Hot deformation behavior study of coarse grained and ultrafine grained QE22 magnesium alloy through development of Constitutive analysis and Johnson –Cook model

Abstract

To enhance high-temperature applicability of ultrafine-grained (UFG) magnesium (Mg) alloy, identifying the optimal stress, strain, and temperature for hot deformation is crucial. This study examines hot deformation behavior of coarse grained (CG) QE22 and UFG QE22 Mg alloy developed through multiple-pass Friction Stir Processing (FSP). The grain size of UFG material following FSP decreased significantly to 0.65 µm, compared to CG material, which had an initial grain size of 38 µm. The true stress for both CG and UFG QE22 Mg alloy decreases with increasing deformation temperature and decreasing strain rate due to enhanced atomic mobility and reduced dislocation density at higher temperatures, as well as less pronounced strain hardening at lower strain rates. The hot deformation behavior was analyzed using the Johnson Cook (JC) model and constitutive analysis on uniaxial compressive tests conducted at temperatures ranging from 250 °C to 450 °C and strain rates from 10^-3 to 10 s^-1. Both models demonstrated a decrease in yield strength, strain hardening exponent, activation energy, and stress exponent for the UFG alloy. These values decreased from 175 MPa, 1.2, 240.5 KJ/mol, and 10.5 in CG material to 164 MPa, 1.05, 120.9 KJ/mol, and 3.6, respectively, in UFG alloy. The dynamic recrystallization (DRX) governing deformation of CG, while grain boundary sliding drives UFG QE22Mg alloy. A correlation between the JC model and the constitutive equation was established through detailed microstructural analysis, offering insights into the mechanisms governing the enhanced high-temperature performance of UFG QE22 Mg alloy.