Influence of grain boundaries on the deformation resistance: insights from an investigation of deformation kinetics and microstructure of copper after predeformation by ECAP

The grain structure of coarse-grained Cu of different degrees of purity was modified by plastic deformation by p = 2, 4 and 8 passes of equal channel angular pressing (ECAP) at an ambient temperature with and without back pressure. The effect of this modification on deformation resistance was tested in rate change tests in compression at elevated temperatures up to 473 K. Electron microscopy and compressive strength evolution with strain confirmed that static recrystallization leads to grain coarsening during heating to test temperatures of 423 K for p > 4 and 473 K for p = 2 and that discontinuous grain coarsening by dynamic recrystallization sets in already at 400 K. The influence of the grain structure on the deformation resistance is discussed in terms of a control loop considering generation and loss of free dislocations. The magnitude and rate dependence of the maximal deformation resistance of unrecrystallized ECAPed Cu were explained in terms of dynamic recovery at low-and high-angle boundaries as proposed previously. The interpretation sheds doubt on the common interpretation of rate change experiments in terms of thermally activated glide.