Pu-Ga Alloys Research Articles

Phase transformation in δ Pu alloys at low temperature: In situ dilatometric study

The purpose of this work is to precisely study the martensitic transformation in a plutonium-gallium alloy. Thus, the thermodynamics and kinetics of the δ→α'+δ phase transformation in a Pu-Ga alloy were studied under isochronal and isothermal conditions. The activation energy of the δ→α'+δ phase transformation at a constant cooling rate (0.5 K.min−1) was determined by using Kissinger and Ozawa models. The average value of the activation energy was found to be at −56 kJ.mol−1. Dilatometry measurement was also used to trace 'in situ' the entire transformation for several temperatures. The kinetics of the δ→α'+δ transformation were modelled under isothermal conditions in the theoretical frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. It is proposed that the transformation consists of three stages. The α' transformation begins with a nucleation of pre-existing embryos. Then, both nucleation and rapid growth of α' occurs simultaneously and finally, the plates width expend. Apparent activation energies for nucleation and growth transformation were determined from the temperature dependence of the constant K at respectively −34 kJ.mol−1 and −60 kJ.mol−1. Adler et al. [1] investigated also the thermodynamics and the kinetics of the martensitic transformation in Pu alloys. These nucleation energies were found by modelling of heterogeneous martensitic nucleation via strain interaction with observed superdislocation-like nucleation sites in PuGa alloys. The values obtain by this model was very close to those we find. Investigations in steels alloys indicate that these energies are of the same order for nucleation near dislocation. Then, it could be indicating a strong relationship between these dislocations and martensitic nucleation sites.

Microstructural analysis of the δ to α' phase transformation in plutonium alloys using X-ray diffraction

The partial martensitic δ→α' transformation in plutonium alloys is sensitive to chemical composition, sample thermal history, as well as crystalline defects. The present work investigates the δ-Pu phase microstructure before and after the martensitic transformation δ→δ+α'. More precisely, microstructural modifications of the host δ-phase, resulting from the stress induced by a cell volume difference of 19% between the δ and α'-phases, were analysed. Microstructural information about crystallite size and microstrain of a highly homogenized Pu-Ga alloy was extracted from x-ray diffraction patterns using a three dimension crystallite size and microstrain model. This is available in Rietveld refinement software and consists in anisotropic broadening analysis of diffraction peaks. Crystallite size doesn't significantly change with the phase transformation contrary to microstrain that is multiplied, on average, by five. Furthermore, internal normal and shear microstress are multiplied, respectively, by 2 and 13 when α'-phase appears. Last, dislocation densities, calculated from crystallite size and microstrain, are compared to TEM results available in the literature.

Evidence for nascent equilibrium nuclei as progenitors of anomalous transformation kinetics in a Pu-Ga alloy

By alloying Pu with Ga, the face-centered-cubic $\ensuremath{\delta}$ phase can be retained down to room temperature in a metstable configuration, which ultimately yields to chemical driving forces by undergoing the $\ensuremath{\delta}\ensuremath{\rightarrow}{\ensuremath{\alpha}}^{\ensuremath{'}}$ isothermal martensitic transformation below ${M}_{s}\ensuremath{\approx}\ensuremath{-}100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. This transformation is found to exhibit anomalous transformation kinetics, the nature of which has remained elusive for over 30 years. Recently, a ``conditioning'' treatment---an isothermal hold above ${M}_{s}$---has been shown to dramatically affect the amount of ${\ensuremath{\alpha}}^{\ensuremath{'}}$ phase formed during the transformation. Herein, we report evidence that the conditioning treatment induces the lower C of the double-C curve and we furthermore implicate the classical nucleation of equilibrium phases as the underlying mechanism of the conditioning effect in Pu-Ga alloys. This mechanism should not be rigorously exclusive to plutonium alloys as it arises from the proximity of energies between the retained metastable phase and the low-energy equilibrium phases.

Phase transformation in δ-Pu alloys at low temperature: An in situ microstructural characterization using X-ray diffraction

In order to investigate the martensitic transformation, an isothermal hold at −130 °C for 48 h was performed on a highly homogenized PuGa alloy. The modifications of the microstructure were characterized in situ thanks to a specific tool. This device was developed at the CEA–Valduc to analyze the crystalline structure of plutonium alloys as a function of temperature and more especially at low temperature using X-ray diffraction. The analysis of the recorded diffraction patterns highlighted that the martensitic transformation for this alloy is the result of a direct δ → α′ + δ phase transformation. Moreover, a significant Bragg’s peaks broadening corresponding to the δ-phase was observed. A microstructural analysis was made to characterize anisotropic microstrain resulting from the stress induced by the unit cell volume difference between the δ and α′ phases. The amount of α′-phase evolved was analyzed within the framework of the Avrami theory in order to characterize the nucleation process. The results suggested that the growth mechanism corresponded to a general mechanism where the nucleation sites were in the δ-grain edges and the α′-phase had a plate-like morphology.

