High strain rate superplasticity of a fine-grained AZ91 magnesium alloy prepared by friction stir processing

Abstract

Friction stir processing (FSP) is a novel severe plastic deformation technique to prepare fine-grained metallic materials. Superplastic behaviors of FSP magnesium alloys have not been fully studied until now. In this study, microstructure and tensile behaviors of a fine-grained Mg-9Al-1Zn alloy prepared by FSP under water are investigated. It is found that the process results in significant grain refinement, and the second phase networks are broken into small particles. The FSP specimens exhibit excellent high strain rate superplasticity, with an elongation of 990% at a strain rate of 2×10 -2 s-1 and 623K. Microstructure evolution and superplastic deformation mechanism of the experimental materials during superplastic tensile test are investigated. Grain boundary sliding is the main mechanism for the superplastic deformation of the specimens. The excellent superplasticity of the FSP material is attributed to its fine grain structures, which contain a larger fraction of grain boundary.