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"Burst-like" Characteristics of the delta/alpha-prime Phase Transformation in Pu-Ga Alloys

Description: The {delta} to {alpha}' phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc d-phase transforms martensitically to the brittle monoclinic {alpha}'-phase at approximately -120 C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of {alpha}' to {delta} involves a large temperature hysteresis beginning just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we examined the {delta}/{alpha}' transformations in a 0.6 wt% Pu-Ga alloy using differential scanning calorimetry (DSC) and resistometry. Both techniques indicate that the martensite start temperature is -120 C and the austenite start temperature is 35 C. The heat of transformation is approximately 3 kJ/mole. During the {alpha}' {yields} {delta} reversion, ''spikes'' and ''steps'' are observed in DSC and resistometry scans, respectively. These spikes and steps are periodic, and their periodicity with respect to temperature does not vary with heating rate. With an appropriate annealing cycle, including a ''rest'' at room temperature, these spikes and steps can be reproduced through many thermal cycles of a single sample.
Date: November 10, 2003
Creator: Blobaum, K; Krenn, C; Haslam, J; Wall, M & Schwartz, A
Partner: UNT Libraries Government Documents Department
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Structure, chemistry, and properties of mineral nanoparticles

Description: Nanoparticle properties can depart markedly from their bulk analog materials, including large differences in chemical reactivity, molecular and electronic structure, and mechanical behavior. The greatest changes are expected at the smallest sizes, e.g. 10 nm and below, where surface effects are expected to dominate bonding, shape and energy considerations. The precise chemistry at nanoparticle interfaces can have a profound effect on structure, phase transformations, strain, and reactivity. Certain phases may exist only as nanoparticles, requiring transformations in chemistry, stoichiometry and structure with evolution to larger sizes. In general, mineralogical nanoparticles have been little studied.
Date: December 2, 2008
Creator: Waychunas, G.A.; Zhang, H. & Gilbert, B.
Partner: UNT Libraries Government Documents Department
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Kinetics of Propagating Phase Transformation in Compressed Bismuth

Description: The authors observed dynamically driven phase transitions in isentropically compressed bismuth. By changing the stress loading conditions they explored two distinct cases one in which the experimental signature of the phase transformation corresponds to phase-boundary crossings initiated at both sample interfaces, and another in which the experimental trace is due to a single advancing transformation front in the bulk of the material. They introduce a coupled kinetics-hydrodynamics model that for this second case enables them, under suitable simplifying assumptions, to directly extract characteristic transition times from the experimental measurements.
Date: August 18, 2004
Creator: Bastea, M.; Bastea, S.; Emig, J.; Springer, P. & Reisman, D.
Partner: UNT Libraries Government Documents Department
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Phase transformation of poled "chem-prep" PZT 95/5-2Nb ceramic under quasi-static loading conditions.

Description: Specimens of poled 'chem-prep' PNZT ceramic from batch HF803 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions at three temperatures of -55, 25, and 75 C and pressures up to 500 MPa. The objective of this experimental study was to obtain the electro-mechanical properties of the ceramic and the criteria of FE (Ferroelectric) to AFE (Antiferroelectric) phase transformations so that grain-scale modeling efforts can develop and test models and codes using realistic parameters. The poled ceramic undergoes anisotropic deformation during the transition from a FE to an AFE structure. The lateral strain measured parallel to the poling direction was typically 35 % greater than the strain measured perpendicular to the poling direction. The rates of increase in the phase transformation pressures per temperature changes were practically identical for both unpoled and poled PNZT HF803 specimens. We observed that the retarding effect of temperature on the kinetics of phase transformation appears to be analogous to the effect of shear stress. We also observed that the FE-to-AFE phase transformation occurs in poled ceramic when the normal compressive stress, acting perpendicular to a crystallographic plane about the polar axis, equals the hydrostatic pressure at which the transformation otherwise takes place.
Date: October 1, 2004
Creator: Lee, Moo Yul; Montgomery, Stephen Tedford & Hofer, John H.
Partner: UNT Libraries Government Documents Department
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Spontaneous Generation of Voltage in Single-Crystal Gd5Si2Ge2 During Magnetostructural Phase Transformations

