Microstructural evolution in high purity aluminum processed by ECAP

High purity (99.99%) aluminum was processed by equal-channel angular pressing (ECAP) through 1–12 passes and examined using orientation imaging microscopy. The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to an essentially equiaxed array of ultrafine grains and from 4 to 12 passes there is no measurable change in the average grain size and grain aspect ratio. The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 12 passes. Anomalously large grains were visible in the central region of the billet pressed through 12 passes due to dynamic recovery and grain growth. The results suggest optimum processing is achieved by pressing through 4–8 passes.