Pu-Ga Alloys Research Articles

Water accelerates parabolic oxidation of Pu-Ga alloy: from mechanism to model

The moisture accelerated oxidation can endanger the safety storage of plutonium, but experiments haven’t provided a complete mechanism explanation, let alone a prediction model. Based on first principles calculations, this work reveals the acceleration mechanism of water on Pu-Ga alloy oxidation in moist air and interprets available experimental findings. Specifically, the OH adsorption after H2O dissociation can promote the O-vacancy formation in PuO2 by the polaron interaction mechanism, and then facilitate the vacancy diffusion of the actual corrosion medium O after O2 dissociation. Therefore, the diffusion kinetics of O in Pu-oxide is decisive, which is correlated to the relative humidity (RH) and temperature (T). Based on the micro mechanisms and the available experiments, we propose a method to modify the oxidation kinetic model by taking into account the coupling effects of RH and T, this model provides a new way to evaluate and predicts the acceleration ratio of RH and T on the oxidation kinetics of Pu-Ga alloy. This work comprehensively reveals the acceleration mechanism of moisture and then proposes the oxidation kinetic model, which is significant to the storage and protective of nuclear materials.

The evolution of radiation-induced point defects near symmetrical tilt Σ5 (3 1 0) 〈0 0 1〉 grain boundary in Ga stabilised δ-Pu: A molecular dynamics study

Molecular dynamics (MD) simulations were performed to investigate the evolution of radiation-induced point defects affected by the symmetrical tilt Σ5 (310) 〈001〉 grain boundary (GB) in Ga stabilised δ-Pu. From the perspective of formation energy, the formation energy of vacancies had no prominent change in the whole region, but it was lower than the formation energy of interstitial atoms. The formation energy of interstitial atoms near GB was lower than that in the bulk. On the other hand, the average formation energy of interstitial Pu atoms was greater than that of interstitial Ga atoms. Furthermore, by comparing the evolution of radiation-induced point defects when primary knock-on atom (PKA) was located at −25 Å and 25 Å far from the GB, respectively, and analyzing the tendency of the number of remaining defects with the distance between PKA and GB changing, it could be found that interstitial atoms were more easily trapped in Σ5 (310) 〈001〉 GB than vacancies for Pu-Ga alloy. Simultaneously, the influence of temperature on the number of point defects near GB was taken into consideration, and the results displayed that the number of residual point defects in Pu-Ga alloy with Σ5 (310) 〈001〉 GB showed a decreasing trend with temperature increasing because temperature had an effect on the diffusion of point defects. Remaining interstitial atoms rather than remaining vacancies, moreover, are more severely affected by temperature, for interstitial atoms were more easily absorbed by GB.

Long-term behavior of vacancy defects in Pu-Ga alloy: Effects of temperature and Ga concentration

Vacancy and its clusters are among the most important defects induced by self-irradiation in plutonium-gallium (Pu-Ga) alloys. For decades-long stockpiles, the generation and evolution of vacancy defects could cause void swelling and helium bubble formation, resulting in the ageing of the Pu-Ga alloys. Therefore, to shed light on the ageing mechanisms caused by vacancies in Pu-Ga alloys, the long-term behaviour of vacancy defects in Pu-Ga alloys is simulated employing an AKMC model parameterized by molecular statics calculations. By tracking the number of vacancy defects, the size distribution of vacancy clusters, and the largest vacancy clusters over time, we found that temperature and Ga concentration significantly influence the evolution of vacancy defects in Pu-Ga alloys. Temperature changes could affect the clustering behaviour of mono vacancies, and critical temperatures initializing the nucleation of vacancy clusters are observed. On the other hand, adding Ga contents in Pu-Ga alloys could increase the migration energy of and attractive interaction among vacancies, leading to increased vacancy cluster numbers at high temperatures.

Parabolic oxidation kinetics of a plutonium alloy at room temperature

Auger electron spectroscopy (AES) was used to characterize the oxide scale formed on an ∼0.5 wt. % Ga plutonium metal alloy. Analysis of changes to the characteristic Auger transitions for Pu and O during sputter depth profiles were used in combination with previously measured sputter yields to determine the thickness of the oxide scale and provide insight into its chemistry. From this data, the parabolic kinetics for the Pu-Ga alloy were determined as a function of relative humidity, RH = 14–92%, and oxygen partial pressure, p(O2) = 31–157 Torr, at 25 °C. For each relative humidity investigated, the parabolic rate constant was found to be independent of oxygen partial pressure. Power-law fits to the relative humidity effect on the rate constant was found to have a power of n = 1.265, indicating the diffusion species which rate-limits the reaction isn’t molecular water, which subsequent dissociates at the oxide-metal interface, or hydroxyl from dissociation at the gas-surface interface.

