The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ECAE

Equal channel angular extrusion (ECAE) has been used to investigate the formation of submicron grain structures in Al-alloys deformed to ultra-high plastic strains by different strain paths. The different strain paths were obtained by rotating billets through 0, 90, and 180° between each extrusion cycle. High resolution EBSD analysis has been employed to measure the boundary misorientations within the deformation structures. This has highlighted great differences in the evolution of the deformed state, as a function of the strain path, even after effective strains as high as 16. It has been demonstrated that the most effective method of forming a submicron grain structure by severe plastic deformation is to maintain a constant strain path. Processing routes involving a 180° rotation reverse the shear strain every second pass and this prevents the build up of significant numbers of new high angle boundaries. When a sample is processed with an alternate clockwise and anticlockwise 90° rotation, between each extrusion cycle the billet is deformed on two shear planes, each of which experiences half the total strain, compared to the single shear plane when there is no rotation. This reduces the rate of formation of high angle boundaries. With a constant clockwise 90° rotation the sample is also deformed on two alternate shear planes, but the total strain becomes redundant every fourth extrusion cycle. However, in this case each shear is reversed out of sequence after first deforming the billet on the alternate shear plane. This appears to be a much more effective means of forming new high angle boundaries than 180° rotation, where the shear strain is immediately reversed each alternate cycle, but is still less efficient than deformation with a constant strain path.