Effect of grain size and specimen dimensions on micro-forming of high purity aluminum

Micro-compression testing was conducted using high purity Al processed by equal-channel angular pressing (ECAP) with both coarse-grained (CG) and ultrafine-grained (UFG) samples. The effects on the flow stresses of the initial grain size and the specimen size were investigated and the results show the initial grain sizes and the specimen dimensions affect the flow stresses during micro-compression for both CG and UFG specimens. There is a transition from strain hardening to strain softening with decreasing grain size during micro-compression but the transition grain size is dependent upon the size of the specimen. These results are interpreted using a model based on the separate influences of dislocation annihilation and dislocation accumulation in the UFG and CG materials, respectively. The results demonstrate that the occurrence of surface roughening is improved when using UFG pure Al and this shows there is a significant potential for using UFG pure Al in micro-forming operations.