech Connect

    Billings, A.; Edwards, T.

    2010-07-08

    As a part of the Waste Acceptance Product Specifications (WAPS) for Vitrified High-Level Waste Forms defined by the Department of Energy - Office of Environmental Management, the waste form stability must be determined for each of the projected high-level waste (HLW) types at the Savannah River Site (SRS). Specifically, WAPS 1.4.1 requires the glass transition temperature (T{sub g}) to be defined and time-temperature-transformation (TTT) diagrams to be developed. The T{sub g} of a glass is an indicator of the approximate temperature where the supercooled liquid converts to a solid on cooling or conversely, where the solid begins to behave as a viscoelastic solid on heating. A TTT diagram identifies the crystalline phases that can form as a function of time and temperature for a given waste type or more specifically, the borosilicate glass waste form. In order to assess durability, the Product Consistency Test (PCT) was used and the durability results compared to the Environmental Assessment (EA) glass. The measurement of glass transition temperature and the development of TTT diagrams have already been performed for the seven Defense Waste Processing Facility (DWPF) projected compositions as defined in the Waste Form Compliance Plan (WCP) and in SRNL-STI-2009-00025. Additional phase transformation information exists for other projected compositions, but overall these compositions did not cover composition regions estimated for future waste processing. To develop TTT diagrams for future waste types, the Savannah River National Laboratory (SRNL) fabricated two caches of glass from reagent grade oxides to simulate glass compositions which would be likely processed with and without Al dissolution. These were used for glass transition temperature measurement and TTT diagram development. The glass transition temperatures of both glasses were measured using differential scanning calorimetry (DSC) and were recorded to be 448 C and 452 C. Using the previous TTT diagrams as

  8. Atomic structure and pressure-induced phase transformations in a phase-change alloy

    NASA Astrophysics Data System (ADS)

    Xu, Ming

    Phase-change materials exist in at least two phases under the ambient condition. One is the amorphous state and another is crystalline phase. These two phases have vastly different physical properties, such as electrical conductivity, optical reflectivity, mass density, thermal conductivity, etc. The distinct physical properties and the fast transformation between amorphous and crystalline phases render these materials the ability to store information. For example, the DVD and the Blue-ray discs take advantage of the optical reflectivity contrast, and the newly developed solid-state memories make use of the large conductivity difference. In addition, both the amorphous and crystalline phases in phase-change memories (PCMs) are very stable at room temperature, and they are easy to be scaled up in the production of devices with large storage density. All these features make phase-change materials the ideal candidates for the next-generation memories. Despite of the fast development of these new memory materials in industry, many fundamental physics problems underlying these interesting materials are still not fully resolved. This thesis is aiming at solving some of the key issues in phase-change materials. Most of phase-change materials are composed of Ge-Sb-Te constituents. Among all these Ge-Sb-Te based materials, Ge2Sb2Te5 (GST) has the best performance and has been frequently studied as a prototypical phase-change material. The first and foremost issue is the structure of the two functioning phases. In this thesis, we investigate the unique atomic structure and bonding nature of amorphous GST (a-GST) and crystalline GST ( c-GST), using ab initio tools and X-ray diffraction (XRD) methods. Their local structures and bonding scenarios are then analyzed using electronic structure calculations. In order to gain insight into the fast phase transformation mechanism, we also carried out a series of high-pressure experiments on GST. Several new polymorphs and their

  9. Internal friction associated with the structural phase transformations in Ni-Mn-Ga alloys

    SciTech Connect

    Cesari, E.; Pons, J.; Segui, C.; Chernenko, V.A.; Kokorin, V.V.

    1997-03-01

    A double peak in the temperature dependence of internal friction (IF) and elastic modulus in some off-stoichiometric Ni{sub 2}MnGa shape memory alloys has been observed both on cooling and heating between the parent and martensite phases. Transmission electron microscopy (TEM) observations have shown that these anomalies are related to structural transformations from the parent cubic phase (P) to an intermediate cubic modulated phase (I) and from the I phase to the martensitic one (M). As for the IF results, the I to M transformation has the characteristics of a first-order phase transition, whereas the P to I transformation shows some distinctive features, such as no temperature-rate dependence.

  10. In-situ phase mapping and direct observations of phase transformations during arc welding of 1045 steel

    NASA Astrophysics Data System (ADS)

    Elmer, J. W.; Palmer, T. A.

    2006-07-01

    In-situ spatially resolved X-ray diffraction (SRXRD) experiments were performed during gas tung-sten arc (GTA) welding of AISI 1045 C-Mn steel. Ferrite (α) and austenite (γ) phases were identified and quantified in the weld heat-affected zone (HAZ) from the real time SRXRD data. The results were compiled with weld temperatures calculated using a coupled thermal fluids model to create a phase map of the HAZ. Kinetics of the α → γ transformation during weld heating and the reverse γ → α transformation during weld cooling were extracted from the map. Superheating as high as 250 °C above the A3 temperature was observed for the α → γ phase transformation to reach completion at locations near the fusion zone (FZ) boundary. The SRXRD experiments revealed that the newly created γ phase exists with two distinct lattice parameters, resulting from the inhomogeneous distribution of carbon and manganese in the starting pearlitic/ferritic microstructure. During cooling, the reverse γ → α phase transformation was shown to depend on the HAZ location. In the fine-grained region of the HAZ, the γ → α transformation begins near the A3 temperature and ends near the A1 temperature. In this region, where the cooling rates are below 40 °C/s, the transformation occurs by nucleation and growth of pearlite. In the coarse-grained region of the HAZ, the γ → α transformation requires 200 °C of undercooling for completion. This high degree of undercooling is caused by the large grains coupled with cooling rates in excess of 50 °C/s that result in a bainitic transformation mechanism.

  11. Ferroelectric Phase Transformations for Energy Conversion and Storage Applications

    NASA Astrophysics Data System (ADS)

    Jo, Hwan Ryul

    Ferroelectric materials possess a spontaneous polarization and actively respond to external mechanical, electrical, and thermal loads. Due to their coupled behavior, ferroelectric materials are used in products such as sensors, actuators, detectors, and transducers. However, most current applications rely on low-energy conversion that involves low magnitude fields. They utilize the low-field linear properties of ferroelectric materials (piezoelectric, pyroelectric) and do not take full advantage of the large-field nonlinear behavior (irreversible domain wall motion, phase transformations) that can occur in ferroelectric materials. When external fields exceed a certain critical level, a structural transformation of the crystal can occur. These phase transformations are accompanied by a much larger response than the linear piezoelectric and pyroelectric responses, by as much as a multiple of ten times in the magnitude. This makes the non-linear behavior in ferroelectric materials promising for energy harvesting and energy storage technologies which will benefit from large-energy conversion. Yet, the ferroelectric phase transformation behavior under large external fields have been less studied and only a few studies have been directed at utilizing this large material response in applications. This dissertation addresses the development ferroelectric phase transformation-based applications, with particular focus on the materials. Development of the ferroelectric phase transformation-based applications was approached in several steps. First, the phase transformation behavior was fully characterized and understood by measuring the phase transformation responses under mechanical, electrical, thermal, and combined loads. Once the behavior was well characterized, systems level applications were addressed. This required assessing the effect of the phase transformation behavior on system performance. The performance of ferroelectric devices is strongly dependent on material

  12. Low Temperature Phase Instability of the Gamma Phase in SnIn Alloys

    SciTech Connect

    Chu, S; Yanar, C; Schwartz, A J; Massalski, T B; Laughlin, D E

    2005-08-19

    The Sn-rich side of the Sn-In phase diagram has been investigated at temperatures ranging from 77 to 500 K by using X-ray diffraction, thermal analysis, and magnetization measurements. It is confirmed that the {beta}-Sn(In)-phase can remain as a metastable phase down to 77 K within the composition range of 86.3-94 at% of Sn. An isothermal displacive (martensitic) transition of the {gamma} phase to the metastable {beta} phase is suggested as the mechanism of the transformation.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

    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 transformation 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. A temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature.

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

    PubMed

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

    2016-10-21

    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 transformation 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. A temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature.

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

    SciTech Connect

    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 transformation 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.

  16. Stress-Induced Cubic-to-Hexagonal Phase Transformation in Perovskite Nanothin Films.

    PubMed

    Cao, Shi-Gu; Li, Yunsong; Wu, Hong-Hui; Wang, Jie; Huang, Baoling; Zhang, Tong-Yi

    2017-08-09

    The strong coupling between crystal structure and mechanical deformation can stabilize low-symmetry phases from high-symmetry phases or induce novel phase transformation in oxide thin films. Stress-induced structural phase transformation in oxide thin films has drawn more and more attention due to its significant influence on the functionalities of the materials. Here, we discovered experimentally a novel stress-induced cubic-to-hexagonal phase transformation in the perovskite nanothin films of barium titanate (BaTiO3) with a special thermomechanical treatment (TMT), where BaTiO3 nanothin films under various stresses are annealed at temperature of 575 °C. Both high-resolution transmission electron microscopy and Raman spectroscopy show a higher density of hexagonal phase in the perovskite thin film under higher tensile stress. Both X-ray photoelectron spectroscopy and electron energy loss spectroscopy does not detect any change in the valence state of Ti atoms, thereby excluding the mechanism of oxygen vacancy induced cubic-to-hexagonal (c-to-h) phase transformation. First-principles calculations show that the c-to-h phase transformation can be completed by lattice shear at elevated temperature, which is consistent with the experimental observation. The applied bending plus the residual tensile stress produces shear stress in the nanothin film. The thermal energy at the elevated temperature assists the shear stress to overcome the energy barriers during the c-to-h phase transformation. The stress-induced phase transformation in perovskite nanothin films with TMT provides materials scientists and engineers a novel approach to tailor nano/microstructures and properties of ferroelectric materials.

  17. Effect of hydrogen on the structure of quenched orthorhombic titanium aluminide-based alloy and phase transformations during subsequent heating

    NASA Astrophysics Data System (ADS)

    Khadzhieva, O. G.; Illarionov, A. G.; Popov, A. A.; Grib, S. V.

    2013-06-01

    The effect of hydrogen on structure formation and changes in the volume fractions of phases in an alloy based on orthorhombic titanium aluminide ( O phase) alloy upon its quenching is studied. X-ray diffraction analysis is used to determine the lattice parameters of phases. It has been shown that hydrogen is dissolved mainly in the β0 phase. Differential thermal analysis is used to determine stages and temperature ranges of phase transformations during heating; it was found that introduced hydrogen shifts the β0 → O and reverse O → β0 transformations into the low-temperature range; the enthalpies of transformation are calculated.

  18. Anomalous plasticity in defect-mediated phase transformations

    NASA Astrophysics Data System (ADS)

    Ghimire, Punam; Ravelo, R.; Germann, T. C.

    2014-03-01

    Large-scale molecular dynamics simulations of shocked wave propagation in metallic single crystals exhibit high elastic limits and are ideally suited for investigating the role defect nucleation and multiplication play on the kinetics of phase transformations. Here we report on the morphology and kinetics of shocked-induced phase transformations in Aluminum single crystals. The atomic interactions were modeled utilizing various embedded atom method (EAM) models of Aluminum, with most models exhibiting an artificial fcc -->bcc phase transformation in the 25-30 GPa range. For cases where plastic deformation precedes the phase transformation, anomalous defect structures atypical of plastic deformation in bcc lattices nucleate early on but anneal out with time. In all cases, the defect-mediated phase transitions proceed at faster rates than defect-free ones. Part of this work supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-12-1-0476.

  19. Hydrogen-Induced Phase Transformations: a Base for a New Sphere of the Science of Metals (an Analytical Review)

    NASA Astrophysics Data System (ADS)

    Goltsov, V. A.

    2017-03-01

    Data on the phase transformations induced in metals by hydrogen are generalized and analyzed. It is suggested to classify hydrogen-induced phase transformations on the basis of allowance for the temperature dependence of the parameters of diffusion of substitutional and interstitial (hydrogen) atoms like in the classical science of metals. A pioneer classification of hydrogen-induced phase transformations by this method is developed.

  20. Conductor requirements for high-temperature superconducting utility power transformers

    SciTech Connect

    Pleva, E. F.; Mehrotra, V.; Schwenterly, S W

    2010-01-01

    High-temperature superconducting (HTS) coated conductors in utility power transformers must satisfy a set of operating requirements that are driven by two major considerations-HTS transformers must be economically competitive with conventional units, and the conductor must be robust enough to be used in a commercial manufacturing environment. The transformer design and manufacturing process will be described in order to highlight the various requirements that it imposes on the HTS conductor. Spreadsheet estimates of HTS transformer costs allow estimates of the conductor cost required for an HTS transformer to be competitive with a similarly performing conventional unit.

  1. Non-gauge phase transformations in quantum transition amplitudes

    NASA Technical Reports Server (NTRS)

    Reiss, H. R.

    1993-01-01

    The prescription for introducing a gauge transformation into a quantum transition amplitude, nominally well known, contains an ambiguous feature. It is presumed by some authors that an appropriate transformation of the phase of a wave function will generate the associated gauge transformation. It is shown that this is a necessary but not sufficient step. Examples from the literature are cited to show the consequences of the failure of this procedure. One must distinguish between true gauge transformations and unitary transformations within a fixed gauge.

  2. Zig-zag twins and helical phase transformations.

    PubMed

    Ganor, Yaniv; Dumitrică, Traian; Feng, Fan; James, Richard D

    2016-04-28

    We demonstrate the large bending deformation induced by an array of permanent magnets (applied field ∼0.02 T) designed to minimize poles in the bent state of the crystal. Planar cantilevers of NiMnGa (5M modulated martensite) ferromagnetic shape memory alloy deform into an arched shape according to theory, with a zig-zag microstructure that complies with the kinematic and magnetic compatibility between adjacent twin variants. A general theory of bent and twisted states is given, applicable to both twinning and austenite/martensite transformations. Some of these configurations achieve order-of-magnitude amplification of rotation and axial strain. We investigate also atomistic analogues of these bent and twisted configurations with perfect interfaces between phases. These mechanisms of large deformation, induced by small magnetic fields or temperature changes, have potential application to the development of new actuation technologies for micro-robotic systems.

  3. White Layer Formation Due to Phase Transformation to Orthogonal machine of AISI 1045 Annealed Steel

    SciTech Connect

    Han, Sangil; Melkote, Shreyes N; Haluska, Dr. Michael S; Watkins, Thomas R

    2008-01-01

    It is commonly believed that the white layer formed during machining of steels is caused primarily by a thermally induced phase transformation resulting from rapid heating and quenching. As a result, it is often assumed that if the temperature at the tool flank-workpiece interface exceeds the nominal phase transformation temperature for the steel, a white layer forms. However, no attempt has been made to actually measure the temperatures produced at the tool flank-workpiece interface and correlate it with microstructural evidence of phase transformation. This paper aims to address these limitations through suitably designed experiments and analysis. Orthogonal machining tests were performed on AISI 1045 annealed steel at different cutting speeds and tool flank wear. During machining, temperature measurements at the tool flank-workpiece interface were made using an exposed thermocouple technique. Metallographic studies of the machined sub-surface and X-ray diffraction (XRD) measurements were performed to determine the presence and depth of white layer, and the presence of the retained austenite phase in the machined surface layer, respectively. Analysis of the data shows that the white layer can form due to phase transformation at temperatures below the nominal austenitization temperature of the steel. Possible causes of this result are presented.

  4. Indentation-induced structural phase transformations of semiconductor materials and applications

    NASA Astrophysics Data System (ADS)

    Khayyat, Maha; Sosa, Norma; Chaudhri, M. Munawar; Cavendish laboratory, University of Cambridge Team; T. J. Watson Research Center, IBM Collaboration

    During hardness indentation materials are subjected to highly localized pressures. These pressures may cause a complete change of the crystal structure of the material within the indented zone. Such structural phase transformations were observed within Vickers indentations made at room temperature in single crystals and amorphous films of Si and Ge. However, when indentations were made at 77 K in Si and Ge, no phase transitions were observed in either. Measurements were also taken from indentations made in silicon single crystals at different temperatures namely 263, 243, 235 and 206 K, and they showed a strong correlation of phase transformation with temperature. It was suggested that during room temperature indentations there is a significant temperature rise approximately to 760 K, which may assist phase transformation. Raman spectroscopy was used as an ex-situ tool monitoring phase transformations in semiconductor materials. In-situ electrical characterizations of indentation-induced metallization in single crystals of silicon were performed using two- and four-contact measurements. The previous work has led to a technique relates to semiconductor device manufacturing, including solar cells, which is a method for controlling the removal of a surface layer from a base substrate utilizing low-temperature. KACST is acknowledged for support.

  5. Phase transformation behaviour and bending property of twisted nickel-titanium endodontic instruments.

    PubMed

    Hou, Xm; Yahata, Y; Hayashi, Y; Ebihara, A; Hanawa, T; Suda, H

    2011-03-01

    To investigate the relationship between phase transformation behaviour and bending property of nickel-titanium endodontic instruments manufactured by a twisting process. The phase transformation behaviour and bending property of Twisted Files (TF; SybronEndo, Orange, CA, USA) and K3 (SybronEndo) with.06 taper and size 30 tip were investigated. K3 was used as control group. Phase transformation behaviour was estimated by differential scanning calorimetry (DSC). Transformation temperatures were calculated from the DSC curve. Bending load of the instruments was measured by cantilever-bending test at 37°C. Data were analysed by Student's t-test and Mann-Whitney U-test. The phase transformation temperatures of TF were significantly higher (P<0.05) than those of K3. The bending load values were significantly lower for TF than that of K3 (P<0.05), both in the elastic and super-elastic ranges. The new method of manufacturing NiTi instruments by twisting coupled with heat treatment might contribute to the increased phase transformation temperatures and superior flexibility. © 2010 International Endodontic Journal.

  6. Effects of phase transformation of steam-water relative permeabilities

    SciTech Connect

    Verma, A.K.

    1986-03-01

    A combined theoretical and experimental study of steam-water relative permeabilities (RPs) was carried out. First, an experimental study of two-phase concurrent flow of steam and water was conducted and a set of RP curves was obtained. These curves were compared with semi-empirical and experimental results obtained by other investigators for two-phase, two-component flow (oil/gas; gas/water; gas/oil). It was found that while the wetting phase RPs were in good agreement, RPs for the steam phase were considerably higher than the non-wetting phase RPs in two-component systems. This enhancement of steam RP is attributed to phase transformation effects at the pore level in flow channels. The effects of phase transformation were studied theoretically. This study indicates that there are two separate mechanisms by which phase transformation affects RP curves: (1) Phase transformation is converging-diverging flow channels can cause an enhancement of steam phase RP. In a channel dominated by steam a fraction of the flowing steam condenses upstream from the constriction, depositing its latent heat of condensation. This heat is conducted through the solid grains around the pore throat, and evaporation takes place downstream from it. Therefore, for a given bulk flow quality; a smaller fraction of steam actually flows through the throat segments. This pore-level effect manifests itself as relative permeability enhancement on a macroscopic level; and (2) phase transformation along the interface of a stagnant phase and the phase flowing around it controls the irreducible phase saturation. Therefore, the irreducible phase saturation in steam-water flow will depend, among other factors, on the boundary conditions of the flow.

  7. Reflectance Changes during Shock-induced Phase Transformations in Metals

    SciTech Connect

    Stevens, G. D.; Turley, W. D.; Veeser, L. R.; Jensen, B. J.; Rigg, P. A.

    2010-06-01

    In performing shock wave experiments to study the characteristics of metals at high pressures, wave profiles (i.e., velocity measurements of the surface of the sample) are an established and useful way to study phase transformations. For example, a sudden change in the velocity or its slope can occur when the phase transformation induces a large volume change leading to a change in particle velocity. Allowing the shock to release into a transparent window that is in contact with the sample surface allows the study of conditions away from the shock Hugoniot. However, in cases where the wave profile is not definitive an additional phase-transformation diagnostic would be useful. Changes in the electronic structure of the atoms in the crystal offer opportunities to develop new phase-change diagnostics. We have studied optical reflectance changes for several phase transformations to see whether reflectance changes might be a generally applicable phase-transformation diagnostic. Shocks were produced by direct contact with explosives or with impacts from guns. Optical wavelengths for the reflectance measurements ranged from 355 to 700 nm. We studied samples of tin, iron, gallium, and cerium as each passed through a phase transformation during shock loading and, if observable, a reversion upon unloading. For solid-solid phase changes in tin and iron we saw small changes in the surface scattering characteristics, perhaps from voids or rough areas frozen into the surface of the sample as it transformed to a new crystal structure. For melt in gallium and cerium we saw changes in the wavelength dependence of the reflectance, and we surmise that these changes may result from changes in the crystal electronic structure. It appears that reflectance measurements can be a significant part of a larger suite of diagnostics to search for difficult-to-detect phase transformations.

  8. Stress-Induced Phase Transformations in Shape-Memory Polycrystals

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Kaushik; Schlömerkemper, Anja

    2010-06-01

    Shape-memory alloys undergo a solid-to-solid phase transformation involving a change of crystal structure. We examine model problems in the scalar setting motivated by the situation when this transformation is induced by the application of stress in a polycrystalline material made of numerous grains of the same crystalline solid with varying orientations. We show that the onset of transformation in a granular polycrystal with homogeneous elasticity is in fact predicted accurately by the so-called Sachs bound based on the ansatz of uniform stress. We also present a simple example where the onset of phase transformation is given by the Sachs bound, and the extent of phase transformation is given by the constant strain Taylor bound. Finally we discuss the stress-strain relations of the general problem using Milton-Serkov bounds.

  9. Influence of Rapid Thermal Ramp Rate on Phase Transformation of Titanium Silicides

    SciTech Connect

    Bailey, Glenn; Hu, Yao, Zhi; Smith, Paul Martin; Tay, Sing Pin; Thakur, Randhir; Yang, Jiting

    1999-05-03

    ULSI technology requires low resistance, stable silicides formed on small geometry lines. Titanium disilicide (TiSiz), which is the most widely used silicide for ULSI applications, exists in two crystallographic phases: the high resistance, metastable C49 phase and the low resistance, stable C54 phase. The major issue with TiSiz is the increasing thermal budget required to transform the C49 phase into the low resistance C54 phase as linewiths decrease below 0.25 pm. Annealing above 900"C to obtain this transformation often results in thermal degradation, so it is desirable to reduce the transformation temperature. The transformation temperature has been shown to be a fi.mction of many factors including microstructure, grain size, and impurities. In this paper we report an investig+ion of rapid thermal silicidation of titanium films (250, 400, and 600 A) on single crystalline silicon at temperatures from 300 to 1000"C. The ramp rates for these experiments are 5, 30, 70, and 200oC/s. The transformation temperature decreases as the ramp rate increases and as the initial film thickness increases. Scanning electron microscopy (SEM) is used to analyze the resultant film microstructure. The ramp rate influence on Ti silicidation is also investigated on polycrystalline Si lines with widths ranging from 0.27 to 3.0 pm.

  10. Design of electromagnetic refractor and phase transformer using coordinate transformation theory.

    PubMed

    Lin, Lan; Wang, Wei; Cui, Jianhua; Du, Chunlei; Luo, Xiangang

    2008-05-12

    We designed an electromagnetic refractor and a phase transformer using form-invariant coordinate transformation of Maxwell's equations. The propagation direction of electromagnetic energy in these devices can be modulated as desired. Unlike the conventional dielectric refractor, electromagnetic fields at our refraction boundary do not conform to the Snell's law in isotropic materials and the impedance at this boundary is matched which makes the reflection extremely low; and the transformation of the wave front from cylindrical to plane can be realized in the phase transformer with a slab structure. Two dimensional finite-element simulations were performed to confirm the theoretical results.

  11. Phase transformations and selective growth in YMnO{sub 3} films

    SciTech Connect

    Iliescu, I.; Boudard, M.; Chaix-Pluchery, O.; Rapenne, L.; Roussel, H.

    2014-12-15

    An irreversible phase transformation from amorphous to crystalline orthorhombic YMnO{sub 3} (o-YMO) phase takes place in Y–Mn–O films deposited by pulsed-injection metal organic chemical vapor deposition (MOCVD) on Si(100) substrate. This phase transformation was studied through ex-situ and in-situ thermal annealings and the corresponding structural changes were analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. It was shown that it takes place at an almost constant temperature (∼700 °C) and in a short period of time (∼min) and that the o-YMO phase thus obtained is stable at least up to 900 °C. We also demonstrate that the selective growth of amorphous, orthorhombic or hexagonal films is made possible by adapting the MOCVD temperature according to the temperature stability region of the different phases. - Graphical abstract: Temperature-dependent Raman spectra of an amorphous as-deposited Y–Mn–O film in the temperature range RT – 900 °C. The red spectrum marks the crystallization of the amorphous phase into the o-YMnO{sub 3} phase. - Highlights: • Phase transformations and selective growth in YMnO{sub 3}/Si(100) MOCVD films are studied. • Transformation from amorphous to crystalline orthorhombic YMnO{sub 3} occurs. • It takes place at an almost constant temperature (∼700 °C). • The orthorhombic YMnO{sub 3} phase thus obtained is stable at least up to 900 °C. • A selective growth is made possible by adapting the MOCVD temperature.

  12. Influence of features of the original structure on the phase transformations and properties of steel in heat treatment

    NASA Astrophysics Data System (ADS)

    D'yachenko, S. S.

    1987-10-01

    The original condition of the steel influences not only the kinetics of phase transformations but also the position of the critical points, the degree of recrystallization at a certain temperature in the intercritical temperature range, the composition of the phases formed, and the morphological characteristics of the structures formed. As the result the use of the Fe-C phase diagram for a quantitative description of the process of the α→γ-transformation even at low heating rates is invalid.

  13. Three phase transformer modelling for fast electromagnetic transient studies

    SciTech Connect

    Papadias, B.C.; Hatziargyriou, N.D.; Bakopoulos, J.A.; Prousalidis, J.M. . Electric Energy Systems Lab.)

    1994-04-01

    In this paper the overvoltages produced by switching the primary side of reactor loaded transformers are simulated using the Electromagnetic Transients Program (EMTP). Attention is focused on transformer modeling. Five general three-phase transformer models are used and from the results obtained, and comparisons with field tests positive conclusions concerning the reliability and the accuracy of these models in the study of switching fast electromagnetic transients are drawn.

  14. Characterization of Solid State Phase Transformation in Continuously Heated and Cooled Ferritic Weld Metal

    SciTech Connect

    Narayana, B; Mills, Michael J.; Specht, Eliot D; Santella, Michael L; Babu, Sudarsanam Suresh

    2010-12-01

    Arc welding processes involve cooling rates that vary over a wide range (1-100 K/s). The final microstructire is thus a product of the heating and cooling cycles experienced by the weld in addition to the weld composition. It has been shown that the first phase to form under weld cooling conditions may not be that predicted by equilibrium calculations. The partitioning of different interstitial/substitutional alloying elements at high temperatures can dramatically affect the subsequent phase transformations. In order to understand the effect of alloying on phase transformation temperatures and final microstructures time-resolved X-ray diffraction technique has been successfully used for characterization. The work by Jacot and Rappaz on pearlitic steels provided insight into austenitization of hypoeutectic steels using a finite volume model. However there is very little work done on the effect of heating and cooling rates on the phase transformation paths in bainitic/martensitic steels and weld metals. Previous work on a weld with higher aluminum content, deposited with a FCAW-S process indicated that even at aluminum levels where the primary phase to solidify from liquid should be delta ferrite, non-equilibrium austenite was observed. The presence of inhomogeneity in composition of the parent microstructure has been attributed to differences in transformation modes, temperatures and microstructures in dual-phase, TRIP steels and ferritic welds. The objectives of the work included the identification of the stability regions of different phases during heating and cooling, differences in the effect of weld heating and cooling rates on the phase transformation temperatures, and the variation in phase fractions of austenite and ferrite in the two phase regions as a function of temperature. The base composition used for the present work is a Fe-1%Al-2%Mn-1%Ni-0.04%C weld metal. A pseudo-binary phase diagram shows the expected solidification path under equilibrium

  15. Texture Evolution and Phase Transformation in Titanium Investigated by In-Situ Neutron Diffraction

    SciTech Connect

    Ma, Dong; Stoica, Alexandru Dan; An, Ke; Yang, Ling; Bei, Hongbin; Mills, Rebecca A; Skorpenske, Harley David; Wang, Xun-Li

    2011-01-01

    We report in-situ neutron diffraction studies of texture evolution and the (hcp) (bcc) phase transformation in commercially pure cold-drawn titanium upon continuous heating and cooling, complemented by differential scanning calorimetry (DSC) measurements. We show that the recrystallization of the phase at elevated temperature enhanced the preexisting fiber texture, which eventually facilitated the nucleation and growth of the phase favored by the Burgers orientation relationship, i.e., {0001} //{110} . More strikingly, upon completion of the transformation, the {110} texture (or preferred orientation) in was eliminated immediately by the rapid grain growth of intergranular allotriomorphs. This resulted in the loss of the original -texture when Ti was transformed back to from to upon subsequent cooling, distinct from the known texture memory effect for rolling textures in titanium. Our present work provides useful experimental results for understanding the mechanisms of texture evolution and phase transformation in titanium and its alloys and, by and large, low-symmetry alloys such as zirconium.

  16. Macroscopic behavior of a bar undergoing the paraelectric-ferroelectric phase transformation

    NASA Astrophysics Data System (ADS)

    Jiang, Qing

    1993-10-01

    R ECENT WORK of the author has developed a continuum model for the paraelectric—ferroelectric phase transformation. The present paper describes the detailed implications of this model when it has been supplemented with constitutive information pertaining to the phase transformation. An explicit Helmholtz potential has been constructed to characterize a hypothetical crystal capable of undergoing the paraelectric-ferroelectric phase transformation under the influence of applied electric fields or thermomechanical loads. The hysteretic macroscopic behavior associated with the phase transformation, induced by variation of temperature, application of electric fields and mechanical loads, has been studied. Some of the predictions have been compared qualitatively with experimental observations. The entire analysis is carried out within a one-dimensional setting.

  17. Molecular dynamics simulation of the martensitic phase transformation in NiAl alloys.

    PubMed

    Pun, G P Purja; Mishin, Y

    2010-10-06

    Using molecular dynamics simulations with an embedded-atom interatomic potential, we study the effect of chemical composition and uniaxial mechanical stresses on the martensitic phase transformation in Ni-rich NiAl alloys. The martensitic phase has a tetragonal crystal structure and can contain multiple twins arranged in domains and plates. The transformation is reversible and is characterized by a significant temperature hysteresis. The magnitude of the hysteresis depends on the chemical composition and stress. We show that applied compressive and tensile stresses reduce and can even eliminate the hysteresis. Crystalline defects such as free surfaces, dislocations and anti-phase boundaries reduce the martensitic transformation temperature and affect the microstructure of the martensite. Their effect can be explained by heterogeneous nucleation of the new phase in defected regions.

  18. Fully phase encrypted memory using cascaded extended fractional Fourier transform

    NASA Astrophysics Data System (ADS)

    Nishchal, Naveen K.; Joseph, Joby; Singh, Kehar

    2003-11-01

    In this paper, we implement a fully phase encrypted memory system using cascaded extended fractional Fourier transform (FRT). We encrypt and decrypt a two-dimensional image obtained from an amplitude image. The fully phase image to be encrypted is fractional Fourier transformed three times and random phase masks are placed in the two intermediate planes. Performing the FRT three times increases the key size, at an added complexity of one more lens. The encrypted image is holographically recorded in a photorefractive crystal and is then decrypted by generating through phase conjugation, conjugate of encrypted image. The decrypted phase image is converted into an amplitude image by using phase contrast technique. A lithium niobate crystal has been used as a phase contrast filter to reconstruct the phase image, alleviating the need of alignment in the Fourier plane, thereby making the system rugged.

  19. Model of High Temperature Phase Transitions in Metals

    NASA Astrophysics Data System (ADS)

    Filippov, E. S.

    2016-04-01

    On the basis of the assumption of the electron density fluctuation at the band degradation, a calculation parameter (the radius R) of the half-width of the probability distribution over the coordinate R is identified at the level of the maximum electron density fluctuation (at a maximum of the Gaussian function). Based on an analysis of the crystallization process and high polymorphic transformations bcc → fcc, the reasons for the formation of bcc, fcc, hexagonal, and tetragonal structures from the liquid phase, as well as for the high temperature bcc → hcp transition in the solid phase are established using the calculated parameter (the radius R) in the solid and liquid phases.

  20. Typical Phases of Transformative Learning: A Practice-Based Model

    ERIC Educational Resources Information Center

    Nohl, Arnd-Michael

    2015-01-01

    Empirical models of transformative learning offer important insights into the core characteristics of this concept. Whereas previous analyses were limited to specific social groups or topical terrains, this article empirically typifies the phases of transformative learning on the basis of a comparative analysis of various social groups and topical…

  1. Typical Phases of Transformative Learning: A Practice-Based Model

    ERIC Educational Resources Information Center

    Nohl, Arnd-Michael

    2015-01-01

    Empirical models of transformative learning offer important insights into the core characteristics of this concept. Whereas previous analyses were limited to specific social groups or topical terrains, this article empirically typifies the phases of transformative learning on the basis of a comparative analysis of various social groups and topical…

  2. Synthesis of general polarization transformers. A geometric phase approach

    NASA Astrophysics Data System (ADS)

    Bhandari, Rajendra

    1989-07-01

    Using a generalized form of Jordan's formulation of the geometric phase problem it is shown that a single gadget capable of realising an arbitrary element of the polarization transformation group SU (2) can be constructed using two half-wave plates and two quarter-wave plates. For special transformations, simpler, practical gadgets are proposed.

  3. Edge Detection in Digital Images Using Dispersive Phase Stretch Transform

    PubMed Central

    Asghari, Mohammad H.; Jalali, Bahram

    2015-01-01

    We describe a new computational approach to edge detection and its application to biomedical images. Our digital algorithm transforms the image by emulating the propagation of light through a physical medium with specific warped diffractive property. We show that the output phase of the transform reveals transitions in image intensity and can be used for edge detection. PMID:25878656

  4. Phase-transformation-induced hardening in Zn-22Al alloys

    NASA Astrophysics Data System (ADS)

    Zhang, Yangming; Zeng, Xuduo; Yang, Lijing; Sun, Keqing; Song, Zhenlun

    2013-06-01

    A phase-transformation-induced hardening effect is reported in Zn-22Al (Al: 22 wt.%) alloys. The Zn-22Al specimens were held at 300 °C for 10 h and then quenched in water. A hardening effect took place in subsequent artificial aging at 100-200 °C, which was accompanied by a phase decomposition of a soft α 2 phase and a grain coarsening. The phase-transformation-induced hardening affects the hardness more than the grain-coarsening-induced softening, which leads to the age-hardening phenomenon.

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

    PubMed

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

    2017-08-31

    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.

  6. Disordered bcc γ-phase to δ-phase transformation in Zr-rich U-Zr alloy

    NASA Astrophysics Data System (ADS)

    Basak, C. B.; Neogy, S.; Srivastava, D.; Dey, G. K.; Banerjee, S.

