AbstractThe effect of flow localisation and specimen thickness on the superplastic deformation of the commercial alloy Suprall 100 (Al–6Cu) is discussed. Optimal flow was accompanied by the formation and spread of intense‘shear bands’along grain/interphase boundaries. The spacing between the shear bands depended on both the strain rate of deformation and the specimen thickness. Superplasticity drastically decreased when the specimen thickness fell below about 10 grain diameters. The apparent activation volume was afunction of strain rate but was independent of specimen thickness, while the strain rate sensitivity index depended on both the strain rate and specimen thickness. The results are explained in terms of the theory that as well as being influenced by specimen geometry, the degree of superplasticity depends on the extent to which cooperative boundary sliding and boundary shear band formation are present during flow.MST/3076