Achieving excellent superplasticity in an ultrafine-grained QE22 alloy at both high strain rate and low-temperature regimes

The high-temperature tensile deformation behavior of ultrafine-grained (UFG) QE22 alloy developed by friction stir processing was investigated at various strain rates (5 x 10(-4) - 1 x 10(-2) s(-1)) and temperatures (300-450 degrees C). The UFG QE22 alloy exhibited excellent high strain rate superplasticity with the maximum elongation of 1630% at a temperature of 450 degrees C and strain rate of 1 x 10(-2) s(-1), which is the highest elongation reported till date in QE22 alloy. The UFG QE22 alloy also shows superior low-temperature superplasticity with a maximum elongation of 850% at a temperature of 350 degrees C and strain rate of 3 x 10(-3) s(-1). Thus, the developed UFG microstructure is found to demonstrate dual-mode superplasticity; both at high strain rate and low-temperature. The primary UFG microstructure and basal texture developed via multi-pass FSP, along with pinning action of Mg12Nd eutectic and fine nano precipitates of Mg12Nd2Ag at grain boundaries played a major role in contributing to the dual-mode superplasticity. The cavitation behavior of the UFG QE22 alloy during superplastic deformation in a dual-mode regime has been studied. The kinetics of superplastic deformation of UFG QE22 alloy was estimated and found to be significantly faster as compared to conventional magnesium alloys processed by various severe plastic deformation routes. (C) 2018 Elsevier B.V. All rights reserved.