Microstructure development in pure aluminium processed by constrained groove pressing

Commercial purity aluminium (99.99 %) plates were processed with a severe plastic deformation technique - the Constrained Groove Pressing (CGP) at room temperature. It has been reported that using this technique, the coarse structure of pure aluminium can be refined to submicron ranges. The plates prior to CGP had a coarse-grained recrystallized structure with large scatter in grain size. Specimens were formed with two passes, each pass introducing the strain of about 0.58 in the deformed regions. The impact of the experimental forming upon microstructure was investigated with light microscopy and transmission electron microscopy of thin foils. Mechanical properties of selected specimens from different regions of the plates were measured by tensile tests. Hardness values (HV 0.3) measured in different segments of deformed specimens matched the strain distribution, and indicated some non-homogeneity in strain distribution. Development of microstructure analysed by transmission electron microscopy (TEM) proved formation of elongated grains with subgrain structure was found already after the first pass. The existence of grains with high angle boundaries already present in deformed structure was surprising and unexpected. The results show that the CGP with two passes resulted in formation of non-uniform microstructures with some new grains. Considering the results, a higher number of passes is required to obtain more advanced microstructure homogenization and grain refining.