Deformation kinetics of ultrafine-grained Cu and Ti

Ultrafine-grained (UFG) pure Cu exhibits softening compared to coarse-grained (CG) Cu in the steady state of deformation at elevated homologous temperatures below 0.31, while commercially pure UFG Ti does not. The softening of UFG Cu can be explained by the fact that the spacing d0 of high-angle grain boundaries is smaller than the subgrain size w∞,CG in CG Cu would be, allowing grain boundaries to reduce the steady-state dislocation density; in addition, grain boundary sliding and diffusive flow by Coble creep are likely to contribute to the softening at elevated temperature. The lack of such softening in the investigated UFG Ti is consistent with d0>w∞,CG, but not with a simple estimate of the rate of Coble creep.