Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. Ga

摘要

The gallium-stabilized Pu-2.0 at. % Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable 8 phase to the gallium-containing, monoclinic a' phase near -100 ℃. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation (TTT) diagram, where two nose temperatures anchoring an upper- and lower-C describe minima in the time for the initiation of transformation. The underlying mechanisms responsible for the double-C behavior are currently unresolved, although recent experiments suggest that a conditioning treatment-wherein, following an anneal at 375 ℃, the sample is held at a sub-anneal temperature for a period of time-significantly influences the upper-C of the TTT diagram. As such, elucidating the effects of the conditioning treatment upon the δ→α' transformation can provide valuable insights into the fundamental mechanisms governing the double-C kinetics of the transition. Following a high-temperature anneal, a differential scanning calorimeter (DSC) was used to establish an optimal conditioning curve that depicts the amount of α' formed during the transformation as a function of conditioning temperature for a specified time. With the optimal conditioning curve as a baseline, the DSC was used to explore the circumstances under which the effects of the conditioning treatment were destroyed, resulting in little or no transformation.