Description: The spontaneous generation of voltage (SGV) in single-crystal and polycrystalline Gd{sub 5}Si{sub 2}Ge{sub 2} during the coupled magnetostructural transformation has been examined. Our experiments show reversible, measurable, and repeatable SGV responses of the materials to the temperature and magnetic field. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic-field changes.
Date: April 19, 2006
Creator: Zou, M.; Tang, H.; Schlagel, D. L.; Lograsso, T. A.; Gschneidner, K. A. Jr. & Pecharsky, V. K.
Partner: UNT Libraries Government Documents Department
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Crystal Level Continuum Modeling of Phase Transformations: The (alpha) <--> (epsilon) Transformation in Iron

Description: We present a crystal level model for thermo-mechanical deformation with phase transformation capabilities. The model is formulated to allow for large pressures (on the order of the elastic moduli) and makes use of a multiplicative decomposition of the deformation gradient. Elastic and thermal lattice distortions are combined into a single lattice stretch to allow the model to be used in conjunction with general equation of state relationships. Phase transformations change the mass fractions of the material constituents. The driving force for phase transformations includes terms arising from mechanical work, from the temperature dependent chemical free energy change on transformation, and from interaction energy among the constituents. Deformation results from both these phase transformations and elasto-viscoplastic deformation of the constituents themselves. Simulation results are given for the {alpha} to {epsilon} phase transformation in iron. Results include simulations of shock induced transformation in single crystals and of compression of polycrystals. Results are compared to available experimental data.
Date: October 18, 2004
Creator: Barton, N R; Benson, D J; Becker, R; Bykov, Y & Caplan, M
Partner: UNT Libraries Government Documents Department
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Micromechanics of Transformation Superplasticity in Ti-6Al-4V/TiBw Composites

Description: Transformation superplasticity is a deformation mechanism induced by thermally-cycling a polymorphic material through the phase transformation range while simultaneously applying an external biasing stress. Unlike microstructural superplasticity, which requires a fine, equiaxed grain structure, this mechanism can be applied to coarse-grained alloys and composites. In this article, we review our research on transformation superplasticity of Ti-6Al-4V/TiB-whisker reinforced composites, during thermal cycling through the titanium {alpha}/{beta} transformation range. The composites exhibit Newtonian flow and superplastic extension under these conditions. We describe the constitutive behavior of composites containing 0, 5 and 10 vol% reinforcing whiskers, and consider the effects of load transfer from matrix to whisker on superplastic deformation using existing rheological models. Additionally, strain hardening due to gradual whisker alignment is observed, and rationalized in terms of increased load transfer for aligned whiskers.
Date: October 16, 2001
Creator: Schuh, C & Dunand, D C
Partner: UNT Libraries Government Documents Department
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Near-Equilibrium Polymorphic Phase Transformations in Praseodymium Under Dynamic Compression

Description: We report the first experimental observation of sequential, multiple polymorphic phase transformations occurring in Praseodymium dynamically compressed using a ramp wave. The experiments also display the signatures of reverse transformations occuring upon pressure release and reveal the presence of small hysteresys loops. The results are in very good agreement with equilibrium hydrodynamic calculations performed using a thermodynamically consistent, multi-phase equation of state for Praseodymium, suggesting a near-equilibrium transformation behavior.
Date: February 12, 2007
Creator: Bastea, M. & Reisman, D.
Partner: UNT Libraries Government Documents Department
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Discontinuous Thermal Expansions and Phase Transformations in Crystals at Higher Temperatures

