Microstructural analysis of the δ to α' phase transformation in plutonium alloys using X-ray diffraction

The partial martensitic delta ->alpha' transformation in plutonium alloys is sensitive to chemical composition, sample thermal history, as well as crystalline defects. The present work investigates the delta-Pu phase microstructure before and after the martensitic transformation delta ->delta+alpha'. More precisely, microstructural modifications of the host delta-phase, resulting from the stress induced by a cell volume difference of 19% between the delta and alpha'-phases, were analysed. Microstructural information about crystallite size and microstrain of a highly homogenized Pu-Ga alloy was extracted from x-ray diffraction patterns using a three dimension crystallite size and microstrain model. This is available in Rietveld refinement software and consists in anisotropic broadening analysis of diffraction peaks. Crystallite size doesn't significantly change with the phase transformation contrary to microstrain that is multiplied, on average, by five. Furthermore, internal normal and shear microstress are multiplied, respectively, by 2 and 13 when alpha'-phase appears. Last, dislocation densities, calculated from crystallite size and microstrain, are compared to TEM results available in the literature.