Pu-Ga Research Articles

Microstructural evidence for conditioning-dependent δ → α′ transformations in retained δ-phase Pu–Ga

The retained δ phase of a Pu–1.9at.% Ga alloy is metastable with respect to the martensitic δ→α′ transformation that occurs at low temperatures. This transformation has been shown to proceed by means of an isothermal martensitic mode, but the kinetics of the transformation are atypical. The transformation exhibits a “double-C” in a time–temperature–transformation diagram, wherein there exist two temperatures at which transformation occurs in a minimum amount of time. The cause of the double-C kinetics remains uncertain, eliciting proposals of multiple mechanisms, multiple paths, or different morphologies as possible origins. Recently, a “conditioning” treatment was found to affect the δ→α′ transformation, but the underlying mechanism by which the conditioning treatment influences the transformation has not yet been resolved. In this study, microstructural characterization as a function of temperature, time, and conditioning has been employed to illuminate the role of conditioning in the δ→α′ transformation. Conditioning is found to enhance transformation in the upper-C and to enable transformation in the lower-C. The data garnered from these experiments suggest that conditioning is intimately linked to nucleation processes, but is of little consequence to the growth and morphology of the α′ product phase.

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.

Aqueous Corrosion Behavior of Plutonium Metal and Plutonium–Gallium Alloys Exposed to Aqueous Nitrate and Chloride Solutions

Previous work in this laboratory examined the polarization behavior of high-purity plutonium metal exposed to various solutions; however, the polarization behavior of plutonium–gallium alloys has not been studied. The objective of this work is to establish the effect of gallium on the electrochemical behavior of plutonium metal exposed to nitric acid, sodium nitrate, and sodium chloride solutions. Results indicated that plutonium–gallium alloys were not passive in solutions, unlike unalloyed plutonium. The dissolution of 0.5 and Ga alloys exposed to , and all materials exposed to NaCl, was found to be controlled by the presence of a corrosion product film. Relative oxide thickness measurements of unalloyed Pu exposed to were obtained. Chloride was found to aggressively attack Pu and its alloys with increasing chloride concentration resulting in increased corrosion. Nitrate ions were shown to have a weak effect on the corrosion behavior of Pu, as compared to protons and chloride ions.

Aqueous Corrosion Behavior of Ce Alloys and an Assessment of Their Adequacy as Surrogates for Pu Alloys

The corrosion behavior of Ce metal and a Ga alloy in aqueous deaerated solutions has been investigated through potentiodynamic and electrochemical impedance experiments. The solutions consist of concentrations of HCl, NaCl, , , , and . It was found that the addition of Ga is generally detrimental to the corrosion resistance of Ce, resulting in the loss of passivity and a decrease in polarization resistance. Ce metal was found to be susceptible to pitting in chloride environments. Increasing acidity was found to promote the passivity of Ce in chloride and sulfate solutions. The opposite was true in nitrate solutions. Dissolution of the Ce–Ga alloy was ohmically limited in certain solutions. The ohmic limitation was not attributable to the solution resistance. Although the addition of Ga was found to be detrimental to the corrosion resistance of Ce and Pu, Ce and Ce–Ga were found to be poor electrochemical surrogates for Pu and Pu–Ga.

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.

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).

Phase transformations in Pu–Ga and Pu–Al alloys: Effects of pressure and temperature on kinetics of δ-phase decomposition

Pressure effected evolution of equilibrium and metastable binary Pu–Ga (Al)-phase diagrams is discussed. With an increase in pressure δ-solid solution region decreases and disappears, which leads to a change of phase diagram type. In experiments and via thermodynamic calculations boundaries of δ-phase region were established at pressure varying from the atmospheric one to ∼1 GPa. Temperature of δ-phase eutectoid transformation versus pressure was determined; a fragment pressure–temperature–composition ( PTC) diagram was constructed, parameters of four-phase equilibrium were found. Influence of static pressure on δ-phase decomposition kinetics is demonstrated, feasibility of decomposing effected by loading with high intensity waves is assessed.

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.

Connections between the Pu–Ga phase diagram in the Pu-rich region and the low temperature phase transformations

Significant progress has been made in recent years regarding the equilibrium phase diagram of the Pu–Ga system. For decades, researchers outside of the former Soviet Union considered the δ phase in Pu–Ga alloys to be in the state of thermodynamic equilibrium down to low temperatures. Recently, decades of experimental work in Russia have been published that indicate the existence of a eutectoid decomposition of the metastable δ phase to α phase and Pu 3Ga. Phase diagram calculations by a number of researchers have predicted this eutectoid transformation as well. In this work, we review the experimental and calculated phase diagrams and also comment on the expected consequences of the IIIrd law of thermodynamics with respect to the form of the delta phase field. Assembled data from the literature on the martensite start ( M S) and reversion ( R S) temperatures for the δ → α′ transformation as a function of Ga content are used to consider the possible trend of the T 0 line describing the equivalence of the free energies of the α and δ phases in the phase diagram. It is suggested that in order to reconcile the need for the existence of a driving force for both the compositionally variant eutectoid reaction, as well as a compositionally invariant martensitic transformation in a Pu—2 at.% Ga alloy, the actual T 0 line may lie higher than that indicated by the present thermodynamical modeling.

Reversible expansion of gallium-stabilized δ-plutonium

It is shown that the transient expansion of plutonium–gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature can be explained by assuming incipient precipitation of Pu 3Ga. However, this ordered ζ′-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized δ-phase, being metastable at ambient temperature, is driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time reverted back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is arranged in ordered ζ′-phase regions.

Density-functional calculations of α-Pu–Ga(Al) alloys

First-principles methods are employed to study the ground-state atomic volumes of α-Pu–Ga(Al) alloys. It is shown that a random distribution of Ga(Al) atoms in the monoclinic lattice of α-Pu results in a maximum expansion of this lattice and creation of the so-called α′-Pu phase. Any kind of ordering of Ga(Al) atoms on the monoclinic lattice results in a shrinking of the lattice constant while the ordered α 8-(Pu–Ga(Al)) configuration yields the smallest lattice constant which is very close to that of pure α-Pu. In addition, energetics of the ordered (unrelaxed and relaxed) and disordered configurations is discussed.

Nucleation and growth of the α′ martensitic phase in Pu–Ga alloys

In a Pu–2.0 at.% Ga alloy, it is observed experimentally that the amount of the martensitic α′ product formed upon cooling the metastable δ phase below the martensite burst temperature ( M b) is a function of the holding temperature and holding time of a prior conditioning (“annealing”) treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the α′ phase, it was first homogenized for 8 h at 375 °C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between −50 and 370 °C for periods of up to 70 h to determine the holding time and temperature that produced the largest volume fraction of α′ upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of α′ formed, the largest amount of α′ was obtained following holding at 25 °C for at least 6 h. Additional time at 25 °C, up to 70 h, did not increase the amount of subsequent α′ formation. At 25 °C, the Pu–2.0 at.% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are α and Pu 3Ga, although a complete eutectoid decomposition of δ to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of α-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic α′ particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classic nucleation theory is presented to describe the formation of α-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more α′ formation on cooling are discussed in terms of the differences in the potency of the embryos created in the δ matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the α′ formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing.