Evidence of transformation bursts during thermal cycling of a Pu-Ga alloy
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I. INTRODUCTION
AT ambient pressures, unalloyed plutonium exhibits six equilibrium solid phases—monoclinic , body-centered monoclinic , face-centered orthorhombic , face-centered cubic , body-centered tetragonal , and body-centered cubic . Early researchers in the United States and Europe found that when plutonium was alloyed with small amounts of specific solutes, such as gallium or aluminum, the phase could be retained at room temperature after cooling. The resulting proposed Pu-rich portion of the Pu-Ga phase diagram is shown in Figure 1(a).[1,2,3] Russian scientists, on the other hand, proposed a phase diagram with a eutectoid decomposition of into Pu3Ga (cubic or tetragonal, depending on temperature) at 97 °C (Figure 1(b)).[3,4] The Russian diagram is now generally accepted as the more likely “equilibrium” phase diagram (note that because plutonium continuously undergoes radioactive decay, the system never attains true thermodynamic equilibrium). The kinetics of the eutectoid reaction, however, are expected to be exceedingly slow, involving times on the order of 10,000 years.[3] Thus, the alloyed phase, although metastable, can be retained at room temperature almost indefinitely.[3,5] When the retained metastable phase is cooled to subambient temperatures, it partially transforms via a martensitic transformation[6,7] to a metastable phase, with the transformation temperature dependent on composition. The phase K.J.M. BLOBAUM, Metallurgist, C.R. KRENN, Metallurgist, J.J. HASLAM, Material Scientist, M.A. WALL, Technical Staff Member, and A.J. SCHWARTZ, Deputy Division Leader, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Contact e-mail: [email protected] J.N. MITCHELL, Team Leader, is with Los Alamos National Laboratory, Los Alamos, NM 87545. T.B. MASSALSKI, Professor Emeritus, is with Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted February 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
is an extension of the monoclinic phase of pure Pu with Ga trapped in the lattice, such that local distortions occur. Given sufficient time and temperature, it is likely that the Ga solute atoms diffuse locally, and they may show a preference for the number 8 lattice site. This may result in relaxation of the phase to the unexpanded phase.[8] The lattice remains alloyed in the phase unless the solute Ga atoms leave the lattice; this process is not likely to occur at temperatures below room temperature under ambient pressures.[8] The lattice parameters of the and phases are similar, but they increase slightly with Ga contents.[8] The martensitic transformation to can continue isothermally at temperatures below an initial onset temperature, which we refer to here as the martensite burst temperature, Mb. Unlike athermal martensitic transformations that exhibit essentially time-independent kinetics, the amount of (martensite) formed from can be represented as a function of isothermal hold times. As a result, the kinetics of the t
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