Microstructural evolution and mechanismsof superplasticity in an Al-4-5%Mg alloy

An AI-4.5%Mg alloy (AA 5083) exhibited superplasticity over the temperature range from 450 to 570°C and strain rate range from 10⁻⁴ to 10⁻² s⁻¹ with high strain rate sensitivities (maximum 0.7). A single activation energy, being close to that for aluminium selfdiffusion, was found for the testing conditions used. Using a technique of quenching and aging under load, the dislocation structures developed during the deformation were retained. A gradual and systematic change was observed between dislocation creep at the high strain rates and diffusion creep at the low strain rates. The grains grew and became elongated during the deformation. The amount of grain extension along the tensile stress in the superplastic range, however, was significantly smaller than the tensile strain, indicating grain boundary sliding (GBS). It is suggested that highly strain rate sensitive plastic flow can be achieved when GBS is controlled by intragranular dislocation climb. Superplasticity in this alloy was a combined action of such a process and conventional dislocation creep.