Transformation crystallography and plasticity of the δ→ α′ transformation in plutonium alloys

In delta phase Pu-Ga alloys, the transformation from the ductile face-centered cubic (fcc) delta phase that is retained at room temperature to the brittle low-temperature monoclinic alpha-prime phase is a thermally activated diffusionless transformation with double-c kinetics. Accurate modeling of the phase transformation requires detailed understanding of the role of plastic flow during the transformation and of the crystallographic transformation path. Using transmission electron microscopy (TEM), we find a significant increase in dislocation density in delta near the alpha' plates, which suggests that plastic deformation contributes to the accommodation of the 20% reduction in volume during the transformation. Analysis of a series of optical micrographs of partially transformed alloys suggests that the alpha' habit plane is usually nearly perpendicular to <111> delta. However, a small number of TEM observations support a habit plane near <112> or <123>, in agreement with earlier work.