Microstructural characterization of the as-cast and annealed Pu-10Zr alloy

Phase diagrams constructed for the Pu-Zr binary system report the existence of θ-(Pu,Zr), δ-(Pu,Zr), γ-Pu, α-Pu, β-Pu, and ζ-Pu₂₈Zr phases in Pu-enriched region, when Pu concentrations exceed 75 at.% (equivalent to 89 wt% Pu). The compound θ-(Pu,Zr) has been said to occur at about 20 at.% Zr, but the regions for θ-(Pu,Zr) on the Pu-rich side have not been well defined. In an effort to understand the phases formed in Pu-Zr binary alloys and define the boundary between θ-(Pu,Zr), (θ+δ), and δ-(Pu,Zr) regions, Pu-10Zr (in wt.%, equivalent 23 at.%) alloys were scrutinized in this contribution. This manuscript details microstructural characterization and phase identification results obtained using electron microscopy-based techniques for a Pu-10Zr alloy before and after annealing at 550 °C. In contradiction with predicted phase diagrams, no θ-(Pu,Zr) was detected in any of the examined specimens. The phases present in as-cast fuel included a δ-(Pu,Zr) matrix with a number of smaller, randomly distributed α-Zr, ZrO₂, Zr₃O, PuO, and κ-PuZr₂ inclusions. Heat treatment annealed out the intermetallic κ-PuZr₂ phase and resulted in formation of small δ′-Pu and β-Pu inclusions. Similar to the as-cast alloys, the matrix of the annealed alloys was consistent with δ-(Pu,Zr) phase and contained small α-Zr, ZrO₂, and PuO₂ inclusions. Two different microscopy-based techniques were used for phase identification, but neither identified any θ-(Pu,Zr) phase. Differential scanning calorimetry was then used to determine the phase transition temperatures and enthalpies of transition for the identified phases. While our data is similar to the existing phase diagrams, a number of discrepancies are reported that call for a careful re-examination of the Pu-Zr system. This contribution discusses different scenarios that could explain the discrepancies between obtained and historical data, and provides the most logical conclusion that could be reached based on the obtained results.