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Opportunities in Plutonium Metallurgical Research

Description: This is an exciting time to be involved in plutonium metallurgical research. Over the past few years, there have been significant advances in our understanding of the fundamental materials science of this unusual metal, particularly in the areas of self-irradiation induced aging of Pu, the equilibrium phase diagram, the homogenization of {delta}-phase alloys, the crystallography and morphology of the {alpha}{prime}-phase resulting from the isothermal martensitic phase transformation, and the phonon dispersion curves, among many others. In addition, tremendous progress has been made, both experimentally and theoretically, in our understanding of the condensed matter physics and chemistry of the actinides, particularly in the area of electronic structure. Although these communities have made substantial progress, many challenges still remain. This brief overview will address a number of important challenges that we face in fully comprehending the metallurgy of Pu with a specific focus on aging and phase transformations.
Date: December 19, 2006
Creator: Schwartz, A J
Partner: UNT Libraries Government Documents Department

Use of strain-annealing to evolve the grain boundary character distribution in polycrystalline copper

Description: We have used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries was {similar_to}70% in the starting recrystallized material. Three different processing conditions were employed: high, moderate, and low temperature. The high-temperature process resulted in a reduction in the fraction of special GBs while both the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction.
Date: November 6, 1996
Creator: King, W.E. & Schwartz, A.J.
Partner: UNT Libraries Government Documents Department

Toward optimization of the grain boundary character distribution in copper by strain annealing

Description: We have used a two step (low and high temperature) strain-annealing process to evolve the two grain boundary character distribution (GBCD) in fully recrystallized oxygen (OFE) copper bar that was forged and rolled. Orientation imaging microscopy (OIM) has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, special fraction was about 70% in the starting recrystallized material. Three different processing conditions were employed: high, moderate, and low temperature. The high temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher-temperature anneal and commensurate increase in the special fraction.
Date: November 10, 1996
Creator: King, W.E. & Schwartz, A.J.
Partner: UNT Libraries Government Documents Department

Overview of Modeling and Simulations of Plutonium Aging

Description: Computer-aided materials research is now an integral part of science and technology. It becomes particularly valuable when comprehensive experimental investigations and materials testing are too costly, hazardous, or of excessive duration; then, theoretical and computational studies can supplement and enhance the information gained from limited experimental data. Such is the case for improving our fundamental understanding of the properties of aging plutonium in the nuclear weapons stockpile. The question of the effects of plutonium aging on the safety, security, and reliability of the nuclear weapons stockpile emerged after the United States closed its plutonium manufacturing facility in 1989 and decided to suspend any further underground testing of nuclear weapons in 1992. To address this, the Department of Energy's National Nuclear Security Administration (NNSA) initiated a research program to investigate plutonium aging, i.e., the changes with time of properties of Pu-Ga alloys employed in the nuclear weapons and to develop models describing these changes sufficiently reliable to forecast them for several decades. The November 26, 2006 press release by the NNSA summarizes the conclusions of the investigation, '...there appear to be no serious or sudden changes occurring, or expected to occur, in plutonium that would affect performance of pits beyond the well-understood, gradual degradation of plutonium materials'. Furthermore, 'These studies show that the degradation of plutonium in our nuclear weapons will not affect warhead reliability for decades', then NNSA Administrator Linton Brooks said. 'It is now clear that although plutonium aging contributes, other factors control the overall life expectancy of nuclear weapons systems'. The origin of plutonium aging is the natural decay of certain plutonium isotopes. Specifically, it is the process of alpha decay in which a plutonium atom spontaneously splits into a 5 MeV alpha particle and an 85keV uranium recoil. The alpha particle traverses the lattice, slowly loosing energy ...
Date: April 24, 2007
Creator: Schwartz, A J & Wolfer, W G
Partner: UNT Libraries Government Documents Department

B+ and D(S)+ Decay Constants from Belle and BaBar

Description: The Belle and Babar experiments have measured the branching fractions for B{sup +} {yields} {tau}{sup +}{nu} and D{sub s}{sup +} {yields} {mu}{sup +}{nu} decays. From these measurements one can extract the B{sup +} and D{sub s}{sup +} decay constants, which can be compared to lattice QCD calculations. For the D{sub s}{sup +} decay constant, there is currently a 2.1 {sigma} difference between the calculated value and the measured value.
Date: April 9, 2012
Creator: Schwartz, A.J. & U., /Cincinnati
Partner: UNT Libraries Government Documents Department

