Plutonium—an Element Never at Equilibrium

The relentless deposition of energy from the α-particle decay of plutonium damages its crystal lattice and transmutes plutonium into other elements over time (principally, helium, americium, uranium, and neptunium). At cryogenic temperatures (4 K), lattice damage causes significant volume expansion of pure plutonium and contraction of face-centered-cubic stabilized alloys, and both appear to lose crystallinity at long irradiation times. At room temperature, much of the lattice damage is annealed out because defects produced by self-irradiation are sufficiently mobile. Nevertheless, plutonium’s delicate balance of stability with changes in temperature, pressure, or chemistry may be affected by self-irradiation. For example, at room temperature the lattice of fcc plutonium alloys expands and exhibits nanoscale bubbles at irradiation levels <0.1 displacements per atom (dpa). In addition to self-irradiation damage, it is now generally agreed that most fcc alloys previously believed to be thermodynamically stable at room temperature are in fact metastable. They undergo eutectoidal decomposition to α-plutonium, plus the nearest intermetallic compound. However, for most practical purposes, the kinetics of phase decomposition are too slow to be of concern. So, although plutonium may not be “far” from equilibrium, it is never at equilibrium because of the very nature of its radioactive decay. Surface reactions in plutonium can be increased catastrophically by the presence of moist air or hydrogen. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

Thermodynamic equilibrium in the low-solute regions of Pu-group MIA metal binary systems

The low-temperature fcc(δ) → monoclinic(α) transformation in Pu-Al and Pu-Ga alloys has been shown to proceedvia a diffusionless, martensitic mechanism. Typically, δ/δ+ α “equilibrium” phase boundaries reported in the literature are based on measurements of the M_s or A_s (forward and reverse) martensite transformation temperatures, which are functions of grain size, strengthening mechanisms, or nucleating defect structures and, hence, do not represent a state of thermodynamic equilibrium.Via a thermodynamic model, in which a regular solution parameter was fit to experimental equilibrium temperatures (T₀) and pressures (P₀) and available solution calorimetry data on two PuAl alloys to define the Gibbs free energy for the δ and α phases, equilibrium phase boundaries were determined using free-energy minimization techniques over the compositional and temperature ranges 0 ≤ X≤ 0.30 and 300 K≤T ≤ 700 K, respectively, for both Pu-Al and Pu-Ga systems. A eutectoid decomposition of δ→ α + Pu₃M is predicted at 335 ± 50 K for the Pu-Al system, while for Pu-Ga the eutectoid occurs at 354 ± 50 K. These findings are consistent with other group III A Pu-M systems (In and TI) which also exhibit the invariant reaction.     

Phase transformations in uranium,plutonium, and neptunium

The actinide metals uranium and neptunium are characterized by three allotropes, and plutonium has six solid phases with two of the allotropes having complex, low symmetry, crystal structures. Further, many phase changes in these metals are accompanied by a large specific volume change and a large enthalpy change. Phase transformations in uranium and plutonium have been studied extensively but very little experimental work has been conducted to understand the phase transformations in neptunium. The low temperature transformations occur isothermally and exhibit simple TTT curves similar to many alloys. The kinetics in the metals are controlled more by purity and thermal history than by any inherent feature of the metals. Mechanisms in the pure metals are inadequately understood, and the crystal-lographic relations are complex. The β → α transformations in uranium and plutonium are diffusional near the α β equilibrium temperature, and they are both reminiscent of shear transformations at low temperatures. Skip transformations have been studied in these metals and the role of grain size on transformation behavior has been investigated more than in other metals. Accordingly, the roles of metal purity, thermal history, microstructure, mechanical properties, and thermodynamic variables are discussed, particularly as they might be related to phase transformations in other systems. Generalities have been difficult to establish for plutonium and uranium alloys. Among well-known metals the zirconium and titanium transformations form the closest analogs to the uranium transformations, but there is no obvious analog to the plutonium transformations. Formerly with the Argonne National Laboratory, Argonne, Ill. 60439, This paper is based on a presentation made at a symposium on “Phase Transformations in Less Common Metals: A Dialogue,” held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.     

Rolling texture of Pu-Ga alloys as a function of temperature

The rolling textures of a series of Pu-Ga fccδ stabilized alloys are reported as a function of both the percent reduction and the temperature of rolling. Though twinning in the cast and homogenized alloys increases with the gallium concentration, no change in rolling texture was observed as a function of gallium for specimens rolled at room temperature. Rolling of these alloys at temperatures down to -196°C caused a texture transition similar to that observed in copper rolled at low temperatures. The stacking fault energy was estimated to be 30 erg per sq cm. For the Pu-3.4 at. pct Ga alloy, this texture transition occurred at a higher temperature than in the other two alloys, and it is known that in this case someα phase was formed during rolling.     

