Microstructural evidence for conditioning-dependent δ → α′ transformations in retained δ-phase Pu-Ga

The retained δ phase of a Pu-1.9 at.% Ga alloy is metastable with respect to the martensitic δ → α′ transformation that occurs at low temperatures. This transformation has been shown to proceed by means of an isothermal martensitic mode, but the kinetics of the transformation are atypical. The transformation exhibits a “double-C” in a time-temperature-transformation diagram, wherein there exist two temperatures at which transformation occurs in a minimum amount of time. The cause of the double-C kinetics remains uncertain, eliciting proposals of multiple mechanisms, multiple paths, or different morphologies as possible origins. Recently, a “conditioning” treatment was found to affect the δ → α′ transformation, but the underlying mechanism by which the conditioning treatment influences the transformation has not yet been resolved. In this study, microstructural characterization as a function of temperature, time, and conditioning has been employed to illuminate the role of conditioning in the δ → α′ transformation. Conditioning is found to enhance transformation in the upper-C and to enable transformation in the lower-C. The data garnered from these experiments suggest that conditioning is intimately linked to nucleation processes, but is of little consequence to the growth and morphology of the α′ product phase.