Toward a Deeper Understanding of Plutonium

Description: Plutonium is a very complex element lying near the middle of the actinide series. On the lower atomic number side of Pu is the element neptunium; its 5f electrons are highly delocalized or itinerant, participating in metallic-like bonding. The electrons in americium, the element to the right of Pu, are localized and do not participant significantly in the bonding. Plutonium is located directly on this rather abrupt transition. In the low-temperature {alpha} phase ground state, the five 5f electrons are mostly delocalized leading to a highly dense monoclinic crystal structure. Increases in temperature take the unalloyed plutonium through a series of five solid-state allotropic phase transformations before melting. One of the high temperature phases, the close-packed face centered cubic {delta} phase, is the least dense of all the phases, including the liquid. Alloying the Pu with Group IIIA elements such as aluminum or gallium retains the {delta} phase in a metastable state at ambient conditions. Ultimately, this metastable {delta} phase will decompose via a eutectoid transformation to {alpha} + Pu{sub 3}Ga. These low solute-containing {delta}-phase Pu alloys are also metastable with respect to low temperature excursions or increases in pressure and will transform to a monoclinic crystal structure at low temperatures via an isothermal martensitic phase transformation or at slightly elevated pressure. The delocalized to localized 5f electron bonding transition that occurs in the light actinides surrounding Pu gives rise to a plethora of unique and anomalous properties but also severely complicates the modeling and simulation. The development of theories and models that are sufficiently sensitive to capture the details of this transition and capable of elucidating the fundamental properties of plutonium and plutonium alloys is currently a grand challenge in actinide science. Recent advances in electronic structure theory, semi-empirical interatomic potentials, and raw computing power have enabled remarkable progress ...
Date: June 21, 2007
Creator: Schwartz, A J & Wolfer, W G
Partner: UNT Libraries Government Documents Department

Mechanical behavior of tantalum and tantalum-tungsten alloys: texture gradients and macro/micro-response

Description: We have examined the mechanical response of unalloyed Ta and Ta-W alloy annealed plates over a wide range of loadings. It was observed in general that Ta exhibits nonuniform mechanical behavior, for example, hourglassing of compression samples and multiple instabilities during tensile deformation. In contrast, the Ta-W alloys do not exhibit any unusual nonuniform behavior. This work presents data revealing the spatial distribution of texture in Ta and Ta-W alloys. Significant variations in texture both through the thickness and from one area of the plate to another were found to be characteristic of Ta. The dominant feature of the texture variations was found to be enhanced <111> crystal direction fractions at the center of the plate, with a decreasing fraction near the surface. We find that the variation in texture in the Ta-W alloys is substantially less than that seen in Ta with primarily a <100> cube texture throughout. This study suggest that the texture gradients are responsible for the nonuniform mechanical response of Ta and that the uniform behavior of the Ta-W alloys is a consequence of the absence of texture gradients.
Date: November 30, 1996
Creator: Lassila, D.H.; Schwartz, A.J.; LeBlanc, M.M. & Wright, S.I.
Partner: UNT Libraries Government Documents Department

Analysis of intergranular impurity concentration and the effects on the ductility of copper shaped charge jets

Description: A geometrical analysis based on an assumed tetrakaidecahedron grain shape is applied to determine the relationship among grain size, bulk impurity content, and breakup time in sulfur-doped, high-precision, 81-mm, oxygen-free electronic (ofe) copper shaped charge liners. The calculations determine the number of impurity atoms as a function of grain size, the number of available sites at the intercrystalline defects, and the intercrystalline impurity concentration. Recent experiments have shown that some larger grain size liners with low impurity contents exhibit better ductility than smaller grain size liners with higher impurity concentrations. Within the range of grain sizes and bulk impurity contents in this study, the analysis suggests that the quadruple nodes and triple lines are saturated with impurities. Over this same range of impurities and grain sizes, only partial filling of a monolayer of impurities exists at the grain boundaries. The analysis suggests that breakup time is fundamentally related to grain boundary impurity concentration.
Date: November 20, 1997
Creator: Schwartz, A.J.; Lassila, D.H. & Baker, E.L.
Partner: UNT Libraries Government Documents Department

Correlating Observations of Deformation Microstructures by TEM and Automated EBSD Techniques

Description: The evolution of the deformed microstructure as a function of imposed plastic strain is of interest as it provides information on the material hardening characteristics and mechanism(s) by which cold work energy is stored. This has been extensively studied using transmission electron microscopy (TEM), where the high spatial and orientational resolution of the technique is used to advantage to study local phenomenon such as dislocation core structures and interactions of dislocations. With the recent emergence of scanning electron microscope (SEM) based automated electron backscatter diffraction (EBSD) techniques, it has now become possible to make mesoscale observations that are statistical in nature and complement the detailed TEM observations. Correlations of such observations will be demonstrated for the case of Ni-base alloys, which are typically non-cell forming solid solution alloys when deformed at ambient temperatures. For instance, planar slip is dominant at low strain levels but evolves into a microstructure where distinct crystallographic dislocation-rich walls form as a function of strain and grain orientation. Observations recorded using both TEM and EBSD techniques are presented and analyzed for their implication on subsequent annealing characteristics.
Date: June 5, 2000
Creator: Schwartz, A.J. & King, W.E.
Partner: UNT Libraries Government Documents Department

High temperature deformation in 2036 Al and 0.2 wt % Zr-2036 A1

Description: 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.
Date: November 1, 1995
Creator: Huang, J.S.; Schwartz, A.J. & Nieh, T.G.
Partner: UNT Libraries Government Documents Department

Effect of interior surface finish on the break-up of commercial shaped charge liners

Description: A series of experiments aimed at understanding the influence of the liner interior surface finish on the break-up of shaped charge jets has been completed. The experiments used a standard 81-mm shaped charge design, loaded with LX-14 high explosive; incorporating high-precision copper shaped charged liners. The results indicate that a significant reduction of jet break-up time occurs between a surface finish of 99.30 microinches and 375.65 microinches. Surface finishes of 4.78, 44.54 and 99.30 microinches produced significantly better ductility and associated break-up times than the 375.65-microinch finish. The baseline production process high-precision liners were measured to have an average surface finish of 44.54 microinches. The results show that for the shaped charge warhead geometry and explosive combination investigated, some care must be taken in respect to surface finish, but that very fine surface finishes do not significantly improve the jet ductility and associated break-up times.
Date: August 11, 1999
Creator: Baker, E L & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Applications of Crystal Plasticity in Multiscale Modeling

Description: Multiscale modeling with crystal plasticity constitutive relations is used to determine the average response of a polycrystal. The measured crystallographic texture of a copper shaped charge liner is used in a crystal plasticity model to construct a yield surface that exhibits normal-shear coupling. Simulations with this yield surface model demonstrate the spinning behavior observed in the spin formed copper shaped charges.
Date: September 24, 2001
Creator: Becker, R.; Busche, M.; Schwartz, A. J. & Kumar, M.
Partner: UNT Libraries Government Documents Department

Grain Size and Pressure Effects on Spall Strength in Copper

Description: We are executing a systematic study to quantify the effects of specific microstructural features on the spall behavior of 99.999% copper. Single crystals with [100] orientation, polycrystals with three grain sizes, and internally oxidized single crystals are shocked with Cu flyers at velocities from 300 to 2000 m/s using a 35-mm single/two-stage light gas gun. VISAR measurements of the free surface velocity are used to characterize the spall pullback signal and details of the ringing. The high purity single crystals exhibit the highest spall strength followed by the large, medium and small grain size polycrystalline samples. Cu-0.15 wt.% Si single crystals have been internally oxidized to produce a fine dispersion of 350 nm silica particles. These samples exhibit the lowest spall strength, a factor of two and greater below the high purity single crystals.
Date: June 1, 2001
Creator: Schwartz, A J; Cazamias, J U; Fiske, P S & Minich, R W
Partner: UNT Libraries Government Documents Department