    2011-08-01

    The transformation mechanism of hexagonal delta phase from the disordered bcc gamma phase has not been reported before in the Zr-rich U-Zr alloy system. With the help of X-ray diffraction, transmission electron microscopy (TEM) and high-resolution TEM analyses it was shown that the gamma to delta conversion takes place by the lattice collapse mechanism of omega transformation. It was also ascertained that a higher aging temperature or time promotes the growth of all four variants of the delta phase within a parent gamma grain. In addition, ab initio electronic structure calculations showed that the bcc to hexagonal transformation, involving partial ordering of the parent bcc phase followed by (111) plane collapse, is energetically favorable.

  7. 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.

  8. 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.

  9. Phase transformations in SrAl2Si2O8 glass

    NASA Technical Reports Server (NTRS)

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

    1992-01-01

    Bulk glass of SrAl2Si2O8 composition crystallized at temperatures below 1000 C into hexacelsian, a hexagonal phase which undergoes a reversible, rapid transformation to an orthorhombic phase at 758 C, and at higher temperatures crystallized as celsian, a monoclinic phase. The glass transition temperature and crystallization onset temperature were determined to be 883 C and 1086 C, respectively, from DSC at a heating rate of 20 C/min. Thermal expansion of the various phases and density and bend strengths of cold isostatically pressed glass powder bars, sintered at various temperatures, were measured. The kinetics of the hexacelsian-to-celsian transformation for SrAl2Si2O8 were studied. Hexacelsian flakes were isothermally heat treated at temperatures from 1025-1200 C for various times. Avrami plots were determined by quantitatively measuring the amount of monoclinic celsian formed at various times using x ray diffraction. The Avrami constant was determined to be 1.1, suggesting a diffusionless, one dimensional transformation mechanism. The activation energy was determined from an Arrhenius plot of 1n k vs. 1/T to be 125 kilocal/mole. This value is consistent with a mechanism which transforms the layered hexacelsian structure to a three dimensional framework celsian structure and involves the breaking of Si-O bonds.

  10. Positive phase space transformation incompatible with classical physics.

    PubMed

    Son, Wonmin; Kofler, Johannes; Kim, M S; Vedral, Vlatko; Brukner, Caslav

    2009-03-20

    Bell conjectured that a positive Wigner function does not allow violation of the inequalities imposed by local hidden variable theories. A requirement for this conjecture is "when phase space measurements are performed." We introduce the theory-independent concept of "operationally local transformations" which refers to the change of the switch on a local measurement apparatus. We show that two separated parties, performing only phase space measurements on a composite quantum system with a positive Wigner function and performing only operationally local transformations that preserve this positivity, can nonetheless violate Bell's inequality. Such operationally local transformations are realized using entangled ancillae.

  11. Pressure-induced phase transformations in cubic Gd[subscript 2]O[subscript 3

    SciTech Connect

    Bai, Ligang; Liu, Jing; Li, Xiaodong; Jiang, Sheng; Xiao, Wansheng; Li, Yanchun; Tang, Lingyun; Zhang, Yufeng; Zhang, Dechun

    2009-10-21

    High-pressure transformation and compression behavior of Gd{sub 2}O{sub 3} were investigated using synchrotron radiation x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. The structural transformation from a cubic to a monoclinic phase occurred during the sample precompression process. Phase transitions from both the cubic and the monoclinic polymorphs to a hexagonal structure were observed. The hexagonal phase was stable up to the highest pressure in this study and was not quenchable and transformed to a monoclinic phase after pressure release. The bulk moduli of Gd{sub 2}O{sub 3} for the cubic, monoclinic, and hexagonal phases were obtained by fitting the compression data to the Birch-Murnaghan equation of state. Moreover, an anomaly of the hexagonal type Gd{sub 2}O{sub 3} was observed.

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

    PubMed

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

    2014-12-21

    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.

  13. 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

  14. Complexion-mediated martensitic phase transformation in Titanium

    NASA Astrophysics Data System (ADS)

    Zhang, J.; Tasan, C. C.; Lai, M. J.; Dippel, A.-C.; Raabe, D.

    2017-02-01

    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α'' (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω, hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface.

  15. Complexion-mediated martensitic phase transformation in Titanium.

    PubMed

    Zhang, J; Tasan, C C; Lai, M J; Dippel, A-C; Raabe, D

    2017-02-01

    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω, hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface.

  16. Complexion-mediated martensitic phase transformation in Titanium

    PubMed Central

    Zhang, J.; Tasan, C. C.; Lai, M. J.; Dippel, A. -C.; Raabe, D.

    2017-01-01

    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a–ω, hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a–ω is stable only at the hetero-interface. PMID:28145484

  17. Chemically Induced Phase Transformation in Austenite by Focused Ion Beam

    NASA Astrophysics Data System (ADS)

    Basa, Adina; Thaulow, Christian; Barnoush, Afrooz

    2013-11-01

    A highly stable austenite phase in a super duplex stainless steel was subjected to a combination of different gallium ion doses at different acceleration voltages. It was shown that contrary to what is expected, an austenite to ferrite phase transformation occurred within the focused ion beam (FIB) milled regions. Chemical analysis of the FIB milled region proved that the gallium implantation preceded the FIB milling. High resolution electron backscatter diffraction analysis also showed that the phase transformation was not followed by the typical shear and plastic deformation expected from the martensitic transformation. On the basis of these observations, it was concluded that the change in the chemical composition of the austenite and the local increase in gallium, which is a ferrite stabilizer, results in the local selective transformation of austenite to ferrite.

  18. Wearless scratch on NiTi shape memory alloy due to phase transformational shakedown

    NASA Astrophysics Data System (ADS)

    Feng, Xi-Qiao; Qian, Linmao; Yan, Wenyi; Sun, Qingping

    2008-03-01

    Cyclic microscratch tests were performed to examine the scratching behavior of NiTi shape memory alloy. It shows a superior wear resistance within the temperature range of 22-120°C, but the corresponding physical mechanisms are different at low and high temperatures. We introduced the concept of phase transformational shakedown to interpret the wear-resistant behavior. At room temperature, a scratch groove may be caused by repeated scratching, but its depth stops increasing after a certain number of scratching cycles once the phase transformational shakedown state has been achieved. The groove will be self-healed upon heating as a result of the shape memory effect. At 60 and 120°C, however, no evident scratch groove is observed under the same load due to the pseudoelastic effect and the increase in the phase transition stress with temperature.

  19. Nucleation, kinetics and morphology of displacive phase transformations in iron

    NASA Astrophysics Data System (ADS)

    Suiker, A. S. J.; Thijsse, B. J.

    2013-11-01

    An extensive, systematic molecular dynamics (MD) study is performed for analysing the nucleation, kinetics and morphology characteristics of thermally-induced, displacive phase transformations from face-centred cubic (fcc) to body-centred cubic (bcc) iron. At the atomic level these transformation characteristics are influenced by a number of factors, including (i) the appearance of free surfaces, (ii) the initial presence of fcc-bcc grain boundaries, (iii) the existence of point defects (i.e., atomic vacancies) near a grain boundary, (iv) the initial thermal velocities of the atoms, and (v) the specific interatomic potential used. Other MD studies that capture the overall transformation behaviour of iron well have often underestimated or ignored the influence by these factors on the transformation response, with the risk of putting the accuracy, generality and physical explanation of the MD results on loose grounds. The present research illustrates the relative contribution of each of the above factors by means of a detailed comparison study for three different interatomic potentials. The accuracy of the interatomic potentials is established by validating for the fcc and bcc phases the calculated elastic moduli, cohesive energy, vacancy formation energy and interfacial energy against experimental and ab initio data reported in the literature. The importance of calibrating material data of both the stable bcc phase and the metastable fcc phase - instead of the stable bcc phase only - is demonstrated. The numerical results call for general caution when interpreting phenomena that start close to instability points and therefore are sensitive to small disturbances; a large spread in the overall transformation time is found under different initial thermal velocities, interfacial lattice incoherence, boundary conditions (free vs. periodic), and interatomic potentials, where for completely transformed atomic systems the discrepancy between the maximum and minimum

  20. Effect of uni- and biaxial strain on phase transformations in Fe thin films

    NASA Astrophysics Data System (ADS)

    Sak-Saracino, Emilia; Urbassek, Herbert M.

    2016-01-01

    Using molecular-dynamics simulation, we study the phase transformations in Fe thin films induced by uni- and biaxial strain. Both the austenitic transformation of a body-centered cubic (bcc) film at the equilibrium temperature of the face-centered cubic (fcc)-bcc transformation and the martensitic transformation of an undercooled fcc film are studied. We demonstrate that different strain states (uni- or biaxial) induce different nucleation kinetics of the new phase and hence different microstructures evolve. For the case of the austenitic transformation, the direction of the applied strain selects the orientation of the nucleated grains of the new phase; the application of biaxial strain leads to a symmetric twinned structure. For the martensitic transformation, the influence of the strain state is even more pronounced, in that it can either inhibit the transformation, induce the homogeneous nucleation of a fine-dispersed array of the new phase resulting in a single-crystalline final state, or lead to the more conventional mechanism of heterogeneous nucleation of grains at the free surfaces, which grow and result in a poly-crystalline microstructure of the transformed material.

  1. Phase transformation in conformational polymorphs of nimesulide.

    PubMed

    Sanphui, Palash; Sarma, Bipul; Nangia, Ashwini

    2011-06-01

    Nimesulide is a nonsteroidal anti-inflammatory drug (NSAID) and a COX-2 inhibitor. The native crystal structure of nimesulide (or Form I) has been characterized in the literature by X-ray powder diffraction (XRPD) lines, whereas full three-dimensional coordinates are known for a second polymorph (Form II). A detailed structural characterization and phase stability of nimesulide polymorphs were carried out. Rod-like crystals of Form I (space group Pca2(1); number of symmetry-independent molecules, Z' = 2) were crystallized from EtOH concomitantly with Form II (C2/c, Z' = 1). These conformational polymorphs have different torsion angles at the phenoxy and sulfonamide groups. The crystal structures are stabilized by N-H · · · O hydrogen bonds and C-H · · · O, C-H · · · π interactions. Phase transition from the metastable Form (II) to the stable modification (I) was studied using differential scanning calorimetry, hot-stage microscopy, solid-state grinding, solvent-drop grinding, and slurry crystallization. The phase transition was monitored by infrared, Raman, and ss-nuclear magnetic resonance spectroscopy; and XRPD and single-crystal X-ray diffraction. The stable polymorph I was obtained in excess during solution crystallization, grinding, and slurry methods. Intrinsic dissolution and equilibrium solubility experiments showed that the metastable Form II dissolves much faster than the stable Form I. Copyright © 2011 Wiley-Liss, Inc.

  2. High temperature growth of Ag phases on Ge(111)

    SciTech Connect

    Mullet, Cory H.; Chiang, Shirley

    2013-03-15

    The growth of the (3 Multiplication-Sign 1) and ({radical}3 Multiplication-Sign {radical}3)R30 Degree-Sign phases of Ag on Ge(111) on substrates at temperatures from 540 to 660 Degree-Sign C is characterized with low energy electron microscopy (LEEM) and low energy electron diffraction (LEED). From 540 Degree-Sign C to the Ag desorption temperature of 575 Degree-Sign C, LEEM images show that growth of the (3 Multiplication-Sign 1) phase begins at step edges. Upon completion of the (3 Multiplication-Sign 1) phase, the ({radical}3 Multiplication-Sign {radical}3)R30 Degree-Sign phase is observed with a dendritic growth morphology that is not much affected by steps. For sufficiently high deposition rates, Ag accumulates on the sample above the desorption temperature. From 575 to 640 Degree-Sign C, the growth proceeded in a manner similar to that at lower temperatures, with growth of the (3 Multiplication-Sign 1) phase to completion, followed by growth of the ({radical}3 Multiplication-Sign {radical}3)R30 Degree-Sign phase. Increasing the substrate temperature to 660 Degree-Sign C resulted in only (3 Multiplication-Sign 1) growth. In addition, for samples with Ag coverage less than 0.375ML, LEEM and LEED images were used to follow a reversible phase transformation near 575 Degree-Sign C, between a mixed high coverage phase of [(4 Multiplication-Sign 4) + (3 Multiplication-Sign 1)] and the high temperature, lower coverage (3 Multiplication-Sign 1) phase.

  3. Kinetics of Propagating Phase Transformation in Compressed Bismuth

    SciTech Connect

    Bastea, M; Bastea, S; Emig, J; Springer, P; Reisman, D

    2004-08-18

    The authors observed dynamically driven phase transitions in isentropically compressed bismuth. By changing the stress loading conditions they explored two distinct cases one in which the experimental signature of the phase transformation corresponds to phase-boundary crossings initiated at both sample interfaces, and another in which the experimental trace is due to a single advancing transformation front in the bulk of the material. They introduce a coupled kinetics-hydrodynamics model that for this second case enables them, under suitable simplifying assumptions, to directly extract characteristic transition times from the experimental measurements.

  4. Efficient multiscale phase unwrapping methodology with modulo wavelet transform.

    PubMed

    Blinder, David; Ottevaere, Heidi; Munteanu, Adrian; Schelkens, Peter

    2016-10-03

    Many robust phase unwrapping algorithms are computationally very time-consuming, making them impractical for handling large datasets or real-time applications. In this paper, we propose a generic framework using a novel wavelet transform that can be combined with many types of phase unwrapping algorithms. By inserting reversible modulo operators in the wavelet transform, the number of coefficients that need to be unwrapped is significantly reduced, which results in large computational gains. The algorithm is tested on various types of wrapped phase imagery, reporting speedup factors of up to 500. The source code of the algorithm is publicly available.

  5. Phase retrieval for attacking fractional Fourier transform encryption.

    PubMed

    Kong, Dezhao; Shen, Xueju; Cao, Liangcai; Jin, Guofan

    2017-04-20

    An advanced iterative phase retrieval algorithm is applied to perform a ciphertext-only attack on the fractional Fourier transform-based double random phase encryption system. With the given complex amplitude of ciphertext and definite support of the object image, the original object image can be recovered by estimating the energy of support area in the recovered image. The encryption system can be attacked by analyzing the sensibility of fractional Fourier transform order keys and evaluating the energy of the object image support area. The proposed algorithm can obtain encrypted fractional order and retrieve two random phase keys. Numerical results demonstrate the efficacy of the proposed attacking method.

  6. A model for heterogeneous materials including phase transformations

    NASA Astrophysics Data System (ADS)

    Addessio, F. L.; Clements, B. E.; Williams, T. O.

    2005-04-01

    A model is developed for particulate composites, which includes phase transformations in one or all of the constituents. The model is an extension of the method of cells formalism. Representative simulations for a single-phase, brittle particulate (SiC) embedded in a ductile material (Ti), which undergoes a solid-solid phase transformation, are provided. Also, simulations for a tungsten heavy alloy (WHA) are included. In the WHA analyses a particulate composite, composed of tungsten particles embedded in a tungsten-iron-nickel alloy matrix, is modeled. A solid-liquid phase transformation of the matrix material is included in the WHA numerical calculations. The example problems also demonstrate two approaches for generating free energies for the material constituents. Simulations for volumetric compression, uniaxial strain, biaxial strain, and pure shear are used to demonstrate the versatility of the model.

  7. Low-temperature criticality of martensitic transformations of Cu nanoprecipitates in α-Fe.

    PubMed

    Erhart, Paul; Sadigh, Babak

    2013-07-12

    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.

  8. 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

  9. α-Phase transformation kinetics of U – 8 wt% Mo established by in situ neutron diffraction

    SciTech Connect

    Garlea, Elena; Steiner, M. A.; Calhoun, C. A.; Klein, R. W.; An, K.; Agnew, S. R.

    2016-05-08

    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 °C 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.

  10. α-Phase transformation kinetics of U – 8 wt% Mo established by in situ neutron diffraction

    SciTech Connect

    Garlea, Elena; Steiner, M. A.; Calhoun, C. A.; Klein, R. W.; An, K.; Agnew, S. R.

    2016-05-08

    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 °C 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.

  11. Phase transformation near the classical limit of stability

    SciTech Connect

    Maibaum, Lutz

    2008-11-06

    Successful theories of phase transformation processes include classical nucleation theory (CNT), which envisions a local equilibrium between coexisting phases, and non--equilibrium kinetic cluster theories. Using computer simulations of the magnetization reversal of the Ising model in three different ensembles we make quantitative connections between these physical pictures. We show that the critical nucleus size of CNT is strongly correlated with a dynamical measure of metastability, and that the metastable phase persists to thermodynamic conditions previously thought of as unstable.

  12. High pressure phase transformation in iron under fast compression

    SciTech Connect

    Bastea, M; Bastea, S; Becker, R

    2009-07-07

    We present experimental results on the solid-solid, {alpha} to {epsilon} phase transformation kinetics of iron under high pressure dynamic compression. We observe kinetic features - velocity loops - similar with the ones recently reported to occur when water is frozen into its ice VII phase under comparable experimental conditions. We analyze this behavior in terms of general ideas coupling the steady sample compression with phase nucleation and growth with a pressure dependent phase interface velocity. The model is used to predict the response of iron when steadily driven across the {alpha} - {epsilon} phase boundary on very short time scales, including those envisioned to be achieved in ultra-fast laser experiments.

  13. Nanoscale Transforming Mineral Phases in Fresh Nacre.

    PubMed

    DeVol, Ross T; Sun, Chang-Yu; Marcus, Matthew A; Coppersmith, Susan N; Myneni, Satish C B; Gilbert, Pupa U P A

    2015-10-21

    Nacre, or mother-of-pearl, the iridescent inner layer of many mollusk shells, is a biomineral lamellar composite of aragonite (CaCO3) and organic sheets. Biomineralization frequently occurs via transient amorphous precursor phases, crystallizing into the final stable biomineral. In nacre, despite extensive attempts, amorphous calcium carbonate (ACC) precursors have remained elusive. They were inferred from non-nacre-forming larval shells, or from a residue of amorphous material surrounding mature gastropod nacre tablets, and have only once been observed in bivalve nacre. Here we present the first direct observation of ACC precursors to nacre formation, obtained from the growth front of nacre in gastropod shells from red abalone (Haliotis rufescens), using synchrotron spectromicroscopy. Surprisingly, the abalone nacre data show the same ACC phases that are precursors to calcite (CaCO3) formation in sea urchin spicules, and not proto-aragonite or poorly crystalline aragonite (pAra), as expected for aragonitic nacre. In contrast, we find pAra in coral.

  14. Nanoscale Transforming Mineral Phases in Fresh Nacre

    DOE PAGES

    DeVol, Ross T.; Sun, Chang-Yu; Marcus, Matthew A.; ...

    2015-09-24

    Nacre, or mother-of-pearl, the iridescent inner layer of many mollusk shells, is a biomineral lamellar composite of aragonite (CaCO3) and organic sheets. Biomineralization frequently occurs via transient amorphous precursor phases, crystallizing into the final stable biomineral. In nacre, despite extensive attempts, amorphous calcium carbonate (ACC) precursors have remained elusive. They were inferred from non-nacre-forming larval shells, or from a residue of amorphous material surrounding mature gastropod nacre tablets, and have only once been observed in bivalve nacre. Here we present the first direct observation of ACC precursors to nacre formation, obtained from the growth front of nacre in gastropod shellsmore » from red abalone (Haliotis rufescens), using synchrotron spectromicroscopy. Surprisingly, the abalone nacre data show the same ACC phases that are precursors to calcite (CaCO3) formation in sea urchin spicules, and not proto-aragonite or poorly crystalline aragonite (pAra), as expected for aragonitic nacre. In contrast, we find pAra in coral.« less

  15. Isothermal phase transformations of a dental porcelain.

    PubMed

    Twiggs, S Warren; Mackert, J Rodway; Oxford, Amalia L; Ergle, Janet W; Lockwood, Petra E

    2005-06-01

    The purpose of this investigation was to determine if the change in the leucite weight fraction during an isothermal heat treatment could be estimated by observing the deformation of PFM strips in a high-heating-rate, computer-controlled bending beam viscometer (BBV). Specimens of a commercial body porcelain were fired according to the manufacturer's instructions-50 disk specimens for quantitative X-ray diffraction (XRD) and 100 bimaterial strip specimens for BBV. The XRD specimens were annealed at temperatures between 650 and 1000 degrees C, and leucite weight fraction was measured using an alumina internal standard. The BBV specimens were annealed in the BBV using time-temperature schedules designed to elucidate the leucite crystallization behavior between 700 and 1000 degrees C. Timoshenko's equation for a bimaterial thermostat was used to estimate the change in the thermal expansion of the porcelain near room temperature. Changes in leucite weight fraction were determined from these thermal expansion changes. The means and SDs were compared to values obtained by quantitative XRD. Good agreement was obtained between values of leucite weight fraction derived from beam deformation and those determined by quantitative XRD (p> or =0.45). The anneal sequence showed that the increase in leucite weight fraction at 800 or 900 degrees C is reversible by an anneal at 1000 degrees C. The BBV technique yields comparable results to quantitative XRD and provides the opportunity to efficiently monitor porcelain leucite changes nondestructively over multiple heat treatments. This technique could prove useful for testing firing schedules designed to stabilize the leucite content in dental porcelain.

  16. Partially transformed relaxor ferroelectric single crystals with distributed phase transformation behavior

    NASA Astrophysics Data System (ADS)

    Gallagher, John A.

    2015-11-01

    Relaxor ferroelectric single crystals such as PMN-PT and PIN-PMN-PT undergo field driven phase transformations when electrically or mechanically loaded in crystallographic directions that provide a positive driving force for the transformation. The observed behavior in certain compositions is a phase transformation distributed over a range of fields without a distinct forward or reverse coercive field. This work focuses on the material behavior that is observed when the crystals are loaded sufficiently to drive a partial transformation and then unloaded, as might occur when driving a transducer to achieve high power levels. Distributed transformations have been modeled using a normal distribution of transformation thresholds. A set of experiments was conducted to characterize the hysteresis loops that occur with the partial transformations. In this work the normal distribution model is extended to include the partial transformations that occur when the field is reversed before the transformation is complete. The resulting hysteresis loops produced by the model are in good agreement with the experimental results.

  17. Phase field approach to martensitic phase transformations with large strains and interface stresses

    NASA Astrophysics Data System (ADS)

    Levitas, Valery I.

    2014-10-01

    Thermodynamically consistent phase field theory for multivariant martensitic transformations, which includes large strains and interface stresses, is developed. Theory is formulated in a way that some geometrically nonlinear terms do not disappear in the geometrically linear limit, which in particular allowed us to introduce the expression for the interface stresses consistent with the sharp interface approach. Namely, for the propagating nonequilibrium interface, a structural part of the interface Cauchy stresses reduces to a biaxial tension with the magnitude equal to the temperature-dependent interface energy. Additional elastic and viscous contributions to the interface stresses do not require separate constitutive equations and are determined by solution of the coupled system of phase field and mechanics equations. Ginzburg-Landau equations are derived for the evolution of the order parameters and temperature evolution equation. Boundary conditions for the order parameters include variation of the surface energy during phase transformation. Because elastic energy is defined per unit volume of unloaded (intermediate) configuration, additional contributions to the Ginzburg-Landau equations and the expression for entropy appear, which are important even for small strains. A complete system of equations for fifth- and sixth-degree polynomials in terms of the order parameters is presented in the reference and actual configurations. An analytical solution for the propagating interface and critical martensitic nucleus which includes distribution of components of interface stresses has been found for the sixth-degree polynomial. This required resolving a fundamental problem in the interface and surface science: how to define the Gibbsian dividing surface, i.e., the sharp interface equivalent to the finite-width interface. An unexpected, simple solution was found utilizing the principle of static equivalence. In fact, even two equations for determination of the

  18. Inducing phase transformations using depth-sensing indentation

    NASA Astrophysics Data System (ADS)

    Juliano, Thomas Frank

    Phase transformations in solids occur during a number of common contact loading situations including polishing, cutting, and grinding. To understand and optimize material response to such processes or to engineer a surface with specific properties (i.e. electrical, optical, mechanical) at a micro-scale, depth-sensing indentation is used to study point loading situations and simulate dynamic processes. Sometimes, evidence of phase transformation can be found from load-displacement curves. Raman spectroscopy or other techniques (i.e. transmission electron microscopy, atomic force microscopy, or x-ray diffraction) on residual impressions often provides useful information to characterize transformations which take place during indentation. In this work, depth-sensing indentation and Raman spectroscopy are used as the primary techniques to identify phase transformations under different maximum applied loads, loading rates and unloading rates. Single-crystal silicon and lead zirconate titanate modified by niobium (PNZT) are used as exemplary materials for study because of their widespread industrial importance. A general technique for studying phase transformations induced by indentation is developed. Silicon undergoes a transformation from the cubic-diamond (Si-I) to beta-tin phase (Si-II) on loading around 12 GPa, and from Si-II to the r8 (Si-XII), bc8 (Si-III) or amorphous silicon on unloading. Using a number of different loading conditions, the stability of Si-II was found to range between 4 and 12 GPa, depending on maximum applied load, unloading rate and indenter tool geometry. 95/5 PNZT undergoes a ferroelectric to antiferroelectric phase change around 300 MPa of contact pressure. Both sharp (Berkovich) and spherical indentation are used to study different stress states for each material, and its influence on phase changes. On silicon, over 4500 indentations are made to determine the statistical nature of phase changes. Differentiation of load-displacement curves

  19. Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

    PubMed Central

    Hudak, Bethany M.; Depner, Sean W.; Waetzig, Gregory R.; Talapatra, Anjana; Arroyave, Raymundo; Banerjee, Sarbajit; Guiton, Beth S.

    2017-01-01

    High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics. PMID:28497788

  20. Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

    DOE PAGES

    Hudak, Bethany M.; Depner, Sean W.; Waetzig, Gregory R.; ...

    2017-05-12

    High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation frommore » twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.« less

  1. Interaction between phase transformations and dislocations at the nanoscale. Part 1. General phase field approach

    NASA Astrophysics Data System (ADS)

    Levitas, Valery I.; Javanbakht, Mahdi

    2015-09-01

    Thermodynamically consistent, three-dimensional (3D) phase field approach (PFA) for coupled multivariant martensitic transformations (PTs), including cyclic PTs, variant-variant transformations (i.e., twinning), and dislocation evolution is developed at large strains. One of our key points is in the justification of the multiplicative decomposition of the deformation gradient into elastic, transformational, and plastic parts. The plastic part includes four mechanisms: dislocation motion in martensite along slip systems of martensite and slip systems of austenite inherited during PT and dislocation motion in austenite along slip systems of austenite and slip systems of martensite inherited during reverse PT. The plastic part of the velocity gradient for all these mechanisms is defined in the crystal lattice of the austenite utilizing just slip systems of austenite and inherited slip systems of martensite, and just two corresponding types of order parameters. The explicit expressions for the Helmholtz free energy and the transformation and plastic deformation gradients are presented to satisfy the formulated conditions related to homogeneous thermodynamic equilibrium states of crystal lattice and their instabilities. In particular, they result in a constant (i.e., stress- and temperature-independent) transformation deformation gradient and Burgers vectors. Thermodynamic treatment resulted in the determination of the driving forces for change of the order parameters for PTs and dislocations. It also determined the boundary conditions for the order parameters that include a variation of the surface energy during PT and exit of dislocations. Ginzburg-Landau equations for dislocations include variation of properties during PTs, which in turn produces additional contributions from dislocations to the Ginzburg-Landau equations for PTs. A complete system of coupled PFA and mechanics equations is presented. A similar theory can be developed for PFA to dislocations and other

  2. Phase equilibria and transformations in the Ti-Al-Nb system

    NASA Astrophysics Data System (ADS)

    Mishurda, Joseph Constantine

    The phase equilibria and transformations in the Nb-Ti-Al system in the vicinity of the Sigma phase field have been examined with respect to the Liquidus Surface (Phase 1), the Phase Equilibria and Equilibrium Transformations (Phase II), and the Phase Transformations, Mechanisms and Kinetics (Phase III). Eight alloy compositions were produced by arc melting. The alloys were characterized by differential thermal analysis, metallography, X-ray diffraction, scanning electron microscopy (BSEI), electron probe microanalysis and transmission electron microscopy. The liquidus examination shed new light on previous microstructural interpretations, opening up new possibilities for microstructural development and control of multiphase alloys. Differential thermal analysis has identified the existence of a beta to sigma + gamma transformation in an alloy where it was not previously thought to exist. The results differed from the calculated diagram by higher titanium solubility in the sigma and delta phases than predicted at lower temperatures and a lower solubility of alpha2 and gamma. The high temperature betao transforms to gamma + sigma in a eutectoid fashion resulting in a desirable lamellar structure of sigma and gamma. The existence of a new body centered tetragonal crystal structure ao = 5.11A and co 28.12A with a point group symmetry of P4/mmm, at 700°C was discovered. A plethel section was found for the sigma + beta two phase alloys. A betao + O + sigma three phase field passes through the alloys between 981 and 1000°C. The plethel section at the transformation has an eutectoid characteristic, however, the nature of the transformation changes to a peritectoid. At temperatures below 970°C, the first transformation to occur is the decomposition of the metastable betao phase to an intermediate metastable phase O'. Reasonable values for Q were obtained, applicable to the diffusion limited region of the TTT-curve. The microstructure evolution of the sigma + beta

  3. Dynamic model of a three-phase power transformer

    SciTech Connect

    Dolinar, D.; Pihler, J.; Grcar, B. . Faculty of Technical Sciences)

    1993-10-01

    An adequate mathematical model of a three-phase power transformer is one of the important elements in the programs for the computer analysis of power system transients. Featured in this paper is the simulation model of a three-phase, three-limb core-type power transformer. Non-linear effects of saturation, hysteresis and eddy currents are considered. Two ways of creating major and minor hysteresis loops are presented. The transformer model, described by a system of time dependent differential equations, is solved by an efficient numerical algorithm. The behavior of the transformer model during switching-in and fault transients, as well as other types of transients, has been tested. The computed transient waveforms are compared with the measured ones of there exists very close agreement between them.

  4. Relativistic Random Phase Approximation At Finite Temperature

    SciTech Connect

    Niu, Y. F.; Paar, N.; Vretenar, D.; Meng, J.

    2009-08-26

    The fully self-consistent finite temperature relativistic random phase approximation (FTRRPA) has been established in the single-nucleon basis of the temperature dependent Dirac-Hartree model (FTDH) based on effective Lagrangian with density dependent meson-nucleon couplings. Illustrative calculations in the FTRRPA framework show the evolution of multipole responses of {sup 132}Sn with temperature. With increased temperature, in both monopole and dipole strength distributions additional transitions appear in the low energy region due to the new opened particle-particle and hole-hole transition channels.

  5. Interaction between phase transformations and dislocations at the nanoscale. Part 2: Phase field simulation examples

    NASA Astrophysics Data System (ADS)

    Javanbakht, Mahdi; Levitas, Valery I.

    2015-09-01

    The complete system of phase field equations for coupled martensitic phase transformations (PTs), dislocation evolution, and mechanics at large strains is presented. Finite element method (FEM) is utilized to solve this system for two important problems. The first one is related to the simulation of shear strain-induced PT at the evolving dislocation pile-ups in a nanosized bicrystal. Plasticity plays a dual part in the interaction with PT. Dislocation pile-ups produce strong stress tensor concentrators that lead to barrierless martensite (M) nucleation. On the other hand, plasticity in the transforming grain relaxes these stress concentrators suppressing PT. The final stationary M morphology is governed by the local thermodynamic equilibrium, either at the interfaces or in terms of stresses averaged over the martensitic region or the entire grain. This is very surprising because of strong heterogeneity of stress fields and is in contrast to previous statements that phase equilibrium conditions do not enter the description of strain-induced PTs. The second problem is devoted to martensitic plate propagation through a bicrystal during temperature-induced PT. For elastic growth (without dislocations) and a large thermal driving force, a complex transformation path with plate branching and direct and reverse PTs is observed, which still ends with the same stationary nanostructure as for a smaller driving force and a traditional transformation path. Sharp grain boundary arrests plate growth at a relatively small driving force, exhibiting an athermal friction. For elastoplastic growth, the generation of dislocations produces athermal friction and arrests the plate below some critical driving force, leading to a morphological transition from plate to lath M. The width of the martensitic plate increases in comparison with elastic growth due to internal stress relaxation. Plate growth is accompanied by the nucleation of dislocations within M and remaining in M, the

  6. Cascade-driven migration of structural interfaces: A new type of irradiation-induced phase transformation

    NASA Astrophysics Data System (ADS)

    Allen, C. W.; Rehn, L. E.; Wiedersich, H.

    1987-06-01

    In Ti-Cr alloys containing both the Laves phase (ordered TiCr2) and the β phase (bcc solid solution), 1 MeV Kr+ irradiation at temperatures <250 K amorphizes the Laves phase but not the β phase. The irradiation-induced amorphous material transforms into polycrystalline β upon warming to room temperature. In contrast, irradiation at room temperature causes the β phase to grow by migration of the β/Laves interface; no change in local composition across the migrating interface is observed. The β phase also grows during prolonged irradiation at 100 K, but the velocity of the β/amorphous interface per unit calculated atomic displacement rate is more than three orders of magnitude lower than that of the β/Laves interface at room temperature. An interpretation of this new type of irradiation-induced phase transformation is presented in terms of displacement cascade formation and dynamic recovery at the structural interface that exists between the two phases.

  7. ISOTHERMAL PHASE TRANSFORMATION CYCLING IN STEEL BY APPLICATION OF A HIGH MAGNETIC FIELD

    SciTech Connect

    Ludtka, Gerard Michael; Jaramillo, Roger A; Ludtka, Gail Mackiewicz-; Kisner, Roger A; Wilgen, John B

    2007-01-01

    A phase transformation reversal via the application and removal of a large magnetic field was investigated. Because a large magnetic field can alter the phase equilibrium between paramagnetic austenite and ferromagnetic ferrite, volume fractions for each phase constituent can be modified at constant temperature by changing the magnetic field strength. In this research elevated temperature isothermal hold experiments were performed for 5160 steel. During the isothermal hold, the magnetic field was cycled between 0 and 30 Tesla. As companion experiments, temperature cycling and isothermal holds were performed without magnetic fields. The resulting microstructures were examined using optical and SEM metallography. These microstructures indicate that a portion of the microstructure experiences isothermal transformation cycling between austenite and ferrite due to the application and removal of the 30T (Tesla) magnetic field.

  8. 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.

  9. Phase transformations in steels: Processing, microstructure, and performance

    SciTech Connect

    Gibbs, Paul J.

    2014-04-03

    In this study, contemporary steel research is revealing new processing avenues to tailor microstructure and properties that, until recently, were only imaginable. Much of the technological versatility facilitating this development is provided by the understanding and utilization of the complex phase transformation sequences available in ferrous alloys. Today we have the opportunity to explore the diverse phenomena displayed by steels with specialized analytical and experimental tools. Advances in multi-scale characterization techniques provide a fresh perspective into microstructural relationships at the macro- and micro-scale, enabling a fundamental understanding of the role of phase transformations during processing and subsequent deformation.

  10. Metallurgical Properties and Phase Transformations of Barium-Strontium Modifier

    NASA Astrophysics Data System (ADS)

    Platonov, M. A.; Sulimova, I. S.; Rozhikhina, I. D.; Dmitrienko, V. I.; Horoshun, G. V.

