Structure and phase stability of a Pu-0.42 wt.% Ga alloy

This paper describes the characterisation of a cast plutonium-gallium (Pu-0.42 wt.% Ga) alloy, both in the as-cast condition as well as following an homogenising heat treatment. The alloy was subjected to density measurements, differential scanning calorimetry (DSC), dilatometry, optical microscopy, electron probe micro-analysis (EPMA), X-ray diffraction (XRD) and hardness measurements. The Ga content is insufficient to retain a wholly delta-Pu (δ-Pu) phase in the as-cast condition. However, the 250-h heat treatment at 450 °C is sufficient to redistribute the Ga resulting in an apparently stable δ-Pu phase. DSC and dilatometry did not indicate the presence of any alpha-Pu (α-Pu) phase in the heat-treated alloy. XRD patterns of the alloys also showed α-Pu to be present, although in the case of the heat-treated alloy this may be a consequence of incomplete removal of the transformed surface layer during the electro-polishing process. The stability of the δ-Pu phase in the heat-treated alloy was evaluated by cooling specimens to sub-zero temperatures. The alloy exhibited a high degree of stability when subjected to cold treatments at temperatures of between −50 °C and −90 °C.