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

Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. % Ga

Description: The gallium-stabilized Pu-2.0 at. % Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable {delta} phase to the gallium-containing, monoclinic {alpha}{prime} phase near -100 C. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation (TTT) diagram, where two nose temperatures anchoring an upper- and lower-C describe minima in the time for the initiation of transformation. The underlying mechanisms responsible for the double-C behavior are currently unresolved, although recent experiments suggest that a conditioning treatment--wherein, following an anneal at 375 C, the sample is held at a sub-anneal temperature for a period of time--significantly influences the upper-C of the TTT diagram. As such, elucidating the effects of the conditioning treatment upon the {delta} {yields} {alpha}{prime} transformation can provide valuable insights into the fundamental mechanisms governing the double-C kinetics of the transition. Following a high-temperature anneal, a differential scanning calorimeter (DSC) was used to establish an optimal conditioning curve that depicts the amount of {alpha}{prime} formed during the transformation as a function of conditioning temperature for a specified time. With the optimal conditioning curve as a baseline, the DSC was used to explore the circumstances under which the effects of the conditioning treatment were destroyed, resulting in little or no transformation.
Date: April 2, 2008
Creator: Jeffries, J R; Blobaum, K M; Wall, M A & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Pu Workshop Letter

Description: In preparation for the upcoming Pu Workshop in Livermore, CA, USA, during July 14 and 15, 2006, we have begun to give some thought as to how the meeting will be structured and what will be discussed. Below, you will find our first proposal as to the agenda and contents of the meeting. From you, we need your feedback and suggestions concerning the desirability of each aspect of our proposal. Hopefully, we will be able to converge to a format that is acceptable to all parties. First, it now appears that we will be limited to three main sessions, Friday morning (July 14), Friday afternoon (July 14) and Saturday morning (July 15). The Pu Futures Meeting will conclude on Thursday, July 13. Following a social excursion, the Russian participants will be transported from Monterey Bay to their hotel in Livermore. We anticipate that the hotel will be the Residence Inn at 1000 Airway Blvd in Livermore. However, the hotel arrangements still need to be confirmed. We expect that many of our participants will begin their travels homeward in the afternoon of Saturday, July 15 and the morning of Sunday, July 16. Associated with the three main sessions, we propose that there be three main topics. Each session will have an individual focus. Because of the limited time available, we will need to make some judicious choices concerning the focus and the speakers for each session. We will also have a poster session associated with each session, to facilitate discussions, and a rotating set of Lab Tours, to maximize participation in the tour and minimize the disruption of the speaking schedule. Presently, we are planning a tour of the Dynamical Transmission Electron Microscope (DTEM) facilities, but this is still in a preliminary stage. We estimate that for each session and topic, ...
Date: March 6, 2006
Creator: Tobin, J G; Schwartz, A J & Fluss, M
Partner: UNT Libraries Government Documents Department

Transmission Electron Microscopy Characterization of Helium Bubbles in Aged Plutonium

Description: The self-irradiation damage generated by alpha decay of plutonium results in the formation of lattice defects, helium, and uranium atoms. Over time, microstructural evolution resulting from the self-irradiation may influence the physical and mechanical properties of the material. In order to assess microstructural changes, we have developed and applied procedures for the specimen preparation, handling, and transmission electron microscopy characterization of Pu alloys. These transmission electron microscopy investigations of Pu-Ga alloys ranging in age up to 42-years old reveal the presence of nanometer-sized helium bubbles. The number density of bubbles and the average size have been determined for eight different aged materials.
Date: November 2, 2004
Creator: Schwartz, A J; Wall, M A; Zocco, T G & Blobaum, K M
Partner: UNT Libraries Government Documents Department

Low Temperature Phase Instability of the Gamma Phase in SnIn Alloys

Description: The Sn-rich side of the Sn-In phase diagram has been investigated at temperatures ranging from 77 to 500 K by using X-ray diffraction, thermal analysis, and magnetization measurements. It is confirmed that the {beta}-Sn(In)-phase can remain as a metastable phase down to 77 K within the composition range of 86.3-94 at% of Sn. An isothermal displacive (martensitic) transition of the {gamma} phase to the metastable {beta} phase is suggested as the mechanism of the transformation.
Date: August 19, 2005
Creator: Chu, S; Yanar, C; Schwartz, A J; Massalski, T B & Laughlin, D E
Partner: UNT Libraries Government Documents Department

