201 Matching Results

Search Results

Advanced search parameters have been applied.

Progress report on DOE research project [Thermodynamic and kinetic behavior of systems with intermetallic and intermediate phases]

Description: A theoretical investigation was made of the coherent displacive phase transformation between two equilibrium single-phase states producing several orientation variants of the product phase. The research was focused on a behavior of coherent systems (martensitic systems, metal and ceramic, and ferroelectric systems) with defects. The computer simulation demonstrated that randomly distributed static defects may drastically affect the thermodynamics, kinetics, and morphology of the transformation. In particular, the interaction of the transformation mode with the defects may be responsible for appearance of two new fields in the phase diagram: (i) the two-phase field describing the tweed microstructure, which consists of the retain parent phase and the variants of the product phase and (ii) the single-phase field describing the tweed microstructure, which consists of the variants of the product phase. These new fields can be attributed to the pre-transitional states observed in some of th e displacive transformations. The microstructure evolution resulting in formation of the thermoelastic equilibrium is path dependent. This unusual behavior is expected in systems with a sharp dependence of the transition temperature on the defect concentration.
Date: December 13, 2000
Creator: Tsakalakos, T.; Semenovskaya-Khachaturyan, S. & Khachaturyan, A.G.
Partner: UNT Libraries Government Documents Department

Dislocation accumulation at large plastic strains -- An approach to the theoretical strength of materials

Description: The usual method of introducing engineers to the concept of dislocations and their role in plastic flow is to compare an estimate of the theoretical strength of solid (of order {micro}/30 where {micro} is the shear modulus) and the observed strength of either single crystals ({mu}/10{sup 4}) or practical engineering material such as structural steels where the yield stress in shear is of order {mu}/10{sup 3}. However, if one considers the problem in reverse, one can consider the accumulation of dislocations as an important mechanism by which one can produce engineering materials in which the strength level approaches the theoretical strength. If one assumes that the flow stress can be expressed in terms of te mean free path between stored dislocations or as the square root of the global dislocation density, then one can see the influence of dislocation density in a diagrammatic form. It is clear that the strengthening by dislocation accumulation due to large imposed plastic strains represents an important approach both to the development of new, potentially valuable, engineering materials and an important area of basic understanding in terms of the mechanical response of materials close to their theoretical strength. Thus, this article will survey some of the factors which influence dislocation accumulation at large strains and the consequences of such accumulation processes.
Date: April 1, 1999
Creator: Embury, J.D. & Han, K.
Partner: UNT Libraries Government Documents Department

DIRECT DETERMINATION OF THE STACKING ORDER IN GD2O3 EPI LAYERS ON GAAS.

Description: We have used Coherent Bragg Rod Analysis (COBRA) to investigate the atomic structure of a 5.6 nm thick Gd{sub 2}O{sub 3} film epitaxially grown on a (100) GaAs substrate. COBRA is a method to directly obtain the structure of systems periodic in two-dimensions by determining the complex scattering factors along the substrate Bragg rods. The system electron density and atomic structure are obtained by Fourier transforming the complex scattering factors into real space. The results show that the stacking order of the first seven Gd{sub 2}O{sub 3} film layers resembles the stacking order of Ga and As layers in GaAs then changes to the stacking order of cubic bulk Gd{sub 2}O{sub 3}. This behavior is distinctly different from the measured stacking order in a 2.7 nm thick Gd{sub 2}O{sub 3} in which the GaAs stacking order persists throughout the entire film.
Date: December 6, 2002
Creator: YACOBY,Y.; SOWWAN,M.; PINDAK,R.; CROSS,J.; WALKO,D.; STERN,E. et al.
Partner: UNT Libraries Government Documents Department

Test data for the calculation of powder paterns for intermetallic phases

Description: Powder diffraction patterns are often calculated from structural parameters to assist in the identification of materials. To ensure that powder pattern calculations are correct, it is useful to have data to test the computer program doing the calculations. this paper contains test data for each of the crystallographic point groups and 63 of the 230 space groups. An important feature of the data is that many tests involve two high-symmetry structures (sodium and magnesium) that are set in successively lower-symmetry space groups. Thus, the calculated powder intensities for sodium, for example, are identical whatever the setting is. Though the data were chosen to be especially useful for the calculation of the powder patterns of metals and intermetallic compounds, the data have wider utility.
Date: September 1, 1996
Creator: Calvert, L.D.; Mueller, M.H.; Wallace, P.L.; Huang, J.A.; Kaduk, J.A.; Dann, J.N. et al.
Partner: UNT Libraries Government Documents Department

Martensitic nature of {delta} {yields} {gamma} allotropic transformation in plutonium

