Phase stability of an aged Pu–0.27 wt.% Ga alloy

Abstract

This paper describes the characterisation of a naturally aged Pu–0.27wt.% Ga alloy 35years of age. The alloy was subjected to bulk chemical analysis, density determination, differential scanning calorimetry (DSC), optical microscopy, electron probe micro-analysis (EPMA) and hardness measurements. Despite the Ga content being only 0.27wt.%, it nevertheless appears to be sufficient to retain the alloy in the δ-Pu phase at ambient temperature. This was demonstrated by optical microscopy, density measurements and DSC. However, the ambient temperature stability of the δ-Pu phase is tenuous. This was demonstrated by the propensity of the alloy to undergo transformation to α-Pu when cooled to sub-ambient temperatures. Indeed, both density measurements and DSC indicate that when the alloy is cooled to −50°C for 1h the alloy has transformed to more than 40% α-Pu. Moreover, a comparison of the Vickers hardness (expressed as a mean pressure) with transformation pressures for Pu–Ga suggests that the alloy has undergone δ→α′ transformation during the indentation process.