Phase stability of α′-phase in a Pu-1 at% Ga alloy following low temperature treatment

Highlights: • Phase stability and transformation behaviour of low temperature induced α′-phase. • Reversion of α′-phase to δ-phase occurs via competing mechanisms and paths. • A direct α′–δ phase transformation that displays a martensitic athermal character. • An indirect transformation path with α′–β, β–γ and γ–δ phase -nucleation and growth. • Different conditions can drive the transformation via one mechanism or the other. - Abstract: A δ-phase (fcc) stabilised plutonium alloy will retain typical metallic characteristics of ductility and malleability from ambient temperature to 450 °C. The α′-phase (monoclinic) which is hard and brittle may be induced to form within the stabilised δ-phase depending on the degree of homogeneity of the alloy and the alloying content, thermal treatment and applied pressure. It is therefore of considerable interest to understand the conditions that will allow α′-phase to form and what impact this may have on the δ-phase properties. The scope of this study was to investigate the phase stability of a plutonium 1 at% gallium alloy (Pu-0.274 wt% Ga alloy) following treatment at low temperatures. The transformation of the induced α′-phase back to the δ-phase has been reported to occur either by an indirect path via the β and γ-phases or via both direct α′–δ phase transformation and indirect transformation paths. This work reports how differential scanning calorimetry (DSC) can be advantageously used to differentiate between the different mechanisms and transformation paths. This study also demonstrates that by conducting recovery anneals, inducing isothermal transformation and optimising the heating rates for each transformation, a different mechanism and transformation path may be favoured