Large strain plastic deformation of commercially pure nickel

Commercially pure nickel was deformed to very large strains by cold rolling. Tensile tests on prestrained sheet were used to establish strain hardening behaviour and residual ductility. The hardening of nickel was found to be peculiar: initial rapid hardening was followed by an apparent plateau, which, in turn, was followed by another stage of rapid hardening at strains larger than 4. The residual elongation dropped precipitously with initial prestrain, levelled off, and then increased again during the final stage of hardening. Nickel rolled to a strain of 6·1 exhibited a remarkable tensile strength of 1400 MN m⁻² with 4% total elongation. Extensive in-plane and limited edge-on transmission electron microscopy was used to examine the evolution of substructure. Dislocation cells formed at strains <0·10 and continued to refine in size with increasing strain. Definite evidence of dynamic recovery was found. The formation of elongated, well defined subgrains appears to coincide with a saturation of hardening in the plateau region. During the final stage of hardening the substructure became cut up into small, more equiaxed domains with subgrains becoming more numerous. The hardening transition may be caused by the re-emergence of original grain boundaries as important structural features at very large strains.