Two-dimensional dislocation dynamics and diffusion kinetics simulations were used to study the different mechanisms of plastic deformation of ultrafine-grained metals at different temperatures. Besides conventional plastic deformation by dislocation glide within the grains, grain-boundary mediated deformation and recovery mechanisms were also considered which were based upon the absorption of dislocations into grain boundaries. The material was modelled as an elastic continuum that contained a defect microstructure consisting of a pre-existing dislocation population, dislocation sources and grain boundaries. The mechanical response of the material to an external load was calculated with this model over a wide range of temperatures. It was found that at low homologous temperatures, the model material behaves in agreement with the classical Hall-Petch law. At high homologous temperatures, however, a pronounced grain-boundary softening and, moreover, a high strain-rate sensitivity of the model material was found. Qualitatively, these numerical results agreed well with experimental results known from the literature. It was therefore concluded that dynamic recovery processes at grain boundaries and grain-boundary diffusion were the rate-limiting processes during the plastic deformation of ultrafine-grained metals.

Mechanisms of Grain Boundary Softening and Strain-Rate Sensitivity in Deformation of Ultrafine-Grained Metals at High Temperatures. N.Ahmed, A.Hartmaier: Acta Materialia, 2011, 59[11], 4323-34