Acoustic emission produced by the delta-to-alpha phase transformation in Pu-Ga alloys

Detailed measurements are reported of acoustic emission (AE) produced during theδ → α phase transformation in plutonium-gallium alloys. During thermal cycling, the number of AE signals produced decreased with each thermal cycle more rapidly than the amount of additional material transformed. Thus, some portion of the transformation does not produce detectable AE. The average amplitude of the signals produced during quenching was not appreciably dependent on gallium content or cooling cycle. The amplitude of the signals is greater for rapid cooling than for slow cooling. The AE behavior is consistent with an isothermal, nonthermoelastic martensitic transformation.     

A solidification diagram for Ni-Cr-Mo-Gd alloys estimated by quantitative microstructural characterization and thermal analysis

A γ-Gd solidification diagram is proposed as an aid to understanding solidification behavior of Ni-Cr-Mo-Gd alloys. In this system, the Ni-Cr-Mo solid solution γ primary austenite phase is treated as the “solvent” and Gd is treated as the solute. The proposed diagram, which has features characteristic of a binary “eutectic” system, was constructed by combining differential thermal analysis and quantitative microstructural analysis data. As a result of the partially divorced solidification microstructure in the ingots studied, determination of the fraction eutectic, and hence the eutectic composition, requires the use of advanced image analysis techniques. The diagram displays a number of features that are very similar to the Ni-Gd binary system and can be used to assess the influence of the Gd concentration on solidification behavior.     

Microstructural evolution in Zr3Al-based alloys during various long-time annealing treatments

The Zr₃Al-based intermetallics have been considered as candidate materials for structural components in pressurized heavy water nuclear reactors due to their attractive properties. However, these alloys could not be used for structural application in the reactors because of their poor room-temperature ductility and irradiation-induced amorphization. Our recent studies of ternary addition of niobium in Zr₃Al have shown some promising results. The present article reports the microstructural evolution in these alloys upon long-time annealing treatments. The formation of various phases, temperature regime of their stability, chemical composition, and volume fraction of these phases during prolonged annealing have also been studied. A pseudobinary phase diagram with varying niobium concentration has also been developed. The morphology and distribution of the Zr₃Al phase have been explained on the basis long-range diffusion as the rate-controlling step. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Prediction of hot tearing tendency for multicomponent aluminum alloys

It is of practical importance to be able to predict the hot tearing tendency for multicomponent aluminum alloys. Hot tearing is one of the most common and serious defects that occurs during the casting of commercial aluminum alloys, almost all of which are multicomponent systems. For many years, the main criterion applied to characterize the hot tearing tendency of an alloy system was based on the solidification interval. However, this criterion cannot explain the susceptibility-composition relation between the limits of the pure base metal and the eutectic composition. Clyne and Davies correlated the susceptibility-composition relationship in binary systems based on the concept of the existence of critical time periods during the solidification process when the structure is most vulnerable to cracking. The Scheil equation was used in their model using constant partition coefficient and constant liquidus slope estimated from the phase diagram. In the current study, the authors followed Clyne and Davies’ general idea, and directly coupled the Scheil solidification simulation with phase diagram calculation via PanEngine, a multicomponent phase equilibria calculation interface, and extended the model to higher order systems. The predicted hot tearing tendencies correlated very well with the experimental results of multicomponent aluminum alloys. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.     

Structure of directionally solidified two phase ternary alloys

Simple theories developed in a series of recent papers, describing solid/liquid interface stability and solute redistribution in three phase ternary alloys are summarized and extended to apply to ternary alloys that are two phase on equilibrium solidification. Calculated results compare well with experiments on alloys from the Al-rich corner of the Al-Cu-Ni ternary phase diagram. These alloys represent the type currently under development for gas turbine applications. At sufficiently high values ofG/R (thermal gradient divided by growth rate) the alloys grow with a fully plane front. At moderate values ofG/r they exhibit a unique structure of single phase cells and two phase intercellular root material which grows with a “plane front”. Further decreases inG/R produce more typical ternary cellular structures with regions of one, two and three phases. Formerly Graduate Student, Massachusetts Institute of Technology, Cambridge, Mass. Formerly Research Associate, Massachusetts Institute of Technology, Cambridge, Mass.     

