Superconductivity: A new window on plutonium’s complexity

Superconductivity has recently been discovered in a plutonium intermetallic compound (PuCoGa₅) at the surprisingly high temperature of 18.5 K. This article discusses the motivation that led to this discovery as well as what it implies for the understanding of both unconventional superconductivity and the metallurgy of plutonium. For more information, contact J.L. Sarrao, Los Alamos National Laboratory, MST-10: Condensed Matter & Thermal Physics, Mail Stop K764, Los Alamos, NM 87545; (505) 665-0481; fax (505) 665-7652; e-mail sarrao@lanl.gov.     

Understanding the unique properties of SPD-induced microstructures

Refining microstructure by severe plastic deformation to a nanometer range changes fundamental properties, such as the Curie and Debye temperatures, and engineering properties of pragmatic significance, such as strength and ductility. These enhancements originate in the combination of very small grain sizes coupled with specific defect structures. Ongoing research is rapidly advancing the understanding of severe plastic deformation-induced microstructures, leading toward the commercialization of these materials. For more information, contact T.C. Lowe, Los Alamos National Laboratory, Materials Science and Technology Division, MD G754, Los Alamos, New Mexico 87545; (505) 665-1131; fax (505) 665-4584; e-mail tlowe@lanl.gov. Authors’ Note: All compositions are given in weight per cent.     

Applying the two-state invar model to delta-phase plutonium

This article describes the authors’ use of the Weiss two-state model for Fe-Ni invar alloys to understand the anomalous thermal expansion of Pu-Ga alloys. Studies on thermal expansion of Pu-Ga are reviewed briefly, and the two-state invar model is described. The authors fit the available neutron-diffraction data for Pu-Ga alloys to the invar model and discuss the consequences. For more information, contact Andrew C. Lawson, Structure and Properties Group: MST-8, MS H-805, Los Alamos National Laboratory, Los Alamos, New Mexico 87545; (505) 667-8844; fax (505) 665-2676; e-mail lawson@lanl.gov.     

The use of severe plastic deformation techniques in grain refinement

Severe plastic deformation (SPD) has emerged as a promising method to produce ultrafine-grained materials with attractive properties. Today, SPD techniques are rapidly developing and are on the verge of moving from lab-scale research into commercial production. This paper discusses new trends in the development of SPD techniques suchas high-pressure torsion and equal-channel angle pressing, as well as new alternative techniques for introducing SPD. The paper also contains a comparative analysis of SPD techniques in terms of their relative capabilities for grain refinement, enhancement of properties, and potential to economically produce ultrafine-grained metals and alloys. For more information, contact Terry C. Lowe, Science and Technology Base Programs, Los Alamos National Laboratory, Los Alamos, NM 87545; (505) 667-7824; fax (505) 665-3199; e-mail tlowe@lanl.gov.     

Using the modified embedded-atom method to calculate the properties of Pu-Ga alloys

In this article, the semi-empirical modified embedded atom method is used to develop a model of Pu-Ga alloys. Employing classical calculations, the model is used to predict thermodynamic properties of these alloys as well as the complex Pu-Ga phase diagram. For more information contact M.I. Baskes, Los Alamos National Laboratory, P.O. Box 1663, MS-G755, Los Alamos, NM 87545, USA; (505) 667-1238; fax (505) 667-8021; baskes@lanl.gov.     

An atomistic study of solid/liquid interfaces in binary systems

In this study, a semi-empirical Lennard-Jones/embedded atom method model is used to capture real materials behavior through the introduction of many-body forces. By means of molecular dynamics calculations, the model is used to study the dependence of the solid-liquid interface velocity on temperature for two alloy compositions. For more information contact M.I. Baskes, Los Alamos National Laboratory, PO Box 1663, MS-G755, Los Alamos, NM 87545, USA; (505) 667-1238; fax (505) 667-8021; baskes@lanl.gov.     