    2016-04-01

    Metallurgical properties and phase transformations of barium-strontium modifier were tested in laboratory conditions resembling steel processing in furnace and ladle. When heating barium-strontium modifier start of melting, kinetics of decomposition, phase and structure transformation were studied. The concentrate under consideration has been revealed to be a complex mineral compound containing barytocalcite, calcite, calciostrontianite, dolomite and siderite. The reaction kinetics of decomposing mineral components of barium-strontium modifier to oxides does not considerably affect slag formation in conditions of out-of-furnace steel processing.

  11. Shock phase transformation and release properties of aluminum nitride

    SciTech Connect

    Kipp, M.E.; Grady, D.E.

    1993-12-31

    Aluminum nitride is characterized by a non-recoverable volume phase transformation from the wurtzite (hexagonal) to the rocksalt (cubic) structure that commences about 22 GPa, accompanied by a volume strain of about 20%. A study of the manifestation of this large volume change under shock loading was made with four uniaxial strain impact experiments. Particle velocity histories of the shock and release states were obtained corresponding to impact pressures of 40 GPa, including the evolution of the shock and release waves with propagation distance. Stress-strain features characterizing aluminum nitride, including the phase transformation, were determined with numerical analysis techniques.

  12. Strain-rate-induced bcc-to-hcp phase transformation of Fe nanowires

    NASA Astrophysics Data System (ADS)

    Xie, Hongxian; Yu, Tao; Fang, Wei; Yin, Fuxing; Faraz Khan, Dil

    2016-12-01

    Using molecular dynamics simulation method, the plastic deformation mechanism of Fe nanowires is studied by applying uniaxial tension along the [110] direction. The simulation result shows that the bcc-to-hcp martensitic phase transformation mechanism controls the plastic deformation of the nanowires at high strain rate or low temperature; however, the plastic deformation mechanism will transform into a dislocation nucleation mechanism at low strain rate and higher temperature. Furthermore, the underlying cause of why the bcc-to-hcp martensitic phase transition mechanism is related to high strain rate and low temperature is also carefully studied. Based on the present study, a strain rate-temperature plastic deformation map for Fe nanowires has been proposed. Project supported by the National Natural Science Foundation of China (Grant No. 51571082) and China Postdoctoral Science Foundation (Grant No. 2015M580191).

  13. Magnetostructural phase transformations in Tb 1-x Mn 2

    SciTech Connect

    Zou, Junding; Paudyal, Durga; Liu, Jing; Mudryk, Yaroslav; Pecharsky, Vitalij K.; Gschneidner, Karl A.

    2015-01-16

    Magnetism and phase transformations in non-stoichiometric Tb1-xMn2 (x = 0.056, 0.039) have been studied as functions of temperature and magnetic field using magnetization, heat capacity, and X-ray powder diffraction measurements. Lowering the temperature, the compounds sequentially order ferrimagnetically and antiferromagnetically, and finally, exhibit spin reorientation transitions. Moreover, these structural distortions from room temperature cubic to low temperature rhombohedral structures occur at TN, and are accompanied by large volume changes reaching ~-1.27% and -1.42%, respectively. First principles electronic structure calculations confirm the phase transformation from the ferrimagnetic cubic structure to the antiferromagnetic rhombohedral structure in TbMn2.

  14. Strain-Temperature-Transformation (STT) Diagram for Soft Solids

    NASA Astrophysics Data System (ADS)

    Li, Shoubo; Xiong, Wentao; Wang, Xiaorong

    Soft materials comprise a variety of physical states that are easily deformed by shear stains or thermal fluctuations. They include suspensions, colloids, polymers, foams, gels, liquid crystals, and a number of biological materials. In this contribution, a generalized strain-temperature-transformation (STT) diagram for many soft materials is presented in which the physical states encountered are related to the strain and temperature changes. The boundary defined for the solid-to-liquid transformation in the STT diagram displays a surprising Z-shaped curve. We discuss this feature with respect to the physical nature of materials.

  15. Thermodynamic Modeling and Experimental Study of Phase Transformations in Alloys Based on γ-TiAl

    NASA Astrophysics Data System (ADS)

    Kuznetsov, A. V.; Sokolovskii, V. S.; Salishchev, G. A.; Belov, N. A.; Nochovnaya, N. A.

    2016-09-01

    Thermo-Calc software is used to model the composition diagram for alloys based on γ-TiAl of the systems Ti - Al - Mo - (4 - 10) at.% Nb and Ti - Al - Nb - X ( X is Cr, Mo, V). The effect of alloying on critical points and sequence of phase transformations is established. Changes in phase composition in relation to alloy TNM-B1 temperature are analyzed using a polythermal section of the Ti - Al - Nb - Mo system.

  16. Phase and structural transformations in the alloy on the basis of the orthorhombic titanium aluminide

    NASA Astrophysics Data System (ADS)

    Popov, A. A.; Illarionov, A. G.; Grib, S. V.; Demakov, S. L.; Karabanalov, M. S.; Elkina, O. A.

    2008-10-01

    Phase and structural transformations in the Ti-24.3 Al-24.8 Nb-1.0 Zr-1.4 V-0.6 Mo-0.3 Si (at %) alloy that take place during heating in the temperature range of 700 1050°C have been investigated. The temperature ranges of existence of the O + β, O + β + α2, β + α2, and β phase fields have been established. A scheme of the relationships between the volume fractions of the O, β, and α2 phases depending on the temperature of heating of the alloy have been investigated. The formation of an ordered incommensurate ω ( V ω) phase has been revealed in the alloy during quenching from 900°C. The existence of a correlation between the hardness properties and changes in the phase composition and morphology of particles precipitating in the alloy has been shown.

  17. Frustrated phase separation and high temperature superconductivity

    SciTech Connect

    Emery, V.J. ); Kivelson, S.A. . Dept. of Physics)

    1992-01-01

    A dilute system of neutral holes in an antiferromagnet separates into a hole-rich and a hole-poor phase. The phase separation is frustrated by long-range Coulomb interactions but, provided the dielectric constant is sufficiently large, there remain large-amplitude low-energy fluctuations in the hole density at intermediate length scales. The extensive experimental evidence showing that this behavior giver, a reasonable picture of high temperature superconductors is surveyed. Further, it is shown that the scattering of mobile holes from the local density fluctuations may account for the anomalous normal-state properties of high temperature superconductors and also provide the mechanism of pairing.

  18. Frustrated phase separation and high temperature superconductivity

    SciTech Connect

    Emery, V.J.; Kivelson, S.A.

    1992-09-01

    A dilute system of neutral holes in an antiferromagnet separates into a hole-rich and a hole-poor phase. The phase separation is frustrated by long-range Coulomb interactions but, provided the dielectric constant is sufficiently large, there remain large-amplitude low-energy fluctuations in the hole density at intermediate length scales. The extensive experimental evidence showing that this behavior giver, a reasonable picture of high temperature superconductors is surveyed. Further, it is shown that the scattering of mobile holes from the local density fluctuations may account for the anomalous normal-state properties of high temperature superconductors and also provide the mechanism of pairing.

  19. Phase Transformation of Calcium Phosphates by Electrodeposition and Heat Treatment

    NASA Astrophysics Data System (ADS)

    Shih, Wei-Jen; Wang, Moo-Chin; Chang, Kuo-Ming; Wang, Cheng-Li; Wang, Szu-Hao; Li, Wang-Long; Huang, Hong-Hsin

    2010-12-01

    The effect of heat treatment on the calcium phosphate deposited on Ti-6Al-4V substrate using an electrolytic process is investigated. The calcium phosphate was deposited in a 0.04 M Ca(H2PO4)2·H2O (MCPM) solution on a Ti-6Al-4V substrate at 333 K (60 °C), 10 V, and 80 Torr for 1 hour, and calcined at various temperatures for 4 hours. The X-ray diffraction (XRD) results demonstrate that the phases are dicalcium phosphate (CaHPO4, DCPD) and hydroxyapatile [Ca(PO4)6 (OH)2, HAP] for the as-deposited samples. When the deposited sample was calcined at 873 K (600 °C) for 4 hours, the XRD results show that the transformation of DCPD to HAP occurs. Moreover, HAP converts to β-TCP, CPP, and CaO. For the sample calcined at 1073 K (800 °C) for 4 hours, the scanning electron microscopy (SEM) micrograph reveals that the crack of the calcined sample propagates with a width of about 3 μm. This result is due to HAP becoming decomposed and converting to β-TCP, CPP, CaO, and H2O. The vaporization of H2O within the calcined sample promotes the crack propagation and growth.

  20. Pressure-induced structural phase transformations in silicon nanowires.

    PubMed

    Poswal, H K; Garg, Nandini; Sharma, Surinder M; Busetto, E; Sikka, S K; Gundiah, Gautam; Deepak, F L; Rao, C N R

    2005-05-01

    High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.

  1. Phase transformations during the growth of paracetamol crystals from the vapor phase

    NASA Astrophysics Data System (ADS)

    Belyaev, A. P.; Rubets, V. P.; Antipov, V. V.; Bordei, N. S.

    2014-07-01

    Phase transformations during the growth of paracetamol crystals from the vapor phase are studied by differential scanning calorimetry. It is found that the vapor-crystal phase transition is actually a superposition of two phase transitions: a first-order phase transition with variable density and a second-order phase transition with variable ordering. The latter, being a diffuse phase transition, results in the formation of a new, "pretransition," phase irreversibly spent in the course of the transition, which ends in the appearance of orthorhombic crystals. X-ray diffraction data and micrograph are presented.

  2. 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.

  3. Low-pressure phase transformation from rhombohedral to cubic BN: Experiment and theory

    NASA Astrophysics Data System (ADS)

    Levitas, Valery I.; Shvedov, Leonid K.

    2002-03-01

    An irreversible phase transformation (PT) from the rhombohedral phase of boron nitride rBN to cubic cBN was recently recorded at the surprisingly low pressure of 5.6 GPa at room temperature. In this paper, a very nontrivial and unexpected explanation of this phenomenon is found, based on our criterion for the PT in plastic materials and approximate solution of corresponding plastic problems. It is found that due to orientational plastic instability and rotational softening in rBN and the higher yield stress of cBN, stresses grow drastically in the transforming region during the PT (despite a volume decrease by a factor of 1.53). This allows the fulfillment of the PT criterion which takes into account the whole stress history during the transformation process. It appears that the above experimental phenomenon is connected to the mechanical behavior of the system of transforming particles+surrounding materials at the millimeter scale.

  4. 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.

  5. In-situ characterization of transformation plasticity during an isothermal austenite-to-bainite phase transformation

    SciTech Connect

    Holzweissig, M.J.; Canadinc, D.; Maier, H.J.

    2012-03-15

    This paper elucidates the stress-induced variant selection process during the isothermal austenite-to-bainite phase transformation in a tool steel. Specifically, a thorough set of experiments combining electron backscatter diffraction and in-situ digital image correlation (DIC) was carried out to establish the role of superimposed stress level on the evolution of transformation plasticity (TP) strains. The important finding is that TP increases concomitant with the superimposed stress level, and strain localization accompanies phase transformation at all stress levels considered. Furthermore, TP strain distribution within the whole material becomes more homogeneous with increasing stress, such that fewer bainitic variants are selected to grow under higher stresses, yielding a more homogeneous strain distribution. In particular, the bainitic variants oriented along [101] and [201] directions are favored to grow parallel to the loading axis and are associated with large TP strains. Overall, this very first in-situ DIC investigation of the austenite-to-bainite phase transformation in steels evidences the clear relationship between the superimposed stress level, variant selection, and evolution of TP strains. - Highlights: Black-Right-Pointing-Pointer Local variations of strain were observed by DIC throughout the phase transformation. Black-Right-Pointing-Pointer The study clearly established the role of the stress-induced variant selection. Black-Right-Pointing-Pointer Variant selection is a key parameter that governs distortion.

  6. Bainite transformation temperatures in high-silicon steels

    SciTech Connect

    Chang, L.C.

    1999-04-01

    The bainite transformation temperatures of eight high-silicon steels were determined metallographically. The bainite start (B{sub s}) temperatures, which define the highest temperature at which bainite can form, all lay below the T{sub 0} loci, where ferrite and austenite of the same chemical compositions have identical free energy. The established method of calculating B{sub s} temperatures gave reasonable agreement with the experimental results. Careful study of the isothermally reacted samples revealed that Widmanstaetten ferrite and bainite could both be observed, even at the beginning of the transformation, at around the B{sub s} temperature. On the other hand, the lower bainite start (LB{sub s}) temperatures of these steels were found to be very close to the martensite start (M{sub s}) temperatures. Silicon is considered to be responsible for depressing the LB{sub s} temperature by retarding the formation of cementite. The coformation of upper and lower bainite near the LB{sub s} temperature is also confirmed. The results indicate that the displacive formation mechanism of bainite is sustainable.

  7. The Binary Temperature-Composition Phase Diagram

    ERIC Educational Resources Information Center

    Sanders, Philip C.; Reeves, James H.; Messina, Michael

    2006-01-01

    The equations for the liquid and gas lines in the binary temperature-composition phase diagram are derived by approximating that delta(H)[subscript vap] of the two liquids are equal. It is shown that within this approximation, the resulting equations are not too difficult to present in an undergraduate physical chemistry lecture.

  8. Anomalous compression behavior of germanium during phase transformation

    SciTech Connect

    Yan, Xiaozhi; Tan, Dayong; Ren, Xiangting; Yang, Wenge E-mail: duanweihe@scu.edu.cn; He, Duanwei E-mail: duanweihe@scu.edu.cn; Mao, Ho-Kwang

    2015-04-27

    In this article, we present the abnormal compression and plastic behavior of germanium during the pressure-induced cubic diamond to β-tin structure transition. Between 8.6 GPa and 13.8 GPa, in which pressure range both phases are co-existing, first softening and followed by hardening for both phases were observed via synchrotron x-ray diffraction and Raman spectroscopy. These unusual behaviors can be interpreted as the volume misfit between different phases. Following Eshelby, the strain energy density reaches the maximum in the middle of the transition zone, where the switch happens from softening to hardening. Insight into these mechanical properties during phase transformation is relevant for the understanding of plasticity and compressibility of crystal materials when different phases coexist during a phase transition.

  9. Ion irradiation-induced diffusion in bixbyite-fluorite related oxides: Dislocations and phase transformation

    NASA Astrophysics Data System (ADS)

    Rolly, Gaboriaud; Fabien, Paumier; Bertrand, Lacroix

    2014-05-01

    Ion-irradiation induced diffusion and the phase transformation of a bixbyite-fluorite related rare earth oxide thin films are studied. This work is focused on yttrium sesquioxide, Y2O3, thin films deposited on Si (1 0 0) substrates using the ion beam sputtering technique (IBS). As-deposited samples were annealed ant then irradiated at cryogenic temperature (80 K) with 260 keV Xe2+ at different fluences. The irradiated thin oxide films are characterized by X-ray diffraction. A cubic to monoclinic phase transformation was observed. Analysis of this phenomenon is done in terms of residual stresses. Stress measurements as a function of irradiation fluences were realised using the XRD-sin2ψ method. Stress evolution and kinetic of the phase transformation are compared and leads to the role-played by the nucleation of point and extended defects.

  10. Microstructural Evidence for Conditioning-dependent (delta) -> (alpha)' Transformations in Retained (delta)-phase Pu-Ga

    SciTech Connect

    Jeffries, J R; Blobaum, K M; Wall, M A; Schwartz, A J

    2008-06-16

    The retained {delta} phase of a Pu-1.9 at.% Ga alloy is metastable with respect to the martensitic {delta} {yields} {alpha}{prime} transformation that occurs at low temperatures. This transformation has been shown to proceed by means of an isothermal martensitic mode, but the kinetics of the transformation are atypical. The transformation exhibits a 'double-C' in a time-temperature-transformation diagram, wherein there exist two temperatures where a given amount of transformation occurs in a minimum amount of time. The cause of the double-C kinetics remains uncertain, eliciting proposals of multiple mechanisms, multiple paths, or different morphologies as possible origins. Recently, a 'conditioning' treatment was found to affect the {delta} {yields} {alpha}{prime} transformation, but the underlying mechanism by which the conditioning treatment influences the transformation has not yet been resolved. In this study, microstructural characterization as a function of temperature, time, and conditioning has been employed to illuminate the role of conditioning in the {delta} {yields} {alpha}{prime} transformation. Conditioning is found to enhance transformation in the upper-C and to enable transformation in the lower-C. The data garnered from these experiments suggest that conditioning is intimately linked to nucleation processes and of little consequence to the growth and morphology of the {alpha}{prime} product phase.

  11. Study of phase transformations in CMSX-6 and CMSX-8 superalloys

    NASA Astrophysics Data System (ADS)

    Szczotok, Agnieszka; Wierzbicka-Miernik, Anna

    2014-09-01

    Nickel-based superalloys are extensively used mainly in the aircraft and aeronautic industry, particularly in the hottest parts of engines or turbo-reactors. The phase reactions occurring in these heat-resistant materials play a crucial role in many aspects of the processing and service of the highly alloyed materials. Cast Ni-based superalloys are obtained in a complex way and their structure is complicated. Differential scanning calorimetry (DSC) technique was applied for determination of temperature ranges of the phase transformations occurring in the CMSX-6 and CMSX-8 superalloys during heating/cooling processes. Thermophysical properties, including temperatures of the phase transformation, are the critical input parameters in mathematical models of solidification and casting of metallic materials. The literature data concerning phase transformations and performance of the heat treatment for CMSX-6 and CMSX-8 are incomplete and ambiguous. DSC results accompanied by scanning electron microscopy characterization of microstructure of CMSX-6 and CMSX-8 superalloy was applied. The present study will improve the understanding of the fundamental mechanisms of phase transformations of single-crystal nickel-based superalloys.

  12. Enhanced reversibility and unusual microstructure of a phase-transforming material.

    PubMed

    Song, Yintao; Chen, Xian; Dabade, Vivekanand; Shield, Thomas W; James, Richard D

    2013-10-03

    Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators, eco-friendly refrigerators and energy conversion devices. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed 'reversibility') is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling. Recently, stronger compatibility conditions called the 'cofactor conditions' have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

  13. In situ phase transformation of Laves phase from Chi-phase in Mo-containing Fe–Cr–Ni alloys

    DOE PAGES

    Tan, L.; Yang, Y.

    2015-11-01

    For an in situ phase transformation of the Chi (χ) phase to the Laves phase we observed in a Fe–Cr–Ni–Mo model alloy. The morphology, composition, and crystal structure of the χ and Laves phases, and their orientation relationship with the matrix austenite phase were investigated. The resulted Laves phase has larger lattice mismatch with the matrix phase than the χ phase, leading to the increase of local strain fields and the formation of dislocations. Moreover, this finding is helpful to understand the precipitation behavior of the intermetallic phases in the Mo-containing austenitic stainless steels.

  14. In situ phase transformation of Laves phase from Chi-phase in Mo-containing Fe–Cr–Ni alloys

    SciTech Connect

    Tan, L.; Yang, Y.

    2015-11-01

    For an in situ phase transformation of the Chi (χ) phase to the Laves phase we observed in a Fe–Cr–Ni–Mo model alloy. The morphology, composition, and crystal structure of the χ and Laves phases, and their orientation relationship with the matrix austenite phase were investigated. The resulted Laves phase has larger lattice mismatch with the matrix phase than the χ phase, leading to the increase of local strain fields and the formation of dislocations. Moreover, this finding is helpful to understand the precipitation behavior of the intermetallic phases in the Mo-containing austenitic stainless steels.

  15. Iterative-Transform Phase Retrieval Using Adaptive Diversity

    NASA Technical Reports Server (NTRS)

    Dean, Bruce H.

    2007-01-01

    A phase-diverse iterative-transform phase-retrieval algorithm enables high spatial-frequency, high-dynamic-range, image-based wavefront sensing. [The terms phase-diverse, phase retrieval, image-based, and wavefront sensing are defined in the first of the two immediately preceding articles, Broadband Phase Retrieval for Image-Based Wavefront Sensing (GSC-14899-1).] As described below, no prior phase-retrieval algorithm has offered both high dynamic range and the capability to recover high spatial-frequency components. Each of the previously developed image-based phase-retrieval techniques can be classified into one of two categories: iterative transform or parametric. Among the modifications of the original iterative-transform approach has been the introduction of a defocus diversity function (also defined in the cited companion article). Modifications of the original parametric approach have included minimizing alternative objective functions as well as implementing a variety of nonlinear optimization methods. The iterative-transform approach offers the advantage of ability to recover low, middle, and high spatial frequencies, but has disadvantage of having a limited dynamic range to one wavelength or less. In contrast, parametric phase retrieval offers the advantage of high dynamic range, but is poorly suited for recovering higher spatial frequency aberrations. The present phase-diverse iterative transform phase-retrieval algorithm offers both the high-spatial-frequency capability of the iterative-transform approach and the high dynamic range of parametric phase-recovery techniques. In implementation, this is a focus-diverse iterative-transform phaseretrieval algorithm that incorporates an adaptive diversity function, which makes it possible to avoid phase unwrapping while preserving high-spatial-frequency recovery. The algorithm includes an inner and an outer loop (see figure). An initial estimate of phase is used to start the algorithm on the inner loop, wherein

  16. Solid-gaseous phase transformation of elemental contaminants during the gasification of biomass.

    PubMed

    Jiang, Ying; Ameh, Abiba; Lei, Mei; Duan, Lunbo; Longhurst, Philip

    2016-09-01

    Disposal of plant biomass removed from heavy metal contaminated land via gasification achieves significant volume reduction and can recover energy. However, these biomass often contain high concentrations of heavy metals leading to hot-corrosion of gasification facilities and toxic gaseous emissions. Therefore, it is of significant interest to gain a further understanding of the solid-gas phase transition of metal(loid)s during gasification. Detailed elemental analyses (C, H, O, N and key metal/metalloid elements) were performed on five plant species collected from a contaminated site. Using multi-phase equilibria modelling software (MTDATA), the analytical data allows modelling of the solid/gas transformation of metal(loid)s during gasification. Thermodynamic modelling based on chemical equilibrium calculations was carried out in this study to predict the fate of metal(loid) elements during typical gasification conditions and to show how these are influenced by metal(loid) composition in the biomass and operational conditions. As, Cd, Zn and Pb tend to transform to their gaseous forms at relatively low temperatures (<1000°C). Ni, Cu, Mn and Co converts to gaseous forms within the typical gasification temperature range of 1000-1200°C. Whereas Cr, Al, Fe and Mg remain in solid phase at higher temperatures (>1200°C). Simulation of pressurised gasification conditions shows that higher pressures increase the temperature at which solid-to-gaseous phase transformations takes place.

  17. Influence of phase transformation on the permanent-magnetic properties of Fe-Pt based alloys

    NASA Astrophysics Data System (ADS)

    Brück, E.; Xiao, Q. F.; Thang, P. D.; Toonen, M. J.; de Boer, F. R.; Buschow, K. H. J.

    2001-07-01

    We have studied the effect of the atomic disorder-order transformation on remanence enhancement and coercivity in Fe-Pt-based materials by isothermal annealing at temperatures well below the transformation point. We also investigated the effect of the annealing temperature and the effect of various types of additives. The relative amount of the low-temperature hard-magnetic face-centered-tetragonal (FCT) phase precipitated in the high-temperature magnetically soft face-centered-cubic (FCC) phase was determined by means of X-ray diffraction. As a function of annealing time and annealing temperature, particle size and relative amount of the FCT phase increased at the cost of the FCC phase. These changes were followed by means of magnetic measurements. We observed a continuous increase in coercivity with increasing annealing time, eventually reaching a maximum. The Kneller-Hawig model was used to explain the occurrence of remanence enhancement and the continuously changing degree of exchange coupling between the magnetically soft and hard phase. The suitability of Fe-Pt based alloys in dental applications is discussed.

  18. Isomorphic phase transformation in shocked cerium using molecular dynamics

    SciTech Connect

    Dupont, Virginie; Germann, Timothy C; Chen, Shao - Ping

    2010-08-12

    Cerium (Ce) undergoes a significant ({approx}16%) volume collapse associated with an isomorphic fcc-fcc phase transformation when subject to compressive loading. We present here a new Embedded Atom Method (EAM) potential for Cerium that models two minima for the two fcc phases. We show results from its use in Molecular Dynamics (MD) simulations of Ce samples subjected to shocks with pressures ranging from 0.5 to 25 GPa. A split wave structure is observed, with an elastic precursor followed by a plastic wave. The plastic wave causes the expected fcc-fcc phase transformation. Comparisons to experiments and MD simulations on Cesium (Cs) indicate that three waves could be observed. The construction of the EAM potential may be the source of the difference.

  19. Reconstructing solute-induced phase transformations within individual nanocrystals.

    PubMed

    Narayan, Tarun C; Baldi, Andrea; Koh, Ai Leen; Sinclair, Robert; Dionne, Jennifer A

    2016-07-01

    Strain and defects can significantly impact the performance of functional nanomaterials. This effect is well exemplified by energy storage systems, in which structural changes such as volume expansion and defect generation govern the phase transformations associated with charging and discharging. The rational design of next-generation storage materials therefore depends crucially on understanding the correlation between the structure of individual nanoparticles and their solute uptake and release. Here, we experimentally reconstruct the spatial distribution of hydride phases within individual palladium nanocrystals during hydrogen absorption, using a combination of electron spectroscopy, dark-field imaging, and electron diffraction in an environmental transmission electron microscope. We show that single-crystalline cubes and pyramids exhibit a uniform hydrogen distribution at equilibrium, whereas multiply twinned icosahedra exclude hydrogen from regions of high compressive strains. Our technique offers unprecedented insight into nanoscale phase transformations in reactive environments and can be extended to a variety of functional nanomaterials.

  20. Atomic Simulations of Alane Phase Transformations and Dehydrogenation Mechanisms

    NASA Astrophysics Data System (ADS)

    Opalka, Susanne; Saxe, Paul; Lovvik, Ole Martin

    2007-03-01

    Density functional theory atomic ground state, molecular dynamics, and direct method lattice dynamic simulations were used to mechanistically probe phase transformations between the various crystallographically refined α, α', β, and γ AlH3 phases. Lattice dynamic predictions of the AlH3 structures provided an ideal test case for systematically accessing the accuracy of the vibrational thermodynamic property contributions with the harmonic approximation. The predicted transformation pathways involved coordinated tilting and rotation mechanisms, similar to that observed in perovskite structures. Further simulations were conducted to elucidate the mechanism for α AlH3 phase decomposition to the Al and H2 products and to identify probable barriers to reversible rehydrogenation.

  1. Nanowire growth by an electron beam induced massive phase transformation

    SciTech Connect

    Sood, Shantanu; Kisslinger, Kim; Gouma, Perena

    2014-11-15

    Tungsten trioxide nanowires of a high aspect ratio have been synthesized in-situ in a TEM under an electron beam of current density 14A/cm² due to a massive polymorphic reaction. Sol-gel processed pseudocubic phase nanocrystals of tungsten trioxide were seen to rapidly transform to one dimensional monoclinic phase configurations, and this reaction was independent of the substrate on which the material was deposited. The mechanism of the self-catalyzed polymorphic transition and accompanying radical shape change is a typical characteristic of metastable to stable phase transformations in nanostructured polymorphic metal oxides. A heuristic model is used to confirm the metastable to stable growth mechanism. The findings are important to the control electron beam deposition of nanowires for functional applications starting from colloidal precursors.

  2. Nanowire growth by an electron beam induced massive phase transformation

    DOE PAGES

    Sood, Shantanu; Kisslinger, Kim; Gouma, Perena

    2014-11-15

    Tungsten trioxide nanowires of a high aspect ratio have been synthesized in-situ in a TEM under an electron beam of current density 14A/cm² due to a massive polymorphic reaction. Sol-gel processed pseudocubic phase nanocrystals of tungsten trioxide were seen to rapidly transform to one dimensional monoclinic phase configurations, and this reaction was independent of the substrate on which the material was deposited. The mechanism of the self-catalyzed polymorphic transition and accompanying radical shape change is a typical characteristic of metastable to stable phase transformations in nanostructured polymorphic metal oxides. A heuristic model is used to confirm the metastable to stablemore » growth mechanism. The findings are important to the control electron beam deposition of nanowires for functional applications starting from colloidal precursors.« less

  3. Natural transformation and phase variation modulation in Neisseria meningitidis.

    PubMed

    Alexander, Heather L; Richardson, Anthony R; Stojiljkovic, Igor

    2004-05-01

    Neisseria meningitidis has evolved the ability to control the expression-state of numerous genes by phase variation. It has been proposed that the process aids this human pathogen in coping with the diversity of microenvironments and host immune systems. Therefore, increased frequencies of phase variation may augment the organism's adaptability and virulence. In this study, we found that DNA derived from various neisserial co-colonizers of the human nasopharynx increased N. meningitidis switching frequencies, indicating that heterologous neisserial DNA modulates phase variation in a transformation-dependent manner. In order to determine whether the effect of heterologous DNA was specific to the Hb receptor, HmbR, we constructed a Universal Rates of Switching cassette (UROS). With this cassette, we demonstrated that heterologous DNA positively affects phase variation throughout the meningococcal genome, as UROS phase variation frequencies were also increased in the presence of neisserial DNA. Overexpressing components of the neisserial mismatch repair system partially alleviated DNA-induced changes in phase variation frequencies, thus implicating mismatch repair titration as a cause of these transformation-dependent increases in switching. The DNA-dependent effect on phase variation was transient and may serve as a mechanism for meningococcal genetic variability that avoids the fitness costs encountered by global mutators.

  4. Martensitic transformation in ZrO 2-based ceramics at cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Li, L.-F.; Hong, C.-S.; Li, Y.-Y.; Zhang, Z.

    The microstructural changes associated with the tetragonal to monoclinic martensitic transformation at cryogenic temperatures in sintered CeO2-ZrO2 ceramics containing 15.5-16.5 mol% CeO2 have been studied by means of TEM observations. X-ray diffraction analysis indicates that the stress-induced martensitic phase increases with decreases in both temperature and CeO2 content. The effects of martensitic morphologies, anti-phase boundaries (APBs) and various dislocation features on mechanical properties are also discussed in the paper.

  5. Phase transformation of the A15 metastable phase of Fe-Cr thin films prepared by ion-beam sputtering

    NASA Astrophysics Data System (ADS)

    Al-Khoury, W.; Eymery, J.-P.; Goudeau, Ph.

    2007-08-01

    Thermal stability of metastable A15 Fe-Cr phase is investigated through the study of its magnetic and structural properties. This phase presents very interesting mechanical properties suggesting that A15-structured films might be of great interest for tribological applications when considering the hardness H over Young's modulus E ratio i.e., a description in terms of "elastic strain to failure" for wear resistance. Indeed, H is multiplied by a factor 2 with respect to the value measured for the bulk cubic centered α phase whereas E remains identical. Then, an improvement by a factor 8 of resistance to plastic deformation may be expected since predictive models stand that this quantity is proportional to the H3/E2 ratio. However, heating problems due to sliding during tribological tests may lead to structural transformation in the film and then a loose of mechanical performance. The formation and the stability of the A15 cubic structure (δ phase) in centered-cubic refractory metals are generally attributed to the presence of oxygen atoms in the unit cell. For equiatomic Fe-Cr thin films elaborated by physical vapor deposition techniques, residual oxygen atoms present in the deposition chamber would be absorbed during the deposition process. In this work, the δ-phase transformation has been studied ex situ in the temperature range 400-650 °C; structural changes have been accurately investigated thanks to the combination of x-ray diffraction and Mössbauer spectroscopy techniques. Thin films were deposited onto quartz substrates and then annealed ex situ under secondary vacuum. From 400 °C, a "structural relaxation" occurring in the δ phase precedes and accompanies the beginning of the phase transformation. Finally, the partially ordered metastable δ-phase transforms into a stable α-phase presenting the precipitation phenomenon at temperature above 550 °C and the presence of a τ-carbide phase is clearly visible from 600 °C. The δ-phase transformation is

  6. Pressure-temperature phase diagrams of biomolecules.

    PubMed

    Smeller, László

    2002-03-25

    The pressure-temperature phase diagram of various biomolecules is reviewed. Special attention is focused on the elliptic phase diagram of proteins. The phenomenological thermodynamic theory describing this diagram explains the heat, cold and pressure denaturations in a unified picture. The limitations and possible developments of this theory are discussed as well. It is pointed out that a more complex diagram can be obtained when the intermolecular interactions are also taken into account. In this case metastable states appear on the pressure-temperature (p-T) diagram due to intermolecular interactions. Pressure-temperature phase diagrams of other biopolymers are also discussed. While the p-T diagrams of helix-coil transition of nucleic acids and of gel-liquid crystal transition of lipid bilayers are non-elliptical, those of gelatinization of starch and of phase separation of some synthetic polymers show an elliptic profile, similar to that of proteins. Finally, the p-T diagram of bacterial inactivation is shown to be elliptic. From the point of view of basic science, this fact shows that the key factor of inactivation should be the protein type, and from the viewpoint of practical applications, it serves as the theoretical basis of pressure treatment of biosystems.

  7. Complex Transformations between Bicontinuous Cubic and Cylinder Phases in a Polystyrene-block-Poly(ethylene oxide) Diblock Copolymer

    NASA Astrophysics Data System (ADS)

    Zhu, Lei; Sun, Lu; Ge, Qing; Quirk, Roderic P.; Cheng, Stephen Z. D.; Hsiao, Benjamin S.; Sics, Igors; Avila-Orta, Carlos

    2004-03-01

    Complex phase transformations between bicontinuous cubic and hexagonal cylinder (Hex) phases in a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer were investigated using small angle X-ray scattering (SAXS), transmission electron microscope (TEM), rheology, and polarized light microscope (PLM). The sample exhibited a typical double gyroid (G) phase, together with a minority plumbers nightmare (P) phase which was only ˜6 vol.% as calculated from the SAXS scattering intensities for each phase. These two bicontinuous cubic phases had the same unit cell dimensions. Under a large-amplitude reciprocating shear, the bicontinuous cubic sample transformed into a single-crystal Hex phase. Annealing this sample at 150 ^oC for 40 min, the Hex phase partially transformed into well-oriented G and P twin structures, as evidenced by two-dimensional synchrotron SAXS experiments. Epitaxial phase transformation relationships between the Hex/G and Hex/P phases were identified. The phase transformations were further confirmed by rheology study and PLM observations. The P phase was metastable with respect to the G phase, and it disappeared when the sample was heated above the order-disorder transition temperature and annealed at 150 ^oC. The mechanism of the Hex arrow G transformation was investigated by TEM. Generally, in a hexagonal cell, three cylinders evolved into left-handed helices, while the other three formed right-handed helices. An intermediate five-fold junction was speculated to facilitate the phase transformation. The Hex -> G phase transformation was observed to follow a nucleation and growth mechanism, and the phase transition zone was less than one unit cell.