Reproducible phase transformation in a single Pu–1.9 at.% Ga specimen

The partial martensitic δ → α ′ transformation in Pu–Ga alloys is sensitive to lattice strains, defects, and dislocations as well as a near–ambient–temperature conditioning treatment. Because the δ → α ′ transformation and reversion inherently induce strains, plastic deformation, and defects, remnants of a previous transformation of a Pu–1.9 at.% Ga alloy can inhibit the phase transformation upon subsequent cooling. On the other hand, a conditioning treatment with isothermal holds as short as 6 h at room temperature can dramatically increase the volumetric amount of transformation. These two factors can prohibit systematic study of the δ → α ′ transformation unless experiments can be performed on multiple identical samples or a single sample can be treated such that these effects are eliminated. The latter approach requires an understanding of the conditions necessary to remove the effects of previous transformation as well as conditioning as they relate to the inhibition or promotion of the δ → α ′ transformation. Herein, we identify and report a thermal procedure, specifically an anneal at 375 °C for 30 min or more, sufficient to remove the effects of conditioning and previous transformation in order to reliably return a sample to an initial state, from which reproducible amounts of δ → α ′ transformation can be achieved with consecutive cycling.

Unconventional δ-phase stabilization in Pu–Ga alloys

Face-centered cubic δ-phase Pu stabilized with less than 3 at.% Ga will partially transform martensitically to monoclinic α′-phase at cryogenic temperatures. Thermal cycling of young alloys in a dilatometer tends to stabilize δ Pu by shifting the transformation to lower temperatures and reducing the amount of transformation. Similar experiments using samples aged naturally or by doping with 238Pu will stabilize δ Pu such that annealing at or above 150 °C is required before normal transformation behavior occurs. We discuss the role that radiation damage and recovery play in this unconventional stabilization of δ-phase Pu.

Effect of Elastic Anisotropy and Inhomogeneity on Coring Structure Evolution in Pu-Ga Alloys – Phase-field modeling

A coring structure in which δ (fcc) grains have Ga-rich cores with Ga-poor shells forms during the \(\varepsilon\) (bcc) to δ (fcc) phase transformations in Pu-Ga alloys. There are considerable differences between the diffusivities of Ga in \(\varepsilon\) and δ and a large and anisotropic stress field exists in the coring structure due to the large lattice mismatch between these two phases and strong elastic anisotropy. We developed a phase-field model for simulating the coring structure evolution in three dimensions, taking into account the inhomogeneities of both Ga diffusivity and elastic properties. It is shown that the elastic interactions among different oriented δ grains and diffusive Ga atoms affect Ga diffusion path, the coring structure, and the growth kinetics of the δ phase.

Atomistic simulations of Ga atom ordering in Pu 5 at. % Ga alloys

We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about ~3–4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.

Orientation Relationship, Habit Plane, Twin Relationship, Interfacial Structure, and Plastic Deformation Resulting from the δ → α′ Isothermal Martensitic Transformation in Pu-Ga Alloys

The orientation relationship, habit plane, parent-product interface at the atomic level, twin relationship, and plastic deformation resulting from the δ→α′ isothermal martensitic transformation in Pu-Ga alloys are examined using optical microscopy, transmission electron microscopy (TEM), and finite element calculations. The δ → α′ transformation exhibits a ∼20 vol pct collapse when the fcc δ phase transforms to the monoclinic α′ phase, which results in unique and intriguing crystallography and morphology. Here, we show that the orientation relationship is very close to that previously reported by Zocco et al. (1990), but has small rotational misalignments between the two phases both parallel and perpendicular to the $$ \left[ {110} \right]_\delta ||\left[ {100} \right]_{\alpha '} $$ direction. The amount of plastic deformation is exceedingly large due to the ∼20 vol pct collapse, and TEM is used to quantify the difference in dislocation density between untransformed δ matrix and regions of δ adjacent to the transformed α′. The twins contained in α′ plates are shown to have a (205) α orientation as the lattice invariant deformation and are found to be composed of two alternating variants that share a common α′ direction, but differ by a 60 deg rotation about α′ . A combination of electron diffraction and optical microscopy has been employed to examine the macroscopic habit plane, and the analysis suggests that a large fraction of the observed habit planes are on or near {111} δ . Finally, high resolution TEM reveals that the interface is faceted on {111} δ exhibiting a series of terrace and ledges.

Phase-field modeling of coring structure evolution in Pu–Ga alloys

During casting of Pu–Ga alloys, the huge difference in Ga diffusivity in ε (body centered cubic) and δ (face centered cubic) phases results in the formation of coring structures which consist of Ga-rich cores with Ga-poor edges. The elastic interaction among different oriented δ grains, matrix ε and diffusive Ga atoms may lead to a directional diffusion and δ grain growth. A phase-field model for modeling the coring structure evolution and Ga homogenization kinetics in three dimensions is developed. The effects of diffusivity inhomogeneity, internal stresses and cooling rate on the coring structure evolution are studied.

Study by XRD of the lattice swelling of PuGa alloys induced by self-irradiation

Plutonium aging leads to the creation of decay products, such as americium, uranium and helium and self-irradiation defects such as vacancies, vacancy clusters, self-interstitials and helium bubbles. As these self-irradiation defects accumulate in plutonium–gallium alloys, the lattice parameters of the material change. Thus, this work is an X-ray diffraction (XRD) study of the lattice parameter changes, such as kinetics and amplitude, as a function of self-irradiation dose for non-homogenized and homogenized samples. The results have shown no incubation time before the beginning of lattice swelling. Moreover, whereas the lattice swelling amplitude seems to be influenced by the gallium segregation to the border of grain in non-homogenized samples, it is not strongly influenced by the gallium concentration. An average lattice parameter increase of about 4.5 × 10 −3 Å is observed for homogenized alloys at saturation. This equilibrium is achieved after a dose of 0.1 dpa. A discussion of possible causes leading to these observations and their effects is presented.

Orientation Relationship, Habit Plane, Twin Relationship, Interfacial Structure, and Plastic Deformation Resulting from the δ → α′ Isothermal Martensitic Transformation in Pu-Ga Alloys

Orientation Relationship, Habit Plane, Twin Relationship, Interfacial Structure, and Plastic Deformation Resulting from the δ → α′ Isothermal Martensitic Transformation in Pu-Ga Alloys

Temperature and time-dependence of the elastic moduli of Pu and Pu–Ga alloys

In previous work, on cooling from 300 K to 10 K the elastic moduli for both α- and δ-Pu dropped 30%. This large change may reflect effects of 5f-electron localization. In this work, the elastic moduli at ambient temperature of several Pu–Ga alloys were measured using resonant ultrasound spectroscopy (RUS). The strong temperature dependence of the bulk and shear modulus and the temperature independence of Poisson's ratio was confirmed and the upper temperature limit for α-Pu was extended to 360 K. Measurements of the time dependence of the shear moduli of Pu and Pu–2.36 at.% Ga were determined with high precision as a function of time and temperature. Using a model for time dependence of point defects, we determined the exponential time constant at ambient temperature for such variations. The low temperature results are consistent with Fluss [1].

The effect of pressure on phase-stability in the Pu–Ga alloy system

In examining and analyzing over three decades of research on the transformations involved in Pu–Ga alloys, it is clear that many incongruities and contradictions exist. New analysis shows that under certain processing conditions large amounts of a non-diffracting disordered state exists in Pu–Ga alloys, which I call the amorphous state. The amorphous state is certainly not an equilibrium state, but it can be formed under certain pressure conditions, and more surprisingly once formed can persist for extended times and temperatures. It is believed that the emergence of the amorphous state is closely linked to the 5f-electrons going into a bonding mode. Being a physical state, the amorphous state has distinct physical properties, meta-stabilities, and sensitivities to Ga content. The existence of an amorphous state requires innovative new tools for identification and analysis. The density and compressibility data presented in this paper is one such tool for analyzing the amorphous state, but it requires other knowledge of the materials for proper analysis. Many studies point to radiation damage producing amorphous regions. New computer modeling studies show the existence of amorphous regions and Pu-rich interstitial defects in Pu–Ga alloys.

Influence of Phase Stability on Radiation Damage Properties: Plutonium-Gallium Alloys

AbstractModeling cascade and fission damage evolution of actinide materials of all kinds is essential for understanding their aging characteristics. As an example of how exotic some of the damage evolution behavior can be, plutonium-gallium (Pu-Ga) alloys in the δ-phase (fcc lattices) are explored. Aging emanates from the wide variety of spontaneous decay and fission products that, in the case of the Pu, are such species as helium (He) and uranium, among others, as well as interstitials, and vacancies. To aid in our understanding, the modified embedded atom method (MEAM) formalism is applied to the Pu-Ga-He system. The behavior of defects in the fcc (δ) phase of Pu-based materials is strongly influenced by the metastability of this phase. The influence of this metastability on minimum displacement threshold energy, point defect characteristics and He bubbles is delineated. The roles of short-range ordering and transformations of voids into stacking fault tetrahedra in the aging process are also examined.

