Evolution of microbands in high purity aluminium–3%magnesium during hot deformation testing in tension–compression

Microbands, as a dislocation substructure formed during deformation both at room and high temperature, have been recognised in both fcc (aluminium and copper) e.g. Refs. and bcc (steel) e.g. Refs. materials by several research groups. The structure consists of 'geometrically necessary dislocations' ', which are dislocations of the same sign. These form walls or 'geometrically necessary' boundaries leading to ingrain lattice curvatures. There is evidence to show that microbands evolve initially at small strains when only single or few slip systems are active and then becomes 'cell blocks' or equiaxed subgrains. The 'geometrically necessary' boundaries have higher misorientation angles across them than other dislocation subboundaries and have a significant effect on subsequent recrystallisation behaviour. During rolling or plane strain compression (PSC) deformation, the microbands form parallel plates oriented at an average 35 deg to the 'rolling' direction. In axisymmetric tension, however, the angle between the microbands and the stress direction varies from 25 deg to 75 deg . The objective of this paper is to clarify the microband orientation in axisymmetric deformation.