Description: Isothermal and isoplethal studies using differential scanning calorimetry have been conducted to characterize the allotropic transformations of plutonium. The {delta}-{gamma} transformation (upon cooling) was observed to have a classic martensitic nature. The work described herein is the first quantitative study of this phenomena in plutonium.
Date: September 1, 1996
Creator: Lopez, P.C.; Cost, J.R. & Axler, K.M.
Partner: UNT Libraries Government Documents Department

Effect of powder characteristics on the {alpha}-TO-{beta} Si{sub 3}N{sub 4} transformation kinetics

Description: {beta}-phase nucleation and growth is an important step in the microstructure development of Si{sub 3}N{sub 4} ceramics. Samples containing different powder types, sintering additives and {alpha}/{beta} ratios were heated at intermediate temperatures and times to monitor the phase changes taking place. The rate of transformation was mainly dependent on the powder surface area and starting {beta}-content. It was observed that additions of {beta}-Si{sub 3}N{sub 4} increased the rate of transformation.
Date: April 1, 1997
Creator: Tiegs, T.N.; Montgomery, F.C.; Schroeder, J.L.; Barker, D.L. & Menchhofer, P.A.
Partner: UNT Libraries Government Documents Department

Cooperative chemical rebonding in the segregation of impurities in silicon grain boundaries

Description: With ab initio calculations the authors show that the experimentally observed large segregation energies of As at Si grain boundaries can be explained by the formation of isolated dimers or ordered chains of dimers of threefold-coordinated As along the cores of grain boundary dislocations. They also find the intriguing possibility that As segregation may drive structural transformation of certain grain boundaries. Recently, they have obtained the first atomic-resolution STEM images of As in a Si grain boundary, consistent with the formation of As dimers. Segregation energy of As dimers was found to be significantly higher in isolated dislocation cores, where larger site-variation in strain than in grain boundaries lead to further lowering of the electronic levels of As deep into the bandgap.
Date: December 1996
Creator: Maiti, A.; Chisholm, M. F.; Pennycook, S. J. & Pantelides, S. T.
Partner: UNT Libraries Government Documents Department

Transformation kinetics in controlled-power and controlled-temperature cycle testing

Description: On-heating transformation kinetics were investigated for several steels by using a Gleeble capable of programmable power input as well as programmable temperature cycling. Transformation kinetics determined in both modes are reported. The temperature cycles are significantly different between the two modes due to the latent heat associated with the phase transformations. Both diffusion rates and transformation driving force increase with temperature above the eutectoid temperature, therefore the latent heat can potentially have a significant impact on the transformation kinetics. Experiments with plain carbon steels illustrate that the latent heat of austenite formation causes an appreciable temperature arrest during transformation, and the dilatation response is similarly altered. A kinetic transformation model, based on the decomposition of pearlite and the diffusional growth of austenite, reproduced the transient dilatation data obtained from both control modes reasonably well using the same kinetic parameter values.
Date: June 1, 1998
Creator: Robino, C.V.; Knorovsky, G.; Dykhuizen, R.C.; MacCallum, D.O. & Damkroger, B.K.
Partner: UNT Libraries Government Documents Department

First-principles simulations and Z-contrast imaging of impurities at <001> tilt grain boundaries in MgO

Description: First-principles density-functional calculations were used to study the effects of CA impurities on the {Sigma} = 5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
Date: December 1, 1997
Creator: Yan, Y.; Chisholm, M.F.; Pennycook, S.J.; Duscher, G.; Maiti, A. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department

A model for the shock-induced phase transition in iron

Description: It has long been known that the shock induced {alpha} to {epsilon} phase transition in iron exhibits significant metastability in the two phase region above 13 GPa. The authors have developed a simple, physically motivated model that accurately describes the metastable phase transition surface for shock loaded iron during numerical simulations. It is demonstrated here that the metastability of Fe can have a substantial impact on numerical simulations of time-resolved experiments.
Date: October 1, 1997
Creator: Boettger, J.C. & Wallace, D.C.
Partner: UNT Libraries Government Documents Department