Selective Actinides Recovery over Lanthanides Using Liquid Gallium Cathode

1. Introduction Pyroprocessing is a promising technology for a fast reactor cycle with a highly concentrated minor actinides (MA: Np, Am and Cm)-bearing metallic fuel. Separation of actinides (An) from fission products such as lanthanide elements (Ln) is one of the difficult challenges to establish practical pyroprocessing. Liquid Cd cathode (LCC) with LiCl-KCl bath has been generally used for An recovery in our process [1], however, An/Ln separation performance using LCC was not high enough to fabricate a highly concentrated MA-bearing fuel from materials having a high ratio of Ln/An. Our recent studies showed that a combination of liquid Ga cathode with LiCl-KCl bath had potential to give a higher An/Ln separation performance, and currently we are developing an innovative pyroprocessing using the liquid Ga electrode. Solubility of U and Pu in liquid Ga is smaller than in liquid Cd. Therefore, the solid precipitates (An-Ga alloy) formation could have a significant influence on designing the process compared to the case of using LCC. However, the influence of the solid precipitates formation on An/Ln separation performance is unclear.In this study, galvanostatic electrolysis test on the recovery of U, Pu and Am in liquid Ga was carried out to investigate the influence. Precipitation of Pu-Ga alloy was observed by SEM/EDX on the cross section of Ga electrode after the electrolysis, and separation factors (SFs) for each metallic elements (M) based on Ce, which is defined as (M/Ce concentration ratio in liquid Ga)/(M/Ce concentration ratio in the salt phase), were calculated to evaluate An/Ln separation performance. 2. Experiment All experiments were performed in glove boxes with high-purity Ar atmosphere. Cd-0.1wt%Li alloy anode, liquid Ga cathode and Ag/AgCl reference electrode were used in LiCl-KCl eutectic melt at 773K. Two runs of galvanostatic electrolysis experiments were carried out applying the constant current of 5~10 mA (cathodic current densities: 3.76~7.53 mA/cm2). In the first experiment, An were recovered in the liquid Ga electrode from LiCl-KCl melt containing An chlorides, where concentrations of recovered An in the liquid Ga were almost the solubility. In the second experiment, two times larger amounts of An than the solubility were recovered in liquid Ga from LiCl-KCl melt containing both An and Ln chlorides. After the electrolysis, the Ga and the molten salt were sampled and dissolved into nitric acid. Concentrations of elements in the samples were analyzed by ICP-AES. γ spectrometry was also used to analyze Am concentration in samples. Cross section of solidified Ga electrode was observed by SEM/EDX. 3. Results and discussion It was confirmed that SFs of An seen in Table 1 were around ten times higher values than those obtained in the case of LCC. Furthermore, the SFs of An in the case of the amount of An recovered in liquid Ga exceeding solubility (second experiment) were comparable to those in the case of the An concentration below solubility. The SEM/EDX observation showed that grain shaped precipitates with 1～10 μm was formed homogeneously inside the Ga and the size of the precipitates increased with the amount of recovered An in liquid Ga. Chemical form of the precipitates was assigned to be Pu-Ga alloy by the EDX. It was also found that the precipitates did not accumulate on the surface of liquid Ga. In the present experimental conditions, the amount of An in the liquid Ga had an influence on the precipitation formation, while the precipitation had no influence on An/Ln separation performance.AcknowledgementThis work is supported by the Innovative Nuclear Research and Development Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Reference [1] S. Kitawaki, et al. Nucl. Technol., 162 (2008) 118-123.[2] T. Murakami et al., J. Nucl. Radiochem. Sci., 16 (2016) 5-10. Figure 1

Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.