Solidification of undercooled Fe-Cr-Ni alloys: Part III. Phase selection in chill casting

In Parts I and II of this series of articles, it was shown that a range of levitation-melted Fe-Cr-Ni alloys, both hypoeutectic and hypereutectic, all solidified with the hypereutectic phase (bcc) as their primary phase, except for the hypoeutectic alloys at low undercoolings. In this article, the effect of heat extraction on phase formation is studied by chill casting the undercooled alloys before nucleation. Two of the previously studied alloys are examined; one hypoeutectic and the other hypereutectic. Chill substrates employed were copper, stainless steel, alumina, zirconia, and a liquid gallium-indium bath. Contrary to the case of levitation melting and solidification, it is found that the dominant primary phase to solidify in both alloys, independent of chill substrate, is the hypoeutectic phase (fcc). It is concluded that chilling the undercooled melt results in nearly concurrent nucleation of bcc and fcc. Two different mechanisms are considered as possible explanations of the subsequent fcc phase selection during growth. These are termed “growth velocity” and “phase stability” mechanisms. Evidence favors a phase stability mechanism, in which the bcc phase massively transforms to fcc early in solidification so that fcc then grows without competition. It is suggested that this mechanism may also explain structures observed in welds and other rapid solidification processes.     

Modeling of structural and compositional homogenization of plutonium-1 weight percent gallium alloys

The microstructure of as-cast Pu-1 wt pct Ga alloys is characterized by extensive Ga microsegregation often referred to as “coring.” This process results in grains that consist of Ga-rich cores (∼1.6 wt pct) with Ga-poor (∼0.1 wt pct) edges. Cored grains can be homogenized at moderately high (i.e., >400 °C) temperatures, though the time required to achieve chemical homogeneity is not well constrained. In this article, we apply several analytical diffusion modeling techniques to characterize the kinetics of alloy homogenization as a function of time and temperature. We also review the experimental investigations that have used analytical tools such as X-ray diffraction, density, dilatometry, and electron microprobe analysis to characterize Pu-Ga alloy homogenization. Data from these studies are used as a basis of comparison with modeling results. In particular, Ga coring-profile modeling appears to be a powerful tool for predicting alloy homogenization.     

Thermal equilibria and mechanical stability of Ti3 Al phase in Ti-Mo-Al alloys

The phase diagram of the isopleth section of the Ti-7 at. pct Mo-Al system has been improved and expanded to include alloys up to 25 at. pct aluminum. The mechanical and thermal stability of alloys aged in the two-phase region, β +Ti₃Al, was correlated to the microstructure. X-ray rocking-curve studies of the polycrystalline specimens showed that after 2 pct deformation of a Ti-7 Mo-16 Al alloy theβ matrix became preferentially plastically deformed, while the Ti₃Al particles functioned as hard particles undergoing little lattice distortions. Formerly a Graduate Student. This paper is based on a thesis submitted by T. Hamajima in partial fulfillment of the requirements for a Ph.D. degree from Rutgers University.     

Modeling of structural and compositional homogenization of plutonium-1 weight percent gallium alloys

The microstructure of as-cast Pu-1 wt pct Ga alloys is characterized by extensive Ga microsegregation often referred to as “coring”. This process results in grains that consist of Ga-rich cores (∼1.6 wt pct) with Ga-poor (∼0.1 wt pct) edges. Cored grains can be homogenized at moderately high (i.e., >400 °C) temperatures, though the time required to achieve chemical homogeneity is not well constrained. In this article, we apply several analytical diffusion modeling techniques to characterize the kinetics of alloy homogenization as a function of time and temperature. We also review the experimental investigations that have used analytical tools such as X-ray diffraction, density, dilatometry, and electron microprobe analysis to characterize Pu-Ga alloy homogenization. Data from these studies are used as a basis of comparison with modeling results. In particular, Ga coring-profile modeling appears to be a powerful tool for predicting alloy homogenization. FRANK E. GIBBS, formerly Technical Staff Member with the Nuclear Materials Technology Division, Los Alamos Laboratory     

Application of free energy-composition diagrams in predicting the sequences of phase transformations in Zr-Nb alloys

The free energy composition diagrams for the Zr-Nb alloy system have been computed from the phase diagram employing Rudman’s technique of phase diagram analysis. The influence of the clustering tendency in the β phase on the sequence of phase transformations in both Zr-rich and Nb (Cb)-rich alloys has been examined. It has been possible to predict different sequences of phase reactions in different ranges of composition and temperature. The experimentally observed phase reactions as reported earlier have been rationalized by the calculated free energy composition plots.     

Phase relations in concentrated Ta-Hf and Nb-Hf alloys

The precipitation reactions occurring in concentrated Ta-Hf and Nb-Hf alloys over the temperature range 600° to 1400°C were investigated by X-ray diffraction and transmission microscopy. In Ta-Hf alloys containing 30 to 62 at. pct Hf and Nb-Hf alloys containing 25 to 50 at. pct Hf a stable bcc solid solution exists at elevated temperature. When quenched from the single phase region and aged at lower temperatures both alloys precipitate α phase, an hcp hafnium-rich phase. Transmission microscopy showed that in the Nb-Hf system the precipitation reaction begins with the formation of coherent zones. Further aging causes coarsening into discs, still at least partially coherent. When coherency is lost the α morphology changes to rod-type. The effect of small amounts of oxygen added to these alloys during aging is an increase in the unit cell size of theα phase. Oxygen concentrations up to ~0.5 at. pct do not cause observable changes in the phase relations. These results cannot be reconciled with the miscibility gap-monotectoid type phase diagrams reported by some earlier investigators and indicate that the proper phase diagram for both systems is theβ-isomorphous type.     

