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1

Creep Strain and Strain Rate Response of 2219 Al Alloy at High Stress Levels  

NASA Technical Reports Server (NTRS)

As a result of high localized plastic deformation experienced during proof testing in an International Space Station connecting module, a study was undertaken to determine the deformation response of a 2219-T851 roll forging. After prestraining 2219-T851 Al specimens to simulate strains observed during the proof testing, creep tests were conducted in the temperature range from ambient temperature to 107 C (225 F) at stress levels approaching the ultimate tensile strength of 2219-T851 Al. Strain-time histories and strain rate responses were examined. The strain rate response was extremely high initially, but decayed rapidly, spanning as much as five orders of magnitude during primary creep. Select specimens were subjected to incremental step loading and exhibited initial creep rates of similar magnitude for each load step. Although the creep rates decreased quickly at all loads, the creep rates dropped faster and reached lower strain rate levels for lower applied loads. The initial creep rate and creep rate decay associated with primary creep were similar for specimens with and without prestrain; however, prestraining (strain hardening) the specimens, as in the aforementioned proof test, resulted in significantly longer creep life.

Taminger, Karen M. B.; Wagner, John A.; Lisagor, W. Barry

1998-01-01

2

Influence of residual stresses on high cycle fatigue strength of Ti6Al4V subjected to foreign object damage  

Microsoft Academic Search

The role of residual stresses in the high cycle fatigue (HCF) strength of Ti6Al4V subjected to foreign object damage (FOD) was evaluated on simulated airfoil and rectangular geometries. Both real and simulated impacts were conducted using spherical projectiles launched at 300 m\\/s and quasi-static chisel indentation, respectively. The spheres used were 1 mm diameter glass beads while the quasi-static indentor

Steven R. Thompson; John J. Ruschau; Theodore Nicholas

2001-01-01

3

Correlating stress generation and sheet resistance in InAlN/GaN nanoribbon high electron mobility transistors  

NASA Astrophysics Data System (ADS)

We report the nanoscale characterization of the mechanical stress in InAlN/GaN nanoribbon-structured high electron mobility transistors (HEMTs) through the combined use of convergent beam electron diffraction (CBED) and elastic mechanical modeling. The splitting of higher order Laue zone lines in CBED patterns obtained along the [540] zone axis indicates the existence of a large strain gradient in the c-direction in both the planar and nanoribbon samples. Finite element models were used to confirm these observations and show that a passivating layer of Al2O3 can induce a tensile stress in the active HEMT layer whose magnitude is dependent on the oxide layer thickness, thus, providing important ramifications for device design and fabrication.

Jones, Eric J.; Azize, Mohamed; Smith, Matthew J.; Palacios, Toms; Grade?ak, Silvija

2012-09-01

4

Subcritical crack growth of Ti-6Al-4V at room temperature under high stress-ratio loading  

SciTech Connect

Ti-6Al-4V is a two phase {alpha}-{beta} titanium alloy commonly used for turbine fan and compressor components. The crack growth behavior of Ti-6Al-4V and the role played by various material, mechanical, and environmental factors has been thoroughly investigated. This alloy is also susceptible to crack growth under sustained loading in air (SLC), and both hydrogen assisted cracking and low temperature creep mechanisms have been used to explain this susceptibility. Very little information is available on high R-ratio fatigue crack growth of Ti-6Al-4V and the role played by SLC on the fatigue process. In order to gain better understanding of the cracking behavior of this alloy under ripple loading conditions, room temperature, high stress-ratio (R {ge} 0.9) fatigue and SLC experiments have been conducted on a Ti-6Al-4V plate forging material in the duplex-annealed (DA) condition. The results of this investigation,namely, fatigue crack growth rates (CGR) as a function of stress intensity; SLC data; and scanning electron microscopy of the fatigue and SLC fracture surfaces are reported below.

Thomas, J.P. [Univ. of Notre Dame, IN (United States). Dept. of Aerospace and Mechanical Engineering] [Univ. of Notre Dame, IN (United States). Dept. of Aerospace and Mechanical Engineering

1998-11-13

5

Stress competition and vortex magnetic anisotropy in FeCoAlO high-frequency soft magnetic films with gradient Al-O contents  

NASA Astrophysics Data System (ADS)

A vortex magnetic anisotropy (VMA) was formed via the competition of residual stresses between radial and tangential directions in the FeCoAlO soft magnetic films (SMFs), prepared by a composition gradient sputtering (CGS) method. The VMA of the magnetic films gives rise to a rotating excitation direction of the ferromagnetic resonance. As a results, the as-deposited FeCoAlO films exhibit good high-frequency ferromagnetic properties with high permeability about 100, cut-off frequency over 2 GHz, and Qm factor over 50 along its individual excitation direction. These SMFs with the VMA are promising in the integration with the circular spiral inductors due to the geometrical match between the excitation direction of the SMFs and the circular inductor lines.

Li, Shandong; Du, Honglei; Xue, Qian; Xie, Shiming; Liu, Ming; Shao, Weiquan; Xu, Jie; Nan, Tianxiang; Sun, Nian X.; Duh, Jenq-Gong

2013-05-01

6

Effects of high electrical stress on GaN/InGaN/AlGaN single-quantum-well light-emitting diodes  

NASA Astrophysics Data System (ADS)

We report on high-electrical-stress testing of Nichia GaN/InGaN/AlGaN single-quantum-well (SQW) light-emitting diodes. In contrast to our earlier experiments with double-heterostructure LEDs, the present SQW devices have been improved to the point that the encapsulating plastic fails under high electrical stress earlier than the diode itself.

Osi?ski, Marek; Barton, Daniel L.; Perlin, Piotr; Lee, Jinhyun

1998-06-01

7

Stress-strain behavior of Cu-Al-Ni single-crystal shape memory alloy at high temperature: shape memory effects  

NASA Astrophysics Data System (ADS)

The experimental results of a preliminary study on stress-strain behavior of Cu-13.3% Al- 4% Ni (by wt.) single crystal shape memory alloy grown along the [001] direction at high temperatures are given. An Instron testing machine with a high temperature environmental chamber has been used to study the quasi-static stress-strain response of 1.5 mm diameter Cu-Al-Ni single crystal wires at different ambient temperatures in the range 100 - 160C. Local strain measurements using a highly sensitive extensometer are compared with overall strain measurements computed from the net displacements between grips. The effect of stress cycles on overall strain on full loading, after unloading and after heating in between stress cycles has been discussed.

Kannarpady, Ganesh K.; Trigwell, Steve; Bhattacharyya, Abhijit; Viahhi, Ivan; Pulnev, Sergei

2004-07-01

8

Influence of high pressure water-jet-assisted machining on surface residual stresses on the work-piece of Ti-6AL-4V alloy  

NASA Astrophysics Data System (ADS)

Titanium alloys are used in aerospace industry owing to their high strength to weight ratio. These alloys are considered to be difficult to machine due to their rigidity and poor thermal conductivity. High-pressure jet-assisted machining of titanium alloys is beneficial. It not only increases production efficiency, by increasing the cutting speed and lowering temperature both in cutting zone and the cutting tool, but also improves chip control and increases tool life. It also produces better surface integrity and compress residual stress, which improves the properties of work-piece. Ti-6Al-4V (Ti-64) rod was machined by turning in two different manners, finishing and roughing. Tests were conducted on a lathe using the same cutting data and different cooling systems, high pressure and conventional. In order to illustrate the effect of high-pressure jet-assisted machining on the properties of the work-piece of Ti-6Al-4V and its residual stress, x-ray defractometery was used. The present study revealed that there were compressive residual stresses in the transversal and longitudinal directions of the cut and the residual stresses in both directions were nearly of the same magnitude. The depth profile of residual stress was measured as well after removing the material by etching. The shear stress was very low, almost negligible. High pressure cooling affected residual stresses in finishing more than in roughing.

Vosough, Manouchehr; Svenningsson, Inge

2005-04-01

9

Time- and space-resolved high-throughput characterization of stresses during sputtering and thermal processing of Al-Cr-N thin films  

NASA Astrophysics Data System (ADS)

(Al100-xCrx)N thin-film materials libraries (x = 31-79 at%) were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analysing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [1?1?1] orientation to [2?0?0]. Further detailed analysis using x-ray diffraction and transmission electron microscopy clearly revealed the presence of a [1?1?1] highly textured face centred cubic (B1 type) Al-Cr-N phase in the as-deposited state as well as the coexistence of the hexagonal [1?1?0] textured Cr2N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries.

Grochla, D.; Siegel, A.; Hamann, S.; Buenconsejo, P. J. S.; Kieschnick, M.; Brunken, H.; Knig, D.; Ludwig, A.

2013-02-01

10

Tree fine root Ca\\/Al molar ratioIndicator of Al and acidity stress  

Microsoft Academic Search

High soil acidity and elevated soil Al concentrations limit plant growth in many terrestrial ecosystems. Aluminium toxicity can be ameliorated by Ca. Thus, Ca\\/Al molar ratios in soil solution and in plant tissues have been proposed as superior indicators than Al concentration itself for evaluating the Al toxicity stress to trees (Cronan & Grigal, J Environ Qual 1995;24:209226). This article

E. I. Vanguelova; Y. Hirano; T. D. Eldhuset; L. Sas-Paszt; M. R. Bakker; . Pttsepp; I. Brunner; K. Lhmus; D. Godbold

2007-01-01

11

Localization of off-stress-induced damage in AlGaN/GaN high electron mobility transistors by means of low frequency 1/f noise measurements  

NASA Astrophysics Data System (ADS)

The location of the time dependent degradation in OFF-state stressed AlGaN/GaN high electron mobility transistors is studied using low frequency 1/f noise measurements, with additional electroluminescence analysis. The gate bias dependence of the 1/f noise is shown to be a powerful tool to illustrate that in addition to the gate edge breakdown, progressive time-dependent trap generation occurs underneath the gate area, possibly extending in the gate-drain access region due to the electric field peak associated with the gate field plate.

Silvestri, Marco; Uren, Michael J.; Killat, Nicole; Marcon, Denis; Kuball, Martin

2013-07-01

12

ALS and Oxidative Stress: The Neurovascular Scenario  

PubMed Central

Oxidative stress and angiogenic factors have been placed as the prime focus of scientific investigations after an establishment of link between vascular endothelial growth factor promoter (VEGF), hypoxia, and amyotrophic lateral sclerosis (ALS) pathogenesis. Deletion of the hypoxia-response element in the vascular endothelial growth factor promoter and mutant superoxide dismutase 1 (SOD1) which are characterised by atrophy and muscle weakness resulted in phenotype resembling human ALS in mice. This results in lower motor neurodegeneration thus establishing an important link between motor neuron degeneration, vasculature, and angiogenic molecules. In this review, we have presented human, animal, and in vitro studies which suggest that molecules like VEGF have a therapeutic, diagnostic, and prognostic potential in ALS. Involvement of vascular growth factors and hypoxia response elements also highlights the converging role of oxidative stress and neurovascular network for understanding and treatment of various neurodegenerative disorders like ALS.

Thakur, Keshav; Gupta, Pawan Kumar

2013-01-01

13

Stress-corrosion resistance of high-strength Al-Zn-Mg-Cu alloys with and without silver additions  

Microsoft Academic Search

The separate and combined effects of adding silver, substituting other elements for chromium, increasing zinc, or varying\\u000a the copper content on the tensile properties and resistance to stress-corrosion cracking of 7075-type alloys were investigated.\\u000a Adding silver produced no increase in strength and marginal increases in the resistance to stress-corrosion cracking. Substituting\\u000a either zirconium or manganese for chromium increased strength and

H. Y. Hunsicker; J. T. Staley; R. H. Brown

1972-01-01

14

Improved Off-State Stress Critical Voltage on AlGaN/GaN High Electron Mobility Transistors Utilizing Pt/Ti/Au Based Gate Metallization  

SciTech Connect

The critical voltage for degradation of AlGaN/GaN high electron mobility transistors (HEMTs) employed with the Pt/Ti/Au gate metallization instead of the commonly used Ni/Au was significantly increased during the off-state stress. The typical critical voltage for HEMTs with Ni/Au gate metallization was around -60V. By sharp contrast, no critical voltage was observed for the HEMTs with Pt/Ti/Au gate metallization, even up to -100V, which was the instrumental limitation in this experiment. Both Schottky forward and reverse gate characteristics of the Ni/Au degraded once the gate voltage passed the critical voltage of around -60V. There was no degradation exhibited for the HEMTs with Pt-gated HEMTs.

Lo, C. F. [University of Florida; Liu, L. [University of Florida, Gainesville; Kang, Tsung Sheng [University of Florida, Gainesville; Davies, Ryan [University of Florida; Gila, Brent P. [University of Florida, Gainesville; Pearton, S. J. [University of Florida; Kravchenko, Ivan I [ORNL; Laboutin, O. [Kopin Corporation, Taunton, MA; Cao, Yu [Kopin Corporation, Taunton, MA; Johnson, Wayne J. [Kopin Corporation, Taunton, MA; Ren, F. [University of Florida

2011-01-01

15

Yield stress anomaly in B2 FeAl  

SciTech Connect

The studies on yield stress anomaly of B2 FeAl single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called yield stress anomaly, is observed in B2 FeAl in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from <111> direction at intermediate temperature to <100> at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly <111> by TEM. Also, it is observed that <111>-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {l_brace}111{r_brace} plane is energetically favorable, which is in agreement with the Flinn`s calculation for the B2 superlattice that APB energy on {l_brace}111{r_brace} plane is lower than that on {l_brace}110{r_brace} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on <111> superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAl single crystals is discussed.

Yoshimi, K.; Hanada, S. [Tohoku Univ., Sendai (Japan). Inst. for Materials Research; Yoo, M.H. [Oak Ridge National Lab., TN (United States). Metals and Ceramics Div.

1996-12-31

16

Stress corrosion of high strength aluminum alloys.  

NASA Technical Reports Server (NTRS)

An investigation has been carried out to examine the relationship of the observed chemical and mechanical properties of Al-Cu and Al-Zn-Mg alloys to the stress corrosion mechanisms which dominate in each case. Two high purity alloys and analogous commercial alloys were selected. Fundamental differences between the behavior of Al-Cu and of Al-Zn-Mg alloys were observed. These differences in the corrosion behavior of the two types of alloys are augmented by substantial differences in their mechanical behavior. The relative cleavage energy of the grain boundaries is of particular importance.

Cocks, F. H.; Brummer, S. B.

1972-01-01

17

Stress Corrosion Cracking of Al-Mg and Mg-Al Alloys  

SciTech Connect

Aluminum and magnesium based alloys are being used for reducing the weight of automobiles. For structural applications they must have adequate stress corrosion resistance and yet, under some circumstances, stress corrosion cracking can occur in both alloy systems. Precipitation of the Mg rich Beta-phase (Al3Mg2) at grain boundaries of Al-Mg alloys and the Beta-phase (Mg17Al12) at grain boundaries of the Mg-Al alloys are critical factors in their stress corrosion performance. In Mg-Al, the Beta-phase is cathodic to the matrix while in the Al-Mg case, the Beta-phase is anodic to the matrix. These phases produce localized galvanic induced-corrosion that leads to intergranular stress corrosion cracking and cracking growth rates of 5 and 103 times faster than the solution treated condition, for Al-Mg and Mg-Al, respectively.

Jones, Russell H.; Vetrano, John S.; Windisch, Charles F.

2004-12-01

18

Short-Duration and Stress-Rupture Strength of Beta-Alloy of Ti-Mo-Cr-Fe-Al System at High Temperatures.  

National Technical Information Service (NTIS)

A beta-alloy of Ti contg. Mo 7, Cr 5.5, Fe 3, and Al 3 percent when quenched from 800 degrees and aged at 550 or 525 degrees for 15 hrs. at 500 degrees for 20 hrs., and at 450 degrees for 50 hrs. exhibits high mech. properties at elevated temps. When expo...

N. V. Ageev S. G. Glazunov L. A. Petrova G. N. Tarasenko L. P. Grankova

1969-01-01

19

Stress-strain behavior of fine-grained Al/Al[sub 3]Ti alloys  

SciTech Connect

It has been recognized that dispersion strengthening with hard, stable, high melting point phases offers an effective technique for improving the stiffness of aluminum alloys over a wider temperature range compared to precipitation strengthening. Recent research indicates that mechanically alloyed (MA) Al-Ti alloys have good ambient and elevated temperature properties. The MA Al-Ti alloys are characterized by the presence of large volume fractions of submicron sized Al[sub 3]Ti dispersoids within a fine grain aluminum matrix. In addition, such alloys contain a significant amount of ultrafine Al[sub 4]C[sub 3] and Al[sub 2]O[sub 3] dispersoids incorporated during the MA process. The carbide and oxide dispersoids reside mainly along grain boundaries and can stabilize a fine-grain structure even after long term exposure at elevated temperature. The high temperature strength of these MA aluminum alloys is strongly controlled by carbide and oxide dispersoids. The Young's modulus of Al[sub 3]Ti phase was determined to be about 220 GPa. Hence, the presence of the Al[sub 3]Ti phase is very effective in increasing the stiffness of the MA aluminum alloys is strongly controlled by carbide and oxide dispersoids. The Young's modulus of Al[sub 3]Ti phase was determined to be about 220 GPa. Hence, the presence of the Al[sub 3]Ti phase is very effective in increasing the stiffness of the MA aluminum alloys. The strength of such alloys increases with increasing Al[sub 3]Ti content; however, the strengthening effect of Al[sub 3]Ti dispersoids diminishes at temperatures above about 400C. On the other hand, such dispersoids also appear responsible for improving the high temperature ductility of the MA Al-Ti alloys. A better understanding of the strengthening mechanisms is desirable. This paper presents the room temperature stress-strain behavior of two Al-Al[sub 3]Ti alloys made by mechanical alloying and hot-pressing.

Wang, S.H.; Kao, P.W.; Chang, C.P. (National Sun Yat-Sen Univ., Kaohsiung (Taiwan, Province of China))

1993-08-01

20

Effect of Stress Level on the High Temperature Deformation and Fracture Mechanisms of Ti-45Al-2Nb-2Mn-0.8 vol. pct TiB2: An In Situ Experimental Study  

NASA Astrophysics Data System (ADS)

The effect of the applied stress on the deformation and crack nucleation and propagation mechanisms of a ?-TiAl intermetallic alloy (Ti-45Al-2Nb-2Mn (at. pct)-0.8 vol. pct TiB2) was examined by means of in situ tensile (constant strain rate) and tensile-creep (constant load) experiments performed at 973 K (700 C) using a scanning electron microscope. Colony boundary cracking developed during the secondary stage in creep tests at 300 and 400 MPa and during the tertiary stage of the creep tests performed at higher stresses. Colony boundary cracking was also observed in the constant strain rate tensile test. Interlamellar ledges were only found during the tensile-creep tests at high stresses ( ? > 400 MPa) and during the constant strain rate tensile test. Quantitative measurements of the nature of the crack propagation path along secondary cracks and along the primary crack indicated that colony boundaries were preferential sites for crack propagation under all the conditions investigated. The frequency of interlamellar cracking increased with stress, but this fracture mechanism was always of secondary importance. Translamellar cracking was only observed along the primary crack.

Muoz-Moreno, Roco; Prez-Prado, M. Teresa; Llorca, Javier; Ruiz-Navas, Elisa Mara; Boehlert, Carl J.

2013-04-01

21

Degradation analysis and characterization of multifilamentary conduction patterns in high-field stressed atomic-layer-deposited TiO2/Al2O3 nanolaminates on GaAs  

NASA Astrophysics Data System (ADS)

In this paper, the effects of applying a high-field electrical stress on TiO2/Al2O3 nanolaminates grown by atomic layer deposition onto a p-type GaAs substrate are investigated. First, it is shown that the current-time (I-t) characteristic of the devices during a constant voltage stress follows the extended Curie-von Schweidler law for dielectric degradation. The application of voltage sweeps from negative to positive bias and back also reveals an hysteretic behavior of the current-voltage (I-V) characteristic typical of the resistive switching mechanism ocurring in these and others high permittivity oxide films. Second, we show that after the detection of the breakdown event the capacitors exhibit a random spot pattern on the top metal electrode (Al) associated with the generation of multifilamentary conduction paths running across the insulating film. The number of generated spots depends on the magnitude of the electrical stress and for a sufficiently large density, it is possible to demonstrate that they are spatially uncorrelated. The analysis is carried out using spatial statistics techniques such as the intensity plot, the interspot distance histogram, and the pair correlation function.

Miranda, E.; Su, J.; Das, T.; Mahata, C.; Maiti, C. K.

2012-09-01

22

Profilin 1 Associates with Stress Granules and ALS-Linked Mutations Alter Stress Granule Dynamics.  

PubMed

Mutations in the PFN1 gene encoding profilin 1 are a rare cause of familial amyotrophic lateral sclerosis (ALS). Profilin 1 is a well studied actin-binding protein but how PFN1 mutations cause ALS is unknown. The budding yeast, Saccharomyces cerevisiae, has one PFN1 ortholog. We expressed the ALS-linked profilin 1 mutant proteins in yeast, demonstrating a loss of protein stability and failure to restore growth to profilin mutant cells, without exhibiting gain-of-function toxicity. This model provides for simple and rapid screening of novel ALS-linked PFN1 variants. To gain insight into potential novel roles for profilin 1, we performed an unbiased, genome-wide synthetic lethal screen with yeast cells lacking profilin (pfy1?). Unexpectedly, deletion of several stress granule and processing body genes, including pbp1?, were found to be synthetic lethal with pfy1?. Mutations in ATXN2, the human ortholog of PBP1, are a known ALS genetic risk factor and ataxin 2 is a stress granule component in mammalian cells. Given this genetic interaction and recent evidence linking stress granule dynamics to ALS pathogenesis, we hypothesized that profilin 1 might also associate with stress granules. Here we report that profilin 1 and related protein profilin 2 are novel stress granule-associated proteins in mouse primary cortical neurons and in human cell lines and that ALS-linked mutations in profilin 1 alter stress granule dynamics, providing further evidence for the potential role of stress granules in ALS pathogenesis. PMID:24920614

Figley, Matthew D; Bieri, Gregor; Kolaitis, Regina-Maria; Taylor, J Paul; Gitler, Aaron D

2014-06-11

23

Stress Development and Relaxation in Al2O3 during Early StageOxidation of beta-NiAl  

SciTech Connect

Using a glancing synchrotron X-ray beam (Advanced Photon Source, Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from thermally grown oxides on NiAl samples were recorded during oxidation at 1000 or 1100 C in air. The diffraction patterns were analyzed to determine strain and phase changes in the oxide scale as it developed and evolved. Strain was obtained from measurements of the elliptical distortion of the Debye-Scherrer rings, where data from several rings of a single phase were used. Results were obtained from {alpha}-Al{sub 2}O{sub 3} as well as from the transition alumina, in this case {theta}-Al{sub 2}O{sub 3}, which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in {alpha}-Al{sub 2}O{sub 3} was tensile, with a magnitude high enough to cause Al{sub 2}O{sub 3} fracture. New {alpha}-Al{sub 2}O{sub 3} patches nucleated at the scale/alloy interface and spread laterally and upward. This transformation not only puts the alpha alumina in tension, but can also cause the transition alumina to be in tension. After a complete {alpha}-Al{sub 2}O{sub 3} layer formed at the interface, the strain level in {alpha}-Al{sub 2}O{sub 3} became compressive, reaching a steady state level around -75 MPa at 1100 C. To study a specimen's response to stress perturbation, samples with different thickness, after several hours of oxidation at 1100 C, were quickly cooled to 950 C to impose a compressive thermal stress in the scale. The rate of stress relaxation was the same for 1 and 3.5 mm thick samples, having a strain rate of {approx} 1 x 10{sup -8}/s. This behavior indicates that oxide creep is the major stress relaxation mechanism.

Hou, P.Y.; Paulikas, A.P.; Veal, B.W.

2005-04-20

24

Constant stress of XD trademark TiAl containing nitrogen  

SciTech Connect

Ti-47at%Al+7vol%(TiN+TiB{sub 2}) alloys, produced by the Martin Marietta XD{trademark} process, were subjected to tensile constant stress creep deformation in the range 760--850{degree}C temperature and 138--276 MPa (20--40 ksi) stress. These alloys have a matrix of TiAl and Ti{sub 3}Al initially containing 1 to 10 {mu}m diameter equiaxed particles of TiB{sub 2} and Ti{sub 2}AlN. Microstructures were examined using optical, SEM and TEM methods in an attempt to correlate the substructures with the property data. In particular, fine coherent particles have been observed to decorate dislocations and the interior of TiAl grains. The presence of these particles is thought to explain the creep resistance of this alloy. 14 refs., 5 figs.

Robertson, E.; Martin, P.L.

1990-01-01

25

Effect of stress reductions on the stress exponent and subgrain size in an Al-Zn alloy  

Microsoft Academic Search

Stress change and uninterrupted tests were performed on an Al-5 wt pct Zn alloy over the high temperature range (0.7 to 0.99 T m, where T mis the melting point) and at a normalized stress range extending from 10-5 to 2 10-4. Two techniques, etch-pit (EP) and transmission electron microscopy (TEM), were used to examine dislocation substructure developed during

Anurag Goel; Timothy J. Ginter; Farghalli A. Mohamed

1983-01-01

26

Stress asymmetry of stoichiometric NiAl single crystals  

SciTech Connect

The yield stress properties of stoichiometric NiAl single crystals were investigated in terms of crystal orientation, temperature and the deformation mode. The calculated critical resolved shear stress (CRSS) was a strong function of crystal orientation, temperature and the deformation mode whether tension or compression. The CRSS was, in a wide range of experimental conditions, higher in the sequence of (110)(100), (100)(100) and (hk0)(100) slips. The CRSS in compression was higher particularly at low temperatures than the CRSS in tension. The tension-compression asymmetry on the CRSS was understood qualitatively as being due to the effect of the normal stress on the core structure of a (001) dislocation and a (111) dislocation. It was suggested that a compressive normal stress makes the core configuration more sessile, resulting in the increased stress effectively at low temperatures.

Takasagu, T.; Kishino, J.; Hanada, S. (Tohoku Univ., Sendai (Japan). Inst. for Materials Research)

1993-04-01

27

High cycle fatigue behaviour of microsphere Al 2O 3Al particulate metal matrix composites  

Microsoft Academic Search

The high-cycle stress-life (SN) curve and fatigue crack growth threshold (?Kth) behaviour of COMRAL-85TM, a 6061 aluminiummagnesiumsilicon alloy reinforced with 20vol.% Al2O3-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated for a stress ratio of R=?1 (fully reversed loading). Fatigue testing was conducted on both smooth round bar (SN) specimens and notched round bar (fatigue

B. G. Park; A. G. Crosky; A. K. Hellier

2008-01-01

28

Multiscale Modeling of MetalMetal Contact Dynamics Under High Electromagnetic Stress: Timescales and Mechanisms for Joule Melting of AlCu Asperities  

Microsoft Academic Search

An analysis and initial results from a multiscale continuum-atomistic simulation of the Joule heating and melting of Cu-Al asperity contacts are presented. An analytic expression is given for the time needed to reach the Al melting point for an asperity as a function of the voltage drop and the asperity contact area. The coupled continuum-atomistic simulations capture the initial stages

Douglas L. Irving; Clifford W. Padgett; Yin Guo; John W. Mintmire; Donald W. Brenner

2009-01-01

29

Multiscale Modeling of Metal-Metal Contact Dynamics under High Electromagnetic Stress: Timescales and Mechanisms for Joule Melting of Al-Cu Asperities  

Microsoft Academic Search

An analysis and initial results from a multiscale continuum-atomistic simulation of the Joule heating and melting of Cu-Al asperity contacts is presented. An analytic expression is given for the time needed to reach the Al melting point for an asperity as a function of the voltage drop and the asperity contact area. The coupled continuum-atomistic simulations capture the initial stages

Douglas Irving; Clifford Padgett; Yin Guo; John Mintmire; Donald Brenner

2008-01-01

30

High Temperature Mechanical Characterization and Analysis of Al2O3 /Al2O3 Composition  

NASA Technical Reports Server (NTRS)

Sixteen ply unidirectional zirconia coated single crystal Al2O3 fiber reinforced polycrystalline Al2O3 was tested in uniaxial tension at temperatures to 1400 C in air. Fiber volume fractions ranged from 26 to 31%. The matrix has primarily open porosity of approximately 40%. Theories for predicting the Young's modulus, first matrix cracking stress, and ultimate strength were applied and evaluated for suitability in predicting the mechanical behavior of Al2O3/Al2O3 composites. The composite exhibited pseudo tough behavior (increased area under the stress/strain curve relative to monolithic alumina) from 22 to 1400 C. The rule-of-mixtures provides a good estimate of the Young's modulus of the composite using the constituent properties from room temperature to approximately 1200 C for short term static tensile tests in air. The ACK theory provides the best approximation of the first matrix cracking stress while accounting for residual stresses at room temperature. Difficulties in determining the fiber/matrix interfacial shear stress at high temperatures prevented the accurate prediction of the first matrix cracking stress above room temperature. The theory of Cao and Thouless, based on Weibull statistics, gave the best prediction for the composite ultimate tensile strength.

Gyekenyesi, John Z.; Jaskowiak, Martha H.

1999-01-01

31

Residual stress in friction stir-welded Al sheets  

Microsoft Academic Search

Friction stir welding (FSW) is a relatively new joining technique particularly for aluminum alloys that are difficult to fusion weld. Although FSW is as a solid-state joining process which produces low-distortion welds of high quality, significant levels of residual stresses can be present in the weld after fabrication. These residual stresses can influence the service performance of the welded components

P. Staron; M. Koak; S. Williams; A. Wescott

2004-01-01

32

Microstructural Evolution and Stress Corrosion Cracking Behavior of Al-5083  

NASA Astrophysics Data System (ADS)

The fine scale microstructure of Al-5083 (H-131) sensitized at 448 K (175 C) for 1, 10, 25, 50, 100, 240, 500, and 1000 hours has been investigated using transmission electron microscopy (TEM) to study the evolution of the ? phase (Al3Mg2) at grain boundaries and on pre-existing intragranular particles. In fully sensitized Al-5083, the ? phase (Al3Mg2) forms heterogeneously both at grain boundaries and on pre-existing particles, which are enriched in manganese. TEM observations showed that the grain boundary precipitation of the ? phase initially occurs between 0 to 1 hour of aging at 448 K (175 C), and that the ? phase grows with a ribbonlike morphology. The grain boundary planes are covered by the ? phase after 240 hours of aging. The contribution of microstructure, defects, and environment on the stress corrosion cracking (SCC) behavior is discussed.

Goswami, R.; Spanos, G.; Pao, P. S.; Holtz, R. L.

2011-02-01

33

The flow stress anomaly in Fe43at%Al single crystals  

Microsoft Academic Search

Fe43at%Al single crystals were deformed in compression between room temperature and 700C. The strain rate sensitivity of the flow stress was measured by stress relaxation tests. The dislocation structure of the deformed samples was studied in a high-voltage transmission electron microscope. In addition, micro-tensile specimens were deformed in situ in this microscope to directly observe the dynamic behaviour of the

U. Messerschmidt; M. Bartsch; Ch. Dietzsch

2006-01-01

34

Commentary: Beyond Stressful Life Events and Depression?--Reflections on Bogdan et al. (2014)  

ERIC Educational Resources Information Center

In light of continuing disagreement, even at the meta-analytic level, as to whether the gene-נ-environment (GE) interaction involving 5-HTTLPR and stressful life events (SLEs) predicts depression, Bogdan and associates (this issue, Bogdan etal., 2014) sought to extend research on what has become a highly controversial general (GxE) and

Belsky, Jay

2014-01-01

35

Influence of high pressure water-jet-assisted machining on surface residual stresses on the work-piece of Ti6AL4V alloy  

Microsoft Academic Search

Titanium alloys are used in aerospace industry owing to their high strength to weight ratio. These alloys are considered to be difficult to machine due to their rigidity and poor thermal conductivity. High-pressure jet-assisted machining of titanium alloys is beneficial. It not only increases production efficiency, by increasing the cutting speed and lowering temperature both in cutting zone and the

Manouchehr Vosough; Inge Svenningsson

2005-01-01

36

Effect of stress reductions on the stress exponent and subgrain size in an Al-Zn alloy  

Microsoft Academic Search

Stress change and uninterrupted tests were performed on an Al-5 wt pct Zn alloy over the high temperature range (0.7 to 0.99T\\u000a m, whereT\\u000a mis the melting point) and at a normalized stress range extending from 10?5 to 2 ? 10?4. Two techniques, etch-pit (EP) and transmission electron microscopy (TEM), were used to examine dislocation substructure\\u000a developed during creep. The

Anurag Goel; Timothy J. Ginter; Farghalli A. Mohamed

1983-01-01

37

Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress states  

Microsoft Academic Search

The effect of different uniaxial and triaxial stress states on the stress-induced martensitic transformation in CuZnAl was\\u000a investigated. Under uniaxial loading, it was found that the compressive stress level required to macroscopically trigger the\\u000a transformation was 34 pct larger than the required tensile stress. The triaxial tests produced effective stress-strain curves\\u000a with critical transformation stress levels in between the tensile

Ken Gall; Kurt Jacobus; Huseyin Sehitoglu; Hans J. Maier

1998-01-01

38

Alloys based on NiAl for high temperature applications  

NASA Technical Reports Server (NTRS)

The NiAl alloys for potential high temperature applications were studied. Alloys were prepared by powder metallurgy techniques. Flow stress values at slow strain rates and high temperatures were measured. Some ternary alloying additions (Hf, Ta and Nb) were identified. The mechanism of strengthening in alloys containing these additions appears to be a form of particle dislocation interaction. The effects of grain size and stoichiometry in binary alloys are also presented.

Vedula, K. M.; Pathare, V.; Aslanidis, I.; Titran, R. H.

1984-01-01

39

Stress-strain behavior of fine-grained Al\\/Al[sub 3]Ti alloys  

Microsoft Academic Search

It has been recognized that dispersion strengthening with hard, stable, high melting point phases offers an effective technique for improving the stiffness of aluminum alloys over a wider temperature range compared to precipitation strengthening. Recent research indicates that mechanically alloyed (MA) Al-Ti alloys have good ambient and elevated temperature properties. The MA Al-Ti alloys are characterized by the presence of

S. H. Wang; P. W. Kao; C. P. Chang

1993-01-01

40

Strengthening of Al-Cu single crystals by stress-oriented Guinier-Preston zones  

Microsoft Academic Search

The strengthening of Al-Cu is examined after introducing Guinier-Preston (GP) zones on particular cube planes by stress ageing. A compressive stress applied during an early stage of ageing promotes formation of the platelet precipitates on the cube planes subjected to the largest normal stress. Selection of a stress axis in the compression tests after the stress ageing enables one to

S. Muraishi; N. Niwa; A. Maekawa; S. Kumai; A. Sato

2002-01-01

41

Stress overshoot in stress-strain curves of Zr65Al10Ni10Cu15 metallic glass  

Microsoft Academic Search

The essential features of the stress overshoot in the stress-strain curves of Zr65Al10Ni10Cu15 (at. %) metallic glass that has a wide supercooled liquid region were revealed. The stress overshoot was dependent on temperature, strain rate, and stress relaxation. During the stretch, a change in strain rate gave rise to stress overshoot or undershoot which was sensitive to the variable quantities

Yoshihito Kawamura; Tsutomu Shibata; Akihisa Inoue; Tsuyoshi Masumoto

1997-01-01

42

Stress Histories of Yttria-Stabilized Zirconia and CoNiCrAlY Coatings during Thermal Spraying  

NASA Astrophysics Data System (ADS)

Thermal barrier coating (TBC) systems which are used for insulating the substrates of gas turbine blades from high temperature can be made by thermal spraying. The TBC system has residual stresses because of high temperature deposition and the thermal expansion mismatch in the system. In this study, how the residual stress occurs in TBC system was examined by both experimental measurement and FEM analysis. The Yttria-stabilized zirconia (YSZ) top coating was deposited by atmospheric plasma spraying (APS). CoNiCrAlY bond coatings were deposited by high velocity oxygen-fuel (HVOF) spraying and APS. The temperatures of YSZ, CoNiCrAlYs and substrates were measured during thermal spraying. The temperature of YSZ was the highest and that of CoNiCrAlY(HVOF) was the lowest among the three types of spray processes. The residual stresses were elastically calculated by FEM based on the measured temperature histories. The residual stress of YSZ and CoNiCrAlYs on two types of substrates were also measured by X-ray diffraction method. It was confirmed from FEM analysis that residual stress consisted of primary quench stress and secondary thermal mismatch stress. The quench stress was caused by the quenching of coating particles during deposition which occurs due to the huge thermal capacity of the substrate. The thermal mismatch stress was caused by the difference in linear expansion coefficients between coating and substrate. It was found that not only these two mechanisms but also microcrack formation caused by quench played an important role in the residual stress. The temperatures at which residual stresses might begin to occur in the coatings were shown based on the stress relaxation by microcrack formation. It was also found that peening effect played an important role in the residual stress of HVOF sprayed coating.

Waki, Hiroyuki; Fujioka, Hidenori; Kobayashi, Akira

43

Stress-Corrosion Cracking of High-Strength Aluminum Alloys.  

National Technical Information Service (NTIS)

A study has been made of the mechanism of hydrogen embrittlement (HE) in Al-Zn-Mg alloys, and of the role of hydrogen in the intergranular stress-corrosion cracking (I-SCC) of these alloys. Up to 300 ppm (1 at pct) hydrogen was introduced into a high-puri...

E. N. Pugh, R. A. Yeske

1980-01-01

44

Quenching, thermal and residual stress in plasma sprayed deposits: NiCrAlY and YSZ coatings  

SciTech Connect

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress magnitudes and/or concentrations can undermine the structural integrity of sprayed parts and impair their functionality. Therefore, it is important to know the stress state, understand its generation and be able to control it. Results of experimental stress determination in plasma sprayed deposits are presented. Neutron diffraction as a non-destructive and phase-distinctive measurement method was used to determine residual stress profiles in thick NiCrAlY and yttria-stabilized zirconia (YSZ) deposits. Measurements were complemented with calculations based on experimentally determined material properties, which allowed for separation of quenching and thermal stress contributions to final residual stress. Since the application of neutron diffraction to plasma sprayed deposits is relatively novel, certain verification measurements were performed. Specimens were prepared at two different deposition temperatures, to determine the effect of temperature on the stresses and relevant deposit properties.

Matejicek, J.; Sampath, S. [State Univ. of New York, Stony Brook, NY (United States). Dept. of Materials Science and Engineering] [State Univ. of New York, Stony Brook, NY (United States). Dept. of Materials Science and Engineering; Brand, P.C.; Prask, H.J. [National Inst. of Standards and Technology, Gaithersburg, MD (United States)] [National Inst. of Standards and Technology, Gaithersburg, MD (United States)

1999-01-15

45

Stress evolution during and after sputter deposition of thin Cu Al alloy films  

NASA Astrophysics Data System (ADS)

The stress evolution during and after sputter deposition of thin Cu-Al alloy films containing 1 and 2 at.% Al onto oxidized Si(100) substrates has been studied up to thicknesses of 300 nm by means of in situ substrate curvature measurements. In order to correlate stress and morphology, the microstructure was investigated by focused ion beam microscopy, scanning electron microscopy, and atomic force microscopy. The evolution of the stress and microstructure of the Cu-Al alloy films is similar to that for sputtered pure Cu films. Film growth proceeds in the Volmer-Weber mode, typical for high mobility metals. It is characterized by nucleation, island, percolation, and channel stages before the films become continuous, as well as lateral grain growth in the compact films. With increasing Al content the overall atom mobility and, thus, the average grain size of the alloy films are reduced. Increase of the sputter pressure from 0.5 to 2 Pa leads to films with larger grain size, rougher surface morphology and higher electrical resistivity.

Pletea, M.; Wendrock, H.; Kaltofen, R.; Schmidt, O. G.; Koch, R.

2008-06-01

46

Joule-Heating-Induced Damage in Cu-Al Wedge Bonds Under Current Stressing  

NASA Astrophysics Data System (ADS)

Copper wires are increasingly used to replace gold wires in wire-bonding technology owing to their better electrical properties and lower cost. However, not many studies have been conducted on electromigration-induced failure of Cu wedge bonds on Al metallization. In this study, we investigated the failure mechanism of Cu-Al wedge bonds under high current stressing from 4 104 A/cm2 to 1 105 A/cm2 at ambient temperature of 175C. The resistance evolution of samples during current stressing and the microstructure of the joint interface between the Cu wire and Al-Si bond pad were examined. The results showed that abnormal crack formation accompanying significant intermetallic compound growth was observed at the second joint of the samples, regardless of the direction of electric current for both current densities of 4 104 A/cm2 and 8 104 A/cm2. We propose that this abnormal crack formation at the second joint is mainly due to the higher temperature induced by the greater Joule heating at the second joint for the same current stressing, because of its smaller bonded area compared with the first joint. The corresponding fluxes induced by the electric current and chemical potential difference between Cu and Al were calculated and compared to explain the failure mechanism. For current density of 1 105 A/cm2, the Cu wire melted within 0.5 h owing to serious Joule heating.

Yang, Tsung-Han; Lin, Yu-Min; Ouyang, Fan-Yi

2014-01-01

47

Research on flow stress of spray formed 70Si30Al alloy under hot compression deformation  

Microsoft Academic Search

The flow stress of spray formed 70Si30Al alloy was studied by hot compression on a Gleeble-1500 test machine. The experimental results indicated that the flow stress depends on the strain rate and the deformation temperature. The flow stress increases with an increase in strain rate at a given deformation temperature. The flow stress decreases with the deformation temperature increasing at

Yanguang WEI; Baiqing XIONG; Yong'an ZHANG; Hongwei LIU; Baohong ZHU; feng WANG

2006-01-01

48

Photosynthesis: Response to high temperature stress.  

PubMed

Global warming has led to increased temperature of the earth which is a major abiotic stress posing a serious threat to the plants. Photosynthesis is amongst the plant cell functions that is highly sensitive to high temperature stress and is often inhibited before other cell functions are impaired. The primary sites of targets of high temperature stress are Photosystem II (PSII), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while Cytochrome b559 (Cytb559) and plastoquinone (PQ) are also affected. As compared to PSII, PSI is stable at higher temperatures. ROS production, generation of heat shock proteins, production of secondary metabolites are some of the consequences of high temperature stress. In this review we have summarized the physiological, biochemical and molecular aspects of high temperature stress on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved. PMID:24796250

Mathur, Sonal; Agrawal, Divya; Jajoo, Anjana

2014-08-01

49

Stress-Induced Grain Growth in an Ultra-Fine Grained Al Alloy  

NASA Astrophysics Data System (ADS)

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.

Lin, Yaojun; Wen, Haiming; Li, Ying; Wen, Bin; Liu, Wei; Lavernia, Enrique J.

2014-03-01

50

Stress-Induced Grain Growth in an Ultra-Fine Grained Al Alloy  

NASA Astrophysics Data System (ADS)

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.

Lin, Yaojun; Wen, Haiming; Li, Ying; Wen, Bin; Liu, Wei; Lavernia, Enrique J.

2014-06-01

51

Thermal residual stresses and their toughening effect in Al 2O 3 platelet reinforced glass  

Microsoft Academic Search

Fluorescence spectroscopy has been used to measure the thermal residual stresses in Al2O3-platelet\\/borosilicate glass composites. Tensile residual stresses were found in the platelets, implying the presence of compressive residual stresses in the glass matrix. Measurements of stresses in the bulk of the composite could be obtained using fluorescence from platelets below the specimen surface. The measured stresses lay between the

R. I. Todd; A. R. Boccaccini; R. Sinclair; R. B. Yallee; R. J. Young

1999-01-01

52

High Density Sliding at Ta/Al and Al/Al Interfaces  

SciTech Connect

We present 3D-nonequilibrium molecular dynamics results for the velocity dependence of the frictional force at smooth sliding interfaces for Ta and Al single crystals. For Ta/Al we consider Al(100)/Ta(100) and Al(111)/Ta(110) interfaces sliding along [001] and [11(bar sign)0]fcc /[001]bcc respectively. These are compared with Al(111)/Al(100) interfaces at the same loads, corresponding to a pressure of 15 GPa. Both interfacial pairs show similar behavior in the velocity dependence of the frictional force: a low velocity regime with an increasing frictional force followed by a strain induced transformation regime at velocities above approximately 1/10 the transverse sound speed, followed by a fluidized interface at high velocities. For both interfacial pairs, the high velocity dependence of the frictional force exhibits power law behavior, Ft {proportional_to} v-{beta} with {beta}=3/4. We discuss the structural changes that influence dissipation in each of these regimes.

Hammerberg, J. E.; Germann, T. C. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Ravelo, R. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Physics Dept., University of Texas -- El Paso, El Paso, TX 79968-0515 (United States); Materials Research Institute, University of Texas - El Paso, El Paso, TX 79968-0515 (United States)

2006-07-28

53

Oxidative stress is an early symptom triggered by aluminum in Al-sensitive potato plantlets  

Microsoft Academic Search

The objective of this study was to evaluate whether the oxidative stress caused by aluminum (Al) toxicity is an early symptom that can trigger root growth inhibition in Macaca (Al-sensitive) and SMIC148-A (Al-tolerant) potato clones. Plantlets were grown in a nutrient solution (pH 4.00) with 0, 100 and 200mg Al L?1. At 24, 72, 120 and 168h after Al addition,

Luciane Almeri Tabaldi; Denise Cargnelutti; Jamile Fabbrin Gonalves; Luciane Belmonte Pereira; Gabriel Y Castro; Joseila Maldaner; Renata Rauber; Liana Vernica Rossato; Dilson Antnio Bisognin; Maria Rosa Chitolina Schetinger; Fernando Teixeira Nicoloso

2009-01-01

54

Suppression of Crack Generation Using High-Compressive-Strain AlN\\/Sapphire Template for Hydride Vapor Phase Epitaxy of Thick AlN Film  

Microsoft Academic Search

A stress control method was proposed to suppress crack generation in the hydride vapor phase epitaxy (HVPE) of a thick AlN film on an AlN\\/sapphire template, i.e., a high-compressive-strain (HCS) AlN\\/sapphire template was employed as a substrate. The AlN films on the HCS AlN\\/sapphire template were crack-free and very smooth. However, the AlN films on the normal AlN\\/sapphire template (with

Kenichi Tsujisawa; Shinya Kishino; Da-Bing Li; Hideto Miyake; Kazumasa Hiramatsu; Tomohiko Shibata; Mitsuhiro Tanaka

2007-01-01

55

Microstructural Evolution and Stress Corrosion Cracking Behavior of Al5083  

Microsoft Academic Search

The fine scale microstructure of Al-5083 (H-131) sensitized at 448K (175C) for 1, 10, 25, 50, 100, 240, 500, and 1000hours\\u000a has been investigated using transmission electron microscopy (TEM) to study the evolution of the ? phase (Al3Mg2) at grain boundaries and on pre-existing intragranular particles. In fully sensitized Al-5083, the ? phase (Al3Mg2) forms heterogeneously both at grain boundaries

R. Goswami; G. Spanos; P. S. Pao; R. L. Holtz

2011-01-01

56

Microstructural Evolution and Stress Corrosion Cracking Behavior of Al5083  

Microsoft Academic Search

The fine scale microstructure of Al-5083 (H-131) sensitized at 448 K (175 C) for 1, 10, 25, 50, 100, 240, 500, and 1000 hours has been investigated using transmission electron microscopy (TEM) to study the evolution of the beta phase (Al3Mg2) at grain boundaries and on pre-existing intragranular particles. In fully sensitized Al-5083, the beta phase (Al3Mg2) forms heterogeneously both

R. Goswami; G. Spanos; P. S. Pao; R. L. Holtz

2011-01-01

57

High electron mobility AlGaN/AlN/GaN HEMT structure with a nano-scale AlN interlayer  

NASA Astrophysics Data System (ADS)

Epitaxies of AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with different thickness of nano-scale AlN interlayers have been realized by metalorganic chemical vapor deposition (MOCVD) technology. After epitaxy, high resolution X-ray diffraction (HRXRD), temperature-dependent Hall Effect and atomic force microscopy (AFM) measurements were used to characterize the properties of these samples. First, it was found that the Al composition of AlGaN layer increases from 21.6 to 34.2% with increasing the thickness of AlN interlayer from 0 to 5 nm under the same AlGaN growth conditions. This result may due to the influences of compressive stress and Al incorporation induced by the AlN interlayer. Then, we also found that the room-temperature (RT) electron mobility stays higher than 1500 cm2/Vs in the samples within AlN interlayer thickness range of 1.5 nm, on the other hand, the low-temperature (80K) electron mobility drops dramatically from 8180 to 5720 cm2/Vs in the samples with AlN interlayer thickness increasing from 1 to 1.5 nm. Furthermore, it was found that the two-dimensional electron gas (2DEG) density increases from 1.151013 to 1.581013 cm-2 beyond the AlN interlayer thickness of 1 nm. It was also found that the temperature independent 2DEG densities are observed in the samples with AlN interlayer thickness of 0.5 and 1 nm. The degenerated characteristics of the samples with AlN thickness thicker than 1.5 nm show the degraded crystalline quality which matched the observation of surface defects and small cracks formations from their AFM images. Finally, the 2DEG mobilities of the proposed structures can be achieved as high as 1705 and 8180 cm2/Vs at RT and 80K, respectively.

