Phase transformations in PuGa 1 at.% alloy: Study of whole reversion process following martensitic transformation

• Direct δ-to-α′ phase transformation at low temperatures without any γ′ phase.
• Competition between direct and indirect reversion at the first stage of reversion.
• Involvement of Ga diffusion associated with indirect reversion mode.
• Pure-Pu β and γ phases emergence during indirect reversion with δ-phase Ga enrichment.
• Reversion process of PuGa 1 at.% alloy following martensitic transformation.

The purpose of this work was to study PuGa 1 at.% alloy behavior from martensitic transformation to the whole reversion process. The relevant phase transformations were investigated as a function of temperature via in situ X-ray diffraction and dilatometry analyses carried out at low and high temperatures. The results show that the δ-to-α′ martensitic transformation occurring at low temperatures is direct and does not involve any intermediate γ′ phase. They also reveal that the reverse transformation of the two-phase δ + α′ alloy during heating involves two competing modes, namely direct and indirect reversion. The latter is associated with a Ga diffusion process that governs the ratio between these reversion modes. More precisely, our study demonstrates that the indirect reversion process consists of a Ga-enrichment of the remaining δ phase as well as the emergence of the β and γ phases of pure plutonium. Every stage of indirect reversion has been discussed in relation to the equilibrium phase diagram of the Pu–Ga binary system.