Effect of Variable Strain Rate on Hot Deformation Behavior of As-Extruded Mg-9Gd-4Y-1Zn-0.5Zr Alloy

The hot deformation flow behavior and microstructure evolution of as-extruded Mg-9Gd-4Y-1Zn-0.5Zr alloy under constant and variable strain rates loading at 450 °C were investigated. It is interesting to find that there are two peak values of the flow stress and the maximum flow stress value is 142 MPa when compressed under the variable strain rate from 1 s⁻¹ to 10 s⁻¹, approaching the peak flow stress of 146 MPa under the constant strain rate condition with 1 s⁻¹, but much smaller than that of 187 MPa under the constant strain rate condition with 10 s⁻¹. Meanwhile, the area proportion of recrystallization grains is the largest but the grain orientation changes slightly under the variable strain rate condition. Analysis of microstructures and slip deformation effects on flow stress revealed that the subgrain dislocation density is the lowest under the variable strain rate, and the activation of pyramidal <c + a> slip with the highest CRSS increases by 6%, contrasting with a 14% increase under the strain rate of constant 10 s⁻¹. These findings indicate that the lower flow stress under the variable strain rate can be attributed to the reduced subgrain dislocation density and the limited activation of pyramidal <c + a> slip systems.