Huang, Shih-Chun; Chen, Wen-Ray; Hsu, Yu-Ting; Lin, Jia-Ching; Chang, Kuo-Jen; Lin, Wen-Jen

2012-10-01

58

Residual stress characterization of Al/SiC nanoscale multilayers using X-ray  

SciTech Connect

Nanolayered composites are used in a variety of applications such as wear resistant coatings, thermal barrier coatings, optical and magnetic thin films, and biological coatings. Residual stresses produced in these materials during processing play an important role in controlling their microstructure and properties. In this paper, we have studied the residual stresses in model metal-ceramic Al/SiC nanoscale multilayers produced by physical vapor deposition (magnetron sputtering). X-ray synchrotron radiation was used to measure stresses in the multilayers using the sin{sup 2} {Psi} technique. The stresses were evaluated as a function of layer thicknesses of Al and SiC and also as a function of the number of layers. The stress state of Al in the multilayer was largely compressive, compared to single layer Al stresses. This is attributed to a peening mechanism due to bombardment of the Al layers by SiC and Ar neutrals during deposition. The stress evolution was numerically modeled by a simplified peening process to qualitatively explain the Al thickness-dependent residual stresses.

Singh, DRP [Arizona State University; Deng, X. [Arizona State University; Chawla, N. [Arizona State University; Bai, J. [Brookhaven National Laboratory (BNL); Hubbard, Camden R [ORNL; Tang, G [University of New Mexico, Albuquerque; Shen, Y-L [University of New Mexico, Albuquerque

2010-01-01

59

Residual Stress Characterization of Al/SiC Nanoscale Multilayers using X-ray  

SciTech Connect

Nanolayered composites are used in a variety of applications such as wear resistant coatings, thermal barrier coatings, optical and magnetic thin films, and biological coatings. Residual stresses produced in these materials during processing play an important role in controlling their microstructure and properties. In this paper, we have studied the residual stresses in model metal-ceramic Al/SiC nanoscale multilayers produced by physical vapor deposition (magnetron sputtering). X-ray synchrotron radiation was used to measure stresses in the multilayers using the sin{sup 2} {psi} technique. The stresses were evaluated as a function of layer thicknesses of Al and SiC and also as a function of the number of layers. The stress state of Al in the multilayer was largely compressive, compared to single layer Al stresses. This is attributed to a peening mechanism due to bombardment of the Al layers by SiC and Ar neutrals during deposition. The stress evolution was numerically modeled by a simplified peening process to qualitatively explain the Al thickness-dependent residual stresses.

D Singh X Deng; N Chawla; J Bai; C Hubbard; G Tang; Y Shen

2011-12-31

60

Measurement of the residual stress field in MIG-welded Al2024 and Al7150 aluminium alloy compact tension specimens  

Microsoft Academic Search

Neutron diffraction has been used to measure the residual-stress field in MIG-welded Al-2024 and Al-7150 aluminium alloy compact-tension (CT) specimens. The measurements were made on the POLDI time-of-flight diffractometer, which uses a multiple frame overlap method. Strains in the three principal directions of each specimen were determined from the measured lattice plane spacing as a function of position through the

S. Pratihar; V. Stelmukh; M. T. Hutchings; M. E. Fitzpatrick; U. Stuhr; L. Edwards

2006-01-01

61

Electrochemical behaviour and corrosion performance of MgLiAlZn anodes with high Al composition  

Microsoft Academic Search

This study investigated the electrochemical and corrosion performance of MgLiAlZn anodes with Al compositions of ?3 wt.% and ?9 wt.%. MgLiAlZn alloy with ?9 wt.% Al had a relatively negative open-circuit potential and a high discharge voltage in MgCl2 electrolyte, owing to the distribution of numerous AlLi particles in the matrix of the alloy. AlLi particles were believed to transform

M. C. Lin; C. Y. Tsai; J. Y. Uan

2009-01-01

62

Utilizing various test methods to study the stress corrosion behavior of Al-Li-Cu alloys  

NASA Technical Reports Server (NTRS)

Recently, much attention has been given to aluminum-lithium alloys because of rather substantial specific-strength and specific-stiffness advantages offered over commercial 2000and 7000-series aluminum alloys. An obstacle to Al-Li alloy development has been inherent limited ductility. In order to obtain a more refined microstructure, powder metallurgy (P/M) has been employed in alloy development programs. As stress corrosion (SC) of high-strength aluminum alloys has been a major problem in the aircraft industry, the possibility of an employment of Al-Li alloys has been considered, taking into account a use of Al-Li-Cu alloys. Attention is given to a research program concerned with the evaluation of the relative SC resistance of two P/M processed Al-Li-Cu alloys. The behavior of the alloys, with and without an addition of magnesium, was studied with the aid of three test methods. The susceptibility to SC was found to depend on the microstructure of the alloys.

Pizzo, P. P.; Galvin, R. P.; Nelson, H. G.

1984-01-01

63

Modelling Of Residual Stresses Induced By High Speed Milling Process  

NASA Astrophysics Data System (ADS)

Maintenance processes used in heavy industries often include high speed milling operations. The reliability of the post-process material state has to be studied. Numerical simulation appears to be a very interesting way to supply an efficient residual stresses (RS) distribution prediction. Because the adiabatic shear band and the serrated chip shaping are features of the austenitic stainless steel high speed machining, a 2D high speed orthogonal cutting model is briefly presented. This finite element model, developed on Forge software, is based on data taken from Outeiro & al.'s paper [1]. A new behaviour law fully coupling Johnson-Cook's constitutive law and Latham and Cockcroft's damage model is detailed in this paper. It ensures results that fit those found in literature. Then, the numerical tools used on the 2D model are integrated to a 3D high speed milling model. Residual stresses distribution is analysed, on the surface and into the depth of the material. Various revolutions and passes of the two teeth hemispheric mill on the workpiece are simulated. Thus the sensitivity of the residual stresses generation to the cutting conditions can be discussed. In order to validate the 3D model, a comparison of the cutting forces measured by EDF R&D to those given by numerical simulations is achieved.

Desmaison, Olivier; Mocellin, Katia; Jardin, Nicolas

2011-05-01

64

Characterisation of stress corrosion cracking (SCC) of MgAl alloys  

Microsoft Academic Search

Stress corrosion cracking (SCC) of the MgAl alloys AZ91, AZ31 and AM30 in distilled water has been characterised using constant extension rate tests (CERTs) and linearly increasing stress tests (LISTs). AZ91 consists of an ?-matrix with extensive ?-particles, whereas AZ31 and AM30 consist only of an ?-matrix with an Al-concentration similar to that in AZ91. The presence of ?-particles in

N. Winzer; A. Atrens; W. Dietzel; V. S. Raja; G. Song; K. U. Kainer

2008-01-01

65

Low-energy deposition of high-strength Al(0) alloys from an ECR plasma  

SciTech Connect

Low-energy deposition of Al(O) alloys from an electron cyclotron resonance (ECR) plasma offers a scaleable method for the synthesis of thick, high-strength Al layers. This work compares alloy layers formed by an ECR-0{sub 2} plasma in conjunction with Al evaporation to 0-implanted Al (ion energies 25-200 keV); and it examines the effects of volume fraction of A1{sub 2}0{sub 3} phase and deposition temperature on the yield stress of the material. TEM showed the Al(O) alloys contain a dense dispersion of small {gamma}-Al{sub 2}0{sub 3} precipitates ({approximately}l nm) in a fine-grain (10-100 nm) fcc Al matrix when deposited at a temperature of {approximately}100C, similar to the microstructure for gigapascal-strength 0-implanted Al. Nanoindentation gave hardnesses for ECR films from 1.1 to 3.2 GPa, and finite-element modeling gave yield stresses up to 1.3 {plus_minus} 0.2 GPa with an elastic modulus of 66 GPa {plus_minus} 6 GPa (similar to pure bulk Al). The yield stress of a polycrystalline pure Al layer was only 0.19 {plus_minus} 0.02 GPa, which was increased to 0.87 {plus_minus} 0.15 GPa by implantation with 5 at. % 0.

Barbour, J.C.; Follstaedt, D.M.; Knapp, J.A.; Myers, S.M. [Sandia National Labs., Albuquerque, NM (United States); Marshall, D.A.; Lad, R.J. [Univ. of Maine, Orono, ME (United States)

1995-12-31

66

Bending fatigue tests on SiC-Al tapes under alternating stress at room temperature  

NASA Technical Reports Server (NTRS)

The development of a testing method for fatigue tests on SiC-Al tapes containing a small amount of SiC filaments under alternating stress is reported. The fatigue strength curves resulting for this composite are discussed. They permit an estimate of its behavior under continuous stress and in combination with various other matrices, especially metal matrices.

Herzog, J. A.

1981-01-01

67

Stress Wave attenuation in SiC3D/Al Composite  

NASA Astrophysics Data System (ADS)

SiC3D/Al composite is a kind of special composite with interpenetrating network microstructure. The attenuation properties of stress wave propagation along the SiC3D/Al composite are studied by a Split Hopkinson Pressure Bar system & FEM simulations, and the attenuation mechanism is discussed in this paper. Results show that the attenuation rate of the stress wave in the composite is up to 1.73MPamm-1. The reduction of the amplitude of waves is caused by that plenty of interfaces between SiC and Al within the composite acting with stress waves. When the incident plane wave reaches the SiC3D/Al interface, reflection wave and transmission wave propagates in different directions along the irregular interface between SiC phase and aluminium phase due to the impedance mismatch of them, which leads to the divergence of stress wave. At the same time, some stress micro-focuses occurs in the aluminium phase for the complex wave superimposition, and some plastic deformation may take place within such micro-regions, which results in the consumption of stress wave energy. In conclusion, the stress wave attenuation is derived from divergence and consumption of stress wave.

Chunyuan, Yuan; Yangwei, Wang; Guoju, Li; Xu, Zhang; Jubin, Gao

2013-03-01

68

Stress corrosion cracking and hydrogen embrittlement of an Al-Zn-Mg-Cu alloy  

SciTech Connect

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al-Zn-Mg-Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al-Zn-Mg-Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al-Zn-Mg-Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement.

Song, R.G.; Dietzel, W.; Zhang, B.J.; Liu, W.J.; Tseng, M.K.; Atrens, A

2004-09-20

69

The Influence of Al-Anon on Stress of Wives of Alcoholics.  

ERIC Educational Resources Information Center

This study assessed Alcoholics Anonymous (Al-Anon) participation as a factor in stress of wives of alcoholics. Additional data focused on attitude and behavior variables. Two groups of 20 subjects each were enlisted from Al-Anon, personal contacts, treatment centers, and referrals in three urban areas in the southeastern United States. Group A

McGregor, Phyllis W.

70

Internal stresses induced by plastic shear deformation of Zr(Cu,Ag)Al bulk metallic glasses  

Microsoft Academic Search

Effective internal shear stress ?i induced by torsional deformation of Zr46(Cu4\\/5Ag1\\/5)46Al8 and Zr46Cu46Al8 bulk metallic glasses different by the glass-forming ability of the maternal melts has been determined by measurements of stress relaxation upon stepwise unloading. It has been found that the ratio ?i\\/?0 (?0 is the initially applied shear stress) decreases upon increasing the temperature from ?0.8 at T=450K

G. V. Afonin; S. V. Khonik; R. A. Konchakov; N. P. Kobelev; A. A. Kaloyan; V. A. Khonik

71

Micromechanical stresses in SiC-reinforced Al2O3 composites  

NASA Technical Reports Server (NTRS)

Applying an Eshelby (1957) approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al2O3 by Predecki, et al. (in press) and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction.

Li, Zhuang; Bradt, Richard C.

1989-01-01

72

Thermal residual strains and stresses in Al{sub 2}O{sub 3}/Al composites with interpenetrating networks  

SciTech Connect

Thermal expansion data for Al{sub 2}O{sub 3}/Al interpenetrating network composites, obtained using a dilatometer, are analyzed to determine the residual stress during, and after, thermal cycling between room temperature and 600 C. A rigorous technique, using the effective medium approximation method, is applied, utilizing unconstrained thermal strains of the individual phases which are determined experimentally. Coefficients of thermal expansion are predicted as a function of metal content and temperature. Residual stresses are calculated using two approaches: (1) a macroscopic approach which considers total composite strain and (2) a micromechanical approach which considers time dependent effects in the metal phase. Predictions agree well with experimental data and neutron diffraction measurements, and provide a mechanistic understanding of the thermo-mechanical behavior of the material.

Hoffman, M. [Univ. of Technology, Darmstadt (Germany). Dept. of Materials Science] [Univ. of Technology, Darmstadt (Germany). Dept. of Materials Science; [Univ. of New South Wales, Sydney, New South Wales (Australia). School of Materials Science and Engineering; Skirl, S.; Roedel, J. [Univ. of Technology, Darmstadt (Germany). Dept. of Materials Science] [Univ. of Technology, Darmstadt (Germany). Dept. of Materials Science; Pompe, W. [Technical Univ. Dresden (Germany). Dept. of Materials Science] [Technical Univ. Dresden (Germany). Dept. of Materials Science

1999-01-15

73

Effects of oxide thickness, Al 2O 3 interlayer and interface asperity on residual stresses in thermal barrier coatings  

Microsoft Academic Search

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat\\/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al2O3 interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in

X. C. Zhang; B. S. Xu; H. D. Wang; Y. X. Wu

2006-01-01

74

Determination and Relaxation of Residual Stress in 2024 Al-30 vol.% Magnesium Borate Whisker Composites  

NASA Astrophysics Data System (ADS)

Residual stresses in 30 vol.% magnesium borate whisker-reinforced 2024 aluminum matrix composites have been determined by a nanoindentation method which takes into consideration pile-up and sink-in effects on indentation contact depth. Owing to the thermal mismatch and the large difference in elasticity modulus between the Al matrix and MBO whiskers, tensile residual stress was introduced to Al matrix material during fabrication. It was found that the solution treatment reduced the tensile residual stress by producing interfacial component and dislocations in the composites. Cryogenic cooling released the stress via reversing the tensile residual stress to compression in the matrix, which was more effective than solution treatment to release the tension stress in the composites. The combination of the solution treatment and the cryogenic cooling provided the most effective procedure to release the residual stress in the composites, which reduced the tensile residual stress from 232.6 to 56.5 MPa, i.e., 76% reduction. Meanwhile, no cracks were observed in the composite when processed with such sudden thermal shocking.

Wang, Zhijiang; Jiang, Zhaohua; Hu, Henry; Nie, Xueyuan

2013-10-01

75

Oxidative stress is an early symptom triggered by aluminum in Al-sensitive potato plantlets.  

PubMed

The objective of this study was to evaluate whether the oxidative stress caused by aluminum (Al) toxicity is an early symptom that can trigger root growth inhibition in Macaca (Al-sensitive) and SMIC148-A (Al-tolerant) potato clones. Plantlets were grown in a nutrient solution (pH 4.00) with 0, 100 and 200mg Al L(-1). At 24, 72, 120 and 168h after Al addition, root length and biochemical parameters were determined. Regardless of exposure time, root length of the Macaca clone was significantly lower at 200mg Al L(-1). For the SMIC148-A clone, root length did not decrease with any Al treatments. Al supply caused lipid peroxidation only in Macaca, in both roots (at 24, 72, 120 and 168h) and shoot (at 120 and 168h). In roots of the Macaca, catalase (CAT) and ascorbate peroxidase (APX) activity decreased at 72 and 120h, and at 24, 72 and 120h, respectively. At 168h, both activities increased upon addition of Al. In roots of the SMIC148-A, CAT activity increased at 72 and 168h, whereas APX activity decreased at 72h and increased at 24, 12 and 168h. The Macaca showed lower root non-protein thiol group (NPSH) concentration at 200mg Al L(-1) in all evaluations, but the SMIC148-A either did not demonstrate any alterations at 24 and 72h or presented higher levels at 120h. This pattern was also observed in root ascorbic acid (AsA) concentration at 24 and 120h. The cellular redox status of these potato clones seems to be affected by Al. Therefore, oxidative stress may be an important mechanism for Al toxicity, mainly in the Al-sensitive Macaca clone. PMID:19570563

Tabaldi, Luciane Almeri; Cargnelutti, Denise; Gonalves, Jamile Fabbrin; Pereira, Luciane Belmonte; Castro, Gabriel Y; Maldaner, Joseila; Rauber, Renata; Rossato, Liana Vernica; Bisognin, Dilson Antnio; Schetinger, Maria Rosa Chitolina; Nicoloso, Fernando Teixeira

2009-09-01

76

The stress state dependence of cavitation in commercial superplastic Al5083 allloy  

Microsoft Academic Search

The cavitation behaviour of superplastic alloys has relatively been less examined under biaxial tension, though it has been demonstrated by some researchers that stress state does affect the cavitation behaviour. In this paper, a commercial superplastic Al5083 alloy was investigated under stress ratios of 1, 0.8293, 0.6667, 0.5985 and 0. It is found that at the same effective strain, the

K. C. Chan; K. K. Chow

2002-01-01

77

Experimental research on unloading properties of clay under high stress  

Microsoft Academic Search

Mechanical properties of clay under high stress are quite different from those under low stress. It is necessary to investigate unloading properties of clay under high stress for the design and construction of deep underground engineering projects. A series of experiments were conducted to investigate the unloading properties of clay under high confining pressures by using a SKA-1 high pressure

Jin-rong MA; Guang-xin CUI; Yong QIN; Guo-qing ZHOU

2008-01-01

78

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys  

NASA Astrophysics Data System (ADS)

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity, K ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity, K,ISCC, decreases and the crack propagation rate, da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.

Lee, Seong-Min; Pyun, Su-Il; Chun, Young-Gab

1991-10-01

79

High-performance nonvolatile Al/AlOx/CdTe:Sb nanowire memory device  

NASA Astrophysics Data System (ADS)

Here we demonstrate a room temperature processed nonvolatile memory device based on an Al/AlOx/CdTe:Sb nanowire (NW) heterojunction. Electrical analysis shows an echelon hysteresis composed of a high-resistance state (HRS) and a low-resistance state (LRS), which can allow it to write and erase data from the device. The conductance ratio is as high as 106, with a retention time of 3 104 s. Moreover, the SET voltages ranged from +6 to +8 V, whilst the RESET voltage ?0 V. In addition, flexible memory nano-devices on PET substrate with comparable switching performance at bending condition were fabricated. XPS analysis of the Al/AlOx/CdTe:Sb NW heterojunction after controlled Ar+ bombardment reveals that this memory behavior is associated with the presence of ultra-thin AlOx film. This Al/AlOx/CdTe:Sb NW heterojunction will open up opportunities for new memory devices with different configurations.

Xie, Chao; Nie, Biao; Zhu, Long; Zeng, Long-Hui; Yu, Yong-Qiang; Wang, Xian-He; Fang, Qun-Ling; Luo, Lin-Bao; Wu, Yu-Cheng

2013-09-01

80

High permittivity Li and Al doped NiO ceramics  

Microsoft Academic Search

High-permittivity dielectric NiO-based ceramics co-doped with Li and Al (LANO) have been prepared by a traditional solid-state synthesis. Analyses of the ceramic microstructure and composition indicate that Al ions are distributed in grain boundaries, and that uniform boundaries indexed as NiAl2O4 surround the grains. The concentration of Al has a remarkable effect on the dielectric properties of the LANO ceramics.

Yuanhua Lin; Jianfei Wang; Lei Jiang; Yu Chen; Ce-Wen Nan

2004-01-01

81

Residual stress in high modulus carbon fibers  

NASA Technical Reports Server (NTRS)

The modulus and residual strain in carbon fibers are measured by successively electrochemically milling away the fiber surface. Electrochemical etching is found to remove the carbon fiber surface very uniformly, in contrast to air and wet oxidation. The precision of fiber diameter measurements is improved by using a laser diffraction technique instead of optical methods. More precise diameter measurements reveal that past correlations of diameter and fiber modulus are largely measurement artifact. The moduli of most carbon fibers decrease after the outer layers of the fibers are removed. Owing to experimental difficulties, the moduli and strengths of the fibers at their centers are not determined, and moduli are estimated on the basis of microstructure. The calculated residual stresses are found to be insensitive to these moduli estimates as well as the exact form of regression equation used to describe the moduli and residual strain distributions. Axial compressive residual stresses are found to be very high for some higher modulus carbon fibers. It is pointed out that the compressive stress makes the fibers insensitive to surface flaws when loaded in tension but it may initiate failure by buckling when loaded in compression.

Chen, K. J.; Diefendorf, R. J.

1982-01-01

82

Nanosintering mechanism of MgAl 2O 4 transparent ceramics under high pressure  

Microsoft Academic Search

Nanosintering behaviour of MgAl2O4 ultra-fine powder was investigated under high pressure. We found that the nanosintering mechanism under high pressure was quite different from that under ambient pressure. To shed light on the mechanism, the residual stress, grain size, and boundary situation were investigated using HRTEM and XRD, respectively. Our results show high pressure can restrain grain growth and initiate

Yongtao Zou; Duanwei He; Xiankui Wei; Richeng Yu; Tiecheng Lu; Xianghui Chang; Shanmin Wang; Li Lei

2010-01-01

83

Modeling stress state dependent damage evolution in a cast AlSiMg aluminum alloy  

Microsoft Academic Search

Internal state variable rate equations are cast in a continuum framework to model void nucleation, growth, and coalescence in a cast AlSiMg aluminum alloy. The kinematics and constitutive relations for damage resulting from void nucleation, growth, and coalescence are discussed. Because damage evolution is intimately coupled with the stress state, internal state variable hardening rate equations are developed to distinguish

M. F. Horstemeyer; J. Lathrop; A. M. Gokhale; M. Dighe

2000-01-01

84

Microstructural Characterization and Stress Corrosion Cracking Susceptibility of Al-Zn-Mg-Ti Alloys.  

National Technical Information Service (NTIS)

The effects of adding small amounts of titanium to age-hardening Al-Zn-Mg alloys have been studied by transmission electron microscopy, mechanical atests, and stress corrosion cracking (SCC) tests. The zinc, magnesium, and titanium content of the alloys w...

C. Chen, G. Judd

1974-01-01

85

ALS Multicenter Cohort Study of Oxidative Stress (ALS COSMOS): Study methodology, recruitment, and baseline demographic and disease characteristics.  

PubMed

Abstract In a multicenter study of newly diagnosed ALS patients without a reported family history of ALS, we are prospectively investigating whether markers of oxidative stress (OS) are associated with disease progression. Methods utilize an extensive structured telephone interview ascertaining environmental, lifestyle, dietary and psychological risk factors associated with OS. Detailed assessments were performed at baseline and at 3-6 month intervals during the ensuing 30 months. Our biorepository includes DNA, plasma, urine, and skin. Three hundred and fifty-five patients were recruited. Subjects were enrolled over a 36-month period at 16 sites. To meet the target number of subjects, the recruitment period was prolonged and additional sites were included. Results showed that demographic and disease characteristics were similar between 477 eligible/non-enrolled and enrolled patients, the only difference being type of health insurance among enrolled patients. Sites were divided into three groups by the number of enrolled subjects. Comparing these three groups, the Columbia site had fewer 'definite ALS' diagnoses. This is the first prospective, interdisciplinary, in-depth, multicenter epidemiological investigation of OS related to ALS progression and has been accomplished by an aggressive recruitment process. The baseline demographic and disease features of the study sample are now fully characterized. PMID:24564738

Mitsumoto, Hiroshi; Factor-Litvak, Pam; Andrews, Howard; Goetz, Raymond R; Andrews, Leslie; Rabkin, Judith G; McElhiney, Martin; Nieves, Jeri; Santella, Regina M; Murphy, Jennifer; Hupf, Jonathan; Singleton, Jess; Merle, David; Kilty, Mary; Heitzman, Daragh; Bedlack, Richard S; Miller, Robert G; Katz, Jonathan S; Forshew, Dallas; Barohn, Richard J; Sorenson, Eric J; Oskarsson, Bjorn; Fernandes Filho, J Americo M; Kasarskis, Edward J; Lomen-Hoerth, Catherine; Mozaffar, Tahseen; Rollins, Yvonne D; Nations, Sharon P; Swenson, Andrea J; Shefner, Jeremy M; Andrews, Jinsy A; Koczon-Jaremko, Boguslawa A

2014-06-01

86

Growth strains and stress relaxation in alumina scales during high temperature oxidation  

SciTech Connect

A novel X-ray technique was used, exploiting synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory, to investigate the growth stresses in {alpha}-Al{sub 2}O{sub 3}. In-situ measurements of Debye-Scherrer diffraction patterns from the scale were recorded during oxidation and cooling, and the elliptical distortion of the diffraction rings was analyzed to yield the in-plane strain. Fe-28Al, Fe-40Al, Fe-40Al-0.2Hf, Fe-20Cr-10Al and Ni-50Al (at. %) were studied. Data were acquired in air at temperatures between 950-1100 C and during cool down. In all cases, the steady stage growth strain was relatively low (<0.1%) and was either tensile or compressive depending on the alloy. A higher tensile strain often existed during the initial oxidation period when transition alumina was present. Thermal stresses imposed on NiAl by reducing the sample temperature to 950 C for a period of time showed noticeable stress relaxation by creep. Different degrees of relaxation were also found during cooling depending on alloy composition and scale microstructure. On all Fe-based alloys, the first formed {alpha}-Al{sub 2}O{sub 3} was highly textured with the degree of texture decreasing with further oxidation. The relationships between stress development, scale wrinkling, oxide phase changes, and the effect of reactive element addition on growth stresses are discussed. Results are compared with other reports of growth stresses in Al{sub 2}O{sub 3} scales.

Hou, P.Y.; Paulikas, A.P.; Veal, B.W.

2004-03-23

87

NiAl-Base Composite Containing High Volume Fraction of AIN Particulate for Advanced Engines  

NASA Technical Reports Server (NTRS)

Cryomilling of prealloyed NiAl containing 53 at. % AJ was carried out to achieve high nitrogen levels. The consolidation of cryomilled powder by extrusion or hot pressing/ hot isostatic pressing resulted in a fully dense NiAl-base composite containing 30 vol. % of inhomogeneously distributed, nanosized AIN particulate. The NiAl-30AIN composite exhibited the highest compression yield strengths at all temperatures between 300 and 1300 K as compared with other compositions of NiAl-AIN composite. The NiAl-30AIN specimens tested under compressive creep loading between 1300 and 1500 K also exhibited the highest creep resistance with very little surface oxidation indicating also their superior elevated temperature oxidation resistance. In the high stress exponent regime, the strength is proportional to the square root of the AIN content and in the low stress exponent regime, the influence of AIN content on strength appears to be less dramatic. The specific creep strength of this material at 1300 K is superior to a first generation Ni-base single crystal superalloy. The improvements in elevated temperature creep strength and oxidation resistance have been achieved without sacrificing the room temperature fracture toughness of the NiAl-base material. Based on its attractive combination of properties, the NiAl-30AIN composite is a potential candidate for advanced engine applications,

Hebsur, Mohan G.; Whittenberger, J. D.; Lowell, C. E.; Garg, A.

1995-01-01

88

Evolution of residual stresses in micro-arc oxidation ceramic coatings on 6061 Al alloy  

NASA Astrophysics Data System (ADS)

Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses. However, residual stresses in the surface coatings of structural components have adverse effects on their properties, such as fatigue life, dimensional stability and corrosion resistance, etc. The micro-arc oxidation ceramic coatings are produced on the surfaces of 6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW. A constant current density of 4.40.1 A/dm2 and a self-regulated composite electrolyte are used. The micro-arc oxidation treatment period ranges from 10 min to 40 min, and the thickness of the ceramic coatings is more than 20 ?m. Residual stresses attributed to ?-Al2O3 constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2 ? method. The analysis results show that the residual stress in the ceramic coatings is compressive in nature, and it increases first and then decreases with micro-arc oxidation time increase. The maximum stress value is 1 66720 MPa for period of 20 min. Through analyzing the coating thickness, surface morphology and phase composition, it is found that the residual stress in the ceramic coatings is linked closely with the coating growth, the phase composition and the micro cracks formed. It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress. The heat treatment makes the residual compressive stress value decrease 1 378 MPa. The ultrasonic action even alters the nature of the residual stress, making the residual compressive stress change into a residual tensile stress.

Shen, Dejiu; Cai, Jingrui; Guo, Changhong; Liu, Peiyu

2013-11-01

89

Time-dependent stress concentration and microcrack nucleation in TiAl  

SciTech Connect

Localized stress evolution associated with the interaction of slip or twinning with an interface is treated by means of a superposition of the {open_quotes}internal loading{close_quotes} of a crystalline subsystem by dynamic dislocation pile-up and the stress relaxation by climb of interfacial dislocations. The peak value of a stress concentration factor depends on both the angular function that includes the effect of mode mixity and the ratio of characteristic times for stress relaxation and internal loading. The available experimental data on orientation and strain-rate dependences of interfacial fracture mode in polysynthetically twinned TiAl crystals are discussed in view of the theoretical concepts presented in this paper.

Yoo, M.H.

1995-07-01

90

Influence of Ti/TiAlN-multilayer designs on their residual stresses and mechanical properties  

NASA Astrophysics Data System (ADS)

In this research work, Ti/TiAlN multilayers of various designs were deposited onto substrates pretreated by different etching procedures. The influence of multilayer design and substrate pretreatment on multilayers adhesion, hardness, wear and friction coefficients was systematically analyzed and correlated with residual stresses of these multilayers as well as with residual stresses on the coating-near substrate region, which were analyzed by synchrotron X-ray diffraction at HZB-BESSYII. These investigations show that the adhesion can be improved by a specific etching procedure, which cause increased compressive stress in the coating-near the substrate region. Additionally, it was found, that the multilayer with the thickest ceramic layers has the highest hardness and the lowest wear coefficients as well as the lowest compressive residual stress within studied multilayers.

Vogli, E.; Tillmann, W.; Selvadurai-Lassl, U.; Fischer, G.; Herper, J.

2011-08-01

91

High permittivity Li and Al doped NiO ceramics  

NASA Astrophysics Data System (ADS)

High-permittivity dielectric NiO-based ceramics co-doped with Li and Al (LANO) have been prepared by a traditional solid-state synthesis. Analyses of the ceramic microstructure and composition indicate that Al ions are distributed in grain boundaries, and that uniform boundaries indexed as NiAl2O4 surround the grains. The concentration of Al has a remarkable effect on the dielectric properties of the LANO ceramics. The dielectric constant remains almost constant (~104-105) at low frequency and has a steplike decrease toward higher frequencies. The high-dielectric-constant response of the LANO ceramics is mainly enhanced by Maxwell-Wagner polarization.

Lin, Yuanhua; Wang, Jianfei; Jiang, Lei; Chen, Yu; Nan, Ce-Wen

2004-12-01

92

Breakdown During High-Field Bias-Temperature Stress.  

National Technical Information Service (NTIS)

Measurements of dielectric breakdown during high-field electrical stress are typically performed at or near room temperature via constant voltage or current stress methods. In this summary they explore whether useful information might also be obtained by ...

D. M. Fleetwood K. S. Krisch F. W. Sexton

1999-01-01

93

Novel high-strength ternary Zr-Al-Sn alloys with martensite structure for nuclear applications  

NASA Astrophysics Data System (ADS)

High strength is essential for the practical application of Zr alloys as structural materials. In this work, Zr-5Al-xSn (x = 2, 3, 4, 5 and 6) alloys have been designed and fabricated through arc melting in order to effectively improve the strength while retaining good ductility. Phase analysis results show that all the samples consist of single phase ?-Zr. The variation trend of lattice constants as a function of Sn content has been analyzed. The microstructural analysis indicates that the Zr-5Al-xSn alloys mainly contain martensite structure. Mechanical tests show that these Zr-5Al-xSn alloys exhibit high compressive strength (1250-1450 MPa), high yield stress (800-1000 MPa), and favorable plastic strain of 18-23%. The fracture mode has been experimentally analyzed. Finally, both Zr-5Al-3Sn and Zr-5Al-5Sn are subjected to heat treatments for further study on the roles of Sn element and controlled heat treatment on the microstructure and mechanical properties of Zr alloys. Sn is found to promote the formation of ZrAl in the Zr-5Al-xSn alloys. Moreover, the martensite laths are observed to evolve into larger strip grains and fine equiaxed grains after heat treatment at 900 C for 2 h. These factors strengthen the Zr-5Al-xSn alloys.

Nie, Li; Zhan, Yongzhong; Hu, Tong; Chen, Xiaoxian; Wang, Chenghui

2013-11-01

94

Mechanical Properties of High Strength Al-Mg Alloy Sheet  

NASA Astrophysics Data System (ADS)

The aim of this research is to develop the high strength Al alloy sheet for the automotive body. For the fabrication Al-Mg alloy sheet, the composition of alloying elements was designed by the properties database and CALPHAD (Calculation Phase Diagram) approach which can predict the phases during solidification using thermodynamic database. Al-Mg alloys were designed using CALPHAD approach according to the high content of Mg with minor alloying elements. After phase predictions by CALPHAD, designed Al-Mg alloys were manufactured. Addition of Mg in Al melts were protected by dry air/Sulphur hexafluoride (SF6) mixture gas which can control the severe Mg ignition and oxidation. After rolling procedure of manufactured Al-Mg alloys, mechanical properties were examined with the variation of the heat treatment conditions.

Choi, Bong-Jae; Hong, Kyung-Eui; Kim, Young-Jig

95

Numerical models for thermal residual stresses in Al 2O 3 platelets\\/borosilicate glass matrix composites  

Microsoft Academic Search

In this paper, we model and numerically study thermal residual stresses in a borosilicate glass reinforced with Al2O3 platelets. This composite material was experimentally characterized in a previous work. The aim of this paper is to investigate further the thermal residual stresses in these composites that are responsible for toughening and to assess the variation of the thermal residual stresses

Valeria Cannillo; Cristina Leonelli; Aldo R Boccaccini

2002-01-01

96

Measuring stress next to Au ball bond during high temperature aging  

Microsoft Academic Search

A real-time signal of the stress caused by a ball bond is recorded during long-term high temperature storage (HTS) without destroying the ball bond, using a piezoresistive integrated CMOS microsensor located next to the Al bond pad (test pad) on a test chip. The sensor is sensitive to in-plane shear stress changes ??xy that arise due to tensile or compressive

M. Mayer; J. T. Moon; J. Persic

2009-01-01

97

Effect of several stress inducing coatings on magnetostriction property of a high induction 3% silicon steel  

Microsoft Academic Search

The stress inducing behavior of various coatings by applying coating solutions mainly composed of colloidal silica and one of (Mg,Ca,Sr,Ba,Fe,Zn and Al) phosphates onto high permeability 3% silicon steel sheet (RG-H) were examined. The results of magnetostriction measurements showed that the coating formed by colloidal silica and Mg phosphate mixed solution gave the largest tensile stress to the steel sheets

H. Shimanaka; T. Ichida; S. Kobayashi; T. Funahashi

1979-01-01

98

Low-frequency noise measurements of electrical stress in InAlN/GaN and AlGaN/GaN heterostructure field-effect transistors  

NASA Astrophysics Data System (ADS)

We report on the low-frequency noise (LFN) measurements on GaN based heterostructure field-effect transistors (HFETs) on sapphire with InAlN and AlGaN barriers to investigate the effects of electrical stress. The HFETs with InAlN barrier undergone a DC stress at bias conditions of VDS=20V and VG= -4.5 for up to 4 hours in aggregate. These devices exhibited an LFN in the form of 1/f? and a significant increase in the noise spectrum up to 15 dB for 2 hours and then the noise saturated for further stress durations. We also monitored the LFN for the HFETs with AlGaN barriers. The devices were stressed by applying 20V DC drain bias for up to 64 hours at various gate voltages. Stressing at a gate bias (VG) of -2V showed negligible degradation. On the other hand, stressing at VG=0V surprisingly reduced the noise power by about 4 to 15 dB in the frequency range of 1 Hz-100 kHz. Additionally, the InAlN-barrier HFETs exhibited 20-25 dB lower noise power than the ones with the AlGaN layer for the tested devices within the entire frequency range. The results suggest that the trap generation increases due to electrical stress in devices with InAlN barrier, whereas the noise power decreases as a function of stress in AlGaN/GaN HFETs due to an increase in the activation energy of the excess traps.

Kayis, Cemil; Zhu, C. Y.; Wu, Mo; Li, Xing; zgr, mit; Morko, Hadis

2011-02-01

99

The rate dependence of the saturation flow stress of Cu and 1100 Al  

SciTech Connect

The strain-rate dependence of the saturation flow stress of OFHC Cu and 1100 Al from 10{sup {minus}3}s{sup {minus}1} to nearly to 10{sup 12}s{sup {minus}1} is examined. The flow stress above 10{sup 9}s{sup {minus}1} is estimated using Wallace's theory of overdriven shocks in metals. A transition to the power-law behavior {Psi} {approximately} {tau}{sub s}{sup 5} occurs at a strain rate of order 10{sup 5}s{sup {minus}1}. 10 refs., 2 figs.

Preston, D.L.; Tonks, D.L.; Wallace, D.C.

1991-01-01

100

Suppression of Crack Generation Using High-Compressive-Strain AlN/Sapphire Template for Hydride Vapor Phase Epitaxy of Thick AlN Film  

NASA Astrophysics Data System (ADS)

A stress control method was proposed to suppress crack generation in the hydride vapor phase epitaxy (HVPE) of a thick AlN film on an AlN/sapphire template, i.e., a high-compressive-strain (HCS) AlN/sapphire template was employed as a substrate. The AlN films on the HCS AlN/sapphire template were crack-free and very smooth. However, the AlN films on the normal AlN/sapphire template (with a reduced or fully relaxed compressive strain) contained many cracks. This is because the HCS in the AlN/sapphire template may balance some of the thermal tensile strain resulting from the different thermal expansion coefficients of AlN and sapphire and thus suppress the crack generation. The typical full width at half maximum (FWHM) values of X-ray rocking curves for the (0002), (10\\bar{1}2), and (10\\bar{1}0) diffractions of the HVPE-grown AlN film on the HCS template were 127, 381, and 520 arcsec, respectively. Accordingly, by atomic force microscopy (AFM), clear atomic steps were observed on the HVPE-grown AlN films on the HCS template, and the root-mean-square (RMS) roughness value of the AlN films was 0.361 nm.

Tsujisawa, Kenichi; Kishino, Shinya; Li, Da-Bing; Miyake, Hideto; Hiramatsu, Kazumasa; Shibata, Tomohiko; Tanaka, Mitsuhiro

2007-06-01

101

Stress corrosion cracking of sensitized AA5083 (Al4.5Mg1.0Mn)  

Microsoft Academic Search

The AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) alloy is a nonheat-treatable aluminum alloy known for its excellent corrosion resistance.\\u000a However, it can become susceptible to intergranular stress corrosion cracking (IGSCC) when exposed to temperatures ranging\\u000a from 50 C to 200 C for sufficient lengths of time. This IGSCC is widely believed to be associated with dissolution of the\\u000a electrochemically active ? phase, Al3Mg2, which

J. L. Searles; P. I. Gouma; R. G. Buchheit

2001-01-01

102

Commentary: Beyond stressful life events and depression? - reflections on Bogdan et al. ().  

PubMed

In light of continuing disagreement, even at the meta-analytic level, as to whether the gene-נ-environment (GE) interaction involving 5-HTTLPR and stressful life events (SLEs) predicts depression, Bogdan and associates (this issue, Bogdan etal., 2014) sought to extend research on what has become a highly controversial general (GxE) and specific (5HTTLPR X SLEs) arena of inquiry. Thus, rather than seeking to replicate this specific GXE interaction in another sample of adolescents or adults, these investigators shifted the developmental focus-to very young children, aged 3-5years of age. This re-direction was motivated by the kindling hypothesis which stipulates that the earliest episodes of depression might be especially sensitive to environmental adversity, with later episodes very much dependent on earlier ones and less a function of later-life environmental provocation. Thus, the investigators reasoned that the controversial GE interaction might actually prove more evident and exert a more pronounced impact early in childhood than at older ages where they have been so extensively studied. PMID:24708477

Belsky, Jay

2014-05-01

103

Stress-induced and thermoelastic properties of Ni-Fe-Al melt-spun ribbon  

NASA Astrophysics Data System (ADS)

Lattice parameters, elastic modulus, internal friction, superelasticity, and shape memory effect are studied in the vicinity of martensitic transformation (MT) exhibited by the Ni55Fe20Al25 melt-spun ribbon. In comparison to the melt-spun ribbons of other ferromagnetic shape memory alloys, Ni-Fe-Al ribbon has abnormally low elastic modulus, a record-breaking strength (>350 MPa), and large recoverable strain (up to 5%) due to both the superelastic and ordinary shape memory effects. The critical stress versus temperature phase diagram consists of two straight lines, indicating the formation of different stress-induced martensitic phases. The structural and thermodynamic results proved to be self-consistent.

Chernenko, V. A.; Kanth, B. Rajini; Mukhopadhyay, P. K.; Kaul, S. N.; Villa, E.; Gambardella, A.; Besseghini, S.

2008-10-01

104

Stress corrosion cracking susceptibilities of Fe 3 Al-based iron aluminides  

Microsoft Academic Search

The susceptibilities to stress corrosion cracking (SCC) were investigated using the constant-potential U-bend SCC test. U-bend\\u000a SCC evaluations were conducted on two iron aluminide compositions based on Fe3Al and containing 2 and 5 at.% Cr in acid-chloride (pH = 4200 ppm Cl), thiosulfate, and tetrathionate solutions at the freely-corroding\\u000a conditions. Cracking failures occurred in the thiosulfate and tetrathionate solutions, but

Jung-Gu Kim; Sung Kyun

1995-01-01

105

Stress corrosion crack velocity and grain boundary precipitates in an Al-Zn-Mg alloy  

Microsoft Academic Search

The fracture kinetics of Al-5.5 Zn-2.5 Mg alloys submersed in 3 pct NaCl-H2O solutions were varied by heat treatment. The steady state velocity, on a plot of velocity vs stress intensity, was compared\\u000a with microstructure and it was found to be inversely proportional to the volume of MgZn2 in the grain boundary. This behavior suggests that grain boundary precipitates can

P. K. Poulose; J. E. Morral; A. J. McEvily

1974-01-01

106

Stress corrosion cracking and hydrogen embrittlement of an AlZnMgCu alloy  

Microsoft Academic Search

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an AlZnMgCu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat

R. G. Song; W. Dietzel; B. J. Zhang; W. J. Liu; M. K. Tseng; A. Atrens

2004-01-01

107

Effect of microstructure on stress corrosion cracking of an Al-Zn-Mg-Cu alloy  

Microsoft Academic Search

The susceptibility to stress corrosion cracking (SCC) of the Al-Zn-Mg-Cu French 7010 alloy of different temper has been investigated by Slow Strain Rate Testing. The tests have been carried out at strain rate ranged between 10?5 and 10?7s?1, at corrosion potential and at cathodic polarisation, in NaCl and NaCl + As2O3 solution. The lowest susceptibility to SCC of overaged alloy

M. Puiggali; A. Zielinski; J. M. Olive; E. Renauld; D. Desjardins; M. Cid

1998-01-01

108

STRESS-CORROSION CRACKING OF Ti5Al2.5Sn  

Microsoft Academic Search

Tests were conducted in which Ti-5Al-2.5Sn alloy was heated to 1150 to ; 1500 deg F in the presence of shop-processing materials such as ; trichloroethylene, air, HF, HNO-HF solutions, and cutting oil. It was ; concluded that stress corrosion cracking of this alloy takes place in the ; presence of halides and oxygen during heat treatment. (J.R.D.);

1960-01-01

109

Residual stress in polycrystalline diamond\\/Ti?6Al?4V systems  

Microsoft Academic Search

Polycrystalline diamond coatings were deposited on Ti?6Al?4V alloy by HF-CVD, at fixed temperature (650 C) for different deposition times. During the process, thick titanium carbide layers were formed at the metal\\/diamond interface. X-ray diffraction (XRD) methods were used to assess coating quality, phase composition, texture, and residual macrostress of the diamond\\/TiC\\/Ti system. For a better evaluation of the residual stress

Paolo Scardi; Matteo Leoni; Giorgio Cappuccio; Vito Sessa; Maria Letizia Terranova

1997-01-01

110

Stresses in alumina scales on NiCrAlY-alloys determined by XSE and LRS  

Microsoft Academic Search

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated. For this purpose cyclic oxidation experiments on model alloys with different additions of silicon or titanium were carried out at temperatures between 950 and 1100 C. Selected samples were analysed by X-ray stress evaluation (XSE) after various times of oxidation.

Volker R. Vosberg; Michael G. Berger; Werner Fischer; Willem J. Quadakkers; Hubertus Nickel

1997-01-01

111

Effect of ion implantation on fracture stress of Al2O3  

Microsoft Academic Search

Effects of ion implantation and subsequent thermal annealing on the fracture stress of single-crystalline alpha-Al2O3 have been investigated for 400 keV N and 300 keV Ni ions. Upon N or Ni implantation, the flexural strength of a specimen 7 25 1 mm3 in size increased by 20 to 60% in the ion dose range from 1 1015

T. Hioki; A. Itoh; S. Noda; H. Doi; J. Kawamoto; O. Kamigaito

1985-01-01

112

High field Maxwell stress-strain characteristics of conventional polymers as actuators  

Microsoft Academic Search

Polymers such as silicone and acrylic elastomers can strain via Maxwell stress. Experiments were carried out to elucidate how the two kinds of polymers would respond to a high electric field. Several kinds of electrode materials were tested to find suitable compliant performances, i.e., Al foils, Cu sheets, conducting stick tapes, conducting rubbers, Ag conducting paste, carbon black (Ketjen black),

Toshikatsu Tanaka; M. Sato; Masahiro Kozako

2004-01-01

113

High-performance nonvolatile Al/AlO(x)/CdTe:Sb nanowire memory device.  