Structure and mechanical properties of Ce-La alloys containing 3-10 wt. % La

This paper describes a study of the microstructure and mechanical properties of a range of cerium-lanthanum (Ce-La) alloys with La contents of between 3 wt. % and 10 wt. %. Ce-La alloys are regarded as non-radioactive surrogates for delta-phase plutonium (δ-Pu) alloys. The Ce-La alloys were characterised using optical microscopy, energy dispersive spectroscopy and X-ray diffraction while the mechanical properties were determined using micro-hardness, ultrasonic velocity and nanoindentation measurements. The presence of La appears to impart a degree of solid solution hardening to the alloys, as indicated by the modest increase in micro-hardness with increasing La content; a small increase in elastic modulus with La content was also seen. The hardness and elastic modulus values of the Ce-La alloys in this study are similar to those of homogenized Pu-Ga alloys in the Ga-lean part of the δ-Pu phase field. Despite the propensity of the Ce-La alloys to undergo oxidation in air, the oxide thicknesses of the alloys, which were determined by energy dispersive spectroscopy, were found to be insufficient to exert a significant influence on the nanoindentation measurements.

Development of a CALPHAD Thermodynamic Database for Pu-U-Fe-Ga Alloys

The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron is of interest to understand the impurity effects on phase stability. A newly developed and self-consistent CALPHAD thermodynamic database is presented which covers the elements: Pu, U, Fe, Ga across their whole composition and temperature ranges. The phase diagram and thermodynamic properties of plutonium-iron (Pu-Fe) and uranium-iron (U-Fe) systems are successfully reassessed, with emphasis on the actinide rich side. Density functional theory (DFT) calculations are performed to validate the stability of the stoichiometric (Pu,U)6Fe and (Pu,U)Fe2 compounds by computing their formation enthalpies. These data are combined to construct the Pu-U-Fe ternary phase diagram. The thermodynamic assessment of Fe-Ga is presented for the first time and application to the quaternary Pu-U-Fe-Ga system is discussed.

Influence mechanisms for phase stability of Pu-Ga alloys: A multi-scale review

In past two decades, we have witnessed significant improvements in theoretical and experimental investigation for metallic Pu, its compounds and alloys, in particular self-irradiation induced aging mechanism, electronic structure, and surface corrosion, however, huge challenges still exist for the phase stability of these Pu-based systems which are sensitive to external factors such as temperature, pressure and chemical doping. In this work we have reviewed the recent progress on the multi-scale characterization for the electronic, structural and magnetic properties related to the phase stability of Pu-based systems, in particular Pu-Ga alloy, such as the phase stability induced aging mechanism, phase diagram, phase transformation, double C curve kinetics of time–temperature-transformation (TTT) diagram for δ → α′ martensitic transformation, and thermodynamical calculation. Finally, future directions on the phase stability of these Pu-based systems are also discussed.

Energetics and diffusion of point defects in Au/Ag metals: A molecular dynamics study**Project supported by the National Natural Science Foundation of China (Grant Nos. 51401237, 11474358, and 51271198), the Fund from Shaanxi Provincial Education Department, China (Grant No. 18JK1207), and the Defence Technology Foundation of China (Grant No. 2301003).

To reveal the potential aging mechanism for self-irradiation in Pu–Ga alloy, we choose Au–Ag alloy as its substitutional material in terms of its mass density and lattice structure. As a first step for understanding the microscopic behavior of point defects in Au–Ag alloy, we perform a molecular dynamics (MD) simulation on energetics and diffusion of point defects in Au and Ag metal. Our results indicate that the octahedral self-interstitial atom (SIA) is more stable than the tetrahedral SIA. The stability sequence of point defects for He atom in Au/Ag is: substitutional site octahedral interstitial site tetrahedral interstitial site. The He–V cluster (, V denotes vacancy) is the most stable at n = m. For the mono-vacancy diffusion, the MD calculation shows that the first nearest neighbour (1NN) site is the most favorable site on the basis of the nudged elastic band (NEB) calculation, which is in agreement with previous experimental data. There are two peaks for the second nearest neighbour (2NN) and the third nearest neighbour (3NN) diffusion curve in octahedral interstitial site for He atom, indicating that the 2NN and 3NN diffusion for octahedral SIA would undergo an intermediate defect structure similar to the 1NN site. The 3NN diffusion for the tetrahedral SIA and He atom would undergo an intermediate site in analogy to its initial structure. For diffusion of point defects, the vacancy, SIA, He atom and He–V cluster may have an analogous effect on the diffusion velocity in Ag.