  8. Mutual transformation between crystalline phases in silicon after treatment in a planetary mill: HRTEM studies.

    PubMed

    Kulnitskiy, Boris; Annenkov, Mikhail; Perezhogin, Igor; Popov, Mikhail; Ovsyannikov, Danila; Blank, Vladimir

    2016-10-01

    High-resolution transmission electron microscopy (HRTEM) studies of silicon after treatment in a planetary mill have been performed. It is shown that along with the initial phase of silicon, Si-I, the sample also contains some high-pressure phases: Si-III (Kasper phase) and Si-IV (lonsdaleite). We studied the orientation relationship between the particles of different phases, finding that there are, in general, two mechanisms of formation of Si-IV: (1) through the stacking faults formation; (2) through the transformation first to the Kasper phase (Si-III), and then from the Kasper phase to Si-IV. Estimations of temperature and pressure conditions in the planetary ball mill made previously are in accordance with the conditions of formation of the above-mentioned phases.

  9. Effect of field driven phase transformations on the loss tangent of relaxor ferroelectric single crystals

    NASA Astrophysics Data System (ADS)

    Gallagher, John A.; Liu, Tieqi; Lynch, Christopher S.

    2013-02-01

    The effect of a bias stress induced phase transformation on the large field dielectric loss in [001] cut and poled single crystal stack actuators of (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT, x = 32) was experimentally characterized. Dielectric loss was observed to increase with compressive preload and electric field amplitude. The dielectric loss was determined by measuring the area within electric displacement vs. electric field hysteresis loops and the measured area was expressed in terms of an effective loss tangent. This approach matches the measured area within the hysteresis loop to an equivalent area ellipse in which the electric displacement lags the electric field by an amount, delta, under sinusoidal loading. The results collapse the measured loss as a function of bias stress and electric field amplitude reasonably close to a single curve. The measured dielectric loss behavior was attributed to the compressive stress preload driving a partial phase transformation from rhombohedral to orthorhombic and the electric field driving the reverse phase transformation from the stress induced orthorhombic phase to the zero stress rhombohedral phase. When the compressive bias stress partially or fully drives this phase transformation, the dielectric loss under unipolar electric field loading increases. This work is focused on quasi-static measurements. The large field dielectric loss is anticipated to be a function of frequency and temperature.

  10. Phase Transformations and Formation of Ultra-fine Microstructure during Hydrogen Sintering and Phase Transformation (HSPT) Processing of Ti-6Al-4V

    SciTech Connect

    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 H-2 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 alpha/alpha(2) within coarse beta grains during an isothermal hold at intermediate temperatures, and (2) the eutectoid transformation of beta -> alpha + delta d at approximately 473 K (200 degrees C). (C) The Minerals, Metals & Materials Society and ASM International 2015

  11. 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

  12. Numerical estimation of phase transformations in solid state during Yb:YAG laser heating of steel sheets

    SciTech Connect

    Kubiak, Marcin Piekarska, Wiesława; Domański, Tomasz; Saternus, Zbigniew; Stano, Sebastian

    2015-03-10

    This work concerns the numerical modeling of heat transfer and phase transformations in solid state occurring during the Yb:YAG laser beam heating process. The temperature field is obtained by the numerical solution into transient heat transfer equation with convective term. The laser beam heat source model is developed using the Kriging interpolation method with experimental measurements of Yb:YAG laser beam profile taken into account. Phase transformations are calculated on the basis of Johnson - Mehl - Avrami (JMA) and Koistinen - Marburger (KM) kinetics models as well as continuous heating transformation (CHT) and continuous cooling transformation (CCT) diagrams for S355 steel. On the basis of developed numerical algorithms 3D computer simulations are performed in order to predict temperature history and phase transformations in Yb:YAG laser heating process.

  13. Effect of chemical etching and mechanical polishing on the transformation-temperature of super elastic shape memory alloys

    NASA Astrophysics Data System (ADS)

    Pattabi, Manjunatha; Murari, M. S.

    2013-06-01

    Effect of chemical etching and mechanical polish on the transformation temperature and growth of R-Phase in super elastic NiTi shape memory alloy have been studied in this paper. It has been observed that the transformation temperature of sample subjected to mechanical polish (HT660MP) is lower than that of samples subjected to chemical etching (HT660CE). With initial thermal cycle the phase transformation temperature get lowered for both the samples and for higher number of thermal cycle the single step transformation A→M changes to A→R→M. Because of the mechanical work the intermediate R-Phase appears at higher number of thermal cycle (N) for HT660MP compared to that of HT660CE.

  14. Effect of oxygen on the thermomechanical behavior of tantalum thin films during the {beta}-{alpha} phase transformation

    SciTech Connect

    Knepper, Robert; Stevens, Blake; Baker, Shefford P.

    2006-12-15

    Tantalum thin films were prepared in the metastable {beta} phase, and their thermomechanical behaviors were investigated in situ in an ultrahigh vacuum environment. Controlled levels of oxygen were incorporated into the films either during deposition, by surface oxidation after deposition, or during thermomechanical testing. The transformation from the {beta} phase to the stable {alpha} phase takes place in conjunction with a distinct increase in tensile stress. The thermomechanical behavior is strongly affected by the amount of oxygen to which the film is exposed and the method of exposure. Increasing oxygen content inhibits the phase transformation, requiring higher temperatures to reach completion. It is shown that the phase transformation takes place by a nucleation and growth process that is limited by growth. Changes in the activation energy for the phase transformation due to solute drag are estimated as a function of oxygen content and the mechanisms behind the stress evolution are elucidated.

  15. The use of Fourier reverse transforms in crystallographic phase refinement

    SciTech Connect

    Ringrose, Sharon

    1997-10-08

    Often a crystallographer obtains an electron density map which shows only part of the structure. In such cases, the phasing of the trial model is poor enough that the electron density map may show peaks in some of the atomic positions, but other atomic positions are not visible. There may also be extraneous peaks present which are not due to atomic positions. A method for determination of crystal structures that have resisted solution through normal crystallographic methods has been developed. PHASER is a series of FORTRAN programs which aids in the structure solution of poorly phased electron density maps by refining the crystallographic phases. It facilitates the refinement of such poorly phased electron density maps for difficult structures which might otherwise not be solvable. The trial model, which serves as the starting point for the phase refinement, may be acquired by several routes such as direct methods or Patterson methods. Modifications are made to the reverse transform process based on several assumptions. First, the starting electron density map is modified based on the fact that physically the electron density map must be non-negative at all points. In practice a small positive cutoff is used. A reverse Fourier transform is computed based on the modified electron density map. Secondly, the authors assume that a better electron density map will result by using the observed magnitudes of the structure factors combined with the phases calculated in the reverse transform. After convergence has been reached, more atomic positions and less extraneous peaks are observed in the refined electron density map. The starting model need not be very large to achieve success with PHASER; successful phase refinement has been achieved with a starting model that consists of only 5% of the total scattering power of the full molecule. The second part of the thesis discusses three crystal structure determinations.

  16. Crystallization kinetics and phase transformations in aluminum ion-implanted electrospun TiO2 nanofibers

    NASA Astrophysics Data System (ADS)

    Albetran, H.; Low, I. M.

    2016-12-01

    Electrospun TiO2 nanofibers were implanted with aluminum ions, and their crystallization kinetics, phase transformations, and activation energies were investigated from 25 to 900 °C by in situ high-temperature synchrotron radiation diffraction. The amorphous non-implanted and Al ion-implanted TiO2 nanofibers transformed to crystalline anatase at 600 °C and to rutile at 700 °C. The TiO2 phase transformation of the Al ion-implanted material was accelerated relative to non-implanted sample. Compared with non-implanted nanofibers, the Al-implanted materials yielded a decreased activation energies from 69(17) to 29(2) kJ/mol for amorphous-to-anatase transformation and from 112(15) to 129(5) kJ/mol for anatase-to-rutile transformation. A substitution of smaller Al ions for Ti in the TiO2 crystal structure results in accelerated titania phase transformation and a concomitant reduction in the activation energies.

  17. Critical kinetic control of non-stoichiometric intermediate phase transformation for efficient perovskite solar cells

    NASA Astrophysics Data System (ADS)

    Rong, Yaoguang; Venkatesan, Swaminathan; Guo, Rui; Wang, Yanan; Bao, Jiming; Li, Wenzhi; Fan, Zhiyong; Yao, Yan

    2016-06-01

    Organometal trihalide perovskites (OTP) have attracted significant attention as a low-cost and high-efficiency solar cell material. Due to the strong coordination between lead iodide (PbI2) and dimethyl sulfoxide (DMSO) solvent, a non-stoichiometric intermediate phase of MA2Pb3I8(DMSO)2 (MA = CH3NH3+) usually forms in the one-step deposition method that plays a critical role in attaining high power conversion efficiency. However, the kinetic understanding of how the non-stoichiometric intermediate phase transforms during thermal annealing is currently absent. In this work, we investigated such a phase transformation and provided a clear picture of three phase transition pathways as a function of annealing conditions. The interdiffusion of MAI and DMSO varies strongly with the annealing temperature and time, thus determining the final film composition and morphology. A surprising finding reveals that the best performing cells contain ~18% of the non-stoichiometric intermediate phase, instead of pure phase OTP. The presence of such an intermediate phase enables smooth surface morphology and enhances the charge carrier lifetime. Our results highlight the importance of the intermediate phase growth kinetics that could lead to large-scale production of efficient solution processed perovskite solar cells.Organometal trihalide perovskites (OTP) have attracted significant attention as a low-cost and high-efficiency solar cell material. Due to the strong coordination between lead iodide (PbI2) and dimethyl sulfoxide (DMSO) solvent, a non-stoichiometric intermediate phase of MA2Pb3I8(DMSO)2 (MA = CH3NH3+) usually forms in the one-step deposition method that plays a critical role in attaining high power conversion efficiency. However, the kinetic understanding of how the non-stoichiometric intermediate phase transforms during thermal annealing is currently absent. In this work, we investigated such a phase transformation and provided a clear picture of three phase transition

  18. Calcium carbonate phase transformations during the carbonation reaction of calcium heavy alkylbenzene sulfonate overbased nanodetergents preparation.

    PubMed

    Chen, Zhaocong; Xiao, Shan; Chen, Feng; Chen, Dongzhong; Fang, Jianglin; Zhao, Min

    2011-07-01

    The preparation and application of overbased nanodetergents with excess alkaline calcium carbonate is a good example of nanotechnology in practice. The phase transformation of calcium carbonate is of extensive concern since CaCO(3) serves both as an important industrial filling material and as the most abundant biomineral in nature. Industrially valuable overbased nanodetergents have been prepared based on calcium salts of heavy alkylbenzene sulfonate by a one-step process under ambient pressure, the carbonation reaction has been monitored by the instantaneous temperature changes and total base number (TBN). A number of analytical techniques such as TGA, DLS, SLS, TEM, FTIR, and XRD have been utilized to explore the carbonation reaction process and phase transformation mechanism of calcium carbonate. An enhanced understanding on the phase transformation of calcium carbonate involved in calcium sulfonate nanodetergents has been achieved and it has been unambiguously demonstrated that amorphous calcium carbonate (ACC) transforms into the vaterite polymorph rather than calcite, which would be of crucial importance for the preparation and quality control of lubricant additives and greases. Our results also show that a certain amount of residual Ca(OH)(2) prevents the phase transformation from ACC to crystalline polymorphs. Moreover, a vaterite nanodetergent has been prepared for the first time with low viscosity, high base number, and uniform particle size, nevertheless a notable improvement on its thermal stability is required for potential applications. Copyright © 2011 Elsevier Inc. All rights reserved.

  19. Phase transformation during silica cluster impact on crystal silicon substrate studied by molecular dynamics simulation

    NASA Astrophysics Data System (ADS)

    Chen, Ruling; Luo, Jianbin; Guo, Dan; Lu, Xinchun

    2008-07-01

    The process of a silica cluster impact on a crystal silicon substrate is studied by molecular dynamics simulation. At the impact loading stage, crystal silicon of the impact zone transforms to a locally ordered molten with increasing the local temperature and pressure of the impact zone. And then the transient molten forms amorphous silicon directly as the local temperature and pressure decrease at the impact unloading stage. Moreover, the phase behavior between the locally ordered molten and amorphous silicon exhibits the reversible structural transition. The transient molten contains not only lots of four-fold atom but also many three- and five-fold atoms. And the five-fold atom is similar to the mixture structure of semi-Si-II and semi-bct5-Si. The structure transformation between five- and four-fold atoms is affected by both pressure and temperature. The structure transformation between three- and four-fold atoms is affected mostly by temperature. The direct structure transformation between five- and three-fold atoms is not observed. Finally, these five- and three-fold atoms are also different from the usual five- and three-fold deficient atoms of amorphous silicon. In addition, according to the change of coordination number of atoms the impact process is divided into six stages: elastic, plastic, hysteresis, phase regressive, adhesion and cooling stages.

  20. Experimental evidence of α → β phase transformation in SiC quantum dots and their size-dependent luminescence

    SciTech Connect

    Guo, Xiaoxiao; Dai, Dejian; Fan, Baolu; Fan, Jiyang

    2014-11-10

    Phase transformation can occur among different SiC polytypes under extreme conditions such as high pressure or temperature. It remains unknown whether phase transformation can occur under normal conditions. We demonstrate that the α → β phase transformation can occur at ambient temperature and pressure in nanoscale SiC. The microstructural characterization and light absorption and emission spectroscopy demonstrate the occurrence of this phase transformation. It is found that the quantum-confinement luminescence dominates in larger SiC quantum dots (QDs) and the surface-defect luminescence dominates in ultrasmall SiC QDs. The rare phenomenon of multiple-phonon-assisted light absorption is observed in the SiC QDs.

  1. Shock-wave response of Ti-Ni shape memory alloys in the transformation temperature range

    NASA Astrophysics Data System (ADS)

    Razorenov, Sergey V.; Garkushin, Gennady V.; Kanel, Gennady I.; Popov, Nikolay N.

    2009-06-01

    The behavior of Ti51.1Ni48.9 and Ti49.4Ni50.6 alloys under shock wave loading was investigated to observe their martensitic transformations. Tested samples had the grain sizes ˜30 μm and 0.05 to 0.3 μm. Reduction of the grain size was done by means of severe plastic deformation methods. In the experiments, the VISAR velocity histories were recorded over the test temperatures range from 193 K to 415 K which involves the temperatures of thermoelastic martensitic transformations of the alloys. Waveforms demonstrate temperature dependences of the Hugoniot elastic limits which is controlled by the critical stress for inducing martensitic transformation, phase transformation without expected so called plateau, and in some cases signatures of pseudo-elastic behavior. The reduction of the material grain size has led to rise in both the HEL values and transformation rates and decrease of the spall strength over whole temperature range.

  2. 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.

  3. 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

  4. Dynamic Response of High Temperature Uranium Phases

    SciTech Connect

    Zaretsky, E.; Herrmann, B.; Shvarts, D.

    2006-07-28

    Unalloyed uranium and uranium-0.78 wt%Ti alloy were studied in planar impact experiments with initial sample temperature ranging from 27 to 860 degree sign C. The velocity of the free surface of the samples was monitored by VISAR. It was found that the dynamic compressive strength of both the materials undergoes two-fold increase in the narrow temperature interval corresponding to the domain of beta-phase of uranium. The increase is followed by abrupt, factor of 3-4, strength drop when the initial state of the tested material is gamma-uranium. Such strength behavior explains the uranium susceptibility to adiabatic shear banding. The spall strength of both the alloys is characterized by similar temperature variations. The strength mechanism (phonon viscosity) acting in gamma-phase of pure uranium seems inherited from its alpha-structure while the strength of beta-uranium is controlled by high resistance to shearing characteristic for material having the structure of intermetallic sigma-phase.

  5. Phase-transformation-induced lubrication of earthquake sliding.

    PubMed

    Green, Harry W

    2017-09-28

    Frictional failure is not possible at depth in Earth, hence earthquakes deeper than 30-50 km cannot initiate by overcoming dry friction. Moreover, the frequency distribution of earthquakes with depth is bimodal, suggesting another change of mechanism at about 350 km. Here I suggest that the change at 30-50 km is from overcoming dry friction to reduction of effective stress by dehydration embrittlement and that the change at 350 km is due to desiccation of slabs and initiation by phase-transformation-induced faulting. High-speed friction experiments at low pressure indicate that exceeding dry friction provokes shear heating that leads to endothermic reactions and pronounced weakening. Higher-pressure studies show nanocrystalline gouge accompanying dehydration and the highest pressure experiments initiate by exothermic polymorphic phase transformation. Here I discuss the characteristic nanostructures of experiments on high-speed friction and high-pressure faulting and show that all simulated earthquake systems yield very weak transformation-induced lubrication, most commonly nanometric gouge or melt. I also show that phase-transformation-induced faulting of olivine to spinel can propagate into material previously transformed to spinel, apparently by triggering melting analogous to high-speed friction studies at low pressure. These experiments taken as a whole suggest that earthquakes at all depths slide at low frictional resistance by a self-healing pulse mechanism with rapid strength recovery.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'. © 2017 The Author(s).

  6. Possible triggers for phase transformation in zirconia hip balls.

    PubMed

    Brown, Sharon S; Green, Douglas D; Pezzotti, Giuseppe; Donaldson, Thomas K; Clarke, Ian C

    2008-05-01

    The clinical history of yttria-stabilized, zirconia (Zr) ceramic has been controversial. In the patient, combinations of hydrothermal and mechanical shocks may trigger detrimental changes in Zr balls that have inferior metastability. Transformations from tetragonal to monoclinic phase may be influenced by impingement, dislocation, and disassociation in certain patients. Hydrothermal stability was measured in Zr balls from four vendors by autoclave and mechanical models that included "cup-impingement," "abrasive" wear, and "3rd-body" wear. Standard simulator tests for polyethylene (PE) wear studies combined pristine and previously transformed Zr and were also used to test lubricant effects (Zr/Zr-serum, Zr/Zr-water, Zr/PE-water). For in-vivo comparisons we studied retrieved Zr balls at 1-15 years follow-up by laser interferometry, SEM, EDS, XRD, and Raman spectroscopy. We found that severe mechanical shock triggered local surface destruction but little transformation. In contrast, hydrothermal processes revealed 5-13% monoclinic by 7 h, increasing at a rate of 0.56%/h for 22-mm balls and 0.81%/h for 26-mm balls. The all-ceramic Zr/Zr bearings were very sensitive to lubrication mode, showing early catastrophic failure when run in water but surviving 20 million cycles when run with serum lubrication. Wear with Zr/PE combination did not trigger phase changes in water or serum but decreased the monoclinic content measured on previously transformed surfaces. Most retrieved Zr balls showed high transformation (30-85% monoclinic) but some showed no transformation. The ball areas with major monoclinic changes corresponded to PE contact, suggesting that tribological conditions under the cup were the trigger. This indicated that we understand little of the hydrothermal conditions operating under Zr/PE hip joints in-vivo. Copyright 2007 Wiley Periodicals, Inc.

  7. Phase transformation induced resistive switching behavior in Al/Cu2Se/Pt

    NASA Astrophysics Data System (ADS)

    Rehman, Shania; Kim, Kihwan; Hur, Ji-Hyun; Kim, Deok-kee

    2017-04-01

    The phase transformation induced resistive switching behavior of an Al/Cu2Se/Pt device was studied. While the device did not demonstrate any resistive switching behavior at room temperature, it exhibited resistive switching behavior at 125 °C, near the transition temperature of copper(I) selenide (Cu2Se) (137 °C), where Cu2Se is known to transform from the monoclinic to superionic phase. The increase in ionic conductivity and structural variations (from ordered to disordered structure) associated with phase transformation were observed to be responsible for the origin of the switching behavior and increase in the on/off resistance ratio near the transition temperature. Thermodynamic calculations showed that a reduction in Gibb’s free energy of nucleation and an increase in the migration speed of the Cu ion associated with the ionic conductivity and order to disorder the transition of the Cu2Se at the transition temperature were the important factors responsible for the reduction in the SET voltages at 155 °C.

  8. Crystalline phase transformation of colloidal cadmium sulfide nanocrystals

    NASA Astrophysics Data System (ADS)

    Ghali, M.; Eissa, A. M.; Mosaad, M. M.

    2017-03-01

    In this paper, we give a microscopic view concerning influence of the growth conditions on the physical properties of nanocrystals (NCs) thin films made of CdS, prepared using chemical bath deposition CBD technique. We show a crystalline phase transformation of CdS NCs from hexagonal wurtzite (W) structure to cubic zincblende (ZB) when the growth conditions change, particularly the solution pH values. This effect was confirmed using X-ray diffraction (XRD), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) measurements. The optical absorption spectra allow calculation of the bandgap value, Eg, where significant increase ˜200 meV in the CdS bandgap when transforming from Hexagonal to Cubic phase was found.

  9. Simplified three-phase transformer model for electromagnetic transient studies

    SciTech Connect

    Chimklai, S.; Marti, J.R.

    1995-07-01

    This paper presents a simplified high-frequency model for three-phase, two- and three-winding transformers. The model is based on the classical 60-Hz equivalent circuit, extended to high frequencies by the addition of the winding capacitances and the synthesis of the frequency-dependent short-circuit branch by an RLC equivalent network. By retaining the T-form of the classical model, it is possible to separate the frequency-dependent series branch from the constant-valued shunt capacitances. Since the short-circuit branch can be synthesized by a minimum-phase-shift rational approximation, the mathematical complications of fitting mutual impedance or admittance functions are avoided and the model is guaranteed to be numerically absolutely stable. Experimental tests were performed on actual power transformers to determine the parameters of the model. EMTP simulation results are also presented.

  10. Phase transformation of Ho[subscript 2]O[subscript 3] at high pressure

    SciTech Connect

    Jiang, Sheng; Liu, Jing; Li, Xiaodong; Bai, Ligang; Xiao, Wansheng; Zhang, Yufeng; Lin, Chuanlong; Li, Yanchun; Tang, Lingyun

    2012-01-20

    The structural stability of cubic Ho{sub 2}O{sub 3} under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with an {approx}8.1% volume collapse. A hexagonal phase begins to appear at {approx}14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B{sub 0}) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B'{sub 0}) of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that the transition pressure from cubic to monoclinic phase, as well as the bulk modulus of the cubic phase, increases with the decreasing of the cation radius of rare-earth sesquioxides.

  11. Phase transformation of PZST-86/14-5-2Nb ceramic under quasi-static loading conditions.

    SciTech Connect

    Broome, Scott Thomas; Scofield, Timothy W.; Montgomery, Stephen Tedford; Bauer, Stephen J.; Hofer, John H.

    2010-02-01

    Specimens of poled and unpoled PZST ceramic were tested under hydrostatic loading conditions at temperatures of -55, 25, and 75 C. The objective of this experimental study was to obtain the electro-mechanical properties of the ceramic and the criteria of FE (Ferroelectric) to AFE (Antiferroelectric) phase transformations of the PZST ceramic to aid grain-scale modeling efforts in developing and testing realistic response models for use in simulation codes. As seen in previous studies, the poled ceramic from PZST undergoes anisotropic deformation during the transition from a FE to an AFE phase at -55 C. Warmer temperature tests exhibit anisotropic deformation in both the FE and AFE phase. The phase transformation is permanent at -55 C for all ceramics tests, whereas the transformation can be completely reversed at 25 and 75 C. The change in the phase transformation pressures at different temperatures were practically identical for both unpoled and poled PZST specimens. Bulk modulus for both poled and unpoled material was lowest in the FE phase, intermediate in the transition phase, and highest in the AFE phase. Additionally, bulk modulus varies with temperature in that PZST is stiffer as temperature decreases. Results from one poled-biased test for PZST and four poled-biased tests from PNZT 95/5-2Nb are presented. A bias of 1kV did not show noticeable differences in phase transformation pressure for the PZST material. However, with PNZT 95/5-2Nb phase transformation pressure increased with increasing voltage bias up to 4.5kV.

  12. Evaluation of colour space transformation suitability to optical temperature measurements

    NASA Astrophysics Data System (ADS)

    Ziemba, A.; Fornalik-Wajs, E.

    2016-09-01

    All optical measurement methods base on the image analysis and relation between the measured parameter and some image features. In Digital Particle Image Thermometry (DPIT), such relation represents a function between the temperature and particles’ colour (i.a. Thermochromic Liquid Crystals). For the quantitative data acquisition the “colour” information is necessary, therefore the colour spaces based on hue H are used. Due to the big number of numerical operations needed in the analysis, the choice of colour space transformation is significant due to the accuracy and computational time. In this paper commonly applied RGB to HSI colour spaces’ transformations were compared and evaluation of their suitability to temperature measurement was performed. Time of obtaining the final results was considered as the main criterion. Appropriate calculations were conducted and presented.

  13. Modeling of variant-interaction during bainitic phase transformation

    NASA Astrophysics Data System (ADS)

    Ehlenbröker, U.; Mahnken, R.; Petersmann, M.; Antretter, T.

    2016-03-01

    In our research, we develop a thermodynamically consistent multi-scale model for phase transformations from austenite into n possible bainite variants. Each material point of the macroscopic configuration represents a polycrystal which describes the mesoscopic configuration. The microscopic configuration consists of an agglomeration of variants which is attached to each single crystal of the mesoscopic configuration. In addition, the model allows simulation of the macroscopic effects of volume change due to phase transformation as well as transformation-induced plasticity (TRIP). In this paper we present the results of recent work in the context of this model, which is concerned with the extension of the model for an effect of variant-interaction between the different crystallographic variants of bainite. For this reason we make use of the theory of transformation hardening. Thereby we are able to include an effect of preferential variant formation. This leads to the simultaneous formation of a selection of variants while the evolution of crystallographic unfavorable variants is handicapped or even completely suppressed. This extension of the model aims for the fact that in general not every crystallographic variant of bainite forms within a single austenite grain.

  14. Structure and thermoelastic martensitic transformations in ternary Ni-Ti-Hf alloys with a high-temperature shape memory effect

    NASA Astrophysics Data System (ADS)

    Pushin, V. G.; Kuranova, N. N.; Pushin, A. V.; Uksusnikov, A. N.; Kourov, N. I.

    2016-07-01

    The effect of alloying by 12-20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi-NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ↔ B19' martensitic transformations, which occur in the temperature range 320-600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19' phases are measured, and the microstructure of the B19' martensite is analyzed.

  15. On phase transformation behavior of porous Shape Memory Alloys.

    PubMed

    Liu, Bingfei; Dui, Guansuo; Zhu, Yuping

    2012-01-01

    This paper is concerned on the phase transformation mechanism of porous Shape Memory Alloys (SMAs). A unit-cell model is adopted to establish the constitutive relation for porous SMAs, the stress distributions, the phase distributions and the martensitic volume fractions for the model are then derived under both pure hydrostatic stress and uniaxial compression. Further, an example for the uniaxial response under compression for a porous Ni-Ti SMA material considering hydrostatic stress is supplied. Good agreement between the theoretical prediction of the proposed model and published experimental data is observed. Crown Copyright © 2011. Published by Elsevier Ltd. All rights reserved.

  16. 2D Hilbert transform for phase retrieval of speckle fields

    NASA Astrophysics Data System (ADS)

    Gorsky, M. P.; Ryabyi, P. A.; Ivanskyi, D. I.

    2016-09-01

    The paper presents principal approaches to diagnosing the structure forming skeleton of the complex optical field. An analysis of optical field singularity algorithms depending on intensity discretization and image resolution has been carried out. An optimal approach is chosen, which allows to bring much closer the solution of the phase problem of localization speckle-field special points. The use of a "window" 2D Hilbert transform for reconstruction of the phase distribution of the intensity of a speckle field is proposed. It is shown that the advantage of this approach consists in the invariance of a phase map to a change of the position of the kernel of transformation and in a possibility to reconstruct the structure-forming elements of the skeleton of an optical field, including singular points and saddle points. We demonstrate the possibility to reconstruct the equi-phase lines within a narrow confidence interval, and introduce an additional algorithm for solving the phase problem for random 2D intensity distributions.

  17. Application of phase coherent transform to cloud clutter suppression

    SciTech Connect

    Ng, L.C.

    1994-11-15

    This paper describes a tracking algorithm using frame-to-frame correlation with frequency domain clutter suppression. Clutter suppression was mechanized via a `Phase Coherent Transform` (PCT) approach. This approach was applied to explore the feasibility of tracking a post-boost rocket from a low earth orbit satellite with real cloud background data. Simulation results show that the PCT/correlation tracking algorithm can perform satisfactorily at signal-to-clutter ratio (SCR) as low as 5 or 7 dB.

  18. Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

    SciTech Connect

    Lee, Hoo-Jeong; Ramirez, Ainissa G.

    2004-08-16

    Amorphous sputtered nickel-titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19{sup '} structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470 deg. C and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35 deg. C. Calorimetric measurements are consistent with the observed crystallization.

  19. Time-temperature-transformation diagrams with more than one nose

    NASA Technical Reports Server (NTRS)

    Weinberg, Michael C.

    1992-01-01

    The structures of time-temperature-transformation diagrams of glasses which crystallize the combined homogeneous and heterogeneous crystallization mechanisms are examined. Considerations are given to the factors which might produce more than one extremum in such diagrams. Specific nucleation and growth models are used, and the influence of the parameters which appear in the nucleation and growth rate expressions upon the structure of the diagrams is evaluated.

  20. Phase Transformation Behavior of Porous TiNi Alloys Produced by Powder Metallurgy Using Magnesium as a Space Holder

    NASA Astrophysics Data System (ADS)

    Aydoğmuş, Tarik; Bor, Elif Tarhan; Bor, Şakir

    2011-09-01

    Porous TiNi alloys with porosities in the range of 51 to 73 pct were prepared successfully applying a new powder metallurgy fabrication route in which magnesium was used as a space holder, resulting in either single austenite phase or a mixture of austenite and martensite phases dictated by the composition of the starting powders, but entirely free from secondary brittle intermetallics, oxides, nitrides, and carbonitrides. Since transformation temperatures are very sensitive to composition, deformation, and oxidation, for the first time, transformation temperatures of porous TiNi alloys were investigated using chemically homogeneous specimens in as-sintered and aged conditions eliminating secondary phase, contamination, and deformation effects. It was found that the porosity content of the foams has no influence on the phase transformation temperatures both in as-sintered and aged conditions, while deformation, oxidation, and aging treatment are severely influential.

  1. Phase stretch transform for super-resolution localization microscopy

    PubMed Central

    Ilovitsh, Tali; Jalali, Bahram; Asghari, Mohammad H.; Zalevsky, Zeev

    2016-01-01

    Super-resolution localization microscopy has revolutionized the observation of living structures at the cellular scale, by achieving a spatial resolution that is improved by more than an order of magnitude compared to the diffraction limit. These methods localize single events from isolated sources in repeated cycles in order to achieve super-resolution. The requirement for sparse distribution of simultaneously activated sources in the field of view dictates the acquisition of thousands of frames in order to construct the full super-resolution image. As a result, these methods have slow temporal resolution which is a major limitation when investigating live-cell dynamics. In this paper we present the use of a phase stretch transform for high-density super-resolution localization microscopy. This is a nonlinear frequency dependent transform that emulates the propagation of light through a physical medium with a specific warped diffractive property and applies a 2D phase function to the image in the frequency domain. By choosing properly the transform parameters and the phase kernel profile, the point spread function of each emitter can be sharpened and narrowed. This enables the localization of overlapping emitters, thus allowing a higher density of activated emitters as well as shorter data collection acquisition rates. The method is validated by numerical simulations and by experimental data obtained using a microtubule sample. PMID:27867725

  2. Phase transformation-induced tetragonal FeCo nanostructures.

    PubMed

    Gong, Maogang; Kirkeminde, Alec; Wuttig, Manfred; Ren, Shenqiang

    2014-11-12

    Tetragonal FeCo nanostructures are becoming particularly attractive because of their high magnetocrystalline anisotropy and magnetization achievable without rare-earth elements, . Yet, controlling their metastable structure, size and stoichiometry is a challenging task. In this study, we demonstrate AuCu templated FeCo shell growth followed by thermally induced phase transformation of AuCu core from face-centered cubic to L10 structure, which triggers the FeCo shell to transform from the body-centered cubic structure to a body-centered tetragonal phase. High coercivity, 846 Oe, and saturation magnetization, 221 emu/g, are achieved in this tetragonal FeCo structure. Beyond a critical FeCo shell thickness, confirmed experimentally and by lattice mismatch calculations, the FeCo shell relaxes. The shell thickness and stoichiometry dictate the magnetic characteristics of the tetragonal FeCo shell. This study provides a general route to utilize phase transformation to fabricate high performance metastable nanomagnets, which could open up their green energy applications.

  3. 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.

  4. Effects of Quenching Media on Phase Transformation Characteristics and Hardness of Cu-Al-Ni-Co Shape Memory Alloys

    NASA Astrophysics Data System (ADS)

    Saud, Safaa N.; Hamzah, E.; Abubakar, T.; Farahany, S.; Bakhsheshi-Rad, H. R.

    2015-04-01

    This paper presents the investigation on the effects of various thermal treatments and quenching media on the phase transformation behaviour of Cu-Al-Ni-Co shape memory alloys (SMAs). The transformation temperatures were determined using a differential scanning calorimeter. The variation of cooling rates had a consequential effect on the phase transformation characteristics of the Cu-Al-Ni-Co SMAs. Nevertheless, the transformation temperature peaks were varied in terms of location as well as heat flow. The results indicated that there was an improvement in transformation temperatures whenever ice water was used as quenching medium. It was also observed that the forward transformation temperatures were higher than the reverse transformation. It was verified that the required heat for the transformation of martensite into austenite was more than the transformation of austenite into martensite. Moreover, thermodynamic parameters, such as enthalpy and entropy, tended to decrease and increase as a result of the changes in the cooling rates of each medium. To clarify the variations of the structures and properties of Cu-Al-Ni-Co SMA quenched samples, x-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, and Vickers hardness were used.

  5. Structural and dynamical transformations between neighboring dense microemulsion phases

    NASA Astrophysics Data System (ADS)

    Kotlarchyk, M.; Sheu, E. Y.; Capel, M.

    1992-07-01

    A small-angle x-ray scattering (SAXS) study of dense AOT-water-decane microemulsions [AOT denotes sodium bis(2-ethylhexyl) sulfosuccinate] was undertaken in order to delineate clearly the phase behavior and corresponding structural transitions for AOT-plus-water volume fractions ranging from φ=0.60 to 0.95. Spectra were collected for temperatures between T=3 and 65 °C. The resulting T-vs-φ phase diagram indicates three distinct structural domains when the water-to-AOT molar ratio is fixed at W=40.8, namely, the previously investigated L2 droplet phase, a high-temperature Lα lamellar phase, and a low-temperature L3 phase consisting of randomly connected lamellar sheets. A significantly wide coexistence region accompanies the droplet-to-lamellar phase transition, which is demonstrated to be first order. For W between 15 and 40, an analysis of the lamellar structure using a one-dimensional paracrystal model produces a Hosemann g factor indicative of an approximately constant variation in the lamellar spacing of about 8%. The SAXS study was supplemented by dielectric-relaxation, shear-viscosity, and quasielastic light-scattering measurements in order to substantiate the observed phase transitions and further our understanding of the structural and dynamical properties of the L3 phase. It was found that the L3 phase exhibits Newtonian behavior up to a shear rate of 790 s-1, in contradiction to previous theoretical considerations. The phase exhibits two distinct relaxation modes. A relaxation time of ~1 ms characterizes the Brownian motion of a single lamellar sheet, while the motion of the entire interconnected sheet assembly has a relaxation time on the order of 1 s.

