Thermodynamics and kinetics of δ→ α martensitic transformation in Pu alloys

A regular solution model is fit to experimental equilibrium temperatures(T 0) and pressures(P 0) for diffusionlessδ→α transformations in Pu-Ga and Pu-Al alloys, in order to define the chemical free energy change ΔG forδ→ α transformation. Analysis of reported isothermalδ→ αtransformation rate data in terms of nucleation-controlled martensitic kinetics gives a nucleation activation energyQ which is a nonlinear function of ΔG. The activation volumeV * defined byδQ/δΔG is of the order of 10 to 40 atomic volumes, suggestive of rate control by an interfacial Peierls barrier. The grain size dependence of the transformation-start temperature at a fixed cooling rate of 2.08 × 10-2 Ks-1 is measured in a Pu-1.7 at. pct Ga alloy, revealing an inhibition of transformation at fine grain sizes. The overall kinetic behavior is characteristic of a martensitic mechanism.