In situ X-ray diffraction study of the δ to α′ isothermal martensitic transformation kinetics in a Pu–Ga alloy

The sequence of phase formation and the isothermal kinetics of the δ to α′ phase transformation in a Pu–1.9 at.% Ga alloy are investigated with in situ X-ray diffraction at the Advanced Photon Source. It has been proposed that the formation of a γ′ intermediate phase may be responsible for the unusual double-C curve kinetics reported for this isothermal martensitic transformation. While a γ′ intermediate phase was not observed at either −120 °C or −155 °C, an unexplained shoulder on the δ (1 1 1) peak was detected at −155 °C. The isothermal kinetics of the δ → α′ transformation appear to fit a Johnson–Mehl–Avrami–Kolmogorov equation, but the exponent is not described by existing JMAK theory. The double-C curve kinetics of the δ to α′ transformation remain unresolved in plutonium science, and understanding this behavior is key to predicting phase stability in aging Pu–Ga alloys.