  6. In Situ Observation of Phase Transformation and Structure Evolution of a 12 pct Cr Ferritic Stainless Steel

    NASA Astrophysics Data System (ADS)

    Song, Changjiang; Guo, Yuanyi; Li, Kefeng; Sun, Fengmei; Han, Qingyou; Zhai, Qijie

    2012-10-01

    This work focuses on an in situ observation of phase transformation of a 12 pct Cr ferritic stainless steel using high-temperature laser scanning confocal microscopy. α→ γ→ δ phase transformation temperatures are determined to be approximately 1073 K and 1423 K (800 °C and 1150 °C), respectively. The onset of phase transformation is found to occur at grain boundaries. When the temperature is beyond 1518 K (1245 °C), the grain growth rate suddenly becomes very high, and the grain growth is related to the self-organizing of adjacent grains. δ→ γ phase transformation has been mostly restrained when cooling rates are in the range of 22.4 K/s to 13.3 K/s (22.4 °C/s to 13.3 °C/s) except for at grain boundaries. Martensitic phase transformation, rather than γ→ α phase transformation, occurs when the cooling rates are in the range of 8.5 K/s to 2.2 K/s (8.5 °C/s to 2.2 °C/s). The starting temperature of martensitic phase transformation is approximately 697 K to 728 K (424 °C to 455 °C) for specimens heated to 1373 K (1100 °C) ( i.e., γ phase field), which is 50 K to 100 K (50 °C to 100 °C) higher than that of specimens heated to 1723 K (1450 °C) ( i.e., δ phase field). Many bulges remain on the surfaces of the specimen heated to 1723 K (1450 °C), and their formation mechanism is analyzed.

  7. Transformation of Hume-Rothery phases under the action of high pressure torsion

    NASA Astrophysics Data System (ADS)

    Straumal, B. B.; Kilmametov, A. R.; Kucheev, Yu. O.; Kolesnikova, K. I.; Korneva, A.; Zieba, P.; Baretzky, B.

    2014-11-01

    It has been revealed experimentally that high-pressure torsion induces phase transformations of certain Hume-Rothery phases (electron compounds) to others. High-pressure torsion induces the ξ → δ + ɛ reaction in copper-tin alloys with the appearance of the δ + ɛ phase mixture as after long-term annealing in the temperature range T eff = 350-589°C. The mass transfer rate driven by high-pressure torsion is 14-18 orders of magnitude higher than the rate of conventional thermal diffusion at the processing temperature T HPT. This phenomenon can be explained by an increased concentration of defects (in particular, vacancies) in the steady state under high-pressure torsion, which is equivalent to an increase in the temperature.

  8. Transformation between spin-Peierls and incommensurate fluctuating phases of Sc-doped TiOCl

    NASA Astrophysics Data System (ADS)

    Zhang, Jian; Wölfel, Alexander; Bykov, Maxim; Schönleber, Andreas; van Smaalen, Sander; Kremer, Reinhard K.; Williamson, Hailey L.

    2014-07-01

    Single crystals of ScxTi1-xOCl(x=0.005) have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4-200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ScxTi1-xOCl gives broadened diffraction maxima at incommensurate positions between Tc1=61.5(3) and ˜90 K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ScxTi1-xOCl(x =0.005) is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of ˜10 K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound CuGeO3. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz-Thouless-type phases in two-dimensional systems.

  9. Martensitic transformation between competing phases in Ti-Ta alloys: a solid-state nudged elastic band study.

    PubMed

    Chakraborty, Tanmoy; Rogal, Jutta; Drautz, Ralf

    2015-03-25

    A combined density functional theory and solid-state nudged elastic band study is presented to investigate the martensitic transformation between β → (α″, ω) phases in the Ti-Ta system. The minimum energy paths along the transformation are calculated and the transformation mechanisms as well as relative stabilities of the different phases are discussed for various compositions. The analysis of the transformation paths is complemented by calculations of phonon spectra to determine the dynamical stability of the β, α″, and ω phase. Our theoretical results confirm the experimental findings that with increasing Ta concentration there is a competition between the destabilisation of the α″ and ω phase and the stabilisation of the high-temperature β phase.

  10. EXAFS measurement of iron bcc-to-hcp phase transformation in nanosecond-laser shocks.

    PubMed

    Yaakobi, B; Boehly, T R; Meyerhofer, D D; Collins, T J B; Remington, B A; Allen, P G; Pollaine, S M; Lorenzana, H E; Eggert, J H

    2005-08-12

    Extended x-ray absorption fine structure (EXAFS) measurements have demonstrated the phase transformation from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometry. This is a direct, atomic-level, and in situ proof of shock-induced transformation in iron, as opposed to the previous indirect proof based on shock-wave splitting.

  11. A magnetic phase transition temperature calibration device

    NASA Astrophysics Data System (ADS)

    Garcia, F.; Cernicchiaro, G. R. C.; Takeuchi, A. Y.

    1999-04-01

    We have devised a technique of manufacturing temperature sensor calibration devices based on the magnetic properties of the pseudobinary compounds of formula (RxR1-x')Co2, where R and R' are heavy rare earth elements. The device is a solid sensor which provides an easily detectable first order magnetic phase transition at fixed temperature points. It is known that a first order magnetic phase transition from ferrimagnetism to paramagnetism is observed in compounds as TmCo2 (3.70 K), ErCo2 (32.05 K), HoCo2 (77.12 K), and DyCo2 (136.55 K). These transitions correspond to a large anomaly in the characteristic properties as function of temperature. In this work, we present the electrical resistivity and magnetization measurements of (Er, Ho)Co2 series and DyCo2 compounds showing the viability of the proposed devices from 32.05 up to 136.55 K. This range can be extended below and above by substitution of the chemical components and the stoichiometric composition. The number of transitions can be fixed by a convenient arrangement of several elements.

  12. Precipitation phase transformation in nanocrystalline Fe-Mo alloys.

    PubMed

    Sarkar, Subhajit; Bansal, Chandrahaas

    2004-01-01

    Precipitation phase transformation was studied in nanocrystalline Fe-rich Fe-Mo alloys with the use of X-ray diffraction and Mössbauer spectroscopy. Alloys up to 5 at% Mo in Fe were synthesized by mechanical alloying and formed in alpha phase bcc solid solutions with average grain sizes in the range of 10-13 nm. The precipitation transformation (alpha-->alpha + lambda) was found to proceed via a Mo clustering that was correlated with the size of the nanograins. This was understood in terms of the Gibbs Thomson effect with a concept of negative surface energy contribution to the Gibbs free energy of mixing in a nanocrystalline alloy with positive internal energy of mixing. This contribution increased the stability of the solid solution for nanosized grains, and the Mo precipitation started once the grains grew beyond a critical size. We argue that the Mo precipitation takes place in the grain boundary regions, and the Mo-rich lambda phase also precipitates directly in the grain boundary regions, in contrast to the microcrystalline alloys, where the Mo clusters formed within the grains and were first dissolved in the Fe matrix before the lambda phase was formed.

  13. 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.

  14. Experimental and computational investigation of the effect of phase transformation on fracture parameters of an SMA

    NASA Astrophysics Data System (ADS)

    Haghgouyan, Behrouz; Shafaghi, Nima; Aydıner, C. Can; Anlas, Gunay

    2016-07-01

    A comprehensive, multi-method experimental characterization of fracture is conducted on shape memory alloy NiTi that exhibits superelasticity due to austenite-to-martensite stress induced phase transformation. This characterization includes (i) load-based measurement of critical stress intensity factor (K max) using ASTM standard E399, (ii) measurement of crack tip opening displacement (CTOD) per ASTM standard E1290, (iii) the digital image correlation (DIC) characterization of the transformation zone as well as the displacement-field based measurement of K max from the DIC data. Samples have also been tested at T = 100 °C to suppress the martensitic transformation to investigate transformation toughening. The experimental investigation is complemented with finite element (FE) analysis that uses Auricchio-Taylor-Lubliner constitutive model. A direct observation with DIC revealed a small scale transformation (K-dominance). K max of the transforming material is higher than that of the transformation-suppressed material tested at 100 °C, suggesting transformation toughening. At 100 °C, the material becomes quite brittle with a very small crack-tip plastic zone when the transformation mechanism is blocked. By measures of critical CTOD, the gap widens even more between the superelastic and transformation-suppressed cases, particularly because of the side effect that, in this very interesting material, material modulus increases with temperature. Evaluating the transformation zone from the DIC strains with reference to the uniaxial stress-strain curve, an equivalent strain form is proposed in conjunction with the plane stress FE prediction.

  15. Anisotropic kinetics of solid phase transition from first principles: alpha-omega phase transformation of Zr.

    PubMed

    Guan, Shu-Hui; Liu, Zhi-Pan

    2016-02-14

    Structural inhomogeneity is ubiquitous in solid crystals and plays critical roles in phase nucleation and propagation. Here, we develop a heterogeneous solid-solid phase transition theory for predicting the prevailing heterophase junctions, the metastable states governing microstructure evolution in solids. Using this theory and first-principles pathway sampling simulation, we determine two types of heterophase junctions pertaining to metal α-ω phase transition at different pressures and predict the reversibility of transformation only at low pressures, i.e. below 7 GPa. The low-pressure transformation is dominated by displacive Martensitic mechanism, while the high-pressure one is controlled by the reconstructive mechanism. The mechanism of α-ω phase transition is thus highly pressure-sensitive, for which the traditional homogeneous model fails to explain the experimental observations. The results provide the first atomic-level evidence on the coexistence of two different solid phase transition mechanisms in one system.

  16. An Asymmetric Image Encryption Based on Phase Truncated Hybrid Transform

    NASA Astrophysics Data System (ADS)

    Khurana, Mehak; Singh, Hukum

    2017-09-01

    To enhance the security of the system and to protect it from the attacker, this paper proposes a new asymmetric cryptosystem based on hybrid approach of Phase Truncated Fourier and Discrete Cosine Transform (PTFDCT) which adds non linearity by including cube and cube root operation in the encryption and decryption path respectively. In this cryptosystem random phase masks are used as encryption keys and phase masks generated after the cube operation in encryption process are reserved as decryption keys and cube root operation is required to decrypt image in decryption process. The cube and cube root operation introduced in the encryption and decryption path makes system resistant against standard attacks. The robustness of the proposed cryptosystem has been analysed and verified on the basis of various parameters by simulating on MATLAB 7.9.0 (R2008a). The experimental results are provided to highlight the effectiveness and suitability of the proposed cryptosystem and prove the system is secure.

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

    SciTech Connect

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

    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 in 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

  18. Crystal-amorphous transformation via defect-templating in phase-change materials

    NASA Astrophysics Data System (ADS)

    Nukala, Pavan

    Phase-change materials (PCM) such as GeTe and Ge-Sb-Te alloys are potential candidates for non-volatile memory applications, because they can reversibly and rapidly transform between a crystalline phase and an amorphous phase with medium-range order. Traditionally, crystal-amorphous transformation in these materials has been carried out via melt-quench pathway, where the crystalline phase is heated beyond its melting point by the rising edge of an electric pulse, and the melt phase is quenched by the falling edge into a glassy phase. Formation of an intermediate melt phase in this transformation pathway requires usage of large switching current densities, resulting in energy wastage, and device degradation issues. Furthermore, melt-quench pathway is a brute force strategy of amorphizing PCM, and does not utilize the peculiar structural properties in crystalline phase. It will be beneficial from a device perspective that crystal-amorphous transformation is carried out via subtler solid-state pathways. Single-crystalline nanowire phase-change memory, owing to its lateral geometry and large volumes of active material, offers a platform to construct a crystal-amorphous transformation pathway via gradually increasing disorder in the crystalline phase, and study it. Using in situ transmission electron microscopy on GeTe and Ge2Sb2Te5 systems, we showed that the application of an electric pulse (heat-shock) creates dislocations in the PCM that migrate with the hole-wind force, and interact with the already existing ferroelectric boundaries in case of GeTe, changing their nature. We adapted novel tools such as optical second harmonic generation polarimety to carefully study these defect interactions. These defects accumulate at a region of local inhomogeneity, and upon addition of defects beyond a critical limit to that region via electrical pulsing, an amorphous phase "nucleates". We also studied the effect of defect dynamics on carrier transport using temperature

  19. High-temperature superconducting transformer performance, cost, and market evaluation

    SciTech Connect

    Dirks, J.A.; Dagle, J.E.; DeSteese, J.G.; Huber, H.D.; Smith, S.A.; Currie, J.W.; Merrick, S.B.; Williams, T.A.

    1993-09-01

    Recent laboratory breakthroughs in high-temperature superconducting (HTS) materials have stimulated both the scientific community and general public with questions regarding how these materials can be used in practical applications. While there are obvious benefits from using HTS materials (most notably the potential for reduced energy losses in the conductors), a number of issues (such as overall system energy losses, cost, and reliability) may limit applications of HTS equipment, even if the well known materials problems are solved. This study examined the future application potential of HTS materials to power transformers. This study effort was part of a US Department of Energy (DOE) Office of Energy Storage and Distribution (OESD) research program, Superconductivity Technology for Electric Power Systems (STEPS). The study took a systems perspective to gain insights to help guide DOE in managing research designed to realize the vision of HTS applications. Specific objectives of the study were as follows: to develop an understanding of the fundamental HTS transformer design issues that can provide guidance for developing practical devices of interest to the electric utility industry; to identify electric utility requirements for HTS transformers and to evaluate the potential for developing a commercial market; to evaluate the market potential and national benefits for HTS transformers that could be achieved by a successful HTS development program; to develop an integrated systems analysis framework, which can be used to support R&D planning by DOE, by identifying how various HTS materials characteristics impact the performance, cost, and national benefits of the HTS application.

  20. Mechanochemical spinodal decomposition: a phenomenological theory of phase transformations in multi-component, crystalline solids

    NASA Astrophysics Data System (ADS)

    Rudraraju, Shiva; van der Ven, Anton; Garikipati, Krishna

    2016-06-01

    We present a phenomenological treatment of diffusion-driven martensitic phase transformations in multi-component crystalline solids that arise from non-convex free energies in mechanical and chemical variables. The treatment describes diffusional phase transformations that are accompanied by symmetry-breaking structural changes of the crystal unit cell and reveals the importance of a mechanochemical spinodal, defined as the region in strain-composition space, where the free-energy density function is non-convex. The approach is relevant to phase transformations wherein the structural order parameters can be expressed as linear combinations of strains relative to a high-symmetry reference crystal. The governing equations describing mechanochemical spinodal decomposition are variationally derived from a free-energy density function that accounts for interfacial energy via gradients of the rapidly varying strain and composition fields. A robust computational framework for treating the coupled, higher-order diffusion and nonlinear strain gradient elasticity problems is presented. Because the local strains in an inhomogeneous, transforming microstructure can be finite, the elasticity problem must account for geometric nonlinearity. An evaluation of available experimental phase diagrams and first-principles free energies suggests that mechanochemical spinodal decomposition should occur in metal hydrides such as ZrH2-2c. The rich physics that ensues is explored in several numerical examples in two and three dimensions, and the relevance of the mechanism is discussed in the context of important electrode materials for Li-ion batteries and high-temperature ceramics.

  1. Texture evolution during nitinol martensite detwinning and phase transformation

    NASA Astrophysics Data System (ADS)

    Cai, S.; Schaffer, J. E.; Ren, Y.; Yu, C.

    2013-12-01

    Nitinol has been widely used to make medical devices for years due to its unique shape memory and superelastic properties. However, the texture of the nitinol wires has been largely ignored due to inherent complexity. In this study, in situ synchrotron X-ray diffraction has been carried out during uniaxial tensile testing to investigate the texture evolution of the nitinol wires during martensite detwinning, variant reorientation, and phase transformation. It was found that the thermal martensitic nitinol wire comprised primarily an axial (1¯20), (120), and (102)-fiber texture. Detwinning initially converted the (120) and (102) fibers to the (1¯20) fiber and progressed to a (1¯30)-fiber texture by rigid body rotation. At strains above 10%, the (1¯30)-fiber was shifted to the (110) fiber by (21¯0) deformation twinning. The austenitic wire exhibited an axial (334)-fiber, which transformed to the near-(1¯30) martensite texture after the stress-induced phase transformation.

  2. Texture evolution during nitinol martensite detwinning and phase transformation

    SciTech Connect

    Cai, S.; Schaffer, J. E.; Ren, Y.

    2013-12-09

    Nitinol has been widely used to make medical devices for years due to its unique shape memory and superelastic properties. However, the texture of the nitinol wires has been largely ignored due to inherent complexity. In this study, in situ synchrotron X-ray diffraction has been carried out during uniaxial tensile testing to investigate the texture evolution of the nitinol wires during martensite detwinning, variant reorientation, and phase transformation. It was found that the thermal martensitic nitinol wire comprised primarily an axial (1{sup ¯}20), (120), and (102)-fiber texture. Detwinning initially converted the (120) and (102) fibers to the (1{sup ¯}20) fiber and progressed to a (1{sup ¯}30)-fiber texture by rigid body rotation. At strains above 10%, the (1{sup ¯}30)-fiber was shifted to the (110) fiber by (21{sup ¯}0) deformation twinning. The austenitic wire exhibited an axial (334)-fiber, which transformed to the near-(1{sup ¯}30) martensite texture after the stress-induced phase transformation.

  3. 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

  4. Nonequilibrium phase transformations in bcc titanium and niobium alloys

    NASA Astrophysics Data System (ADS)

    Doherty, Kevin James

    The major goal throughout this entire study was to find a bulk beta-titanium amorphous system. In this case, the feasibility of bulk amorphization by destabilizing the crystalline phase in bcc titanium alloys is developed. The binary Ti-Cr system was previously reported, by others, to undergo spontaneous vitrification. This work was later proven to be irreproducible by several other groups. With the proper alloying additions to the Ti-Cr system, the resultant bcc matrix is extremely unstable, however, the formation of alpha, o, and intermetallics is inhibited. Powders of the complex system Ti65Cr13Cu 16Mn4Fe2 transform to a fully amorphous structure after just 3 to 4 hours of mechanical milling. In bulk, this system forms nanoscale disordered regions, totaling 20 to 30% of the microstructure, upon annealing of the metastable bcc phase. The phase separation, beta → beta + beta' accompanies this transformation and induces strain into the matrix. Analytical high resolution transmission electron microscopy (TEM) is used to characterize the decomposition behavior by obtaining physical measurements of the microstructure and chemistry, and to determine the mechanism of the phase separation. High resolution and analytical TEM data map the development of successive chromium rich (copper poor) and chromium poor (copper rich) regions formed in <100> directions during heat treatment. This reaction is shown to occur by spinodal decomposition. A known bcc, binary spinodal decomposition system, Nb-Zr, was chosen as a reference system to verify the spinodal mechanism in the 5-component titanium system and to validate the use of analytical TEM to characterize spinodal decomposition. The Ti-Cr system is also investigated for comparison with the complex Ti-Cr-Cu-Mn-Fe system and to resolve some of the issues presented during the earlier spontaneous vitrification studies. Finally, a combination of high resolution TEM and chemical analysis is utilized to differentiate between the

  5. Influence of water concentrations on the phase transformation of a model surfactant/co-surfactant/water system

    NASA Astrophysics Data System (ADS)

    Lunkad, Raju; Srivastava, Arpita; Debnath, Ananya

    2017-02-01

    The influence of water concentrations on phase transformations of a surfactant/co-surfactant/water system is investigated by using all atom molecular dynamics simulations. At higher water concentrations, where surfactant (behenyl trimethyl ammonium chloride, BTMAC) to co-surfactant (stearyl alcohol, SA) ratio is fixed, BTMAC and SA self-assemble into spherical micelles, which transform into strongly interdigitated one dimensional rippled lamellar phases upon decreasing water concentrations. Fragmentation or fusions of spherical micelles of different sizes are evident from the radial distribution functions at different temperatures. However, at lower water concentrations rippled lamellar phase transforms into an LβI phase upon heating. Our simulations reveal that the concentrations of water can influence available space around the head groups which couple with critical thickness to accommodate the packing fraction required for respective phases. This directs towards obtaining a controlling factor to design desired phases important for industrial and medical applications in the future.

  6. Correlating Hardness Retention and Phase Transformations of Al and Mg Cast Alloys for Aerospace Applications

    NASA Astrophysics Data System (ADS)

    Kasprzak, W.; Czerwinski, F.; Niewczas, M.; Chen, D. L.

    2015-03-01

    The methodology based on correlating hardness and phase transformations was developed and applied to determine the maximum temperature of hardness retention of selected Al-based and Mg-based alloys for aerospace applications. The Al alloys: A356, F357, and C355 experienced 34-66% reduction of the initial hardness, in comparison to 4-22% hardness reduction observed in Mg alloys: QE22A, EV31A, ZE41A, and WE43B after the same annealing to 450 °C. For Al alloys the hardness reduction showed a steep transition between 220 and 238 °C. In contrast, Mg alloys showed a gradual hardness decrease occurring at somewhat higher temperatures between 238 and 250 °C. The hardness data were correlated with corresponding phase transformation kinetics examined by dilatometer and electrical resistivity measurements. Although Mg alloys preserved hardness to higher temperatures, their room temperature tensile strength and hardness were lower than Al alloys. The experimental methodology used in the present studies appears to be very useful in evaluating the softening temperature of commercial Al- and Mg-based alloys, permitting to assess their suitability for high-temperature applications.

  7. Large-strain cyclic response and martensitic transformation of austenitic stainless steel at elevated temperatures

    NASA Astrophysics Data System (ADS)

    Hamasaki, H.; Nakano, T.; Ishimaru, E.; Yoshida, F.

    2016-08-01

    Cyclic tension-compression tests were carried out for austenitic stainless steel (SUS304) at elevated temperatures. The significant Bauschinger effect was found in the obtained stress-strain curve. In addition, stagnation of deformation induced martensitic transformation was observed just after stress reversal until the equivalent stress reached the maximum value in the course of experiment. The constitutive model for SUS304 at room temperature was developed, in which homogenized stress of SUS304 was expressed by the weighed summation of stresses of austenite and martensite phases. The calculated stress-strain curves and predicted martensite volume fraction were well correlated with those experimental results.

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

    SciTech Connect

    Christien, F.; Telling, M.T.F.; Knight, K.S.

    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 data 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)

  9. Dynamics of Structural Transformations between Lamellar and Inverse Bicontinuous Cubic Lyotropic Phases

    SciTech Connect

    Conn, Charlotte E.; Ces, Oscar; Mulet, Xavier; Seddon, John M.; Templer, Richard H.; Finet, Stephanie; Winter, Roland

    2006-03-17

    The liquid crystalline lamellar (L{sub {alpha}}) to double-diamond inverse bicontinuous cubic (Q{sub II}{sup D}) phase transition for the amphiphile monoelaidin in excess water exhibits a remarkable sequence of structural transformations for pressure or temperature jumps. Our data imply that the transition dynamics depends on a coupling between changes in molecular shape and the geometrical and topological constraints of domain size. We propose a qualitative model for this coupling based on theories of membrane fusion via stalks and existing knowledge of the structure and energetics of bicontinuous cubic phases.

  10. Two-phase visualization at cryogenic temperature

    NASA Astrophysics Data System (ADS)

    Rousset, Bernard; Chatain, Denis; Beysens, Daniel; Jager, Bernard

    2001-05-01

    This paper presents two different applications for two-phase visualization at low temperature. In the first application, a CCD video camera located inside vacuum is directly supported by the Pyrex pipe containing a two-phase superfluid flow. In the case of slightly positive slopes in which the flow is co-current but ascending, two different flow patterns have been seen, stratified and intermittent, depending on the vapor mass flow. Experimental investigations from stratified to intermittent flow have been made visually and compared to a code derived from the Taitler/Dukler model. The second application concerns phase transition of hydrogen near critical point (33 K) in zero gravity. The experiments have been performed in a cryostat equipped with a 10 T superconducting coil allowing the gravity compensation for hydrogen. Images of the condensation cell are shifted to the top of the cryostat with a specific cryogenic endoscope because CCD cameras do not work in high magnetic fields. The sample was enlightened with diffuse or parallel (coherent) light using a second endoscope. Images obtained in this apparatus are similar with those obtained in space.

  11. Two-dimensional phase transformation probed by second harmonic generation: Oscillatory transformation of the K/Al(111) system

    SciTech Connect

    Ying, Z.C.; Plummer, E.W. |

    1995-12-31

    The technique of optical second harmonic generation is used to study phase transformations at two-dimensional surfaces and interfaces. Examples are given to illustrate that changes in surface symmetry, adsorption configuration, and electronic structure can be detected by this nonlinear optical technique. An oscillatory phase transformation of potassium adsorbed atoms on Al(111) probed by second harmonic generation is analyzed in detail.

  12. A New, More Stable Polymorphic Form of Otilonium Bromide: Solubility, Crystal Structure, and Phase Transformation.

    PubMed

    Vega, Daniel R; Halac, Emilia; Segovia, Luciano; Baggio, Ricardo

    2016-10-01

    A new polymorphic form of otilonium bromide is presented (Form I), and a thorough analysis of its crystal and molecular structure is performed. The compound suffers a temperature-driven first-order phase transition at about 396 K, which transforms it into the polymorph reported by Dapporto P and Sega A (Acta Cryst. 1986;C42:474-478) (Form II). Through thermal analysis and solubility experiments the relative stability of both crystal modifications were determined, confirming that at room temperature this new Form I is the more stable one, Form II existing just in a metastable state.

  13. Study of phase transformation and crystal structure for 1D carbon-modified titania ribbons

    SciTech Connect

    Zhou, Lihui Zhang, Fang; Li, Jinxia

    2014-02-15

    One-dimensional hydrogen titanate ribbons were successfully prepared with hydrothermal reaction in a highly basic solution. A series of one-dimensional carbon-modified TiO{sub 2} ribbons were prepared via calcination of the mixture of hydrogen titanate ribbons and sucrose solution under N{sub 2} flow at different temperatures. The phase transformation process of hydrogen titanate ribbons was investigated by in-situ X-ray diffraction at various temperatures. Besides, one-dimensional carbon-modified TiO{sub 2} ribbons calcined at different temperatures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption isotherms, diffuse reflectance ultraviolet–visible spectroscopy, and so on. Carbon-modified TiO{sub 2} ribbons showed one-dimensional ribbon crystal structure and various crystal phases of TiO{sub 2}. After being modified with carbon, a layer of uniform carbon film was coated on the surface of TiO{sub 2} ribbons, which improved their adsorption capacity for methyl orange as a model organic pollutant. One-dimensional carbon-modified TiO{sub 2} ribbons also exhibited enhanced visible-light absorbance with the increase of calcination temperatures. - Highlights: • The synthesis of 1D carbon-modified TiO{sub 2} ribbons. • The phase transformation of 1D carbon-modified TiO{sub 2} ribbons. • 1D carbon-modified TiO{sub 2} exhibites enhanced visible-light absorbance.

  14. Lattice instability during phase transformations under multiaxial stress: Modified transformation work criterion

    NASA Astrophysics Data System (ADS)

    Levitas, Valery I.; Chen, Hao; Xiong, Liming

    2017-08-01

    A continuum/atomistic approach for predicting lattice instability during crystal-crystal phase transformations (PTs) is developed for the general loading with an arbitrary stress tensor and large strains. It is based on a synergistic combination of the generalized Landau-type theory for PTs and molecular dynamics (MD) simulations. The continuum approach describes the entire dissipative transformation process in terms of an order parameter, and the general form of the instability criterion is derived utilizing the second law of thermodynamics. The feedback from MD allowed us to present the instability criterion for both direct and reverse PTs in terms of the critical value of the modified transformation work, which is linear in components of the true stress tensor. It was calibrated by MD simulations for direct and reverse PTs between semiconducting silicon Si i and metallic Si ii phases under just two different stress states. Then, it describes hundreds of MD simulations under various combinations of three normal and three shear stresses. In particular, the atomistic simulations show that the effects of all three shear stresses along cubic axes on lattice instability of Si i are negligible, which is in agreement with our criterion.

  15. Controllable Phase Transformation in Extracting Valuable Metals from Chinese Low-Grade Nickel Sulphide Ore

    NASA Astrophysics Data System (ADS)

    Cui, Fuhui; Mu, Wenning; Wang, Shuai; Xu, Qian; Zhai, Yuchun; Luo, Shaohua

    2017-10-01

    In this work, a two-stage sulphuric acid roasting and water leaching system was chosen to extract valuable metals from Chinese low-grade nickel sulphide ore. By optimizing the two-stage roasting process, first roasting temperature at 295°C with particle size of 80-91 μm with an acid-ore ratio of 1.1:1 for 2 h, and second roasting temperature at 620°C for 2 h, it was found that more than 98% of the nickel and 99% of the copper but less than 14.38% of Fe were leached into the water. Attempts were made via x-ray diffraction analysis, scanning electron microscopy, chemical phase analysis, and differential thermal and thermogravimetric analysis to reveal the phase transformations for Ni, Cu, and Mg, which could be expressed as mineral phases → sulphates hydrate → sulphates and for iron as mineral phases → hydrated ferrous sulphate → ferric sulphates and ferric oxide → oxide. The results of this work suggest that a controllable phase transformation by using a two-stage sulphuric acid roasting process is a feasible method for efficiently extracting valuable metals from Chinese nickel sulphide ore.

  16. Controllable Phase Transformation in Extracting Valuable Metals from Chinese Low-Grade Nickel Sulphide Ore

    NASA Astrophysics Data System (ADS)

    Cui, Fuhui; Mu, Wenning; Wang, Shuai; Xu, Qian; Zhai, Yuchun; Luo, Shaohua

    2017-06-01

    In this work, a two-stage sulphuric acid roasting and water leaching system was chosen to extract valuable metals from Chinese low-grade nickel sulphide ore. By optimizing the two-stage roasting process, first roasting temperature at 295°C with particle size of 80-91 μm with an acid-ore ratio of 1.1:1 for 2 h, and second roasting temperature at 620°C for 2 h, it was found that more than 98% of the nickel and 99% of the copper but less than 14.38% of Fe were leached into the water. Attempts were made via x-ray diffraction analysis, scanning electron microscopy, chemical phase analysis, and differential thermal and thermogravimetric analysis to reveal the phase transformations for Ni, Cu, and Mg, which could be expressed as mineral phases → sulphates hydrate → sulphates and for iron as mineral phases → hydrated ferrous sulphate → ferric sulphates and ferric oxide → oxide. The results of this work suggest that a controllable phase transformation by using a two-stage sulphuric acid roasting process is a feasible method for efficiently extracting valuable metals from Chinese nickel sulphide ore.

  17. Statistical Thermodynamics of Phase Transformations in Lithium Alanates with Release of Hydrogen

    NASA Astrophysics Data System (ADS)

    Zaginaichenko, S. Yu.; Matysina, Z. A.; Shchur, D. V.; Pomytkin, A. P.; Gabdullin, M. T.; Zaritskii, D. A.

    2017-02-01

    Based on the molecular and kinetic concepts, the paper presents the theory of phase transformations in lithium alanates with the release of hydrogen. The calculations are given for free energies of phases and their dependences on pressure, temperature, hydrogen concentration, and energy parameters are determined. The equations are derived for the thermodynamically-equilibrium states which determine the Pressure-Temperature-Concentration diagram and estimate the energy parameters with the use of experimental results taken from the literature. The investigation of the detected temperature/concentration dependence in crystals shows the impossibility of a complete hydrogen release from alanates. The paper contains isotherm and isopleth plots. A possibility is established for the hysteresis effect. A comparison is given to the theoretical and experimental results.

  18. Proposed Giaever transformer to probe the pseudogap phase of cuprates.

    SciTech Connect

    Levchenko, A.; Norman, M. R.

    2011-03-01

    We develop a theory of the rectification effect in a double-layer system where both layers are superconductors or one of the layers is a normal metal. The Coulomb interaction is assumed to provide the dominant coupling between the layers. We find that superconducting fluctuations strongly enhance the drag conductivity, with rectification most pronounced when both layers are superconductors. In view of their distinct dependence on temperature near T{sub c} and layer separation, drag measurements based on a Giaever transformer could distinguish whether rectification occurs due to fluctuating pairs or inductively coupled fluctuating vortices.

  19. Phase Transformation of VO2 Nanoparticles Assisted by Microwave Heating

    PubMed Central

    Sikong, Lek.

    2014-01-01

    The microwave assisted synthesis nowadays attracts a great deal of attention. Monoclinic phase VO2 (M) was prepared from NH4VO3 and H2C2O4 · 2H2O by a rapid microwave assisted technique. The synthesis parameters, microwave irradiation time, microwave power, and calcinations temperature were systematically varied and their influences on the structure and morphology were evaluated. The microwave power level has been carried out in range 180–600 W. TEM analysis demonstrated nanosized samples. The structural and morphological properties were measured using XRD, TEM, and thermal analyses. The variations of vanadium phase led to thermochromic properties. PMID:24688438

  20. Phase Transformation Volume Change Control of Strain and Instability Mechanisms

    NASA Astrophysics Data System (ADS)

    Green, H. W.

    2011-12-01

    Phase transformations that are accompanied by significant change in volume self-organize the transformation process under stress to enhance strain and/or decrease stress. When the kinetics of the reaction are reasonably rapid, this occurs by nucleation and growth of phase(s) on grain boundaries of appropriate orientation to facilitate strain. When the kinetics of reaction are sluggish (ratio of nucleation rate to growth rate >> 1) and the reaction is polymorphic and exothermic, under specific conditions this process can lead to shearing instability in which the volume change of reaction drives runaway nucleation at stress concentrations leading to macroscopic faulting. Under these conditions, the detailed processes and microstructures produced depend on the sign of volume change but the failure is very similar whether volume change is positive or negative. The microstructures confirm that the mesoscale physics of this process (self-organization of primary features and nucleation/propagation of a fault) is the same as for brittle shear fracture, despite the large difference in the fundamental microscopic physics (formation of nanocrystalline microlenses instead of tensile cracks, and fault propagation by grain-boundary sliding rather than frictional sliding). When volume change is negative and the kinetics of reaction are less sluggish (ratio of nucleation rate to growth rate of order 1), nucleation of the new (denser) phase occurs preferentially on grain boundaries normal to maximum compression and new crystals nucleated on any orientation of grain boundary grow parallel to maximum compression. The result is stylolite-like volume loss normal to maximum compression. Observations of polyphase metamorphic rocks suggest that similar creep can occur during prograde metamorphism by nucleation of more-dense phases and less-dense phases on different populations of phase-boundary orientations such that the overall pattern achieves a macroscopic strain by volume transfer

  1. Effect of phase transformation on optical and dielectric properties of pulsed laser deposited ZnTiO3 thin films

    NASA Astrophysics Data System (ADS)

    Jain, Praveen K.; Salim, Mohammad; Kaur, Davinder

    2016-04-01

    Zinc titanate (ZnTiO3) ceramics were prepared by conventional solid state reaction method using ZnO and TiO2 in a molar ratio of 1:1 with optimized parameters. It was found that the sample sintered at 800 °C for 12 h exhibit single hexagonal phase of ZnTiO3. ZnTiO3 thin film have been deposited on ITO coated glass substrate using pulsed laser deposition (PLD) technique employing a KrF laser source (λ = 248 nm). In present work, the effect of substrate temperature, which leads to transformation of hexagonal phase to cubic phase, has been studied. The XRD pattern revealed that pure hexagonal phase of ZnTiO3 appear upto 400 °C and more increment in substrate temperature leads to transformation of hexagonal phase to cubic phase. We have observed the blue shift in absorption edge at lower temperature. When the substrate temperature increases from 300 to 400 °C the band gap decreases due to strong hexagonal phase, but more increment in substrate temperature increases the band gap causes by change of phase from hexagonal to cubic. The dielectric constant of ZnTiO3 thin film increases as the substrate temperature increases due to the enhancement in crystallinity and improved morphology.

