Enhanced superplasticity in an extruded high strength Mg–Gd–Y–Zr alloy with Ag addition

Abstract

The effect of 2wt% Ag addition on the superplastic behavior of an extruded Mg–8.5Gd–2.5Y–0.5Zr (wt%) alloy was investigated by impression testing in the temperature range of 523–598K. The average sizes of the dynamically recrystallized grains of the Ag-free and Ag-containing alloys were about 8 and 3μm, respectively. Analysis of electron backscattered diffraction (EBSD) data confirmed the higher fractions of high-angle grain boundaries (HAGBs) in the Ag-containing alloy. The deformation response of this alloy in proper temperature range conforms to regions I, II and III, typical of superplastic deformation behavior. The addition of Ag to the base alloys led to enhanced superplasticity in region II by increasing the strain rate sensitivity (SRS) indices (m-values) from 0.25 to 0.51 and 0.36 to 0.46 at 573 and 598K, respectively. These high m-values together with the activation energy of 181kJ/mol suggest that the major mechanism involved in superplastic deformation is grain boundary sliding (GBS) accommodated by lattice diffusion at temperatures above 573K.