Microstructural evolution and mechanical properties of high purity aluminium processed by equal-channel angular pressing

High purity (99.999%) aluminium was subjected to Equal-Channel Angular Pressing (ECAP) at room temperature for up to 8 passes through route B _c using a die having a channel angle of 90°. Hardness measurements and tensile tests were conducted at room temperature with respect to the number of ECAP passes. It is shown that the hardness and the tensile strength increased to take a maximum at 2 passes. However, unusual behaviour was observed such that further straining by ECAP for more than 2 passes decreased the hardness and tensile strength. Furthermore, the total elongation and the uniform elongation were enhanced with straining by ECAP. Microstructural observation using transmission electron microscopy revealed that straining by ECAP for 2 passes led to the development of a subgrain structure having dislocations within some subgrains and tangled dislocations near subgrain boundaries. After 8 passes, the grains became lager with the grain size of ∼15 μm and with the grain boundaries sharpened and clearly defined. Within such grains, subgrain boundaries were formed with an average separation of ∼3 μm and with the dislocation density very low. It is considered that the high dislocation mobility arising from high stacking fault energy and very low level of impurities were responsible for the unusual tensile behaviour and the difficulty in reducing the grain size to the submicrometer level. It is also considered that the grain refinement occurred due to evolution of subgrain boundaries to high angle boundaries, initially mutual coalescence of dislocations to subgrain boundaries followed by increasing misorientation angles by merging nearbydislocations.