Isentropic compression experiments on the Sandia Z accelerator

Description: A long-standing goal of the equation of state (EOS) community has been the development of a loading capability for direct measurement of material properties along an isentrope. Previous efforts on smooth bore launchers have been somewhat successful, but quite difficult to accurately reproduce, had pressure limitations, or tended to be a series of small shocks as opposed to a smoothly increasing pressure load. A technique has recently been developed on the Sandia National Laboratories Z accelerator which makes use of the high current densities and magnetic fields available to produce nearly isentropic compression of samples that are approximately 1 mm in thickness over approximately 120 ns. Velocity interferometry is used to measure the rear surface motion of these samples. The resulting time resolved velocity profiles from multiple sample thicknesses provide information about mechanical response under isentropic loading conditions and phase transition kinetics. Feasibility experiments have been performed to pressures of approximately 130 kbar in copper and 300 kbar in iron with effects of the {alpha}-{var_epsilon} phase change kinetics in iron clearly observed. Work is in progress to achieve 1--2% accuracy in P-v space along an isentrope, provide uniaxial strain, and to eliminate magnetic field and current diffusion within the sample of interest.
Date: February 21, 2000
Creator: HALL,CLINT A.
Partner: UNT Libraries Government Documents Department

Gibbsite to Boehmite Transformation in Strongly Caustic and Nitrate Environments

Description: The transformation of gibbsite to boehmite in strongly caustic solutions was studied using quantitative X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy techniques. Under hydrothermal conditions we identified two transformation mechanisms; dehydration and in-situ nucleation and dissolution and nucleation. If the reaction container was not completely sealed, dehydration of gibbsite followed by in-situ nucleation of boehmite was the preferred mechanism. Boehmite produced fibrous boehmite particles within the amorphous matrix of the decomposed gibbsite particles, which exhibited a poorly crystalline structure and smaller size than the initial gibbsite particles. In a closed environment, the preferred mechanism was the dissolution of gibbsite along (001) planes. The final boehmite particles were not morphologically related to the initial gibbsite particles and could be many times larger than the gibbsite particles.
Date: November 26, 2002
Creator: Hobbs, D.T.
Partner: UNT Libraries Government Documents Department

Oriented lead zirconate titanate thin films: Characterization of film crystallization

Description: Film processing temperature and time was varied to characterize the pyrochlore-to-perovskite crystallization of solution-derived PZT 20/80 thin films. 3000 {Angstrom} thick films were prepared by spin deposition using <100> single crystal MgO as substrate. By controlled rapid thermal processing, films at different stages in the perovskite crystallization process were prepared with the tetragonal PZT 20/80 phase being <100>/<001> oriented relative to the MgO surface. An activation energy for the conversion process of 326 kJ/mole was determined by use of an Arrhenius expression using rate constants found by application of the method of Avrami. Activation energy for formation of the PZT 20/80 perovskite phase of the solution-derived films compared favorably with that calculated from data by Kwok and Desu for sputter-deposited 3500 {Angstrom} thick PZT 55/45 films. Similarity in activation energies indicates that the energetics of the conversion process are not strongly dependent on the method used for film deposition.
Date: November 1, 1993
Creator: Voigt, J. A.; Tuttle, B. A.; Headley, T. J.; Eatough, M. O.; Lamppa, D. L. & Goodnow, D.
Partner: UNT Libraries Government Documents Department

Effect of initial composition distribution on the phase transformation behavior in the Fe-Cr-Ni system

Description: A finite-difference implicit numerical model was used to study the diffusion-controlled {alpha}{minus}{gamma} (ferrite-to-austenite) solid-state phase transformation in the Fe-Cr-Ni system. The influence of a nonuniform initial composition distribution was examined in order to assess the impact that nonuniform solute profiles resulting from solidification may have on subsequent transformation behavior in weldments and castings. A significant impact on the transformation kinetics and transformation path was found in some cases. Factors that affect the degree of influence are discussed.
Date: December 31, 1995
Creator: Vitek, J.M. & David, S.A.
Partner: UNT Libraries Government Documents Department

Coherent V{sub 2}O{sub 3} precipitates in {alpha}-Al{sub 2}O{sub 3} co-implanted with vanadium and oxygen

Description: The oxides of vanadium VO{sub 2} and V{sub 2}O{sub 3} are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, the authors report the formation of buried precipitates of V{sub 2}O{sub 3} in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V{sub 2}O{sub 3} precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V{sub 2}O{sub 3}//(0001) {alpha}-Al{sub 2}O{sub 3} and (11--20)V{sub 2}O{sub 3}//(0001) {alpha}-Al{sub 2}O{sub 3}. This finding is in agreement with results obtained elsewhere for thin films of V{sub 2}O{sub 3} deposited on c-axis-oriented sapphire.
Date: December 31, 1994
Creator: Gea, L.A.; Boatner, L.A.; Budai, J.D. & Rankin, J.
Partner: UNT Libraries Government Documents Department

