Multiple necking in superplastic Zn–22Al under uniaxial tension

The phenomenon of multiple necking in superplastic Zn–22Al was studied experimentally and analytically. Experiments revealed a proportional relation between the wavelength of multiple necking and the square root of the ratio between the strain rate and strain, which is explained on the basis of diffusion of defects under inhomogeneous strain distributions. The order of magnitude of the diffusion coefficient and the activation energy suggest that strain diffusion can result from the flow of atomic vacancies accompanied by grain boundary sliding in the superplastic state. Moreover, the analytic derivation shows the relation between the present model and second-order strain gradient theory.