The fundamentals of nanostructured materials processed by severe plastic deformation

Nanostructured materials produced by severe plastic deformation (SPD) are 100% dense, contamination-free, and sufficiently large for use in real commercial structural applications. These materials are found to have high strength, good ductility, superior superplasticity, a low friction coefficient, high wear resistance, enhanced high-cycle fatigue life, and good corrosion resistance. This article reviews the structures and properties of nanostructured materials produced by SPD and reports recent progress in determining the deformation mechanisms that lead to these superior mechanical properties. For more information, contact Yuntian T. Zhu, Los Alamos National Laboratory, Materials Science and Technology Division, Los Alamos, NM 87545; (505) 667-4029; fax (505) 667-2264; e-mail yzhu@lanl.gov.     

A brief history of TEM observations of plutonium and its alloys

Transmission electron microscopy (TEM) has proven to be a useful tool to investigate the morphological and crystallographic nature of plutonium metal and alloys. Its unique ability to provide direct visual as well as crystallographic information on a very fine scale has given new insight into the complex nature of plutonium. Because of the limited number of TEM observations performed to date, there is little doubt that this metal’s most intriguing microstructural secrets are yet to be revealed. For more information, contact T.G. Zocco, Los Alamos National Laboratory, NMT-10, Manufacturing Process Science and Technology Group, P.O. Box 1663, Mail Stop E506, Los Alamos, NM 87545; (505) 667-4481; fax (505) 667-8528; e-mail zocco@lanl.gov.     

Plutonium: Aging mechanisms and weapon pit lifetime assessment

Planning for future refurbishment and manufacturing needs of the U.S. nuclear weapons complex critically depends on credible estimates for component lifetimes. One of the most important of these components is the pit, that portion of the weapon that contains the fissile element plutonium. The U.S. government has proposed construction of a new Modern Pit Facility, and a key variable in planning both the size and schedule for this facility is the minimum estimated lifetime for stockpile pits. This article describes the current understanding of aging effects in plutonium, provides a lifetime estimate range, and outlines in some detail methodology that will improve this estimate over the next few years. For more information, contact J.C. Martz, Los Alamos National Laboratory, Enhanced Surveillance, MST-DO, Los Alamos, New Mexico 87545; (505) 667-2323; e-mail jmartz@lanl.gov. Editor’s Note: A hypertext-enhanced version of this article is available on-line at www.tms.org/pubs/journals/JOM/0309/Martz-0309.html     

The corrosion of materials in spallation neutron sources

Efforts to measure the real-time corrosion rates of alloy 718 during 800 MeV proton radiation at currents up to 1 mA are reported. Specially designed corrosion probes, which incorporate ceramic seals, were mounted in a water manifold that allowed samples to be directly exposed to the proton beam at the Los Alamos Neutron Science Center. The water system that supplied the manifold provided a means for controlling water chemistry, measuring dissolved hydrogen concentration, and measuring the effects of water radiolysis and water quality on corrosion rate. Real-time corrosion rate measurements during proton irradiation showed an exponential increase in corrosion rate with proton-beam current. These results are discussed within the context of water radiolysis at the diffusion boundary layer/beam-spot interface. However, additional factors that may influence these parameters, such as oxide spallation and charge build-up in the passive film, are not ruled out. Scott Lillard earned his Ph.D. in materials science and engineering from Johns Hopkins University in 1992. He is currently a technical staff member at the Materials Corrosion and Environmental Effects Laboratory, Los Alamos National Laboratory. Darryl P. Butt earned his Ph.D. in ceramic science from Pennsylvania State University in 1991. He is currently a technical staff member at the Non-Proliferation and International Security Division, Los Alamos National Laboratory.     

