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

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