Evolution Of Microstructure In Al-Cu Alloy Processed By High-pressure Torsion And Multi-axial Compression

The homogeneous evolution of microstructure in Al-Cu binary alloy during multi-axial compression and high-pressure torsion was investigated by transmission electron microscopy and Vickers hardness,and the best process parameters were obtained to fabricate ultrafine-grained Al-Cu. Fourthmore, characteristics and homogenization mechanism of microstructure processed by HPT and MAC are illustrated respectively.The main results are as follows:(1)Al-Cu alloys respectively containing θ,θ’and θ" phases were processed by high pressure torsion at room temperatue. The results show there is a non-uniform distribution of microhardness values where r<3mm across the sample diameter, and microhardness near the edge reaches a saturation level after1revolution. After5whole revolutions, a reasonably homogeneous distribution of microhardness and microstructures can be realized, and ultrafine grains about250nm were obtained.(2)After4whole turns, the values of HV for the sample containing0’ phase decrease abruptly where r<3mm and for the sample containing0" phase, decrease on the whole. The former is related to the rapid and massive re-dissolution of0’phase and the latter to the dynamic recovery during HPT processing.(3)The ulrafine-grained Al-Cu can be fabricated by MAC processing through adjusting the process parameters. When processed by MAC with35whole turns at60℃, the structure of Al-Cu alloy containing0" phase was greatly refined (～100nm). When Al-Cu alloy was processed by MAC with35whole turns at100℃, the grain size of the finished sample is about500nm.(4)The particular technical characteristics in MAC and HPT result in the inherent inhomogeneity. The distribution of shear strain under torsion dependence on the distance from the centre of the disc reflects the non-uniform deformation process. However, the homogenous structure can be realized by adjusting the process parameters. The rigid zones subjected to friction are continuely expanded and deformation models are unsteady and inhomogeneous during every compresion so that the structure processed by MAC is extremely non-uniform. This conditon can be improved by increasing the number of compressing steps, but not eliminated. Estrin considers that HPT deformation technique provides near uniform simple shear which is favorable for dynamic recovery and recrystallization as compared to the pure shear deformation. This suggests HPT-processed microstructure is more homogenous and cotains lower dislocation in contrast to MAC.