Description: The purpose of this investigation is to make more detailed studies of transformations. Fourteen compounds have been examined by high temperature X-ray diffraction for this purpose. The investigations have been carried out in such a way as to reveal: 1. the existence of transformations, 2. the influence of polarizability on thermal expansion, 3. the anisotropy of expansion, and 4. the discontinuity of thermal expansion.
Date: 1967
Creator: Hsu, Yuan Tsun
Partner: UNT Libraries
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Neutron Scattering Studies of Pre-Transitional Effects in Solid-Solid Phase Transformations

Description: Neutron scattering studies have played a fundamental role in understanding solid-solid phase transformations, particularly in studying the lattice dynamical behavior associated with precursor effects. A review of the studies performed on solids exhibiting Martensitic transformations is given below. The mode softening and associated elastic diffuse scattering, previously observed in NiAl alloys, will be discussed as well as more recent work on Ni{sub 2}MnGa, a system exhibiting magnetic order as well as a Martensitic transformation. Also, new results on the precursor effects in ordered and disordered FePt alloys will be presented.
Date: June 30, 1999
Creator: Shapiro, S. M.
Partner: UNT Libraries Government Documents Department
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On the Micromechanisms of Shock-Induced Martensitic Transformation in Tantalum

Description: Shock-induced twinning and martensitic transformation in tantalum, which exhibits no solid-state phase transformation under hydrostatic pressures up to 100 GPa, have been further investigated. Since the volume fraction and size of twin and phase domains are small in scale, they are considered foming by heterogeneous nucleation that is catalyzed by high density lattice dislocations. A dynamic dislocation mechanism is accordingly proposed based upon the observation of dense dislocation clustering within shock-recovered tantalum. The dense dislocation clustering can cause a significant increase of strain energy in local regions of {beta} (bcc) matrix, which renders mechanical instability and initiates the nucleation of twin and phase domains through the spontaneous reactions of dislocation dissociation within the dislocation clusters. That is, twin domains can be nucleated within the clusters through the homogeneous dissociation of 1/2&lt;111&gt; dislocations into 1/6&lt;111&gt; partial dislocations, and {omega} phase domains can be nucleated within the closters through the inhomogeneous dissociation of 1/2&lt;111&gt; dislocations into 1/12&lt;111&gt;, 1/3&lt;111&gt; and 1/12&lt;111&gt; partial dislocations.
Date: December 7, 2005
Creator: Hsiung, L L
Partner: UNT Libraries Government Documents Department
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Pressure Induced Phase Transformation of Pb(Zr(0.95)Ti(0.05))O(3) Based Ceramics: Grain Size Dependence

Description: A substantial decrease in hydrostatic ferroelectric (FE) to antiferroelectric (AFE) transformation pressure was measured for Pb(Zr{sub 0.949}Ti{sub 0.051}){sub 0.989}Nb{sub 0.0182}O{sub 3} ceramics with decreasing grain size. The 150 MPa decrease in hydrostatic FE to AFE transformation pressure over the grain size range of 8.5 {micro}m to 0.7{micro}m was shown to be consistent with enhanced internal stress with decreasing grain size. Further, the Curie Point decreased and the dielectric constant measured at 25 C increased with decreasing grain size. All three properties: dielectric constant magnitude, Curie point shift and FE to AFE phase transformation pressure were shown to be semi-quantitatively consistent with internal stress differences on the order of 100 MPa. Calculations of Curie point shifts from the Clausius-Clapeyron equation, using internal stress levels derived from the hydrostatic depoling characteristics, were consistent with measured values.
Date: December 21, 1999
Creator: Tuttle, Bruce A.; Voigt, James A.; Scofield, Timothy W.; Aselage, Terrence L.; Rodriguez, Mark A.; Yang, Pin et al.
Partner: UNT Libraries Government Documents Department
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Transformation Crystallography and Plasticity of the Delta to Alpha Prime Transformation in Plutonium Alloys

