Connections between the Pu–Ga phase diagram in the Pu-rich region and the low temperature phase transformations

Significant progress has been made in recent years regarding the equilibrium phase diagram of the Pu–Ga system. For decades, researchers outside of the former Soviet Union considered the δ phase in Pu–Ga alloys to be in the state of thermodynamic equilibrium down to low temperatures. Recently, decades of experimental work in Russia have been published that indicate the existence of a eutectoid decomposition of the metastable δ phase to α phase and Pu3Ga. Phase diagram calculations by a number of researchers have predicted this eutectoid transformation as well. In this work, we review the experimental and calculated phase diagrams and also comment on the expected consequences of the IIIrd law of thermodynamics with respect to the form of the delta phase field. Assembled data from the literature on the martensite start (MS) and reversion (RS) temperatures for the δ → α′ transformation as a function of Ga content are used to consider the possible trend of the T0 line describing the equivalence of the free energies of the α and δ phases in the phase diagram. It is suggested that in order to reconcile the need for the existence of a driving force for both the compositionally variant eutectoid reaction, as well as a compositionally invariant martensitic transformation in a Pu—2 at.% Ga alloy, the actual T0 line may lie higher than that indicated by the present thermodynamical modeling.