Phase transformation in δ-Pu alloys at low temperature: An in situ microstructural characterization using X-ray diffraction

Abstract In order to investigate the martensitic transformation, an isothermal hold at −130 °C for 48 h was performed on a highly homogenized PuGa alloy. The modifications of the microstructure were characterized in situ thanks to a specific tool. This device was developed at the CEA–Valduc to analyze the crystalline structure of plutonium alloys as a function of temperature and more especially at low temperature using X-ray diffraction. The analysis of the recorded diffraction patterns highlighted that the martensitic transformation for this alloy is the result of a direct δ → α ′ + δ phase transformation. Moreover, a significant Bragg’s peaks broadening corresponding to the δ -phase was observed. A microstructural analysis was made to characterize anisotropic microstrain resulting from the stress induced by the unit cell volume difference between the δ and α ′ phases. The amount of α ′-phase evolved was analyzed within the framework of the Avrami theory in order to characterize the nucleation process. The results suggested that the growth mechanism corresponded to a general mechanism where the nucleation sites were in the δ -grain edges and the α ′-phase had a plate-like morphology.