Developing impact and fatigue damage prognosis solutions for composites

An approach to developing a damage prognosis solution that integrates advanced sensing technology, data interrogation procedures for damage detection, novel model validation and uncertainty quantification techniques, and reliability-based decision-making algorithms is summarized in this article. In parallel, experimental efforts are underway to deliver a proof-of-principle technology demonstration by assessing impact damage and predicting the subsequent fatigue damage accumulation in a composite plate. This article provides an overview of the various technologies that are being integrated to address this damage prognosis problem. Editor’s Note: Presentation of this paper is supported by the Air Force Research Laboratory, under agreement number F33615-01-D-5801. The U.S. Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Research Laboratory or the U.S. Government. For more information, contact Charles Farrar, Los Alamos National Laboratory, MST-006, Los Alamos, NM 87545; e-mail farrar@lanl.gov.     

The Nb-Pu (niobium-plutonium) system

Work supported by a contract from Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and ASM International. Literature searched through September 1991.     

Truchas – a multi-physics tool for casting simulation

Abstract

The Truchas code was developed at Los Alamos National Laboratory under the Advanced Simulation and Computing Program. This open source multi-physics simulation software is designed to run in a scalable parallel computing environment. The capabilities of the code and numerical implementation are briefly described. The advantages and limitations of large three dimensional simulations will be discussed, and two example simulations are shown that demonstrate the utility of the fluid flow, heat transfer, phase change and solid mechanics capabilities. Validation of a code such as Truchas is a difficult task because of the complexity of the coupling between different physical phenomena being modelled and the poor understanding of phenomena such as heat transfer across interfaces. The challenges associated with verification and validation of complex simulation tools and integration into the design process are also discussed.     

Texture changes in the hcp → bcc → hcp transformation of zirconium studied in situ by neutron diffraction

The crystallographic texture of hot-rolled polycrystalline zirconium has been studied below and above the hcp–bcc transition temperature with HIPPO, the new time-of-flight neutron diffractometer at Los Alamos Neutron Science Center, making use of the multidetector capabilities and a vacuum furnace. Incomplete pole figures were extracted from diffraction spectra to determine the orientation distribution function and recalculate complete pole figures in situ at various temperatures. The texture analysis reveals that the orientation of grains in the new high-temperature (bcc) phase is related to the texture of the low-temperature (hcp) phase by Burgers relation, but with both an orientation selection and a symmetry variant selection. The cubic transformation texture is best explained if we assume preferential nucleation of the bcc phase in the hcp grain orientations that are most subject to mechanical twinning. After cooling, the new hcp texture closely resembles the original texture. Thermal cycling repeats this process with slight strengthening of the texture. The hexagonal transformation texture (after cooling) may be caused by nucleation and growth of untransformed domains or through variant selection by stresses imposed by neighboring grains.     

Neutron diffraction study of the deformation behavior of beryllium–aluminum composites

Neutron diffraction measurements of the mean phase elastic strains during loading were performed on two different beryllium–aluminum composites. The first was Be–47.5Al–2.5Ag produced from hot isostatically pressed powder fabricated by rapid solidification using a centrifugal atomization process; the second was a commercially available Be–38Al extruded powder product. Under rapid solidification, Be–Al undergoes liquid phase separation, resulting in a three-dimensional interpenetrating morphology in which both phases are continuous. Elastic strains in the individual phases were measured as a function of tensile and compressive load at the Los Alamos Neutron Scattering Center using a pulsed neutron source. The deformation behavior was unusual due to the unique composite microstructure and elastic properties of beryllium. Notably, the aluminum developed compressive elastic strains in the direction perpendicular to the compressive loading axis. These strains were compared with predictions from finite element analyses to clarify the stress state in each phase during deformation.     