PubMed

Here we demonstrate a room temperature processed nonvolatile memory device based on an Al/AlO(x)/CdTe:Sb nanowire (NW) heterojunction. Electrical analysis shows an echelon hysteresis composed of a high-resistance state (HRS) and a low-resistance state (LRS), which can allow it to write and erase data from the device. The conductance ratio is as high as 10?, with a retention time of 3 10? s. Moreover, the SET voltages ranged from +6 to +8 V, whilst the RESET voltage ?0 V. In addition, flexible memory nano-devices on PET substrate with comparable switching performance at bending condition were fabricated. XPS analysis of the Al/AlO(x)/CdTe:Sb NW heterojunction after controlled Ar? bombardment reveals that this memory behavior is associated with the presence of ultra-thin AlO(x) film. This Al/AlO(x)/CdTe:Sb NW heterojunction will open up opportunities for new memory devices with different configurations. PMID:23924708

Xie, Chao; Nie, Biao; Zhu, Long; Zeng, Long-Hui; Yu, Yong-Qiang; Wang, Xian-He; Fang, Qun-Ling; Luo, Lin-Bao; Wu, Yu-Cheng

2013-09-01

114

Crystallization of MgAl and Al-based metallic liquids under ultra-high gravity  

Microsoft Academic Search

We present results from high-temperature centrifugal processing, which is a new method to detect the sequence of crystallization and to physically isolate eutectic compositions in multi-component alloys. Alloys of composition Mg50Al30Y6Li7Cu7, Mg50Al30Y6Li7Cu7, and Al52.6Cu13.4Ge28Si6 were processed for 2 h in a centrifuge above their melting point at an inertial acceleration of 60,000 g (g=gravitational acceleration) and then slowly cooled to

Jrg F Lffler; William L Johnson

2002-01-01

115

Nrf2/ARE Signaling Pathway: Key Mediator in Oxidative Stress and Potential Therapeutic Target in ALS  

PubMed Central

Nrf2 (nuclear erythroid 2-related factor 2) is a basic region leucine-zipper transcription factor which binds to the antioxidant response element (ARE) and thereby regulates the expression of a large battery of genes involved in the cellular antioxidant and anti-inflammatory defence as well as mitochondrial protection. As oxidative stress, inflammation and mitochondrial dysfunctions have been identified as important pathomechanisms in amyotrophic lateral sclerosis (ALS), this signaling cascade has gained interest both with respect to ALS pathogenesis and therapy. Nrf2 and Keap1 expressions are reduced in motor neurons in postmortem ALS tissue. Nrf2-activating compounds have shown therapeutic efficacy in the ALS mouse model and other neurodegenerative disease models. Alterations in Nrf2 and Keap1 expression and dysregulation of the Nrf2/ARE signalling program could contribute to the chronic motor neuron degeneration in ALS and other neurodegenerative diseases. Therefore, Nrf2 emerges as a key neuroprotective molecule in neurodegenerative diseases. Our recent studies strongly support that the Nrf2/ARE signalling pathway is an important mediator of neuroprotection and therefore represents a promising target for development of novel therapies against ALS, Parkinson's disease (PD), Huntington's disease (HD), and Alzheimer's disease (AD).

Petri, Susanne; Korner, Sonja; Kiaei, Mahmoud

2012-01-01

116

Defect Band Characteristics in Mg-Al and Al-Si High-Pressure Die Castings  

Microsoft Academic Search

Bands of positive macrosegregation and porosity commonly follow the surface contour of components produced by high-pressure\\u000a die casting (HPDC). In this article, Al alloy AlSi7Mg and Mg alloys AZ91 and AM60 were cast into tensile test bars using cold-chamber\\u000a (cc) HPDC. Microstructural characterization revealed that externally solidified crystals (ESCs) are not necessary for defect\\u000a band formation, and that defect bands

C. M. Gourlay; H. I. Laukli; A. K. Dahle

2007-01-01

117

Magnetic properties and high frequency characteristics of sputtered FeAl and FeAlB  

Microsoft Academic Search

The magnetic properties and high frequency characteristics of sputtered Fe81Al19 and Fe81Al10B9 thin films have been investigated. The above two thin films were sputtered at different oblique deposition angles in order to induce magnetic anisotropy in the film. The saturation magnetization of the films is easily above 12 kG, while the coercivity and the magnetic anisotropy field of the films

C. C. Hsieh; M. Y. Jian; H. W. Chang; X. G. Zhao; W. C. Chang

2011-01-01

118

Static High Pressure X-Ray Diffraction of TI-6AL-4V  

SciTech Connect

Ti-6Al-4V was examined under static-high pressure conditions using a diamond anvil cell. The angle-dispersive x-ray diffraction experiments were performed at the Advanced Photon Source, Argonne National Laboratory. Radial and axial geometry were used to examine multiple samples. The purpose of the experiment was to generate pressure-volume data at room temperature (which is non-existent in literature) and to examine deviatoric stress effects on such a hard alloy.

Chesnut, Gary N.; Velisavljevic, Nenad; Sanchez, Lilliana (LANL)

2008-01-10

119

ZERODUR glass ceramics for high stress applications  

NASA Astrophysics Data System (ADS)

Recently SCHOTT has shown in a series of investigations the suitability of the zero expansion glass ceramic material ZERODUR for applications like mirrors and support structures of complicated design used at high mechanical loads. Examples are vibrations during rocket launches, bonded elements to support single mirrors or mirrors of a large array, or controlled deformations for optical image correction, i.e. adaptive mirrors. Additional measurements have been performed on the behavior of ZERODUR with respect to the etching process, which is capable of increasing strength significantly. It has been determined, which minimum layer thickness has to be removed in order to achieve the strength increase reliably. New data for the strength of the material variant ZERODUR K20 prepared with a diamond grain tool D151 are available and compared with the data of ZERODUR specimens prepared in the same way. Data for the stress corrosion coefficient n of ZERODUR for dry and normal humid environment have been measured already in the 1980s. It has been remeasured with the alternative double cleavage drilled compression (DCDC) method.

Hartmann, Peter; Nattermann, Kurt; Dhring, Thorsten; Jedamzik, Ralf; Kuhr, Markus; Thomas, Peter; Kling, Guenther; Lucarelli, Stefano

2009-08-01

120

Microstructure and properties of a new super-high-strength AlZnMgCu alloy C912  

Microsoft Academic Search

Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the microstructure of a new super-high-strength AlZnMgCu alloy (C912) has been investigated. Compared with some other high-strength aluminum alloys, the C912 alloy exhibits higher strength and good stress-corrosion resistance and its specific strength is even higher than some AlLi alloys. Its potential for use in the Chinese AE100 airplane is

Y. L Wu; F. H Froes; A Alvarez; C. G Li; J Liu

1997-01-01

121

Fiber damage and high temperature tensile properties of Al 2O 3 fiber reinforced NiAl-matrix composites with and without hBN-interlayer  

Microsoft Academic Search

The fiber damage caused by thermal residual stress in NiAl-matrix composites reinforced by single-crystal Al2O3 fibers (sapphire) was studied. In the NiAl\\/Al2O3 (Vf=0.3) composites, more fibers were broken into shorter segments because of the larger thermal axial compressive stress exerted on the fibers which was caused by the different thermal expansion coefficients of Al2O3 fibers and NiAl matrix. An increasing

Y. Zhong; H. Chen; W. Hu; G. Gottstein

2007-01-01

122

Effect of Stress in GaN/AlInGaN Grown on GaN Templates with Different Stress States  

NASA Astrophysics Data System (ADS)

We clarify the effect of the stress in GaN templates on the subsequent AlInGaN deposition by simply growing 150nm AlInGaN on a 30 ?m GaN template (sample 1) prepared by hydride vapor phase epitaxy and a 2.3 ?m thin control GaN template (sample 2) prepared by metalorganic chemical vapor deposition. X-ray diffraction and secondary iron mass spectroscopy measurements reveal the stress states (tensile stress and full relaxed for samples 1 and 2, respectively) and compositions (Al0.169In0.01 Ga0.821N, Al0.171In0.006Ga0.823N for samples 1 and 2, respectively) of AlInGaN. By carefully eliminating other possible factor, as template surface roughness, it is concluded that different stress states of AlInGaN should stem from different stress states of GaN templates.

Feng, Xiang-Xu; Liu, Nai-Xin; Zhang, Ning; Wei, Tong-Bo; Wang, Jun-Xi; Li, Jin-Min

2014-05-01

123

Stress corrosion cracking of sensitized AA5083 (Al-4.5Mg-1.0Mn)  

NASA Astrophysics Data System (ADS)

The AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) alloy is a nonheat-treatable aluminum alloy known for its excellent corrosion resistance. However, it can become susceptible to intergranular stress corrosion cracking (IGSCC) when exposed to temperatures ranging from 50 C to 200 C for sufficient lengths of time. This IGSCC is widely believed to be associated with dissolution of the electrochemically active ? phase, Al3Mg2, which is precipitated on grain boundaries. Recently, alternative mechanisms have been invoked related to hydrogen effects and/or free Mg segregation or depletion in the grainboundary regions. To establish a baseline for the sensitization effect, constant-extension-rate tests (CERTs) were conducted under open-circuit conditions and under potential control in 3.5 pct NaCl on samples isothermally treated at 150 C. To aid in interpreting the CERT results, grain-boundary precipitation and solute depletion were characterized by transmission electron microscopy (TEM). Additionally, the electrochemical behavior of the ? phase was characterized by anodic polarization of the intermetallic compound synthesized in bulk form. In CERTs under open-circuit conditions, the measured ductility depended strongly on sensitization time, reaching a minimum at 189 hours, followed by a slight increase at longer times. This trend correlated well with the fractional coverage of ? phase on grain boundaries, which increased up to 189 hours, where it existed with nearly continuous coverage. At longer times, this film coarsened and became discontinuous. Correspondingly, some resistance to IGSCC was recovered. In polarization experiments, bulk synthesized ? phase was found to be spontaneously passive from its corrosion potential (-1.40 VSCE) up to about -0.92 VSCE, where passivity was observed to break down. Sensitized AA5083 samples polarized below the ?-phase breakdown potential showed almost no evidence of IGSCC, indicating that a high ? dissolution rate is a requirement for IGSCC. Mg-depleted zones were observed along grain boundaries in sensitized alloys, but a clear role for solute depletion in IGSCC could not be defined on the basis of the results developed in this study.

Searles, J. L.; Gouma, P. I.; Buchheit, R. G.

2001-11-01

124

Characterization of interface states in Al2O3/AlGaN/GaN structures for improved performance of high-electron-mobility transistors  

NASA Astrophysics Data System (ADS)

We have investigated the relationship between improved electrical properties of Al2O3/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al2O3/AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al2O3/AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N2O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al2O3 insulator. As compared to the sample without the treatment, the N2O-radical treated Al2O3/AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al2O3/AlGaN interface were estimated to be 1 1012 cm-2 eV-1 or less around the midgap and 8 1012 cm-2 eV-1 near the conduction-band edge. In addition, we observed higher maximum drain current at the positive gate bias and suppressed threshold voltage instability under the negative gate bias stress even at 150 C. Results presented in this paper indicated that the N2O-radical treatment is effective both in reducing the interface states and improving the electrical properties of the Al2O3/AlGaN/GaN MOS-HEMTs.

Hori, Y.; Yatabe, Z.; Hashizume, T.

2013-12-01

125

Thermal stress in high temperature cylindrical fasteners  

SciTech Connect

Uninsulated structures fabricated from carbon or silicon-based materials, which are allowed to become hot during flight, are attractive for the design of some components of hypersonic vehicles. They have the potential to reduce weight and increase vehicle efficiency. Because of manufacturing contraints, these structures will consist of parts which must be fastened together. The thermal expansion mismatch between conventional metal fasteners and carbon or silicon-based structural materials may make it difficult to design a structural joint which is tight over the operational temperature range without exceeding allowable stress limits. In this study, algebraic, closed-form solutions for calculating the thermal stresses resulting from radial thermal expansion mismatch around a cylindrical fastener are developed. These solutions permit a designer to quickly evaluate many combinations of materials for the fastener and the structure. Using the algebraic equations developed, material properties and joint geometry were varied to determine their effect on thermal stresses. Finite element analyses were used to verify that the closed-form solutions derived give the correct thermal stress distribution around a cylindrical fastener and to investigate the effect of some of the simplifying assumptions made in developing the closed-form solutions for thermal stresses.

Blosser, M.L.

1988-05-01

126

Quenching, thermal and residual stress in plasma sprayed deposits: NiCrAlY and YSZ coatings  

Microsoft Academic Search

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress magnitudes and\\/or concentrations can undermine the structural integrity of sprayed parts and impair their functionality. Therefore, it is important to know the stress state, understand its generation and be able to control it. Results of experimental stress determination in plasma sprayed deposits

J. Matejicek; S. Sampath; P. C. Brand; H. J. Prask

1999-01-01

127

Reliability studies of AlGaN/GaN high electron mobility transistors  

NASA Astrophysics Data System (ADS)

AlGaN/GaN high electron mobility transistors are gaining commercial acceptance for use in high power and high frequency applications, but the degradation mechanisms that drive failure in the field are not completely understood. Since some of these mechanisms are current or field driven, reliability studies must go beyond the typical Arrhenius-accelerated life tests. In this paper, we summarize recent work on electric field or current driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plates) and the effect of device fabrication processes for both dc and RF stress conditions.

Cheney, D. J.; Douglas, E. A.; Liu, L.; Lo, C. F.; Xi, Y. Y.; Gila, B. P.; Ren, F.; Horton, David; Law, M. E.; Smith, David J.; Pearton, S. J.

2013-07-01

128

Ab initio local energy and local stress: application to tilt and twist grain boundaries in Cu and Al.  

PubMed

The energy-density and stress-density schemes (Shiihara etal 2010 Phys. Rev. B 81 075441) within the projector augmented wave (PAW) method based on the generalized gradient approximation (GGA) have been applied to tilt and twist grain boundaries (GBs) and single vacancies in Cu and Al. Local energy and local stress at GBs and defects are obtained by integrating the energy and stress densities in each local region by the Bader integration using a recent algorithm (Yu etal 2011 J. Chem. Phys. 134 064111) as well as by the layer-by-layer integration so as to settle the gauge-dependent problem in the kinetic terms. Results are compared with those by the fuzzy-Voronoi integration and by the embedded atom method (EAM). The features of local energy and local stress at GBs and vacancies depend on the bonding nature of each material. Valence electrons in Al mainly located in the interatomic regions show remarkable response to structural disorder as significant valence charge redistribution or bond reconstruction, often leading to long-range variations of charges, energies and stresses, quite differently from d electrons in Cu mainly located near nuclei. All these features can be well represented by our local energy and local stress. The EAM potential for Al does not reproduce correct local energy or local stress, while the EAM potential for Cu provides satisfactory results. PMID:23835349

Wang, Hao; Kohyama, Masanori; Tanaka, Shingo; Shiihara, Yoshinori

2013-07-31

129

Ab initio local energy and local stress: application to tilt and twist grain boundaries in Cu and Al  

NASA Astrophysics Data System (ADS)

The energy-density and stress-density schemes (Shiihara et al 2010 Phys. Rev. B 81 075441) within the projector augmented wave (PAW) method based on the generalized gradient approximation (GGA) have been applied to tilt and twist grain boundaries (GBs) and single vacancies in Cu and Al. Local energy and local stress at GBs and defects are obtained by integrating the energy and stress densities in each local region by the Bader integration using a recent algorithm (Yu et al 2011 J. Chem. Phys. 134 064111) as well as by the layer-by-layer integration so as to settle the gauge-dependent problem in the kinetic terms. Results are compared with those by the fuzzy-Voronoi integration and by the embedded atom method (EAM). The features of local energy and local stress at GBs and vacancies depend on the bonding nature of each material. Valence electrons in Al mainly located in the interatomic regions show remarkable response to structural disorder as significant valence charge redistribution or bond reconstruction, often leading to long-range variations of charges, energies and stresses, quite differently from d electrons in Cu mainly located near nuclei. All these features can be well represented by our local energy and local stress. The EAM potential for Al does not reproduce correct local energy or local stress, while the EAM potential for Cu provides satisfactory results.

Wang, Hao; Kohyama, Masanori; Tanaka, Shingo; Shiihara, Yoshinori

2013-07-01

130

Stress relaxation of low pressure plasma-sprayed NiCrAlY alloys  

NASA Technical Reports Server (NTRS)

The stress relaxation behavior of three NiCrAlY alloys that are commonly used as bond coats for thermal barrier coatings (TBCs) has been directly measured. The relaxation study was conducted at temperatures of 800-1000 C and over a wide range of stresses. It was established that all three bond coat alloys relaxed quite rapidly at temperatures of 900 C and above. Since the upper use temperatures for bond coats in gas turbine engines are between 900 and 1000 C, bond coat relaxation is expected to occur in service. Therefore, relaxation of the bond coat has the potential to affect TBC life. Furthermore, the relaxation differences observed between the three alloys offers a possible explanation for the differences in TBC life observed for these bond coats. While bond coat relaxation is expected to occur for a TBC in service, the mechanism for a relaxation effect on TBC life, if it exists, is yet to be determined.

Brindley, W. J.; Whittenberger, J. D.

1993-01-01

131

Thermal residual stresses and their toughening effect in Al{sub 2}O{sub 3} platelet reinforced glass  

SciTech Connect

Fluorescence spectroscopy has been used to measure the thermal residual stresses in Al{sub 2}O{sub 3}-platelet/borosilicate glass composites. Tensile residual stresses were found in the platelets, implying the presence of compressive residual stresses in the glass matrix. Measurements of stresses in the bulk of the composite could be obtained using fluorescence from platelets below the specimen surface. The measured stresses lay between the predictions of models for spherical particles and thin platelets, but were closer to the former for the range of platelet contents investigated (5--30 vol.%). Estimates of the increase in toughness associated with the compressive residual stresses in the matrix suggest that this mechanism makes a significant contribution to the toughening effect of the Al{sub 2}O{sub 3} platelets.

Todd, R.I.; Sinclair, R.; Yallee, R.B.; Young, R.J. [Univ. of Manchester and UMIST (United Kingdom). Manchester Materials Science Centre] [Univ. of Manchester and UMIST (United Kingdom). Manchester Materials Science Centre; Boccaccini, A.R. [Technische Universitaet Ilmenau (Germany)] [Technische Universitaet Ilmenau (Germany)

1999-09-08

132

Grain boundary segregation in Al-Zn-Mg alloysImplications to stress corrosion cracking  

NASA Astrophysics Data System (ADS)

Prior studies on grain boundary segregation in Al-Zn-Mg have shown that free Mg is present along the grain boundaries in these alloys under all heat treatment conditions. In this paper, a physical basis for the observed segregation profiles is provided through the vacancy-pump model developed by Anthony. The implications of the observed segregation to stress corrosion cracking (SCC) and other environmentally induced embrittling phenomena (where hydrogen could possibly play a dominant role) are discussed in terms of a possible Mg-H complex formation.

Viswanadham, R. K.; Sun, T. S.; Green, J. A. S.

1980-12-01

133

Stresses in Selectively Oxidized GaAs/(AlGa){sub x}O{sub y} Structures  

SciTech Connect

Raman scattering spectroscopy is used to study the process of selective oxidation of Al{sub 0.97}Ga{sub 0.03}As layers. Stresses arising in GaAs/(AlGa){sub x}O{sub y} layers as a result of selective oxidation under different conditions are determined. The effects of local heating of the samples with laser radiation during measurements of the Raman signals, photoresist hardening resulting from the oxidation, and overoxidation are analyzed. The instrumentation and method of selective oxidation are optimized; as a result, arrays of vertical-cavity surface-emitting lasers are fabricated. The active region of these lasers is based on two InGaAs quantum wells with top oxidized and bottom semiconductor distributed Bragg reflectors.

Blokhin, S.A.; Smirnov, A.N.; Sakharov, A.V.; Gladyshev, A.G.; Kryzhanovskaya, N.V.; Maleev, N.A.; Zhukov, A.E.; Semenova, E.S.; Bedarev, D.A.; Nikitina, E.V.; Kulagina, M.M.; Maksimov, M.V.; Ledentsov, N.N.; Ustinov, V.M. [Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 (Russian Federation)

2005-07-15

134

Yield Strength of Transparent MgAl 2 O 4 Nano-Ceramic at High Pressure and Temperature  

Microsoft Academic Search

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced\\u000a from peak broadening increased significantly with pressure up to 2GPa, with no clear indication of strain saturation. However,\\u000a by then, warming the sample above 400C under 4GPa, stress relaxation was

Jie Zhang; Tiecheng Lu; Xianghui Chang; Shengli Jiang; Nian Wei; Jianqi Qi

2010-01-01

135

Stress Corrosion Cracking of High Strength Steels.  

National Technical Information Service (NTIS)

The stress corrosion cracking (SCC) was investigated for AerMet 100 and 300M steels in four aqueous NaCl solutions of different concentrations (0. 035-3.5%), three of which had an identical electrical conductivity (12.44 x 10 to the -4th power sq m S/mol)...

E. U. Lee H. Sanders B. Sarkar

1995-01-01

136

On the stress corrosion cracking of Al-Li alloys: The role of grain boundary precipitates  

NASA Astrophysics Data System (ADS)

The stress corrosion cracking (SCC) behavior of precipitation-hardened alloys may depend on a large number of microstructural parameters that vary during fabrication and heat treatment, such as grain size, grain boundary (GB) solute segregation, matrix precipitate size, GB precipitate size, precipitate-free zone size, and matrix slip character.[1] Since all of these factors vary simultaneously during normal heat treatments, it is difficult to assess independently the contribution of each microstructural factor to the SCC behavior of an alloy. In particular, a series of experiments was designed which would allow the evaluation of the role of GB precipitates in the SCC behavior of Al-Li and Al-Li-Cu alloys independent of the other factors (such as matrix precipitate microstructure) that normally vary during aging treatments. For these experiments, the matrix precipitates of a binary Al-Li alloy were held constant, keeping the yield strength constant, while the GB precipitate size and volume percent were systematically varied. In contrast, to keep the yield strength of the ternary Al-Li-Cu alloy constant at the same level as the binary alloy, the matrix precipitate size and distribution were varied with the GB precipitate size.

Ricker, R. E.; Fink, J. L.; Vasudvan, A. K.

1991-01-01

137

Lateral gradients of phases, residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal  

PubMed Central

The influence of a local thermal treatment on the properties of TiAlN coatings is not understood. In the present work, a Ti0.52Al0.48N coating on a WCCo substrate was heated with a diode laser up to 900C for 30s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the TiAlN decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about ?200 to 500MPa down to a depth of 6?m is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti0.52Al0.48N coating on a WCCo substrate was performed in the range of 251003C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structureproperty relationships in complex materials.

Bartosik, M.; Daniel, R.; Zhang, Z.; Deluca, M.; Ecker, W.; Stefenelli, M.; Klaus, M.; Genzel, C.; Mitterer, C.; Keckes, J.

2012-01-01

138

Lateral gradients of phases, residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal.  

PubMed

The influence of a local thermal treatment on the properties of Ti-Al-N coatings is not understood. In the present work, a Ti0.52Al0.48N coating on a WC-Co substrate was heated with a diode laser up to 900C for 30s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti-Al-N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about -200 to 500MPa down to a depth of 6?m is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti0.52Al0.48N coating on a WC-Co substrate was performed in the range of 25-1003C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure-property relationships in complex materials. PMID:23471140

Bartosik, M; Daniel, R; Zhang, Z; Deluca, M; Ecker, W; Stefenelli, M; Klaus, M; Genzel, C; Mitterer, C; Keckes, J

2012-06-25

139

Microstructural effects on ignition sensitivity in Ni/Al systems subjected to high strain rate impacts  

NASA Astrophysics Data System (ADS)

The effect of microstructural refinement on the sensitivity of the Ni/Al (1:1 mol%) system to ignition via high strain rate impacts is investigated. The tested microstructures include compacts of irregularly convoluted lamellar structures with nanometric features created through high-energy ball milling (HEBM) of micron size Ni/Al powders and compacts of nanometric Ni and Al powders. The test materials were subjected to high strain rate impacts through Asay shear experiments powered by a light gas gun. Muzzle velocities up to 1.1 km/s were used. It was found that the nanometric powder exhibited a greater sensitivity to ignition via impact than the HEBM material, despite greater thermal sensitivity of the HEBM. A previously unseen fast reaction mode where the reaction front traveled at the speed of the input stress wave was also observed in the nanometric mixtures at high muzzle energies. This fast mode is considered to be a mechanically induced thermal explosion mode dependent on the magnitude of the traveling stress wave, rather than a self-propagating detonation, since its propagation rate decreases rapidly across the sample. A similar mode is not exhibited by HEBM samples, although local, nonpropagating reaction zones shear bands formed during the impact event are observed.

Reeves, Robert V.; Mukasyan, Alexander S.; Son, Steven

2012-03-01

140

Very small critical current spreads in Nb\\/Al-AlOx\\/Nb integrated circuits using low-temperature and low-stress ECR PECVD silicon oxide films  

Microsoft Academic Search

We have developed a new Nb\\/Al-AlOx\\/Nb IC process with very small critical current spread. Low-temperature and low-stress ECR (Electron Cyclotron Resonance) PECVD (Plasma Enhanced Chemical Vapor Deposition) silicon oxide films have been used in the Nb IC process for all dielectric insulating layers to replace e-beam evaporated silicon monooxide films and RF reactive sputtered silicon oxide. Since ECR PECVD silicon

Xiaofan Meng; Anupama Bhat; Theodore Van Duzer

1999-01-01

141

Characterization of stress induced in SOS and Si\\/?-Al 2O 3\\/Si heteroepitaxial thin films by Raman spectroscopy  

Microsoft Academic Search

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si\\/?-Al2O3\\/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si\\/?-Al2O3\\/Si thin

Qi-Yuan Wang; Jun Wang; Jian-Huan Wang; Zhong-Li Liu; Lan-Ying Lin

2005-01-01

142

Through-thickness determination of phase composition and residual stresses in thermall barrier coatings using high- energy x-rays.  

SciTech Connect

High-energy X-rays were used to determine the local phase composition and residual stresses through the thickness of as-sprayed and heat-treated plasma-sprayed thermal barrier coatings consisting of a NiCoCrAlY bond coat and an yttria-stabilized zirconia (YSZ) topcoat produced with through-thickness segmentation cracks. The as-sprayed residual stresses reflected the combined influence of quenching stresses from the plasma spray process, thermal expansion mismatch between the topcoat, bond coat and substrate, and stress relief from the segmentation cracks. Heat treatments led to the formation of a thermally grown oxide (TGO) which was in compression in the plane, as well as relief of quenching stresses and development of a stress gradient in the YSZ topcoat. The high-energy X-ray technique used in this study revealed the effects that TGO and segmentation cracks have on the in-plane stress state of the entire coating.

Weyant, , C. M.; Almer, J. D.; Faber, K. T.; Stony Brook Univ.

2009-01-01

143

Effect of k/[Theta]-phase proportion on the flow stress of Al-Cu alloys at 540[degree]C  

SciTech Connect

Al-Cu alloys have been extensively investigated for their strength and deformation behavior. In two-phase systems, the strength of an alloy is estimated by the sum of the strengths of the component phases in proportion to their volume fractions. The hardness values of the K and (Theta) phases present in two-phases present in two-phase Al-Cu alloys of various compositions have also indicated an increase in the hardness of the soft k phase and a decrease in the hard [Theta] phase with respect to the hardness of the corresponding single phase systems. A change in the proportion of the two phases in an alloy also influences the grain size, in which case, the evaluation of the effect of any one of these variables on the flow stress is complicated. However, high log(stress) - log(strain rate) curves with the flow stress being independent of such conditions, the effect of k/[Theta] - phase proportion in the Al-Cu alloys can be studied without being affected by grain size. Therefore, the effect of k/[Theta]-phase proportion on the flow stress in the Al-Cu alloys was investigated under such conditions in the present work.

Bakshi, P.K.; Kashyap, B.P. (Indian Inst. of Tech., Bombay, (India). Dept. of Metallurgical Engineering)

1993-10-15

144

Relation between surface slip topography and stress corrosion cracking in Ti-8 wt % Al  

NASA Technical Reports Server (NTRS)

The deformation behavior of Ti-8 wt % Al has been investigated in an inert environment (air), and an aggressive environment (salt water). Details of surface slip geometry were examined by high resolution surface replicas at various stages of deformation in both environments. Specimens aged to contain a fine dispersion of Ti3Al precipitates failed by subcritical crack growth in salt water, whereas specimens in the single phase condition showed no effects of environment on the yield or fracture characteristics. The Ti3Al precipitates produce little change in strength level or slip character compared to the single phase alloy, and there is no evidence of any effects of environment on the character of surface slip. Rather, the presence of trenches along slip bands on the surface of aged specimens suggest that the specific effect of the Ti3Al precipitates is to render the surface slip steps chemically active relative to the surrounding matrix by slip induced dissolution of the particles.

Boyd, J. D.; Hoagland, R. G.

1974-01-01

145

Incorporation of Residual Stresses in the Fatigue Performance Design of Ti6Al4V  

Microsoft Academic Search

The high cycle fatigue (HCF) performance of turbine engine components has long been improved by the introduction of a surface layer of compressive residual stress, usually by shot peening. However, credit has not been taken for the improved fatigue performance in component design; rather shot peening is used primarily as an additional safe guard against fatigue failure. Recently, laser shock

P. S. Prevy; N. Jayaraman; R. Ravindranath

146

High temperature coatings based on beta-NiAl  

NASA Astrophysics Data System (ADS)

Ni-based superalloys have been the workhorse materials in the high temperature alloy industry. Further increases in operating temperatures of these alloys have been limited due to low melting temperatures of constituent phases. A new alloy system has been proposed based on the Mo-Ni-Al system, which appears to be a promising alternative alloy. However, the intrinsic oxidation resistance of these alloys is relatively poor compared to current Ni-based superalloys. A two-step coating process based on electroplating and pack cementation was used to synthesize beta-NiAl coatings on top of the Mo-Ni-Al alloys. These coatings were seen to be stable when exposed to expected operating conditions, and substantially improved the oxidation resistance of the Mo-Ni-Al alloys. Grain size of beta-NiAl has been shown to be important during oxidation. Platinum group metals and Hf have been identified as effective alloying additions. However, less expensive additions need to be identified. Additions of 2 nd phase TiB2 particles were found to decrease the initial grain size of beta-NiAl, and limit grain growth after long term exposure at 1300C.

Severs, Kevin Richard

147

Regional aspect of highly water-stressed population estimated by world water resources under SRES scenarios  

NASA Astrophysics Data System (ADS)

Global and regional populations lived in highly water-stressed basin for a function of the temperature were estimated using the socio-economical data and the outputs of GCMs. In global, the highly water-stressed population in scenario A2 rapidly increased when the anomaly of temperature is exceeding to about +1.5 deg.. In the case of the scenario A1b, the gradient of increase of highly water-stressed population was less than that in the case of the scenario A2. When the value of temperature anomaly was exceeding to about + 1.5 deg., the gradient of increase of highly water-stressed population became loose. On the other hand, the highly water-stressed population in the scenario B1 decreased when the temperature anomaly was exceeding to about +1.2 deg.. According to the estimation of the highly water-stressed population when the effect of climate change was ignored (that is, runoff was not changed), the highly water-stressed population was almost same. This result implied that it is strongly contributed by not the climate change but the socio- economical factors (ex; an irrigated area, increase of industrial water use, increase of population itself). Parry et al. (2001) pointed out that the influence on the water risk by the climate change is serious when the anomaly of temperature is exceeding to +2 deg.. However, our assessment disagrees with their assignment. When the highly water-stressed population assessment is estimated within the social elements (ex., the increase of irrigation demand, industrial demand, and population itself), the climate change is not so effective. Part of this study was financially supported by the Global Environment Research Fund, "S-4" and "S-5", from the Japanese Ministry of the Environment.

Kiguchi, M.; Shen, Y.; Kanae, S.; Oki, T.

2008-12-01

148

High resolution, 2-dimensional shear stress measurements from imaging polarimetry  

Microsoft Academic Search

Here we present a novel method for obtaining high-resolution, 2-dimensional shear stress measurements over aerodynamic surfaces, based upon the dynamic birefringence of a shear sensitive, liquid crystal coating. Air flowing over the surface of a liquid crystal coating causes a measurable change in the birefringence of the coating that is dependent upon the magnitude of the shear stress of the

Fletcher Kimura; Werner Kaminsky; Gamal Khalil; James Riley; James Callis

2004-01-01

149

Residual stresses in high-temperature ceramic eutectics  

Microsoft Academic Search

This paper explores residual thermal stresses in directionally solidified ceramic eutectics, a class of materials that has much promise for high-temperature structural applications. Residual strain tensors of both phases in a eutectic composite are measured by single crystal X-ray diffraction techniques. In the analysis the material is treated as fully anisotropic and the strain tensors, subsequently converted to stress tensors,

Elizabeth C. Dickey; Colleen S. Frazer; Thomas R. Watkins; Camden R. Hubbard

1999-01-01

150

Simulation of distortion and residual stress in high pressure die casting - modelling and experiments  

NASA Astrophysics Data System (ADS)

Two individual high-pressure die-casting geometries were developed in order to study the influence of process parameters and different alloys on the distortion behaviour of castings. These geometries were a stress lattice and a V-shaped sample tending to form residual stress due to different wall thickness respectively by a deliberate massive gating system. In the experimental castings the influence of the most important process parameters such as die temperature and die opening time and the cooling regime was examined. The time evolution of process temperatures was measured using thermal imaging. The heat transfer coefficients were adapted to the observed temperature distributions. Castings were produced from the two alloys AlSi12 and AlSi10MnMg. The distortion of the castings was measured by means of a tactile measuring device. For the alloy AlSi10MnMg thermo-physical and thermo-mechanical data were obtained using differential scanning calorimetry, laser flash technique, dilatometry and tensile testing at elevated temperatures. These data were used for modelling the material behaviour of the AlSi10MnMg alloy in the numerical model while for the alloy AlSi12(Fe) literature data were used. Process and stress simulation were conducted using the commercial FEM software ANSYS Workbench. A survey on the results of the comparison between simulation and experiment is given for both alloys.

Hofer, P.; Kaschnitz, E.; Schumacher, P.

2012-07-01

151

High sea-floor stress induced by extreme hurricane waves  

NASA Astrophysics Data System (ADS)

Strong surface waves and currents generated by major hurricanes can produce extreme forces at the seabed that scour the seafloor and cause massive underwater mudslides. Our understanding of these forces is poor due to lack of concurrent measurements of waves and currents under these storms. Using unique observations collected during the passage of a category-4 hurricane, Ivan, bottom stress due to currents and waves over the outer continental shelf in the Gulf of Mexico was examined. During the passage of Ivan, the bottom stress was highly correlated with the wind with a maximum of about 40% of the wind stress. The bottom stress was dominated by the wave-induced stresses, and exceeded critical levels at depths as large as 90 m. Surprisingly, the bottom damaging stress persisted after the passage of Ivan for about a week, and was modulated by near-inertial waves.

Wijesekera, Hemantha W.; Wang, David W.; Teague, William J.; Jarosz, Ewa

2010-06-01

152

Residual Stress on Ti6Al4V Hybrid and Laser Welded Joints  

Microsoft Academic Search

\\u000a Ti6Al4V alloy combines mechanical strength, deformability, excellent fatigue and corrosion resistance and high strength to\\u000a weight ratio. Furthermore, the mechanical behavior remains excellent at high temperature. Such characteristics make this material\\u000a attractive for numerous applications (structural, aerospace and naval) because of recent improvements in welding techniques\\u000a (laser, hybrid laser\\/MIG) that allow realizing high quality titanium welded joints. However some problems

C. Casavola; C. Pappalettere; F. Tursi

153

Residual stress distribution of Ti-6Al-4V alloy under different ns-LSP processing parameters  

NASA Astrophysics Data System (ADS)

A finite element modeling (FEM) model is developed to simulate the effects of overlapping rate, laser spot diameter and laser power density on residual stress of Ti-6Al-4V alloy at the surface and in the depth direction using ABQUAS software, and a simulated residual stress field is analyzed and compared with the experimental data. Results showed the fluctuation ratio of surface residual stress decreases gradually with increasing overlapping rate. The increment of laser power density can increase the affected depth of residual stress, and the higher value of surface residual stress can be obtained by increasing laser spot diameter when laser power density remains unchanged. In addition, the influence mechanism of overlapping LSP impacts on Ti-6Al-4V alloy is clearly also investigated.

Luo, K. Y.; Lu, J. Z.; Wang, Q. W.; Luo, M.; Qi, H.; Zhou, J. Z.

2013-11-01

154

High Pressure Structural Study of Quasicrystalline Al-Mn.  

National Technical Information Service (NTIS)

High pressure X-ray diffraction studies have been performed on rapidly solidified Al-Mn alloys (35 and 40 wt% Mn) containing the quasicrystalline phase. Pressure experiments were undertaken using a diamond anvil cell, and X-ray diffraction analysis in the...

E. Johnson J. Staun Olsen J. V. Wood L. Gerward

1987-01-01

155

Characterization of Al2O3-HfO2-Al2O3 sandwiched MIM capacitor under DC and AC stresses  

NASA Astrophysics Data System (ADS)

In this paper, electrical properties and reliability of high capacitance density Metal-Insulator-Metal (MIM) capacitor with sandwiched hafnium-based dielectric is analyzed using three kinds of voltage stress; constant voltage stress (CVS), unipolar voltage and bipolar voltage stresses. The fabricated MIM capacitor shows not only high capacitance density but also low leakage current density of about 10 nA/cm2 at room temperature and 1 V. The relative variation of capacitance (?C/C0) increases and the variation of voltage linearity (?/?0) gradually decreases with stress-time due to the charge trapping effect in the high-k dielectric. The relative variations of capacitance and voltage linearity show the greater change by the bipolar voltage stress than CVS and unipolar voltage stresses.

Kwak, Ho-Young; Kwon, Hyuk-Min; Jung, Yi-Jung; Kwon, Sung-Kyu; Jang, Jae-Hyung; Choi, Woon-Il; Ha, Man-Lyun; Lee, Ju-Il; Lee, Song-Jae; Lee, Hi-Deok

2013-01-01

156

Brittle/Ductile Stress Relaxation During MOCVD Growth of AlGaN/GaN Heterostructures  

NASA Astrophysics Data System (ADS)

III-nitride materials are the subject of intense study due to their novel optoelectronic properties, but our understanding of the fundamental coupling between microstructure and mechanical behavior lags far behind the technology. Hexagonal AlGaN alloys grow heteroepitaxially on GaN in a state of biaxial tensile stress due to the lattice mismatch. Above some critical thickness, the AlGaN layer undergoes a combined relaxation mode in which a low density channel crack array is introduced initially, followed by the formation of a dense but well-ordered misfit dislocation network within the epitaxial interface. The competition between fracture and dislocation introduction appears to be determined by limitations on defect nucleation. Once cracks do form, they then foster limited nucleation of misfit dislocations that rapidly glide large distances from the cracks. Subsequently, multiplication processes take over that produce pure interfacial misfit segments which complete the relaxation process. I will discuss the many challenges and opportunities in this research area with regard to understanding the specific slip systems, cross-slip, the role of climb, dissociation into partials, dislocation reactions, multiplication, and ultra-rapid relaxation processes. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

Floro, Jerrold

2002-03-01

157

NiAl alloys for high-temperature structural applications  

Microsoft Academic Search

Intermetallic compounds containing aluminum, such as NiAl, offer new opportunities for developing low-density, high-strength structural alloys which might be used at temperatures higher than currently possible with conventional titanium- and nickel-base alloys. Once developed, the intermetallic alloys and their composites will enable the design and production of higher performance, lighter (high thrust-to-weight ratio) engines for future military aircraft and supersonic

Darolia

1991-01-01

158

Effects of Crystal Orientation on Stress Rupture Behavior of a Ni3Al-BASE Single Crystal Superalloy IC6SX  

NASA Astrophysics Data System (ADS)

The effect of the crystal orientation on the stress rupture behavior of the Ni3Al-base Single Crystal alloy IC6SX under the test condition of 1100C/120Mpa has been studied. The results showed that the stress rupture lives of the specimens with [001], [011] and [111] crystal orientations were 205.45h, 268.6h and 485h, respectively, i.e., the specimen with [111] crystal orientation had the longest stress rupture life. The results of the tests also showed the significant different of high temperature stress rupture elongation and area reduction for different crystal orientation specimens, i.e., the ruptured elongations for the specimens with [001], [011] and [111] crystal orientations were 61.9%, 22.9% and 28.8%, and the values of area reduction for the specimens with [001], [011]and [111] crystal orientations were 11.7%, 12.2%and 7.3% respectively.

Jiang, Liwu; Li, Shusuo; Wu, Meiling; Han, Yafang

159

An enhanced integrated stress response ameliorates mutant SOD1-induced ALS.  

PubMed

Varied stresses to cells can lead to a repression in translation by triggering phosphorylation of eukaryotic translation initiator factor 2? (eIF2?), which is central to a process known as the integrated stress response (ISR). PKR-like ER-localized eIF2 kinase (PERK), one of the kinases that phosphorylates eIF2? and coordinates the ISR, is activated by stress occurring from the accumulation of misfolded or unfolded proteins in the endoplasmic reticulum (ER). Mutant Cu/Zn superoxide dismutase (mtSOD1) is thought to cause familial amyotrophic lateral sclerosis (FALS) because it misfolds and aggregates. Published studies have suggested that ER stress is involved in FALS pathogenesis since mtSOD1 accumulates inside the ER and activates PERK leading to phosphorylated eIF2? (p-eIF2?). We previously used a genetic approach to show that haploinsufficiency of PERK significantly accelerates disease onset and shortens survival of G85R mtSOD1 FALS transgenic mice. We now show that G85R mice that express reduced levels of active GADD34, which normally dephosphorylates p-eIF2? and allows recovery from the global suppression of protein synthesis, markedly ameliorates disease. These studies emphasize the importance of the ISR, and specifically the PERK pathway, in the pathogenesis of mtSOD1-induced FALS and as a target for treatment. Furthermore, the ISR may be an appropriate therapeutic target for sporadic ALS and other neurodegenerative diseases since misfolded proteins have been implicated in these disorders. PMID:24368417

Wang, Lijun; Popko, Brian; Roos, Raymond P

2014-05-15

160

Degradation of AlGaN/GaN high electron mobility transistors related to hot electrons  

NASA Astrophysics Data System (ADS)

This paper reports on an extensive analysis of the degradation of AlGaN/GaN high electron mobility transistors (HEMTs) submitted to on-state and off-state stresses. The results of this study indicate that: (i) the analyzed devices do not show any degradation when submitted to off-state stress tests up to 100 V (ii) during on-state stress, devices show a significant decrease in drain current and in the electroluminescence (EL) signal; (iii) degradation rate is strongly dependent on the intensity of the EL signal measured during stress on the devices, which is related to the concentration of hot electrons in the channel. Based on the experimental evidence collected within this paper, the on-state degradation of HEMTs is ascribed to the trapping of negative charge in the gate-drain access region, triggered by hot electrons. By considering the intensity of the EL signal as a measure of the stress acceleration factor, we have derived an acceleration law for the hot-electron induced degradation.

Meneghini, Matteo; Stocco, Antonio; Silvestri, Riccardo; Meneghesso, Gaudenzio; Zanoni, Enrico

2012-06-01

161

High-Temperature Creep Degradation of the AM1/NiAlPt/EBPVD YSZ System  

NASA Astrophysics Data System (ADS)

The failure mechanisms of a NiAlPt/electron beam physical vapor deposition yttria-stabilized-zirconia thermal barrier coating system deposited on the AM1 single crystalline substrate have been investigated under pure creep conditions in the temperature range from 1273 K to 1373 K (1000 C to 1100 C) and for durations up to 1000 hours. Doubly tapered specimens were used allowing for the analysis of different stress states and different accumulated viscoplastic strains for a given creep condition. Under such experiments, two kinds of damage mechanisms were observed. Under low applied stress conditions ( i.e., long creep tests), microcracking is localized in the vicinity of the thermally grown oxide (TGO). Under high applied stress conditions, an unconventional failure mechanism at the substrate/bond coat interface is observed because of large creep strains and fast creep deformation, hence leading to a limited TGO growth. This unconventional failure mechanism is observed although the interfacial bond coat/top coat TGO thickening is accelerated by the mechanical applied stress beyond a given stress threshold.

Riallant, Fanny; Cormier, Jonathan; Longuet, Arnaud; Milhet, Xavier; Mendez, Jos

2014-01-01

162

Strength of VGCF/Al Composites for High Thermal Conductivity  

NASA Astrophysics Data System (ADS)

In this paper, the evaluation of the strength of the VGCF/Aluminum composites which have high thermal conductivity is reported. VGCF (Vapor Growth Carbon Fiber) is a kind of the Carbon nanotube (CNT) which has very high thermal conductivity as well as CNT. The composites are made by spark plasma sintering. The stress-strain curves of the composites are obtained by the tensile tests and show that the composites have brittle behavior. The brittleness of the composites increases with increase in the volume fraction of VGCF. A numerical simulation based on the micromechanics is conducted to estimate nonlinear behavior in the elastic deformation and plastic deformation of the stress-strain relations of the composites. The theories of Eshelby, Mori-Tanaka, Weibull, and Ramberg-Osgood are employed for the numerical simulation. The simulations give some information of the microstructural change in the composite related to the volume fraction of VGCF.

Fukuchi, Kohei; Sasaki, Katsuhiko; Imanishi, Terumitsu; Katagiri, Kazuaki; Kakitsuji, Atsushi; Shimizu, Akiyuki

163

The high pressure high shear stress rheology of liquid lubricants  

NASA Astrophysics Data System (ADS)

A limiting shear stress model of liquid lubricant shear rheology is offered which accurately represents all available primary data. The model is of the nonlinear Maxwell type with shear modulus taken into the time derivative and broadening of the viscous-plastic transition with pressure. Property relations for viscosity, limiting stress and shear modulus are refined for a polyphenyl ether in particular. The model, with simplifying assumptions, is compared with disk machine results. This model, with change of yield criterion, may be applicable to some shear thinning liquids at low pressure. Limiting shear stress varies with pressure in the same manner as the ultimate shear strength of solid polymers.

Bair, S.; Winer, W. O.

1992-01-01

164

Characterization of stress induced in SOS and Si/?-Al 2O 3/Si heteroepitaxial thin films by Raman spectroscopy  

NASA Astrophysics Data System (ADS)

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si/?-Al 2O 3/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si/?-Al 2O 3/Si thin film is much smaller than that of as-grown SOS and SPE upgraded SOS films. The stress decrease for double heteroepitaxial film Si/?-Al 2O 3/Si mainly arises from the smaller lattice mismatching of 2.4% between silicon top layer and the ?-Al 2O 3/Si epitaxial composite substrate, comparing with the large lattice mismatch of 13% for SOS films. It indicated that ?-Al 2O 3/Si as a silicon-based epitaxial substrate benefits for reducing the residual stress for further growth of silicon layer, compared with on bulk sapphire substrate.

Wang, Qi-Yuan; Wang, Jun; Wang, Jian-Huan; Liu, Zhong-Li; Lin, Lan-Ying

2005-06-01

165

High-risk stress fractures: evaluation and treatment.  

PubMed

Stress fractures are common overuse injuries seen in athletes and military recruits. The pathogenesis is multifactorial and usually involves repetitive submaximal stresses. Intrinsic factors, such as hormonal imbalances, may also contribute to the onset of stress fractures, especially in women. The classic presentation is a patient who experiences the insidious onset of pain after an abrupt increase in the duration or intensity of exercise. The diagnosis is primarily clinical, but imaging modalities such as plain radiography, scintigraphy, computed tomography, and magnetic resonance imaging may provide confirmation. Most stress fractures are uncomplicated and can be managed by rest and restriction from the precipitating activity. A subset of stress fractures can present a high risk for progression to complete fracture, delayed union, or nonunion. Specific sites for this type of stress fracture are the femoral neck (tension side), the patella, the anterior cortex of the tibia, the medial malleolus, the talus, the tarsal navicular, the fifth metatarsal, and the great toe sesamoids. Tensile forces and the relative avascularity at the site of a stress-induced fracture often lead to poor healing. Therefore, high-risk stress fractures require aggressive treatment. PMID:11104398

Boden, B P; Osbahr, D C

2000-01-01

166

Residual stresses in Al[sub 2]O[sub 3]-ZrO[sub 2] composites: A test of stochastic stress models  

SciTech Connect

In cooling sintered composites of Al[sub 2]O[sub 3]-ZrO[sub 2] from their fabrication temperature residual stresses are created as a result of both the difference in thermal expansion between the two phases and the crystallographically anisotropic thermal expansion of the Al[sub 2]O[sub 3] phase. In this work the first and second moments of the residual stress distribution have been measured as a function of volume fraction of zirconia from 0.01 to 0.90. The measurement technique used is piezo-spectroscopy based on the optical fluorescence from Cr[sup 3+] dopants in the alumina phase. For zirconia volume fractions up to 0.35 the average stress accurately fits the predictions given by the upper Hashin bound and this fit provides a value of the average thermal strain in the composites. Using this value, the effective medium approximation produces an excellent description of the average stress over the entire volume fraction. It is shown that the fluorescence broadening due to stress fluctuations lies close to the predicted upper and lower Hashin bounds modified by the restrictions imposed by the principle of maximum entropy. The measured moments and those predicted by stochastic stress analysis compare well suggesting that the stochastic analysis provides a reliable method of calculating residual stress in composites.

