Influence of processing route on microstructure and grain boundary development during equal-channel angular pressing of pure aluminum

High-purity (99.99%) aluminum that had been subjected to equal-channel angular pressing (ECAP) was analyzed by orientation imaging microscopy (OIM). The analysis of microtexture and microstructure by OIM will be reviewed. The ECAP pressing was conducted at room temperature with a die that had a 90° angle between the die channels and repetitive pressings followed either route A, B_C or C. Billets were examined after one pass and after four or twelve passes by each of the ECAP routes. After one pressing operation, the deformation-induced microstructure was inhomogeneous at the resolution of OIM and consisted mostly of subgrains. Following four pressings by each of the routes, the microstructures were homogeneous and exhibited similar (sub)grain sizes (∼1.3 μm). Elongation and alignment of the (sub)grains with the shearing direction of the last pressing operation was observed in all cases. The corresponding disorientation distributions showed significant increases in the fractions of high-angle boundaries (θ>15°) although processing route had little apparent effect on the observed distributions. However, distinct differences were noted in the microtexture data. After 12 pressing operations by any of the different routes the grain size was reduced further to about 1.0μm. The fraction of high-angle boundaries also increased slightly but, again, processing route had little effect on the disorientation distribution. Different textures were apparent for each processing route, although texture data for all routes indicated that a <111> tended to align with the shear plane of the final pressing pass.