THERMODYNAMIC-EQUILIBRIUM IN THE LOW-SOLUTE REGIONS OF PU-GROUP-IIIA METAL BINARY-SYSTEMS

The low-temperature fcc(delta) --> monoclinic(alpha) transformation in Pu-Al and Pu-Ga alloys has been shown to proceed via a diffusionless, martensitic mechanism. Typically, delta/delta + alpha "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 delta and alpha phases, equilibrium phase boundaries were determined using free-energy minimization techniques over the compositional and temperature ranges 0 less-than-or-equal-to X less-than-or-equal-to 0.30 and 300 K less-than-or-equal-to T less-than-or-equal-to 700 K, respectively, for both Pu-Al and Pu-Ga systems. A eutectoid decomposition of delta --> alpha + 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 IIIA Pu-M systems (In and Tl) which also exhibit the invariant reaction.