Ma, Qing; Clarke, D.R. (Univ. of California, Santa Barbara, CA (United States). Materials Dept.); Pompe, W. (Technical Univ. of Dresden (Germany). Materials Science Dept.); French, J.D. (National Inst. of Standards and Technology, Gaithersburg, MD (United States). Ceramics Div.)

1994-05-01

167

Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior  

NASA Astrophysics Data System (ADS)

Transmission electron microscopy (TEM) was employed to investigate the microchemistry and microstructure of grain boundary precipitates in Al 7075 aged at room temperature for several hours, at 393 K (120 C) for 12 hours (under aged), at peak aged (T651) and over aged (T73) conditions. High resolution TEM analysis of precipitates at grain boundaries and fine probe energy dispersive spectrometry showed that the grain boundary precipitates at peak and over aged conditions are hexagonal ? phase with stoichiometry Mg(Cu x Zn1- x )2. Considerable increase in Cu content in the grain boundary ? in the over aged condition compared to the peak aged condition was observed. The average Cu content in the over aged condition was found to be 20 at. pct. The higher Cu content of the precipitate is associated with a lower stress corrosion cracking plateau velocity.

Goswami, Ramasis; Lynch, Stanley; Holroyd, N. J. Henry; Knight, Steven P.; Holtz, Ronald L.

2013-03-01

168

High Performance Polarized Electron Photocathodes Based on InGaAlAs/AlGaAs Superlattices  

SciTech Connect

Highly efficient emitters of polarized electrons based on the InAlGaAs/AlGaAs superlattice give an optimistic prognosis to explorations of such structures as the sources for accelerators. A new set of these SL structures with minimized conduction band offset was designed and recently tested. A new technology of surface protection in MBE growth leads to a significantly reduced heat-cleaning temperature. At these lowered cleaning temperatures, the thermal degradation of the working structure parameters is avoided. As a result a polarization P of up to 91% at corresponding quantum efficiency (QE) of 0.3% was achieved at room temperature. A 50% increase in the photocathode lifetime has been achieved with Sb coverage.

Mamaev, Yu.

2004-12-10

169

High-pressure and high-temperature stability field of hydrous phase delta-AlOOH  

Microsoft Academic Search

Stability field of hydrous phases is a key for understanding water concentration in the earth's mantle. delta-AlOOH is a high-pressure polymorph of diaspore (alpha-AlOOH) and boehmite (gamma-AlOOH). The space group of this phase is Pnn2 and it is similar to CaCl2-type SiO2 which is a high-pressure polymorph of stishovite; edge-sharing Al-O octahedra make single-chain along c-axis. Although it has a

A. Sano; E. Ohtani; T. Kondo; N. Hirao; T. Sone; T. Kikegawa; N. Sata; Y. Ohishi

2005-01-01

170

Life expectancy of modular Ti6Al4V hip implants: influence of stress and environment.  

PubMed

Stress dependent electrochemical dissolution is identified as one of the key mechanisms governing surface degradation in fretting and crevice corrosion of biomedical implants. The present study focuses on delineating the roles of mechanical stress and chemical conditions on the life expectancy of modular hip implants. First, material removal on a stressed surface of Ti6Al4V subjected to single asperity contact is investigated experimentally to identify the influence of contact load, in-plane stress and chemical environment on mean wear rates. A range of known stress levels are applied to the specimen while its surface is mechanically stimulated in different non-reactive to oxidizing aqueous environments. Evolution of surface degradation is monitored, and its mechanism is elucidated. This phase allows estimation of Preston Constant which is later used in the analysis. Second phase of the work is semi-analytical and computational, where, based on the estimated Preston constant and other material and process parameters, the scratch propensity (consisting of magnitude of scratch depth and their frequency per unit area) due to micro-motion in modular hip implants is estimated. The third phase views these scratches as initial notches and utilizes a mixed-mode fatigue crack propagation model to estimate the critical crack length for onset of instability. The number of loading cycles needed to reach this critical crack length is then labeled as the expected life of the implant under given mechanical and chemical conditions. Implications of different material and process conditions to life expectancy of orthopedic implants are discussed. It is observed that transverse micro-motion, compared to longitudinal micro-motion, plays a far more critical role in determining the implant life. Patient body weight, as well as proximity of the joint fluid to its iso-electric point play key roles in determining wear rates and associated life expectancies of modular hip implants. Sustained aeration of joint fluid, as well as proper tolerancing of mating surfaces, along with a proper choice of material microstructure may be utilized to extend implant life. PMID:22098898

Chandra, A; Ryu, J J; Karra, P; Shrotriya, P; Tvergaard, V; Gaisser, M; Weik, T

2011-11-01

171

High strain rate testing of AP/Al/HTPB solid propellants  

NASA Astrophysics Data System (ADS)

Ammonium perchlorate (AP), aluminum (Al), and hydroxy-terminater polybutadiene (HTPB) solid propellant samples were subjected to strain rates up to 700 sec-1 using the Split-Hopkinson Pressure Bar apparatus. The issues are the effects of temperature on the mechanical behavior of these propellants at high strain rates. Strain rates were between 100 sec-1 and 700 sec-1 over wide range of temperature regions. Included in this paper are the strain rates, strain, and stress curves and material response properties for Aluminum, AP, and HTPB based propellants formulation.

John, Henry J.; Robbs, Rodney

1998-07-01

172

NiAl-base composite containing high volume fraction of AlN for advanced engines  

NASA Technical Reports Server (NTRS)

A particulate reinforced NiAl-AlN composite alloy has a NiAl matrix and greater than about 13 volume percent fine particles of AlN within the matrix. The particles preferably have a diameter from about 15 nanometers to about 50 nanometers. The particulate reinforced NiAl-AlN composite alloy may be prepared by cryomilling prealloyed NiAl in liquid nitrogen using grinding media having a diameter of from about 2 to 6 mm at an impeller speed of from about 450 RPM to about 800 RPM. The cryomilling may be done for a duration of from about 4 hours to about 20 hours to obtain a cryomilled powder. The cryomilled powder may be consolidated to form the particulate reinforced NiAl-AlN composite alloy. The particulate reinforced alloy can further include a toughening alloy. The toughening alloy may include NiCrAlY, FeCrAlY, and FeAl.

Hebsur, Mohan (inventor); Whittenbeger, John D. (inventor); Lowell, Carl F. (inventor)

1994-01-01

173

High temperature, oxidation resistant noble metal-Al alloy thermocouple  

NASA Technical Reports Server (NTRS)

A thermocouple is disclosed. The thermocouple is comprised of an electropositive leg formed of a noble metal-Al alloy and an electronegative leg electrically joined to form a thermocouple junction. The thermocouple provides for accurate and reproducible measurement of high temperatures (600 - 1300 C) in inert, oxidizing or reducing environments, gases, or vacuum. Furthermore, the thermocouple circumvents the need for expensive, strategic precious metals such as rhodium as a constituent component. Selective oxidation of rhodium is also thereby precluded.

Smialek, James L. (inventor); Gedwill, Michael G. (inventor)

1994-01-01

174

Effect of whisker orientation on the stress corrosion cracking behavior of alumina borate whisker reinforced pure Al composite  

Microsoft Academic Search

Stress corrosion cracking (SCC) behavior of as-extruded alumina borate whisker (Al18B4O33)w reinforced pure aluminum composite in 3.5%NaCl solution was investigated using double cantilever beam (DCB) method. The effect of whisker orientation on the stress corrosion cracking behavior of the composite after extrusion was examined. The results indicated that the crack propagated easily along the interface between whisker and matrix in

J Hu; C. S Chen; L. X Xu; C. K Yao; L. C Zhao

2002-01-01

175

Damage Assessment of Stress-Thermal Cycled High Temperature  

NASA Technical Reports Server (NTRS)

We report on the characterization of bismaleimide and polyimide carbon fiber composite, microcrack development under stress thermal cycling loading. Such cycle range from cryogenic temperatures associated with cryogenic fuel (LN, LOX) containment to high temperatures of 300 degrees associated with future hypervelocity aeropropulsion systems. Microcrack development thresholds as a function of temperature range of the thermal cycle, the number of cycles, and the applied stress level imposed on the composite are reported. We have conducted stress-thermal cycles on thin bismaleimide-woven carbon fiber foils for three temperature range cycles. The principle findings are that the full cycles from -196 degree celsius up to 250 degree celsius cause the most significant microcrack development. These observations indicate that the high temperature portion of the cycle under load causes fiber-matrix interface failure and subsequent exposure to higher stresses at the cryogenic, low temperature region results in composite matrix microcracking as a result of the additional stresses associated with the fiber-matrix thermal expansion mismatch. Our initial studies for 12 ply PMR-II-50 polyimide/M6OJB carbon fabric [0f, 90f, 90f, 0f, 0f, 90f] composites will be presented. The stress-thermal cycle test procedure for these will be described. Moisture absorption characteristics between cycles will be used to monitor interconnected microcrack development. The applied stress level will be 75 percent of the composite cryogenic -196 degree celsius ultimate strength.

Ju, Jach-Yung; Prochazka, Michael; Ronke, Ben; Morgan, Roger; Shin, Eugene

2003-01-01

176

Energy dissipation of highly charged ions on Al oxide films.  

PubMed

Slow highly charged ions (HCIs) carry a large amount of potential energy that can be dissipated within femtoseconds upon interaction with a surface. HCI-insulator collisions result in high sputter yields and surface nanofeature creation due to strong coupling between the solid's electronic system and lattice. For HCIs interacting with Al oxide, combined experiments and theory indicate that defect mediated desorption can explain reasonably well preferential O atom removal and an observed threshold for sputtering due to potential energy. These studies have relied on measuring mass loss on the target substrate or probing craters left after desorption. Our approach is to extract highly charged ions onto the Al oxide barriers of metal-insulator-metal tunnel junctions and measure the increased conductance in a finished device after the irradiated interface is buried under the top metal layer. Such transport measurements constrain dynamic surface processes and provide large sets of statistics concerning the way individual HCI projectiles dissipate their potential energy. Results for Xe(q + ) for q = 32, 40, 44 extracted onto Al oxide films are discussed in terms of postirradiation electrical device characteristics. Future work will elucidate the relationship between potential energy dissipation and tunneling phenomena through HCI modified oxides. PMID:21389384

Lake, R E; Pomeroy, J M; Sosolik, C E

2010-02-01

177

High strain rate deformation of NiAl  

SciTech Connect

NiAl is a potential high temperature structural material. Applications for which NiAl is being considered (such as rotating components in jet engines) requires knowledge of mechanical properties over a wide range of strain rates. Single crystal NiAl (stoichiometric and Ni 49.75Al 0.25Fe) has been deformed in compression along [100] at strain rates of 0.001, 0.1/s and 2000/s and temperatures of 76,298 and 773K. <111> slip was observed after 76K testing at a strain rate of 0.001/s and 298K testing at a strain rate of 2000/s. Kinking was observed after deformation at 298K and a strain rate of 0.001/s and sometimes at 298 K and a strain rate of 0.1/s. Strain hardening rates of 8200 and 4000 MPa were observed after 773 and 298K testing respectively, at a strain rate of 2000/s. Results are discussed in reference to resulting dislocation substructure.

Maloy, S.A.; Gray, G.T. III [Los Alamos National Lab., NM (United States); Darolia, R. [General Electric Co., Cincinnati, OH (United States). Aircraft Engines

1994-07-01

178

The stress-corrosion behavior of Al-Li-Cu alloys: A comparison of test methods  

NASA Technical Reports Server (NTRS)

Two powder metallurgy processed (Al-Li-Cu) alloys with and without Mg addition were studied in aqueous 3.5% NaCl solution during the alternate immersion testing of tuning fork specimens, slow crack growth tests using fracture mechanics specimens, and the slow strain rate testing of straining electrode specimens. Scanning electron microscopy and optical metallography were used to demonstrate the character of the interaction between the Al-Li-Cu alloys and the selected environment. Both alloys are susceptible to SC in an aqueous 3.5% NaCl solution under the right electrochemical and microstructural conditions. Each test method yields important information on the character of the SC behavior. Under all conditions investigated, second phase particles strung out in rows along the extrusion direction in the alloys were rapidly attacked, and played principal role in the SC process. With time, larger pits developed from these rows of smaller pits and under certain electrochemical conditions surface cracks initiated from the larger pits and contributed directly to the fracture process. Evidence to support slow crack growth was observed in both the slow strain rate tests and the sustained immersion tests of precracked fracture mechanics specimens. The possible role of H2 in the stress corrosion cracking process is suggested.

Rizzo, P. P.; Galvin, R. P.; Nelson, H. G.

1982-01-01

179

Proteomics of rice grain under high temperature stress  

PubMed Central

Recent proteomic analyses revealed dynamic changes of metabolisms during rice grain development. Interestingly, proteins involved in glycolysis, citric acid cycle, lipid metabolism, and proteolysis were accumulated at higher levels in mature grain than those of developing stages. High temperature (HT) stress in rice ripening period causes damaged (chalky) grains which have loosely packed round shape starch granules. The HT stress response on protein expression is complicated, and the molecular mechanism of the chalking of grain is obscure yet. Here, the current state on the proteomics research of rice grain grown under HT stress is briefly overviewed.

Mitsui, Toshiaki; Shiraya, Takeshi; Kaneko, Kentaro; Wada, Kaede

2013-01-01

180

Growth and characterization of a high-quality Al0.45Ga0.55N layer with AlGaN/AlN superlattices  

NASA Astrophysics Data System (ADS)

We report the growth and characterization of a Si-doped, n-type AlGaN layer with 45% Al composition. For the application of n-type AlGaN layers with high Al compositions in ultraviolet emitters, we fabricated an n-Al0.45Ga0.55N layer with high crystalline quality and high electrical conductivity by inserting Al0.85Ga0.15N/AlN superlattices (SLs) to prevent cracks prior to growing the n-type AlGaN layer. The dislocation density in the n-AlGaN layer with a 45% Al composition and SLs was less than 2.4 1010 cm-2, which was lower than the dislocation density of 5.3 1010 cm-2 for the n-AlGaN layer without SLs. The resistivity, mobility, and free-electron concentration in the n-type Al0.45Ga0.55N layer with SLs were 2.2 10-2 ?cm, 55.0 cm2/V-s, and 5.0 1018 cm-3 at room temperature, respectively.

Jeon, Seong-Ran; Park, Si-Hyun

2013-10-01

181

Effect of Stress Ratio on the Fatigue Behavior of a Friction Stir Processed Cast Al-Si-Mg Alloy  

SciTech Connect

The effect of friction stir processing (FSP) on the fatigue life of a cast Al-7Si-0.6Mg alloy at a stress ratio of R=0 was evaluated. Two types of specimen geometry were used for the FSPed condition, through-thickness processed and partial thickness processed. FSP enhanced the fatigue life by a factor of 15 for the through thickness processed samples at lower stress amplitudes. This is different from the FSP specimens tested at R=-1 and similar stress amplitudes where a 5 times improvement in fatigue life was observed. In light of these observations, various closure mechanisms were examined.

Jana, Saumyadeep; Mishra, Rajiv S.; Baumann, John B.; Grant, Glenn J.

2009-11-01

182

High voltage bushing having weathershed and surrounding stress relief collar  

DOEpatents

A high voltage electric bushing comprises a hollow elongated dielectric weathershed which encloses a high voltage conductor. A collar formed of high voltage dielectric material is positioned over the weathershed and is bonded thereto by an interface material which precludes moisture-like contaminants from entering between the bonded portions. The collar is substantially thicker than the adjacent weathershed which it surrounds, providing relief of the electric stresses which would otherwise appear on the outer surface of the weathershed. The collar may include a conductive ring or capacitive foil to further relieve electric stresses experienced by the bushing.

Cookson, Alan H. (Pittsburgh, PA)

1981-01-01

183

Stress-State dependence of strain-hardening behavior in 2014 Al\\/15 vol Pct Al 2 O 3 composite  

Microsoft Academic Search

The purpose of this investigation was to examine if the effective stress-strain function for discontinuously reinforced aluminum\\u000a (DRA) matrix composites is independent of stress state, as they are for aluminum alloys. The rationale for such work is provided\\u000a by the need to develop constitutive equations for applications in metal forming and forging problems. Experimental effectiveas curves at room temperature were

D.-G. C. Syu; A. K. Ghosh

1994-01-01

184

Uncertainties in obtaining high reliability from stress-strength models  

NASA Technical Reports Server (NTRS)

There has been a recent interest in determining high statistical reliability in risk assessment of aircraft components. The potential consequences are identified of incorrectly assuming a particular statistical distribution for stress or strength data used in obtaining the high reliability values. The computation of the reliability is defined as the probability of the strength being greater than the stress over the range of stress values. This method is often referred to as the stress-strength model. A sensitivity analysis was performed involving a comparison of reliability results in order to evaluate the effects of assuming specific statistical distributions. Both known population distributions, and those that differed slightly from the known, were considered. Results showed substantial differences in reliability estimates even for almost nondetectable differences in the assumed distributions. These differences represent a potential problem in using the stress-strength model for high reliability computations, since in practice it is impossible to ever know the exact (population) distribution. An alternative reliability computation procedure is examined involving determination of a lower bound on the reliability values using extreme value distributions. This procedure reduces the possibility of obtaining nonconservative reliability estimates. Results indicated the method can provide conservative bounds when computing high reliability. An alternative reliability computation procedure is examined involving determination of a lower bound on the reliability values using extreme value distributions. This procedure reduces the possibility of obtaining nonconservative reliability estimates. Results indicated the method can provide conservative bounds when computing high reliability.

Neal, Donald M.; Matthews, William T.; Vangel, Mark G.

1992-01-01

185

Nanocrystalline Ni-Al ferrites for high frequency applications  

NASA Astrophysics Data System (ADS)

Nanocrystalline Ni0.94Co0.03Mn0.04Cu0.03Fe1.96-xAlxO4(x = 0,0.1,0.3,0.5,0.7and0.9) were synthesized using microwave hydrothermal method at 160C/40 min. The synthesized powders were characterized using XRD, TEM and FTIR. The nanopowders were sintered using microwave sintering method at 950C/30 min. The prepared samples were characterized using XRD and SEM. The substitution of Al3+ ions in place of Fe3+ ions results decrease of the density and lattice constant. The increase in D.C resistivity and reduction in the initial permeability, Ms and TC has been observed with an increase of Al3+ ions. High D.C resistivity, thereby decrease in the dielectric losses and an excellent temperature stability of Magnetization (4?Ms) are desired characteristics of ferrites used for fabrication of microwave devices.

Ramesh, T.; Bharadwaj, S.; Shinde, R. S.; Murthy, S. R.

2013-02-01

186

Time-dependent degradation of AlGaN/GaN high electron mobility transistors under reverse bias  

NASA Astrophysics Data System (ADS)

This paper describes a detailed analysis of the time-dependent degradation kinetics of GaN-based high electron mobility transistors submitted to reverse-bias stress. We show that: (1) exposure to reverse-bias may induce recoverable changes in gate leakage and threshold voltage, due to the accumulation of negative charge within the AlGaN layer, and of positive charge at the AlGaN/GaN interface. (2) Permanent degradation consists in the generation of parasitic leakage paths. Several findings support the hypothesis that permanent degradation is due to a defect percolation process: (2(a)) for sufficiently long stress times, degradation occurs even below the ``critical voltage'' estimated by step stress experiments; (2(b)) before permanent degradation, gate current becomes noisy, indicating an increase in defect concentration; and (2(c)) time to breakdown strongly depends on the initial defectiveness of the samples.

Meneghini, Matteo; Stocco, Antonio; Bertin, Marco; Marcon, Denis; Chini, Alessandro; Meneghesso, Gaudenzio; Zanoni, Enrico

2012-01-01

187

Yield Strength of Transparent MgAl2O4 Nano-Ceramic at High Pressure and Temperature  

NASA Astrophysics Data System (ADS)

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl2O4 nano-ceramics as an engineering material.

Zhang, Jie; Lu, Tiecheng; Chang, Xianghui; Jiang, Shengli; Wei, Nian; Qi, Jianqi

2010-08-01

188

Yield Strength of Transparent MgAl2O4 Nano-Ceramic at High Pressure and Temperature  

PubMed Central

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl2O4 nano-ceramics as an engineering material.

2010-01-01

189

Yield Strength of Transparent MgAl2O4 Nano-Ceramic at High Pressure and Temperature.  

PubMed

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl2O4 nano-ceramics as an engineering material. PMID:20676198

Zhang, Jie; Lu, Tiecheng; Chang, Xianghui; Jiang, Shengli; Wei, Nian; Qi, Jianqi

2010-01-01

190

High Toughness Ceramic Laminates by Design of Residual Stresses  

Microsoft Academic Search

Multilayered ceramic composites are very promising materials for different engineering applications. Laminates with strong interfaces can provide high apparent fracture toughness and damage tolerance along with the high strength and reliability. The control over the mechanical behavior of laminates can be obtained through design of residual stresses in separate layers. Here we report a development of tough silicon nitride based

Nina A. Orlovskaya; Jakob Kuebler; Vladimir I. Subotin; Mykola Lugovy

191

Effects of High-Temperature Exposure on Mechanical Properties of Thermal Sprayed CoNiCrAlY Alloy Coating  

NASA Astrophysics Data System (ADS)

Thermal spraying is widely used in various industrial fields as effective surface modification method. In this study, mechanical properties of thermal sprayed CoNiCrAlY alloy coating were investigated. The coatings were prepared by three kinds of thermal spray methods. High-temperature exposure of the coated specimens was carried out in the atmosphere. After high-temperature exposure, apparent Young's modulus, strain tolerance, residual stress and hardness of the CoNiCrAlY coating were measured. These tests were also carried out using specimens without high-temperature exposure. As a result, the changes of mechanical properties caused by high-temperature exposure seemed to be correlated with properties of boundary between sprayed particles. From the results, effects of high-temperature exposure on mechanical properties of thermal sprayed CoNiCrAlY alloy coating were discussed.

Yamano, Hideaki; Takabatake, Takeshi; Tani, Kazumi

192

High temperature stress-strain analysis  

NASA Technical Reports Server (NTRS)

The objectives of the high temperature structures program are threefold: to assist in the development of analytical tools needed to improve design analysis and procedures for the efficient and accurate prediction of the nonlinear structural response of hot-section components; to aid in the calibration, validation, and evaluation of the analytical tools by comparing predictions with experimental data; and to evaluate existing as well as advanced temperature and strain measurement instrumentation.

Thompson, Robert L.

1985-01-01

193

Texture Evolution and Residual Stress Relaxation in a Cold-Rolled Al-Mg-Si-Cu Alloy Using Vibratory Stress Relief Technique  

NASA Astrophysics Data System (ADS)

Over the last half century, vibratory stress relief (VSR) has come to be recognized as a technique with several unique benefits, and it has found applications in various industries. However, the mechanisms involved remain unclear, and the textures corresponding to residual stress relaxation were rarely reported in the existing literature. The purpose of this study is to discuss the texture evolution and residual stress relaxation in a cold-rolled Al-Mg-Si-Cu alloy using VSR technique. All the residual stress measurements were performed using a standard X-ray diffraction (XRD) technique. Measurement of texture was performed on the specimen surface using conventional pole figure (PF) as well as orientation distribution functions (ODFs) methods. Results indicate that the VSR technique can be applied to weaken the ?-fiber and cause the residual stress of the rolled samples to gradually approach uniformity in XRD analysis. The best relaxation of the compressive residual stress reaches about 52.6 pct, but relaxation of the tensile residual stress is less than 10 pct. After the VSR process for 20 minutes, the texture intensities of PFs (200) and (220) as compared to those in nonvibration are nearly homogeneously distributed. Furthermore, the texture of (111) PF perpendicular to normal direction (ND), which is affected by vibratory force parallel to ND, almost disappears.

Wang, Jia-Siang; Hsieh, Chih-Chun; Lin, Chi-Ming; Kuo, Che-Wei; Wu, Weite

2013-02-01

194

High resolution, 2-dimensional shear stress measurements from imaging polarimetry  

NASA Astrophysics Data System (ADS)

Here we present a novel method for obtaining high-resolution, 2-dimensional shear stress measurements over aerodynamic surfaces, based upon the dynamic birefringence of a shear sensitive, liquid crystal coating. Air flowing over the surface of a liquid crystal coating causes a measurable change in the birefringence of the coating that is dependent upon the magnitude of the shear stress of the airflow. The liquid crystal molecules also orient themselves in the direction of the applied shear stress. Using circularly polarized, monochromatic light and linear polarizers, the birefringence (|sin(?)|) and molecular orientation (extinction angle, ?) can be quantified from relatively straightforward intensity measurements. Through the use of a new imaging device developed in our lab known as Millipol, quantitative birefringence and extinction angle measurements can be made at the rate of 29 Hz. The birefringence and extinction angle data is then calibrated, resulting in real time, 2-dimensional shear stress vector maps with resolutions limited only by the CCD imager.

Kimura, Fletcher; Kaminsky, Werner; Khalil, Gamal; Riley, James; Callis, James

2004-11-01

195

Advances in Swept High Frequency Eddy Current Residual Stress Characterization  

NASA Astrophysics Data System (ADS)

We report on recent progress of a swept high frequency eddy current (SHFEC) technique for nondestructive characterization of residual stresses in engine materials with specific applications to shot-peened superalloy components. The technique determines conductivity depth profiles by model-based inversion of SHFEC data, and converts them into residual stress profiles using a material-based model which generally includes the piezoresistivity (PR) and possibly other non-PR effects. Improvement in detection coil and upgrades to the measurement system have extended the effective frequency upper bound to 70 MHz. A modified piezoresistivity theory, including the effect of texture on stress-induced conductivity changes, was developed in light of the experimental evidences of shot-induced changes of texture in peened Inconel 718 samples. The theory was applied to convert the conductivity profile to a residual stress depth profile, which agrees with the data taken by the standard layer removal XRD method.

Shen, Y.; Frishman, A. M.; Lee, S. J.; Lo, C. C. H.; Nakagawa, N.

2008-02-01

196

Cryogenic Treatment of Al-Li Alloys for Improved Weldability, Repairability, and Reduction of Residual Stresses  

NASA Technical Reports Server (NTRS)

Service performance has shown that cryogenic treatment of some metals provides improved strength, fatigue life, and wear resistance to the processed material. Effects such as these were initially discovered by NASA engineers while evaluating spacecraft that had returned from the cold vacuum of space. Factors such as high cost, poor repairability, and poor machinability are currently prohibitive for wide range use of some aerospace aluminum alloys. Application of a cryogenic treatment process to these alloys is expected provide improvements in weldability and weld properties coupled with a reduction in repairs resulting in a significant reduction in the cost to manufacture and life cycle cost of aerospace hardware. The primary purpose of this effort was to evaluate the effects of deep cryogenic treatment of some aluminum alloy plate products, welds, and weld repairs, and optimize a process for the treatment of these materials. The optimized process is being evaluated for improvements in properties of plate and welds, improvements in weldability and repairability of treated materials, and as an alternative technique for the reduction of residual stresses in repaired welds. This paper will present the results of testing and evaluation conducted in this effort. These results will include assessments of changes in strength, toughness, stress corrosion susceptability, weldability, repairability, and reduction in residual stresses of repaired welds.

Malone, Tina W.; Graham, Benny F.; Gentz, Steven J. (Technical Monitor)

2001-01-01

197

High temperature stress-strain analysis  

NASA Technical Reports Server (NTRS)

The objectives of the high-temperature structures program are threefold: to assist in the development of analytical tools needed to improve design analyses and procedures for the efficient and accurate prediction of the nonlinear structural response of hot-section components; to aid in the calibration, validation, and evaluation of the analytical tools by comparing predictions with experimental data; and to evaluate existing as well as advanced temperature and strain measurement instrumentation. As the analytical tools, test methods, tests, instrumentations, as well as data acquisition, management, and analysis methods are developed and evaluated, a proven, integrated analysis and experiment method will result in a more accurate prediction of the cyclic life of hot section components.

Thompson, Robert L.; Moorhead, Paul E.

1986-01-01

198

Effect of multiaxial stresses on the high-temperature behavior and rupture of advanced alloys  

NASA Astrophysics Data System (ADS)

The evolution and effect of multiaxial stress states on the high temperature deformation and rupture behavior of materials with non-uniform microstructures has been investigated. Through a detailed description of the role that multiaxial stresses play on damage evolution and rupture, the abundant existing data for uniaxial rupture can be used to more successfully design for the life of high temperature components. Three dimensional finite element calculations of primary creep deformation were performed for particulate reinforced metal matrix composites under a variety of multiaxial loading conditions. A quasi-steady state stress distribution develops during primary creep for each of the conditions considered. The results indicate that higher stresses exist in regions above and below the particles and accommodate the development of creep damage. The nature of the stress state within these regions is not significantly altered by the presence of the particles. The strain fields show a distribution similar to the stress fields. Despite significantly large regions of enhanced stress, the overall creep strain rates for all models are decreased by the presence of the particles. The applied effective stress does not have a unique relationship with overall effective strain rate for particulate reinforced composites under different applied stress states. The failure of sections of turbine rotor disks formed from the superalloy V-57 which operate under highly multiaxial stresses has been investigated. Optical microscopy of a turbine rotor disk removed from service after 30,000 hrs showed an intergranular crack that initiated at the root of a fir-tree turbine rotor blade attachment. Transmission electron microscopy studies showed heavy grain boundary oxidation that could account for the cracking and failure of the rotor disks. Heat treatments of a TiAl alloy have been established for producing a microstructure suitable for high temperature multiaxial rupture testing. The morphology of tested material has a fully lamellar microstructure with interlocking colony boundaries. Despite the interlocked boundaries, rupture time data correlates well with the principal facet stress, which indicates that a localized deformation mechanism controls the rupture process. Possible deformation mechanisms include lath boundary sliding and fast creep within the softer alphasb2 laths.

Johnson, Nancy Louise

1998-05-01

199

Properties of Fe-Al-Si high-damping steel  

NASA Astrophysics Data System (ADS)

Several high-damping materials have been developed in recent years to decrease vibration and noise levels of machines and structures. The most important damping steels have been Fe- 12Cr-based alloys. These steels exhibit high damping capacity combined with rather good mechanical and corrosion properties. A new vibration damping Fe-2.5Al-0.5Si steel has been developed by NKK-Corporation in Japan, and it is produced under a trade name of NKK- SERENA. This steel is a potential multi-purpose damping steel, because it is more economical than the previous steels. Damping capacity of NKK-SERENA is very high in wide temperature and frequency ranges, and its mechanical properties are similar to those of common structural steels. In this study, mechanical, welding and corrosion properties, and the results of the microstructural characterization are presented.

Aaltio, Ilkka; Ullakko, Kari; Hanninen, Hannu

1996-05-01

200

Specific heat of UNiAl in high magnetic fields  

NASA Astrophysics Data System (ADS)

Specific heat measurements on UNiAl in magnetic fields up to 20 T demonstrate that the Nel temperature ( TN = 19 K) is drastically reduced by magnetic fields B ? c-axis. AF ordering disappears for B > 11.35 T, the field of the metamagnetic transition. A step in ? versus B (to - 250 mJ/mol K 2) observed just above the critical field separates the low-field increasing tendency from the high-field decrease. At 20 T, a ?-value smaller than the zero-field value of 163 mJ/mol K 2 is found.

Brck, E.; Van der Meulen, H. P.; Menovsky, A. A.; De Boer, F. R.; De Chtel, P. F.; Franse, J. J. M.; Perenboom, J. A. A. J.; Berendschot, T. T. J. M.; Van Kempen, H.; Havela, L.; Sechovsk, V.

1992-02-01

201

Anodizing of High Electrically Stressed Components  

SciTech Connect

Anodizing creates an aluminum oxide coating that penetrates into the surface as well as builds above the surface of aluminum creating a very hard ceramic-type coating with good dielectric properties. Over time and use, the electrical carrying components (or spools in this case) experience electrical breakdown, yielding undesirable x-ray dosages or failure. The spool is located in the high vacuum region of a rod pinch diode section of an x-ray producing machine. Machine operators have recorded decreases in x-ray dosages over numerous shots using the reusable spool component, and re-anodizing the interior surface of the spool does not provide the expected improvement. A machine operation subject matter expert coated the anodized surface with diffusion pump oil to eliminate electrical breakdown as a temporary fix. It is known that an anodized surface is very porous, and it is because of this porosity that the surface may trap air that becomes a catalyst for electrical breakdown. In this paper we present a solution of mitigating electrical breakdown by oiling. We will also present results of surface anodizing improvements achieved by surface finish preparation and surface sealing. We conclude that oiling the anodized surface and using anodized hot dip sealing processes will have similar results.

Flores, P. [NSTec; Henderson, D. J. [NSTec; Good, D. E. [NSTec; Hogge, K. [NSTec; Mitton, C. V. [NSTec; Molina, I. [NSTec; Naffziger, C. [NSTec; Codova, S. R. [SNL; Ormond, E. U. [SNL

2013-06-01

202

High Temperature Deformation Behavior of As-Cast Al-3Mg-0.25Sc Alloy  

NASA Astrophysics Data System (ADS)

High temperature deformation behavior of as-cast Al-3Mg-0.25Sc alloy was characterized in a wide range of temperatures 573 K to 773 K (300 C to 500 C) and strain rates (0.001 to 10 s-1). A threshold stress behavior is observed in the temperature range of 573 K to 673 K (300 C to 400 C) due to interaction of dislocations with precipitates. In the temperature range of 573 K to 623 K (300 C to 350 C), the dislocations are pinned by precipitates, following Orowan-bowing mechanism. At 673 K (400 C), the dislocations bypass the particles by climb process. At high temperature and low strain rate of 723 K (450 C), and 0.001 s-1, respectively, the alloy exhibited high efficiency of power dissipation for optimal hot working process.

Suresh Kumar, M.; Ambresha; Venkateswarlu, K.; Ranganath, V. R.

2014-07-01

203

Transport characteristics of AlGaN/GaN/AlGaN double heterostructures with high electron mobility  

NASA Astrophysics Data System (ADS)

The AlGaN/GaN/AlGaN double heterostructure (DH) with high electron mobility of 1862 cm2/Vs at room temperature and 478 cm2/Vs at 573 K high temperature was obtained by a combination of optimization schemes considering scattering mechanisms. First, a composite buffer layer structure, including GaN and AlGaN layer, was used to improve the crystal quality of the AlGaN/GaN/AlGaN DH. Second, interface roughness scattering was reduced by increasing the channel thickness, thus the two-dimensional electron gas mobility was further improved. Moreover, an ultrathin AlN interlayer was inserted between the GaN channel layer and the AlGaN buffer layer to decrease the alloy disorder scattering. The Hall effect measurements showed that the DH had better transport characteristics at high temperatures, and an electron mobility of 478 cm2/Vs was achieved at 573 K, which is twice larger than that of the conventional single heterostructure (~200 cm2/Vs at 573 K). Therefore, AlGaN/GaN/AlGaN DH is more suitable for the applications in high temperature electronic devices.

Meng, Fanna; Zhang, Jincheng; Zhou, Hao; Ma, Juncai; Xue, Junshuai; Dang, Lisha; Zhang, Linxia; Lu, Ming; Ai, Shan; Li, Xiaogang; Hao, Yue

2012-07-01

204

The High Secure Psychiatric Hospitals Nursing Staff Stress Survey 3: Identifying stress resistance resources in the stress process of physical assault  

Microsoft Academic Search

The present paper forms part of the High Secure Psychiatric Hospitals Nursing Staff Stress Survey and explores the causal role of generalized and specific stress resistance resources (SRRs) in the stress process following physical assault. Confidential self-report questionnaires were administered to all nurses working in English and Scottish High Secure Psychiatric Hospitals. A total of 636 nurses responded to questionnaires

Ulrich Reininghaus; Tom Craig; Kevin Gournay; Patrick Hopkinson; Jerome Carson

2007-01-01

205

TorsinA rescues ER-associated stress and locomotive defects in C. elegans models of ALS  

PubMed Central

ABSTRACT Molecular mechanisms underlying neurodegenerative diseases converge at the interface of pathways impacting cellular stress, protein homeostasis and aging. Targeting the intrinsic capacities of neuroprotective proteins to restore neuronal function and/or attenuate degeneration represents a potential means toward therapeutic intervention. The product of the human DYT1 gene, torsinA, is a member of the functionally diverse AAA+ family of proteins and exhibits robust molecular-chaperone-like activity, both in vitro and in vivo. Although mutations in DYT1 are associated with a rare form of heritable generalized dystonia, the native function of torsinA seems to be cytoprotective in maintaining the cellular threshold to endoplasmic reticulum (ER) stress. Here we explore the potential for torsinA to serve as a buffer to attenuate the cellular consequences of misfolded-protein stress as it pertains to the neurodegenerative disease amyotrophic lateral sclerosis (ALS). The selective vulnerability of motor neurons to degeneration in ALS mouse models harboring mutations in superoxide dismutase (SOD1) has been found to correlate with regional-specific ER stress in brains. Using Caenorhabditis elegans as a system to model ER stress, we generated transgenic nematodes overexpressing either wild-type or mutant human SOD1 to evaluate their relative impact on ER stress induction in vivo. These studies revealed a mutant-SOD1-specific increase in ER stress that was further exacerbated by changes in temperature, all of which was robustly attenuated by co-expression of torsinA. Moreover, through complementary behavioral analysis, torsinA was able to restore normal neuronal function in mutant G85R SOD1 animals. Furthermore, torsinA targeted mutant SOD1 for degradation via the proteasome, representing mechanistic insight on the activity that torsinA has on aggregate-prone proteins. These results expand our understanding of proteostatic mechanisms influencing neuronal dysfunction in ALS, while simultaneously highlighting the potential for torsinA as a novel target for therapeutic development.

Thompson, Michelle L.; Chen, Pan; Yan, Xiaohui; Kim, Hanna; Borom, Akeem R.; Roberts, Nathan B.; Caldwell, Kim A.; Caldwell, Guy A.

2014-01-01

206

Stress evolution during growth of GaN (0001)/Al2O3(0001) by reactive dc magnetron sputter epitaxy  

NASA Astrophysics Data System (ADS)

We study the real time stress evolution, by in situ curvature measurements, during magnetron sputter epitaxy of GaN (0?0?0?1) epilayers at different growth temperatures, directly on Al2O3(0?0?0?1) substrates. The epilayers are grown by sputtering from a liquid Ga target in a mixed N2/Ar discharge. For 600 C, a tensile biaxial stress evolution is observed, while for 700 C and 800 C, compressive stress evolutions are observed. Structural characterization by cross-sectional transmission electron microscopy, and atomic force microscopy, revealed that films grew at 700 C and 800 C in a layer-by-layer mode while a growth temperature of 600 C led to an island growth mode. High resolution x-ray diffraction data showed that edge and screw threading dislocation densities decreased with increasing growth temperature, with a total density of 5.5 1010 cm?2 at 800 C. The observed stress evolution and growth modes are explained by a high surface mobility during magnetron sputter epitaxy at 700800 C. Other possible reasons for the different stress evolutions are also discussed.

Junaid, M.; Sandstrm, P.; Palisaitis, J.; Darakchieva, V.; Hsiao, C.-L.; Persson, P. O. .; Hultman, L.; Birch, J.

2014-04-01

207

Internal stress and solid solubility effects on the thermal expansivity of Al-Si eutectic alloys  

SciTech Connect

Thermal expansion measurements are reported for a number of as-cast Al-Si eutectic alloys including a Sr-modified alloy which gives nearly spherical Si particles. The measurements were obtained by heating and cooling over repeated temperature cycles between room temperature and 500/degree/C. In general, lower expansivity values were measured on the cooling cycle as compared with the heating cycle, resulting in a net positive permanent deformation at room temperature. Analytical solutions are described for the thermal expansivity of a concentric-spheres model for a Si particle contained within an Al matrix. The effect of plastic flow in the Al is included. Overall, the predictions show reasonable agreement with the measured expansivities. The observed differences between heating and cooling are of the same order as that which is predicted. At high temperatures, the measured increase in expansivities is smaller than calculated. The latter effect is explained by the decrease in expansivity which results from an increasing solid solubility of silicon in aluminum with increasing temperature.

Hahn, T.A.; Armstrong, R.W.

1988-03-01

208

Quantitative prediction of twinning stress in fcc alloys: Application to Cu-Al  

NASA Astrophysics Data System (ADS)

Twinning is one of most prevalent deformation mechanisms in materials. Having established a quantitative theory to predict onset twinning stress ?crit in fcc elemental metals from their generalized planar-fault-energy (GPFE) surface, we exemplify its use in alloys where the Suzuki effect (i.e., solute energetically favors residing at and near planar faults) is operative; specifically, we apply it in Cu-xAl ( x is 0, 5, and 8.3at.% ) in comparison with experimental data. We compute the GPFE via density-functional theory, and we predict the solute dependence of the GPFE and ?crit , in agreement with measured values. We show that ?crit correlates monotonically with the unstable twin fault energies (the barriers to twin nucleation) rather than the stable intrinsic stacking-fault energies typically suggested. We correlate the twinning behavior and electronic structure with changes in solute content and proximity to the fault planes through charge-density redistribution at the fault and changes to the layer- and site-resolved density of states, where increased bonding charge correlates with decrease in fault energies and ?crit .

Kibey, Sandeep A.; Wang, L. L.; Liu, J. B.; Johnson, H. T.; Sehitoglu, H.; Johnson, D. D.

2009-06-01

209

Stress Concentration and Fracture at Inter-variant Boundaries in an Al-Li Alloy  

NASA Technical Reports Server (NTRS)

Delamination fracture has limited the use of lightweight Al-Li alloys. Studies of secondary, delamination cracks in alloy 2090, L-T fracture toughness samples showed grain boundary failure between variants of the brass texture component. Although the adjacent texture variants, designated B(sub s1) and B(sub s2), behave similarly during rolling, their plastic responses to mechanical tests can be quite different. EBSD data from through-thickness scans were used to generate Taylor factor maps. When a combined boundary normal and shear tensor was used in the calculation, the delaminating grains showed the greatest Taylor Factor differences of any grain pairs. Kernel Average Misorientation (KAM) maps also showed damage accumulation on one side of the interface. Both of these are consistent with poor slip accommodation from a crystallographically softer grain to a harder one. Transmission electron microscopy was used to confirm the EBSD observations and to show the role of slip bands in the development of large, interfacial stress concentrations. A viewgraph presentation accompanies the provided abstract.

Crooks, Roy; Tayon, Wes; Domack, Marcia; Wagner, John; Beaudoin, Armand

2009-01-01

210

Damage Assessment of Stress-Thermal Cycled high temperature  

NASA Technical Reports Server (NTRS)

We report on the characterization of bismaleimide and polyimide carbon fiber composite, microcrack development under stress thermal cycling loading. Such cycles range from cryogenic temperatures associated with cryogenic fuel (LN, LOX) containment to high temperatures of 300 degrees Celsius associated with future hypervelocity aeropropulsion systems. Microcrack development thresholds as a function of temperature range of the thermal cycle; the number of cycles; the applied stress level imposed on the composite are reported. We have conducted stress-thermal cycles on thin bismaleimide-woven carbon fiber foils for three temperature range cycles: 1. Ambient temperature - -196 degrees celsius. 2. Ambient temperature - 150 degrees Celsius; 200 degrees Celsius; 250 degrees Celsius. 3. -196 degrees Celsius - 250 degrees Celsius. The Principle findings are that the full cycles from -196 degrees Celsius to to 250 degrees Celsius cause the most significant microcrack of development. These observations indicate that the high temperature portion of the cycle under load causes fiber-matrix interface failure and subsequent exposure to higher stresses at the cryogenic, low temperature region results in composite matrix microcracking as a result of the additional stresses associate with the fiber-matrix thermal expansion mismatch. Our initial studies for 12 ply PMR-II-50 polyimide/M60JB carbon fabric [0f,90f,90f,0f,0f,90f]ls composites will be presented. The stress-thermal cycle test procedure for these will be described. Moisture absorption characteristics between cycles will be used to monitor interconnected microcrack development. The applied stress level will be 75% of the composite cryogenic (-196 degrees Celsius) ultimate strength.

Ju, Jae-Hyung; Prochazka, Michael; Ronke, Ben; Morgan, Roger; Shin, Eugence

2004-01-01

211

Hydriding behavior of MgAl and leached MgAl compounds prepared by high-energy ball-milling  

Microsoft Academic Search

The structure and hydrogen absorption properties of Mg:Al alloys prepared by high-energy ball milling were studied over the whole compositional range. These materials were prepared in their as-milled and Al-leached forms. The latter are obtained from the former materials by leaching out Al in a 1 N NaOH solution. The structure of the various alloys was determined by X-ray diffraction.

S. Bouaricha; J. P. Dodelet; D. Guay; J. Huot; S. Boily; R. Schulz

2000-01-01

212

Creep and rupture of an ods alloy with high stress rupture ductility  

NASA Astrophysics Data System (ADS)

The creep and stress rupture properties of an oxide (Y2O3) dispersion strengthened nickel-base alloy, which also is strengthened by ?' precipitates, was studied at 760 C and 1093 C. At both temperatures the alloy YDNiCrAl exhibits unusually high stress rupture ductility as measured by both elongation and reduction in area. Failure was transgranular, and different modes of failure were observed including crystallographic fracture at intermediate temperatures and tearing or necking almost to a chisel point at higher temperatures. While the rupture ductility was high, the creep strength of the alloy was low relative to conventional ?' strengthened superalloys in the intermediate temperature range and to ODS alloys in the higher temperature range. These findings are discussed with respect to the alloy composition; the strengthening oxide phases, which are inhomogeneously dispersed; the grain morphology, which is coarse and elongated and exhibits many included grains; and the second phase inclusion particles occurring at grain boundaries and in the matrix. The creep properties, in particular the high stress dependencies and high creep activation energies measured, are discussed with respect to the resisting stress model of creep in particle strengthened alloys.

McAlarney, Mona E.; Arons, Richard M.; Howson, Tim E.; Tien, John K.; Baranow, Sanford

1982-08-01

213

A novel surface modification technique for the introduction of compressive residual stress and preliminary studies on Al alloy AA6063  

Microsoft Academic Search

A new peening process for introducing compressive residual stresses with less erosion and good surface finish using a high pressure oil jet was developed. Aluminium alloy, AA 6063 was peened with an oil pressure of 50MPa at different stand-off-distances. Residual stresses measured using X-ray diffraction indicated the induction of about 50MPa (compressive) residual stresses on the surface. The depth of

A. Sahaya Grinspan; R. Gnanamoorthy

2006-01-01

214

Characterization of Detergent-Insoluble Proteins in ALS Indicates a Causal Link between Nitrative Stress and Aggregation in Pathogenesis  

PubMed Central

Background Amyotrophic lateral sclerosis (ALS) is a progressive and fatal motor neuron disease, and protein aggregation has been proposed as a possible pathogenetic mechanism. However, the aggregate protein constituents are poorly characterized so knowledge on the role of aggregation in pathogenesis is limited. Methodology/Principal Findings We carried out a proteomic analysis of the protein composition of the insoluble fraction, as a model of protein aggregates, from familial ALS (fALS) mouse model at different disease stages. We identified several proteins enriched in the detergent-insoluble fraction already at a preclinical stage, including intermediate filaments, chaperones and mitochondrial proteins. Aconitase, HSC70 and cyclophilin A were also significantly enriched in the insoluble fraction of spinal cords of ALS patients. Moreover, we found that the majority of proteins in mice and HSP90 in patients were tyrosine-nitrated. We therefore investigated the role of nitrative stress in aggregate formation in fALS-like murine motor neuron-neuroblastoma (NSC-34) cell lines. By inhibiting nitric oxide synthesis the amount of insoluble proteins, particularly aconitase, HSC70, cyclophilin A and SOD1 can be substantially reduced. Conclusion/Significance Analysis of the insoluble fractions from cellular/mouse models and human tissues revealed novel aggregation-prone proteins and suggests that nitrative stress contribute to protein aggregate formation in ALS.

