Superplasticity and the development of dislocation structures in an Al-4.5% Mg alloy

The mechanical behaviour and development of microstructure in tension of an Al-4.5% Mg alloy at temperatures from 450 to 570 degrees C and strain rates from 10(-6) to 10(-2) s(-1) has been investigated. High strain rate sensitivities, up to m = 0.7, but relatively small elongations, between 120 and 280%, were observed. A single activation energy for deformation, close to reported values for self diffusion in aluminium, was obtained over the temperature and strain rate rang used, and the strain rate and stress could be related using a hyperbolic sine function at constant strain. Tensile specimens were quenched and aged under stress to preserve the dislocation structures for examination by transmission electron microscopy. The dislocation structures observed showed a gradual and systematic change between regimes of power law creep at higher strain rates and diffusional creep at lower strain rates, and in the superplastic regime there was a tendency for small grains to be free of dislocations. Many of the dislocations had Burgers vectors inclined at about 45 degrees to the stress axis, which suggests that they were active directly in response to the applied stress rather than as the result of a relaxation process accommodating grain boundary sliding.