Supporting evidence for double-C curve kinetics in the isothermal δ → α′ phase transformation in a Pu–Ga alloy

Time-temperature-transformation (TTT) diagrams for the δ → α′ transformation in a number of Pu–Ga alloys were first reported in 1975 by Orme et al. Unlike typical single-C curve kinetics observed in most isothermal martensitic transformations, the Pu–1.9 at.% Ga alloy exhibits two noses, and thus double-C curve kinetics. The authors attributed the occurrence of the double-C to a difference in mechanism: a massive transformation for the upper C and a martensitic transformation for the lower C. Since that time, the nature, and the existence of the double C have received only limited attention. The results of Deloffre et al. suggest a confirmation of this behavior, but the fundamental origin of the double C remains unknown. Here, we apply differential scanning calorimetry (DSC) as an alternative approach to acquiring the TTT data and our experimental evidence suggests a confirmation of the double-C behavior after 18 h of isothermal hold time. In addition, we report three exothermic peaks corresponding to transformations during cooling at 20 °C/min prior to the isothermal holds. These three peaks are reproducible and suggest a number of possibilities for the origin of the unique kinetics: α′ forms with different morphologies, or from different embryos in the upper and lower C curves; α′ forms directly in one C curve and forms via an intermediate phase in the other C curve; the two C curves result from α′ forming by two or more distinct mechanisms (e.g., massive and martensitic transformations).