Description: In delta phase Pu-Ga alloys, the transformation from the ductile face-centered cubic (fcc) {delta} phase that is retained at room temperature to the brittle low-temperature monoclinic alpha' phase is a thermally activated diffusionless transformation with double-c kinetics. Accurate modeling of the phase transformation requires detailed understanding of the role of plastic flow during the transformation and of the crystallographic transformation path. Using transmission electron microscopy (TEM), we find a significant increase in dislocation density in {delta} near the {alpha}' plates, which suggests that plastic deformation contributes to the accommodation of the 20% reduction in volume during the transformation. Analysis of a series of optical micrographs of partially transformed alloys suggests that the {alpha}' habit plane is usually nearly perpendicular to &lt;111&gt; {delta}. However, a small number of TEM observations support a habit plane near &lt;112&gt; or &lt;123&gt;, in agreement with earlier work.
Date: December 18, 2003
Creator: Krenn, C R; Wall, M A & Schwartz, A J
Partner: UNT Libraries Government Documents Department
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In Situ Spectroscopic Observation of Activation and Transformation of Tantalum Suboxides

Description: Using ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS), we were able to observe the process of oxidation of tantalum with different morphological parameters. Being able to trace surface evolution during oxidation, we evaluated activation energy of oxidation under the influence of strain and grain boundaries. It was found that the metal oxidized through three different stages and there was a transition stage where the phase transformation from suboxides to the equilibrium state of pentoxide. The applied stress and surface defects reduced the activation energy oxidation.
Date: December 16, 2009
Creator: Wang, Ke; Liu, Zhi; Cruz, Tirma Herranz; Salmeron, Miquel & Liang, Hong
Partner: UNT Libraries Government Documents Department
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Spinodal Decomposition and Ordering Transformation in U-6 wt% Nb

Description: Phase stability and aging mechanisms in a water-quenched (WQ) U-6wt% Nb (WQ-U6Nb) alloy artificially aged at 200 C (16 hours) and naturally aged at room temperature for 15 years have been investigated. Age hardening/softening phenomenon is recorded from the artificially aged samples by microhardness measurement. The age hardening can be readily rationalized by the occurrence of spinodal decomposition (or fine-scaled Nb segregation), which results in the formation of a modulated structure in the aged samples. Prolonged aging leads to age softening of the alloy by coarsening of the modulated structure. Disorder-order or chemical ordering transformation is found within the naturally aged alloy according to TEM observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. The formation of a partially ordered phase in the naturally aged alloy is proposed and identified.
Date: August 15, 2005
Creator: Hsiung, L M
Partner: UNT Libraries Government Documents Department
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Burst Martensitic Transformations in a Steel and in a Pu-Ga Alloy

Description: Upon cooling a Pu-2.0 at% Ga alloy from the ambient temperature, the metastable delta phase partially transforms martensitically to the alpha-prime phase. Because this transformation involves a 25% volume contraction, plastic accommodation by the delta matrix must occur. When the material is isochronally heated or isothermally annealed above ambient temperatures, the reversion of alpha-prime to delta is likely to occur by the alpha-prime/delta interface moving to consume the alpha-prime particles. This reversion exhibits a burst martensitic mode and is observed as sharp spikes in differential scanning calorimetry data and as steps in resistometry data. These large bursts appear to be the result of an interplay between the autocatalytically driven transformation of individual alpha-prime particles and self-quenching caused by small changes in temperature and/or stress accompanying each burst. The behavior of this Pu-Ga alloy is compared to that of a steel referred to as a ''burst martensite'' in the literature, which also exhibits bursts during both thermal cycling and isothermal holds.
Date: June 14, 2005
Creator: Blobaum, K; Krenn, C; Wall, M & Schwartz, A
Partner: UNT Libraries Government Documents Department
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Transformation Mechanism and Kinetics for the Pressure-Induced Phase Transition in Shocked CdS