Basso, Manuela; Samengo, Giuseppina; Nardo, Giovanni; Massignan, Tania; D'Alessandro, Giuseppina; Tartari, Silvia; Cantoni, Lavinia; Marino, Marianna; Cheroni, Cristina; De Biasi, Silvia; Giordana, Maria Teresa; Strong, Michael J.; Estevez, Alvaro G.; Salmona, Mario; Bendotti, Caterina; Bonetto, Valentina

2009-01-01

215

High quality AlGaN grown on ELO AlN/sapphire templates  

NASA Astrophysics Data System (ADS)

The defect structure and the homogeneity of 1-3 m thick AlxGa1-xN layers grown on epitaxially laterally overgrown (ELO) AlN on patterned AlN/sapphire templates have been investigated in dependence on the miscut direction of the c-plane sapphire substrates, the etching depth into the sapphire and the Al concentration. It was found that shallowly etched AlN/sapphire templates with a 0.25 miscut toward the a-plane provide a smooth surface of ELO AlN and therefore a good Al homogeneity in the overgrown Al0.8Ga0.2N layer. The threading dislocation density in these layers is as low as 5108 cm-2.

Zeimer, U.; Kueller, V.; Knauer, A.; Mogilatenko, A.; Weyers, M.; Kneissl, M.

2013-08-01

216

Gas Evolution From Transformer Oils Under High-Voltage Stress  

Microsoft Academic Search

Some insulating oils when subjected to high-voltage stress undergo chemical changes that result in the formation of various gases. Since there are different insulating oils available to users that are made by various finishing techniques resulting in oils of significantly varied composition, a study of a range of these materials was made to show if they produced different amounts or

T. K. Sloat; J. L. Johnson; G. M. L. Sommerman

1967-01-01

217

LIQUEFACTION OF DEEP SATURATED SANDS UNDER HIGH EFFECTIVE CONFINING STRESS  

Microsoft Academic Search

This paper describes the findings of an ongoing experimental study supported by the U.S. Army Centrifuge Research Center and Engineer Earthquake Engineering Research Program (EQEN) into the behavior of saturated sands under high initial effective confining stresses subjected to strong ground shaking. The research was conducted using the Army Centrifuge at the U.S. Army Engineering Research and Development Center (ERDC),

R Scott Steedman; Michael Sharp

218

The Effect of Ballistic Impacts on the High Cycle Fatigue Properties of Ti-48Al-2Nb-2Cr (at.%)  

NASA Technical Reports Server (NTRS)

The ability of gamma - TiAl to withstand potential foreign and/or domestic object damage is a technical risk to the implementation of gamma - TiAl in low pressure turbine (LPT) blade applications. The overall purpose of the present study was to determine the influence of ballistic impact damage on the high cycle fatigue strength of gamma - TiAl simulated LPT blades. Impact and specimen variables included ballistic impact energy, projectile hardness, impact temperature, impact location, and leading edge thickness. The level of damage induced by the ballistic impacting was studied and quantified on both the impact (front) and backside of the specimens. Multiple linear regression was used to model the cracking and fatigue response as a function of the impact variables. Of the impact variables studied, impact energy had the largest influence on the response of gamma - TiAl to ballistic impacting. Backside crack length was the best predictor of remnant fatigue strength for low energy impacts (<0.74J) whereas Hertzian crack length (impact side damage) was the best predictor for higher energy impacts. The impacted gamma - TiAl samples displayed a classical mean stress dependence on the fatigue strength. For the fatigue design stresses of a 6th stage LPT blade in a GE90 engine, a Ti-48Al-2Nb-2Cr LPT blade would survive an impact of normal service conditions.

Draper, S. L.; Lerch, B. A.; Pereira, J. M.; Nathal, M. V.; Austin, C. M.; Erdman, O.

2000-01-01

219

ZERODUR glass ceramics: design of structures with high mechanical stresses  

NASA Astrophysics Data System (ADS)

Designing highly mechanically loaded structures made of the zero expansion glass ceramic material ZERODUR means to analyze the stress for the whole loaded surface, considering changes of the stress state occurring over the total lifetime. Strength data are obtained from specimens with small size and relatively short loading duration, making them not directly applicable to the much larger areas that occur in practical cases. This publication gives guidelines for calculating a fracture probability for mirrors and structures on the basis of existing strength data.

Nattermann, Kurt; Hartmann, Peter; Kling, Guenther; Gath, Peter; Lucarelli, Stefano; Messerschmidt, Boris

2008-07-01

220

Preparation of TiAl 3 Al Composite Coating by Cold Spray and Its High Temperature Oxidation Behavior  

Microsoft Academic Search

A novel TiAl3-Al coating was prepared by cold spray for high temperature protection of titanium aluminum-based alloy. The substrate alloy\\u000a was orthorhombic-Ti-22Al-26Nb (at.%). The composite coating was mainly composed of TiAl3 embedded in the matrix of residual aluminum. An interlayer about 10?m was formed between the coating and the substrate.\\u000a The oxidation test indicated that this composite coating was very

L. Y. Kong; L. Shen; B. Lu; R. Yang; X. Y. Cui; T. F. Li; T. Y. Xiong

2010-01-01

221

Trap states in InAlN/AlN/GaN-based double-channel high electron mobility transistors  

NASA Astrophysics Data System (ADS)

We present a detailed analysis of trap states in InAlN/AlN/GaN double-channel high electron mobility transistors grown by pulsed metal organic chemical vapor deposition. By frequency dependent conductance measurements, trap densities and time constants at both InAlN/AlN/GaN interfaces were determined. Two types of traps, with a high density of up to ~1014 cm-2 eV-1, were observed existing at the higher InAlN/AlN/GaN interface. On the other hand, the density dramatically decreased to ~1012 cm-2 eV-1 for traps located at lower InAlN/AlN/GaN interface on which a low-temperature grown GaN (LT-GaN) layer was deposited. Additionally, photo-assisted capacitance-voltage measurements were performed to estimate deep-level defects, yielding a low density of 1.79 1011 cm-2 acting as negative fixed charges at the LT-GaN and lower InAlN interface.

Zhang, Kai; Xue, JunShuai; Cao, MengYi; Yang, LiYuan; Chen, YongHe; Zhang, JinCheng; Ma, XiaoHua; Hao, Yue

2013-05-01

222

Fatigue crack closure behavior at high stress ratios  

NASA Technical Reports Server (NTRS)

Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.

Turner, C. Christopher; Carman, C. Davis; Hillberry, Ben M.

1988-01-01

223

Resilience in highly stressed urban children: concepts and findings.  

PubMed Central

The Rochester Child Resilience Project is a coordinated set of studies of the correlates and antecedents of outcomes relating to resilience among profoundly stressed urban children. The studies have been conducted over the course of the past decade. Based on child test data, parent, teacher, and self ratings of child adjustment, and in-depth individual interviews with parents and children, a cohesive picture has developed of child and family milieu variables that consistently differentiate children with resilient versus stress-affected outcomes within this highly stressed sample. Resilient children are characterized by an easy temperament and higher IQ; sound parent/child relationships; a parent's sense of efficacy; the parent's own wellness, especially mental health; and the child's perceived competence, realistic control, empathy, and social problem-solving.

Cowen, E. L.; Wyman, P. A.; Work, W. C.

1996-01-01

224

A High-Resolution Tectonic Stress Map of Iceland  

NASA Astrophysics Data System (ADS)

Iceland is one of few locations in the world where oceanic transform faulting processes can be observed on-land. The plate boundary through Iceland consists of oblique rift zones and transform segments with and without well-defined strike-slip faults. Since the installation of the South Iceland Lowland (SIL) seismic network in 1990, approximately half a million earthquakes (M>-2) have been recorded along the Icelandic plate boundary. Here we use the focal mechanisms of more than 200,000 earthquakes to map the state of stress along the plate boundary in detail. In addition, we study temporal variations in the stress field throughout the recent magnitude 6.5 earthquake sequence in the South Iceland Seismic Zone. We demonstrate here how a three-dimensional gridding method ('octree') can be used to examine temporal changes in stress by determining stress parameters at common points at different epochs. Our results provide a high-resolution map of the tectonic stress field in Iceland and enable us to examine how recent large earthquakes have affected adjacent areas.

Lund, B.; Townend, J.

2013-12-01

225

Influence of Al2O3 layer thickness on high-temperature stability of TiAlN/Al2O3 multilayers  

NASA Astrophysics Data System (ADS)

TiAlN/Al2O3 multilayers which have constant TiAlN layer thickness (10 nm) and various Al2O3 layer thicknesses ranging from 0.5 nm to 4.5 nm were synthesized on alumina substrate by magnetron sputtering. The effects of annealing on the mechanical and structural properties of the multilayers were investigated using X-ray diffractometry (XRD), X-ray reflection (XRR), X-ray photoelectron spectroscopy (XPS), and Nanoindenter. It was found that the hardness for the multilayers with Al2O3 layer thickness from 0.5 nm to 4.5 nm was much higher than TiAlN or Al2O3 monolayer and their hardness values were over 36 GPa. The annealed multilayers displayed high-temperature stable hardness and elastic modulus. The hardness increases from 36 GPa of as-deposited to 39 GPa of annealed multilayer at 700 C in the case of lO=1.2?nm. It also indicates the highest elastic modulus of 560 GPa after 700 C annealing. The multilayers had polycrystallines of TiAlN(1 1 1) and TiAlN(2 2 2) textures. Compared with as-deposited multilayers, the annealed multilayers exhibited unchanged textures. The interface and layered structure also showed good high-temperature stability.

Gao, C. K.; Yan, J. Y.; Dong, L.; Li, D. J.

2013-11-01

226

High aluminum resistance in buckwheat. I. Al-induced specific secretion of oxalic acid from root tips  

PubMed

High Al resistance in buckwheat (Fagopyrum esculentum Moench. cv Jianxi) has been suggested to be associated with both internal and external detoxification mechanisms. In this study the characteristics of the external detoxification mechanism, Al-induced secretion of oxalic acid, were investigated. Eleven days of P depletion failed to induce secretion of oxalic acid. Exposure to 50 &mgr;M LaCl3 also did not induce the secretion of oxalic acid, suggesting that this secretion is a specific response to Al stress. Secretion of oxalic acid was maintained for 8 h by a 3-h pulse treatment with 150 &mgr;M Al. A nondestructive method was developed to determine the site of the secretion along the root. Oxalic acid was found to be secreted in the region 0 to 10 mm from the root tip. Experiments using excised roots also showed that secretion was located on the root tip. Four kinds of anion-channel inhibitors showed different effects on Al-induced secretion of oxalic acid: 10 &mgr;M anthracene-9-carboxylic acid and 4, 4'-diisothiocyanatostilbene-2,2'-disulfonate had no effect, niflumic acid stimulated the secretion 4-fold, and phenylglyoxal inhibited the secretion by 50%. Root elongation in buckwheat was not inhibited by 25 &mgr;M Al or 10 &mgr;M phenylglyoxal alone but was inhibited by 40% in the presence of Al and phenylglyoxal, confirming that secretion of oxalic acid is associated with Al resistance. PMID:9662517

Jian Zheng S; Feng Ma J; Matsumoto

1998-07-01

227

Proteomic analysis of a high aluminum tolerant yeast Rhodotorula taiwanensis RS1 in response to aluminum stress.  

PubMed

Rhodotorula taiwanensis RS1 is a high-aluminum (Al)-tolerant yeast that can survive in Al concentrations up to 200mM. The mechanisms for the high Al tolerance of R. taiwanensis RS1 are not well understood. To investigate the molecular mechanisms underlying Al tolerance and toxicity in R. taiwanensis RS1, Al toxicity-induced changes in the total soluble protein profile were analyzed using two-dimensional gel electrophoresis (2-DE) coupled with mass spectrometry. A total of 33 differentially expressed proteins responding to Al stress were identified from approximately 850 reproducibly detected proteins. Among them, the abundance of 29 proteins decreased and 4 increased. In the presence of 100mM Al, the abundance of proteins involved in DNA transcription, protein translation, DNA defense, Golgi functions and glucose metabolism was decreased. By contrast, Al treatment led to increased abundance of malate dehydrogenase, which correlated with increased malate dehydrogenase activity and the accumulation of intracellular citrate, suggesting that Al-induced intracellular citrate could play an important role in detoxification of Al in R. taiwanensis RS1. PMID:23831152

Wang, Chao; Wang, Chang Yi; Zhao, Xue Qiang; Chen, Rong Fu; Lan, Ping; Shen, Ren Fang

2013-10-01

228

High-temperature mechanical properties and deformation behavior of high Nb containing TiAl alloys fabricated by spark plasma sintering  

NASA Astrophysics Data System (ADS)

A high Nb containing TiAl alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y (at%) by spark plasma sintering (SPS). Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050C and a strain rate range of 0.002 to 0.2 s-1. The results show that the high-temperature mechanical properties of the high Nb containing TiAl alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900C, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900C exhibit obvious flow softening after the peak stress. Under the deformation condition of 900-1050C and 0.002-0.2 s-1, the interrelations of peak flow stress, strain rate, and deformation temperature follow the Arrhenius' equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJmol-1.

Lu, Xin; Zhao, Li-hua; Zhu, Lang-ping; Zhang, Bin; Qu, Xuan-hui

2012-04-01

229

Al-rich precipitation in CoNiCrAlY bondcoat at high temperature  

NASA Astrophysics Data System (ADS)

A thermal barrier coating (TBC) is applied on a surface of a gas turbine blade to provide a thermal barrier and oxidation resistant properties for the components. The ability to resist oxidation of the coating arises from the self-healing, protective Al2O3 scale on top of the bondcoat, which is formed during service. However, if Al depletion occurs within the bondcoat, the protective scale will lose its self-healing ability, and hence, its oxidation-resistant property. This paper investigated the depletion of Al within the bondcoat by studying the microstructure of the bondcoat on a gas turbine blade after it has been in 4000 h service at 1200 C. The results showed that Al depletion had occurred at different levels throughout the turbine blade. In the area where Al depletion had not yet occurred, precipitation of an Al-rich phase was detected. Most of the Al was contained within this phase, leaving only small amount of Al in the surrounding matrix. A well-defined boundary was observed between the depleted and non-depleted regions.

Koomparkping, T.; Damrongrat, S.; Niranatlumpong, P.

2005-06-01

230

Magnetotransport properties of high equivalent Al composition AlGaN/GaN heterostructures using AlN/GaN superlattice as a barrier  

NASA Astrophysics Data System (ADS)

Magnetotransport properties of the two-dimensional electron gas (2DEG) in high equivalent Al composition AlGaN/GaN heterostructures using AlN/GaN superlattice as a barrier have been studied at low temperatures and high magnetic fields. Well resolved Shubnikov-de Haas oscillations were observed, indicating excellent quality of the quasi-AlGaN/GaN heterostructures. It is measured that the energy separation between the two subbands in the GaN triangular quantum well can be as large as 180.5 meV, depicting strong quantum confinement at the heterointerface. The strong quantum confinement results in a high 2DEG density of 2 1013 cm-2. The persistent photoconductivity investigation also indicates that the superlattice barrier layer has a low density of impurities/defects. It is believed that the AlN/GaN superlattice, instead of high Al composition alloy AlGaN layer, could greatly improve the device performance.

Liu, S. D.; Tang, N.; Shen, X. Q.; Duan, J. X.; Lu, F. C.; Yang, X. L.; Xu, F. J.; Wang, X. Q.; Ide, T.; Shimizu, M.; Ge, W. K.; Shen, B.

2013-07-01

231

Structure, stress, and magnetic properties of high saturation magnetization films of FeCo  

Microsoft Academic Search

Thin (?2000 ) films of Fe60Co40 have been prepared by sputtering on Si\\/SiO2, Si\\/AlOx, AlTiC\\/AlOx, AlTiC\\/AlOx\\/NiFe, and AlTiC\\/AlOx\\/NiFeCr substrates using an alloy Fe60Co40 target. The structure of the films was characterized by means of X-ray diffraction and transmission electron microscopy (TEM). Surface roughness was characterized by atomic force microscopy (AFM). Stress, transport, and magnetic measurements were conducted. We observed correlation

V. A. Vas'ko; James O. Rantschler; Mark T. Kief

2004-01-01

232

Crack growth and residual stress in Al-Li metal matrix composites under far-field cyclic compression  

Microsoft Academic Search

The growth of cracks under far-field cyclic compressive loading in aluminium-lithium (Al-Li) alloys reinforced with SiC particulates is investigated in notched compact tension specimens (CT). When cracks were initiated from the root of the notch, progressive deceleration occurred with the initial crack growth being largest. After crack arrest, analysis indicated that the initial residual stress diminished as the crack became

R. Hermann

1995-01-01

233

Stress-induced martensitic transformation in a Ti45Zr38Al17 cast rod  

NASA Astrophysics Data System (ADS)

An ordered B2 intermetallic compound with composition Ti45Zr38Al17 was prepared by copper mold casting. The density of the alloy is 5.078 g/cm3, whereas the Poisson's ratio and the Young's modulus are 0.38 and 70 GPa, respectively. Room temperature compression tests show good mechanical properties, namely, a high strength of 1650 MPa combined with a plastic strain of about 19.5 %. These parameters make the material an interesting candidate for biomedical applications like prostheses. Structure investigations reveal that during compression the alloy undergoes a martensitic transformation which proceeds from the parent B2 structure to the orthorhombic B19 structure through an intermediate step involving a hexagonal phase.

Nikolowski, K.; Scudino, S.; Stoica, M.; Surreddi, K. B.; Das, J.; Eckert, J.

2009-01-01

234

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs  

SciTech Connect

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 10{sup 8} cm{sup -2}. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.

Coerekci, S., E-mail: scorekci@kirklareli.edu.tr [K Latin-Small-Letter-Dotless-I rklareli University, Department of Physics (Turkey); Oeztuerk, M. K. [Gazi University, Department of Physics (Turkey); Yu, Hongbo [Bilkent University, Nanotechnology Research Center (Turkey); Cakmak, M.; Oezcelik, S. [Gazi University, Department of Physics (Turkey); Oezbay, E. [Bilkent University, Nanotechnology Research Center, Department of Physics, Department of Electrical and Electronics Engineering (Turkey)

2013-06-15

235

The influence of water on the Peierls stress of olivine at high pressures  

NASA Astrophysics Data System (ADS)

To investigate the influence of water on the low-temperature plasticity of olivine under lithospheric conditions, we carried out a series of creep experiments on polycrystalline olivine at high pressures (~6 GPa), relatively low temperatures (873 ? T ? 1173 K), and hydrous conditions using a deformation-DIA. Samples were fabricated from fine powdered San Carlos olivine under hydrous conditions. In the experiments, a sample column composed of a sample and alumina pistons was assembled with a talc sleeve and graphite resistance heater into a 6.2-mm edge length cubic pressure medium. Experiments were carried out at the National Synchrotron Light Source at Brookhaven National Laboratory. In a run, differential stress and sample displacement were monitored in-situ using synchrotron x-ray diffraction and radiography, respectively. The low-temperature plasticity of olivine under hydrous conditions is constrained by our data with a Peierls stress of 4.2 0.3 GPa. This value is much lower than those reported the Peierls stress for olivine under anhydrous conditions (~ 6 - 15 GPa, Evans and Goetze, 1979; Raterron et al., 2004; Mei at al., 2010), indicating a significant influence of water on the low-temperature plasticity of olivine. The low-temperature flow behavior of olivine under hydrous conditions quantified in this study provides a necessary constraint for modeling the dynamic activities occurring within lithospheric mantle especially for those regions with the presence of water such as beneath a mid-ocean ridge and along a subducting slab.

Mei, S.; Suzuki, A. M.; Xu, L.; Kohlstedt, D. L.; Dixon, N. A.; Durham, W. B.

2012-04-01

236

Finite element stress analysis of polymers at high strains  

NASA Technical Reports Server (NTRS)

A numerical analysis is presented for the problem of a flat rectangular rubber membrane with a circular rigid inclusion undergoing high strains due to the action of an axial load. The neo-hookean constitutive equations are introduced into the general purpose TITUS program by means of equivalent hookean constants and initial strains. The convergence is achieved after a few iterations. The method is not limited to any specific program. The results are in good agreement with those of a company sponsored photoelastic stress analysis. The theoretical and experimental deformed shapes also agree very closely with one another. For high strains it is demonstrated that using the conventional HOOKE law the stress concentration factor obtained is unreliable in the case of rubberlike material.

Durand, M.; Jankovich, E.

1973-01-01

237

High-pressure and high-temperature stability field of hydrous phase delta-AlOOH  

NASA Astrophysics Data System (ADS)

Stability field of hydrous phases is a key for understanding water concentration in the earth's mantle. ?-AlOOH is a high-pressure polymorph of diaspore (?-AlOOH) and boehmite (?-AlOOH). The space group of this phase is Pnn2 and it is similar to CaCl2-type SiO2 which is a high-pressure polymorph of stishovite; edge-sharing Al-O octahedra make single-chain along c-axis. Although it has a large stability field in pressure range from 18 GPa to 32 GPa and temperature of up to 1473 K, the high-pressure stability limits has not yet clarified. In this study, we investigated the stability field of ?-AlOOH up to 130 GPa. The high-pressure experiments were performed using a laser-heated diamond-anvil cell. Starting material was gibbsite (Al(OH)3) powder mixed with platinum black as a laser absorber. The sample was sandwiched by pure gibbsite layers and loaded into a rhenium gasket. Pressures were measured with ruby-fluorescence technique before and after heating. The sample was heated from both sides by a Nd:YAG laser operated in multimode. After experiment, stable phase in each condition was determined using the X-ray diffraction and Raman spectroscopy method. We also conducted in-situ X-ray diffraction experiments under high-pressure and temperature conditions together with the X-ray diffraction of the recovered samples both at BL10XU in SPring-8 and BL-13A in Photon Factory. In these runs, pressures were also calculated using the equation of state of platinum. The experimental conditions were in the pressures between 50-130 GPa and temperatures to 1800 K. X-ray analysis shows the sample recovered from the pressure and temperature range from 40 GPa and 1577 K to 130 GPa and 1800 K consists of ?-AlOOH. In the in-situ experiments, crystallization of ?-AlOOH was observed at 65 GPa and 1300 K and it was stable up to 1700 K. ?-AlOOH is stable in the large pressure range with a dehydration temperature around 1800K. It can be a water reservoir in subducting slabs in the deep lower mantle conditions beyond the dehydration conditions of the other hydrous phases such as dense hydrous magnesium silicates.

Sano, A.; Ohtani, E.; Kondo, T.; Hirao, N.; Sone, T.; Kikegawa, T.; Sata, N.; Ohishi, Y.

2005-12-01

238

High cycle fatigue behavior of implant Ti-6Al-4V in air and simulated body fluid.  

PubMed

Ti-6Al-4V implants that function as artificial joints are usually subjected to long-term cyclic loading. To study long-term fatigue behaviors of implant Ti-6Al-4V in vitro and in vivo conditions exceeding 107 cycles, constant stress amplitude fatigue experiments were carried out at ultrasonic frequency (20 kHz) with two different surface conditions (ground and polished) in ambient air and in a simulated body fluid. The initiation mechanisms of fatigue cracks were investigated with scanning electron microscopy. Improvement of fatigue strength is pronounced for polished specimens below 106 cycles in ambient air since fatigue cracks are initiated from surfaces of specimens. While the cycles exceed 106, surface conditions have no effect on fatigue behaviors because the defects located within the specimens become favorable sites for crack initiation. The endurance limit at 108 cycles of polished Ti-6Al-4V specimens decreases by 7% if it is cycled in simulated body fluid instead of ambient air. Fracture surfaces show that fatigue failure is initiated from surfaces in simulated body fluid. Surface improvement has a beneficial effect on fatigue behaviors of Ti-6Al-4V at high stress amplitudes. The fatigue properties of Ti-6Al-4V deteriorate and the mean endurance limits decrease significantly in simulated body fluid. PMID:24211906

Liu, Yong-jie; Cui, Shi-ming; He, Chao; Li, Jiu-kai; Wang, Qing-yuan

2014-01-01

239

Space charge accumulation process in PET film at high temperature under high electric stress  

Microsoft Academic Search

Dynamic behavior of charge accumulation in PET film at high temperature under high electric stress has been investigated using pulsed electro-acoustic (PEA) method. Since dielectric materials such as an electric motor and a generator are sometimes used in environment of high temperature, the performance of the dielectric strength at high temperature is important. There are a few reports, however, about

Seiichi Ishizaki; Kazuma Miyatake; Yasuhiro Tanaka; Tatsuo Takada

2002-01-01

240

Electrical and optical characteristics of water under high electric stress  

Microsoft Academic Search

In order to determine the electrical characteristics of water under high electric stress, we have measured the current-electric field (I-E) characteristics of distilled water with a resistivity of 200 k?cm, up to electric field intensities of 1 MV\\/cm. The gap between a 1.7 mm diameter sphere and a plane stainless steel electrodes was varied between 50 ?m to 400 ?m.

S. Katsuki; R. P. Joshi; M. Laroussi; F. Leipold; K. H. Schoenbach

2002-01-01

241

Stress corrosion failure of high-pressure gas pipeline  

Microsoft Academic Search

Incidents of failure due to corrosion\\/stress corrosion cracking of high-pressure gas pipelines in Pakistan have been observed to occur after about 1520 years of service. The present paper constitutes the failure analysis of an 18-inch diameter electric resistance-welded gas pipeline. The failure was characterized, on the basis of all the available evidence and the metallurgical examination carried out on the

F. Hasan; J. Iqbal; F. Ahmed

2007-01-01

242

Low-strain plasticity in a high pressure die cast Mg-Al alloy  

NASA Astrophysics Data System (ADS)

The Kocks-Mecking method was used to compare the strain-hardening behavior at low strains of high pressure die cast Mg-9 mass% Al alloy and gravity cast fine grained pure Mg specimens. The alloy specimens exhibited a rounded flow curve in contrast with the pure metal's for which macroscopic yielding occurred at a well-defined stress. Microhardness mapping of the cross-section of an alloy specimen showed a surface layer, or skin, with hardness values 20 HV above those of the centre or core region. On the assumption that the core strain hardens at the same rate as the pure Mg specimen, it was estimated that 20% of the alloy specimen's cross-section was still elastic when the core reached full plasticity. The micromechanics of the elasto-plastic transition in the alloy specimens are discussed.

Vanna Yang, K.; Cceres, C. H.; Nagasekhar, A. V.; Easton, M. A.

2012-03-01

243

High Strain Rate Testing of AP/Al/HTPB Solid Propellant  

NASA Astrophysics Data System (ADS)

Ammonium perchlorate (AP), aluminum (Al), and hydroxy- terminater polybutadiene (HTPB) solid propellant samples were subjected to high rates of strain using the Split-Hopkinson Pressure Bar (SHPB) apparatus. The key experimental issues for this study are the effects of temperature on the mechanical behavior of this propellant at high strain rates. Strain rates were between 100 sec-1 and 700 sec-1 over wide range of temperature regions. Included in this paper are the strain rates, strain, and stress curves and material response properties for Aluminum, AP, and HTPB based propellant formulation. SHPB data collected and analyzed under this experiment showed typical results that would be expected from filled elastomers. As the temperature increased the failure and yield stress decreased. The elastic modulus was relatively constant at each varying strain rate and the modulus changed slightly with temperature. The large increase in the elastic modulus at the lowest temperature indicates that the sample was near or below the glass transition temperature of the material and this corresponds with DSC testing.

John, Henry J., Jr.; Hudson, Frank E., III; Robbs, Rodney L.

1997-07-01

244

Dopant-free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors.  

PubMed

We report the rational synthesis of dopant-free GaN/AlN/AlGaN radial nanowire heterostructures and their implementation as high electron mobility transistors (HEMTs). The radial nanowire heterostructures were prepared by sequential shell growth immediately following nanowire elongation using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM) studies reveal that the GaN/AlN/AlGaN radial nanowire heterostructures are dislocation-free single crystals. In addition, the thicknesses and compositions of the individual AlN and AlGaN shells were unambiguously identified using cross-sectional high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM). Transport measurements carried out on GaN/AlN/AlGaN and GaN nanowires prepared using similar conditions demonstrate the existence of electron gas in the undoped GaN/AlN/AlGaN nanowire heterostructures and also yield an intrinsic electron mobility of 3100 cm(2)/Vs and 21,000 cm(2)/Vs at room temperature and 5 K, respectively, for the heterostructure. Field-effect transistors fabricated with ZrO(2) dielectrics and metal top gates showed excellent gate coupling with near ideal subthreshold slopes of 68 mV/dec, an on/off current ratio of 10(7), and scaled on-current and transconductance values of 500 mA/mm and 420 mS/mm. The ability to control synthetically the electronic properties of nanowires using band structure design in III-nitride radial nanowire heterostructures opens up new opportunities for nanoelectronics and provides a new platform to study the physics of low-dimensional electron gases. PMID:16834431

Li, Yat; Xiang, Jie; Qian, Fang; Gradecak, Silvija; Wu, Yue; Yan, Hao; Blom, Douglas A; Lieber, Charles M

2006-07-01

245

Nonlinear Elastic Constitutive Model of Soil-Structure Interfaces Under Relatively High Normal Stress  

Microsoft Academic Search

The shear characteristics of soil-structure interfaces with different roughness are studied systematically by using the DRS-1 high normal stress and residual shear apparatus. The experimental results indicate that, under a relatively high normal stress, normal stress and the coefficient of structural roughness are the most important factors affecting the mechanical interface characteristics. The relationship between shear stress and shear displacement

Guo-qing ZHOU; Hong-chun XIA; Guang-si ZHAO; Jie ZHOU

2007-01-01

246

Improved Performance of Highly Scaled AlGaN/GaN High-Electron-Mobility Transistors Using an AlN Back Barrier  

NASA Astrophysics Data System (ADS)

An ultrathin AlN layer is inserted between the GaN channel and buffer in the fabrication of deep-submicrometer AlGaN/GaN high-electron-mobility transistors (HEMTs). The wide bandgap of AlN establishes a high back barrier and thus enhances the confinement of two-dimensional electron gas under high drain bias voltages. Owning to the effective suppression of short-channel effects in the device with a highly scaled gate length, the fabricated AlN back barrier HEMTs show better pinch-off quality, lower subthreshold current, lower drain-induced barrier lowering factor, and better high-frequency response than the reference device without an AlN back barrier.

Kong, Xin; Wei, Ke; Liu, Guoguo; Liu, Xinyu; Wang, Cuimei; Wang, Xiaoliang

2013-05-01

247

Effects of aging on stress-induced martensitic transformation in ductile CuAlMn-based shape memory alloys  

Microsoft Academic Search

Effects of aging at 473573K on stress-induced martensitic transformation for textured Cu71.9Al16.6Mn9.3Ni2B0.2 and random-textured Cu72.1Al16.9Mn10.5Co0.5 shape memory alloy (SMA) wires with a large relative grain size d\\/D=6 were investigated by cyclic tensile testing at room temperature, where d and D indicate mean grain size and wire diameter, respectively. The random-textured Cu72.1Al16.9Mn10.5Co0.5 wire cannot be uniformly deformed and the ductility is

Y. Sutou; N. Koeda; T. Omori; R. Kainuma; K. Ishida

2009-01-01

248

Influence of bias variation on residual stress and texture in TiAlN PVD coatings  

Microsoft Academic Search

It is recognized that the quality and performance of PVD coatings on tungsten carbide based cutting tools are strongly affected by the level of residual stress as it may cause the coatings to delaminate from the substrate and affect the tool life. Here we report on the substrate bias as an important parameter to affect the compressive stress and the

M. Ahlgren; H. Blomqvist

2005-01-01

249

High-strain-rate superplasticity in ultrahigh-carbon steel containing 10 wt.% Al (UHCS-10Al)  

Microsoft Academic Search

The present study represents a new processing route by which high-strain-rate superplasticity can be obtained in a two-phase, Fe-base alloy. For this study, an ultrahigh-carbon steel containing 10 wt.% Al (UHCS-10Al) was processed by a powder-metallurgy technique. Mechanical attrition was used to introduce a large degree of cold work into pre-alloyed powders, creating the very fine microstructural features necessary for

Eric M. Taleff; Mamoru Nagao; Kenji Higashi; Oleg D. Sherby

1996-01-01

250

Sliding wear behavior of FeAl and FeAl\\/WC coatings prepared by high velocity arc spraying  

Microsoft Academic Search

A comparative study was carried out to investigate the microstructure and tribological behavior of FeAl and FeAl\\/WC iron aluminide based coatings against Si3N4 under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings

Binshi Xu; Zixin Zhu; Shining Ma; Wei Zhang; Weimin Liu

2004-01-01

251

TiAlN/TiAlON/Si3N4 tandem absorber for high temperature solar selective applications  

NASA Astrophysics Data System (ADS)

A tandem absorber of TiAlN/TiAlON/Si3N4 is prepared using a magnetron sputtering process. The graded composition of the individual component layers of the tandem absorber produces a film with a refractive index increasing from the surface to the substrate, which exhibits a high absorptance (0.95) and a low emittance (0.07). The tandem absorber is stable in air up to 600 C for 2 h, indicating its importance for high temperature solar selective applications. The thermal stability of the tandem absorber is attributed to high oxidation resistance and microstructural stability of the component materials at higher temperatures.

Barshilia, Harish C.; Selvakumar, N.; Rajam, K. S.; Sridhara Rao, D. V.; Muraleedharan, K.; Biswas, A.

2006-11-01

252

Fatigue Life and Short Crack Behavior in Ti-6Al-4V Alloy; Interactions of Foreign Object Damage, Stress, and Temperature  

NASA Astrophysics Data System (ADS)

High-cycle fatigue (HCF) failures associated with foreign object damage (FOD) in turbine engines of military aircrafts have been of major concern for the aeronautic industry in recent years. The present work is focused on characterizing the effects of FOD on crack initiation and small crack growth of a Ti-6Al-4V alloy at ambient and also elevated temperatures. Results show that the preferred crack initiation site depends on applied stress and temperature as maximum fractions of cracks emanating from the simulated damage site, and naturally initiated cracks are observed at 25 C under the maximum stress of 700 MPa and at 300 C under the maximum stress of 300 MPa. The fatigue crack growth rate is influenced by increasing temperature, and the FCG rate at 300 C is higher than that at room temperature under the same ? K, whereas this effect for FOD-site initiated cracks is not so remarkable. This observation seems to be due to the effect of stress relaxation at 300 C. Results also indicate that fatigue crack initiation life ( N i ) and fatigue life ( N f ) are expressed by three-parameter Weibull distribution function.

Majidi, Behzad

2008-04-01

253

High (pressure, temperature) phase diagrams of ZnO and AlN from second harmonic generation measurements  

NASA Astrophysics Data System (ADS)

The pressure-induced B4 --> B1 structural phase boundaries of ZnO and AlN have been determined with the second harmonic generation (SHG) technique at high temperature. The SHG measurements of AlN show that between 15.6 and 18 GPa, the phase boundary has a negative slope of nearly -627 K/GPa, and that below 15.6 GPa, the slope is significantly smaller (-77 K/GPa). ZnO has a phase boundary with a negative slope of nearly -1427 K/GPa around 5.3-6 GPa and -228 K/GPa below 5 GPa. The phase transition pressure of AlN is sensitive to deviatoric stress and varies from 18 to 24.5 GPa.

Bayarjargal, Lkhamsuren; Winkler, Bjrn

2012-01-01

254

Comparison of the transport properties of high quality AlGaN/AlN/GaN and AlInN/AlN/GaN two-dimensional electron gas heterostructures  

NASA Astrophysics Data System (ADS)

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm2/V s along with 8.441012 cm-2 electron density, which resulted in a two-dimensional sheet resistance of 435 ?/? for the Al0.2Ga0.8N/AlN/GaN heterostructure. The sample designed with an Al0.88In0.12N barrier exhibited very high sheet electron density of 4.231013 cm-2 with a corresponding electron mobility of 812 cm2/V s at room temperature. A record two-dimensional sheet resistance of 182 ?/? was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al0.88In0.12N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.

Tlek, Remziye; Ilgaz, Aykut; Gkden, Sibel; Teke, Ali; ztrk, Mustafa K.; Kasap, Mehmet; zelik, Sleyman; Arslan, Engin; zbay, Ekmel

2009-01-01

255

Caspase12 cleavage and increased oxidative stress during motoneuron degeneration in transgenic mouse model of ALS  

Microsoft Academic Search

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by loss of motoneurons in the spinal cord and brain stem. We have characterized motoneuron death in transgenic mice carrying the mutant human copper\\/zinc superoxide dismutase, as a model for familial ALS. Previous studies have shown the involvement of mitochondria in nerve cell demise in these animals. We report here an

Hanna Wootz; Inga Hansson; Laura Korhonen; Ulla Npnkangas; Dan Lindholm

2004-01-01

256

Grain Boundary Segregation in Al-Zn-Mg Alloys -- Implications to Stress Corrosion Cracking.  

National Technical Information Service (NTIS)

Prior studies on grain boundary segregation in Al-Zn-Mg have shown that free Mg is present along the grain boundaries in these alloys under all heat-treat conditions. In this paper, further studies on grain boundary segregation in Al-Zn-Mg alloys are outl...

R. K. Viswanadham, T. S. Sun, J. A. S. Green

1978-01-01

257

Thermal Fatigue Life of Glidcop Al-15 High-Heat-Load Components  

NASA Astrophysics Data System (ADS)

Shanghai Synchrotron Radiation Facility (SSRF) is a third-generation of synchrotron radiation light source and is presently under construction as a large scale national scientific project in China. Depending on the outstanding thermal and mechanical performance, Glidcop Al-15, a dispersion strengthened copper alloy, is chosen to serve for the high-heat-load components at beam line front end in SSRF. Present study is to investigate the thermal fatigue lives of critical SSRF components. A nonlinear finite element method (FEM) is used to simulate the nonlinear three dimensional stress-strain fields of the critical component-Mask2 at SSRF beam line. The method consists of transient temperature analyses followed by elastic-plastic stress analyses. Then, a critical plane approach is used to predict the thermal fatigue life of mask2. The critical plane approach is appropriate for estimating service life of critical SSRF components since the results are in good consistent with the experimental ones taken at the Advanced Photon Source (APS).

Jin, J. F.; Xiao, W. L.; Chen, H. B.

2010-05-01

258

Stress Corrosion Cracking Susceptibility and Aging Characteristics of Al-Zn-Mg-Ti Alloys.  

National Technical Information Service (NTIS)

A comparison of the stress corrosion cracking (SCC) susceptibility of four aluminum-zinc-magnesium alloys was determined as a function of processing and composition variables, particularly aging treatment and titanium additions. The SCC tests were perform...

C. Chen, G. Judd

1973-01-01

259

Structural Differences between Al-Fe-Cu and Al-Li-Cu Quasicrystals Studied by High-Resolution Electron Microscopy  

NASA Astrophysics Data System (ADS)

Stable icosahedral phases of Al-Fe-Cu and Al-Li-Cu, which are recognized as icosahedral quasicrystals corresponding to face-centered and simple 6D-hypercubic lattices, respectively, were examined by high-resolution electron microscopy with the incident beam parallel to the two-fold symmetry axis. Images observed were analyzed in detail with the aid of computerized image processing. In the image of Al-Li-Cu, a contrast modulation of about ?3 times distance of lattice spacing was observed on lattice fringes perpendicular to the 5-fold symmetry axes, whereas in Al-Fe-Cu, complex contrast modulations with various distances of 2, 3, and 5 lattice spacings were observed on the lattice fringes perpendicular to the same axes.

Hiraga, Kenji; Shindo, Daisuke

1989-12-01

260

Very high channel conductivity in ultra-thin channel N-polar GaN/(AlN, InAlN, AlGaN) high electron mobility hetero-junctions grown by metalorganic chemical vapor deposition  

NASA Astrophysics Data System (ADS)

Different back barrier designs comprising of AlN, AlGaN, and InAlN layers are investigated for ultra-thin GaN channel N-polar high-electron-mobility-transistors grown by metalorganic chemical vapor deposition. A combinational back barrier with both AlGaN and InAlN materials is proposed. The dependence of channel conductivity on channel thickness is investigated for different back barrier designs. The study demonstrated that the back barrier design of AlN/InAlN/AlGaN is capable of retaining high channel conductivity for ultra-scaled channel thicknesses. For devices with 5-nm-thick channel, a sheet resistance of ~230 ?/? and mobility ~1400 cm2/V-s are achieved when measured parallel to the multi-step direction of the epi-surface.

Lu, Jing; Denninghoff, Dan; Yeluri, Ramya; Lal, Shalini; Gupta, Geetak; Laurent, Matthew; Keller, Stacia; DenBaars, Steven P.; Mishra, Umesh K.

2013-06-01

261

High Temperature Sulfidation Properties of FeAl Alloys  

Microsoft Academic Search

The sulfidation properties of the Fe-Al alloys containing 6, 9, 18 and 28 atomic percent Al were investigated in alloy\\/FeS diffusion couples, in sulfur vapour at the dissociation pressure of the FeS and in H?S+H atmosphereat 1173K. The reaction kinetics were determined thermogravitmetrically and by layer thickness. measurements. The reaction specimens were analyzed using light microscopy, X-ray diffraction and electron

Prakash Chandra Patnaik

1984-01-01

262

STRESS  

NSDL National Science Digital Library

During this activity, students will use the available resources to learn more about stress, the effects of stress and how to handle stress. This activity focuses on the Utah fifth grade health core Standard 1: The students will learn ways to improve mental health and manage stress. During this project, students are given two different scenarios and ...

Hancey, Ms.

2010-04-27

263

Stress  

PubMed Central

The concept of stress is often mistaken. Its correct usage stems from the meaning in physical medicine, and many facets of stress are missed because this is not generally realized. This paper defines a psychiatric model of stress and describes a method by which stress can be quantified in various individuals.

Pearce, K. I.

1973-01-01

264

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy.  

PubMed

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100 degrees C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures. PMID:15045361

Vosberg, V R; Fischer, W; Berger, M G; Clemens, D; Quadakkers, W J; Nickel, H

1996-06-01

265

Deformation behavior of high Cr ferritic steel upon abrupt stress loading  

Microsoft Academic Search

Microscopic deformation behavior after abrupt stress loading under several applied stresses was investigated at high temperature for ASME-P92 steel. The estimation method for elastic limit was proposed based on analyzing the deformation behavior. Instantaneous plastic strain was not observed upon the abrupt stress loading under the applied stress from 60 to 200MPa. Anelastic deformation originating from martensitic lath structure was

K. Sawada; F. Abe; K. Kimura

2004-01-01

266

Physcomitrella patens is highly tolerant against drought, salt and osmotic stress  

Microsoft Academic Search

In order to determine the degree of tolerance of the moss Physcomitrella patens to different abiotic stress conditions, we examined its tolerance against salt, os- motic and dehydration stress. Compared to other plants like Arabidopsis thaliana, P. patens exhibits a high degree of abiotic stress tolerance, making it a valuable source for the identification of genes effecting the stress adap-

W. Frank; D. Ratnadewi; R. Reski

267

Physcomitrella patens is highly tolerant against drought, salt and osmotic stress  

Microsoft Academic Search

In order to determine the degree of tolerance of the moss Physcomitrella patens to different abiotic stress conditions, we examined its tolerance against salt, osmotic and dehydration stress. Compared to other plants like Arabidopsis thaliana, P. patens exhibits a high degree of abiotic stress tolerance, making it a valuable source for the identification of genes effecting the stress adaptation. Plants

Wolfgang Frank; Diah Ratnadewi; Ralf Reski

2005-01-01

268

The Influence of Different Heat Treatments on Stress Corrosion Cracking in the Aluminum Alloys al-7050 and Az 7461 de Invloed van Verschillende Warmtebehandelingen OP de Spanningscorrosie-Gevoeligheid van de Aluminium Legeringen al-7050 en Az 74.61.  

National Technical Information Service (NTIS)

Stress corrosion cracking (SCC) susceptibility of two Al-Zn-Mg forging alloys, Al-7050 and AZ 74.61, was investigated for different heat treatments. The constant strain rate (CSR) test method was used. Results of the CSR tests were compared with published...

H. F. Dejong, R. R. Kalmann

1982-01-01

269

An Inducer of VGF Protects Cells against ER Stress-Induced Cell Death and Prolongs Survival in the Mutant SOD1 Animal Models of Familial ALS  

Microsoft Academic Search

Amyotrophic lateral sclerosis (ALS) is the most frequent adult-onset motor neuron disease, and recent evidence has suggested that endoplasmic reticulum (ER) stress signaling is involved in the pathogenesis of ALS. Here we identified a small molecule, SUN N8075, which has a marked protective effect on ER stress-induced cell death, in an in vitro cell-based screening, and its protective mechanism was

Masamitsu Shimazawa; Hirotaka Tanaka; Yasushi Ito; Nobutaka Morimoto; Kazuhiro Tsuruma; Michinori Kadokura; Shigeki Tamura; Teruyoshi Inoue; Mitsunori Yamada; Hitoshi Takahashi; Hitoshi Warita; Masashi Aoki; Hideaki Hara; Rafael Linden

2010-01-01

270

High temperature oxidation of Ni sub 3 Al composites  

SciTech Connect

As part of an effort to develop oxidation-resistant metal matrix composites, the influence of reinforcement materials on the oxidation of Ni{sub 3}Al was studied. Thermogravimetric measurements and microstructural analyses were used to determine the 800{degree}C oxidation behavior of Ni{sub 3}Al and composites based on this material. The presence of fiber (Al{sub 2}O{sub 3})- or particle (TiN or TiC)- reinforcement material in the Ni{sub 3}Al increased susceptibility to oxidation as measured by the amount of reacted material, rate constants, and/or extent of internal attack. The orientation of fibers with respect to the free surfaces was found to affect the oxidation behavior by providing, in certain cases, reactive paths along the fiber-matrix interface that led to significant internal penetration. The results suggest that the choice of reinforcement material and the method of materials processing will be important considerations in the design of oxidation-resistant Ni{sub 3}Al composites. However, ways to minimize the detrimental influence of the reinforcement material on oxidation resistance often conflict with requirements for obtaining attractive mechanical and physical properties of the composite. 13 refs., 6 figs.

Tortorelli, P.F.; DeVan, J.H.; McKamey, C.G.; Howell, M.

1990-01-01

271

The influence of buffer layer coalescence on stress evolution in GaN grown on ion implanted AlN/Si(111) substrates  

NASA Astrophysics Data System (ADS)

The effect of AlN buffer layer morphology on the evolution of growth stress in GaN epilayers deposited by metalorganic chemical vapor deposition on N+ ion-implanted AlN/Si(111) substrates was investigated. AlN buffer layers were grown using either a continuous or pulsed source flow process which altered the grain size and extent of coalescence of the films. In situ stress measurements revealed that substrate implantation reduced the initial compressive stress in the GaN epilayers likely due to a decoupling of the AlN lattice from the underlying crystalline Si substrate. The buffer layer morphology was found to significantly alter the influence of ion-implantation on the film properties. GaN epilayers grown on ion-implanted AlN/Si(111) substrates prepared with the pulsed conditions exhibited a 63% decrease in threading dislocation (TD) density compared to unimplanted substrates. In addition, these films were observed to grow under a low overall stress compared to the other samples which exhibited a more typical compressive to tensile stress transition during growth. The low overall growth stress of the GaN grown on the implanted pulsed AlN/Si(111) was explained in terms of a reduced strain gradient from dislocation inclination.

Gagnon, Jarod C.; Leathersich, Jeffrey M.; Shahedipour-Sandvik, Fatemeh (Shadi); Redwing, Joan M.

2014-05-01

272

High pressure and high temperature stabilization of cubic AlN in Ti0.60Al0.40N  

NASA Astrophysics Data System (ADS)

In the present work, the decomposition of unstable arc evaporated Ti0.6Al0.4N at elevated temperatures and quasihydrostatic pressures has been studied both experimentally and by first-principles calculations. High pressure and high temperature (HPHT) treatment of the samples was realized using the multi anvil press and diamond anvil cell techniques. The products of the HPHT treatment of Ti0.6Al0.4N were investigated using x-ray diffractometry and transmission electron microscopy. Complimentary calculations show that both hydrostatic pressure and high temperature stabilize the cubic phase of AlN, which is one of the decomposition products of Ti0.6Al0.4N. This is in agreement with the experimental results which in addition suggest that the presence of Ti in the system serves to increase the stability region of the cubic c-AlN phase. The results are industrially important as they show that Ti0.6Al0.4N coatings on cutting inserts do not deteriorate faster under pressure due to the cubic AlN to hexagonal AlN transformation.

