In search of θ-(Pu,Zr) in binary Pu–Zr: Thermal and microstructural analyses of Pu − 30Zr alloy

The existing Pu–Zr binary phase diagrams report the stability of the compound θ-(Pu,Zr) in the low temperature region between 300 and 0 °C. Furthermore, the current understanding is that θ-(Pu–Zr) is thermodynamically favored over the metastable δ-(Pu,Zr) phase. In an effort to shed light on the phases formed in Pu–Zr binary alloys and reduce uncertainties in the poorly defined boundary between the θ-(Pu,Zr), (θ+δ), δ-(Pu,Zr), and (θ+α-Zr) regions, Pu − 30Zr (in wt.%, equivalent 53 at.%) alloys were subjected to microstructural characterization, annealing, and differential scanning calorimetry (DSC). The results indicate that the alloy is composed of δ-(Pu,Zr) matrix, with a number of smaller, randomly distributed α-Zr precipitates. The phase transition temperatures (determined based on the DSC data) and phases identified in Pu − 30Zr alloys (based on crystallographic data) compare well to those predicted by the phase diagrams with the exception of the θ-(Pu–Zr) phase. Our data indicates that θ-(Pu–Zr) is metastable and can be observed only within a small temperature window (100–300 °C). The consecutive heating cycles remove θ-(Pu–Zr) from the system, and no traces of θ-(Pu–Zr) remain at room temperature, as evidenced by microstructural characterization. This calls for reevaluation of the binary Pu–Zr phase diagram, with particular attention paid to the existence of θ-(Pu–Zr) and its stability.