Shear strength of nanocrystalline δ-phase Pu-Ga alloys: Atomistic simulations

The mechanical properties of nanocrystalline materials differ dramatically from those of conventional ones. In particular, the yield strength noticeably grows as grain size decreases. On the other hand, the grain size is not the only factor affecting the yield strength. Extended ingrain defects such as dislocations and stacking faults also contribute to the strength of materials. Modification of the mechanical properties of plutonium and its alloys via grain refinement is of interest because of their use in civil and military nuclear technologies, e.g., as a component of metallic nuclear fuel. But the experimental study of plutonium is hampered by its high chemical and radiological activity harmful to human health. That is why theoretical and computational techniques proved to be useful for investigation into plutonium properties must be applied first, prior to experimental ones. In the paper the atomistic simulation approach is applied to directly calculate the mechanical properties of nanocrystalline face centered cubic δ-phase Pu-Ga alloys, in particular, the ambient conditions quasi-static yield stress dependence on grain size and extended defect concentration. The range of grain sizes studied is 40−200nm. A deviation from the Hall-Petch relation is demonstrated. The effect of the alloying addition redistribution inside grains as well as between the grains is also evaluated.

A molecular dynamics simulation of energetics and diffusion of point defects in a Au–Ag alloy

For revealing an aging mechanism for self-irradiation in a Pu–Ga alloy, we carried out a molecular dynamics (MD) simulation on a substitutional material, i.e., Au–Ag alloy. In this work, we estimate physical and microscopic properties of the Au–Ag alloy containing various point defects using a MD method, in particular, formation energy for point defects, migration energy for point defects diffusion into interstitial sites, and diffusion coefficient for the Au–Ag alloy containing point defects, such as vacancy, He atom and He-vacancy (He-V) cluster. The results indicate that volumetric heat capacity and linear expansion coefficient would decrease due to the various point defects, and He atom has the most remarkable influence on the physical properties of the Au–Ag alloy for point defects considered in this work. The formation energy of Au and Ag self-interstitial atom indicates that Octa1 is the most stable site, and structural stability of octahedral (Octa) interstitial sites for the He atom obeys \(\hbox {Octa1}> \hbox {Octa2}> \hbox {Octa4} > \hbox {Octa3}\). For the \(\hbox {He}_{n}\hbox {V}_{m}\) cluster, the formation energy of the defect structure is most stable at \(n = m\). The diffusion coefficient of the He-V cluster is relatively smaller, showing that vacancy defects would further decrease atomic diffusion. An influence of various point defects on the diffusion velocity in the Au–Ag alloy obeys the He-V \(\hbox {cluster}> \hbox {He}> \hbox {vacancy}> \hbox {Ag} > \hbox {Au}\).

Probing He bubbles in naturally aged and annealed δ-Pu alloys using ultra-small-angle x-ray scattering

The self-irradiation of Pu alloys generates He that is trapped within the metal matrix in the form of He bubbles. The distribution of these He bubbles in δ-phase Pu-Ga alloys exhibits a peak near a radius of 0.7 nm, and this size is remarkably stable as function of time. When annealed, the He bubbles in δ-Pu alloys grow, coarsening the distribution. However, the magnitude of this coarsening is uncertain, as different experimental methods reveal bubbles that differ by at least one order of magnitude. Small-angle x-ray scattering results, which can probe a wide range of bubble sizes, imply only a mild coarsening of the He bubble distribution for an annealing treatment of 425 ∘C for 24 h, and analysis of the He bubble content suggests that He is actually lost from the bubbles with annealing.

Atomistic simulations of dislocation dynamics in δ-Pu-Ga alloys

Molecular dynamics with the modified embedded atom model (MEAM) for interatomic interaction is applied to direct simulations of dislocation dynamics in fcc δ-phase Pu-Ga alloys. First, parameters of the MEAM potential are fitted to accurately reproduce experimental phonon dispersion curves and phonon density of states at ambient conditions. Then the stress-velocity dependence for edge dislocations as well as Pierls stress are obtained in direct MD modeling of dislocation motion using the shear stress relaxation technique. The simulations are performed for different gallium concentrations and the dependence of static yield stress on Ga concentration derived demonstrates good agreement with experimental data. Finally, the influence of radiation defects (primary radiation defects, nano-pores, and radiogenic helium bubbles) on dislocation dynamics is investigated. It is demonstrated that uniformly distributed vacancies and nano-pores have little effect on dislocation dynamics in comparison with that of helium bubbles. The results of the MD simulations evidence that the accumulation of the radiogenic helium in the form of nanometer-sized bubbles is the main factor affecting strength properties during long-term storage. The calculated dependence of static yield stress on helium bubbles concentration for fcc Pu 1wt.% Ga is in good agreement with that obtained in experiments on accelerated aging. The developed technique of static yield stress evaluation is applicable to δ-phase Pu-Ga alloys with arbitrary Ga concentrations.

