Effect of twinning on the deformation behavior of an extruded Mg-Zn-Zr alloy during hot compression testing

The research was aimed to understand the formation and evolution of twins during hot deformation of a wrought magnesium alloy and the effect of twinning on strain hardening. Hot compression tests of an extruded Mg-Zn-Zr alloy were performed at temperatures of 350-450 °C and strain rates of 0.01-10 s−1. A concave stress-strain curve shape was observed, when a high value of the Zener-Hollomon parameter was applied. Such a stress-strain curve shape was however not observable during tests at a low value of the Z parameter. Microstructure examination of specimens obtained from interrupted compression tests at high values of the Z parameter showed that a large number of the extension twins formed and then reached saturation when the deforming material was in the work-hardening regime. Most of these twins in each elongated grain were found to be parallel to each other. After the extension twins completely consumed the elongated grain matrix, a number of contraction twins emerged, leading to an increased strain-hardening rate through the Hall-Petch effect and finally to a necklace-like microstructure after deformation reached the steady state. However, at low values of the Z parameter, the formation and evolution of twins hardly occurred, leading to a uniform equiaxial grain structure after the compression tests.