Stability and thermodynamic properties of supersaturated solid solution and amorphous phase formed by ball milling in the Zr- Al system

Zr ₁₀₀-_xA1_x (x ≤ 40) metastable alloys were synthesized by high- energy ball milling of elemental Zr and Al powders: supersaturated substitutional cph solid solution for x ≤ 15 and an amorphous phase for x ≥ 17.5. We performed a calorimetric study of the thermodynamics and kinetics of the metastable- to- equilibrium transformations of these phases. Their stability range (temperature/composition), as well as the apparent activation energies associated with the transformations, were determined. The transformation enthalpies were measured and used to determine the enthalpy of formation for these metastable phases. For both as- milled and relaxed amorphous phases, the measured enthalpy of crystallization is compared with those estimated for an undercooled liquid. Different amounts of retained entropy at the glass transition temperature were used to estimate the enthalpy loss upon undercooling due to the excess specific heat. This paper was presented at the Thermodynamics and Phase Equilibria of Metastable Phases Symposium at the Spring TMS Meeting, March 1-4,1992, in San Diego. The symposium was organized by Philip Nash, Illinois Institute of Technology, and Ricardo Schwarz, Los Alamos National Laboratory.     

An odyssey through the rare earths. A quest for knowledge

Research highlights on rare earth materials are described from the earliest days as a graduate student during the mid 1950s, through the six years at Los Alamos, to the last 28 at Iowa State University. The early work was concerned with the rare earth carbides which led to an appreciation of systematics of the properties of rare earth materials. Use of the systematic variation of properties and behaviors is a powerful tool in understanding the nature of certain phenomena - in particular solid solution formation and the role of 4f electrons in bonding (4f hybridization). Some of the anomalous 4f properties of cerium metal and cerium compounds, and the quenching of spin fluctuations in exchanged enhanced materials, are also discussed.     

The alpha prime to delta reversion transformation in Pu-Ga alloys

The α′→δ reversion transformation in stabilized δ phase Pu-Ga alloys containing 0.5 wt.% and 0.6 wt.% Ga was studied using dilatometry and differential scanning calorimetry. The α′→δ reversion was observed to occur as a series of transformation bursts spread over a 20°C to 40°C temperature range. The enthalpy of the endothermic α′→δ reversion transformation was estimated to be approximately 0.56 J/g for the 0.6 wt.% Ga alloy. Lower gallium was found to raise the α′→δ reversion temperature in specimens cooled to −155°C from 44–48°C for 0.6 wt.% Ga to 92°C for 0.5 wt.% Ga. Repeated cycling to −155°C caused the reversion onset temperature to decrease by ∼10°C. Raising the minimum hold temperature from −150°C to −125°C caused the reversion onset temperature to decrease by 5–10°C. Annealing at 300–400°C was required to erase all traces of the effects of cold exposure; material heated to only 200°C after thermal cycling displayed decreased reversion onset temperatures during subsequent thermal cycling. For more information, contact D.S. Schwartz, Los Alamos National Laboratory, NMT Division, Los Alamos, New Mexico 87545.     

Engineering Applications of Time-of-Flight Neutron Diffraction

Time-of-flight neutron diffraction is widely used in characterizing the microstructure and mechanical response of heterogeneous systems. Microstructural characterization techniques include spatial or temporal mapping of the phases and determination of grain size, dislocation structure, and grain orientations (texture) within these phases. Mechanical response analysis utilizes the crystallographic selectivity of the diffraction process to measure the partitioning of strain within the system. The microstructural and mechanical response information is then used to develop more realistic constitutive models. In this article we review some examples of such measurements, based on our experiences at the Lujan Center of Los Alamos National Laboratory.     

Evolution of nanoscale porosity during equal-channel angular pressing of titanium

Small-angle neutron scattering (SANS) analysis and transmission electron microscopy evidence suggest the occurrence of nanoscale porosity in commercial-purity titanium processed by equal-channel angular pressing (ECAP). SANS data were produced at two different facilities (GKSS, Germany; and Los Alamos, USA) and were analysed using three different methods. The results are consistent and yield a conclusive picture of the distribution of the scattering centres, which are believed to be associated with nanoporosity. Back pressure applied during ECAP tends to reduce the average pore size, which also depends on the processing route used. The results of the study strongly suggest that ECAP leaves a footprint in titanium in the form of a population of polydispersed nanovoids, which may play an important role in subsequent processing of the material.