  2. An experimental method for investigating phase transformations in the heat affected zone of welds using synchrotron radiation

    SciTech Connect

    Elmer, J.W.; Wong, J.; Froba, M.; Waide, P.A.; Larson, E.M.

    1995-05-26

    Although welding is an established technology used in many industrial settings, it is least understand terms of the phases that actually exist, the variation of their spatial disposition with time, and the rate of transformation from one phase to another at various thermal coordinates in the vicinity of the weld. With the availability of high flux and, more recently, high brightness synchrotron x-radiation sources, a number of diffraction and spectroscopic methods have been developed for structural characterization with improved spatial and temporal resolutions to enable in-situ measurements of phases under extreme temperature, pressure and other processing conditions not readily accessible with conventional sources. This paper describes the application of spatially resolved x-ray diffraction (SRXRD) for in-situ investigations of phase transformations in the heat affected zone (HAZ) of fusion welds. Results are presented for gas tungsten (GTA) welds in commercially pure titanium that show the existence of the high temperature bcc {beta}-phase in a 3.3 {plus_minus} 0.3 mm wide HA band adjacent to the liquid weld pool. Phase concentration profiles derived from the SRXRD data further show the co-existence of both the low temperature hcp ({alpha}-phase and the {beta}-phase in the partially, transformed region of the HA. These results represent the first direct observations of solid state phase transformations and mapping of phase boundaries in fusion welds. SRXRD experiments of this type are needed as experimental input for modeling of kinetics of phase transformations and microstructural evolution under the highly non-isothermal conditions produced during welding.

  3. Instrumental phase-based method for Fourier transform spectrometer measurements processing

    SciTech Connect

    Saggin, Bortolino; Scaccabarozzi, Diego; Tarabini, Marco

    2011-04-20

    Phase correction is a critical procedure for most space-borne Fourier transform spectrometers (FTSs) whose accuracy (owing to often poor signal-to-noise ratio, SNR) can be jeopardized from many uncontrollable environmental conditions. This work considers the phase correction in an FTS working under significant temperature change during the measurement and affected by mechanical disturbances. The implemented method is based on the identification of an instrumental phase that is dependent on the interferometer temperature and on the extraction of a linear phase component through a least-squares approach. The use of an instrumental phase parameterized with the interferometer temperature eases the determination of the linear phase that can be extracted using only a narrow spectral region selected to be immune from disturbances. The procedure, in this way, is made robust against phase errors arising from instrumental effects, a key feature to reduce the disturbances through spectra averaging. The method was specifically developed for the Mars IR Mapper spectrometer, that was designed for operation onboard a rover on the Mars surface; the validation was performed using ground and in-flight measurements of the Fourier transform IR spectrometer planetary Fourier spectrometer, onboard the MarsExpress mission. The symmetrization has been exploited also for the spectra calibration, highlighting the issues deriving from the cases of relevant beamsplitter emission. The applicability of this procedure to other instruments is conditional to the presence in the spectra of at least one spectral region with a large SNR along with a negligible (or known) beamsplitter emission. For the PFS instrument, the processing of data with relevant beamsplitter emission has been performed exploiting the absorption carbon dioxide bands present in Martian spectra.

  4. Modelling a single phase voltage controlled rectifier using Laplace transforms

    NASA Technical Reports Server (NTRS)

    Kraft, L. Alan; Kankam, M. David

    1992-01-01

    The development of a 20 kHz, AC power system by NASA for large space projects has spurred a need to develop models for the equipment which will be used on these single phase systems. To date, models for the AC source (i.e., inverters) have been developed. It is the intent of this paper to develop a method to model the single phase voltage controlled rectifiers which will be attached to the AC power grid as an interface for connected loads. A modified version of EPRI's HARMFLO program is used as the shell for these models. The results obtained from the model developed in this paper are quite adequate for the analysis of problems such as voltage resonance. The unique technique presented in this paper uses the Laplace transforms to determine the harmonic content of the load current of the rectifier rather than a curve fitting technique. Laplace transforms yield the coefficient of the differential equations which model the line current to the rectifier directly.

  5. Phase-Constrained Spectrum Deconvolution for Fourier Transform Mass Spectrometry.

    PubMed

    Grinfeld, Dmitry; Aizikov, Konstantin; Kreutzmann, Arne; Damoc, Eugen; Makarov, Alexander

    2017-01-17

    This Article introduces a new computationally efficient noise-tolerant signal processing method, referred to as phased spectrum deconvolution method (ΦSDM), designed for Fourier transform mass spectrometry (FT MS). ΦSDM produces interference-free mass spectra with resolution beyond the Fourier transform (FT) uncertainty limit. With a presumption that the oscillation phases are preserved, the method deconvolves an observed FT spectrum into a distribution of harmonic components bound to a fixed frequency grid, which is several times finer than that of FT. The approach shows stability under noisy conditions, and the noise levels in the resulting spectra are lower than those of the original FT spectra. Although requiring more computational power than standard FT algorithms, ΦSDM runs in a quasilinear time. The method was tested on both synthetic and experimental data, and consistently demonstrated performance superior to the FT-based methodologies, be it across the entire mass range or on a selected mass window of interest. ΦSDM promises substantial improvements in the spectral quality and the speed of FT MS instruments. It might also be beneficial for other spectroscopy approaches which require harmonic analysis for data processing.

  6. Optical image encryption in Fresnel domain using spiral phase transform

    NASA Astrophysics Data System (ADS)

    Kumar, Ravi; Bhaduri, Basanta

    2017-09-01

    In this study, we propose a new nonlinear optical image encryption technique using spiral phase transform (SPT). First, the primary image is phase encoded and multiplied with a random amplitude mask (RAM), and using power function, the product is then powered to m. This powered output is Fresnel propagated with distance z 1 and then modulated with a random phase mask (RPM). The modulated image is further Fresnel propagated with distance z 2. Similarly, a security image is also modulated with another RAM and then Fresnel propagated with distance z 3. Next, the two modulated images after Fresnel propagations, are interfered and further Fresnel propagated with distance z 4 to get a complex image. Finally, this complex image is SPT with particular spiral phase function (SPF), to get the final encrypted image for transmission. In the proposed technique, the security keys are Fresnel propagation distances, the security image, RPM, RAMs, power order, m, and order of SPF, q. Numerical simulation results confirm the validity and effectiveness of the proposed technique. The proposed technique is robust against noise and brutal force attacks.

  7. Thermodynamic properties, nonstoichiometry and phase transformation parameters of oxides in the Eu-O system

    NASA Astrophysics Data System (ADS)

    Sukhushina, I.; Vasiljeva, I.; Balabajeva, R.

    1998-01-01

    The influence of nonstoichiometry on the partial thermodynamic properties of oxygen and the {C to B} transformation parameters of europium sesquioxide within a temperature range from 1200 K to 1400 K using e.m.f., DTA and DSC methods, has been determined. A tentative phase diagram in the {Cto B} transformation region is proposed. L'influence de la nonstoechiométrie sur les propriétés thermodynamiques partielles de l'oxygène et sur les paramètres de la transition {C to B} de Eu2O3 dans l'intervalle de températures de 1200 K à 1400 K a été étudiée par les méthodes EMF, DTA, DSC. Une variante de diagramme de phases dans la région de la transition {Cto B} est présentée.

  8. Phase Transformation of Nano-sized ZrO2 upon Thermal Annealing and Intense Radiation

    SciTech Connect

    Lu, Fengyuan; Zhang, Jiaming; Huang, Mengbing; Namavar, Fereydoon; Ewing, Rodney C.; Lian, Jie

    2011-03-25

    The phase stability and microstructure evolution of zirconia nanofilms on Si substrates prepared by ion beam assisted deposition (IBAD) upon thermal annealing and intensive radiation have been studied by in situ transmission electron microscopy (TEM), ex situ X-ray diffraction, and Raman spectroscopy. For as-prepared amorphous-dominant ZrO2 thin films, a phase transformation sequence of amorphous-to-tetragonal and tetragonal-to-monoclinic has been identified upon increasing annealing temperature from 500, 850, to 1000 °C. This phase transformation sequence varying with annealing temperature is accompanied by concomitant grain growth from ~5 to ~50 nm, consistent with the grain-size-controlled phase stability as a result of total energy crossover among different zirconia polymorphs. Upon ion bombardments of 350 KeV O+ and 1 MeV Kr2+ at room temperature, a monoclinic-to-tetragonal phase transformation was observed in the monoclinic-dominant ZrO2. This monoclinic-to-tetragonal phase transformation may be attributed to the oxygen vacancy accumulation in ZrO2 upon irradiation. Furthermore, both 1 MeV Kr2+ and 350 KeV O+ bombardments on the amorphous-dominant ZrO2 lead to an amorphous-to-tetragonal phase transformation as a result of radiation-induced recrystallization process. Thermodynamically metastable tetragonal ZrO2 phase can be stabilized at room temperature under intensive radiation by relatively low-energy ion bombardments. These results suggest a method of combining both thermal annealing and ion beam technique for controlling ZrO2 phase stability and thus tailoring materials properties for many engineering applications including actinide host matrix for advanced nuclear energy systems.

  9. Transformation toughened ceramics for the heavy duty diesel engine technology program, phase 2

    NASA Technical Reports Server (NTRS)

    Musikant, S.; Samanta, S. C.; Architetto, P.; Feingold, E.

    1985-01-01

    The objective of this program is to develop an insulating structural ceramic for application in a heavy duty adiabatic diesel engine. The approach is to employ transformation toughening (TT) by additions of zirconia-hafnia solid solution (ZHSS). The feasibility of using ZHSS as a toughening agent in mullite and alumina has been demonstrated in Phase 1 of this work. Based on Phase 1 results, a decision was made to concentrate the Phase 2 effort on process optimization of the TT mullite. A strong factor in that decision was the low thermal conductivity and high thermal shock resistance of the mullite. Results of the Phase 2 effort indicate that optimum toughening of mullite by additions of ZHSS is difficult to achieve due to apparent sensitivity to morphology. The 48 ksi room temperature modulus-of-rupture (MOR) achieved in selected specimens is approximately 50% of the original strength target. The MOR deteriorated to 34 ksi at 800 C.

  10. Isotope fractionation induced by phase transformation: First-principles investigation for Mg2SiO4

    NASA Astrophysics Data System (ADS)

    Wu, Zhongqing; Huang, Fang; Huang, Shichun

    2015-01-01

    With increasing pressure, olivine transforms to wadsleyite, ringwoodite, and finally dissociates into bridgmanite and ferropericlase, producing dramatic structure changes along these phase transformations. Here we investigated the equilibrium isotope fractionation of Mg, Si, and O among these phases using first-principles calculations. Both Mg and Si have measurable isotope fractionations among these phases, even at mantle's pressure-temperature conditions. Wadsleyite and ringwoodite are depleted in heavy Si but enriched in heavy Mg relative to olivine. Bridgmanite is depleted in heavy Si and Mg among all phases. Increasing pressure can slightly reduce the size of Si isotope fractionation but significantly depress Mg isotope fractionation among these phases. Mg and Si isotope fractionation among Mg2SiO4 polymorphs may provide a promising way to "probe" the depth and temperature of origin of mantle xenoliths.

  11. Phase transformation of Ca4[Al6O12]SO4 and its disordered crystal structure at 1073 K

    NASA Astrophysics Data System (ADS)

    Kurokawa, Daisuke; Takeda, Seiya; Colas, Maggy; Asaka, Toru; Thomas, Philippe; Fukuda, Koichiro

    2014-07-01

    The phase transformation of Ca4[Al6O12]SO4 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 (CuKα1). 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 I4bar3m with the unit-cell dimensions of a=0.92426(2) nm and V=0.78955(2) nm3 (Z=2) at 1073 K. The initial structural model was derived by the direct methods and further refined by the Rietveld method. The final structural model showed the orientational disordering of SO4 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 SO4 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 SO4 tetrahedra, without distinct dynamical reorientation.

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

    SciTech Connect

    Lone, A. G. 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 at 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.

  13. 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.

  14. 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.

  15. Phase transformation and negative thermal expansion in TaVO5.

    PubMed

    Wang, Xiaowei; Huang, Qingzhen; Deng, Jinxia; Yu, Ranbo; Chen, Jun; Xing, Xianran

    2011-03-21

    Two phase transformations of TaVO(5) were observed by DSC and/or dilatometry measurements in the studied temperature range. X-ray diffraction and neutron powder diffraction structure refinements indicated a phase transformation at -14 °C from a monoclinic symmetry with space group P2(1)/c to an orthorhombic symmetry with space group Pnma above this temperature. The rigid TaO(6) octahedron in orthorhombic phase becomes nonregular at -14 °C, which results in the transition from Pnma to P2(1)/c. TaVO(5) was found to be a negative thermal expansion material above room temperature. The calculated volumetric thermal expansion coefficients (TECs) are -8.92 × 10(-6) °C(-1) in the range of 20-600 °C, and -2.19 × 10(-5) °C(-1) above 600 °C, respectively. The negative thermal expansion behavior is accounted for by the tilt of the TaO(6) and VO(4) polyhedra, where the shrinkage of the VO(4) tetrahedra result in the increase of Ta-O-V angles on heating, while the angle of Ta-O1-Ta maintains at 180° in the framework.

  16. Heat Estimation from Infrared Measurement Compared to DSC for Austenite to R Phase Transformation in a NiTi Alloy

    NASA Astrophysics Data System (ADS)

    Delobelle, V.; Favier, D.; Louche, H.

    2013-06-01

    Heat source estimations from temperature field measurements deduced from infrared imaging are increasingly used to study thermo-mechanical coupling during materials' deformation. These estimations are based on approximations of the derivative terms with respect to time and space which are involved in the heat diffusion equation. This paper proposes a first experimental validation of this method by applying it to an experimental uniform air cooling of a NiTi Shape Memory Alloy thin plate. In the studied cooling temperate range, heat sources are due to Austenite to R phase transformation. Transformation temperatures, heat sources, and energies are estimated from infrared temperature measurements and compared successfully to differential scanning calorimetry results.

  17. 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.

  18. Electric-field-induced paraelectric to ferroelectric phase transformation in prototypical polycrystalline BaTiO{sub 3}

    SciTech Connect

    Wang, Zhiyang; Hinterstein, Manuel; Daniels, John E.; Webber, Kyle G.; Hudspeth, Jessica M.

    2014-10-20

    An electric-field-induced paraelectric cubic to ferroelectric tetragonal phase transformation has been directly observed in prototypical polycrystalline BaTiO{sub 3} at temperatures above the Curie point (T{sub C}) using in situ high-energy synchrotron X-ray diffraction. The transformation persisted to a maximum temperature of 4 °C above T{sub C}. The nature of the observed field-induced transformation and the resulting development of domain texture within the induced phase were dependent on the proximity to the transition temperature, corresponding well to previous macroscopic measurements. The transition electric field increased with increasing temperature above T{sub C}, while the magnitude of the resultant tetragonal domain texture at the maximum electric field (4 kV mm{sup −1}) decreased at higher temperatures. These results provide insights into the phase transformation behavior of a prototypical ferroelectric and have important implications for the development of future large-strain phase-change actuator materials.

  19. Ultrasonic Observations of Elasticity Changes Across Phase Transformations in MgSiO3 Pyroxenes

    SciTech Connect

    Kung,J.; Li, B.; Liebermann, R.

    2006-01-01

    Using ultrasonic interferometry in conjunction with synchrotron X-radiation techniques in a high-pressure, multi-anvil apparatus, the elastic behavior of MgSiO{sub 3} pyroxene has been continuously monitored as the specimens undergo phase transformations from the orthoenstatite (OEN) to the high-pressure clinoenstatite phase (HP-CEN) and from HP-CEN to the low-pressure clinoenstatite phase (LP-CEN). In the former case, elastic softening and amplitude attenuation is observed for both compresssional (P) and shear (S) waves when the pressure exceeds 9 GPa at room temperature, which we suggest is associated with a transition to a metastable phase intermediate between OEN and HP-CEN. In the latter case, both P and S wave velocities decrease rapidly as the back-transformation from HP-CEN to LP-CEN occurs on decrease of pressure below 4 GPa at room temperature; this is accompanied by an increase in attenuation of the P waves in the specimen, but not the S waves.

  20. Phase transformations of optically levitated microparticles of atmospheric relevance

    NASA Astrophysics Data System (ADS)

    Davis, Ryan D.

    The phase state and water content of atmospheric particles influences climate, air quality, and the hydrologic cycle through impacts on particle optical properties, heterogeneous chemistry, and the partitioning of water between the particle and gas phase. A significant fraction of liquid atmospheric particles contain soluble inorganic compounds that can undergo efflorescence, i.e., the process of particle crystallization with concurrent loss of particle-phase water. Despite the importance of particle phase state, there is no comprehensive understanding of particle crystallization and empirical observations are a necessity. In the laboratory studies presented here, phase transformations of aqueous and crystalline inorganic microparticles were studied using a novel long-working distance optical trap. The optical trap was developed with the motivation of probing crystallization induced by particle collisions (coagulation). A key result from these studies was the first observations of a previously unexplored particle crystallization pathway that we termed “contact efflorescence”. Contact efflorescence describes the crystallization of a metastable aqueous microdroplet initiated by surface contact with an externally located particle. These studies demonstrate that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient relative humidity (≤80%). The occurrence of contact efflorescence is demonstrated using soluble crystalline contact nuclei as well as amorphous (non-crystalline) organic contact nuclei. These results point toward a transient crystal nucleation pathway relevant at the moment of aerosol contact and suggest far more particle types can induce crystallization of atmospheric aerosols via coagulation than is currently considered in atmospheric chemistry and climate models.

  1. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Stein, S.; Okal, E.A.; Rubie, David C.

    1996-01-01

    should cease at depths near 700 km, because the seismogenic phase transformations in the slab are completed or can no longer occur. Substantial metastability is expected only in old, cold slabs, consistent with the observed restriction of deep earthquakes to those settings. Earthquakes should be restricted to the cold cores of slabs, as in any model in which the seismicity is temperature controlled, via the distribution of metastability. However, the geometries of recent large deep earthquakes pose a challenge for any such models. Transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics, including focal mechanisms indicating localized shear failure rather than implosive deformation, are so similar to those of shallow earthquakes. Finally, metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab. Such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and/or are detached from subducting slabs. Metastability in downgoing slabs could have considerable geodynamic significance. Metastable wedges would reduce the negative buoyancy of slabs, decrease the driving force for subduction, and influence the state of stress in slabs. Heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage, causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity. Because wedge formation should occur only for fast subducting slabs, it may act as a "parachute" and contribute to regulating plate speeds. Wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge tra

  2. Phase transformation dependence on initial plastic deformation mode in Si via nanoindentation

    SciTech Connect

    Wong, Sherman; Haberl, Bianca; Williams, James S.; Bradby, Jodie E.

    2016-09-30

    Silicon in its diamond-cubic phase is known to phase transform to a technologically interesting mixture of the body-centred cubic and rhombohedral phases under nanoindentation pressure. In this study, we demonstrate that during plastic deformation the sample can traverse two distinct pathways, one that initially nucleates a phase transformation while the other initially nucleates crystalline defects. These two pathways remain distinct even after sufficient pressure is applied such that both deformation mechanisms are present within the sample. Here, it is further shown that the indents that initially nucleate a phase transformation generate larger, more uniform volumes of the phase transformed material than indents that initially nucleate crystalline defects.

  3. Phase transformation dependence on initial plastic deformation mode in Si via nanoindentation

    SciTech Connect

    Wong, Sherman; Haberl, Bianca; Williams, James S.; Bradby, Jodie E.

    2016-09-30

    Silicon in its diamond-cubic phase is known to phase transform to a technologically interesting mixture of the body-centred cubic and rhombohedral phases under nanoindentation pressure. In this study, we demonstrate that during plastic deformation the sample can traverse two distinct pathways, one that initially nucleates a phase transformation while the other initially nucleates crystalline defects. These two pathways remain distinct even after sufficient pressure is applied such that both deformation mechanisms are present within the sample. Here, it is further shown that the indents that initially nucleate a phase transformation generate larger, more uniform volumes of the phase transformed material than indents that initially nucleate crystalline defects.

  4. Phase transformation considerations during process development and manufacture of solid oral dosage forms.

    PubMed

    Zhang, Geoff G Z; Law, Devalina; Schmitt, Eric A; Qiu, Yihong

    2004-02-23

    The quality and performance of a solid oral dosage form depends on the choice of the solid phase, the formulation design, and the manufacturing process. The potential for process-induced solid phase transformations must be evaluated during design and development of formulations and manufacturing processes. This article briefly reviews the basic principles of polymorphism, defines the classes of phase transformation and the underlying transformation mechanisms, and discusses respective kinetic factors. The potential phase transformations associated with common unit operations employed in manufacturing solid oral dosage forms are highlighted. Specific examples are given to illustrate the importance of solid phases, and process-induced phase transitions in formulation and process development.

  5. [Raman characterization of rutile phase transitions under high-pressure and high-temperature].

    PubMed

    Xiao, Wan-Sheng; Zhang, Hong; Tan, Da-Yong; Weng, Ke-Nan; Li, Yan-Chun; Luo, Chong-Ju; Liu, Jing; Xie, Hong-Sen

    2007-07-01

    The pressure-induced phase transition of rutile-structured TiO2 was investigated by in-situ Raman spectrum method in a laser-heated diamond anvil cell (DAC). The experiment was conducted at 35 GPa under quasihydrostatic conditions using argon as medium. At room temperature, the rutile-type TiO2 begins to transform to baddeleyite-type phase at 13.4 GPa and completes at 21 GPa, and this new high-pressure structure retains up to 35 GPa, the upmost pressure used in this study. At the pressure of 29.4 GPa the sample of baddeleyite-type TiO2 was heated by an YAG laser to about 1 000-1500 degrees C, and then the baddeleyite phase transformed to a Pbca phase. The Pbca phase was heated again at 35.0 GPa and it was still stable. The sample then began to be decompressed, and the Pbca phase of TiO2 transformed to baddeleyite structure at 26.3 GPa, which stayed stable to 11.4 GPa. The formation of Pbca phase from baddeleyite phase needs the condition of high temperature, it transforms back to badde-leyite structure completely at pressure of a little below that on its formation, which suggests the boundary of the two phases can be determined at about 28 GPa. At 7. 6 GPa, and the Raman spectrum shows the characteristics of the mixture of two phases of baddeleyite-type and alpha-PbO2-type, which indicates that the baddeleyite phase transforms to alpha-PbO2 phase at about 7 GPa. The alpha-PbO2-type TiO2 is metastable under ambient condition.

  6. Single beam Fourier transform digital holographic quantitative phase microscopy

    SciTech Connect

    Anand, A. Chhaniwal, V. K.; Mahajan, S.; Trivedi, V.; Faridian, A.; Pedrini, G.; Osten, W.; Dubey, S. K.; Javidi, B.

    2014-03-10

    Quantitative phase contrast microscopy reveals thickness or height information of a biological or technical micro-object under investigation. The information obtained from this process provides a means to study their dynamics. Digital holographic (DH) microscopy is one of the most used, state of the art single-shot quantitative techniques for three dimensional imaging of living cells. Conventional off axis DH microscopy directly provides phase contrast images of the objects. However, this process requires two separate beams and their ratio adjustment for high contrast interference fringes. Also the use of two separate beams may make the system more vulnerable to vibrations. Single beam techniques can overcome these hurdles while remaining compact as well. Here, we describe the development of a single beam DH microscope providing whole field imaging of micro-objects. A hologram of the magnified object projected on to a diffuser co-located with a pinhole is recorded with the use of a commercially available diode laser and an arrayed sensor. A Fourier transform of the recorded hologram directly yields the complex amplitude at the image plane. The method proposed was investigated using various phase objects. It was also used to image the dynamics of human red blood cells in which sub-micrometer level thickness variation were measurable.

  7. Fourier transform infrared phase shift cavity ring down spectrometer

    NASA Astrophysics Data System (ADS)

    Schundler, Elizabeth; Mansur, David J.; Vaillancourt, Robert; Benedict-Gill, Ryan; Newbry, Scott P.; Engel, James R.; Dupuis, Julia Rentz

    2014-05-01

    OPTRA has developed a Fourier transform infrared phase shift cavity ring down spectrometer (FTIR-PS-CRDS) system under a U.S. EPA SBIR contract. This system uses the inherent wavelength-dependent modulation imposed by the FTIR on a broadband thermal source for the phase shift measurement. This spectrally-dependent phase shift is proportional to the spectrally-dependent ring down time. The spectral dependence of both of these values is introduced by the losses of the cavity including those due to the molecular absorption of the sample. OPTRA's approach allows broadband detection of chemicals across the feature-rich fingerprint region of the long-wave infrared. This represents a broadband and spectral range enhancement to conventional CRDS which is typically done at a single wavelength in the near IR; at the same time the approach is a sensitivity enhancement to traditional FTIR, owing to the long effective path of the resonant cavity. In previous papers1,2, OPTRA has presented a breadboard system aimed at demonstrating the feasibility of the approach and a prototype design implementing performance enhancements based on the results of breadboard testing. In this final paper in the series, we will present test results illustrating the realized performance of the fully assembled and integrated breadboard, thereby demonstrating the utility of the approach.

  8. Phase-field modeling of the beta to omega phase transformation in Zr–Nb alloys

    DOE PAGES

    Yeddu, Hemantha Kumar; Lookman, Turab

    2015-05-01

    A three-dimensional elastoplastic phase-field model is developed, using the Finite Element Method (FEM), for modeling the athermal beta to omega phase transformation in Zr–Nb alloys by including plastic deformation and strain hardening of the material. The microstructure evolution during athermal transformation as well as under different stress states, e.g. uni-axial tensile and compressive, bi-axial tensile and compressive, shear and tri-axial loadings, is studied. The effects of plasticity, stress states and the stress loading direction on the microstructure evolution as well as on the mechanical properties are studied. The input data corresponding to a Zr – 8 at.% Nb alloy aremore » acquired from experimental studies as well as by using the CALPHAD method. Our simulations show that the four different omega variants grow as ellipsoidal shaped particles. Our results show that due to stress relaxation, the athermal phase transformation occurs slightly more readily in the presence of plasticity compared to that in its absence. The evolution of omega phase is different under different stress states, which leads to the differences in the mechanical properties of the material. The variant selection mechanism, i.e. formation of different variants under different stress loading directions, is also nicely captured by our model.« less

  9. On Inclusion-Matrix Interfacial Stresses in Composites Containing Phase-Transforming Phases

    NASA Astrophysics Data System (ADS)

    Wang, Yun-Che; Ko, Chi-Ching

    2010-05-01

    Recent development in composites containing phase-transforming particles, such as vanadium dioxide or barium titanate, reveals the overall stiffness and viscoelastic damping of the composites may be unbounded. Negative stiffness is induced from phase transformation predicted by the Landau phase transformation theory. Although this unbounded phenomenon is theoretically supported with the composite homogenization theory, detailed stress analyses of the composites are still lacking. In this work, we analyze the two-dimensional plane stress elasticity problem of a square plate containing a circular inclusion, under the assumption that the Young's modulus of the inclusion is negative. Assumption of negative stiffness is a priori in the present analysis. A static loading condition is adopted to estimate the effective modulus of the composites by the ratio of applied stress to averaged strain on the loading edges. It is found that the interfacial stresses between the circular inclusion and matrix increase dramatically when the negative stiffness is so tuned that overall stiffness is unbounded. Furthermore, it is found that stress distributions in the inclusion are not uniform, contrary to Eshelby's theorem, which states, for two-phase, infinite composites, the inclusion's stress distribution is uniform when the shape of the inclusion has higher symmetry than an ellipse. The rationale for this nonuniform stress distributions is due to nonlocal effects induced from negative stiffness.

  10. Phase-field modeling of the beta to omega phase transformation in Zr–Nb alloys

    SciTech Connect

    Yeddu, Hemantha Kumar; Lookman, Turab

    2015-05-01

    A three-dimensional elastoplastic phase-field model is developed, using the Finite Element Method (FEM), for modeling the athermal beta to omega phase transformation in Zr–Nb alloys by including plastic deformation and strain hardening of the material. The microstructure evolution during athermal transformation as well as under different stress states, e.g. uni-axial tensile and compressive, bi-axial tensile and compressive, shear and tri-axial loadings, is studied. The effects of plasticity, stress states and the stress loading direction on the microstructure evolution as well as on the mechanical properties are studied. The input data corresponding to a Zr – 8 at.% Nb alloy are acquired from experimental studies as well as by using the CALPHAD method. Our simulations show that the four different omega variants grow as ellipsoidal shaped particles. Our results show that due to stress relaxation, the athermal phase transformation occurs slightly more readily in the presence of plasticity compared to that in its absence. The evolution of omega phase is different under different stress states, which leads to the differences in the mechanical properties of the material. The variant selection mechanism, i.e. formation of different variants under different stress loading directions, is also nicely captured by our model.

  11. Iron phase transformations resulting from the respiration of Shewanella putrefaciens on a mixed mineral phase

    NASA Astrophysics Data System (ADS)

    Boyanov, M. I.; O'Loughlin, E. J.; Kemner, K. M.

    2009-11-01

    The initial Fe(III) minerals and the secondary mineralization products of Shewanella putrefaciens CN32 grown in the presence of dissolved phosphate and a commercial Fe(III) oxide, nominally nanoparticulate lepidocrocite, were determined using XRD and XAFS. The starting material was transformed by the bacteria from a reddish brown, rust colour mineral to a dark green phase over 90 days. Acid extraction of the bioreduced solids with 0.75 M HCl recovered 83% of the total iron as Fe(II), leaving a solid, acid-resistant phase. The latter was identified as nanoparticulate hematite by EXAFS. Subsequently, the starting Fe(III) phase was determined to be a mixture of 60% lepidocrocite, 26% ferrihydrite, and 14% hematite, using linear combination EXAFS analysis. For the acid-extractable phase, XANES and EXAFS indicated a predominantly Fe(II) valence state and a spectrum consistent with a mixture of brucite-type minerals(e.g., green rust or ferrous hydroxide) and siderite. The observed transformations suggest that in this mixed-mineral system, lepidocrocite and ferrihydrite are readily reducible to green rust and siderite, whereas hematite is less amenable to bacterial reduction. This study also demonstrates the utility of XAFS spectroscopy in the quantitative characterization of dissimilatory metal transformations, particularly in complex systems such as nanoparticulate minerals in hydrated mineral-bacteria assemblages.

  12. Natural demodulation of two-dimensional fringe patterns. II. Stationary phase analysis of the spiral phase quadrature transform.

    PubMed

    Larkin, K G

    2001-08-01

    Utilizing the asymptotic method of stationary phase, I derive expressions for the Fourier transform of a two-dimensional fringe pattern. The method assumes that both the amplitude and the phase of the fringe pattern are well-behaved differentiable functions. Applying the limits in two distinct ways, I show, first, that the spiral phase (or vortex) transform approaches the ideal quadrature transform asymptotically and, second, that the approximation errors increase with the relative curvature of the fringes. The results confirm the validity of the recently proposed spiral phase transform method for the direct demodulation of closed fringe patterns.

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

    SciTech Connect

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

    2013-08-28

    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.

  14. Phase transformation and phonon anomalies in Ni{sub 2}MnGa

    SciTech Connect

    Zheludev, A.; Shapiro, S.M.; Wochner, P.; Schwartz, A.; Wall, M.; Tanner, L.E.

    1995-07-01

    Inelastic neutron scattering experiments and transmission electron microscopy have been used to study a single crystal of the Ni{sub 2}MnGa shape memory Hustler alloy in a wide temperature range covering the parent phase (T>T{sub 1}=265 K), a recently discovered pemartensitic (T{sub 1}T>T{sub M}) and martensitic (Tphase regions. A temperature-dependent anomaly in the TA{sub 2} phonon dispersion in the parent phase was observed and related to the phase transformations. The premartensitic phase involves a transverse modulation of the parent cubic structure with a simple periodicity of 1/3 [110]. The approximately tetragonal lattice of the low-temperature martensite is distorted by transverse modulations with incommensurate wave vectors [{zeta}M {zeta}M {sup 0}] and [{sup 2}{zeta}M {sup 2}{zeta}M {sup 0}], {zeta}M {approx}0.43. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.

  15. Analysis of temperature rise for piezoelectric transformer using finite-element method.

    PubMed

    Joo, Hyun-Woo; Lee, Chang-Hwan; Rho, Jong-Seok; Jung, Hyun-Kyo

    2006-08-01

    Analysis of heat problem and temperature field of a piezoelectric transformer, operated at steady-state conditions, is described. The resonance frequency of the transformer is calculated from impedance and electrical gain analysis using a finite-element method. Mechanical displacement and electric potential of the transformer at the calculated resonance frequency are used to calculate the loss distribution of the transformer. Temperature distribution using discretized heat transfer equation is calculated from the obtained losses of the transformer. Properties of the piezoelectric material, dependent on the temperature field, are measured to recalculate the losses, temperature distribution, and new resonance characteristics of the transformer. Iterative method is adopted to recalculate the losses and resonance frequency due to the changes of the material constants from temperature increase. Computed temperature distributions and new resonance characteristics of the transformer at steady-state temperature are verified by comparison with experimental results.

  16. Effect of heat treatment on transformation temperatures and bending properties of nickel-titanium endodontic instruments.

    PubMed

    Yahata, Y; Yoneyama, T; Hayashi, Y; Ebihara, A; Doi, H; Hanawa, T; Suda, H

    2009-07-01

    To investigate the effect of heat treatment on the bending properties of nickel-titanium endodontic instruments in relation to their transformation behaviour. Nickel-titanium super-elastic alloy wire (1.00 mm Ø) was processed into a conical shape with a 0.30 mm diameter tip and 0.06 taper. The heat treatment temperature was set at 440 or 500 degrees C for a period of 10 or 30 min. Nonheat-treated specimens were used as controls. The phase transformation behaviour was examined using differential scanning calorimetry. A cantilever-bending test was used to evaluate the bending properties of the specimens. Data were analyzed by ANOVA and the Tukey-Kramer test (P = 0.05). The transformation temperature was higher for each heat treatment condition compared with the control. Two clear thermal peaks were observed for the heat treatment at 440 degrees C. The specimen heated at 440 degrees C for 30 min exhibited the highest temperatures for M(s) and A(f), with subsequently lower temperatures observed for specimens heated at 440 degrees C for 10 min, 500 degrees C for 30 min, 500 degrees C for 10 min, and control specimens. The sample heated at 440 degrees C for 30 min had the lowest bending load values (P < 0.05), both in the elastic range (0.5 mm deflection) and in the super-elastic range (2.0 mm deflection). The influence of heat treatment time was less than that of heat treatment temperature. Change in the transformation behaviour by heat treatment may be effective in increasing the flexibility of nickel-titanium endodontic instruments.