Norrby, N.; Lind, H.; Parakhonskiy, G.; Johansson, M. P.; Tasndi, F.; Dubrovinsky, L. S.; Dubrovinskaia, N.; Abrikosov, I. A.; Odn, M.

2013-02-01

273

Stress-induced and thermoelastic properties of Ni-Fe-Al melt-spun ribbon  

Microsoft Academic Search

Lattice parameters, elastic modulus, internal friction, superelasticity, and shape memory effect are studied in the vicinity of martensitic transformation (MT) exhibited by the Ni55Fe20Al25 melt-spun ribbon. In comparison to the melt-spun ribbons of other ferromagnetic shape memory alloys, Ni-Fe-Al ribbon has abnormally low elastic modulus, a record-breaking strength (>350 MPa), and large recoverable strain (up to 5%) due to both

V. A. Chernenko; B. Rajini Kanth; P. K. Mukhopadhyay; S. N. Kaul; E. Villa; A. Gambardella; S. Besseghini

2008-01-01

274

Stress-induced martensite formation in Cu?Al?Ni alloys  

Microsoft Academic Search

A study has been made of superelasticity and the strain-memory effect in Cu?Al?Ni alloys in the composition range 14 wt pct\\u000a Al and 2 to 3 wt pct Ni. These alloys have a bcc structure on quenching and show a low temperature martensitic transformation\\u000a which is responsible for both the superelastic and strain-memory effects. Tests on both single and polycrystalline

K. Oishi; L. C. Brown

1971-01-01

275

Frequency comparison of two high-accuracy Al+ optical clocks.  

PubMed

We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{-18}, based on quantum logic spectroscopy of an Al+ ion. A simultaneously trapped Mg+ ion serves to sympathetically laser cool the Al+ ion and detect its quantum state. The frequency of the {1}S{0}<-->{3}P{0} clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0x10{-18}. The two clocks exhibit a relative stability of 2.8x10{-15}tau{-1/2}, and a fractional frequency difference of -1.8x10{-17}, consistent with the accuracy limit of the older clock. PMID:20366869

Chou, C W; Hume, D B; Koelemeij, J C J; Wineland, D J; Rosenband, T

2010-02-19

276

Stress corrosion cracking of duplex stainless steel in high?temperature\\/high?pressure water  

Microsoft Academic Search

This paper describes an investigation of the relationship between the microstructure and stress corrosion cracking of duplex stainless steel in high?temperature\\/high?pressure water by transmission electron microscopy and fractography.The specimens were solution?treated at various temperatures and then sensitised at 923 K 72 ksec. Slow strain rate tests (SSRT) were run at 562 K in high?temperature\\/high?pressure water with 8 ppm dissolved

T. Kuroda; F. Matsuda; K. Bunno

1995-01-01

277

The thermal effect due to stress-induced martensite formation in ?-CuAlNi single crystals  

Microsoft Academic Search

The thermal effect associated with the formation and reversion of stress-induced?? martensite has been studied in single crystals of?\\u000a 1-CuAlNi. The maximum temperature increase for the?\\u000a 1 ??\\u000a 1? transformation in a specimen at room temperature ranged from 1.2C at a strain-rate of 0.025 min?1 to 14C at a strain-rate of 25 min?1. The large temperature increase is due to

C. Rodriguez; L. C. Brown

1980-01-01

278

Finite element analysis of high strain rate superplastic forming (SPF) of AlTi alloys  

Microsoft Academic Search

This paper presents the numerical results obtained from the finite element analyses of the superplastic forming (SPF) of AlTi alloys. The models are used to optimise the process and predict forming times in terms of deformed shapes, stressstrain distributions and thickness evolution across the facets of fully formed surfaces. Unlike earlier studies that have used membrane elements, this is an

Prasad K. D. V. Yarlagadda; Prasad Gudimetla; Clayton Adam

2002-01-01

279

Influence of incomplete shape memory deformation on the generation of reactive stresses in single crystals of the Cu-Al-Ni alloy  

NASA Astrophysics Data System (ADS)

The influence of preliminary incomplete shape memory (SM) deformation on the generation of reactive stresses in single crystals of the Cu-13.6 wt % Al-4.0 wt % Ni alloy under conditions of constrained SM deformation has been studied. It has been found that, with an increase in the deformation, the value of reactive stresses increases linearly with increasing stress of preliminary deformation of the alloy.

Nikolaev, V. I.; Averkin, A. I.; Egorov, V. M.; Malygin, G. A.; Pul'nev, S. A.

2014-03-01

280

Time evolution of off-state degradation of AlGaN/GaN high electron mobility transistors  

NASA Astrophysics Data System (ADS)

The evolution of AlGaN/GaN high electron mobility transistors under off-state stress conditions is studied by gate leakage current (Ig) monitoring, electroluminescence (EL), and atomic force microscope (AFM) imaging at room temperature. It is found that the number of off-state failure sites as identified by EL increases over time during stress until it reaches a saturation value. Ig increases accordingly during stress until this saturation number of failure sites is reached. AFM scanning of the device surface stripped of metal contacts and passivation reveals surface pits corresponding to the location of the EL spots. These pits have an elongated shape oriented towards the drain contact whose length is correlated with the distance to the adjacent pits and with the time since their appearance during the stress experiment. A model for the generation and evolution of the off-state stress-related failure sites is proposed consistent with the experimental results, bringing together surface migration of electrochemical species with trap-based leakage mechanisms and resulting in the formation of an exclusion zone around each failure site.

Bajo, M. Montes; Sun, H.; Uren, M. J.; Kuball, M.

2014-06-01

281

Study on Mold Slag with High Al2O3 Content for High Aluminum Steel  

NASA Astrophysics Data System (ADS)

The slag-steel equilibrium reaction between the newly developed mold slag ND-MSL and 20Mn23AlV steel has been studied at high temperatures in the laboratory. The crystal morphology, microanalysis, and phase analysis of the original and final ND-MSL slags were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that, in the final ND-MSL slag, the constitution of SiO2 decreased by 0.7 wt pct and Al2O3 increased by 6.46 wt pct, while the melting temperature, viscosity, and crystallization rate increased by 62 K, 0.66 dPa s, and 15 pct, respectively. NaAlSi3O8 and CaAl2Si2O8 were found to be precipitated in the final ND-MSL slag. Both the original and final ND-MSL slags have a small amount of LiF crystal and good glass form. The ND-MSL slag has little change in the composition and properties compared with the two currently used mold slags.

Wang, Qiang; Sun, Min; Qiu, Shengtao; Tian, Zhiling; Zhu, Guoling; Wang, Longmei; Zhao, Pei

2014-04-01

282

High level compressive residual stresses produced in aluminum alloys by laser shock processing  

NASA Astrophysics Data System (ADS)

Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results for metal surface treatments in underwater laser irradiation at 1064 nm. A convergent lens is used to deliver 1.2 J/cm 2 in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG, two laser spot diameters were used: 0.8 and 1.5 mm. Results using pulse densities of 2500 pulses/cm 2 in 6061-T6 aluminum samples and 5000 pulses/cm 2 in 2024 aluminum samples are presented. High level of compressive residual stresses are produced -1600 MPa for 6061-T6 Al alloy, and -1400 MPa for 2024 Al alloy. It has been shown that surface residual stress level is higher than that achieved by conventional shot peening and with greater depths. This method can be applied to surface treatment of final metal products.

Gomez-Rosas, G.; Rubio-Gonzalez, C.; Ocaa, J. L.; Molpeceres, C.; Porro, J. A.; Chi-Moreno, W.; Morales, M.

2005-11-01

283

Characterization of the reliability and uniformity of an anodization-free fabrication process for high-quality Nb/Al-AlOx/Nb Josephson junctions  

NASA Astrophysics Data System (ADS)

We have developed a reliable and reproducible fabrication process for high-quality Nb/Al-AlOx/Nb Josephson junctions that completely avoids anodization techniques, that are typically used to define the junction area, to electrically insulate the base electrode as well as the sidewalls of the counter-electrode and to protect the tunnel barrier. Hence, this process is well suited for the fabrication of electrically floating junction-based devices such as non-hysteretic rf-SQUIDs. Josephson junctions of various sizes have been produced and characterized at 4.2 K. We found that our junctions have a high quality, which is confirmed by measured gap voltages Vg and Ic Rn products up to 2.9 and 1.8 mV and on-wafer average values of the resistance ratio Rsg/Rn above 30 in most cases. Here, Rsg and Rn denote the subgap and the normal state resistance of a Josephson junction. Although the uniformity of the properties of the Josephson junctions across a wafer is high, we observe some systematic variations of the critical current density and the gap voltage over an entire wafer. These variations are most likely to be attributed to residual stress in the Nb films as well as the surface roughness of the Nb base electrode.

Kempf, S.; Ferring, A.; Fleischmann, A.; Gastaldo, L.; Enss, C.

2013-06-01

284

High temperature oxidation resistant coatings for the directionally solidified Ni-Nb-Cr-Al eutectic superalloy  

NASA Technical Reports Server (NTRS)

Protective coatings required for the Ni-Nb-Cr-Al directionally solidified eutectic superalloy were developed and evaluated on the basis of oxidation resistance, diffusional stability, thermal fatigue, and creep resistance. NiCrAlY+Pt and NiCrAlY physical vapor-deposition coating systems exhibited the best combination of properties. Burner-rig testing indicated that the useful life of a 127-micron-thick NiCrAlY+Pt coating exceeds 1000 h at 1366 K. Eutectic-alloy creep lives at 1311 K and a stress of 151.7 MN/sq m were greater for NiCrAlY+Pt-coated specimens than for uncoated specimens by a factor of two.

Strangman, T. E.; Ulion, N. E.; Felten, E. J.

1977-01-01

285

Polarization effects on gate leakage in InAlN/AlN/GaN high-electron-mobility transistors  

NASA Astrophysics Data System (ADS)

Lattice-matched InAlN/AlN/GaN high electron mobility transistors offer high performance with attractive electronic and thermal properties. For high-voltage applications, gate leakage currents under reverse bias voltages remain a serious challenge. This current flow is dominated by field enhanced thermal emission from trap states or direct tunneling. We experimentally measure reverse-bias gate leakage currents in InAlN/AlN/GaN transistors at various temperatures and find that the conventional trap-assisted Frenkel-Poole model fails to explain the experimental data. Unlike the non-polar semiconductors Si, Ge, large polarization-induced electric fields exist in III-nitride heterojunctions. When the large polarization fields are accounted for, a modified Frenkel-Poole model is found to accurately explain the measured data at low reverse bias voltages. At high reverse bias voltages, we identify that the direct Fowler-Nordheim tunneling mechanism dominates. The accurate identification of the gate leakage current flow mechanism in these structures leads to the extraction of several useful physical parameters, highlights the importance of polarization fields, and leads to suggestions for improved behavior.

Ganguly, Satyaki; Konar, Aniruddha; Hu, Zongyang; Xing, Huili; Jena, Debdeep

2012-12-01

286

Delineating the genetic heterogeneity of ALS using targeted high-throughput sequencing  

PubMed Central

Background Over 100 genes have been implicated in the aetiology of amyotrophic lateral sclerosis (ALS). A detailed understanding of their independent and cumulative contributions to disease burden may help guide various clinical and research efforts. Methods Using targeted high-throughput sequencing, we characterised the variation of 10 Mendelian and 23 low penetrance/tentative ALS genes within a population-based cohort of 444 Irish ALS cases (50 fALS, 394 sALS) and 311 age-matched and geographically matched controls. Results Known or potential high-penetrance ALS variants were identified within 17.1% of patients (38% of fALS, 14.5% of sALS). 12.8% carried variants of Mendelian disease genes (C9orf72 8.78%; SETX 2.48%; ALS2 1.58%; FUS 0.45%; TARDBP 0.45%; OPTN 0.23%; VCP 0.23%. ANG, SOD1, VAPB 0%), 4.7% carried variants of low penetrance/tentative ALS genes and 9.7% (30% of fALS, 7.1% of sALS) carried previously described ALS variants (C9orf72 8.78%; FUS 0.45%; TARDBP 0.45%). 1.6% of patients carried multiple known/potential disease variants, including all identified carriers of an established ALS variant (p<0.01); TARDBP:c.859G>A(p.[G287S]) (n=2/2 sALS). Comparison of our results with those from studies of other European populations revealed significant differences in the spectrum of disease variation (p=1.710?4). Conclusions Up to 17% of Irish ALS cases may carry high-penetrance variants within the investigated genes. However, the precise nature of genetic susceptibility differs significantly from that reported within other European populations. Certain variants may not cause disease in isolation and concomitant analysis of disease genes may prove highly important.

Kenna, Kevin P; McLaughlin, Russell L; Byrne, Susan; Elamin, Marwa; Heverin, Mark; Kenny, Elaine M; Cormican, Paul; Morris, Derek W; Donaghy, Colette G; Bradley, Daniel G; Hardiman, Orla

2013-01-01

287

A Study of Relaxation Techniques and Coping Skills with Moderately to Highly Stressed Middle and High School Students.  

ERIC Educational Resources Information Center

This report describes a program for heightening awareness of stress and reducing stress levels while improving learning. The targeted population comprised seventh and ninth grade students in middle schools and high schools located in affluent suburban communities of a large Midwest city. The problem of moderate to high levels of stress was

Credit, Alison; Garcia, Mary

288

High lung volume increases stress failure in pulmonary capillaries  

NASA Technical Reports Server (NTRS)

We previously showed that when pulmonary capillaries in anesthetized rabbits are exposed to a transmural pressure (Ptm) of approximately 40 mmHg, stress failure of the walls occurs with disruption of the capillary endothelium, alveolar epithelium, or sometimes all layers. The present study was designed to test whether stress failure occurred more frequently at high than at low lung volumes for the same Ptm. Lungs of anesthetized rabbits were inflated to a transpulmonary pressure of 20 cmH2O, perfused with autologous blood at 32.5 or 2.5 cmH2O Ptm, and fixed by intravascular perfusion. Samples were examined by both transmission and scanning electron microscopy. The results were compared with those of a previous study in which the lung was inflated to a transpulmonary pressure of 5 cmH2O. There was a large increase in the frequency of stress failure of the capillary walls at the higher lung volume. For example, at 32.5 cmH2O Ptm, the number of endothelial breaks per millimeter cell lining was 7.1 +/- 2.2 at the high lung volume compared with 0.7 +/- 0.4 at the low lung volume. The corresponding values for epithelium were 8.5 +/- 1.6 and 0.9 +/- 0.6. Both differences were significant (P less than 0.05). At 52.5 cmH2O Ptm, the results for endothelium were 20.7 +/- 7.6 (high volume) and 7.1 +/- 2.1 (low volume), and the corresponding results for epithelium were 32.8 +/- 11.9 and 11.4 +/- 3.7. At 32.5 cmH2O Ptm, the thickness of the blood-gas barrier was greater at the higher lung volume, consistent with the development of more interstitial edema. Ballooning of the epithelium caused by accumulation of edema fluid between the epithelial cell and its basement membrane was seen at 32.5 and 52.5 cmH2O Ptm. At high lung volume, the breaks tended to be narrower and fewer were oriented perpendicular to the axis of the pulmonary capillaries than at low lung volumes. Transmission and scanning electron microscopy measurements agreed well. Our findings provide a physiological mechanism for other studies showing increased capillary permeability at high states of lung inflation.

Fu, Z.; Costello, M. L.; Tsukimoto, K.; Prediletto, R.; Elliott, A. R.; Mathieu-Costello, O.; West, J. B.

1992-01-01

289

The Effect of High Temperature Corrosion on Mechanical Behavior of a GAMMA-TiAl Alloy  

NASA Astrophysics Data System (ADS)

The mechanical properties of Ti-48Al-2Cr-2Nb alloy were discussed after the high temperature corrosion tests carried out with salt mixture of 75wt. % Na2SO4 and 25wt. % NaCl at 800C. The microstructure of the alloy after corrosion was observed by SEM and the fracture behavior of the corroded and uncorroded alloys was investigated by means of the three-point bending tests. It has been shown that the corrosion path was mainly along the lamellar structure and rough surface with a large number of corrosion pits formed during the high temperature corrosion. The experimental results also indicated that the bearing capacity of bending fracture descended evidently due to the molten salt corrosion at high temperature, which only had remarkable effects on the surface state of the alloy. The microcracks inside the alloy always propagated along the phase interfaces and grain boundaries while the corrosion pits on salt-deposited surface became the main crack initiation location in corroded alloy. The stress concentration caused by corrosion was considered as the essential reason of the property reduction, which decreased the energy barrier of crack nucleation and shortened the incubation period.

Zhao, Wenyue; Ma, Yue; Gong, Shengkai

290

High power AlGaN ultraviolet light emitters  

NASA Astrophysics Data System (ADS)

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

Shatalov, Max; Sun, Wenhong; Jain, Rakesh; Lunev, Alex; Hu, Xuhong; Dobrinsky, Alex; Bilenko, Yuri; Yang, Jinwei; Garrett, Gregory A.; Rodak, Lee E.; Wraback, Michael; Shur, Michael; Gaska, Remis

2014-06-01

291

TEM investigation of age-hardenable Al 2519 alloy subjected to stress corrosion cracking tests  

Microsoft Academic Search

The influence of changes in chemical composition and pre-aging deformation on the resistance to stress corrosion cracking in the age-hardenable aluminum alloy 2519 was investigated by transmission electron microscopy. The improvement of this resistance may be accomplished by keeping the Cu concentration on the lower side of the allowed limit for the 2519 alloy. Also, plastic deformation prior to aging,

Stanis?aw Dymek; Marek Dollar

2003-01-01

292

Stress  

MedlinePLUS

... sudden negative change, such as losing a job, divorce, or illness Traumatic stress, which happens when you ... stress, so you can avoid more serious health effects. NIH: National Institute of Mental Health

293

Stress.  

National Technical Information Service (NTIS)

A brief section describes some areas of possible military interest in studies of stress--selection, classification, training, human factors and systems research and, of course, stress as related to the combat situation, among others. Since there is diffic...

J. Weitz

1966-01-01

294

Virtual melting as a new mechanism of stress relaxation under high strain rate loading  

PubMed Central

Generation and motion of dislocations and twinning are the main mechanisms of plastic deformation. A new mechanism of plastic deformation and stress relaxation at high strain rates (1091012s-1) is proposed, under which virtual melting occurs at temperatures much below the melting temperature. Virtual melting is predicted using a developed, advanced thermodynamic approach and confirmed by large-scale molecular dynamics simulations of shockwave propagation and quasi-isentropic compression in both single and defective crystals. The work and energy of nonhydrostatic stresses at the shock front drastically increase the driving force for melting from the uniaxially compressed solid state, reducing the melting temperature by 80% or 4,000K. After melting, the relaxation of nonhydrostatic stresses leads to an undercooled and unstable liquid, which recrystallizes in picosecond time scales to a hydrostatically loaded crystal. Characteristic parameters for virtual melting are determined from molecular dynamics simulations of Cu shocked/compressed along the and directions and Al shocked/compressed along the direction.

Levitas, Valery I.; Ravelo, Ramon

2012-01-01

295

A Thermal-Stress Model of a High Pressure Diamond-Anvil Cell  

NASA Astrophysics Data System (ADS)

A time-dependent mathematical model to describe both thermal and stress propagation in a material compressed to high pressures in a diamond-anvil cell has been formulated. The cell is treated as an axisymmetric composite solid with temperature-dependent thermal conductivity. The effects of various boundary conditions on the interior temperature have been modeled and investigated. A finite-element code (FlexPDE) is utilized to solve the heat-conduction equation and the associated stress propagation equations. The model is utilized to interpret recent experiments* that investigate the transport of thermal and elastic waves in compressed media. The mathematical analysis and experimental data reveal that both thermal and stress propagation simultaneously occurs over two distinct time scales. The thermal propagation model utilizes the familiar parabolic heat-conduction equation, which implies that the effects of the thermal disturbance are instantaneously felt throughout the computational domain. The validity of the parabolic heat-conduction model is examined and compared with available experimental data. A one-dimensional transient analytic approximation for the temperature is also discussed. * Ladouceur, et.al., 1999 AIRAPT Proc.

Ladouceur, H. D.; Pangilinan, G. I.; Russell, T. P.

2000-03-01

296

Inhibition of photosynthesis and energy dissipation induced by water and high light stresses in rice  

Microsoft Academic Search

Photoprotection mechanisms of rice plants were stu- died when its seedlings were subjected to the com- bined stress of water and high light. The imposition of water stress, induced by PEG 6000 which was applied to roots, resulted in substantial inhibition of stomatal conductance and net photosynthesis under all irradiance treatments. Under high light stress, the rapid decline of photosynthesis

Yanhong Zhou; Hon Ming Lam; Jianhua Zhang

2007-01-01

297

The Effects of a High School Stress Management Unit on Student's Heart Rate and Muscle Tension.  

ERIC Educational Resources Information Center

Attempted to determine effectiveness of three-week stress management unit for high school students. Measures of heart rate and muscle tension were used to determine the amount of control demonstrated during a period of relaxation and a period of induced stress. Results of the study indicate that a stress management unit for high school students

Richardson, Glenn E.; And Others

1982-01-01

298

The Voices of High School Counselors: Lived Experience of Job Stress  

ERIC Educational Resources Information Center

There is a paucity of literature addressing high school counselors' experiences of job stress. Our qualitative phenomenological study adds to the professions' knowledge of job stress as experienced by counselors in large suburban high schools. Our study illustrates the job stress phenomenon in the counselors' own voices, identifies situations

Falls, Leigh; Nichter, Mary

2007-01-01

299

Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires  

NASA Astrophysics Data System (ADS)

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 \\mathrm {B2} \\to \\mathrm {body} -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 ~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 ~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 , whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Sutrakar, Vijay Kumar; Mahapatra, D. Roy

2009-07-01

300

Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires.  

PubMed

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. PMID:19567964

Sutrakar, Vijay Kumar; Mahapatra, D Roy

2009-07-22

301

Oxidative stress and autophagic alteration in brainstem of SOD1-G93A mouse model of ALS.  

PubMed

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease involving both upper and lower motor neurons. The mechanism of motor neuron degeneration is still unknown. Although many studies have been performed on spinal motor neurons, few have been reported on brainstem and its motor nuclei. The aim of this study was to investigate oxidative stress and autophagic changes in the brainstem and representative motor nuclei of superoxide dismutase 1 (SOD1)-G93A mouse model of ALS. The expression levels of cluster of differentiation molecule 11b (CD11b), glial fibrillary acidic protein, glutamate-cysteine ligase catalytic subunit, heme oxygenase-1, NAD(P)H: quinone oxidoreductase 1, voltage-dependent anion-selective channel protein 1, Sequestosome 1/p62 (p62), microtubule-associated protein 1 light chain 3B (LC3), and SOD1 proteins in brainstem were examined by Western blot analysis. Immunohistochemistry and immunofluorescence were performed to identify the cellular localization of SOD1, p62, and LC3B, respectively. The results showed that there were progressive asctrocytic proliferation and microglial activation, induction of antioxidant proteins, and increased p62 and LC3II expression in brainstem of SOD1-G93A mice. Additionally, SOD1 and p62 accumulated in hypoglossal, facial, and red nuclei, but not in oculomotor nucleus. Furthermore, electron microscope showed increased autophagic vacuoles in affected brainstem motor nuclei. Our results indicate that brainstem share similar gliosis, oxidative stress, and autophagic changes as the spinal cord in SOD1-G93A mice. Thus, SOD1 accumulation in astrocytes and neurons, oxidative stress, and altered autophagy are involved in motor neuron degeneration in the brainstem, similar to the motor neurons in spinal cord. Therefore, therapeutic trials in the SOD1G93A mice need to target the brainstem in addition to the spinal cord. PMID:24390572

An, Ting; Shi, Pengxiao; Duan, Weisong; Zhang, Shipan; Yuan, Pin; Li, Zhongyao; Wu, Dongxia; Xu, Zuoshang; Li, Chunyan; Guo, Yansu

2014-06-01

302

Review of stress corrosion cracking of pipeline steels in low and high pH solutions  

Microsoft Academic Search

This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the high pH stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the low pH stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is

B. Y. Fang; A. Atrens; J. Q. Wang; E. H. Han; Z. Y. Zhu; W. Ke

2003-01-01

303

Defects and stresses in MBE-grown GaN and Al0.3Ga0.7N layers doped by silicon using silane  

NASA Astrophysics Data System (ADS)

The electric and structural characteristics of silicon-doped GaN and Al0.3Ga0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 1020 cm-3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al0.3Ga0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 1019 cm-3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al0.3Ga0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.

Ratnikov, V. V.; Kyutt, R. N.; Smirnov, A. N.; Davydov, V. Yu.; Shcheglov, M. P.; Malin, T. V.; Zhuravlev, K. S.

2013-12-01

304

Genetic dissection of Al tolerance QTLs in the maize genome by high density SNP scan  

PubMed Central

Background Aluminum (Al) toxicity is an important limitation to food security in tropical and subtropical regions. High Al saturation on acid soils limits root development, reducing water and nutrient uptake. In addition to naturally occurring acid soils, agricultural practices may decrease soil pH, leading to yield losses due to Al toxicity. Elucidating the genetic and molecular mechanisms underlying maize Al tolerance is expected to accelerate the development of Al-tolerant cultivars. Results Five genomic regions were significantly associated with Al tolerance, using 54,455 SNP markers in a recombinant inbred line population derived from Cateto Al237. Candidate genes co-localized with Al tolerance QTLs were further investigated. Near-isogenic lines (NILs) developed for ZmMATE2 were as Al-sensitive as the recurrent line, indicating that this candidate gene was not responsible for the Al tolerance QTL on chromosome 5, qALT5. However, ZmNrat1, a maize homolog to OsNrat1, which encodes an Al3+ specific transporter previously implicated in rice Al tolerance, was mapped at ~40 Mbp from qALT5. We demonstrate for the first time that ZmNrat1 is preferentially expressed in maize root tips and is up-regulated by Al, similarly to OsNrat1 in rice, suggesting a role of this gene in maize Al tolerance. The strongest-effect QTL was mapped on chromosome 6 (qALT6), within a 0.5 Mbp region where three copies of the Al tolerance gene, ZmMATE1, were found in tandem configuration. qALT6 was shown to increase Al tolerance in maize; the qALT6-NILs carrying three copies of ZmMATE1 exhibited a two-fold increase in Al tolerance, and higher expression of ZmMATE1 compared to the Al sensitive recurrent parent. Interestingly, a new source of Al tolerance via ZmMATE1 was identified in a Brazilian elite line that showed high expression of ZmMATE1 but carries a single copy of ZmMATE1. Conclusions High ZmMATE1 expression, controlled either by three copies of the target gene or by an unknown molecular mechanism, is responsible for Al tolerance mediated by qALT6. As Al tolerant alleles at qALT6 are rare in maize, marker-assisted introgression of this QTL is an important strategy to improve maize adaptation to acid soils worldwide.

2014-01-01

305

Relationships among Stress, Coping, and Mental Health in High-Achieving High School Students  

ERIC Educational Resources Information Center

This study investigates the relationships among stress, coping, and mental health in 139 students participating in an International Baccalaureate (IB) high school diploma program. Mental health was assessed using both positive indicators (life satisfaction, academic achievement, academic self-efficacy) and negative indicators (psychopathology) of

Suldo, Shannon M.; Shaunessy, Elizabeth; Hardesty, Robin

2008-01-01

306

Synthesis and stress relaxation of ZnO/Al-doped ZnO core-shell nanowires  

NASA Astrophysics Data System (ADS)

Doping nanostructures is an effective method to tune their electrical and photoelectric properties. Taking ZnO nanowires (NWs) as a model system, we demonstrate that atomic layer deposition (ALD) can be adopted for the realization of a doping process by the homo-epitaxial growth of a doped shell on the NW core. The Al-doped ZnO NWs have a layered superlattice structure with dopants mainly occupying the interstitial positions. After annealing, Al3+ ions diffuse into the ZnO matrix and occupy substitutional locations, which is desirable for dopant activation. The stress accumulated during epitaxial growth is relaxed by the nucleation of dislocations, dislocation dipoles and anti-phase boundaries. We note that the proposed method can be easily adopted for doping different types of nanostructures, and fabricating superlattices and multiple quantum wells on NWs in a controllable way.Doping nanostructures is an effective method to tune their electrical and photoelectric properties. Taking ZnO nanowires (NWs) as a model system, we demonstrate that atomic layer deposition (ALD) can be adopted for the realization of a doping process by the homo-epitaxial growth of a doped shell on the NW core. The Al-doped ZnO NWs have a layered superlattice structure with dopants mainly occupying the interstitial positions. After annealing, Al3+ ions diffuse into the ZnO matrix and occupy substitutional locations, which is desirable for dopant activation. The stress accumulated during epitaxial growth is relaxed by the nucleation of dislocations, dislocation dipoles and anti-phase boundaries. We note that the proposed method can be easily adopted for doping different types of nanostructures, and fabricating superlattices and multiple quantum wells on NWs in a controllable way. Electronic supplementary information (ESI) available: Schematic diagram of the deposition process, TEM images of the as-grown ZnO NWs, the distribution profiles of the elements Zn, O and Al, and the panoramic XPS curves after calibration by C 1s at 284.6 eV. See DOI: 10.1039/c3nr33584d

Wang, Hong-Bo; Ma, Fei; Li, Qian-Qian; Dong, Ce-Zhou; Ma, Da-Yan; Wang, Hong-Tao; Xu, Ke-Wei

2013-03-01

307

Preparation of Nb3Al by high-energy ball milling and superconductivity  

NASA Astrophysics Data System (ADS)

The A15 phase superconductor Nb3Al has been considered as an alternative to Nb3Sn for high field and large scale applications. However, to prepare a stoichiometric Nb3Al with fine grain structures is very difficult. High-energy ball milling is a solid state powder processing technique and is a very useful for preparing Nb-Al alloys (Nb3Al). The effects of ball milling time and annealing temperature on the formation of Nb3Al superconducting phase have been studied. Pure Nb and Al powders with stoichiometric ratio of Nb3Al were mixed and milled, and the charging and milling were performed in an inert atmosphere. Phase formation and structural evolution during high-energy ball milling have been examined by X-ray diffraction. Al disappeared and Nb peaks broadened after about one hour of milling. With increasing milling time, the peaks of Nb became considerably broader and intensities decreased, the Nb-Al solid solution phase was extensive when milled about 3 hours. In order to obtain Nb3Al superconducting phase, a subsequent anneal was required. We have annealed the as-milled powders at 800-900C for different times to prepared Nb3Al superconducting alloy. The results indicated that Nb3Al with small amount of impurity phase can be obtained on annealing the Nb-Al solid solution phase and the superconducting transition temperature was about 15K, but it is difficult to obtain a homogeneous Nb3Al phase by annealing the amorphous powder.

Chen, Yongliang; Liu, Zhao; Li, Pingyuan; Zhang, Xiaolan; Yang, Suhua; Yang, Dawei; Du, Lupeng; Cui, Yajing; Pan, Xifeng; Yan, Guo; Zhao, Yong

2014-05-01

308

In-situ studies of the TGO growth stresses and the martensitic transformation in the B2 phase in commercial Pt-modified NiAl and NiCoCrAlY bond coat alloys.  

SciTech Connect

Oxide growth stresses were measured in situ at 1100 C on commercial Pt-modified NiAl and NiCoCrAlY bond coat alloys using synchrotron X-rays. Measurements were taken on samples that had no preoxidation, as well as on samples that had experienced 24 one-hour thermal exposures at 1150 C, a condition known to induce rumpling in the Pt-modified NiAl alloy, but not in the NiCoCrAlY alloy. The NiCoCrAlY alloy showed continuous stress relaxation under all conditions, whereas the Pt-modified NiAl alloys would typically stabilize at a fixed (often non-zero) stress suggesting a higher creep strength in the 'Thermally Grown Oxide' on the latter alloy, though the precise behavior was dependent on initial surface preparation. The formation of martensite in the Pt-modified NiAl alloys was also observed upon cooling and occurred at temperatures below 200 C for all of the samples observed. Based on existing models, this M{sub s} temperature is too low to account for the rumpling observed in these alloys.

Hovis, D.; Hu, L.; Reddy, A.; Heuer, A. H.; Paulikas, A. P.; Veal, B. W. (Materials Science Division); (Case Western Reserve Univ.)

2007-12-01

309

Impact of electrical stress on the electrical characteristics of 2 MeV electron irradiated metal-oxide-silicon capacitors with atomic layer deposited Al2O3, HfO2 and nanolaminated dielectrics  

NASA Astrophysics Data System (ADS)

In this work, the impact of electrical stress on the electrical characteristics of 2 MeV electron irradiated metal-oxide-silicon capacitors with atomic layer deposited (ALD) high permittivity (high-k) dielectric layers of Al2O3, HfO2 and a nanolaminate of them is evaluated. The aim is to investigate the susceptibility to electrical stress of the radiation effects created in irradiated MOS structures, paying especial attention to any possible interaction between the radiation-induced damage and the subsequent electrical stress degradation. For this study, MOS capacitors with a nominal dielectric physical thickness of 10 nm (equivalent oxide thickness (EOT) between 3 nm and 7 nm) on different p-type and n-type silicon substrates were investigated. An exponentially increasing stress current was forced to flow in accumulation through the different dielectric layers, registering the evolution of gate voltage versus stress time until dielectric breakdown occurred. Capacitance-voltage characteristics of the different irradiated and non-irradiated structures are analyzed as a function of electrical stress. Different charge trapping behaviors and significant polarity dependence in interface state generation are observed for the dielectric layers subjected to substrate and gate injections. No clear interaction between radiation and electrical stress damages is noticed under substrate injection (n-type samples). However, higher negative charge trapping near the metal/dielectric interface is registered for the irradiated Al2O3 and nanolaminate layers subjected to gate injection and, in the case of the most irradiated samples, the radiation-induced interface states damage is found to dominate against the damage generated in early stages of the electrical stress.

Raf, J. M.; Gonzlez, M. B.; Takakura, K.; Tsunoda, I.; Yoneoka, M.; Beldarrain, O.; Zabala, M.; Campabadal, F.

2013-11-01

310

Stress  

NSDL National Science Digital Library

This project collects resources for studying mental health and stress issues with middle schoolers. Teens and stress Science NetLinks: The Laughing Brain 2: A Good Laugh Dealing with anger Stress-o-meter Look at each of the above sites. Choose one and read the content. Write a one-paragraph summary. Play interactive games and take quizzes. Keep a log of what you do. Tell which site you liked best and why. Watch the following video for positive things teens do to reduce the stress ...

Deaton, Mrs.

2011-06-10

311

Performance evaluation of AIP-TiAlN coated tool for high speed machining  

Microsoft Academic Search

Ti0.67Al0.33N single-layered and Ti0.52Al0.48N\\/TiN double-layered coatings were applied to end-mill tools made of WCCo material by an arc ion plating technique. Their performances under high speed cutting conditions were evaluated about cutting force, tool wear, and surface roughness of workpiece. With TiAlN coatings, tool life and performance were much increased under high speed machining. The Ti0.67Al0.33N single-layer coated tool showed

Myung Chang Kang; In-Wook Park; Kwang Ho Kim

2003-01-01

312

Effect of Zn Content on the Microstructure and Properties of Super-High Strength Al-Zn-Mg-Cu Alloys  

NASA Astrophysics Data System (ADS)

The microstructure and properties of three different Al-Zn-Mg-Cu alloys with high Zn content (9 wt pct, 10 wt pct, and 11 wt pct, marked as 9Zn, 10Zn, and 11Zn, respectively) were investigated. The strength of alloys increases as the Zn content increases from 9 wt pct to 10 wt pct, while it does not increase any more as the Zn content increases continuously from 10 wt pct to 11 wt pct. The stress-corrosion cracking (SCC) resistance decreases as the Zn content increases from 9 wt pct to 10 wt pct, while it changes unobviously as the Zn content increases continuously from 10 wt pct to 11 wt pct. The elongation and fracture toughness of alloys decrease as the Zn content increases in these Al-Zn-Mg-Cu alloys. The Zn content has little effect on the precipitation reaction of Al-Zn-Mg-Cu alloys that contain the mixture of GP zones, and ?' are the main Matrix Precipitates (MPt) in the peak-aging state, and the mixture of ?' and ? are the main MPt in the over-aging state. The amount of MPt and coarse T (AlZnMgCu) phases are shown to increase with the increasing Zn content in Al-Zn-Mg-Cu alloys. The coarse T phases hardly dissolve into the matrix and are the source for the crack initiation, which may be the responsibility for the negative effect on the properties of high Zn content Al-Zn-Mg-Cu alloys.

Chen, Ziyong; Mo, Yuanke; Nie, Zuoren

2013-08-01

313

High damping Al-Fe-Mo-Si/Zn-Al composites produced by rapidly solidified powder metallurgy process  

SciTech Connect

The metallic materials commonly used in aircraft and aerospace fields, such as aluminum and titanium alloys, steels, etc., show extremely low damping capacity (usually of the order of or less than 10{sup {minus}3}). Thus, some problems related to vibration may emerge and influence the reliability, safety and life of airplanes, satellites, etc. It has been reported that almost two thirds of errors for rockets and satellites are related to vibration and noise. One effective way to solve these vibration-related problems is to adopt high damping metallic materials. Conventional high damping alloys exhibit damping capacity above 10{sup {minus}2}, however, their densities are usually great than 5 x 10{sup 3} kg m{sup {minus}3}, or their strengths are less than 200 MPa (for alloys based on dislocation damping), making them impossible to be applied to aircraft and aerospace areas. Recently, some low-density high-damping metal/metal composites based on aluminum and high damping alloys have been developed in Beijing Institute of Aeronautical Materials (BIAM) by the rapidly solidified power metallurgy process. This paper aims to report the properties of the composites based on a high temperature Al-Fe-Mo-Si alloy and a high damping Zn-Al alloy, and compare them with that of 2618-T61 alloy produced by the ingot metallurgy process.

Li, P.Y.; Dai, S.L.; Chai, S.C.; Li, Y.R.

2000-05-10

314

High-power AlGaN\\/InGaN\\/AlGaN\\/GaN recessed gate heterostructure field-effect transistors  

Microsoft Academic Search

We demonstrate the use of high-quality thin InGaN films as the reactive ion etching (RIE) stop layer for fabrication of recessed gate high-microwave-power AlGaN\\/InGaN\\/AlGaN\\/GaN heterostructure field-effect transistors. We used migration-enhanced-metalorganic-chemical-vapor-deposition grown InGaN layer sandwiched in AlGaN barrier yields better than 10 times RIE selectivity of AlGaN and InGaN compared to our conventional standard AlGaN\\/GaN high electron mobility transistors. The fabricated

R. S. Qhalid Fareed; X. Hu; A. Tarakji; J. Deng; R. Gaska; M. Shur; M. A. Khan

2005-01-01

315

Evolution of stress failure resulting from high-temperature stress-corrosion cracking in a hot isostatically pressed silicon nitride  

Microsoft Academic Search

Stress-corrosion cracking in a commercially available, hot isostatically pressed (HIPed), yttria-fluxed, silicon nitride was the prevalent mode of failure in specimens creep-ruptured at 1,370 C. High-temperature diffusional processes associated with oxygen were responsible for the creation of an advancing stress-corrosion front that had formed at the specimen surface and advanced radially inward. The volume of material in the wake of

Andrew A. Wereszczak; Mattison K. Ferber; Timothy P. Kirkland; Karren L. More; Michael R. Foley; Russell L. Yeckley

1995-01-01

316

Investigation of the current collapse induced in InGaN back barrier AlGaN/GaN high electron mobility transistors  

NASA Astrophysics Data System (ADS)

Current collapses were studied, which were observed in AlGaN/GaN high electron mobility transistors (HEMTs) with and without InGaN back barrier (BB) as a result of short-term bias stress. More serious drain current collapses were observed in InGaN BB AlGaN/GaN HEMTs compared with the traditional HEMTs. The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse. The surface states may be the primary mechanism of the origination of current collapse in AlGaN/GaN HEMTs for short-term direct current stress.

Xiaojia, Wan; Xiaoliang, Wang; Hongling, Xiao; Chun, Feng; Lijuan, Jiang; Shenqi, Qu; Zhanguo, Wang; Xun, Hou

2013-10-01

317

Straight-sided, buckling-driven delamination of thin films at high stress levels  

Microsoft Academic Search

The fracture mechanics of a straight-sided, thin film delamination at stress levels, which are high compared to the stress required to initiate the delamination is investigated. Buckling at a bifurcation point of the delaminated region, resulting from incompletely relieved stresses in this region, is analysed by a semi- analytical approach for delaminations of infinite extent. The results are compared to

Henrik Myhre Jensen; Izhak Sheinman

2001-01-01

318

Efficacy of two relaxation techniques in reducing pulse rate among highly stressed females  

Microsoft Academic Search

The present study aimed at comparing the efficacy of two relaxation techniques on stressed females.30 highly stressed female subjects were selected and randomly assigned to three groups (N=10 each) (a) group 1 receiving galvanic skin resistance biofeedback, (b) group 2 receiving progressive muscle relaxation training, (c) group 3 control. The stress level was determined using Comprehensive Anxiety Test Questionnaire. Pulse

Archana Khanna; Maman Paul; Jaspal S. Sandhu

2007-01-01

319

High trait anxiety in healthy subjects is associated with low neuroendocrine activity during psychosocial stress  

Microsoft Academic Search

Altered stress responsiveness has been repeatedly related to mood and anxiety disorders. In a traditional view, a reduction of the stress response has been thought favorable. The goal of the present study was to verify the hypothesis that high anxiety is accompanied by enhanced hormone release during stress. Healthy subjects at the upper (anxious, n=15) and lower (non-anxious, n=12) limits

Daniela Jezova; Aikaterini Makatsori; Roman Duncko; Fedor Moncek; Martin Jakubek

2004-01-01

320

High Glucose-Mediated Oxidative Stress Impairs Cell Migration  

PubMed Central

Deficient wound healing in diabetic patients is very frequent, but the cellular and molecular causes are poorly defined. In this study, we evaluate the hypothesis that high glucose concentrations inhibit cell migration. Using CHO.K1 cells, NIH-3T3 fibroblasts, mouse embryonic fibroblasts and primary skin fibroblasts from control and diabetic rats cultured in 5 mM D-glucose (low glucose, LG), 25 mM D-glucose (high glucose, HG) or 25 mM L-glucose medium (osmotic control - OC), we analyzed the migration speed, protrusion stability, cell polarity, adhesion maturation and the activity of the small Rho GTPase Rac1. We also analyzed the effects of reactive oxygen species by incubating cells with the antioxidant N-Acetyl-Cysteine (NAC). We observed that HG conditions inhibited cell migration when compared to LG or OC. This inhibition resulted from impaired cell polarity, protrusion destabilization and inhibition of adhesion maturation. Conversely, Rac1 activity, which promotes protrusion and blocks adhesion maturation, was increased in HG conditions, thus providing a mechanistic basis for the HG phenotype. Most of the HG effects were partially or completely rescued by treatment with NAC. These findings demonstrate that HG impairs cell migration due to an increase in oxidative stress that causes polarity loss, deficient adhesion and protrusion. These alterations arise, in large part, from increased Rac1 activity and may contribute to the poor wound healing observed in diabetic patients.

Lamers, Marcelo L.; Almeida, Maira E. S.; Vicente-Manzanares, Miguel; Horwitz, Alan F.; Santos, Marinilce F.

2011-01-01

321

High strain rate deformation of Ti-48Al-2Cr-2Nb in the duplex morphology  

SciTech Connect

The compressive deformation behavior of Ti-48Al-2Nb-2Cr in the duplex microstructural morphology has been studied at strain rates of 0.001/s and 2000/s over the temperature range from -196 to 1100{degrees}C. The material was cast, homogenized, extruded and heat treated to obtain the duplex microstructure. The yield stress is strain rate sensitive at 25{degrees}C and increases with temperature at a strain rate of 2000/s from 500 to 1100{degrees}C. TEM investigations reveal that deformation occurs in {gamma}-TiAl by means of [111]<112> twinning, 1/2<110> slip, and <101> superdislocations under all conditions depending on the orientation of the grain with respect to the deformation axis. Optical metallography reveals that twinning increases with increasing strain rate. TEM results revealing the dislocation substructure are used to explain the yield stress anomaly.

Maloy, S.A.; Gray, G.T. III

1995-03-01

322

Skin solidification during high pressure die casting of Al11Si2Cu1Fe alloy  

Microsoft Academic Search

Surface regions of Al11Si2Cu1Fe alloy castings produced by high pressure die casting (HPDC) were examined and three microstructural features were observed. In locations of direct melt impingement, there was not an ?-Al rich layer. In most surface locations, a porosity-free ?-Al rich layer was observed. However, in surface locations of low impact of melt, a porosity-free layer being a mixture

Z. W. Chen

2003-01-01

323

Local structural orders in nanostructured Al2O3 prepared by high-energy ball milling  

Microsoft Academic Search

Nanostructured Al2O3 powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al3+ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe3+ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the 27Al nuclei and the zero-field splitting

G. Scholz; R. Stsser; J. Klein; G. Silly; J. Y. Buzar; Y. Laligant; B. Ziemer

2002-01-01

324

Notch size effects on high cycle fatigue limit stress of Udimet 720  

Microsoft Academic Search

Notch size effects on the high cycle fatigue (HCF) limit stress of Ni-base superalloy Udimet 720 were investigated on cylindrical specimens with three notch sizes of the same stress concentration factor Kt=2.74. The HCF limit stress corresponding to a life of 106 cycles was experimentally determined at a stress ratio of 0.1 and a frequency of 25 Hz at room

Weiju Ren; Theodore Nicholas

2003-01-01

325

Degradation of blue AlGaN/InGaN/GaN LEDs subjected to high current pulses  

SciTech Connect

Short-wavelength, visible-light emitting optoelectronic devices are needed for a wide range of commercial applications, including high-density optical data storage, full-color displays, and underwater communications. In 1994, high-brightness blue LEDs based on gallium nitride and related compounds (InGaN/AlGaN) were introduced by Nichia Chemical Industries. The Nichia diodes are 100 times brighter than the previously available SiC blue LEDs. Group-III nitrides combine a wide, direct bandgap with refractory properties and high physical strength. So far, no studies of degradation of GaN based LEDs have been reported. The authors study, reported in this paper, focuses on the performance of GaN LEDs under high electrical stress conditions. Their observations indicate that, in spite of a high defect density, which normally would have been fatal to other III-V devices, defects in group-III nitrides are not mobile even under high electrical stress. Defect tubes, however, can offer a preferential path for contact metals to electromigrate towards the p-n junction, eventually resulting in a short. The proposed mechanism of GaN diode degradation raises concern for prospects of reliable lasers in the group-III nitrides grown on sapphire.

Barton, D.L. [Sandia National Labs., Albuquerque, NM (United States); Zeller, J.; Phillips, B.S.; Chiu, P.C.; Askar, S.; Lee, D.S.; Osinski, M.; Malloy, K.J. [Univ. of New Mexico, Albuquerque, NM (United States). Center for High Technology Materials

1994-12-31

326

Degradation of blue AlGaN/InGaN/GaN LEDs subjected to high current pulses  

NASA Astrophysics Data System (ADS)

Short-wavelength, visible-light emitting optoelectronic devices are needed for a wide range of commercial applications, including high-density optical data storage, full-color displays, and underwater communications. In 1994, high-brightness blue LED's based on gallium nitride and related compounds (InGaN/AlGaN) were introduced by Nichia Chemical Industries. The Nichia diodes are 100 times brighter than the previously available SiC blue LED's. Group-3 nitrides combine a wide, direct bandgap with refractory properties and high physical strength. So far, no studies of degradation of GaN based LED's have been reported. The authors study, reported in this paper, focuses on the performance of GaN LED's under high electrical stress conditions. Their observations indicate that, in spite of a high defect density, which normally would have been fatal to other 3-5 devices, defects in group-3 nitrides are not mobile even under high electrical stress. Defect tubes, however, can offer a preferential path for contact metals to electromigrate towards the p-n junction, eventually resulting in a short. The proposed mechanism of GaN diode degradation raises concern for prospects of reliable lasers in the group-3 nitrides grown on sapphire.