Supporting evidence for double-C curve kinetics in the isothermal δ → α′ phase transformation in a Pu–Ga alloy

Time-temperature-transformation (TTT) diagrams for the δ → α′ transformation in a number of Pu–Ga alloys were first reported in 1975 by Orme et al. Unlike typical single-C curve kinetics observed in most isothermal martensitic transformations, the Pu–1.9 at.% Ga alloy exhibits two noses, and thus double-C curve kinetics. The authors attributed the occurrence of the double-C to a difference in mechanism: a massive transformation for the upper C and a martensitic transformation for the lower C. Since that time, the nature, and the existence of the double C have received only limited attention. The results of Deloffre et al. suggest a confirmation of this behavior, but the fundamental origin of the double C remains unknown. Here, we apply differential scanning calorimetry (DSC) as an alternative approach to acquiring the TTT data and our experimental evidence suggests a confirmation of the double-C behavior after 18 h of isothermal hold time. In addition, we report three exothermic peaks corresponding to transformations during cooling at 20 °C/min prior to the isothermal holds. These three peaks are reproducible and suggest a number of possibilities for the origin of the unique kinetics: α′ forms with different morphologies, or from different embryos in the upper and lower C curves; α′ forms directly in one C curve and forms via an intermediate phase in the other C curve; the two C curves result from α′ forming by two or more distinct mechanisms (e.g., massive and martensitic transformations).

Formation and recovery of irradiation and mechanical damage in stabilized δ-plutonium alloys

Based on the results of thermal expansion measurements of aged δ-phase Pu–Ga alloys and comparisons with historical bulk density, thermal expansion, and X-ray diffraction data of δ-phase Pu–Al and Pu–Ga alloys, it is being proposed that Pu-solute atom mixed 〈1 0 0〉 dumbbell interstitials, generated by damage processes, induce volumetric swelling and impede δ → α′ Martensitic transformation in these alloys. Damage induced volumetric swelling in δ-phase Pu–Al and Pu–Ga alloys is distinct from swelling induced by He bubble formation in that it appears to saturate as a function of damage eventually reaching a constant value which is linearly dependent on solute element atomic concentration. Based on arguments offered in this work, each Pu-solute atom mixed 〈1 0 0〉 dumbbell interstitial creates a strain field of approximately 4.8 times the Pu atomic volume over some 20 Å in extent. As the interstitial concentration increases with damage, so does the volumetric swelling up to the point of saturation with a concentration of 1% of the solute atomic concentration. These interstitials appear to anneal out at temperatures of 150 to 200 °C. This relatively high thermal stability is possibly due to an altered local bonding environment for the Pu-solute atom interstitial pair and first-nearest neighbors in response to reduced coordination number and lattice strain field.

Instability of the electronic structure of actinides under the high pressure

The paper discusses phase transitions in Pu–Ga alloy under the pressure up to 1 GPa and analyzes electronic phases of unalloyed actinides using data of static experiments at pressure of 100–300 GPa.

Thermostatics and kinetics of transformations in Pu-based alloys

CALPHAD assessment of the thermodynamic properties of a series of Pu-based alloys is briefly presented together with some results on the kinetics of phase formation and transformations in Pu–Ga alloys.

Magnetic state of f electrons in δ-phase of Pu–Ga alloys studied by Ga NMR

69Ga nuclear magnetic resonance (NMR) line shift ( 69 K) and nuclear spin–lattice relaxation rate ( 69 T 1 −1) are measured for Pu 0.95Ga 0.05 alloy, stabilized in δ-phase, in the temperature range 10 and 650 K at magnetic field of 9.4 T. The shift and 69 T 1 −1 are determined correspondingly by the static and fluctuating-in-time parts of the local magnetic fields arisen at Ga due to transferred hyperfine coupling with the nearest f electron environment of more magnetic Pu. At T > 200 K, the temperature dependent part of the shift 69 K( T) scales macroscopic magnetic susceptibility χ( T), following the Curie–Weiss law, and the product ( 69 T 1 T) increases with temperature proportionally ( T + 255) 1.5(1). Both of the NMR observations are typical of the incoherent spin fluctuation regime of f electrons in nonmagnetic 3D Kondo lattice. An estimate of the effective magnetic moment μ eff,5f( g e = 2) = 0.15(5) μ B per Pu atom points out a strong suppression of the spin magnetism in the alloy.