Evidence for Conditioning as a Progenitor of Double-C Transformation Mechanisms in Pu-Ga Alloys.*

By alloying Pu with Ga, the fcc $\delta$ phase can be retained down to room temperature. This metastable $\delta $ phase is realized due to slow Ga diffusion, which prevents the $\delta$ phase from decomposing into the equilibrium mixed phase structures. The metastable $\delta$ phase in a Pu-1.9 at.{\%} Ga alloy, however, does yield to chemical driving forces by undergoing the $\delta \quad \to \quad \alpha $' isothermal martensitic transformation below M$_{s} \quad \approx $ -100 \r{ }C. This transformation exhibits poorly understood double-C behavior in the time-temperature-transformation diagram. Recently, a ``conditioning'' treatment---which entails isothermally holding a specimen at sub-anneal temperatures but above M$_{s}$ ---has been shown to dramatically affect the amount of $\alpha $' phase formed at low temperature. We report evidence that the conditioning treatment induces the lower-C of the double-C curve, and we implicate the classical nucleation of equilibrium phases as the underlying mechanism behind conditioning in Pu-Ga alloys.