Influence of deformation path on the strain hardening behavior and microstructure evolution in low SFE FCC metals

Following our recent studies of the influence of mechanical twinning on the strain hardening of low SFE FCC metals deformed by simple compression, the investigation was extended to two different deformation modes. These were plane strain compression and simple shear carried out on 70/30 brass, which exhibits only strain hardening, and on MP35N, a Co-Ni based alloy that als o shows secondary hardening by deformation promoted precipitation. It was f ound that the magnitude of the primary strain hardening in both alloys, and the secondary hardening in MP35N, was dramatically reduced under simple sh ear compared to the other deformation paths. This reduced hardening in simp le shear appears to be a consequence of the bulk of the deformation twins, and also the secondary hardening precipitates, forming on planes that were parallel to the primary {111} slip planes in this deformation path. These h ypotheses are supported by deformation path change tests in which the shear samples that show low flow stress under continued shear, when subjected to simple compression showed a significant increase (jump) in the flow stress , reaching values that are similar to those of the alloy continuously compr essed to the same equivalent strain. That is, the reduced strain hardening in shear deformation is due not to reduced twinning, but to the twins produ ced by shear providing only limited barriers to continued strain by simple shear. Shear banding was found to be more marked in plane strain compressio n than in simple compression after cold working, and particularly after the additional secondary hardening in MP35N. (C) 2001 Elsevier Science Ltd. Al l rights reserved.