Reactive sputter deposition of boron nitride

Description: The preparation of fully dense, boron targets for use in planar magnetron sources has lead to the synthesis of Boron Nitride (BN) films by reactive rf sputtering. The deposition parameters of gas pressure, flow and composition are varied along with substrate temperature and applied bias. The films are characterized for composition using Auger electron spectroscopy, for chemical bonding using Raman spectroscopy and for crystalline structure using transmission electron microscopy. The deposition conditions are established which lead to the growth of crystalline BN phases. In particular, the growth of an adherent cubic BN coating requires 400--500 C substrate heating and an applied {minus}300 V dc bias.
Date: October 1, 1995
Creator: Jankowski, A.F.; Hayes, J.P.; McKernan, M.A. & Makowiecki, D.M.
Partner: UNT Libraries Government Documents Department

Phase stability of transition metals and alloys

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project was focused on resolving unexplained differences in calculated and measured phase transition pressures in transition metals. Part of the approach was to do new, higher accuracy calculations of transmission pressures for group 4B and group 6B metals. Theory indicates that the transition pressures for these baseline metals should change if alloyed with a d-electron donor metal, and calculations done using the Local Density Approximation (LDA) and the Virtual Crystal Approximation (VCA) indicate that this is true. Alloy systems were calculated for Ti, Zr and Hf based alloys with various solute concentrations. The second part of the program was to do new Diamond Anvil Cell (DAC) measurements to experimentally verify calculational results. Alloys were prepared for these systems with grain size suitable for Diamond Anvil Cell experiments. Experiments were done on pure Ti as well as Ti-V and Ti-Ta alloys. Measuring unambiguous transition pressures for these systems proved difficult, but a new technique developed yielded good results.
Date: June 1, 1997
Creator: Hixson, R.S.; Schiferl, D.; Wills, J.M. & Hill, M.A.
Partner: UNT Libraries Government Documents Department

Particle-induced amorphization of complex ceramics. Final report

Description: The crystalline-to-amorphous (c-a) phase transition is of fundamental importance. Particle irradiations provide an important, highly controlled means of investigating this phase transformation and the structure of the amorphous state. The interaction of heavy-particles with ceramics is complex because these materials have a wide range of structure types, complex compositions, and because chemical bonding is variable. Radiation damage and annealing can produce diverse results, but most commonly, single crystals become aperiodic or break down into a polycrystalline aggregate. The authors continued the studies of the transition from the periodic-to-aperiodic state in natural materials that have been damaged by {alpha}-recoil nuclei in the uranium and thorium decay series and in synthetic, analogous structures. The transition from the periodic to aperiodic state was followed by detailed x-ray diffraction analysis, in-situ irradiation/transmission electron microscopy, high resolution transmission electron microscopy, extended x-ray absorption fine structure spectroscopy/x-ray absorption near edge spectroscopy and other spectroscopic techniques. These studies were completed in conjunction with bulk irradiations that can be completed at Los Alamos National Laboratory or Sandia National Laboratories. Principal questions addressed in this research program included: (1) What is the process at the atomic level by which a ceramic material is transformed into a disordered or aperiodic state? (2) What are the controlling effects of structural topology, bond-type, dose rate, and irradiation temperature on the final state of the irradiated material? (3) What is the structure of the damaged material? (4) What are the mechanisms and kinetics for the annealing of interstitial and aggregate defects in these irradiated ceramic materials? (5) What general criteria may be applied to the prediction of amorphization in complex ceramics?
Date: August 1, 1998
Creator: Ewing, R. C. & Wang, L. M.
Partner: UNT Libraries Government Documents Department

Austenite to ferrite transformation kinetics during continuous cooling

Description: The austenite decomposition has been investigated in a hypo-eutectoid plain carbon steel under continuous cooling conditions using a dilatometer and a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary systems Fe-C-Mn. The austenite to ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates. The formation of ferrite nuclei takes place with equilibrium composition on austenite grain boundaries. The nuclei are assumed to have a pill box shape in accordance with minimal interfacial energy. For higher cooling rates, early growth has to be taken into account to describe the transformation start. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium; i.e. only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute drag effect on the boundary movement. Thus, growth kinetics is controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.
Date: December 31, 1994
Creator: Militzer, M.; Pandi, R. & Hawbolt, E.B.
Partner: UNT Libraries Government Documents Department

Char crystalline transformations during coal combustion and their implications for carbon burnout. Semiannual technical progress report, 1 January 1996--1 July 1996

