Evolution of anelastic behaviour and twinning in cyclic loading of extruded magnesium alloy ZM21

In this study, the cyclic response of the wrought magnesium alloy ZM21 is investigated. The microstructure of the original extruded bar consists of equiaxed grains with a grain size range of 15–18 μm. The anelasticity was measured using a method in which the elastic modulus in every loop is obtained by calculating the slope of lines fitted to the loading and unloading parts of the curves. The cyclic test for ZM21 shows that the anelastic strain increases with the strain, reaching its maximum between 2-4% applied strain and then levels off. The anelastic strain reaches a maximum between 2-4% at which point the fraction of extension twins also reaches a maximum. Twins were separated from their parent grains in order to quantify twin evolution during cyclic loading. Overall, it was found that most of the twins observed are extension twins up to 8% accumulative strain. Beyond 8% strain, exhaustion of 90% of extension twins occurred and contraction twins appeared in the microstructure. Moreover, double twins were identified after a strain of 8%. These twins were composed of extension twins and contraction twins which can contribute to softening. Such softening can result in the reduction of uniform elongation in tension for the cyclic loading in comparison with the monotonic loading. It was possible for the material to sustain continued straining without failure to over double this value, that is, 15%.