Superplasticity governed by effective grain size and its distribution in fine-grained aluminum alloys

Although many efforts have been made to improve superplasticity in aluminum alloys by reducing grain size, the present study indicated that just reducing grain size was not sufficient. Three different grain-sized Al–Mg–Sc alloys with a similar misorientation distribution were fabricated by friction stir processing (FSP) and subjected to superplastic investigation. It was revealed that the medium-grained sample exhibited the largest elongation at the highest strain rate due to the optimum combination of proper fine grain and thermal stability. In-depth analyses showed that superplasticity was governed by the effective grain size (deff) and its distribution just before tension rather than the initial grain size (dini). The analyses on the superplastic data indicated that the large variation range in the dimensionless constant for superplastic deformation of FSP aluminum alloys was associated with the use of dini rather than deff. A modified constitutive equation was developed to elucidate the deff dependencies of superplastic flow stress which contributed to obtaining the true dimensionless constant and supporting the suggestion that the enhanced kinetics of grain boundary sliding was associated with high fraction of high-angle grain boundaries.