Phase equilibria in the Cr-Ni-C system and their use for developing physicochemical principles for design of hard alloys based on chromium carbide

The Cr—Ni—C phase diagram at the melting point was plotted by a combination of procedures (metallography, x-ray, microprobe, differential thermal analysis, Pirani—Alterthum method, etc.). A general feature of this system is the existence of equilibria between the nickel-based phase and all the other phases. The temperature of the quasibinary (Ni)+(Cr₇C₃) eutectic was determined to be 1324±6°C. Based on both the phase diagram of the Cr—Ni—C system and the bending strength and Rockwell hardness of the alloys, the optimal composition of the initial carbide ingredient for production of hard alloys based on Cr₃C₂ with nickel—phosphorus binder was estimated as 13.0–13.3 at.%, substoichiometric with respect to Cr₃C₂. Institute of Problems in Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya. No. 5/6(395), pp. 13–24, May–June, 1997.     

The Grain Refining of Aluminum and Phase Relationships in the Al-Ti-B System

A number of researchers have suggested that there may be phase relationships in the Al-Ti-B system, which can be used to explain why boron improves the grain refining ability of aluminum-titanium master alloys. In this paper, the available information on the phase diagram is reviewed and theoretical calculations are made to establish the ternary Al-Ti-B phase diagram. The phase diagram cannot explain the important effect of boron. It appears to be necessary to seek another explanation.     

Structure of directionally solidified two phase ternary alloys

Simple theories developed in a series of recent papers, describing solid/liquid interface stability and solute redistribution in three phase ternary alloys are summarized and extended to apply to ternary alloys that are two phase on equilibrium solidification. Calculated results compare well with experiments on alloys from the Al-rich corner of the Al-Cu-Ni ternary phase diagram. These alloys represent the type currently under development for gas turbine applications. At sufficiently high values ofG/R (thermal gradient divided by growth rate) the alloys grow with a fully plane front. At moderate values ofG/r they exhibit a unique structure of single phase cells and two phase intercellular root material which grows with a “plane front”. Further decreases inG/R produce more typical ternary cellular structures with regions of one, two and three phases.     

Molybdenum in Nb-C alloys

The effects of molybdenum alloying additions to niobium on the carbide phases and their precipitation behavior were investigated. The experimental alloys included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After selected heat treatments the microstructural changes were determined by metallography and the carbide phases were extracted and identified by X-ray diffraction and chemical analysis. The results are essentially in agreement with recent phase diagram determinations. Additions of 30 wt pct Mo appears to slightly increase the solubility of carbon in niobium at temperatures around 1650°C. The solubility of molybdenum in Nb₂C is very small. Discontinuous precipitation of β-Nb₂C was found to occur in the Nb-30Mo-0.1C alloy during annealing at 1200°C. The important, overall effect of molybdenum in Nb-C alloys is to decrease the rate of niobium carbide precipitation so that appreciable carbon supersaturation can be achieved even after comparatively slow furnace cooling.     

Nonequilibrium behavior in the Al-Ge alloy system: Insights into the metastable phase diagram

The competitive formation of metastable and stable phases during nonequilibrium processing of Al-Ge alloys and the corresponding metastable phase equilibria have been investigated. For germanium concentrations in the range 30 to 50 at. pct, it is shown that the four metastable phases can be ranked in order of decreasing stability as follows: monoclinic (P2₁/c), rhombohedral (R-C), orthorhombic (Pbca), and hexagonal (P6/mmm). Their formation depends not only on the transformation temperature(e.g., the liquid undercooling), but also on the presence of appropriate heterogeneous nucleation sites. For example, the orthorhombic phase has only been observed in amorphous films after rapid annealing/crystallization treatments. It is also shown that all of these phases form metastable equilibria with α-aluminum only,i.e., no metastable phase equilibria appear to exist between any metastable phase and β-germanium or between any two metastable phases. Consequently, it is not possible to draw a single metastable phase diagram that incorporates all of these phases with phase boundaries that represent their metastable equilibria; rather, separate diagrams should be drawn for each metastable phase. It is noted that these diagrams should extend only to the metastable phase field rather than all the way to pure germanium: for compositions richer in germanium, the results indicate that the metastable phase forms and then remelts upon the formation of germanium or a more stable, germanium-enriched metastable phase. Furthermore, it is proposed that this behavior is rather general in nature. Finally, it is concluded that the production of metastable phases in bulk form, in systems such as this where so many reactions occur simultaneously and competitively, might be impossible using solidification processing approaches. Formerly with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195