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.