Phase transformations in Pu-Ga alloy: Synergy between simulations and experiments to elucidate direct and indirect reversion competition

Reverse transformation in plutonium alloyed with a small amount of gallium (Pu-1 at.% Ga) was investigated in detail by coupling CALPHAD-based simulations with experiments. As observed previously, reverse transformation in a two-phase δ + α′ alloy exhibits the existence of competition between direct reversion (α1at.%′→δ1at.%) and indirect reversion (α1at.%′→βpure Pu(+δe)→γpure Pu(+δe)→δ1at.%). The latter leads to Ga enrichment of the δ phase (δe) as well as the emergence of several phases of pure Pu. The present paper focuses on the thermodynamic and kinetic aspects of the competition between these two reversion modes, especially between direct reversion and the first step of indirect reversion, whenever competition occurs. Thermodynamic simulations indicated that both reversion modes were possible and that indirect reversion was more favorable. Specific isothermal experiments combined with simulations revealed the existence of two potential Ga diffusion paths during indirect reversion: (i) from the α′ phase to the δ phase and (ii) from a new transient β′ phase, preceding the appearance of β phase of pure Pu, to the δ phase. These newly discovered mechanisms have enabled the present authors to fully understand the competition between the reversion modes. Finally, the ratio of direct to indirect reversion was found to depend on three factors, namely heating rate, initial α′ phase fraction and Ga content. All simulations were confirmed experimentally and highlighted key aspects of reversion mechanisms.