Detection and quantification of residual α-phase in δ-stabilized plutonium

The temperature range of the δ-phase field of plutonium can be expanded by alloying with Group IIIA elements. Ga is a particularly potent δ-stabilizer and effectively stabilizes the δ-phase to room temperature. Due to a strong propensity to wards s olute redistribution during cooling through the e→δ phase field, regions of the material often do not contain enough solute to stabilize the δ-phase even after extensive homogenization annealing in the δ-phase field. The result is a small, but persistent, fraction of α-phase in the material. A technique using differential scanning calorimetry to measure the enthalpy of transformation of the plutonium α→β transformation is described which can detect and quantify α-phase in a δ-phase matrix at levels as low as ~0.1 wt. %. Complications arise due to interference from the pressure-induced α-phase, and a peak separation method was developed to accurately measure the heat signal from each phase. A set of δ-stabilized Pu-1.7 atomic % Ga alloys was examined using the technique and found to contain 0.32 ± 0.06 weight % α-phase. The onset temperature of the α→β transformation in these specimens was found to be 140.2°C, significantly higher than that for the transformation in pure plutonium, 126.2°C. This increase in onset temperature is a consequence of significant Ga content in the α-phase.