Influence of short time annealing on strain hardening rate and flow stress of ultrafine grained material processed by severe plastic deformation

摘要

In this paper, the ultrafine grained (UFG) copper achieved by the equal channel angular pressingnmethod was annealed at 473 K for 2 min and compressed at room temperature under quasi-staticnloading condition. The experimental results show that virtually no strain hardening is observednafter an initial rapid strain hardening over a small plastic strain regime. While for the as recievednmaterial, slight strain softening is even observed at room temperature. At the same time, thenannealed UFG copper also displays higher flow stress than its as received counterpart. Althoughnthere are several mechanisms including dynamic recovery and dynamic recrystallisationnproposed to explain the strain softening in nanocrystalline or UFG metals, the fact thatnenhancements in strain hardening and flow stress after annealing cannot be explained. Based onnthe experimental results, it is proposed that residual internal stress formed during the fabricationnof the UFG material may be a main reason for the enhancements of strain hardening and flownstress of annealed UFG copper.