ISOTHERMAL (DELTA)/(ALPHA-PRIME) TRANSFORMATION AND TTT DIAGRAM IN A PLUTONIUM GALLIUM ALLOY

Description: Differential scanning calorimetry (DSC) is used as an alternative approach to determining the tine-temperature-transformation (TTT) diagram for the martensitic delta to alpha-prime transformation in a Pu-2.0 at% Ga alloy. Previous work suggests that the TTT diagram for a similar alloy exhibits an unusual double-C curve for isothermal holds of less than 100 minutes. Here, we extend this diagram to 18 hours, and confirm the double-C curve behavior. When the sample is cooled prior to the isothermal holds, the delta to alpha-prime transformation is observed as several overlapping exothermic peaks. These peaks are very reproducible, and they are believed to be the result of different kinds of delta to alpha-prime martensitic transformation. This may be due to the presence of different nucleation sites and/or different morphologies.
Date: November 11, 2005
Creator: Oudot, B P; Blobaum, K M; Wall, M A & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Extrinsic Paramagnetic Meissner Effect in Multiphase Indium-Tin Alloys

Description: A well-known effect in superconducting materials below their critical temperatures (T{sub c}) is the reduction to zero of their electrical resistivities. Concomitantly, the materials become perfect diamagnets for small fields. This effect, termed the Meissner Effect, allows for the direct measurement of the transition temperature (T{sub c}) by magnetic techniques such as the superconducting quantum interference device (SQUID). A Paramagnetic Meissner Effect (PME), i.e., the unexpected observation of positive magnetic moment in a superconductor below its critical temperature during field cooling (FC), was first reported in 1989 by Svedlindh et al. (1). The origin of PME in high T{sub c} superconductors has been discussed by numerous investigators as possibly resulting from {pi}-junctions, d-wave behavior, giant vortex states, flux compression, or weak links. In conventional superconductors like Nb, the PME was ascribed to the inhomogeneous nature of such samples, whereby their surface is sufficiently different from the interior and becomes superconducting at a higher temperature than the interior on cooling, thereby trapping the magnetic flux. There remains significant controversy regarding the fundamental origin of the PME. Here, we show that the PME in two-phase and three-phase In-Sn alloys is a property resulting from the morphological distribution of the multiple phases. We propose that PME in these alloys results from microstructural encapsulation of the grains of one superconducting phase inside the grains of another (e.g., the matrix) which has a higher T{sub c}. Hence the PME in this case is extrinsic in nature rather than intrinsic to the material, and could be described as an Extrinsic Paramagnetic Meissner Effect (EPME). It may be expected to occur in multiple-phase alloy samples where more than one of the phases is superconducting, or in nominally single-phase materials where the surface of the specimen, grain boundaries, or other defects have different superconducting properties. This discovery opens ...
Date: December 2, 2005
Creator: Chu, S; Schwartz, A J; Massalski, T B & Laughlin, D E
Partner: UNT Libraries Government Documents Department

Phase transformations and phase relations in Ti{sub 50}Pd{sub (50-x)}TM{sub x} alloys

Description: The effect of transition metal (TM) substitution for Pd in Ti{sub 5O}Pd{sub (50-x}TM{sub x} alloys with x between 5 and 37.5 at.% and TM = V, Cr, Mn and Fe are being characterized by transmission electron microscopy and First-Principles Alloy Theory modeling. The goal is to obtain detailed structural information related to the ternary phase relations and transformations that are necessary for effective shape-memory alloy development. Thus far, the authors have found that the tend to have pseudobinary eutectoid-like configurations with a terminal TiPd and a non-close-packed long period ordered structure type crystal structure) based on the stoichiometry Ti{sub 2}PdTM. The systems exhibit a conventional martensitic transformation, as well as a new type of displacive transformation that shear-modulates B2 to produce a periodically distorted, but non-close-packed metastable product phase.
Date: July 15, 1994
Creator: Schwartz, A. J.; Sluiter, M. H.; Harmon, B. N. & Tanner, L. E.
Partner: UNT Libraries Government Documents Department