Thermal cycling as a novel method to experimentally derive the “equilibrium” Pu–Ga phase diagram

The feasibility of using thermal cycling to experimentally derive the “equilibrium” Pu–Ga phase diagram in the Pu-rich end is discussed, in particular the effect of thermal cycling on the decomposition of δ-stabilized Pu alloys at ambient pressure. This is an alternative to the Russian method and was originally proposed by the author on the basis of a thorough analysis of data from a systematic thermal cycling experiment performed on annealed and aged δ-stabilized Pu–Ga alloys. These were subject to repetitive thermal cycling in the temperature range 25–180 °C, which was found to drive and accelerate the Pu–Ga system towards its “equilibrium” state, as demonstrated by continuous α-phase in-growth with each thermal cycle. An experimental programme to confirm these initial observations is proposed, and the “constrained” equilibrium Pu–Ga phase diagram that might be expected from this method is outlined.

Phase stability and mechanical property of gallium-stabilized <x content-type="symbol">δ</x>-plutonium

Plutonium (Pu) is a very important nuclear material, acting as a key fissile component in nuclear weapons as well as in the uranium Pu mixed oxide fuel. The existence of six equilibrium solid phases at ambient pressure in its phase diagram makes Pu one of the most complex elements in the Periodic Table. The phase stability and mechanical properties play key roles in its application in nuclear industry. As a radioactive element, Pu is vulnerable to aging, decaying to uranium by emitting an α particle. This process eventually results in abundant crystal defects and helium bubbles. To maintain the effectiveness of Pu-based materials in practical applications, it is necessary to study the mechanical and structural properties of Pu. However, it is challenging to obtain the atomistic mechanism of the evolution of Pu-based materials in experimental studies due to the limitations of spatial and temporal resolution ratio. Recently, numerical simulation especially classical molecular dynamics simulation has become an important tool to study the basic properties of materials at nanoscale. The two phases of greatest interest are the monoclinic α -phase, which is the basic form of unalloyed Pu at room temperature, and the face-centered-cubic (FCC) δ -phase, which can be stabilized to room temperature by alloying with a few atomic percent (at.%) of aluminum (Al) or gallium (Ga). In this work, a molecular dynamics simulation model is set up to investigate the phase stability of Ga-stabilized FCC phase Pu ( δ -Pu). A widely accepted modified embedded atom model (MEAM) potentials are implemented to describe the interactions of the binary system using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The results indicate that unalloyed Pu deviates substantially from a typical FCC structure with lattice distortion and domain decomposition at room temperature and ambient pressure, while it is much more stable at 700 K. Furthermore, it is found that doping Ga can remarkably improve the phase stability of Pu at room temperature, and the stability increases with the atomic percent of Ga in Pu-Ga alloy. Energetically, the average potential energy per atom and the total potential energy decreases obviously with increasing the atomic percentage of Ga in the alloy. Based on the stability results above, we study the elastic property of Ga-stabilized δ -Pu alloy. According to our simulation results, δ -Pu is highly anisotropic with a Zener Index of 6.89, a much higher value than those of other metals with FCC structure such as copper, aluminum and gold. In addition, we perform uniaxial tension on single crystal nanowires with different crystallographic orientations and transverse directions. The elastic moduli of the typical low index crystal orientations [100], [110] and [111] are calculated to be 12.86, 33.47 and 74.37 GPa, respectively. Our simulation results reveal that increasing the concentration of Ga can decrease the anisotropy but increase the elastic moduli conversely, which is in good agreement with available experimental data for non-aged Ga stabilized polycrystalline δ -Pu. It is expected that our findings could lead to an improvement of fundamental understanding of the phase stability and anisotropy in Pu-Ga alloy, which is of great significance to understand the self-radiation effects and aging mechanisms of Pu-based nuclear materials.

Equilibrium thermodynamics of radiation defect clusters in δ-phase Pu–Ga alloys

The paper presents a theoretical investigation into the response of δ-phase Pu–Ga alloys to self-irradiation. Using classical molecular dynamics we investigate the long-term behavior of primary radiation defects (vacancies) in the face-centered cubic lattice of the alloys under ambient conditions. High diffusive migration energy barriers and the corresponding low mobility of vacancies do not allow us to track their dynamics in the lattice by direct molecular dynamics simulations. Instead, we use the Helmholtz free energy to investigate the equilibrium thermodynamics of metastable microconfigurations of Pu–Ga crystals with artificially introduced vacancy clusters in various regular and random configurations. The Helmholtz free energy of the microconfigurations are calculated with the thermodynamic integration method. Based on the free energy evaluation we draw conclusions about the relative thermodynamic stability of various microconfigurations under ambient conditions. The equilibrium parameters of vacancy clusters in the bulk of the lattice and in the presence of edge dislocations are estimated.