  17. A new type of Cu-Al-Ta shape memory alloy with high martensitic transformation temperature

    NASA Astrophysics Data System (ADS)

    Wang, C. P.; Su, Y.; Y Yang, S.; Shi, Z.; Liu, X. J.

    2014-02-01

    In this study, a new type of Cu-Al-Ta (Cu86Al12Ta2 wt%) shape memory alloy with high martensitic transformation temperature is explored. The microstructure, reversible martensitic transformation and shape memory properties are investigated by means of optical microscopy, back-scattered electron, electron probe microanalysis, x-ray diffraction, differential scanning calorimetry and tensile tests. It is proposed that Cu86Al12Ta2 alloy consists of a mixture of primarily {\\beta }_{1}^{\\prime} martensite and a little {\\gamma }_{1}^{\\prime} martensite and some different precipitates. The tiny thin-striped Ta2(Al,Cu)3 precipitate is predominant in the as-quenched condition, whereas the particle-shaped Cu(Al, Ta) precipitate is dominant after hot-rolling. Additionally, the dendritic-shaped γ1(Cu9Al4) phase begins to appear after hot-rolling, but it disappears when the sample is re-quenched. All studied samples have reversible martensitic transformation temperatures higher than 450 ° C. The results show that two-step martensitic transformation behavior is observed for Cu86Al12Ta2 alloy in all three different conditions due to the transformations between ({\\beta }_{1}^{\\prime}+{\\gamma }_{1}^{\\prime}) martensites and the austenite parent phase. The results further show that the recovery ratios are almost 100% when the pre-strains are ≤2.5%, then they gradually decrease with further increase of the pre-strains. The shape memory effects clearly increase as a result of increase of the pre-strains, up to a maximum value of 3.2%.

  18. Thickness-dependent phase transformation in nanoindented germanium thin films.

    PubMed

    Oliver, D J; Bradby, J E; Williams, J S; Swain, M V; Munroe, P

    2008-11-26

    We investigate the mechanical response of 50-600 nm epitaxial Ge films on a Si substrate using nanoindentation with a nominally spherical (R≈4.3 µm) diamond tip. The inelastic deformation mechanism is found to depend critically on the film thickness. Sub-100 nm Ge films deform by pressure-induced phase transformation, whereas thicker films deform only by shear-induced dislocation slip and twinning. Nanoindentation fracture response is similarly dependent on film thickness. Elastic stress modelling shows that differing stress modes vary in their spatial distribution, and consequently the film thickness governs the stress state in the film, in conjunction with the radius of the nanoindenter tip. This opens the prospect of tailoring the contact response of Ge and related materials in thin film form by varying film thickness and indenter radius.

  19. Phase transformation of poled "chem-prep" PZT 95/5-2Nb ceramic under quasi-static loading conditions.

    SciTech Connect

    Lee, Moo Yul; Montgomery, Stephen Tedford; Hofer, John H.

    2004-10-01

    Specimens of poled 'chem-prep' PNZT ceramic from batch HF803 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions at three temperatures of -55, 25, and 75 C and pressures up to 500 MPa. The objective of this experimental study was to obtain the electro-mechanical properties of the ceramic and the criteria of FE (Ferroelectric) to AFE (Antiferroelectric) phase transformations so that grain-scale modeling efforts can develop and test models and codes using realistic parameters. The poled ceramic undergoes anisotropic deformation during the transition from a FE to an AFE structure. The lateral strain measured parallel to the poling direction was typically 35 % greater than the strain measured perpendicular to the poling direction. The rates of increase in the phase transformation pressures per temperature changes were practically identical for both unpoled and poled PNZT HF803 specimens. We observed that the retarding effect of temperature on the kinetics of phase transformation appears to be analogous to the effect of shear stress. We also observed that the FE-to-AFE phase transformation occurs in poled ceramic when the normal compressive stress, acting perpendicular to a crystallographic plane about the polar axis, equals the hydrostatic pressure at which the transformation otherwise takes place.

  20. Kinetics of phase transformation in glass forming systems

    NASA Technical Reports Server (NTRS)

    Ray, Chandra S.

    1994-01-01

    The objectives of this research were to (1) develop computer models for realistic simulations of nucleation and crystal growth in glasses, which would also have the flexibility to accomodate the different variables related to sample characteristics and experimental conditions, and (2) design and perform nucleation and crystallization experiments using calorimetric measurements, such as differential scanning calorimetry (DSC) and differential thermal analysis (DTA) to verify these models. The variables related to sample characteristics mentioned in (1) above include size of the glass particles, nucleating agents, and the relative concentration of the surface and internal nuclei. A change in any of these variables changes the mode of the transformation (crystallization) kinetics. A variation in experimental conditions includes isothermal and nonisothermal DSC/DTA measurements. This research would lead to develop improved, more realistic methods for analysis of the DSC/DTA peak profiles to determine the kinetic parameters for nucleation and crystal growth as well as to assess the relative merits and demerits of the thermoanalytical models presently used to study the phase transformation in glasses.

  1. Structural transformations in Pb/Si(111) phases induced by C₆₀ adsorption.

    PubMed

    Matetskiy, A V; Bondarenko, L V; Gruznev, D V; Zotov, A V; Saranin, A A; Tringides, M C

    2013-10-02

    Structural transformations at the Pb/Si(111) surface occurring upon C₆₀ adsorption onto Pb/Si(111)1 × 1 phase at room temperature and Pb/Si(111)[Formula: see text] at low temperatures between 30 and 210 K, have been studied using scanning tunneling microscopy and low-energy electron diffraction observations. Typically, C₆₀ fullerenes agglomerate into random molecular islands nucleated at the surface defects. C₆₀ island formation is accompanied by expelling Pb atoms to the surrounding surface area where more dense Pb/Si(111) phases form. Productivity of C₆₀-induced expelling of Pb atoms is controlled by surface defects and is suppressed dramatically when regular ('crystalline') C₆₀ islands self-assemble at the defect-free Pb/Si(111) surface. When Pb atoms are ejected by the random C₆₀ islands, extended structural transformations involving reordering of numerous Pb atoms are fully completed at the surface within the shortest possible time (a few dozen seconds) to reapproach and image the surface after C₆₀ deposition. Estimations show that the observed transformations cannot be controlled by random walk diffusion of Pb adatoms, which implies a highly correlated motion of the Pb atom displacements within the layer.

  2. 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.

  3. Phase transformation and thermoelectric properties of bismuth-telluride nanowires

    NASA Astrophysics Data System (ADS)

    Hsin, Cheng-Lun; Wingert, Matthew; Huang, Chun-Wei; Guo, Hua; Shih, Ten-Jen; Suh, Joonki; Wang, Kevin; Wu, Junqiao; Wu, Wen-Wei; Chen, Renkun

    2013-05-01

    Thermoelectric materials have attracted much attention due to the current interest in energy conversion and recent advancements in nano-engineering. A simple approach to synthesize BiTe and Bi2Te3 micro/nanowires was developed by combining solution chemistry reactions and catalyst-free vapor-solid growth. A pathway to transform the as-grown BiTe nanostructures into Bi2Te3 can be identified through the Bi-Te phase diagram. Structural characterization of these products was identified using standard microscopy practices. Meanwhile, thermoelectric properties of individual Bi-Te compound micro/nanowires were determined by the suspended microdevice technique. This approach provides an applicable route to synthesize advanced high performance thermoelectric materials in quantities and can be used for a wide range of low-dimensional structures.Thermoelectric materials have attracted much attention due to the current interest in energy conversion and recent advancements in nano-engineering. A simple approach to synthesize BiTe and Bi2Te3 micro/nanowires was developed by combining solution chemistry reactions and catalyst-free vapor-solid growth. A pathway to transform the as-grown BiTe nanostructures into Bi2Te3 can be identified through the Bi-Te phase diagram. Structural characterization of these products was identified using standard microscopy practices. Meanwhile, thermoelectric properties of individual Bi-Te compound micro/nanowires were determined by the suspended microdevice technique. This approach provides an applicable route to synthesize advanced high performance thermoelectric materials in quantities and can be used for a wide range of low-dimensional structures. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr00876b

  4. Controllable Phase Transformation and Mid-infrared Emission from Er3+-Doped Hexagonal-/Cubic-NaYF4 Nanocrystals

    PubMed Central

    Yang, Dandan; Chen, Dongdan; He, Huilin; Pan, Qiwen; Xiao, Quanlan; Qiu, Jianrong; Dong, Guoping

    2016-01-01

    The morphology of hexagonal phase NaYF4:Er3+ nanorods synthesized by hydrothermal method changed greatly after a continuing calcination, along with a phase transformation to cubic phase. Photoluminescence (PL) spectra indicated that mid-infrared (MIR) emission was obtained in both hexagonal and cubic phase NaYF4:Er3+ nanocrystals for the first time. And the MIR emission of NaYF4:Er3+ nanocrystals enhanced remarkably at higher calcination temperature. To prevent uncontrollable morphology from phase transformation, the cubic phase NaYF4:Er3+ nanospheres with an average size of ~100 nm were prepared via a co-precipitation method directly. In contrast, the results showed better morphology and size of cubic phase NaYF4:Er3+ nanocrystals have realized when calcined at different temperatures. And PL spectra demonstrated a more intense MIR emission in the cubic phase NaYF4:Er3+ nanocrystals with an increasing temperature. Besides, the MIR emission peak of Er3+ ions had an obvious splitting in cubic phase NaYF4. Therefore, cubic phase NaYF4:Er3+ nanospheres with more excellent MIR luminescent properties seems to provide a new material for nanocrystal-glass composites, which is expected to open a broad new field for the realization of MIR lasers gain medium. PMID:27453150

  5. α-Phase transformation kinetics of U – 8 wt% Mo established by in situ neutron diffraction

    DOE PAGES

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

    2016-05-08

    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

  6. Phase transformation behaviour and bending properties of hybrid nickel-titanium rotary endodontic instruments.

    PubMed

    Hayashi, Y; Yoneyama, T; Yahata, Y; Miyai, K; Doi, H; Hanawa, T; Ebihara, A; Suda, H

    2007-04-01

    To investigate the bending properties of hybrid rotary nickel-titanium endodontic instruments in relation to their transformation behaviour. Four types of nickel-titanium rotary endodontic instruments with different cross-sectional shapes (triangular-based and rectangular-based) and different heat treatment conditions (super-elastic type and hybrid type with shape memory effect) were selected to investigate bending properties and phase transformation behaviour. Bending load of the instruments was measured in a cantilever-bending test at 37 degrees C with the maximum deflection of 3.0 mm. A commercial rotary instrument, ProFile (PF; Dentsply Maillefer, Ballaigues, Switzerland) was used as a reference for the bending test. Phase transformation temperatures were calculated from the diagrams obtained from differential scanning calorimetry. Data were analysed by anova and Scheffe's test. The bending load values of the hybrid type that had undergone additional heat treatment at the tip were significantly lower (P < 0.05) than those of the super-elastic type with no additional heat treatment. The bending load values of rectangular-based cross-sectional shaped instruments were significantly lower (P < 0.05) than those of triangular-based cross-sectional shaped instruments. Phase transformation temperatures (M(s) and A(f) points) of the hybrid type were significantly higher (P < 0.05) than the super-elastic type. The M(f) and A(s) points of the tip part were significantly higher (P < 0.05) than those of the whole part of the hybrid instrument. Additional heat treatment of hybrid nickel-titanium instruments may be effective in increasing the flexibility of nickel-titanium rotary instruments.

  7. Superplasticity in silicon nitride through the α to β phase transformation

    NASA Astrophysics Data System (ADS)

    Rossignol, Fabrice; Roussel, Tanguy; Besson, Jean-Louis; Goursat, Paul; Lespade, Pierre

    1995-02-01

    Superplasticity is observed in silicon for temperatures ranging between 1500 and 1600 ^{circ}C in damage resistant materials with a submicron grain size. The microstructure evolves rapidly in the temperature range. This study shows that it is an advantage to start with an α-Si{3}N{4} rich ceramic in order to derive benefit from both the equiaxed α-grain shape and the transient liquid phase that forms during deformation at an intermediate stage of the αtoβ transformation. The kinetics of this transformation is analyzed and direct correlation is made with the deformation curves (elongation > 80%). La superplasticité du nitrure de silicium se manifeste entre 1500 et 1600 ^{circ}C pour des matériaux à grains submicroniques présentant une bonne résistance à l'endommagement. Dans ce domaine de température la microstructure évolue rapidement. Cette étude montre qu'il est avantageux de partir d'une céramique principalement constituée de la phase α-Si{3}N{4} afin de bénéficier de la forme équiaxe des grains α ainsi que la phase liquide transitoire qui se forme au cours de la déformation durant le processus de transformation αtoβ. La cinétique de cette transformation est anlysée et son incidence sur les courbes de déformation à chaud (allongement > 80%) est mise en évidence.

  8. 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.

  9. Transformation of two-line ferrihydrite to goethite and hematite as a function of pH and temperature.

    PubMed

    Das, Soumya; Hendry, M Jim; Essilfie-Dughan, Joseph

    2011-01-01

    Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.

  10. Dynamically driven phase transformations in heterogeneous materials. II. Applications including damage

    NASA Astrophysics Data System (ADS)

    Plohr, JeeYeon N.; Clements, B. E.; Addessio, F. L.

    2006-12-01

    A model, developed for heterogeneous materials undergoing dynamically driven phase transformations in its constituents, has been extended to include the evolution of damage. Damage is described by two mechanisms: interfacial debonding between the constituents and brittle failure micro-crack growth within the constituents. The analysis is applied to silicon carbide-titanium (SiC-Ti) unidirectional metal matrix composites that undergo the following phenomena: Ti has a yield stress of approximately 0.5 GPa and above a pressure of about 2 GPa undergoes a solid-solid phase transformation. The inelastic work from plastic dissipation contributes to the temperature and pressure rise in the Ti. SiC behaves elastically below a critical stress, above which it is damaged by microcrack growth. Finally, under tensile loading, the interface between Ti and SiC debonds according to an interfacial decohesion law. Each process is first examined independently in order to understand how its characteristic behavior is manifested in the stress-strain response of the composite. The complex interplay between loading states, viscoplasticity, damage, and solid-solid phase transformations is then studied at both the micromechanics and macromechanics levels.

  11. The Effect of Hardenability Variation on Phase Transformation of Spiral Bevel Gear in Quenching Process

    NASA Astrophysics Data System (ADS)

    Zhang, Yingtao; Shi, Wankai; Yang, Lin; Gu, Zhifei; Li, Zhichao

    2016-07-01

    The hardenability of gear steel is dependent on the composition of alloying elements and is one of important criteria to assess process of phase transformation. The variation of hardenability has to be considered in control of the microstructures and distortion during gear quenching. In this paper, the quantitative effect of hardenability has been investigated on phase transformations of spiral bevel gears in die quenching. The hardenability deviation of 22CrMoH steel was assessed by using Jominy test. The dilatometry experiments were conducted to build phase transformation kinetic models for steels with low and high hardenability, respectively. The complete die quenching process of spiral bevel gear was modeled to reveal the significant difference on microstructures and temperature history with variation of hardenability. The final microstructures of the gear are martensite in surface layer after quenching process. There are bainite inside the gear tooth and the mixture of bainite and ferrite inside gear for the gear with low hardenability. The microstructure is bainite inside the gear with high hardenability.

  12. Volume and Mass Estimation of Three-Phase High Power Transformers for Space Applications

    NASA Technical Reports Server (NTRS)

    Kimnach, Greg L.

    2004-01-01

    Spacecraft historically have had sub-1kW(sub e), electrical requirements for GN&C, science, and communications: Galileo at 600W(sub e), and Cassini at 900W(sub e), for example. Because most missions have had the same order of magnitude power requirements, the Power Distribution Systems (PDS) use existing, space-qualified technology and are DC. As science payload and mission duration requirements increase, however, the required electrical power increases. Subsequently, this requires a change from a passive energy conversion (solar arrays and batteries) to dynamic (alternator, solar dynamic, etc.), because dynamic conversion has higher thermal and conversion efficiencies, has higher power densities, and scales more readily to higher power levels. Furthermore, increased power requirements and physical distribution lengths are best served with high-voltage, multi-phase AC to maintain distribution efficiency and minimize voltage drops. The generated AC-voltage must be stepped-up (or down) to interface with various subsystems or electrical hardware. Part of the trade-space design for AC distribution systems is volume and mass estimation of high-power transformers. The volume and mass are functions of the power rating, operating frequency, the ambient and allowable temperature rise, the types and amount of heat transfer available, the core material and shape, the required flux density in a core, the maximum current density, etc. McLyman has tabulated the performance of a number of transformers cores and derived a "cookbook" methodology to determine the volume of transformers, whereas Schawrze had derived an empirical method to estimate the mass of single-phase transformers. Based on the work of McLyman and Schwarze, it is the intent herein to derive an empirical solution to the volume and mass estimation of three-phase, laminated EI-core power transformers, having radiated and conducted heat transfer mechanisms available. Estimation of the mounting hardware, connectors

  13. Membrane lipid phase transitions and phase organization studied by Fourier transform infrared spectroscopy.

    PubMed

    Lewis, Ruthven N A H; McElhaney, Ronald N

    2013-10-01

    Fourier transform infrared (FTIR) spectroscopy is a powerful yet relatively inexpensive and convenient technique for studying the structure and organization of membrane lipids in their various polymorphic phases. This spectroscopic technique yields information about the conformation and dynamics of all regions of the lipid molecule simultaneously without the necessity of introducing extrinsic probes. In this review, we summarize some relatively recent FTIR spectroscopic studies of the structure and organization primarily of fully hydrated phospholipids in their biologically relevant lamellar crystalline, gel and liquid-crystalline phases, and show that interconversions between these bilayer phases can be accurately monitored by this technique. We also briefly discuss how the structure and organization of potentially biologically relevant nonlamellar micellar or reversed hexagonal lipid phases can be studied and how phase transitions between lamellar and nonlamellar phases, or between various nonlamellar phases, can be followed as well. In addition, we discuss the potential for FTIR spectroscopy to yield fairly high resolution structural information about phospholipid packing in lamellar crystalline or gel phases. Finally, we show that many, but not all of these FTIR approaches can also yield valuable information about lipid-protein interactions in membrane protein- or peptide-containing lipid membrane bilayer model or even in biological membranes. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

  14. Distortion induced band gap and phase transformation in Tix Ag(1 - x) O2 system

    NASA Astrophysics Data System (ADS)

    Mathpal, Mohan Chandra; Tripathi, Anand Kumar; Kumar, Promod; Agrahari, Vivek; Singh, Manish Kumar; Agarwal, Arvind

    2014-10-01

    The polymorphic crystallization and optical properties of Ag-doped TiO2 nanoparticles have been investigated. Sol-gel method has been used to prepare anatase, rutile, and the anatase-rutile mixed phase of TiO2 nanoparticles by Ag doping on Ti(4+) sites. The doped TiO2 nanoparticles exhibit different phase of TiO2 for the different concentration of dopants. The Ag-doping affects the luminescence and morphological properties of TiO2 nanoparticles. The anatase to rutile phase transformation temperature (ART) has reduced significantly. The Ag doping in TiO2 lattice gives a large red shift for a particular concentration of Ag dopants.

  15. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    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.

  17. Temperature-dependent formation and transformation of mesostructures in water-ionic liquid mixtures.

    PubMed

    Kattnig, Daniel R; Hinderberger, Dariush

    2012-05-01

    The temperature-dependent formation and transformation of mesostructures in binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim(+)][BF(4)(-)]) and water are characterized. Through addition of nitroxide radicals as paramagnetic spin probes, the temperature dependence of the solution structure can be assessed by using electron paramagnetic resonance (EPR) spectroscopy from typical solutes' points of view. Additionally, the phase behavior on cooling and reheating is probed by differential scanning calorimetry (DSC). Thermal hysteresis and memory effects are observed, and DSC is used to identify the crystallization and thawing of ice as the pertinent phase transition. The EPR data of the nitroxide radicals before and after freezing and thawing reveal a transformation of the mesostructures, probably triggered by the crystallization of water pools to ice. A more polar state results after thawing, thereby suggesting a rupture and dissolution of the ordered IL-rich mesostructures. If the thawed solutions are not agitated, the system relaxes very slowly, that is, at room temperature with a time constant of approximately 90 h, to its equilibrium state of mesophase-separated IL-rich and bulk-like water regions. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  18. Grain boundary enrichment in the FePt polymorphic A1 to L1(0) phase transformation.

    PubMed

    Torres, K L; Thompson, G B

    2009-04-01

    A series of Fe(54+/-1)Pt(46+/-1) thin films have been sputter-deposited and annealed at various times and temperatures to facilitate the A1 to L1(0) polymorphic phase transformation. The annealing times span one minute to tens of minutes over temperatures of 300-800 degrees C. The films were characterized by X-ray and electron diffraction and atom probe tomography. This time-temperature regime provides 'snap-shots' into the compositional segregation evolution at the grain boundaries during the polymorphic phase transformation. The as-deposited A1 phase showed a preferential segregation of Pt to the grain boundaries. The reduction of Pt enrichment at the boundaries was observed for all L1(0) ordered films.

  19. Evaluation of the high temperature texture of the [Beta] phase of a TA6V sample from the individual orientations of grains of the low temperature [alpha] phase

    SciTech Connect

    Humbert, M.; Moustahfid, H.; Wagner, F.; Philippe, M.J. )

    1994-02-01

    The texture of the low temperature state is an important state is an important parameter, which is greatly involved in the plastic deformation of polycrystals. In this schema, the first step is to determine the texture of the high temperature state, using various high temperature processes. Unfortunately, the direct determination of the texture of the high temperature phase is not easy and sometimes impossible. This contribution describes an indirect method of determination of this [beta] texture, based on the fact that a grain of the high temperature [beta] phase transforms into different [alpha] plates during the phase transformation, where the [alpha] plate orientation is correlated with the orientation of the former [beta] grain. The determination of the orientation of the parent [beta] grain is possible, provided that the boundaries of the parent [beta] grain are clear, the number of different plate orientations sufficient, and that a strict orientation relation between the [alpha] and [beta] lattices exists. Using this method, the authors have obtained the texture of the [beta] phase of an alloy of TA6V from the determination of the orientations of a population of parent [beta] grains, the orientation of a given [beta] grain, deduced by correlating the orientations of the corresponding [alpha] plates measured by E.B.S.P.

  20. TRANSFORMATION

    SciTech Connect

    LACKS,S.A.

    2003-10-09

    Transformation, which alters the genetic makeup of an individual, is a concept that intrigues the human imagination. In Streptococcus pneumoniae such transformation was first demonstrated. Perhaps our fascination with genetics derived from our ancestors observing their own progeny, with its retention and assortment of parental traits, but such interest must have been accelerated after the dawn of agriculture. It was in pea plants that Gregor Mendel in the late 1800s examined inherited traits and found them to be determined by physical elements, or genes, passed from parents to progeny. In our day, the material basis of these genetic determinants was revealed to be DNA by the lowly bacteria, in particular, the pneumococcus. For this species, transformation by free DNA is a sexual process that enables cells to sport new combinations of genes and traits. Genetic transformation of the type found in S. pneumoniae occurs naturally in many species of bacteria (70), but, initially only a few other transformable species were found, namely, Haemophilus influenzae, Neisseria meningitides, Neisseria gonorrheae, and Bacillus subtilis (96). Natural transformation, which requires a set of genes evolved for the purpose, contrasts with artificial transformation, which is accomplished by shocking cells either electrically, as in electroporation, or by ionic and temperature shifts. Although such artificial treatments can introduce very small amounts of DNA into virtually any type of cell, the amounts introduced by natural transformation are a million-fold greater, and S. pneumoniae can take up as much as 10% of its cellular DNA content (40).

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

    SciTech Connect

    Frenzel, J.; George, Easo P; Dlouhy, A.; Somsen, Ch.; Wagner, M. F.-X; Eggeler, G.

    2010-01-01

    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 rationalized 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.

  2. A comparative first-principles study of martensitic phase transformations in TiPd2 and TiPd

    SciTech Connect

    Krcmar, Maja; Morris, James R

    2014-01-01

    Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2 B19) and subsequent monoclinic transformations (B19 B19 ) as the system is cooled. To evaluate the transition temperature for TiPd2 we employ a theoretical approach based on a phenomenological Landau theory of the structural phase transition and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b oI6 transition temperature is in good agreement with reported experimental results. To investigate the TiPd B2 B19 transformation, we employ both the Cauchy-Born rule and a soft-mode- based approach, and elucidate on the importance of coupling of lattice distortion and atomic displacements (i.e., shuffling) in the formation of the final structure. The estimated B2 B19 transition temperature for TiPd system agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19 B19 structural phase transformation.

  3. Electronic correlations determine the phase stability of iron up to the melting temperature

    PubMed Central

    Leonov, I.; Poteryaev, A. I.; Gornostyrev, Yu. N.; Lichtenstein, A. I.; Katsnelson, M. I.; Anisimov, V. I.; Vollhardt, D.

    2014-01-01

    We present theoretical results on the high-temperature phase stability and phonon spectra of paramagnetic bcc iron which explicitly take into account many-body effects. Several peculiarities, including a pronounced softening of the [110] transverse (T1) mode and a dynamical instability of the bcc lattice in harmonic approximation are identified. We relate these features to the α-to-γ and γ-to-δ phase transformations in iron. The high-temperature bcc phase is found to be highly anharmonic and appears to be stabilized by the lattice entropy. PMID:24998330

  4. On the Concordance of Static High Pressure Phase Transformation Data on Minerals With Shock Wave Data

    NASA Astrophysics Data System (ADS)

    de Carli, P. S.; El Goresy, A.; Xie, Z.; Sharp, T. G.

    2006-12-01

    About 50 years ago, shock wave researchers were confounded by the apparent rapidity of shock-induced phase transformations and invoked special mechanisms as an explanation. Some workers speculatively interpret shock wave data as indicative of very rapid reconstructive phase transformations of minerals at modest temperatures. The belief that kinetic effects are negligible has justified the use of microsecond duration shock recovery experiments to interpret much longer duration shock effects in terrestrial impact craters and in meteorites. Here we summarize the extensive evidence that phase transformations under shock compression are governed by the same kinetics as phase transitions under static compression. Hugoniot measurements on quartz and feldspars have been interpreted as indicating phase transitions to dense phases at pressures near 35 GPa. Release adiabat measurements imply that these phases persist on release of pressure down to about 7 GPa, below which there is expansion to a final density appropriate to a glass. Microsecond-duration shock recovery experiments support this interpretation; a low density amorphous phase (diaplectic glass) is recovered. A similar effect is observed in static compression experiments conducted at room temperature at a lower pressure, 12-15 GPa. The pressure difference is a kinetic effect; the transition requires higher shock pressures (implying higher shock temperatures) to run to completion on a microsecond time scale. Direct evidence for kinetic effects on the tectosilicate transformation is found in studies of naturally shocked meteorites that contain melt veins. When the melt veins solidify under pressure, the resultant mineralogy together with vein cooling calculations constrains the shock pressure-time history. In some samples, in which the shock pressure is constrained by vein mineralogy to a range of 17-25 Gpa, one may observe the presence of diaplectic glass in only near-vein regions heated by conduction from the vein

  5. Nutrient transformation during aerobic composting of pig manure with biochar prepared at different temperatures.

    PubMed

    Li, Ronghua; Wang, Quan; Zhang, Zengqiang; Zhang, Guangjie; Li, Zhonghong; Wang, Li; Zheng, Jianzhong

    2015-01-01

    The effects of the corn stalk charred biomass (CB) prepared at different pyrolysis temperatures as additives on nutrient transformation during aerobic composting of pig manure were investigated. The results showed that the addition of CB carbonized at different temperatures to pig manure compost significantly influenced the compost temperature, moisture, pH, electrical conductivity, organic matter degradation, total nitrogen, [Formula: see text] and NH3 variations during composting. Compared with control and adding CB charred at lower temperature treatments, the addition of CB prepared over 700°C resulted in higher pH (over 9.2) and NH3 emission and lower potherb mustard seed germination index value during the thermophilic phase. Peak temperatures of composts appeared at 7 days for control and 11 days for CB added treatments. During 90 days composting, the organic matter degradation could be increased over 14.8-29.6% after adding of CB in the compost mixture. The introduction of CB in pig manure could prolong the thermophilic phase, inhibit moisture reduce, facilitate the organic matter decomposition, reduce diethylene triamine pentaacetic acid (DTPA) extractable Zn and Cu contents in pig manure composts and increase ryegrass growth. The study indicated that the corn stalk CB prepared around 500°C was a suitable additive in pig manure composting.

  6. Phase Transformation of U3O8 and Enhanced Structural Stability at Extreme Conditions

    NASA Astrophysics Data System (ADS)

    Zhang, Fuxiang; Lang, Maik; Ewing, Rodney

    2013-06-01

    A powder sample of β-U3O8 was pressurized at room temperature up to 37.5 GPa with a symmetric diamond anvil cell. XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The fluorite-like structure is stable up to 37.5 GPa. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U3O8 is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 Å at ambient conditions, which is smaller than that of the stoichiometric UO2. Previous experiments have shown that the stoichiometric uranium dioxide (UO2) is not stable at high pressure conditions and starts to transform to a cotunnite structure at ~30 GPa. When heating the sample at high pressure, the critical transtion pressure is greatly reduced. However, the fluorite-like high-pressure phase of U3O8 is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure. This work was supported by Materials Science of Actinides, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001089.

  7. Nanoalloying and phase transformations during thermal treatment of physical mixtures of Pd and Cu nanoparticles

    PubMed Central

    Mukundan, Vineetha; Yin, Jun; Joseph, Pharrah; Luo, Jin; Shan, Shiyao; Zakharov, Dmitri N; Zhong, Chuan-Jian; Malis, Oana

    2014-01-01

    Nanoscale alloying and phase transformations in physical mixtures of Pd and Cu ultrafine nanoparticles are investigated in real time with in situ synchrotron-based x-ray diffraction complemented by ex situ high-resolution transmission electron microscopy. The combination of metal–support interaction and reactive/non-reactive environment was found to determine the thermal evolution and ultimate structure of this binary system. At 300 °C, the nanoparticles supported on silica and carbon black intermix to form a chemically ordered CsCl-type (B2) alloy phase. The B2 phase transforms into a disordered fcc alloy at higher temperature (> 450 °C). The alloy nanoparticles supported on silica and carbon black are homogeneous in volume, but evidence was found of Pd surface enrichment. In sharp contrast, when supported on alumina, the two metals segregated at 300 °C to produce almost pure fcc Cu and Pd phases. Upon further annealing of the mixture on alumina above 600 °C, the two metals interdiffused, forming two distinct disordered alloys of compositions 30% and 90% Pd. The annealing atmosphere also plays a major role in the structural evolution of these bimetallic nanoparticles. The nanoparticles annealed in forming gas are larger than the nanoparticles annealing in helium due to reduction of the surface oxides that promotes coalescence and sintering. PMID:27877663

  8. Phase transformations and the spectral reflectance of solid sulfur - Can metastable sulfur allotropes exist on Io?

    NASA Technical Reports Server (NTRS)

    Moses, Julianne I.; Nash, Douglas B.

    1991-01-01

    Laboratory investigations have been conducted on the effects of variations in sulfur sample histories on their solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. The temporal variations in question may be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification, as well as to the physics of the phase-transformation process itself. The results obtained are pertinent to the physical behavior and spectral variation of such freshly solidified sulfur as may exist on the Jupiter moon Io; this would initially solidify into a glassy solid or monoclinic crystalline lattice, then approach ambient dayside temperatures. Laboratory results imply that the monoclinic or polymeric allotropes can in these circumstances be maintained, and will take years to convert to the stable orthorhombic crystalline form.

  9. Phase transformations and the spectral reflectance of solid sulfur - Can metastable sulfur allotropes exist on Io

    SciTech Connect

    Moses, J.I.; Nash, D.B. San Juan Capistrano Research Institute, CA )

    1991-02-01

    Laboratory investigations have been conducted on the effects of variations in sulfur sample histories on their solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. The temporal variations in question may be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification, as well as to the physics of the phase-transformation process itself. The results obtained are pertinent to the physical behavior and spectral variation of such freshly solidified sulfur as may exist on the Jupiter moon Io; this would initially solidify into a glassy solid or monoclinic crystalline lattice, then approach ambient dayside temperatures. Laboratory results imply that the monoclinic or polymeric allotropes can in these circumstances be maintained, and will take years to convert to the stable orthorhombic crystalline form. 114 refs.

  10. Phase transformations and the spectral reflectance of solid sulfur - Can metastable sulfur allotropes exist on Io?

    NASA Astrophysics Data System (ADS)

    Moses, J. I.; Nash, D. B.

    1991-02-01

    Laboratory investigations have been conducted on the effects of variations in sulfur sample histories on their solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. The temporal variations in question may be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification, as well as to the physics of the phase-transformation process itself. The results obtained are pertinent to the physical behavior and spectral variation of such freshly solidified sulfur as may exist on the Jupiter moon Io; this would initially solidify into a glassy solid or monoclinic crystalline lattice, then approach ambient dayside temperatures. Laboratory results imply that the monoclinic or polymeric allotropes can in these circumstances be maintained, and will take years to convert to the stable orthorhombic crystalline form.

  11. Phase transformations and the spectral reflectance of solid sulfur - Can metastable sulfur allotropes exist on Io?

    NASA Technical Reports Server (NTRS)

    Moses, Julianne I.; Nash, Douglas B.

    1991-01-01

    Laboratory investigations have been conducted on the effects of variations in sulfur sample histories on their solid-state transformation rate and the corresponding spectral variation of freshly frozen sulfur. The temporal variations in question may be due to differences in the amount and type of metastable allotropes present in the sulfur after solidification, as well as to the physics of the phase-transformation process itself. The results obtained are pertinent to the physical behavior and spectral variation of such freshly solidified sulfur as may exist on the Jupiter moon Io; this would initially solidify into a glassy solid or monoclinic crystalline lattice, then approach ambient dayside temperatures. Laboratory results imply that the monoclinic or polymeric allotropes can in these circumstances be maintained, and will take years to convert to the stable orthorhombic crystalline form.