Description: Recent work at Sandia National Laboratories, Imperial College, and the U.K. utility PowerGen, has identified an important mechanism believed to have a large influence on unburned carbon levels from pulverized coal fired boilers. That mechanism is char carbon crystalline rearrangements on subsecond times scales at temperatures of 1800 - 2500 K, which lead to char deactivation in the flame zones of furnaces. The so-called thermal annealing of carbons is a well known phenomenon, but its key role in carbon burnout has only recently been appreciated, and there is a lack of quantitative data in this time/temperature range. In addition, a new fundamental tool has recently become available to study crystalline transformations, namely high resolution transmission electron microscopy (HRTEM) fringe imaging, which provides a wealth of information on the nature and degree of crystallinity in carbon materials such as coal chars. Motivated by these new developments, this University Coal Research project has been initiated with the following three goals: to determine transient, high-temperature thermal deactivation kinetics as a function of parent coal and temperature history; and to characterize the effect of this thermal treatment on carbon crystalline structure through high-resolution transmission electron microscopy and specialized, quantitative image analysis. Work is currently underway on the following three tasks: (1) experimental technique development; (2) thermal deactivation kinetics; and (3) crystal structure characterization. In this second project period, progress was made on subtasks 1 and 3, in both cases in the areas of equipment and technique development. These activities are discussed in detail in this report.
Date: November 1, 1996
Creator: Hurt, R.H.
Partner: UNT Libraries Government Documents Department

Shock-induced defects in bulk materials

Description: In this paper examples of the shock-induced defects produced during shock compression which correlate with microstructure/mechanical property changes induced in materials due to shock prestraining are discussed. The characteristics of the shock impulse(peak shock pressure, pulse duration, and rarefaction rate) imparted to the material under investigation and the shock-induced defects produced in numerous metals and alloys are compared with their deformation behavior at ordinary rates of deformation. Examples of the range of defects observed in shock-recovered metals and alloys, include: dislocations, deformation twins, point defects, and residual metastable remnants from pressure-induced phase transformations. Results concerning the influence of interstitial content on the propensity of {omega}-phase formation and its structure in high-purity and A-7O Ti are presented. The influence of shock-wave deformation on the phase stability and substructure evolution of high-purity (low-interstitial) titanium and A-7O (3,700 ppm oxygen) titanium were probed utilizing real-time velocity interferometry (VISAR) and soft shock-recovery techniques. Suppression of the {alpha}-{omega} pressure-induced phase transformation in A-70 Ti, containing a high interstitial oxygen content, is seen to simultaneously correspond with the suppression of deformation twinning.
Date: March 1, 1998
Creator: Gray, G.T.
Partner: UNT Libraries Government Documents Department

VO{sub 2} precipitates for self-protected optical surfaces

Description: A method for forming crystallographically coherent precipitates of vanadium dioxide in the near-surface region of sapphire and the resulting product is disclosed. Ions of vanadium and oxygen are stoichiometrically implanted into a sapphire substrate (Al{sub 2}O{sub 3}), and subsequently annealed to form vanadium dioxide precipitates in the substrate. The embedded VO{sub 2} precipitates, which are three-dimensionally oriented with respect to the crystal axes of the Al{sub 2}O{sub 3} host lattice, undergo a first-order monoclinic-to-tetragonal (and also semiconducting-to- metallic) phase transition at {minus}77 C. This transformation is accompanied by a significant variation in the optical transmission of the implanted region and results in the formation of an optically active, thermally switchable surface region on Al{sub 2}O{sub 3}.
Date: December 1, 1997
Creator: Gea, Laurence A. & Boatner, Lynn A.
Partner: UNT Libraries Government Documents Department

Slip, twinning, and transformation in Laves phases. Final technical report, 1 August 1990--January 31, 1997

Description: Currently, intermetallic compounds are being studied as potential high-temperature structural materials. Most of these studies are on phases such as Ni{sub 3}Al, TiAl, NiAl, and Ti{sub 3}Al, which have structures that are crystallographic derivatives of fcc, bcc, or hcp structures. However, most intermetallics have more complex crystal structures, and little is known about their deformation mechanisms and mechanical properties. By far the largest class of such compounds is that of the Laves phases, AB{sub 2} compounds having three different crystal structures: C15 (cubic), C14 (hexagonal), and C36 (dihexagonal). This research program explored room-temperature compressive deformation of two-phase alloys containing substantial amounts of Laves phase, and, by transmission electron microscopy, characterized the deformation-induced defects in these alloys. The results include: transmission electron microscopy analysis of defect structures to determine deformation mechanisms, use of indentation experiments to gain deeper understanding of deformation mechanisms, systematic study of effects of ternary alloying additions on mechanical properties, and assessment of effects of departure from stoichiometry on phase stability and mechanical properties of Laves phases.
Date: June 1, 1998
Creator: Allen, S.M.
Partner: UNT Libraries Government Documents Department