Consequences of the Special Geometry of $Uranium's Yield Surface on the Analysis of Dislocation Activity

αUranium formed by the β -> α phase transformation is stressed due to the volume mismatch between the parent and the product phases. It has been suggested that sub grains formed in the a phase are the result of dislocation slip to relax these transformation stresses. This hypothesis is analyzed by combination of computational tools and transmission electron microscopy. Methods for the calculations of the texture and microstructure developed during plastic forming is utilized to this microscopic phenomena. The theory is required to describe the structures formed due to the activity and interactions of huge numbers of dislocations for a wide range of materials and processing conditions. Although dislocation theory can give good predictions regarding the general plastic properties of materials. it cannot be easily applied to plastic forming because of the complex interactions and the dense dislocation structures formed during such processes. For many years the cell formation and texture development of highly symmetric 'simple' materials were studied. The high crystallographic symmetry gives rise to an intrinsic ambiguity concerning the type of dislocations trapped at the cell walls. Our work demonstrates that low symmetry materials are favorable as model systems for the investigation of dislocation boundary formation due to the small number of slip systems composing every vertex of the yield surface. U-0.1%Cr samples were heat treated at 720 degree C (β phase) for 0.5 hours and then furnace cooled to room temperature. The samples were examined using optical and scanning electron microscopy followed by TEM characterization. Using single crystal plasticity theory. the yield surface of a-uranium was calculated. Than the stresses prevailing in a newly formed a particle in the β matrix were calculated by solving the elasto-plastic inclusion problem. Both results were utilized to predict the relative activity of each slip system during the growth of a-uranium. Finally it is shown how the subgrain-boundaries in the a-uranium can be constructed by the operative slip dislocations