MD study of the finite temperature effects on the phase ordering, stacking fault energy, and edge dislocation core structure in elemental Pu and Pu–Ga alloys

The Modified Embedded Atom Model (MEAM) of elemental plutonium and plutonium–gallium alloys has been tested for its ability to reproduce the correct ordering of the hcp and fcc phases that is crucial from the point of view of molecular dynamics simulation of elastic–plastic phenomena in the material. Stacking fault energy obtained with the MEAM is in agreement with experimental data. Results of the edge dislocation modeling at the ambient conditions evidence for rather wide dislocation core, namely, 5–6 Burgers vectors. The results of the MD simulation have been compared with those obtained early with recently developed Multi State-MEAM potential.

Equilibrium thermodynamics of helium in [formula omitted]-phase Pu–Ga alloys

The paper presents a theoretical study of the response of δ-phase Pu–Ga alloys to self-irradiation. We investigate the long-term behaviour of He atoms in the face-centred cubic (fcc) lattice under ambient conditions by means of classical molecular dynamics. Because of the relatively low diffusive mobility of He atoms in the lattice their dynamics cannot be tracked directly in molecular dynamics simulations. Instead, we use the Helmholtz free energy function to investigate the equilibrium thermodynamics of metastable microconfigurations of perfect crystals with artificially introduced He atoms in different configurations. The Helmholtz free energy of the microconfigurations are calculated using the thermodynamic integration method. Based on the free energy evaluation, inferences about the relative thermodynamic stability of various microconfigurations under ambient conditions are drawn. Estimates of the equilibrium parameters of He bubbles in the lattice and the distribution of He bubble radii are calculated and compared to those observed experimentally.

Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastableδfcc Pu-Ga alloys

The scientifically fascinating question of the spatial extent and bonding of the 5$f$ orbitals of Pu and its six different phases extends to its \ensuremath{\delta}-retained alloys and the mechanism by which Ga and a number of other unrelated elements stabilize its low density face-centered-cubic (fcc) structure. This issue of phase stability is also important technologically because of its significance to Science-Based Stockpile Stewardship. Answering these questions requires information on the local order and structure around the Ga and its effects on the Pu. We have addressed this by characterizing the structures of a large number of Pu-Ga and two Pu-In and one Pu-Ce \ensuremath{\delta} alloys, including a set of high purity \ensuremath{\delta} Pu${}_{1\ensuremath{-}x}$Ga${}_{x}$ materials with 1.7 \ensuremath{\le} $x$ \ensuremath{\le} 6.4 at. % Ga that span the low [Ga] portion of the \ensuremath{\delta} region of the phase diagram across the \ensuremath{\sim}3.3 at. % Ga metastability boundary, with extended x-ray absorption fine structure (EXAFS) spectroscopy that probes the element specific local structure, supplemented by x-ray pair distribution function analysis that gives the total local structure to longer distances, and x-ray diffraction that gives the long-range average structure of the periodic component of the materials. Detailed analyses indicate that the alloys at and below a nominal composition of \ensuremath{\sim}3.3 at. % Ga are heterogeneous and in addition to the \ensuremath{\delta} phase also contain up to \ensuremath{\sim}20% of a novel, coexisting ``\ensuremath{\sigma}'' structure for Pu that forms in nanometer scale domains that are locally depleted in Ga. The invariance of the Ga EXAFS with composition indicates that this \ensuremath{\sigma} structure forms in Ga-depleted domains that result from the Ga atoms in the \ensuremath{\delta} phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu${}_{25\ensuremath{-}35}$Ga so that \ensuremath{\delta} Pu-Ga is neither a random solid solution nor the more stable Pu${}_{3}$Ga + \ensuremath{\alpha}. Above this 3.3 at. % Ga nominal composition, the \ensuremath{\delta} Pu-Ga alloy is homogeneous, and no \ensuremath{\sigma} phase is present. These results that demonstrate that collective and cooperative behavior in the interactions between the alloy elements as well as local elastic forces are crucial in determining the properties of complex materials and contradict the conventional mechanism for martensitic transformations, in this case indicating that nucleation is not the rate limiting step.