The deformation behaviors of ultrafine-grained pure material with a grain size of about 350nm produced by severe plastic deformation, and of coarse-grained material, were investigated as a function of temperature and strain rate. The grain structure was fairly stable below 450K (33% of the melting point). Here, the grain boundaries in ultrafine-grained material had a strengthening effect with regard to plastic yielding. With increasing strain, increasing temperature and decreasing strain rate, however, the strengthening was turned to softening by grain boundaries. This transition was related to a distinctly higher strain-rate sensitivity of ultrafine-grained material as compared with coarse-grained material, and an activation energy for flow of the order of 100kJ/mol. In the steady state, the range of softening by grain boundaries extended from the stability limit of the grain structure of ultrafine-grained material down to a temperature which was close to room temperature; where ultrafine-grained material was produced. The transition from strengthening to softening was explained in terms of the generation and grain-boundary diffusion-controlled annihilation of lattice dislocations at grain boundaries.

Transition from Strengthening to Softening by Grain Boundaries in Ultrafine-Grained Cu. Y.J.Li, X.H.Zeng, W.Blum: Acta Materialia, 2004, 52[17], 5009-18