Description: The pressure-induced phase transition in CdS was investigated using picosecond time-resolved electronic spectroscopy in plate impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in single crystals of CdS shocked along the c and a axes to peak stresses between 35 and 90 kbar (above the phase transition stress of approximately 30 kbar measured in continuum studies). When shocked to stresses above approximately 50 kbar along the crystal c axis and 60 to 70 kbar along the crystal a axis, the crystals undergo a very rapid change in electronic structure, indicating that significant structural changes occur within the first 100 ps. These results, along with previous ns continuum measurements, make a strong case for a metastable state during the phase transition in shocked CdS. Ab-initio periodic Hartree-Fock calculations (with DFT correlation corrections) were employed to examine the compression of CdS and to determine a possible lattice structure for the proposed metastable structure. These results, along with details of the transformation kinetics and orientational dependence, will be discussed. Work supported by ONR.
Date: June 24, 1999
Creator: Gupta, Y.M.; Knudson, M.D. & Kunz, A.B.
Partner: UNT Libraries Government Documents Department
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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
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First-principles approaches to materials stability

Description: Parameter-free electronic structure approaches are now being used to predict chemical order, and to a lesser extent, structural transformations in multi-component alloys, as a function of temperature, concentration and pressure. The underlying state-of-the-art framework will be briefly reviewed, and applications to specific aspects of the statics and kinetics of alloy transformations will be discussed. Finally special emphasis will be put on the relations between stability and mechanical properties in substitutional alloys with examples pertaining to the energetics of antiphase boundaries and interfaces.
Date: December 1, 1994
Creator: Turchi, P.E.A.
Partner: UNT Libraries Government Documents Department
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Investigation of the Kinetics of the Ferrite/Austenite Phase Transformation in the HAZ of a 2205 Duplex Stainless Steel Weldment

Description: A semi-quantitative map based on a series of spatially resolved X-ray diffraction (SRXRD) scans shows the progression of the ferrite ({delta})/austenite ({gamma}) phase balance throughout the HAZ during GTA welding of a 2205 duplex stainless steel (DSS). This map shows an unexpected decrease in the ferrite fraction on heating, followed by a recovery to the original ferrite fraction on cooling at locations within the HAZ. Even though such behavior is supported by thermodynamic calculations, it has not been confirmed by either experimental methods or have the kinetics been evaluated. Both Gleeble thermal simulations and time resolved x-ray diffraction measurements on spot welds in the 2205 DSS provide further evidence for this rather low-temperature transformation. On the other hand, calculations of the diffusion of alloying elements across the 6/y interface under a variety of conditions shed no further light on the driving force for this transformation. Further work on the mechanisms and driving forces for this transformation is on-going.
Date: March 14, 2002
Creator: Palmer, T A; Elmer, J W; Wong, J; Babu, S S & Vitek, J M
Partner: UNT Libraries Government Documents Department
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Phase Transformation Hysteresis in a Plutonium Alloy System: Modeling the Resistivity during the Transformation

Description: We have induced, measured, and modeled the {delta}-{alpha}' martensitic transformation in a Pu-Ga alloy by a resistivity technique on a 2.8-mm diameter disk sample. Our measurements of the resistance by a 4-probe technique were consistent with the expected resistance obtained from a finite element analysis of the 4-point measurement of resistivity in our round disk configuration. Analysis by finite element methods of the postulated configuration of {alpha}' particles within model {delta} grains suggests that a considerable anisotropy in the resistivity may be obtained depending on the arrangement of the {alpha}' lens shaped particles within the grains. The resistivity of these grains departs from the series resistance model and can lead to significant errors in the predicted amount of the {alpha}' phase present in the microstructure. An underestimation of the amount of {alpha}' in the sample by 15%, or more, appears to be possible.
Date: November 14, 2001
Creator: Haslam, J. J.; Wall, M. A.; Johnson, D. L.; Mayhall, D. J. & Schwartz, A. J.
Partner: UNT Libraries Government Documents Department
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