Stability of ultrafine-grained Cu to subgrain coarsening and recrystallization in annealing and deformation at elevated temperatures

Pure, ultrafine-grained (UFG) Cu produced by equal channel angular pressing (route BCBC) at ambient temperature was deformed in uniaxial compression at elevated temperatures up to 418K=0.31Tm (TmTm: melting point). The deformation is accompanied by softening. Analysis of the deformation behavior and microstructural observations provide evidence that the softening is related to homogeneous growth of the subgrain size towards a steady-state value which depends on stress as in materials of conventional grain size and is related to growth of the spacing of high-angle boundaries. The softening can be explained by a recently published dislocation model of deformation of UFG materials. In addition, local coarsening of the structure by recrystallization produces bimodal grain structures and leads to strong softening when the temperature is sufficiently high during slow deformation after prior fast deformation and during annealing.