Structure evolution and deformation resistance in production and application of ultrafine-grained materials - The concept of steady-state grains

Severe plastic predeformation of crystalline materials leads not only to formation of a steady-state dislocation structure including low-angle boundaries, but also brings the high-angle boundary structure into a steady state. When the steady-state flow stress is high enough, the material becomes ultrafine-grained or even nanocrystalline. The change from coarse-grained to ultrafine-grained is accompanied by a distinct change in the steady-state deformation resistance that is measured after predeformation. This is explained by two opposing effects of high-angle boundaries, namely enhanced dislocation storage and accelerated dislocation recovery. The first one causes net hardening at high temperature-normalized strain rate Z (Zener-Hollomon), the second one net softening at low Z. This means that the rate-sensitivity of the flow stress is enhanced, which causes the paradoxon of enhanced strength at enhanced ductility. Tests with abrupt large changes of deformation conditions bring the strain associated with dynamic recovery into the focus. The results of such tests indicate that the boundaries, low-angle as well as high-angle ones, migrate under concentrated stress during deformation and thereby contribute to straining and recovery. The corresponding system of differential equations needed to model structure evolution and deformation kinetics on a semi-empirical basis is briefly discussed. © (2011) Trans Tech Publications, Switzerland.