Barton, Daniel L.; Zeller, Joachim; Phillips, B. Scott; Chiu, Pei-Chih; Askar, Sabrina; Lee, Dong-Seung; Osinski, Lee Marek; Malloy, Kevin J.

1994-12-01

327

INVESTIGATION OF STRESS-CORROSION CRACKING OF HIGH-STRENGTH ALLOYS. Report No. L0414-01-20. Progress Report No. 20, November 1November 30, 1962  

Microsoft Academic Search

Chemical compositions and mechanical properties are tabulated for 6Al-4V ; titanium, 20%-nickel maraging steel, and high-titanium 18%-nickel maraging steel. ; The status o the stress-corrosion cracking tests and development of protective ; coatings is outlined. (D.L.C.);

1963-01-01

328

The influence of growth rate and temperature on high cycle fatigue of Al-Al3Ni  

NASA Technical Reports Server (NTRS)

High-cycle fatigue tests have been conducted on specimens of an Al-Al3Ni eutectic alloy, unidirectionally solidified at selected rates from 0.000139 to 0.3 cm/sec. Tests were conducted in air at 298, 458 and 683 K. Room temperature fatigue lives were independent of growth rate at low solidification rates but were markedly improved in samples grown at 0.3 cm/sec. Materials grown at 0.00833 cm/sec exhibited fatigue lives similar to those of the lower growth rates, despite gross misalignment due to cellular growth. The dependence of fatigue life on growth rate at elevated temperatures appears to be due primarily to differences in cyclic creep rates as a result of varying interfiber spacings. Crack initiation and propagation mechanisms were established by metallographic and fractographic examination. Dislocation substructure-fiber interactions were studied by transmission electron microscopy.

Maurer, G. E.; Duquette, D. J.; Stoloff, N. S.

1976-01-01

329

Stress Corrosion Cracking of High Strength Skills. Abstract Only.  

National Technical Information Service (NTIS)

The stress corrosion cracking of AerMet 100 and 300M steels was investigated in aqueous NaCl solutions of different concentrations (0.035-3.5%) but an identical electrical conductivity, employing rising step load test method. The threshold stress intensit...

E. Lee H. Sanders B. Sarkar

1999-01-01

330

Stress-corrosion crack-growth study of titanium alloy Ti-6Al-4V exposed to freon PCA and nitrogen tetroxide MON-1  

NASA Technical Reports Server (NTRS)

An experimental fracture mechanics program was performed to determine the stress corrosion crack growth sensitivity of the propellant tank material, titanium alloy Ti-6Al-4V, for aerospace satellite applications involving long term exposure to Freon PCA and nitrogen tetroxide MON-1. Sustained load tests were made at a 49 C (120 F) constant temperature using thin gauge tensile test specimens containing semielliptical surface flaws. Test specimen types included parent metal, center of weld, and weld heat affected zone. It was concluded that Ti-6Al-4V alloy is not adversely affected in a stress environment when exposed to Freon PCA for 1000 hours followed by exposure to nitrogen tetroxide MON-1 for 2000 hours at stress levels up to 80% of the experimental critical plane strain stress intensity factor.

Bjorklund, R. A.

1983-01-01

331

Cavity Optomechanics with High-Stress Silicon Nitride Films  

NASA Astrophysics Data System (ADS)

There has been a barrage of interest in recent years to marry the fields of nanomechanics and quantum optics. Mechanical systems provide sensitive and scalable architectures for sensing applications ranging from atomic force microscopy to gravity wave interferometry. Optical resonators driven by low noise lasers provide a quiet and well-understood means to read-out and manipulate mechanical motion, by way of the radiation pressure force. Taken to an extreme, a device consisting of a high-Q nanomechanical oscillator coupled to a high-finesse optical cavity may enable ground-state preparation of the mechanical element, thus paving the way for a new class of quantum technology based on chip-scale phononic devices coupled to optical photons. By way of mutual coupling to the optical field, this architecture may enable coupling of single phonons to real or artificial atoms, an enticing prospect because of the vast "quantum optics toolbox" already developed for cavity quantum electrodynamics. The first step towards these goals --- ground-state cooling of the mechanical element in a "cavity optomechanical" system --- has very recently been realized in a cryogenic setup. The work presented in this thesis describes an effort to extend this capability to a room temperature apparatus, so that the usual panoply of table-top optical/atomic physics tools can be brought to bear. This requires a mechanical oscillator with exceptionally low dissipation, as well as careful attention to extraneous sources of noise in both the optical and mechanical componentry. Our particular system is based on a high- Q, high-stress silicon nitride membrane coupled to a high-finesse Fabry-Perot cavity. The purpose of this thesis is to record in detail the procedure for characterizing/modeling the physical properties of the membrane resonator, the optical cavity, and their mutual interaction, as well as extraneous sources of noise related to multimode thermal motion of the oscillator, thermal motion of the cavity apparatus, optical absorption, and laser phase fluctuations. Our principle experimental result is the radiation pressure-based cooling of a high order, ? 4.8 MHz drum mode of the membrane from room temperature to ? 100 mK (? 500 phonons). Secondary results include an investigation of the Q-factor of membrane oscillators with various geometries, some of which exhibit state-of-the-art Qxfrequency products of 3 x 1013 Hz, and a novel technique to suppress extraneous radiation pressure noise using electro-optic feedback.

Wilson, Dalziel Joseph

332

Tunable High efficiency Resonant Tunneling GaN/AlGaN MQW UV Detectors.  

National Technical Information Service (NTIS)

The objective of this project is to develop tunable high efficiency resonant tunneling GaN/AlGaN multiple quantum well (MQW)-based UV photodetectors. During the grant period, more than twenty six GaN/AlGaN MQW phoptodetectors with different types of devic...

W. B. Wang S. K. Zhang R. R. Alfano

2005-01-01

333

Microstructure Characterization and Stress Corrosion Evaluation of Autogenous and Hybrid Friction Stir Welded Al-Cu-Li 2195 Alloy  

NASA Technical Reports Server (NTRS)

Friction stir welding process is being evaluated for application on the Al-Cu-Li 2195 Super-Light Weight External Tank of the Space Transportation System. In the present investigation Al-Cu-Li 2195 plates were joined by autogenous friction stir welding (FSW) and hybrid FSW (friction stir welding over existing variable polarity plasma arc weld). Optical microscopy and transmission electron microscopy (TEM) were utilized to characterize microstructures of the weldments processed by both welding methods. TEM observations of autogenous FSW coupons in the center section of the dynamically-recrystallized zone showed an equiaxed recrystallized microstructure with an average grain size of approx. 3.8 microns. No T(sub 1), precipitates were present in the above-mentioned zone. Instead, T(sub B) and alpha precipitates were found in this zone with a lower population. Alternate immersion, anodic polarization, constant load, and slow strain tests were carried out to evaluate the general corrosion and stress-corrosion properties of autogenous and hybrid FSW prepared coupons. The experimental results will be discussed.

Li, Zhixian; Arbegast, William J.; Meletis, Efstathios I.

1997-01-01

334

The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates  

Microsoft Academic Search

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of ?6.5GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated ?3.8GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel

L. A Donohue; D. B Lewis; W-D Mnz; M. M Stack; S. B Lyon; H-W Wang; D Rafaja

1999-01-01

335

Development of a novel Ca\\/Al carbonates for medium-high temperature CO 2 capture  

Microsoft Academic Search

This study describes the synthesis and characterization of a layered material CaAlCO3 as CO2 sorbent at medium-high temperature conditions. The CaAl mixed oxides were prepared by co-precipitation of binary metal nitrates, i.e., Ca(NO3)2 and Al(NO3)3 under alkaline pH condition, and the concentration of calcium oxide was increased by varying the molar ratio of Ca\\/Al at 1, 3, 5, 7, 13

Ching-Tsung Yu; Chi-Hung Wang; Min-Jung Hsu; Yau-Pin Chyou

2011-01-01

336

Theoretical study of thermally stable SiO2/AlN/SiO2 Lamb wave resonators at high temperatures  

NASA Astrophysics Data System (ADS)

Aluminum nitride (AlN) and silicon dioxide (SiO2) bilayer structure has been widely utilized in temperature-compensated micromechanical resonators as SiO2 has unique positive temperature coefficients of elasticity. However, the thermal expansion mismatch would cause large bending deformation and stress distribution in the resonant plate. In this study, a symmetrical SiO2/AlN/SiO2 sandwiched structure is proposed to reduce the temperature-induced deformation in the asymmetrical AlN/SiO2 bilayer plate. The thermal compensation at high temperatures for the Lamb wave resonators utilizing the lowest-order symmetric (S0) mode in the SiO2/AlN/SiO2 sandwiched structure is theoretically investigated herein. While operation temperature rises from room temperature to 600 C, the temperature-induced bending deformation in the symmetrical SiO2/AlN/SiO2 composite plate is much less than that in the AlN/SiO2 composite plate conventionally used for temperature compensation. Furthermore, the different material properties of the AlN and SiO2 layers make the displacements of the S0 mode not purely symmetric with respect to the neutral axis, whereas the symmetrical SiO2/AlN/SiO2 sandwiched membrane still can enable a pure S0 mode which shows higher phase velocity and larger electromechanical coupling coefficient than the lowest-order quasi-symmetric (QS0) mode traveling in the AlN/SiO2 bilayer membrane. With proper thickness selection of AlN and SiO2, the S0 mode in the symmetrical SiO2/AlN/SiO2 sandwiched membrane can simultaneously offer excellent thermal compensation, high phase velocity, large electromechanical coupling coefficient, and small thermally induced deformation at high temperatures.

Zou, Jie; Lin, Chih-Ming; Chen, Yung-Yu; Pisano, Albert P.

2014-03-01

337

Polishing and characterization of thick AlN layers grown on SiC substrates by stress control hydride vapor phase epitaxy  

Microsoft Academic Search

In this paper, we report new results on polishing and characterization of thick aluminium nitride (AlN) layers grown on 2- and 3-inch electrically conducting 4H-SiC on-axis substrate. As thick as 10-22 mum AlN layers were deposited on Si-face by stress control hydride vapor phase epitaxy. Rough as-grown layers were polished to provide epi-ready surface for device epitaxy. Surface morphology and

H. Mank; B. Amstatt; D. Turover; E. Bellet-Amalric; B. Daudin; V. Ivantsov; V. Dmitriev; V. Maslennikov

2006-01-01

338

Influence of residual stresses on the evolution of micro-structure during the partial reduction of NiAl 2O 4  

Microsoft Academic Search

Metal-ceramic microstructures were formed in situ by the partial reduction (i.e. the reduction of only one of the metallic elements) of the spinel compound NiAl2O4. Depending on reduction conditions, these microstructures consist of Ni particles embedded in an ?-Al2O3 or a multiphase matrix called defect spinel. The volume shrinkage that accompanies the reaction generates residual stresses which profoundly affect the

E. stndag; Z. Zhang; M. L. Stocker; P. Rangaswamy; M. A. M. Bourke; S. Subramanian; K. E. Sickafus; J. A. Roberts; S. L. Sass

1997-01-01

339

High temperature degradation behavior of sputtered nanostructured Co-Al coatings on superalloy  

NASA Astrophysics Data System (ADS)

Microstructure and cyclic high temperature oxidation behavior of nanostructured Co-Al coatings on Superni-718 substrate have been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 800 C and 900 C for up to 100 thermal cycles between the peak and room temperatures. The results showed that bare substrate has higher oxidation resistance at 800 C as compared to 900 C and coated sample has slightly higher oxidation resistance at 900 C. The external scale of both coated sample exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co-Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co-Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of ?-CoAl phase as Al reservoir and the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in the scale. The oxidation results confirmed an improved oxidation resistance of the Co-Al coating on superalloy as compared to bare substrate up to 100 cycles.

Rahman, Atikur; Jayaganthan, R.; Chandra, Ramesh; Ambardar, R.

2013-01-01

340

Highly transparent and conductive ZnO:Al thin films prepared by vacuum arc plasma evaporation  

NASA Astrophysics Data System (ADS)

A vacuum arc plasma evaporation (VAPE) method using both oxide fragments and gas sources as the source materials is demonstrated to be very effective for the preparation of multicomponent oxide thin films. Highly transparent and conductive Al-doped ZnO (AZO) thin films were prepared by the VAPE method using a ZnO fragment target and a gas source Al dopant, aluminum acethylacetonate (Al(C5H7O2)3) contained in a stainless steel vessel. The Al content in the AZO films was altered by controlling the partial pressure (or flow rate) of the Al dopant gas. High deposition rates as well as uniform distributions of resistivity and thickness on the substrate surface were obtained on large area glass substrates. A low resistivity on the order of 10-4 ? cm and an average transmittance above 80% in the visible range were obtained in AZO thin films deposited on glass substrates. .

Miyata, Toshihiro; Minamino, Youhei; Ida, Satoshi; Minami, Tadatsugu

2004-07-01

341

Expression and Function of a Highly Conserved Water Stress Protein.  

National Technical Information Service (NTIS)

New Mexico agriculture depends upon irrigation to avoid water deficit stress in agricultural crops. Targeted manipulation of plant genomes for enhanced salinity and water deficit tolerance through genetic engineering will require a sophisticated understan...

P. J. Lammers J. Bongianni S. Mori

1996-01-01

342

High-Power AlGaAs Channeled Substrate Planar Diode Lasers for Spaceborne Communications.  

National Technical Information Service (NTIS)

A high power channeled substrate planar AlGaAs diode laser with an emission wavelength of 8600 to 8800 A was developed. The optoelectronic behavior (power current, single spatial and spectral behavior, far field characteristics, modulation, and astigmatis...

J. C. Connolly B. Goldstein G. N. Pultz S. E. Slavin D. B. Carlin

1988-01-01

343

Development of a high specific stiffness mechanically milled FeAl intermetallic alloy.  

National Technical Information Service (NTIS)

Powder metallurgy techniques such as gas atomization and mechanical milling have been used to develop a FeAl alloy with enhanced ductility and strength at both low and high temperature. The improvement method combines ductility increase by grain boundary ...

R. Baccino D. San Filippo P. Martel F. Moret

1995-01-01

344

Electrode Potential Relaxation in High Temperature Li-Al/FeS Engineering Cells.  

National Technical Information Service (NTIS)

The cell resistance and electrode polarization of high temperature Li-Al/FeS cells were investigated using a current-interruption, potential-relaxation tecnique. The special instrumentation possesses excellent long term stability and is capable of resolvi...

L. Redey S. Higuchi T. Kaun F. Martino D. R. Vissers

1980-01-01

345

Induction of engineered residual stresses fields and enhancement of fatigue life of high reliability metallic components by laser shock processing  

NASA Astrophysics Data System (ADS)

Laser shock processing (LSP) is being increasingly applied as an effective technology for the improvement of metallic materials mechanical and surface properties in different types of components as a means of enhancement of their corrosion and fatigue life behavior. As reported in previous contributions by the authors, a main effect resulting from the application of the LSP technique consists on the generation of relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Additional results accomplished by the authors in the line of practical development of the LSP technique at an experimental level (aiming its integral assessment from an interrelated theoretical and experimental point of view) are presented in this paper. Concretely, follow-on experimental results on the residual stress profiles and associated surface properties modification successfully reached in typical materials (especially Al and Ti alloys characteristic of high reliability components in the aerospace, nuclear and biomedical sectors) under different LSP irradiation conditions are presented along with a practical correlated analysis on the protective character of the residual stress profiles obtained under different irradiation strategies. Additional remarks on the improved character of the LSP technique over the traditional "shot peening" technique in what concerns depth of induced compressive residual stresses fields are also made through the paper.

Ocaa, J. L.; Porro, J. A.; Daz, M.; Ruiz de Lara, L.; Correa, C.; Gil-Santos, A.; Peral, D.

2013-02-01

346

Effects of high mean stress on high-cycle fatigue behavior of PWA 1480  

NASA Technical Reports Server (NTRS)

PWA 1480 is a potential candidate material for use in the high-pressure fuel turbine blade of the space shuttle main engine. As an engine material it will be subjected to high-cycle fatigue loading superimposed on a high mean stress due to combined centrifugal and thermal loadings. The present paper describes the results obtained in an ongoing program at the Argonne National Laboratory, sponsored by NASA Lewis, to determine the effects of a high mean stress on the high-cycle fatigue behavior of this material. Straight-gauge high-cycle fatigue specimens, 0.2 inch in diameter and with the specimen axis in the 001 direction, were supplied by NASA Lewis. The nominal room temperature yield and ultimate strength of the material were 146 and 154 ksi, respectively. Each specimen was polished with 1-micron diamond paste prior to testing. However, the surface of each specimen contained many pores, some of which were as large as 50 micron. In the initial tests, specimens were subjected to axial-strain-controlled cycles. However, very little cyclic plasticity was observed.

Majumdar, S.; Antolovich, S. D.; Milligan, W. W.

1985-01-01

347

High-resolution electron microscope observation of interface microstructure of a cast Al-Mg-Si-Bi-Pb(6262)/Al2O3p composite.  

PubMed

High-resolution electron microscopy was employed to characterize the interface structure of a cast Al-Mg-Si-Bi-Pb aluminium(6262)-based composite reinforced by alpha alumina particles with a trace of beta alumina in order to investigate the behaviour of alloying elements in cast composites. Except for a few primary Mg2Si particles, few reaction products were detected at the interface of Al/alpha-Al2O3 due to the unfavourable reaction kinetics during the squeeze-casting process. The Mg2Si particle has an orientation relationship with alpha-Al2O3 of [011]Mg2Si//[1210]alpha-Al2O3 (111)Mg2Si//(0006)alpha-Al2O3. A significant amount of MgAl2O4 was found on the surface of the beta-Al2O3 particles, which is in contrast to the small degree of reaction found on alpha-Al2O3 particles. MgAl2O4 and beta-Al2O3 particles have the following orientation relationship: [011]MgAl2O4//[1210]beta-Al2O3 (111) MgAl2O4//(0006)beta-Al2O3. The similar crystal structure of beta-Al2O3 to MgAl2O4 favours MgAl2O4 nucleation and growth on the surface of beta-Al2O3. Interfacial energy minimization dominates the atomic structure of the interface with the result that close packed planes and directions in the Al2O3 reinforcement and reaction products are parallel to the interfaces. Bi and Pb were found in the form of metallic nanometre particles between Al2O3 particles, or between the MgAl2O4 and Al2O3 particles, or in the open channels of beta-Al2O3 filled by the Al matrix. PMID:11207916

Zhou, Z.; Fan, Z.; Peng, H. X.; Li, D. X.

2001-02-01

348

Stress corrosion cracking of several high strength ferrous and nickel alloys  

NASA Technical Reports Server (NTRS)

The stress corrosion cracking resistance of several high strength ferrous and nickel base alloys has been determined in a sodium chloride solution. Results indicate that under these test conditions Multiphase MP35N, Unitemp L605, Inconel 718, Carpenter 20Cb and 20Cb-3 are highly resistant to stress corrosion cracking. AISI 410 and 431 stainless steels, 18 Ni maraging steel (250 grade) and AISI 4130 steel are susceptible to stress corrosion cracking under some conditions.

Nelson, E. E.

1971-01-01

349

In situ stress heterogeneity induced by weak natural fractures and faults with high slip-tendency  

NASA Astrophysics Data System (ADS)

In situ stress measurements typically conducted using several geotechnical methods (hydraulic fracturing and overcoring) often show quite scattered and inconsistent stress magnitudes. Would they be artifacts from test errors or genuine stress signals in the crust? I report two field examples of stress measurements, in which lateral stresses represented by maximum (SHmax) and minimum (Shmin) horizontal principal stresses scatter quite widely, to investigate the cause of the observed stress heterogeneity. Hydraulic fracturing stress measurements were conducted in vertical boreholes at two different locations in South Korea. The boreholes are 300 and 400 m deep, respectively, both penetrating granites. Several isolated intervals of intact rocks over depths in the boreholes were vertically fractured by injecting water. Magnitudes of Shmin were determined from shut-in pressures. Magnitudes of SHmax were estimated based on the Hubbert-Willis (1957) equation using reliably determined hydraulic fracturing tensile strengths. The stress states in both locations are in reverse faulting stress regimes, in which vertical stress (Sv) is the least principal stress. The magnitudes of SHmax are generally within or close to stress range limited by frictional coefficients of 0.6-1.0 of nearby faults. However, SHmax do not increase consistently with depth, but rather scatter quite significantly. Over only a few tens of meter depth interval, the SHmax magnitudes jump up and down by an order of ~10 MPa, often resulting in lower SHmax values at deeper depths. Near the depths of relatively low stress, natural fractures and faults with wide apertures (for instance, wider than ~10 mm, observed from borehole image logs) are abundant, and near those of relatively high stress, those wide discontinuities are scarce. Such wide discontinuities are inferred to be filled with weak gouges or rock fragments, and thus tend to have relatively low frictional coefficients. In particular, the wide fractures and faults are oriented predominantly in the directions of high slip-tendency. If excessive stress is exerted, those weak fractures and faults would slip to release the stress, which would reduce the stress magnitudes to the values that can only be sustained by the discontinuities. This observation suggests that stress magnitudes are constrained quite locally by such weak, high slip-tendency discontinuities. Although the remote stress field might be uniformly applied, the stress state in the crust seems inherently heterogeneous because of the heterogeneous distribution of weak natural fractures and faults.

Chang, Chandong

2014-05-01

350

Fabrication of High TCR TaAl-N Thin Film by Reactive Sputtering Method  

NASA Astrophysics Data System (ADS)

The electric characteristics of TaAl-N thin film were investigated. The resistivity of the TaAl-N thin films, having been prepared by DC reactive sputtering of Ta and Al (8:2 area ratio) target in a gas mixture of argon and nitrogen, showed pressure-ratio dependence of nitrogen (PN2/Ptotal). The condition of high PN2/Ptotal was effective for fabricating the films, having a high TCR (the temperature coefficient of the resistivity). As a result, in the condition of PN2/Ptotal=75%, the TCR values were (-)35000 ppm/C (at 100C) and (-)25000 ppm/C (at 200C, resistivity).

Okano, Yukiko; Tajiri, Shuichi; Aozono, Takashi; Okamoto, Akio; Ogawa, Soichi; Mima, Hiroshi

351

High quality InAsSb grown on InP substrates using AlSb/AlAsSb buffer layers  

SciTech Connect

High quality InAsSb grown on semi-insulating InP substrates by molecular beam epitaxy was achieved using AlSb/AlAsSb structure as the buffer layer. A 1000 A InAsSb layer grown on top of 1 {mu}m AlSb/AlAsSb buffer layer showed a room temperature electron mobility of {approx}12 000 cm{sup 2}/V s. High structural quality and low misfit defect density were also demonstrated in the InAsSb layer. This novel AlSb/AlAsSb buffer layer structure with the AlAsSb layer lattice matched to InP substrates could enhance the performance of optoelectronic devices utilizing 6.1 A family of compound semiconductor alloys.

Wu, B.-R.; Liao, C.; Cheng, K. Y. [Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, Urbana, Illinois 61801 (United States)

2008-02-11

352

Effect of Hf-Rich Particles on the Creep Life of a High-Strength NiAl Single Crystal Alloy  

NASA Technical Reports Server (NTRS)

Additions of small amounts of Hf and Si to NiAl single crystals significantly improve their high-temperature strength and creep properties. However, if large Hf-rich dendritic particles formed during casting of the alloyed single crystals are not dissolved completely during homogenization heat treatment, a large variation in creep rupture life can occur. This behavior, observed in five samples of a Hf containing NiAl single crystal alloy tested at 1144 K under an initial stress of 241.4 MPa, is described in detail highlighting the role of interdendritic Hf-rich particles in limiting creep rupture life.

Garg, A.; Raj, S. V.; Darolia, R.

1995-01-01

353

High-temperature ultra-high cycle fatigue damage of notched single crystal superalloys at high mean stresses  

Microsoft Academic Search

Blade nickel superalloy CMSX-4 widely used in the aero industry and its potential low cost alternative, superalloy CM186LC intended for use in the industrial gas turbines, were subjected to ultra-high-cycle fatigue at high mean stresses to model the effect of vibrations superimposed on sustained load. Circumferentially notched cylindrical specimens of single crystals with the axis orientation of [001] were tested

P. Luk; L. Kunz; M. Svoboda

2005-01-01

354

Effect of multiaxial stresses on the high-temperature behavior and rupture of advanced alloys  

Microsoft Academic Search

The evolution and effect of multiaxial stress states on the high temperature deformation and rupture behavior of materials with non-uniform microstructures has been investigated. Through a detailed description of the role that multiaxial stresses play on damage evolution and rupture, the abundant existing data for uniaxial rupture can be used to more successfully design for the life of high temperature

Nancy Louise Johnson

1998-01-01

355

[Raman characterization of high-pressure phase transition in AlN nanowires].  

PubMed

High-pressure phase transition of AlN nanowires was investigated in the range of 0-33.1 GPa by in situ Raman spectrum method in diamond anvil cell (DAC). The A1 (LO) vibration mode exhibits considerably asymmetry and broadening, indicating the occurrence of wurtzite-to-rocksalt phase transition. The Raman signal of high-pressure phase can be assigned to the disorder activated Raman scattering of rocksalt AlN. After fully releasing pressure, the Raman characterization of high-pressure phase was quenched. According to the pressure dependence of phonon frequency of AlN nanowires, the difference of transiton path between AlN nanowires and bulk materials was discussed and the mode Grneisen parameters were determined. PMID:21595221

Shen, Long-hai; Li, Qian; Wu, Li-jun; Ma, Yan-mei; Cui, Qi-liang

2011-03-01

356

Families OverComing under Stress (FOCUS) for Early Childhood: Building Resilience for Young Children in High Stress Families  

ERIC Educational Resources Information Center

Parental distress and trauma affects the entire family, including the youngest children. Families OverComing Under Stress (FOCUS) is a targeted prevention program for high-risk families that aims to enhance family cohesion, support the parent-child relationship, and build emotional regulation, communication, and problem-solving skills across the

Mogil, Catherine; Paley, Blair; Doud, Tricia; Havens, Linda; Moore-Tyson, Jessica; Beardslee, William R.; Lester, Patricia

2010-01-01

357

Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy An experimental study  

Microsoft Academic Search

This paper presents the experimental results of fatigue life enhancement and the residual stresses around the cold expanded holes in Al 2024, a widely used aerospace alloy. Two techniques for cold expansion of holes, namely split-sleeve with taper pin technique and split-sleeve with ball technique were considered for comparison, as the former involves surface contact and the latter has line

H. D. Gopalakrishna; H. N. Narasimha Murthy; M. Krishna; M. S. Vinod; A. V. Suresh

2010-01-01

358

Nitrogen Plasma Ion Implantation of Al and Ti alloys in the High Voltage Glow Discharge Mode  

NASA Astrophysics Data System (ADS)

Enhanced surface properties can be attained for aluminum and its alloys (mechanical and tribological) and Ti6Al4V (mainly tribological) by Plasma Immersion Ion Implantation (PIII) technique. The main problem here, more severe for Al case, is the rapid oxygen contamination even in low O partial pressure. High energy nitrogen ions during PIII are demanded for this situation, in order to enable the ions to pass through the formed oxide layer. We have developed a PIII system that can operate at energies in excess of 50keV, using a Stacked Blumlein (SB) pulser which can nominally provide up to 100 kV pulses. Initially, we are using this system in the High Voltage Glow Discharge (HVGD) mode, to implant nitrogen ions into Al5052 alloy with energies in the range of 30 to 50keV, with 1.5?s duration pulses at a repetition rate of 100Hz. AES, pin-on-disc, nanoindentation measurements are under way but x-ray diffraction results already indicated abundant formation of AlN in the surface for Al5052 treated with this HVGD mode. Our major aim in this PIII experiment is to achieve this difficult to produce stable and highly reliable AlN rich surface layer with high hardness, high corrosion resistance and very low wear rate.

Oliveira, R. M.; Ueda, M.; Rossi, J. O.; Reuther, H.; Lepienski, C. M.; Beloto, A. F.

2006-11-01

359

Stress response of Barkhausen noise in high carbon steel cables and ropes  

NASA Astrophysics Data System (ADS)

Stress response of three magnetic parameters related to Barkhausen noise (BN) for three types of steel concrete reinforcement cables, in as-delivered and heat treated conditions, and three steel ropes, in the as-delivered state, is investigated. In all materials, rms value of BN and peak voltage of noise burst steadily decrease as a function of tensile stress up to a level of 1000 MPa, whereas peak position of noise burst is simultaneously delayed on the time axis, indicating an increase in coercive force. Stress response of rms and peak voltage of noise is strong in all reinforcement cables but weaker in ropes. Stress dependence of magnetic parameters is caused by the stress gradient effect at high stresses and at low stresses, either by <110> wire texture or by a decrease in mobility of Bloch walls oriented along the cementite lamellae or rows and, simultaneously, along the cables or ropes.

Rautioaho, R.; Kivimaa, J.; Moilanen, M.

1994-01-01

360

Heat Pipe Embedded AlSiC Plates for High Conductivity - Low CTE Heat Spreaders  

SciTech Connect

Heat pipe embedded aluminum silicon carbide (AlSiC) plates are innovative heat spreaders that provide high thermal conductivity and low coefficient of thermal expansion (CTE). Since heat pipes are two phase devices, they demonstrate effective thermal conductivities ranging between 50,000 and 200,000 W/m-K, depending on the heat pipe length. Installing heat pipes into an AlSiC plate dramatically increases the plates effective thermal conductivity. AlSiC plates alone have a thermal conductivity of roughly 200 W/m-K and a CTE ranging from 7-12 ppm/ deg C, similar to that of silicon. An equivalent sized heat pipe embedded AlSiC plate has effective thermal conductivity ranging from 400 to 500 W/m-K and retains the CTE of AlSiC.

Johnson, Matthew (DOE/NNSA Kansas City Plant (United States)); Weyant, J.; Garner, S. (Advanced Cooling Technologies, Inc. (Lancaster, PA (United States)); Occhionero, M. (CPS Technologies Corporation, Norton, MA (United States))

2010-01-07

361

High-brightness AlGaInP orange light-emitting diodes  

NASA Astrophysics Data System (ADS)

Orange AlGaInP high brightness light emitting diodes (LEDs) were fabricated by low pressure metalorganic chemical vapor deposition technology. AlGaInP double heterojunction structure was used as active layer. 15 pairs of Al0.5Ga0.5As/AlAs distributed Bragg reflector and 7 micrometers Al0.8Ga0.2As current spreading layer were employed to reduce the absorption of GaAs substrate and upper anode respectively. At 20 mA the LEDs emitting wavelength was between 600 - 610 nm with 18.3 nm FWHM, 0.45 mW radiation power and 1.7% external quantum efficiency. Brightness of the LED chips and LED lamps with 15 degree(s) viewing angle (2(theta) 1/2) reached 30 mcd and 1000 mcd respectively.

Li, Yuzhang; Wang, Guohong; Ma, Xiaoyu; Peng, Huaide; Wang, Shutang; Chen, Lianghui

1998-08-01

362

Fundamental studies of stress distributions and stress relaxation in oxide scales on high temperature alloys. [Final progress report  

SciTech Connect

This report summarizes a three-year study of stresses arising in the oxide scale and underlying metal during high temperature oxidation and of scale cracking. In-situ XRD was developed to measure strains during oxidation over 1000{degrees}C on pure metals. Acoustic emission was used to observe scale fracture during isothermal oxidation and cooling, and statistical analysis was used to infer mechanical aspects of cracking. A microscratch technique was used to measure the fracture toughness of scale/metal interface. A theoretical model was evaluated for the development and relaxation of stresses in scale and metal substrate during oxidation.

Shores, D.A.; Stout, J.H.; Gerberich, W.W.

1993-06-01

363

Cast Aluminum Alloys for High Temperature Applications Using Nanoparticles Al2O3 and Al3-X Compounds (X = Ti, V, Zr)  

NASA Technical Reports Server (NTRS)

In this paper, the effect of nanoparticles Al2O3 and Al3-X compounds (X = Ti, V, Zr) on the improvement of mechanical properties of aluminum alloys for elevated temperature applications is presented. These nanoparticles were selected based on their low cost, chemical stability and low diffusions rates in aluminum at high temperatures. The strengthening mechanism at high temperature for aluminum alloy is based on the mechanical blocking of dislocation movements by these nanoparticles. For Al2O3 nanoparticles, the test samples were prepared from special Al2O3 preforms, which were produced using ceramic injection molding process and then pressure infiltrated by molten aluminum. In another method, Al2O3 nanoparticles can also be homogeneously mixed with fine aluminum powder and consolidated into test samples through hot pressing and sintering. With the Al3-X nanoparticles, the test samples are produced as precipitates from in-situ reactions with molten aluminum using conventional permanent mold or die casting techniques. It is found that cast aluminum alloy using nanoparticles Al3-X is the most cost effective method to produce high strength aluminum alloys for high temperature applications in comparison to nanoparticles Al2O3. Furthermore, significant mechanical properties retention in high temperature environment could be achieved with Al3-X nanoparticles, resulting in tensile strength of nearly 3 times higher than most 300- series conventional cast aluminum alloys tested at 600 F.

Lee, Jonathan A.

2009-01-01

364

Stress corrosion in high-strength aluminum alloys  

NASA Technical Reports Server (NTRS)

Report describes results of stress-corrosion tests on aluminum alloys 7075, 7475, 7050, and 7049. Tests compare performance of original stress-corrosion-resistant (SCR) aluminum, 7075, with newer, higher-strength SCR alloys. Alloys 7050 and 7049 are found superior in short-transverse cross-corrosion resistance to older 7075 alloy; all alloys are subject to self-loading effect caused by wedging of corrosion products in cracks. Effect causes cracks to continue to grow, even at very-low externally applied loads.

Dorward, R. C.; Hasse, K. R.

1980-01-01

365

Ulnar Shaft Stress Fracture in a High School Softball Pitcher  

PubMed Central

This article presents a case of a 17-year-old softball pitcher with insidious onset of right forearm pain. On presentation, the patient had tenderness on palpation of the midshaft of the ulna, pain with resisted pronation, and pain with fulcrum-type stressing of the forearm. A bone scan revealed increased uptake in the right ulna, and a subsequent magnetic resonance imaging revealed bone marrow edema and numerous small ulnar stress fractures. She was treated with bone stimulation and complete rest and is in the process of returning to pitching.

Bigosinski, Krystian; Palmer, Trish; Weber, Kathleen; Evola, Jennifer

2010-01-01

366

Acute stress differentially affects spatial configuration learning in high and low cortisol-responding healthy adults  

PubMed Central

Background Stress and stress hormones modulate memory formation in various ways that are relevant to our understanding of stress-related psychopathology, such as posttraumatic stress disorder (PTSD). Particular relevance is attributed to efficient memory formation sustained by the hippocampus and parahippocampus. This process is thought to reduce the occurrence of intrusions and flashbacks following trauma, but may be negatively affected by acute stress. Moreover, recent evidence suggests that the efficiency of visuo-spatial processing and learning based on the hippocampal area is related to PTSD symptoms. Objective The current study investigated the effect of acute stress on spatial configuration learning using a spatial contextual cueing task (SCCT) known to heavily rely on structures in the parahippocampus. Method Acute stress was induced by subjecting participants (N = 34) to the Maastricht Acute Stress Test (MAST). Following a counterbalanced within-subject approach, the effects of stress and the ensuing hormonal (i.e., cortisol) activity on subsequent SCCT performance were compared to SCCT performance following a no-stress control condition. Results Acute stress did not impact SCCT learning overall, but opposing effects emerged for high versus low cortisol responders to the MAST. Learning scores following stress were reduced in low cortisol responders, while high cortisol-responding participants showed improved learning. Conclusions The effects of stress on spatial configuration learning were moderated by the magnitude of endogenous cortisol secretion. These findings suggest a possible mechanism by which cortisol responses serve an adaptive function during stress and trauma, and this may prove to be a promising route for future research in this area.

Meyer, Thomas; Smeets, Tom; Giesbrecht, Timo; Quaedflieg, Conny W. E. M.; Merckelbach, Harald

2013-01-01

367

Preliminary Thermal Stress Analysis of a High-Pressure Cryogenic Storage Tank  

NASA Technical Reports Server (NTRS)

The thermal stresses on a cryogenic storage tank strongly affect the condition of the tank and its ability to withstand operational stresses. These thermal stresses also affect the growth of any surface damage that might occur in the tank walls. These stresses are particularly of concern during the initial cooldown period for a new tank placed into service, and during any subsequent thermal cycles. A preliminary thermal stress analysis of a high-pressure cryogenic storage tank was performed. Stresses during normal operation were determined, as well as the transient temperature distribution. An elastic analysis was used to determine the thermal stresses in the inner wall based on the temperature data. The results of this elastic analysis indicate that the inner wall of the storage tank will experience thermal stresses of approximately 145,000 psi (1000 MPa). This stress level is well above the room-temperature yield strength of 304L stainless steel, which is about 25,000 psi (170 MPa). For this preliminary analysis, several important factors have not yet been considered. These factors include increased strength of 304L stainless steel at cryogenic temperatures, plastic material behavior, and increased strength due to strain hardening. In order to more accurately determine the thermal stresses and their affect on the tank material, further investigation is required, particularly in the area of material properties and their relationship to stress.

Baker, J. Mark

2003-01-01

368

Growth stresses and cracking in GaN films on (111) Si grown by metal-organic chemical-vapor deposition. I. AlN buffer layers  

NASA Astrophysics Data System (ADS)

Intrinsic stress evolution during the growth of GaN by metal-organic chemical-vapor deposition on (111) Si, using an AlN buffer layer, was monitored in situ with a multiple-beam optical stress sensor. Data show that stress evolution takes place in two stages: an initial compressive regime up to about 100 nm in thickness followed by a transition to a constant tensile stress, ~0.3 GPa, in films up to 1 ?m thick. Correlation of the stress evolution with surface morphological evolution by sequential atomic force microscopy images clearly shows that the incremental stress remains compressive in spite of grain coalescence, which is generally considered to be the dominant source of tensile stress in GaN films on sapphire. Rather, the most dominant feature accompanying the transition in stress from compressive to tensile, which takes place after grain coalescence, is an increase in the lateral size of individual islands. It is shown that this incremental tensile stress accompanied by an increase in lateral grain size can be accounted for by the annihilation of free volume associated with the grain boundaries. On samples cooled to room temperature, surface cracks mainly on the (1010) planes are observed to have channeled in films thicker than 250 nm. Analysis of cracking using the theory of brittle fracture, using the measured growth stress profile and value for the critical thickness, yields a thermal-expansion mismatch stress off 1.1 GPa for GaN films deposited at 1100 C and cooled to room temperature.

Raghavan, Srinivasan; Redwing, Joan M.

2005-07-01

369

Pt-Al2O3 nanocoatings for high temperature concentrated solar thermal power applications  

NASA Astrophysics Data System (ADS)

Nano-phased structures based on metal-dielectric composites, also called cermets (ceramic-metal), are considered among the most effective spectral selective solar absorbers. For high temperature applications (stable up to 650 C) noble metal nanoparticles and refractory oxide host matrices are ideal as per their high temperature chemical inertness and stability: Pt/Al2O3 cermet nano-composites are a representative family. This contribution reports on the optical properties of Pt/Al2O3 cermet nano-composites deposited in a multilayered tandem structure. The radio-frequency sputtering optimized Pt/Al2O3 solar absorbers consist of stainless steel substrate/ Mo coating layer/ Pt-Al2O3/ protective Al2O3 layer and stainless steel substrate/ Mo coating layer /Pt-Al2O3 for different composition and thickness of the Pt-Al2O3 cermet coatings. The microstructure, morphology, theoretical modeling and optical properties of the coatings were analyzed by the x-ray diffraction, atomic force, microscopy, effective medium approximation and UV-vis specular and diffuse reflectance.

Nuru, Zebib. Y.; Arendse, C. J.; Nemutudi, R.; Nemraoui, O.; Maaza, M.

2012-05-01

370

Thermal treatment and utilization of Al-rich waste in high calcium fly ash geopolymeric materials  

NASA Astrophysics Data System (ADS)

The Al-rich waste with aluminium and hydrocarbon as the major contaminant is generated at the wastewater treatment unit of a polymer processing plant. In this research, the heat treatment of this Al-rich waste and its use to adjust the silica/alumina ratio of the high calcium fly ash geopolymer were studied. To recycle the raw Al-rich waste, the waste was dried at 110C and calcined at 400 to 1000C. Mineralogical analyses were conducted using X-ray diffraction (XRD) to study the phase change. The increase in calcination temperature to 600, 800, and 1000C resulted in the phase transformation. The more active alumina phase of active ?-Al2O3 was obtained with the increase in calcination temperature. The calcined Al-rich waste was then used as an additive to the fly ash geopolymer by mixing with high calcium fly ash, water glass, 10 M sodium hydroxide (NaOH), and sand. Test results indicated that the calcined Al-rich waste could be used as an aluminium source to adjust the silica/alumina ratio and the strength of geopolymeric materials. The fly ash geopolymer mortar with 2.5wt% of the Al-rich waste calcined at 1000C possessed the 7-d compressive strength of 34.2 MPa.

Chindaprasirt, Prinya; Rattanasak, Ubolluk; Vongvoradit, Pimdao; Jenjirapanya, Supichart

2012-09-01

371

Stress corrosion cracking of Alloy 600 in high temperature water  

Microsoft Academic Search

Slow strain rate stress corrosion tests have been performed on specimens cut from four separate heats of alloy 600 steam generator tubing. Material was tested in the mill annealed and thermally stabilized conditions and after various low temperature aging treatments. Only limited cracking was observed, even for tests at 340°C, but the initiation of intergranular cracking was easier on the

J. Congleton; R. N. Parkins; B. Hemsworth

1987-01-01

372

Botulinum toxin detection using AlGaN/GaN high electron mobility transistors  

NASA Astrophysics Data System (ADS)

Antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1 to 10 ng/ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN/GaN HEMTs for botulinum toxin detection.

Wang, Yu-Lin; Chu, B. H.; Chen, K. H.; Chang, C. Y.; Lele, T. P.; Tseng, Y.; Pearton, S. J.; Ramage, J.; Hooten, D.; Dabiran, A.; Chow, P. P.; Ren, F.

2008-12-01

373

Strain state of InAlAs\\/InP layers subjected to high pressure treatment  

Microsoft Academic Search

Strain state of InAlAs\\/InP structures subjected to annealing under high hydrostatic pressure was investigated by X-ray diffraction methods. The InxAl1-xAs (0.50 x 0.53) layers were grown on the (001) oriented InP substrates by molecular beam epitaxy. Due to differences in lattice parameters of the compounds it was possible to induce tensile as well as compressive strains in InxAl1-xAs layers by

A. Shalimov; J. Bak-Misiuk; J. Kaniewski; A. Misiuk; J. Domagala; E. Kowalczyk

2005-01-01

374

High temperature deformation in 2036 Al and 0.2 wt % Zr-2036 A1  

SciTech Connect

The microstructure and high-temperature deformation of 2036 Al and a 0.2 wt % Zr modified 2036 Al were characterized. A particle-simulated- nucleation process was applied to refine grain structure in both alloys. Thermomechanically processed materials were tested from 450 to 500 C and strain rates from 2{times}10{sup {minus}1} to 2{times}10{sup {minus}4}s{sup {minus}1}. Strain rate sensitivity exponent, activation energy, and total elongation were measured, and the deformation mechanism was proposed. Effect of Zr on microstructure and deformation of 2036 Al at elevated temperatures was discussed.

Huang, J.S.; Schwartz, A.J.; Nieh, T.G.

1995-11-01

375

Pitting and Stress Corrosion Cracking Susceptibility of Nanostructured Al-Mg Alloys in Natural and Artificial Environments  

NASA Astrophysics Data System (ADS)

The stress corrosion cracking (SCC) behavior of two developmental nanocrystalline 5083 alloys with varied composition and processing conditions was studied. The results were compared to a commercial aluminum AA 5083 (H111) alloy. The pitting densities, size and depths, and residual tensile strengths were measured after alternate immersion in artificial seawater and atmospheric exposure under different loading conditions. Optical and scanning electron microscopy (SEM) with EDX was used to analyze the fracture surfaces of failed specimen after removal at selected intervals and tensile testing. One of the nanostructured Al-Mg alloys exhibited significantly superior pitting resistance when compared to conventional microstructured AA 5083. Under conditions where pitting corrosion showed up as local tunnels toward phase inclusions, transgranular cracking was observed, whereas under conditions when pitting corrosion evolved along grain boundaries, intergranular cracking inside the pit was observed. Pit initiation resistance of the nano alloys appears to be better than that of the conventional alloys. However, long-term pit propagation is a concern and warrants further study. The objective of this investigation was to obtain information regarding the role that ultra-fine microstructures play in their degradation in marine environments and to provide insight into the corrosion mechanisms and damage processes of these alloys.

Sharma, Mala M.; Ziemian, Constance W.

2008-12-01

376

Role of high zinc levels in the stress defense of common carp  

Microsoft Academic Search

We studied the link between high zinc levels and the extreme stress tolerance of common carp. Fish under stress showed much\\u000a higher plasma cortisol levels than controls. Stress or cortisol injection induced large changes in zinc levels in the common\\u000a carp but not in grass carp, silver carp or tilapia. The effect of 5days of anoxia and 4 subsequent days

Tzu-Yung Lin; Yen-Hua Chen; Chien-Lin Liu; Sen-Shyong Jeng

377

Nonlinear stress analysis of composite-jacketed steel cylinders subjected to high internal pressure  

NASA Astrophysics Data System (ADS)

A nonlinear stress analysis of a thick-walled compound tube subjected to high internal pressure is presented. The compound tube is constructed of a steel liner and a graphite-bismaleimide outer shell. Analytical expressions for the stresses, strains, and displacements are derived for all loading ranges up to failure. Numerical results for the stresses and the maximum value that the compound tube can contain without failure are presented.

Chen, Peter C.

1992-03-01

378

Traps and defects in pre- and post-proton irradiated AlGaN-GaN high electron mobility transistors and AlGaN Schottky diodes  

NASA Astrophysics Data System (ADS)

High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for both commercial and military applications that require high voltage, high power, and high efficiency operation. Study of reliability and radiation effects of AlGaN-GaN HEMTs is necessary before solid state power amplifiers based on GaN HEMT technology are successfully deployed in satellite communication systems. Several AlGaN HEMT manufacturers have recently reported encouraging reliability data, but long-term reliability of these devices in the space environment still remains a major concern because a large number of traps and defects are present both in the bulk as well as at the surface leading to undesirable characteristics. This study is to investigate the effects of the AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons.

Sin, Yongkun; Foran, Brendan; Presser, Nathan; LaLumondiere, Stephen; Lotshaw, William; Moss, Steven C.

2013-03-01

379

Assessment of microalloying effects on the high temperature fatigue behavior of NiAl  

NASA Technical Reports Server (NTRS)

Binary NiAl suffers from a lack of strength and poor creep properties at and above 1000 K. Poor creep resistance in turn affects low cycle fatigue (LCF) lives at low strain ranges due to the additional interactions of creep damage. One approach for improving these properties involved microalloying with either Zr or N. As an integral part of a much larger alloying program the low cycle fatigue behavior of Zr and N doped nickel aluminides produced by extrusion of prealloyed powders has been investigated. Strain controlled LCF tests were performed in air at 1000 K. The influence of these microalloying additions on the fatigue life and cyclic stress response of polycrystalline NiAl are discussed.

Noebe, R. D.; Lerch, B. A.; Rao, K. B. S.

