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MM: Metall- und Materialphysik

MM 17: Symposium Severe Plastic Deformation IV

MM 17.1: Fachvortrag

Dienstag, 28. März 2006, 14:45–15:15, IFW A

Creep properties of ultrafine-grained metals — •Wolfgang Blum — University of Erlangen-Nürnberg, Inst. f. Werkstoffwissenschaften LS1, Martensstr. 5, 91058 Erlangen, Germany

At elevated temperatures ultrafine-grained (ufg) materials produced by severe plastic deformation attain a relative maximum of deformation resistance short after yielding. The relative maximum of flow stress at constant strain rate corresponds to a relative minimum in the rate of creep at constant stress. It is observed not only in tension, but also in compression and consequently respresents a deformation characteristic which is independent of fracture through necking, but means that deformation occurs close to steady state conditions where the dislocation structure is in a state of dynamic equilibrium of generation and annihilation of dislocations. The relatively high strain rate sensitivity of the flow stress corresponds to a low stress exponent of the creep rate. It is a major reason for the good ductility of ufg metals and even leads to superplastic behaviour in cases where the ufg structure is stable enough for the necessary rise in deformation temperature. The special properties of ufg metals come into apparence as the fraction of high-angle boundaries in the deformation induced subgrain structure becomes significant. They are explained by modification of dislocation structure evolution through relatively easy annihilation of dislocations at high-angle grain boundaries and direct strain contributions through grain boundary sliding and diffusive flow.