Specimens were tested in tension, at temperatures ranging from 450 to 570C, using strain rates of 10-6 to 10-2/s. High (up to 0.7) strain-rate sensitivities, but relatively small elongations (120 to 280%), were observed. A single activation energy for deformation was found, within the present temperature and strain-rate ranges, which was close to the reported value for self-diffusion in Al. The strain rate and stress could be related by using a hyperbolic sine function at constant strain. Tensile specimens were quenched, and aged under stress, in order to preserve the dislocation structures. Transmission electron microscopy of the dislocation structures revealed a gradual and systematic change between power-law creep at higher strain rates and diffusional creep at lower strain rates. Within the superplastic regime, there was a tendency for small grains to be free of dislocations. Many of the dislocations had Burgers vectors which were inclined at about 45 to the stress axis. This suggested that they were activated in direct response to the applied stress, rather than as the result of a relaxation process which was associated with grain-boundary sliding.

F.Li, W.T.Roberts, P.S.Bate: Acta Materialia, 1996, 44[1], 217-33