Effects of changes in strain path on work-hardening in CP aluminium and an AlCuMg alloy

Abstract

Effects of abrupt changes in the direction of plastic deformation on work-hardening behaviour have been investigated in two-stage stretching of sheets of AA1050 and a heat-treated 2014 aluminium alloy aged at various temperatures up to 300°C. The results show that reorganisation of dislocation distribution after a change in strain path can result in transient changes in work-hardening behaviour of two kinds. Changes of the first kind, which tend to increase the hardening rate in early stages of the second mode of deformation, are associated with reorientation of internal stresses. Changes in the second kind, which tend to cause transient reductions in hardening rate, are believed to be associated with partial dissolution of the original dislocation substructure. The relative magnitudes and strain dependencies of these two kinds of change depend on the deformation sequence and on material variables. The change in hardening rate of CP aluminium after a change in strain path is dominated by changes of the second kind which, after moderate prestrains, cause reductions in the limits of stable elongation. In similar tests on overaged conditions of the 2014 alloy the overall changes in hardening rate are dominated by changes of the first kind, so that the limit of uniform elongation is increased by a change in strain path. When dynamic ageing is active in the 2014 alloy changes of the second kind can be suppressed so that reductions in the hardening rate do not occur.