  12. A flow-through hydrothermal cell for in situ neutron diffraction studies of phase transformations

    NASA Astrophysics Data System (ADS)

    O'Neill, Brian; Tenailleau, Christophe; Nogthai, Yung; Studer, Andrew; Brugger, Joël; Pring, Allan

    2006-11-01

    A flow-through hydrothermal cell for the in situ neutron diffraction study of crystallisation and phase transitions has been developed. It can be used for kinetic studies on materials that exhibit structural transformations under hydrothermal conditions. It is specifically designed for use on the medium-resolution powder diffractometer (MRPD) at ANSTO, Lucas Heights, Sydney. But it is planned to adapt the design for the Polaris beamline at ISIS and the new high-intensity powder diffractometer (Wombat) at the new Australian reactor Opal. The cell will operate in a flow-through mode over the temperature range from 25-300 °C and up to pressures of 100 bar. The first results of a successful transformation of pentlandite (Fe,Ni) 9S 8 to violarite (Fe,Ni) 3S 4 under mild conditions (pH∼4) at 120 °C and 3 bar using in situ neutron diffraction measurements are presented.

  13. In Situ Study of the Influence of Nickel on the Phase Transformation Kinetics in Austempered Ductile Iron

    NASA Astrophysics Data System (ADS)

    Saal, Patrick; Meier, Leopold; Li, Xiaohu; Hofmann, Michael; Hoelzel, Markus; Wagner, Julia N.; Volk, Wolfram

    2016-02-01

    Phase fractions and austenite carbon contents in austempered ductile iron samples with three different nickel contents were determined by in situ neutron diffraction. The samples were austenitized at 1178 K (905 °C) for 30 minutes and austempered for 3.5 hours at temperatures between 523 K and 723 K (250 °C and 450 °C) using a mirror furnace. Based on the in situ neutron diffraction studies, plateau times were derived, which determine the end of stage I reaction. The austenite contents increase for higher austempering temperatures when the austempering times are selected properly, considering the accelerated phase transformation at higher temperature. Appropriate austempering times were derived for austempering temperatures between 523 K and 723 K (250 °C and 450 °C). Increased nickel contents lead to higher austenite phase fractions. Moreover the retarding effect of nickel on the phase transformation was quantified. The plateau values of phase fraction and the according austempering times were converted to TTT diagrams. The evolution of the austenite carbon content shows a maximum at 623 K (350 °C) austempering temperature. This can be explained by temperature-dependent carbide precipitation and carbon diffusion into lattice defects. Fine carbides within the ferrite could be found by preliminary APT analysis.

  14. 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.

  15. 630 kVA high temperature superconducting transformer

    NASA Astrophysics Data System (ADS)

    Zueger, H.

    This document describes the 630 KVA HTS transformer project made by ABB jointly with EDF and ASC. The project started April 1994 and its goal was to manufacture a real scale superconducting distribution transformer and to operate it during one year in the grid of Geneva's utility (SIG). The conclusion highlights the future perspective of HTS transformers.

  16. Dynamic observation of phase transformation behaviors in indium(III) selenide nanowire based phase change memory.

    PubMed

    Huang, Yu-Ting; Huang, Chun-Wei; Chen, Jui-Yuan; Ting, Yi-Hsin; Lu, Kuo-Chang; Chueh, Yu-Lun; Wu, Wen-Wei

    2014-09-23

    Phase change random access memory (PCRAM) has been extensively investigated for its potential applications in next-generation nonvolatile memory. In this study, indium(III) selenide (In2Se3) was selected due to its high resistivity ratio and lower programming current. Au/In2Se3-nanowire/Au phase change memory devices were fabricated and measured systematically in an in situ transmission electron microscope to perform a RESET/SET process under pulsed and dc voltage swept mode, respectively. During the switching, we observed the dynamic evolution of the phase transformation process. The switching behavior resulted from crystalline/amorphous change and revealed that a long pulse width would induce the amorphous or polycrystalline state by different pulse amplitudes, supporting the improvement of the writing speed, retention, and endurance of PCRAM.

  17. Collective Atomic Displacements during Complex Phase Boundary Migration in Solid-Solid Phase Transformations.

    PubMed

    Duncan, Juliana; Harjunmaa, Ari; Terrell, Rye; Drautz, Ralf; Henkelman, Graeme; Rogal, Jutta

    2016-01-22

    The A15 to bcc phase transition is simulated at the atomic scale based on an interatomic potential for molybdenum. The migration of the phase boundary proceeds via long-range collective displacements of entire groups of atoms across the interface. To capture the kinetics of these complex atomic rearrangements over extended time scales we use the adaptive kinetic Monte Carlo approach. An effective barrier of 0.5 eV is determined for the formation of each new bcc layer. This barrier is not associated with any particular atomistic process that governs the dynamics of the phase boundary migration. Instead, the effective layer transformation barrier represents a collective property of the complex potential energy surface.

  18. Phase transformation of "chem-prep" PZT 95/5-2Nb HF1035 ceramic under quasi-static loading conditions.

    SciTech Connect

    Montgomery, Stephen Tedford; Lee, Moo Yul; Meier, Diane A.; Hofer, John H.

    2006-07-01

    Specimens of poled and unpoled ''chem-prep'' PNZT ceramic from batch HF1035 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions at -55, 25, and 75 C. The objective of this experimental study was to characterize the mechanical properties and conditions for the ferroelectric (FE) to antiferroelectric (AFE) phase transformations of this ''chem-prep'' PNZT ceramic to aid grain-scale modeling efforts in developing and testing realistic response models for use in simulation codes. As seen from a previously characterized material (batch HF803), poled ceramic from HF1035 was seen to undergo anisotropic deformation during the transition from a FE to an AFE phase. Also, the phase transformation was found to be permanent for the two low temperature conditions, whereas the transformation can be completely reversed at the highest temperature. The rates of increase in the phase transformation pressures with temperature were practically identical for both unpoled and poled PNZT HF1035 specimens. We observed that temperature spread the phase transformation over mean stress analogous to the observed spread over mean stress due to shear stress. Additionally, for poled ceramic samples, the FE to AFE phase transformation was seen to occur when the normal compressive stress, acting perpendicular to a crystallographic plane about the polar axis, equals the hydrostatic pressure at which the transformation otherwise takes place.

  19. Three-dimensional phase transformation by impedance-matched dielectric slabs and generation of hollow beams based on transformation optics

    NASA Astrophysics Data System (ADS)

    Wang, Lei; Yang, Shuaisai; Tang, Zhixiang; Shu, Weixing

    2016-10-01

    We propose a three-dimensional (3D) phase transformation method by an impedance-matched dielectric slab and apply it to generating hollow beams. We first employ transformation optics to establish a method for the transformation between two arbitrary 3D wavefronts through a flat dielectric and impedance-matched material. Then the method is used to convert a solid beam into a hollow beam with desired wavefront. By tuning the transformation surface, different hollow beams can be produced. The results are further validated by 3D finite-difference time-domain simulations.

  20. Magnetic and structural phase transitions in erbium at low temperatures and high pressures

    SciTech Connect

    Thomas, Sarah A.; Tsoi, Georgiy M.; Wenger, Lowell E.; Vohra, Yogesh K.

    2012-02-07

    Electrical resistance and crystal structure measurements have been carried out on polycrystalline erbium (Er) at temperatures down to 10 K and pressures up to 20 GPa. An abrupt change in the slope of the resistance is observed with decreasing temperature below 84 K, which is associated with the c-axis modulated (CAM) antiferromagnetic (AFM) ordering of the Er moments. With increasing pressure the temperature of the resistance slope change and the corresponding AFM ordering temperature decrease until vanishing above 10.6 GPa. The disappearance of the slope change in the resistance occurs at similar pressures where the hcp structural phase of Er is transformed to a nine-layer {alpha}-Sm structural phase, as confirmed by our high-pressure synchrotron x-ray diffraction studies. These results suggest that the disappearance in the AFM ordering of Er moments is strongly correlated to the structural phase transition at high pressures and low temperatures.

  1. Anomalous Change in Temperature during the Pressure-Induced Phase Transition of KI

    NASA Astrophysics Data System (ADS)

    Nomura, Motoyuki; Harino, Hideo; Itoh, Tsukasa

    1990-11-01

    Temperature changes associated with the pressure-induced phase transition of KI were observed. Two kinds of temperature signals were detected, one of which is endothermic due to the adiabatic expansion of the pressure transmitting liquid brought about by the discontinuous decrease in the sample volume. The other is an exothermic one transmitted from the sample by radiation, which means heating of a sample at the forward transition. The latter signal consists of a spike accompanied by a small but long-tailed signal. The heating at the forward transition is inconsistent with the negative slope of the phase boundary in the accepted pressure-temperature phase diagram. Although the pattern of temperature signals observed seems to support the transition mechanism of the nucleation of B2 and its growth, this mechanism contradicts the behavior of the fracture plane of KI which suggests some kind of Martesite transformation. In a reverse transition, complicated temperature signals were detected.

  2. Variation of thermal conductivity of DPPC lipid bilayer membranes around the phase transition temperature

    PubMed Central

    Youssefian, Sina; Lambert, Christopher R.; Van Dessel, Steven

    2017-01-01

    Given their amphiphilic nature and chemical structure, phospholipids exhibit a strong thermotropic and lyotropic phase behaviour in an aqueous environment. Around the phase transition temperature, phospholipids transform from a gel-like state to a fluid crystalline structure. In this transition, many key characteristics of the lipid bilayers such as structure and thermal properties alter. In this study, we employed atomistic simulation techniques to study the structure and underlying mechanisms of heat transfer in dipalmitoylphosphatidylcholine (DPPC) lipid bilayers around the fluid–gel phase transformation. To investigate this phenomenon, we performed non-equilibrium molecular dynamics simulations for a range of different temperature gradients. The results show that the thermal properties of the DPPC bilayer are highly dependent on the temperature gradient. Higher temperature gradients cause an increase in the thermal conductivity of the DPPC lipid bilayer. We also found that the thermal conductivity of DPPC is lowest at the transition temperature whereby one lipid leaflet is in the gel phase and the other is in the liquid crystalline phase. This is essentially related to a growth in thermal resistance between the two leaflets of lipid at the transition temperature. These results provide significant new insights into developing new thermal insulation for engineering applications. PMID:28539484

  3. Dependency of working temperature and equivalent constant of concentric disk-type piezoelectric transformer

    NASA Astrophysics Data System (ADS)

    Chou, I.-Mu; Lai, Yi-Ying; Wu, Wen-Jong; Lee, Chih-Kung

    2011-03-01

    This paper presents the effect of equivalent constant and output power on working temperature of concentric disk-type piezoelectric transformer. To analyze the energy loss in the piezoelectric transformer, the equivalent circuit model was built. Losses in the piezoelectric transformer are considered generally having two different parts: dielectric loss and mechanical loss. First of all, a measurement circuit based on an impedance analyzer was built. Then, the circuit simulation software PSIM was employed to verify the experimental results obtained. Secondly, according to the experimental results, temperature and input voltage are the two factors which influenced the energy loss in a piezoelectric transformer. As the input voltage and temperature increased, the energy loss rises, as well. In addition, when the input voltage is low, the temperature becomes the main influencing factor for energy loss of the piezoelectric transformer. On the other hand, when the input voltage is high, the main factor for energy loss of the piezoelectric transformer is the input voltage other than the temperature. Furthermore, the control loop that dealt with the energy loss of the piezoelectric transformer was proposed. At different temperatures, the variations of losses of the piezoelectric transformer are presented in this paper. Finally, the dielectric loss and mechanical loss are combined to analyze the losses within piezoelectric transformers. Then, the relationship between the output power of the piezoelectric transformer and the temperature was revealed. The result showed that as the temperature increased, the output power decreased.

  4. Texture of primary recrystallization on nonoriented electrical steel sheet with phase transformation

    NASA Astrophysics Data System (ADS)

    Kumano, T.; Kubota, T.; Takahashi, N.

    1995-08-01

    The magnetic properties of nonoriented (NO) electrical steel sheet are commonly improved if the texture of their products possesses “cube texture” (e.g., {100 <0vw>, “goss texture”) (i.e., ¹10<011>, and less ²22 texture). Industrially “cube type” has not been obtained, but “goss texture” has been. In a greater or lesser degree, {222} texture exists. To improve “goss texture” and reduce ²22 texture, the grain size of the material prior to cold rolling should be larger. When the grain size before cold rolling is larger, during primary recrystallization, “goss texture” is enriched, ²22 texture is decreased, and the grain grows so easily that higher induction and lower core loss can be obtained. This does not depend on the presence of phase transformation. In case of NO steel with phase transformation, heat treatment before cold rolling has been done below the austenite transition temperature (Acin1) in order to prevent the fine grain size caused by α → γ(+α) → α transformation. By using material that was heated over Acin1 and cooled with changing cooling rates, this study describes (a) the relationship between textures before cold rolling and the texture of the final product, and (b) the development of the magnetic properties.

  5. Texture of primary recrystallization on nonoriented electrical steel sheet with phase transformation

    SciTech Connect

    Kumano, T.; Kubota, T.; Takahashi, N.

    1995-08-01

    The magnetic properties of nonoriented (NO) electrical steel sheet are commonly improved if the texture of their products possesses cube texture e.g., {l_brace}100{r_brace} <0vw>, ``goss texture`` i.e., {l_brace}110{r_brace}<011>, and less {l_brace}222{r_brace} texture. Industrially ``cube type`` has not been obtained, but ``goss texture`` has been. In a greater or lesser degree, {l_brace}222{r_brace} texture exists. To improve ``goss texture`` and reduce {l_brace}222{r_brace} texture, the grain size of the material prior to cold rolling should be larger. When the grain size before cold rolling is larger, during primary recrystallization, ``goss texture`` is enriched, {l_brace}222{r_brace} texture is decreased, and the grain grows so easily that higher induction and lower core loss can be obtained. This does not depend on the presence of phase transformation. In case of NO steel with phase transformation, heat treatment before cold rolling has been done below the austenite transition temperature (Ac{sub 1}) in order to prevent the fine grain size caused by {alpha} {yields} {gamma}(+{alpha}) {yields} {alpha} transformation. By using material that was heated over Ac{sub 1} and cooled with changing cool rates, this study describes (a) the relationship between textures before cold rolling and the texture of the final product, and (b) the development of the magnetic properties.

  6. Synthesis of iron oxide nanorods via chemical scavenging and phase transformations of intermediates at ambient conditions

    NASA Astrophysics Data System (ADS)

    Deshmukh, Ruchi; Mehra, Anurag; Thaokar, Rochish

    2017-01-01

    Chemically induced shape transformations of isotropic seeds, comprised of iron oxyhydroxides and iron oxide borate into nanorods, is reported. Transient growth studies show that the nanorods are formed via phase transformation and aggregation of various metastable species. Addition of tetra- methyl-ammonium hydroxide (TMAH) to the in situ synthesized seeds ensures a typical reaction pathway that favors formation of magnetite (Fe 3 O 4) via the steps of chemical etching, phase transformation of intermediates, and crystal consolidation. Whereas, with addition of sodium hydroxide (NaOH), either magnetite (Fe 3 O 4) or a mixture of ( γ-Fe 2 O 3 + α-FeOOH) is obtained. The shape with both the additives is always that of nanorods. When the seeds treated with TMAH were aged in an ultrasonication bath, rods with almost twice the length and diameter (length = 2800 nm, diameter = 345 nm) are obtained as compared to the sample aged without ultrasonication (length = 1535 nm, diameter = 172 nm). The morphology of nanostructures depending upon other experimental conditions such as, aging the sample at 60 ∘C, seeds synthesized under ultrasonication/ stirring or externally added are also examined and discussed in detail. All the samples show high coercivity and strong ferromagnetic behavior at room temperature and should be promising candidates as ferro-fluids for various applications.

  7. An investigation of phase transformation of titania slag using microwave heating

    NASA Astrophysics Data System (ADS)

    Chen, Guo; Chen, Jin; Peng, Jinhui

    2016-09-01

    The influences of microwave heating on the phase transformation of titania slag were systematically investigated. The thermal stability, surface chemical functional groups and microstructure of the titania slag before and after microwave heating, at a temperature of 950 °C for 60 min, were also analyzed using thermogravimetry and differential thermal analysis (TG-DSC), Fourier transform infrared spectroscopy (FT-IR) spectrum and scanning electron microscope (SEM), respectively. The TG-DSC analysis revealed that the phase transformation of the titania slag from anatase TiO2 to rutile TiO2 occurred between 750 and 1000 °C. The FT-IR rustles demonstrate that the banding form of Ti4+, Ti3+ and Ti2+ ions and the methyl groups on the surface of the titania slag has changed and a new chemical bond Ti-OH was formed. The results of SEM showed that a large number of regulation rutile TiO2 crystals were found on the surface of the microwave-treated samples and the synthetic rutile has been synthesized successfully using microwave heating.

  8. 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.

  9. Phase change based cooling for high burst mode heat loads with temperature regulation above the phase change temperature

    SciTech Connect

    The United States of America as represented by the United States Department of Energy

    2009-12-15

    An apparatus and method for transferring thermal energy from a heat load is disclosed. In particular, use of a phase change material and specific flow designs enables cooling with temperature regulation well above the fusion temperature of the phase change material for medium and high heat loads from devices operated intermittently (in burst mode). Exemplary heat loads include burst mode lasers and laser diodes, flight avionics, and high power space instruments. Thermal energy is transferred from the heat load to liquid phase change material from a phase change material reservoir. The liquid phase change material is split into two flows. Thermal energy is transferred from the first flow via a phase change material heat sink. The second flow bypasses the phase change material heat sink and joins with liquid phase change material exiting from the phase change material heat sink. The combined liquid phase change material is returned to the liquid phase change material reservoir. The ratio of bypass flow to flow into the phase change material heat sink can be varied to adjust the temperature of the liquid phase change material returned to the liquid phase change material reservoir. Varying the flowrate and temperature of the liquid phase change material presented to the heat load determines the magnitude of thermal energy transferred from the heat load.

  10. 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.

  11. Structures, Phase Transformations, and Dielectric Properties of BiTaO4 Ceramics.

    PubMed

    Zhou, Di; Fan, Xiao-Qin; Jin, Xiao-Wei; He, Duan-Wei; Chen, Guo-Hua

    2016-11-21

    Low (α)- and high-temperature (β) forms of BiTaO4 have attracted much attention due to their dielectric and photocatalytic properties. In the present work, a third form, the so-called HP-BiTaO4, was synthesized at high temperature and pressure. The phase evolution, phase transformations, and dielectric properties of α- and β-BiTaO4 and HP-BiTaO4 ceramics are studied in detail. β-BiTaO4 ceramics densified at 1300 °C with the microwave permittivity εr ≈ 53, the microwave quality factor Qf ≈ 12070 GHz, and the temperature coefficient of resonant frequency τf ≈ -200 ppm/°C. HP-BiTaO4 ceramics were synthesized at 5 GPa and 1300 °C followed by annealing at 600 °C. In contrast with the α phase, HP-BiTaO4 exhibited εr ≈ 195 at 1 kHz to 10 MHz, accompanied by a low dielectric loss of ∼0.004. The relation between structure and dielectric properties is discussed in the context of Shannon's additive rule and bond theory.

  12. Low-temperature transformation and electronic structure of (Ni 1-xCo x) 2MnGa

    NASA Astrophysics Data System (ADS)

    Takeda, Yoichi; Kodera, Kazushi; Yoshimura, Satoru; Asano, Hidefumi; Matsui, Masaaki

    2007-03-01

    Low-temperature structural transformation of (Ni 1-xCo x) 2MnGa was investigated experimentally and theoretically. Magnetization measurement as a function of temperature in the low magnetic field revealed that the transformation temperature T1 disappeared for x⩾0.2. The band calculation predicted that the stabilization of austenite L2 1 phase occurred for x>0.3, which was in rough agreement with the experimental result. Furthermore, the instability of austenite phase was discussed from the viewpoint of Hume-Rothery regime and the prediction was in good agreement with the results. It was concluded that the Galanakis curve was a good prediction for half-metal Heusler alloy and a new Heusler alloy was proposed for 1.0>x⩾0.7.

  13. 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.

  14. In situ HVEM studies of phase transformation in Zr alloys and compounds under irradiation

    SciTech Connect

    Motta, A.T.; Faldowski, J.A.; Howe, L.M.; Okamoto, P.R.

    1996-01-01

    The High Voltage Electron Microscope (HVEM)/Tandem facility at Argonne National Laboratory has been used to conduct detailed studies of the phase stability and microstructural evolution in zirconium alloys and compounds under ion and electron irradiation. Detailed kinetic studies of the crystalline-to-amorphous transformation of the intermetallic compounds Zr{sub 3}(Fe{sub 1-x}Ni{sub x}), Zr(Fe{sub 1-x},Cr{sub x}){sub 2}, Zr{sub 3}Fe, and Zr{sub 1.5} Nb{sub 1.5} Fe, both as second phase precipitates and in bulk form, have been performed using the in-situ capabilities of the Argonne facility, under a variety of irradiation conditions (temperature, dose rate). Results include a verification of a dose rate effect on amorphization and the influence of material variables (stoichiometry x, presence of stacking faults, crystal structure) on the critical temperature and on the critical dose for amorphization. Studies were also conducted of the microstructural evolution under irradiation of specially tailored binary and ternary model alloys. The stability of the {omega}-phase in Zr-20%Nb under electron and Ar ion irradiation was investigated as well as the {beta}-phase precipitation in Zr-2.5%Nb under Ar ion irradiation. The ensemble of these results is discussed in terms of theoretical models of amorphization and of irradiation-altered solubility.

  15. Fe-Solid Phase Transformations Under Highly Basic Conditions

    SciTech Connect

    Qafoku, Nik; Qafoku, Odeta; Ainsworth, Calvin C.; Dohnalkova, Alice; McKinley, Susan G.

    2007-09-01

    Hyperalkaline and saline radioactive waste fluids with elevated temperatures from S-SX high-level waste tank farm at Hanford, WA accidentally leaked into sediments beneath the tanks, initiating a series of geochemical processes and reactions whose significance and extent was unknown. Among the most important processes was the dissolution of soil minerals and precipitation of stable secondary phases. The objective of this investigation was to study the release of Fe into the aqueous phase upon dissolution of Fe-bearing soil minerals, and the subsequent formation of Fe rich precipitates. Batch reactors were used to conduct experiments at 50 0C using solutions similar in composition to the waste fluids. Results clearly showed that, similarly to Si and Al, Fe was released from the dissolution of soil minerals (most likely phyllosilicates such as biotite, smectite, and chlorite). The extent of Fe release increased with base concentration and decreased with Al concentration in the contacting solution. The maximum apparent rate of Fe release (0.566 × 10-13 mol m-2 s-1) was measured in the treatment with no Al and a concentration of 4.32 mol L-1 NaOH in the contacting solution. Results from electron microscopy indicated that while Si and Al precipitated together to form feldspathoids in the groups of cancrinite and/or sodalite, Fe precipitation followed a different pathway leading to the formation of hematite and goethite. The newly formed Fe oxy-hydroxides may increase the sorption capacity of the sediments, promote surface mediated reactions such as precipitation and heterogeneous redox reactions, and affect the phase distribution of contaminant and radionuclides.

  16. Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires.

    PubMed

    Sutrakar, Vijay Kumar; Mahapatra, D Roy

    2009-07-22

    Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced [Formula: see text]-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of approximately 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of approximately 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 A, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

  17. Fatigue properties of a metastable beta-type titanium alloy with reversible phase transformation.

    PubMed

    Li, S J; Cui, T C; Hao, Y L; Yang, R

    2008-03-01

    Due to recent concern about allergic and toxic effects of Ni ions released from TiNi alloy into human body, much attention has been focused on the development of new Ni-free, metastable beta-type biomedical titanium alloys with a reversible phase transformation between the beta phase and the alpha'' martensite. This study investigates the effect of the stress-induced alpha'' martensite on the mechanical and fatigue properties of Ti-24Nb-4Zr-7.6Sn (wt.%) alloy. The results show that the as-forged alloy has a low dynamic Young's modulus of 55GPa and a recoverable tensile strain of approximately 3%. Compared with Ti-6Al-4V ELI, the studied alloy has quite a high low-cycle fatigue strength because of the effective suppression of microplastic deformation by the reversible martensitic transformation. Due to the low critical stress required to induce the martensitic transformation, it has low fatigue endurance comparable to that of Ti-6Al-4V ELI. Cold rolling produces a beta+alpha'' two-phase microstructure that is characterized by regions of nano-size beta grains interspersed with coarse grains containing alpha'' martensite plates. Cold rolling increases fatigue endurance by approximately 50% while decreasing the Young's modulus to 49GPa along the rolling direction but increasing it to 68GPa along the transverse direction. Due to the effective suppression of the brittle isothermal omega phase, balanced properties of high strength, low Young's modulus and good ductility can be achieved through ageing treatment at intermediate temperature.

  18. High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures

    SciTech Connect

    Vohra, Yogesh K.; Sangala, Bagvanth Reddy; Stemshorn, Andrew K.; Hope, Kevin M.

    2008-07-01

    High-pressure studies have been performed on heavy rare earth metals Terbium (Tb) to 155 GPa and Holmium (Ho) to 134 GPa in a diamond anvil cell at room temperature. The following crystal structure sequence was observed in both metals hcp {yields} Sm-type {yields} dhcp {yields} distorted fcc (hR-24) {yields} monoclinic (C2/m) with increasing pressure. The last transformation to a low symmetry monoclinic phase is accompanied by a volume collapse of 5 % for Tb at 51 GPa and a volume collapse of 3 % for Ho at 103 GPa. This volume collapse under high pressure is reminiscent of f-shell delocalization in light rare earth metal Cerium (Ce), Praseodymium (Pr), and heavy actinide metals Americium (Am) and Curium (Cm). The orthorhombic Pnma phase that has been reported in Am and Cm after f-shell delocalization is not observed in heavy rare earth metals under high pressures. (authors)

  19. Domain wall modeling of bcc to hcp reconstructive phase transformation in early transition metals

    NASA Astrophysics Data System (ADS)

    Sanati, Mahdi; Saxena, A.; Lookman, T.

    2001-09-01

    The bcc (body-centered-cubic) phase to hcp (hexagonal-close-packed) phase transformation in certain elements and alloys is induced either by quenching or the application of pressure. Following the Burgers mechanism and through first-principles calculations we show that the bcc structure of Sc, Y, Ti, Zr, and Hf is unstable with respect to the shuffle of atoms rather than the shear. We therefore reduce the two-order-parameter (two-OP) Ginzburg-Landau (GL) free energy to an effective free energy in the shuffle OP. From the phonon dispersion experiments and the change of entropy at transition temperature we found the GL free energy coefficients for Ti and Zr. By using this information we obtain the domain wall width and energy for Ti and Zr.

  20. Multiscale continuum mechanics model for phase transformations in SMA single crystals

    NASA Astrophysics Data System (ADS)

    Stoilov, Vesselin M.; Bhattacharyya, Abhijit

    2004-07-01

    This work aims to connect atomistic model with continuum theory of phase transformations in Shape memory alloys(SMA). A formulation of the Helmholtz free energy potential based on the Lennard-Jones potential has been developed. Lennard-Jones potential was used to describe the inter-atomic interactions in bi-atomic crystal of NiTi. The microscopic expressions of the instantaneous mechanical (continuum) variables of mass, momentum, internal energy and temperature have been derived in terms of the atomic variables. The developed Helmholtz thermodynamic potential is used in the context of the sharp phase front-based continuum framework proposed by Stoilov et. el.(Acta Mat. 2002) to study the micro-macro transition during the thermomechanical response of NiTi crystals. The developed model has been successfully used to predict the response of 1D single crystal system.

  1. Phase transformation mechanisms in (AuCu)1-xPdx pseudobinary alloys by direct aging method.

    PubMed

    Hernandez, R I; Udoh, K; Tanaka, Y; Takuma, Y; Winn, H; Hisatsune, K

    1999-09-01

    Phase transformation mechanisms in the AuCu-Pd pseudobinary system were studied by means of electrical resistivity measurements, hardness tests, X-ray diffraction and transmission electron microscopy. A direct aging method was employed to eliminate the otherwise unavoidable ordering that takes place rapidly during quenching into ice brine, hence it is important to distinguish the ordering processes with and without an incubation period. Three phase transformation modes occurred, namely; ordering at grain boundaries and in the grain interior with nucleation and growth mechanism after incubation, and spinodal ordering without any incubation period. The age-hardening of the alloys examined was attributed to AuCu I ordering. Nucleation and growth mechanism followed by twinning occurred in the specimens aged at higher temperatures, while spinodal ordering was seen in specimens aged in lower temperature. The spinodal ordering temperature of AuCu-Pd alloys increased according to Pd content.

  2. Phase transformations in a Cu−Cr alloy induced by high pressure torsion

    SciTech Connect

    Korneva, Anna; Straumal, Boris; Kilmametov, Askar; Chulist, Robert; Straumal, Piotr; Zięba, Paweł

    2016-04-15

    Phase transformations induced by high pressure torsion (HPT) at room temperature in two samples of the Cu-0.86 at.% Cr alloy, pre-annealed at 550 °C and 1000 °C, were studied in order to obtain two different initial states for the HPT procedure. Observation of microstructure of the samples before HPT revealed that the sample annealed at 550 °C contained two types of Cr precipitates in the Cu matrix: large particles (size about 500 nm) and small ones (size about 70 nm). The sample annealed at 1000 °C showed only a little fraction of Cr precipitates (size about 2 μm). The subsequent HPT process resulted in the partial dissolution of Cr precipitates in the first sample and dissolution of Cr precipitates with simultaneous decomposition of the supersaturated solid solution in another. However, the resulting microstructure of the samples after HPT was very similar from the standpoint of grain size, phase composition, texture analysis and hardness measurements. - Highlights: • Cu−Cr alloy with two different initial states was deformed by HPT. • Phase transformations in the deformed materials were studied. • SEM, TEM and X-ray diffraction techniques were used for microstructure analysis. • HPT leads to formation the same microstructure independent of the initial state.

  3. Cluster Variation Method as a Theoretical Tool for the Study of Phase Transformation

    NASA Astrophysics Data System (ADS)

    Mohri, Tetsuo

    2017-02-01

    Cluster variation method (CVM) has been widely employed to calculate alloy phase diagrams. The atomistic feature of the CVM is consistent with first-principles electronic structure calculations, and the combination of CVM with electronic structure calculation enables one to formulate free energy from the first-principles. CVM free energy conveys affluent information of a given system, and the second-order derivative traces the stability locus against configurational fluctuation. The kinetic extension of the CVM is the path probability method (PPM) which is utilized to calculate transformation and relaxation kinetics associated with the temperature change. Hence, the CVM and PPM are coherent methods to perform a synthetic study from initial non-equilibrium to final equilibrium states. By utilizing CVM free energy as a homogeneous free energy density term, one can calculate the time evolution of ordered domains within the phase field method. Finally, continuous displacement cluster variation method (CDCVM) is discussed as the recent development of CVM. CDCVM is capable of introducing the local lattice displacement into the free energy. Moreover, it is shown that CDCVM can be extended to study collective atomic displacements leading to displacive phase transformation.

  4. Cluster Variation Method as a Theoretical Tool for the Study of Phase Transformation

    NASA Astrophysics Data System (ADS)

    Mohri, Tetsuo

    2017-06-01

    Cluster variation method (CVM) has been widely employed to calculate alloy phase diagrams. The atomistic feature of the CVM is consistent with first-principles electronic structure calculations, and the combination of CVM with electronic structure calculation enables one to formulate free energy from the first-principles. CVM free energy conveys affluent information of a given system, and the second-order derivative traces the stability locus against configurational fluctuation. The kinetic extension of the CVM is the path probability method (PPM) which is utilized to calculate transformation and relaxation kinetics associated with the temperature change. Hence, the CVM and PPM are coherent methods to perform a synthetic study from initial non-equilibrium to final equilibrium states. By utilizing CVM free energy as a homogeneous free energy density term, one can calculate the time evolution of ordered domains within the phase field method. Finally, continuous displacement cluster variation method (CDCVM) is discussed as the recent development of CVM. CDCVM is capable of introducing the local lattice displacement into the free energy. Moreover, it is shown that CDCVM can be extended to study collective atomic displacements leading to displacive phase transformation.

  5. Numerical modeling of dielectrics electrocaloric effect near the ferroelectric-paraelectric phase transformation

    NASA Astrophysics Data System (ADS)

    Wang, Yixing; Liu, Liwu; Liu, Yanju; Leng, Jinsong

    2013-08-01

    Dielectrics with great electrocaloric effect (ECE) have great potential to be applied in modern refrigeration industry. Compared with the traditional refrigeration technology, it is environmentally friendly and has a higher efficiency. Researchers have found that compared with ECE occurring in ferroelectric phase, ECE in paraelectric state is giant. This paper is determined on calculating the ECE of several kinds of polar dielectric material so as to find some materials with giant ECE. First, we investigate the theoretical framework of ECE near the Ferroelectric-Paraelectric phase transformation, and we show the formula derivation of ECE near the Ferroelectric-Paraelectric phase transformation in the analytical method of the calculus derivation. Then we deduce the expression of phenomenological study parameters. Finally, we calculate the maximum temperature change, entropy change and the mechanical work of several kinds of dielectrics based on the expression deduced. We successfully find some dielectrics with giant ECE. The paper should offer great help in finding the dielectrics with giant ECE, which is of great value in application.

  6. 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

  7. Phase-Transformation Ductilization of Brittle High-Entropy Alloys via Metastability Engineering.

    PubMed

    Huang, Hailong; Wu, Yuan; He, Junyang; Wang, Hui; Liu, Xiongjun; An, Ke; Wu, Wei; Lu, Zhaoping

    2017-08-01

    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. Here, 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. This not only sheds new insights on the development of HEAs with excellent combination of strength and ductility, but also has great implications on overcoming the long-standing strength-ductility tradeoff of metallic materials in general. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  8. Phase transformation and microstructural evolution of nanostructured oxides and nitrides under ion irradiations

    NASA Astrophysics Data System (ADS)

    Lu, Fengyuan

    Material design at the nanometer scale is an effective strategy for developing advanced materails with enhanced radiation tolerance for advanced nuclear energy systems as high densities of surfaces and interfaces of the nanostructured materials may behave as effective sinks for defect recovery. However, nanostructured materials may not be intrinsically radiation tolerant, and the interplay among the factors of crystal size, temperature, chemical composition, surface energy and radiation conditions may eventually determine material radiation behaviors. Therefore, it is necessary to understand the radiation effects of nanostructured materials and the underlying physics for the design of advanced nanostructured nuclear materials. The main objective of this doctoral thesis is to study the behavior of nanostructured oxides and nitrides used as fuel matrix and waste forms under extreme radiation conditions with the focus of phase transformation, microstructural evolution and damage mechanisms. Radiation experiments were performed using energetic ion beam techniques to simulate radiation damage resulting from energetic neutrons, alpha-decay events and fission fragments, and various experimental approaches were employed to characterize materials’ microstructural evolution and phase stability upon intense radiation environments including transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Thermal annealing experiments indicated that nanostructured ZrO2 phase stability is strongly affected by the grain size. Radiation results on nanostructured ZrO2 indicated that thermodynamically unstable or metastable high temperature phases can be induced by energetic beam irradiation at room temperature. Various phase transformation among different polymorphs of monoclinic, tetragonal and amorphous states can be induced, and different mechanisms are responsible for structural transformations including oxygen vacancies accumulation upon displacive

  9. 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.