A proposed deformation mechanism for high strain-rate superplasticity

Superplasticity at high deformation rates is desirable in order to make superplastic forming more practical. Recently, superplasticity was achieved at strain rates between 10{sup {minus}2} and 100 s{sup {minus}1} in powder-metallurgy (PM) processed and mechanically alloyed (MA) aluminum alloys. The purpose of this paper is to understand the rate-controlling-deformation mechanisms for ultrafine-grained aluminum materials where high-strain-rate superplasticity is observed. The stress-strain rate relations for IM, PM and MA materials are analyzed in terms of deformation mechanism maps, and in terms of the contribution of subgrain boundaries to the enhancement of grain boundary sliding. Conclusions are as follows: (1) Lattice-diffusion-controlled grain boundary sliding is believed to be the rate-controlling mechanisms for high-strain-rate superplasticity in PM and MA aluminum alloys. (2) Construction of deformation mechanisms maps based on diffusional creep, grain boundary sliding and dislocation creep illustrate the importance of grain size in achieving high-strain- rate-superplasticity. (3) The Ball-Hutchinson model for grain-boundary sliding has been modified to include the influence of subgrain size in enhancing the grain boundary sliding process. (4) The introduction of threshold stress for grain boundary sliding and the important contribution of grain and subgrains permit unification of all superplastic IM, PM and MA aluminum data available.