Coupled map lattice model of jet breakup

Description: An alternative approach is described to evaluate the statistical nature of the breakup of shaped charge liners. Experimental data from ductile and brittle copper jets are analyzed in terms of velocity gradient, deviation of {Delta}V from linearity, R/S analysis, and the Hurst exponent within the coupled map lattice model. One-dimensional simulations containing 600 zones of equal mass and using distinctly different force-displacement curves are generated to simulate ductile and brittle behavior. A particle separates from the stretching jet when an element of material reaches the failure criterion. A simple model of a stretching rod using brittle, semi-brittle, and ductile force-displacement curves is in agreement with the experimental results for the Hurst exponent and the phase portraits and indicates that breakup is a correlated phenomenon.
Date: January 25, 2001
Creator: Minich, R W; Schwartz, A J & Baker, E L
Partner: UNT Libraries Government Documents Department

Delta/Alpha-Prime Phase Transformations in a Pu-Ga Alloy

Description: In pure plutonium, the monoclinic {alpha} phase is the equilibrium phase at ambient temperature and pressure. The addition of a few percent of gallium, however, allows the fcc {delta} phase to be retained metastablely at ambient conditions. When the metastable {delta} phase is cooled to subambient temperatures, it partially transforms to the monoclinic {alpha}' phase, which has gallium supersaturated in the lattice. The {alpha}' phase reverts to the {delta} phase when the sample is heated above the ambient temperature. The martensite burst (M{sub b}) and reversion start (R{sub s}) temperatures are functions of the composition, heating rate, and prior thermal history. For a Pu-2.0 at% Ga alloy, the transformation hysteresis is approximately 150 C, which is large compared with other solid-solid phase transformations. Both the forward and reverse transformations are martensitic and proceed via a burst mode. Here, we use differential scanning calorimetry (DSC) and resistometry to perform fundamental studies of the {alpha}'/{delta} transformations with the goal of understanding how aging may affect {delta} phase stability, particularly the M{sub b} temperature. Because materials properties of the {alpha}' and {delta} phases are considerably different (including a density increase of 25% and an accompanying resistivity increase of 46% upon transformation from {delta} to {alpha}'), unexpected transformation to the {alpha}' phase is of particular interest to the stockpile stewardship community.
Date: March 7, 2005
Creator: Blobaum, K M; Krenn, C R; Wall, M A & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Advanced Transmission Electron Microscopy of Pu Alloys

Description: The characterization of microstructural changes in Pu-Ga alloys resulting from storage and aging phenomena is an important technical challenge to the nuclear Stockpile Stewardship program. We have identified at least two age-related phenomena that may occur in Pu alloys, dimensional changes due to the initial transient, helium accumulation, and void swelling, and phase instability. The initial transient is a well-known effect that results from the initial cascade damage. This form of dimensional change tends to saturate within approximately two years. A second contributor to dimensional change is the build-up of helium as a result of the alpha decay. Helium is generated at a rate of approximately 40 parts per million per year. Positron annihilation results by Howell indicate that the helium atoms will quickly fill a nearby vacancy and diffuse through the lattice as a helium filled vacancy. Void swelling is potentially the most severe mechanism of dimensional change in Pu alloys. It has been observed in all-materials exposed to irradiation, but has yet to be seen in naturally aged Pu. Phase instability is a potential concern due to the fact that the {delta}-phase is thermodynamically metastable at room temperature. Timofeeva has shown that the {delta}-phase will decompose to {delta}-phase and Pu{sub 3}Ga given enough time at ambient temperature. At sub-ambient temperatures, the {delta}-phase undergoes a displacive or martensitic phase transformation to the monoclinic {alpha}{prime}-phase, which is approximately 20% more dense. Phase transformations such as these would result in density changes, dimensional changes, and Changes in mechanical properties. Traditional characterization techniques such as optical microscopy, x-ray diffraction and scanning electron microscopy are insensitive to many of the age-related microstructural changes. In this investigation, we have applied advanced transmission electron microscopy (TEM) to investigate the microstructure and bonding of Pu alloys. A 300 keV Phillips CM300FEG with a field emission gun ...
Date: January 27, 2003
Creator: Schwartz, A J; Wall, M A; Wolfer, W G & Moore, K T
Partner: UNT Libraries Government Documents Department