1995-01-01

380

Rapidly solidified Al-Fe-Mo-Si alloys for high temperatures applications  

Microsoft Academic Search

Iron solubility in aluminium can be extended through the rapid solidification route, allowing the precipitation of a high volume fraction of intermetallic phases. In the ternary Al-Fe-Si system, the observed metastable hardening phase Al13Fe3Si is sensitive to ripening. Thermal stability can be improved by modification of the surface energy of the precipitate, i.e. by lattice parameter variation, through addition of

M. C. S. DE MACEDO; G. MICHOT

1993-01-01

381

Feasibility study of Nb3Al Rutherford cable for high field accelerator magnet application  

Microsoft Academic Search

Feasibility study of Cu stabilized NbAl strand and Rutherford cable for the application to high field accelerator magnets are being done at Fermilab in collaboration with NIMS. The NbAl strand, which was developed and manufactured at NIMS in Japan, has a non-copper Jc of about 844 A\\/mm at 15 Tesla at 4.2 K, a copper content of 50%, and filament

R. Yamada; A. Kikuchi; G. Ambrosio; N. Andreev; E. Barzi; C. Cooper; S. Feher; V. V. Kashikhin; M. Lamm; I. Novitski; T. Takeuchi; M. Tartaglia; D. Turrioni; A. P. Verweij; M. Wake; G Willering; A. V. Zlobin

2006-01-01

382

Feasibility Study of Nb3Al Rutherford Cable for High Field Accelerator Magnet Application  

Microsoft Academic Search

Feasibility study of Cu stabilized Nb3Al strand and Rutherford cable for the application to high field accelerator magnets are being done at Fermilab in collaboration with NIMS. The Nb3Al strand, which was developed and manufactured at NIMS in Japan, has a non-copper Jc of about 844 A\\/mm2 at 15 Tesla at 4.2 K, a copper content of 50%, and filament

R. Yamada; A. Kikuchi; G. Ambrosio; N. Andreev; E. Barzi; C. Cooper; S. Feher; V. V. Kashikin; M. Lamm; I. Novitski; T. Takeuchi; M. Tartaglia; D. Turrioni; A. P. Verweij; M. Wake; G. Willering; A. V. Zlobin

2007-01-01

383

Highly Uniform Bipolar Resistive Switching With Buffer Layer in Robust NbAlO-Based RRAM  

Microsoft Academic Search

The bipolar resistive switching characteristics of atomic-layer-deposited NbAlO-based devices have been investigated for nonvolatile memory applications. With the help of a thin Al2O3 buffer layer, highly uniform and reproducible bipolar resistance switching cycles could be observed. Four typical multilevel operations, with resistances being at 1000, 350, 145, and 75 ??, respectively, are also successfully demonstrated by varying the current compliance

Lin Chen; Yan Xu; Qing-Qing Sun; Han Liu; Jing-Jing Gu; Shi-Jin Ding; David Wei Zhang

2010-01-01

384

Formation of uniform passive oxide layers on high Si content Al alloy by plasma electrolytic oxidation  

Microsoft Academic Search

In this research, we demonstrated the successful fabrication of conformal passive oxide layers on a high Si content Al alloy by a plasma electrolytic oxidation (PEO) process. The PEO process of Al12wt%Si alloy was performed in an electrolyte containing sodium hexametaphosphate ((NaPO3)6) and ammonium metavanadate (NH4VO3), and the variation of the morphology of the electrochemically generated oxide layer was investigated.

In Jun Hwang; Duck Young Hwang; Yong Min Kim; Bongyoung Yoo; Dong Hyuk Shin

2010-01-01

385

Al NMR study of molten aluminum oxide compounds and mixtures, measured at ultra high temperatures  

Microsoft Academic Search

The technique of ultra high-temperature nuclear magnetic resonance (NMR) has provided insight into the chemical structure and properties of molten aluminum-bearing refractory ceramics, at temperatures in excess of 2000 ^oC. Through application of standard NMR measurements we have studied molten aluminum-bearing ceramics via ^27Al NMR. We have measured spin-lattice (T_1) and spin-spin (T_2) relaxation times, have studied Al-O-P chemical bonding

J. Piwowarczyk; R. F. Marzke; G. H. Wolf; W. T. Petuskey; B. Takulapalli

2002-01-01

386

Preparation of nanostructured Ni\\/Al 2O 3 composite coatings in high magnetic field  

Microsoft Academic Search

Ni\\/Al2O3 composite coatings were prepared by a novel method from a modified Watt's type electrolyte containing nano-Al2O3 particles, where a high magnetic field was imposed in the direction parallel to an electrolytic current instead of mechanical agitation. Effects of magnetic field on the content of particles, surface morphology, microhardness and wear resistance of plating layer were investigated. It was found

Qiuyuan Feng; Tingju Li; Zhongtao Zhang; Jian Zhang; Mei Liu; Junze Jin

2007-01-01

387

Effect of Nb on the high temperature oxidation of Ti(050 at.%)Al  

Microsoft Academic Search

The oxidation behavior of TiNb, Ti3AlNb and TiAlNb (Nb: 030 at.%) has been investigated at 1173 K in air. When Nb is in solid solution with TiO2, the addition of Nb can improve the oxidation resistance of the alloys by impeding mass transfer in TiO2. However, Nb decreases the oxidation resistance when the amount of Nb is too high and

Huiren Jiang; Mitsuji Hirohasi; Yun Lu; Hitoshi Imanari

2002-01-01

388

High pressure rhombohedral perovskite phase Ca 2 AlSiO 5.5  

Microsoft Academic Search

A new high pressure phase with the composition Ca2AlSiO5.5 has been synthesized using an MA-8 apparatus operating at 1700 C and 16 GPa. The phase possesses a structure analagous to CaSiO3 perovskite but with half the Si atoms replaced by Al, and charge balance provided by vacancies in the oxygen sub-lattice. The unit cell possesses a lattice parameter, ao =

J. D. Fitz Gerald; A. E. Ringwood

1991-01-01

389

Some observations on the high temperature oxidation behaviour of plasma sprayed Ni 3Al coatings  

Microsoft Academic Search

High temperature oxidation resistance of the superalloys can be greatly enhanced by plasma sprayed coatings and this is a growing industry of considerable economic importance. The purpose of these coatings is to form long-lasting oxidation protective scales. In the current investigation, Ni3Al powder was prepared by mechanical mixing of pure nickel and aluminium powders in a ball mill. Subsequently Ni3Al

H. Singh; S. Prakash; D. Puri

2007-01-01

390

Derivation of Apollo 14 High-Al Basalts at Discrete Times: Rb-Sr Isotopic Constraints  

NASA Technical Reports Server (NTRS)

Pristine Apollo 14 (A-14) high-Al basalts represent the oldest volcanic deposits returned from the Moon [1,2] and are relatively enriched in Al2O3 (>11 wt%) compared to other mare basalts (7-11 wt%). Literature Rb-Sr isotopic data suggest there are at least three different eruption episodes for the A-14 high-Al basalts spanning the age range approx.4.3 Ga to approx.3.95 Ga [1,3]. Therefore, the high-Al basalts may record lunar mantle evolution between the formation of lunar crust (approx.4.4 Ga) and the main basin-filling mare volcanism (<3.85 Ga) [4]. The high-Al basalts were originally classified into five compositional groups [5,6], and then regrouped into three with a possible fourth comprising 14072 based on the whole-rock incompatible trace element (ITE) ratios and Rb-Sr radiometric ages [7]. However, Rb-Sr ages of these basalts from different laboratories may not be consistent with each other because of the use of different 87Rb decay constants [8] and different isochron derivation methods over the last four decades. This study involved a literature search for Rb-Sr isotopic data previously reported for the high-Al basalts. With the re-calculated Rb-Sr radiometric ages, eruption episodes of A-14 high-Al basalts were determined, and their petrogenesis was investigated in light of the "new" Rb-Sr isotopic data and published trace element abundances of these basalts.

Hui. Hejiu; Neal, Clive, R.; Shih, Chi-Yu; Nyquist, Laurence E.

2012-01-01

391

High-power high-brightness tapered lasers with an Al-free active region at 915 nm  

Microsoft Academic Search

We have developed high power and high brightness tapered lasers based on an Al-free active region at 915 nm. The material structure, which was grown by MOCVD (Metallorganic Chemical Vapor Deposition), has very low internal losses of 0.5 cm-1, a very low transparency current density of 86 A\\/cm2, a high internal quantum efficiency of 86%, and a high characteristic temperature

N. Michel; I. Hassiaoui; M. Lecomte; O. Parillaud; M. Calligaro; M. Krakowski

2006-01-01

392

Stress-Strain Behaviors Simulation of High Chromium Steel at Elevated Temperatures  

NASA Astrophysics Data System (ADS)

It is important but difficult to study the constitutive equations describing the mechanical properties of steels. In this work, a thermal/mechanical simulator was used in conjunction with the Anand model to obtain the stress-strain curves for a high chromium steel associated with different temperature/strain rate pairs. The finite element software, ANSYS, was used to simulate the stress-strain behavior of a high chromium steel during casting at the strain rates of 1, 0.1, and 0.01 1/s, and to validate the Anand model. The results show that the high-temperature deformation is mainly plastic and the von Mises stresses are small, the stresses introduced at elevated temperatures have little effect on the residual stress fields, and the simulation of the mechanical behavior of steels using an elastic-plastic model at low temperatures during cooling is acceptable.

Liu, Ligang; Li, Qiang; Liao, Bo; Gao, Yukui; Wang, Yuhui; Ren, Xuejun; Yang, Qingxiang

2010-10-01

393

A New High-Pressure Phase With Mg2Al2O5 Composition  

NASA Astrophysics Data System (ADS)

As MgAl2O4 is one of major components of both spinel phase and calcium ferrite phase in the Earth"fs mantle, many researchers have studied its high-pressure phase relations. However, the high- pressure high-temperature phase relation has been unclear at temperature higher than 2000 C. By performing high-pressure experiments, we found that MgAl2O4 spinel decomposes to Mg2Al2O5 and Al2O3 at 20-25 GPa and temperature higher than 2000 C. This is the first report of a compound with the composition of Mg2Al2O5. The powder X-ray diffraction pattern can not be explained by crystal structures of already known high-pressure phases like brownmillerite-type Ca2Al2O5. In this study, we tried to construct a crystal structure model of the new Mg2Al2O5 phase. The obtained structure model was used for structure refinement by the Rietveld analysis. The Mg2Al2O5 phase for a powder XRD measurement was synthesized using a Kawai-type multi-anvil high-pressure apparatus at Gakushuin University. The starting material of a mixture of MgO:Al2O3 = 2:1 (in molar ratio) was hold at 23 GPa and 2500 C for 10 min. After quenching, the sample was recoverd to ambient pressure. The sintered polycrystalline sample was crushed by a die and ground into powder. The powder X-ray diffraction profile was measured using Rigaku RINT2500V (Cr K?, 45 kV, 250 mA) with the step size of 0.02. The Rietveld analysis was made using RIETAN-2000 program. The XRD profile showed that symmetry of the Mg2Al2O5 phase is orthorhombic. The extinction rules of the powder XRD peaks suggested the space group of Pba2 or Pbam. Since it was found that the XRD pattern is close to that of (Mg, Fe2+)2(Fe3+, Al)(BO3)O2 ludwigite, a structure model of the Mg2Al2O5 phase was constructed based on it. It should be noted that planner three coordinated sites in ludwigite in which boron places were moved by 0.5 in atomic coordinate along c-axis, because interatomic distances between cation and oxygen in the planner three coordinated sites are too short for Mg2+ and Al3+. In this structure model, (Mg, Al)O6 octahedra connected by edge-sharing and corner-sharing form triangular tunnels along c-axis. Cation sites occupied by only Mg are placed in the tunnels. The sites have a coordination environment of trigonal prism- type MgO6. By the Rietveld analysis, lattice parameters were determined as a = 9.3710(6) , b = 12.1952(6) , c = 2.7916(2) . The calculated density of 3.801(1) g/cm3 is consistent with the high-pressure phase relations. R factor by adopting the space group of Pbam is smaller than that by Pba2. Therefore, it is suggested that the space group of the Mg2Al2O5 phase is Pbam

Kojitani, H.; Enomoto, A.; Tsukamoto, S.; Akaogi, M.; Miura, H.

2008-12-01

394

Electrical characteristics of AlGaN-GaN high electron mobility transistors and AlGaN Schottky diodes irradiated with protons  

NASA Astrophysics Data System (ADS)

AlGaN-GaN high electron mobility transistors (HEMTs) are most suitable for commercial and military applications requiring high voltage, high power, and high efficiency operation. In recent years, leading AlGaN HEMT manufacturers have reported encouraging reliability of these devices, but their long-term reliability especially in the space environment still remains a major concern. In addition, degradation mechanisms in AlGaN HEMT devices are still not well understood, and a large number of traps and defects present both in the bulk and at the surface lead to undesirable characteristics. Study of reliability and radiation effects of AlGaN-GaN HEMTs is therefore necessary before GaN HEMT technology is successfully employed in satellite communication systems. For the present study, we investigated electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons. We studied two types of MOCVD-grown AlGaN HEMTs on semi-insulating SiC substrates (HEMT-1 and HEMT-2) as well as MOCVD-grown Al0.27Ga 0.73N Schottky diodes on conducting SiC substrates. Our HEMT-1 structure consisted of a GaN cap, AlGaN/AlN barrier, and 2 ?m GaN buffer layers. Our HEMT-2 structure consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer showed a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500 cm2 /V.sec. Our HEMT-1 devices had a Pt-Au Schottky gate length of 0.2 ?m, a total gate width of 200-400 ?m periphery, and SiNx passivation. Electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes were compared before and after they were proton irradiated with different energies and fluences. Current-mode deep level transient spectroscopy (DLTS) and capacitance-mode DLTS were employed to study pre-proton irradiation trap characteristics in the AlGaN-GaN HEMTs and AlGaN Schottky diodes, respectively. Focused ion beam (FIB) was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM. In addition, electrical characteristics of GaAs MESFETs used as reference devices were compared before and after they were proton irradiated.

Sin, Yongkun; Presser, Nathan; Foran, Brendan; LaLumondiere, Stephen; Lotshaw, William; Moss, Steven C.

2014-03-01

395

High-Temperature Deformation and Ductility of a Modified 5083 Al Alloy  

NASA Astrophysics Data System (ADS)

The high-temperature deformation of a 5.5% Mg and 0.6% Ca modified 5083 aluminum alloy was investigated in the temperature range from 573 to 723 K at strain rates in the range of 10-5-10-1 s-1. Ca was added to form an insoluble second phase in the range of temperatures tested to improve the high-temperature characteristics of this alloy. It was shown that the deformation behavior of the alloy could be divided into two regions with stress exponent, n of 3.5 and 13 at low and high strain rates, respectively. The apparent activation energy determined in both regions suggested that the deformation process is diffusion controlled in both regions. The slightly high value of n at the low-strain rate region (viscous glide) was attributed to the presence of threshold stress. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 16.5 kJ mol-1. Analysis of creep data in terms of threshold stress and using diffusivity of Mg in normalizing the strain rates, revealed two types of deformation behavior. At high values of normalized strain rate (kT} {DGb}}} > 10^{{ - 9}} ), a high value of stress exponent of n = 10 is observed, and the exponential law creep takes place. At low normalized strain rates ?10-9, the n value is 3 and the true activation energy, Q, is equal to 123 kJ mol-1 suggesting viscous glide of dislocations as rate-controlling mechanism. Enhanced ductility has been observed in the region of viscous-glide controlled deformation as a result of high strain-rate sensitivity.

El-Danaf, Ehab A.; Almajid, Abdulhakim A.; Soliman, Mahmoud S.

2008-08-01

396

Cyclically-Induced Pore Pressure at High Confining Stress  

Microsoft Academic Search

Experiments were conducted by the ERDC Centrifuge Research Team to investigate effective confining stress effects on liquefaction potential of fine, clean, Nevada sand, under the boundary and loading conditions of a centrifuge model. For this test series, twenty-six level ground models with either a dense layer over a loose layer or homogeneous profile were tested in an equivalent-shear-beam box. Some

Michael K. Sharp; R. Scott Steedman

397

High-intensity acoustic tests of a thermally stressed plate  

NASA Technical Reports Server (NTRS)

An investigation was conducted in the Thermal Acoustic Fatigue Apparatus at the Langley Research Center to study the acoustically excited random motion of an aluminum plate which is buckled due to thermal stresses. The thermal buckling displacements were measured and compared with theory. The general trends of the changes in resonances frequencies and random responses of the plate agree with previous theoretical prediction and experimental results for a mechanically buckled plate.

Ng, Chung Fai; Clevenson, Sherman A.

1991-01-01

398

Stress Corrosion Resistance of a New High Strength Aluminum Alloy Developed for Use in Thick Sections.  

National Technical Information Service (NTIS)

The objective of the investigation was to evaluate the stress corrosion behavior of a new high strength aluminum alloy plate in the T6+35 hours at 325F temper. Using smooth specimens, stress corrosion tests were conducted in two different laboratory envir...

I. S. Shaffer

1971-01-01

399

Influence of high temperature and terminal moisture stress on dormancy in wheat ( Triticum aestivum L.)  

Microsoft Academic Search

Preharvest sprouting is common in cereals that lack grain dormancy if maturing grain is exposed to rain. Over three successive seasons wheat genotypes with a range of dormancy levels, were exposed to moisture stress and periods of high temperature stress (>30C) in controlled field trials. Dormancy assessments were based on a germination index of hand threshed grain throughout grain filling.

Thomas B. Biddulph; Julie A. Plummer; Tim L. Setter; Daryl J. Mares

2007-01-01

400

STRESS-CORROSION CRACKING OF HIGH-STRENGTH STAINLESS STEELS IN ATMOSPHERIC ENVIRONMENTS  

Microsoft Academic Search

Available information on the stress-corrosion cracking of the high-; strength stainless steels is tabulated and discussed. Data are included for ; austenitic, martensitic, martensitic precipitation-hardenable, and semiaustenitic ; precipitation-hardenable grades. Some tentative guidelines are indicated. ; Stress-corrosion cracking appears to be strongly influenced by prior thermal ; history. (auth);

Slunder

1961-01-01

401

Stress Factors and Their Alleviation in Parents of High Risk Preterm Infants.  

ERIC Educational Resources Information Center

The word "stress" all too often describes the emotional state of a parent after the premature birth of a high risk infant. Research studies indicate specific reasons for parental stress in the context and settings associated with the premature birth of a child. Such reasons include lack of preparation for the early birth, the immature appearance

Lowenthal, Barbara

402

Bone cell responses to high-frequency vibration stress: does the nucleus oscillate within the cytoplasm?  

Microsoft Academic Search

Mechanosensing by cells directs changes in bone mass and structure in response to the chal- lenges of mechanical loading. Low-amplitude, high- frequency loading stimulates bone growth by enhancing bone formation and inhibiting disuse osteoporosis. However, how bone cells sense vibration stress is unknown. Hence, we investigated bone cell responses to vibration stress at a wide frequency range (5-100 Hz). We

Rommel G. Bacabac; Theo H. Smit; Jack J. W. A. Van Loon; Behrouz Zandieh Doulabi; Marco Helder; Jenneke Klein-Nulend

2006-01-01

403

Investigation of the Interface Phenomena Due to Interaction of High Intensity Stress Waves with Material Boundaries.  

National Technical Information Service (NTIS)

A theoretical model for the interface between two geologically different rock layers, subjected to high intensity stress waves is developed in this research study The purpose for this model is to study the stress waves that are generated at the interface ...

A. Amini I. S. Majtenyi

1993-01-01

404

Residual Stress Distribution of 600MPa Grade High Tensile Strength Steel Pipe Using Welding fe Simulation  

NASA Astrophysics Data System (ADS)

This paper aims to determine the residual stress distribution of 600MPa grade high tensile strength steel pipe (STKT590) by girth welding. Welding FE simulation is achieved considering temperature dependent physical constants and mechanical properties, obtained by the temperature elevated tensile tests. Comparative analyses clarify the characteristics of residual stress profile near weld joint of STKT590 pipe.

Chang, Kyong-Ho; Jang, Gab-Chul

2011-06-01

405

Bimetallic Pd/Al particles for highly efficient hydrodechlorination of 2-chlorobiphenyl in acidic aqueous solution.  

PubMed

Pd-based bimetallic materials have been widely studied for the effective hydrodechlorination (HDC) of aqueous chloroorganic compounds. However, the reaction functions of metal substrates and mechanism responsible for changes in reactivity have not been fully elucidated. Here, we synthesized Pd-based bimetals with Mg, Al, Mn, Zn, Fe, Sn, and Cu to explore the influence of metal substrates on HDC reactivity of 2-chlorobiphenyl (2-PCB). Bimetals exhibited disparate reactivity toward 2-PCB in acidic solution. Among these bimetals, Pd/Al particles presented the highest stability and relatively high reactivity to remove 2-PCB. The maintenance and regeneration of Al substrate are attributed to its particular corrosion properties which provide an efficient recycling between Al element and its oxide layer. The fresh and reacted Pd/Al samples were characterized by ICP-OES, SEM, XRD, and BET. The investigation of the pH effect on 2-PCB HDC further revealed the particular behaviour of Al surface. The effect of Pd loading amount on the HDC indicated that the optimal Pd content in terms of catalytic activity was related to the Pd dispersion degree. Finally, a mechanism for 2-PCB HDC on the Pd/Al surface was proposed. PMID:21377789

Yang, Bo; Deng, Shubo; Yu, Gang; Zhang, Hong; Wu, Jinhua; Zhuo, Qiongfang

2011-05-15

406

Penta-coordinated Al 3+ ions as preferential nucleation sites for BaO on ?-Al 2O 3: An ultra-high-magnetic field 27Al MAS NMR study  

Microsoft Academic Search

We report the first observation of preferential anchoring of an impregnated catalytic phase onto penta-coordinated Al3+ sites on the surface of ?-Al2O3. The interaction of barium oxide with a ?-alumina support was investigated by high resolution solid state 27Al magic angle spinning NMR at an ultra-high magnetic field of 21.1 T and at sample spinning rates of up to 23

Ja Hun Kwak; Jian Zhi Hu; Do Heui Kim; Janos Szanyi; Charles H. F. Peden

2007-01-01

407

Highly textured growth of AlN films on sapphire by magnetron sputtering for high temperature surface acoustic wave applications  

SciTech Connect

Piezoelectric aluminum nitride films were deposited onto 3 in. [0001] sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly (002)-textured AlN films to be used in surface acoustic wave (SAW) devices for high temperature applications. The synthesized films, typically 1 {mu}m thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly (002)-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3 deg. can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250 deg. C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800 deg. C for 90 min. The frequency response, recorded after the annealing process, confirmed that the thin films were still strong in a high temperature environment and that they had retained their piezoelectric properties.

Aubert, T.; Assouar, M. B.; Legrani, O.; Elmazria, O.; Tiusan, C.; Robert, S. [Institut Jean Lamour, UMR 7198, Nancy University-CNRS, BP 70239, Bd des Aiguillettes, 54506 Vandoeuvre Cedex (France)

2011-03-15

408

Synchronized Stress-strain Measurements in Dynamic Loading at High Pressure using D-DIA  

SciTech Connect

A new data collection protocol for forced oscillation experiments using a multianvil high pressure device is reported. We derive the stress of the sample at high pressure and temperature from synchrotron x-ray diffraction that is synchronized with sample strain measurements from x-ray radiographs. This method yields stress directly from the sample rather than a stress proxy. Furthermore, the diffraction pattern yields useful information concerning time evolution of structurally related phenomena. Here we illustrate some of these possibilities with high pressure experimental data.

L Li; D Weidner

2011-12-31

409

High-Performance Laser Peening for Effective Mitigation of Stress Corrosion Cracking  

SciTech Connect

Stress corrosion cracking (SCC) in the Yucca Mountain waste package closure welds is believed to be the greatest threat to long-term containment. Use of stress mitigation to eliminate tensile stresses resulting from welding can prevent SCC. A laser technology with sufficient average power to achieve high throughput has been developed and commercially deployed with high peak power and sufficiently high average power to be an effective laser peening system. An appropriately applied version of this process could be applied to eliminate SCC in the waste package closure welds.

Hackel, L; Hao-Lin, C; Wong, F; Hill, M

2002-10-02

410

Neutron diffraction analysis of residual strain/stress distribution in the vicinity of high strength welds  

NASA Astrophysics Data System (ADS)

Residual stresses resulting from non homogeneous heat distribution during welding process belong to most significant factor influencing behavior of welded structures. These stresses are responsible for defect occurrence during welding and they are also responsible for crack initiation and propagation at the either static or dynamic load. The significant effect of weld metal chemical composition as well as the effect of fatigue load and local plastic deformation on residual stress distribution and fatigue life have been recognized for high strength steels welds. The changes in residual stress distribution have then positive effect on cold cracking behavior and also on fatigue properties of the welds [1-3]. Several experimental methods, both destructive and non-destructive, such as hole drilling method, X-ray diffraction, neutron diffraction and others, have been used to examine residual stress distribution in all three significant orientations in the vicinity of the welds. The present contribution summarizes the results of neutron diffraction measurements of residual stress distribution in the vicinity of single-pass high-strength-steel welds having different chemical composition as well as the influence of fatigue load and local plastic deformation. It has been observed that the chemical composition of the weld metal has a significant influence on the stress distribution around the weld. Similarly, by aplying both cyclic load or pre-stress load on the specimens, stress relaxation was observed even in the region of approximately 40 mm far from the weld toe.

Mrz, L.; Karlsson, L.; Hamk, I.; Vrna, M.; Mikula, P.

2010-06-01

411

Mechanical alloying and high pressure processing of a TiAl-V intermetallic alloy.  

PubMed

An alloy with a chemical composition of Ti-45Al-5V (at.%) was synthesized by mechanical alloying in a Szegvari-type attritor from elemental powders of high purity. Before compaction, the powders were characterized by X-ray diffraction and scanning as well as transmission electron microscopy. The compaction of powders was carried out by hot isostatic pressing and hot isostatic extrusion. The resulting material was subjected to microstructural and mechanical characterization. The microstructure investigated by transmission and scanning electron microscopy supplemented by X-ray diffraction revealed that the bulk material was composed of a mixture of TiAl- and Ti(3)Al-based phases, however, the typical lamellar microstructure for such alloys was not observed. The materials exhibited exceptionally high yield strength together with satisfactory ductility and fracture toughness. The high strength was unequivocally due to grain refinement and the presence of oxide dispersoid. PMID:20500422

Dymek, S; Wrbel, M; Witczak, Z; Blicharski, M

2010-03-01

412

Effect of High-Pressure Inert Gas Annealing on AlON/Ge Gate Stacks  

NASA Astrophysics Data System (ADS)

We investigated germanium (Ge) metal--insulator--semiconductor (MIS) gate stacks with aluminum oxynitride (AlON) thin dielectric film. We found that high-pressure inert gas post deposition annealing (PDA) using N2 or Ar gas dramatically improved the electrical properties of AlON/Ge MIS gate stacks. The advantage of this process over high-pressure O2 oxidation or annealing, which produce excellent Ge gate stacks, is that no further interface layer growth in the N2 or Ar PDA is expected. We expect that thin AlON films combined with high-pressure inert gas PDA will provide a new way to achieve excellent Ge-MIS gate stacks with scalable equivalent oxide thickness.

Tabata, Toshiyuki; Nagashio, Kosuke; Toriumi, Akira

2012-09-01

413

High Temperature Hall sensors using AlGaN/GaN HEMT Structures  

NASA Astrophysics Data System (ADS)

Hall effect sensors are the most widely used magnetic sensors, and are commonly fabricated using narrow band-gap semiconductors such as InSb and GaAs. However, the operation of InSb and GaAs-based Hall sensors is unstable at elevated temperatures. Here, we exploited the fact that gallium nitride- (GaN) based wide band-gap (3.4eV) semiconductors have high breakdown electric field strength and are robust at elevated temperatures and tested the high temperature electrical characteristics of AlGaN/GaN heterostructure micro-Hall effect sensors with a two-dimensional electron gas (2DEG). The AlGaN/GaN heterostructures had an electron mobility of 1360 cm2/Vs and a 2DEG density of 1.0 1013 cm-2. The supply-current-related sensitivity (SCRS) was 77VA-1T-1 at room temperature. Notably, the temperature coefficient of the Hall voltage was +0.05 %/C near room temperature, and SCRS was 67 VA-1T-1 at 400C. The results for AlGaN/GaN were compared with those for AlGaAs/GaAs and AlInSb/InAsSb/AlInSb QW Hall effect sensors.

Koide, S.; Takahashi, H.; Abderrahmane, A.; Shibasaki, I.; Sandhu, A.

2012-03-01

414

High elemental selectivity to Sn submonolayers embedded in Al using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

In the present work, we demonstrate that metal layers in the submonolayer range embedded in a matrix are revealed with unprecedented sensitivity by coincident Doppler-broadening spectroscopy of the positron annihilation using a monoenergetic positron beam. The measured electron momentum distribution specific for Sn is clearly observable in Al/Sn/Al -layered samples even at a Sn area density of as low as 7.310-2?g/cm2 below 200nm Al. An explanation for the high elemental selectivity for the thin Sn layers is set forward in terms of efficient positron trapping due to the changing positron affinity at the Al/Sn -interface and quantum-dot-like positron states in Sn nanoparticles.

Hugenschmidt, C.; Pikart, P.; Stadlbauer, M.; Schreckenbach, K.

2008-03-01

415

Preliminary study of the characteristics of a high Mg containing Al-Mg-Si alloy  

NASA Astrophysics Data System (ADS)

An Al-20Mg-4Si high Mg containing alloy has been produced and its characteristics investigated. The as-cast alloy revealed primary Mg2Si particles evenly distributed throughout an ?-Al matrix with a ?-Al3Mg2 fully divorced eutectic phase observed in interdendritic regions. The Mg2Si particles displayed octahedral, truncated octahedral, and hopper morphologies. Additions of Sb, Ti and Zr had a refining influence reducing the size of the Mg2Si from 52 4 ?m to 25 0.1 ?m, 35 1 ?m and 34 1 ?m respectively. HPDC tensile test samples could be produced with a 0.6 wt.% Mn addition which prevented die soldering. Solution heating for 1 hr was found to dissolve the majority of the Al3Mg2 eutectic phase with no evidence of any effect on the primary Mg2Si. Preliminary results indicate that the heat treatment has a beneficial effect on the elongation and the UTS.

Yan, F.; McKay, B. J.; Fan, Z.; Chen, M. F.

2012-01-01

416

High-temperature powder synchrotron diffraction studies of synthetic cryolite Na 3AlF 6  

NASA Astrophysics Data System (ADS)

A high-resolution synchrotron diffraction study of the structures of a synthetic sample of cryolite Na 3AlF 6 from room temperature to 800C is reported. At room temperature Na 3AlF 6 is monoclinic and the structure is described in space group P2 1/n. Heating the sample to 560C results in only minor changes to the structure. A first-order transition from this monoclinic structure to a high-temperature cubic structure is observed near 567C. The cubic Fm 3m structure is characterized by disorder of the fluoride atoms.

Zhou, Qingdi; Kennedy, Brendan J.

2004-03-01

417

A new high-pressure form of MgAl2O4  

Microsoft Academic Search

The transformation of MgAl2O4 spinel to a new high-pressure form at pressures above 25 GPa in a multianvil high-pressure apparatus is reported. The new MgAl2O4 phase has a structure similar to that of CaFe2O4 (calcium ferrite) and its zero-pressure density is 3.937(3) g\\/cu cm, which is about 2 percent denser than the lower-pressure assemblage of periclase + corundum. It is

T. Irifune; K. Fujino; E. Ohtani

1991-01-01

418

Control of stresses in highly doped multilayer polysilicon structures used in MEMS applications  

NASA Astrophysics Data System (ADS)

Influence of LPCVD deposition condition, substrate, film thickness, crystallized degree and pre-annealing on residual stress in LPCVD polysilicon films was studied. The polysilicon deposited on PSG substrate shows the lowest residual stress. The relationship between crystallized degree of polysilicon films and the film thickness was investigated with the aid of Raman Scattering Spectrum. The residual stress shows a significant dependence on the film thickness because crystallized degree raises with the film thickness increase. The test results show that (1) for a thinner film (0.20 micrometer), even if to use a higher deposition temperature (630 degrees Celsius), its crystallized degree is still quite low and a quite higher residual tensile stress is resulted in the film. (2) for a thick film (4 micrometer), even if to use a lower deposition temperature (580 degrees Celsius), a significant crystallization still will occur in as-deposited films and a residual tensile stress is resulted in the films. A pre-annealing step before polysilicon boron doping is brought into the fabrication process of multi-layer diaphragm structure. It can be used as a method to control stress in highly doped polysilicon films. The stress control test of highly boron doped polysilicon/oxide diaphragm structure was carried out. The result shows that the property and magnitude of the stresses in highly boron doped polysilicon-oxide diaphragm can be arbitrarily changed in certain range by varying the holding time of final annealing.

Chen, Longqing; Guo, Li Hui; Miao, Jianmin; Lin, Rongming

2001-04-01

419

Laminated metal composite formed from low flow stress layers and high flow stress layers using flow constraining elements and making same  

DOEpatents

A laminated metal composite of low flow stress layers and high flow stress layers is described which is formed using flow constraining elements, preferably in the shape of rings, individually placed around each of the low flow stress layers while pressure is applied to the stack to bond the layers of the composite together, to thereby restrain the flow of the low flow stress layers from the stack during the bonding. The laminated metal composite of the invention is made by the steps of forming a stack of alternate layers of low flow stress layers and high flow stress layers with each layer of low flow stress material surrounded by an individual flow constraining element, such as a ring, and then applying pressure to the top and bottom surfaces of the resulting stack to bond the dissimilar layers together, for example, by compression rolling the stack. In a preferred embodiment, the individual flow constraining elements surrounding the layers of low flow stress material are formed of a material which may either be the same material as the material comprising the high flow stress layers, or have similar flow stress characteristics to the material comprising the high flow stress layers. Additional sacrificial layers may be added to the top and bottom of the stack to avoid damage to the stack during the bonding step; and these additional layers may then be removed after the bonding step. 5 figs.

Syn, C.K.; Lesuer, D.R.

1995-07-04

420

Laminated metal composite formed from low flow stress layers and high flow stress layers using flow constraining elements and making same  

DOEpatents

A laminated metal composite of low flow stress layers and high flow stress layers is described which is formed using flow constraining elements, preferably in the shape of rings, individually placed around each of the low flow stress layers while pressure is applied to the stack to bond the layers of the composite together, to thereby restrain the flow of the low flow stress layers from the stack during the bonding. The laminated metal composite of the invention is made by the steps of forming a stack of alternate layers of low flow stress layers and high flow stress layers with each layer of low flow stress material surrounded by an individual flow constraining element, such as a ring, and then applying pressure to the top and bottom surfaces of the resulting stack to bond the dissimilar layers together, for example, by compression rolling the stack. In a preferred embodiment, the individual flow constraining elements surrounding the layers of low flow stress material are formed of a material which may either be the same material as the material comprising the high flow stress layers, or have similar flow stress characteristics to the material comprising the high flow stress layers. Additional sacrificial layers may be added to the top and bottom of the stack to avoid damage to the stack during the bonding step; and these additional layers may then be removed after the bonding step.

Syn, Chol K. (Moraga, CA) [Moraga, CA; Lesuer, Donald R. (Livermore, CA) [Livermore, CA

1995-01-01

421

Perturbed angular correlation studies of uniaxial compressive stressed zinc, titanium, rutile, Ti2AlN, and Nb2AlC.  

PubMed

We use the perturbed angular correlation method with (111)In-(111)Cd probe atoms to in situ study the changes in the electric field gradient at room temperature of polycrystalline Ti2AlN and Nb2AlC, titanium and zinc, and rutile samples, as a function of cyclic uniaxial compressive loads. The load dependence of the quadrupole coupling constant ?Q was found to be large in titanium and zinc but small in Ti2AlN, Nb2AlC and rutile. Reversible and irreversible increases in the electric field gradient distribution widths were found under load and after releasing the load, respectively. Annihilation of dislocations, as well as elastic deformation, are considered to contribute to the reversible behavior. The irreversible response must be caused by a permanent increase in dislocation and point defect densities. The deformation induced broadening of the electric field gradient can be recovered by post-annealing of the deformed sample. PMID:24957211

Brsewitz, C; Vetter, U; Hofsss, H; Barsoum, M W

2014-07-23

422

Interfacial shear stresses in SiC and Al/sub 2/O/sub 3/ fiber-reinforced glasses  

SciTech Connect

Single fiber pullout tests were performed on silicon carbide and sapphire filaments embedded in glass matrices to determine the interfacial shear strengths both as a function of residual stress and as a function of fiber coating. The effect of residual stress on the interfacial shear stress was studied by altering the glass matrix composition, and hence, the thermal mismatch between fiber and matrix. The presence of a carbon coating on sapphire is sufficient to reduce the interfacial shear stress to measurable levels, while excess carbon at the SiC/glass interface increases the interfacial shear stress. 15 references.

Goettler, R.W.; Faber, K.T.

1988-08-01

423

High temperature deformation of ZrB 2SiCAlN ceramic composite  

Microsoft Academic Search

ZrB2 ceramic composites are powerful candidates for use in ultra-high temperature structural materials given their strength under high temperatures, superb ablation resistance, and good oxidation resistance. Unfortunately, ZrB2 ceramic composites are difficult to deform due to inherent brittle property, which impedes large-scale application. High temperature deformation of ZrB2SiCAlN ceramic composite at 1900C with inert gas protection is investigated using deep

Wenbo Han; Xinghong Zhang; Weibin Tai; Jiecai Han

2009-01-01

424

Constitutive equation for Ti6Al4V at high temperatures measured using the SHPB technique  

Microsoft Academic Search

A high temperature split Hopkinson pressure bar (SHPB) test system is used to investigate the effects of temperature as well as those of strain and strain-rate. Effects of temperature for the titanium alloy (Ti6Al4V) are investigated by developing a high temperature SHPB test system. In this work, high temperatures greater than 1000C are attained in the SHPB test specimens by

Songwon Seo; Oakkey Min; Hyunmo Yang

2005-01-01

425

Ultrasonic elastic wave velocity measurements of polycrystalline MgAl2O4 spinel at high pressure and high temperature  

NASA Astrophysics Data System (ADS)

Magnesium aluminate (MgAl2O4) spinel is considered an important functional/structural material widely used at extreme conditions, due to its good mechanical strength, high resistance to chemical attack, good stability, and excellent optical and dielectric properties. Moreover, MgAl2O4 spinel also plays a significant role in geophysics as an important rock-forming mineral, which constitutes peridotites from the uppermost of the Earth's mantle. Therefore, understanding the elasticity and sound velocities of MgAl2O4 spinel are of great interest in the fields ranging from materials physics to geophysics by various experimental techniques. Previous experimental studies on MgAl2O4 spinel were carried out either at high temperature or high temperature. To date, there are no direct measurements of the elastic wave velocities of MgAl2O4 spinel at simultaneous high-pressure and high-temperature conditions. Recently, elasticity and sound velocities of polycrystalline MgAl2O4 spinel have been firstly measured up to 14 GPa and 900 K using ultrasonic interferometry in conjunction with energy-dispersive synchrotron X-ray diffraction. It is found that compressional wave velocity (VP) increases with pressure and decreases with temperature, whereas the shear wave velocity (VS) decreases with both pressure and temperature. Two-dimensional linear fittings of the present data give: KS0 =195 (1) GPa, dKs/dP = 4.4(1), dKs/dT = -0.023(1) GPa/K, G0 = 108(1) GPa, dG/dP = 0.38(1), dG/dT = -0.014(1) GPa/K. Moreover, the bulk modulus (KS) exhibits obvious increase with pressure, while the shear modulus (G) shows a weak variation with pressure, which may be due to the shear deformation relaxation by the coupling between atomic displacements and shear strains.

Irifune, T.; Zou, Y.; Greaux, S.; Zhou, C.; Whitaker, M. L.; Higo, Y.; Li, B.

2012-12-01

426

Formation of High Quality AlN Tunnel Barriers via an Inductively Couple Plasma  

NASA Astrophysics Data System (ADS)

Increasing operating frequencies of SIS receivers requires junctions that can operate at higher current densities. A major limiting factor of higher current density junctions is the increase in subgap leakage that occurs in AlOX barriers as current densities approach and exceed 10kA/cm2. AlN insulators are a promising alternative due to their lower leakage current at these high current densities. In this paper we present a more detailed analysis of the formation of AlN barriers using our previously reported inductively coupled plasma (ICP) source growth technique. The ICP allows for independent control of ion energy and current density in the plasma. Additionally, plasmas with very low ion energy (~20eV) and a high degree of dissociation (~80%) can be achieved. This improved control allows for the repeatable formation of high quality barriers. In particular, we report on the relationship between barrier thickness and plasma conditions as determined by in-situ discrete ellipsometry. Ellipsometry results were verified by fabricating Nb/Al-AlN/Nb junctions and measuring current-voltage, I(V), curves. dc I(V) curves for a range of current densities are presented.

Cecil, Thomas W.; Lichtenberger, Arthur W.; Kerr, Anthony R.

2008-04-01

427

Hydride vapor phase epitaxy-grown AlGaN/GaN high electron mobility transistors  

NASA Astrophysics Data System (ADS)

High quality undoped AlGaN/GaN high electron mobility transistors (HEMTs) structures have been grown by hydride vapor phase epitaxy (HVPE), for the first time. The morphology of the films grown on Al 2O 3 substrates is excellent, with a root-mean-square roughness of 0.2 nm over 1010 ?m 2 measurement area. Capacitance-voltage measurements show the formation of a dense sheet of charge at the AlGaN/GaN interface. This is the first ever report of the formation of a two-dimensional electron gas in a nitride structure grown by HVPE. HEMTs with 1 ?m gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6 A/mm.

Mastro, M. A.; Tsvetkov, D.; Soukhoveev, V.; Usikov, A.; Dmitriev, V.; Luo, B.; Ren, F.; Baik, K. H.; Pearton, S. J.

2003-06-01

428

Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation.  

PubMed

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made. PMID:20433182

Shteinberg, Alexander S; Lin, Ya-Cheng; Son, Steven F; Mukasyan, Alexander S

2010-05-27

429

Rhombohedral AlPt films formed by self-propagating, high temperature synthesis.  

SciTech Connect

High-purity AlPt thin films prepared by self-propagating, high temperature combustion synthesis show evidence for a new rhombohedral phase. Sputter deposited Al/Pt multilayers of various designs are reacted at different rates in air and in vacuum, and each form a new trigonal/hexagonal aluminide phase with unit cell parameters a = 15.571(8) {angstrom}, c = 5.304(1) {angstrom}, space group R-3 (148), and Z, the number of formula units within a unit cell, = 39. The lattice is isostructural to that of the AlPd R-3 lattice as reported by Matkovic and Schubert (Matkovic, 1977). Reacted films have a random in-plane crystallographic texture, a modest out-of-plane (001) texture, and equiaxed grains with dimensions on the order of film thickness.

Adams, David Price; Rodriguez, Mark Andrew; Kotula, Paul Gabriel

2005-11-01

430

Effect of antimony on the growth kinetics of high purity Al-Si alloys  

SciTech Connect

More recently antimony is extensively used as an alternative to sodium and strontium in Europe and Japan in the production of Al-Si alloys by the permanent mold process. It has been found that antimony can effectively refine the Al-Si eutectic structure at relatively high cooling rates, providing a low susceptibility to gassing and excellent casting properties. This paper studied the effects of antimony on the solidification characteristics of a high purity Al-12{sup wt}%Si alloy. The adsorptivity of Sb on {l_brace}111{r_brace}{sub Si} and {l_brace}100{r_brace}{sub Si} surfaces and the influence of antimony on the epitaxial growth of silicon phase from single crystal silicon substrates were also examined.

Liu, Q. [South China Univ. of Technology (China). Mechanical Engineering Dept. II] [South China Univ. of Technology (China). Mechanical Engineering Dept. II

1998-03-03

431

T91 cladding tubes with and without modified FeCrAlY coatings exposed in LBE at different flow, stress and temperature conditions  

NASA Astrophysics Data System (ADS)

Corrosion tests of 2000 h duration are conducted on tubes consisting of the steel T91 in liquid metal loops containing eutectic lead-bismuth melt with 10 -6 wt% oxygen in solution. The experiments include tests at temperatures of 480-600 C, at liquid metal flow velocities of 1, 2 and 3 m/s and under mechanical stress due to an internal pressure of 15 MPa. The surface of tubes exposed to 600 C and to different flow velocities are coated with a FeCrAlY alloy to examine its suitability as a protective coating for high loaded parts like cladding tubes. The coating was remelted by an electron pulse of GESA to homogenize the coating and improve its bonding to the bulk material. In all of the tests no liquid metal attack was observed. As received steel specimens developed multilayer oxide scales of a thickness increasing with temperature and internal pressure, while coated tubes had a thin protective alumina scale. Flow velocities above 2 m/s permanently removed formed magnetite at 550 C. No influence of the flow velocity was observed for the coated surfaces which keep their stable thin alumina scale. The internal pressure of 15 MPa caused a strain of 0.7% in the tube wall, which obviously increases iron diffusion and enhances magnetite formation.

Weisenburger, A.; Heinzel, A.; Mller, G.; Muscher, H.; Rousanov, A.

2008-06-01

432

Amorphous and nanocrystalline phase formation in highly-driven Al-based binary alloys  

SciTech Connect

Remarkable advances have been made since rapid solidification was first introduced to the field of materials science and technology. New types of materials such as amorphous alloys and nanostructure materials have been developed as a result of rapid solidification techniques. While these advances are, in many respects, ground breaking, much remains to be discerned concerning the fundamental relationships that exist between a liquid and a rapidly solidified solid. The scope of the current dissertation involves an extensive set of experimental, analytical, and computational studies designed to increase the overall understanding of morphological selection, phase competition, and structural hierarchy that occurs under far-from equilibrium conditions. High pressure gas atomization and Cu-block melt-spinning are the two different rapid solidification techniques applied in this study. The research is mainly focused on Al-Si and Al-Sm alloy systems. Silicon and samarium produce different, yet favorable, systems for exploration when alloyed with aluminum under far-from equilibrium conditions. One of the main differences comes from the positions of their respective T{sub 0} curves, which makes Al-Si a good candidate for solubility extension while the plunging T{sub 0} line in Al-Sm promotes glass formation. The rapidly solidified gas-atomized Al-Si powders within a composition range of 15 to 50 wt% Si are examined using scanning and transmission electron microscopy. The non-equilibrium partitioning and morphological selection observed by examining powders at different size classes are described via a microstructure map. The interface velocities and the amount of undercooling present in the powders are estimated from measured eutectic spacings based on Jackson-Hunt (JH) and Trivedi-Magnin-Kurz (TMK) models, which permit a direct comparison of theoretical predictions. For an average particle size of 10 {micro}m with a Peclet number of {approx}0.2, JH and TMK deviate from each other. This deviation indicates an adiabatic type solidification path where heat of fusion is reabsorbed. It is interesting that this particle size range is also consistent with the appearance of a microcellular growth. While no glass formation is observed within this system, the smallest size powders appear to consist of a mixture of nanocrystalline Si and Al. Al-Sm alloys have been investigated within a composition range of 34 to 42 wt% Sm. Gas atomized powders of Al-Sm are investigated to explore the morphological and structural hierarchy that correlates with different degrees of departure from full equilibrium conditions. The resultant powders show a variety of structural selection with respect to amount of undercooling, with an amorphous structure appearing at the highest cooling rates. Because of the chaotic nature of gas atomization, Cu-block melt-spinning is used to produce a homogeneous amorphous structure. The as-quenched structure within Al-34 to 42 wt% Sm consists of nanocrystalline fcc-Al (on the order of 5 nm) embedded in an amorphous matrix. The nucleation density of fcc-Al after initial crystallization is on the order of 10{sup 22}-10{sup 23} m{sup -3}, which is 10{sup 5}-10{sup 6} orders of magnitude higher than what classical nucleation theory predicts. Detailed analysis of liquid and as-quenched structures using high energy synchrotron X-ray diffraction, high energy transmission electron microscopy, and atom probe tomography techniques revealed an Al-Sm network similar in appearance to a medium range order (MRO) structure. A model whereby these MRO clusters promote the observed high nucleation density of fcc-Al nanocrystals is proposed. The devitrification path was identified using high temperature, in-situ, high energy synchrotron X-ray diffraction techniques and the crystallization kinetics were described using an analytical Johnson-Mehl-Avrami (JMA) approach.

Kalay, Yunus Eren

2008-10-15