Formation of macroscopic twin bands and inhomogeneous deformation during cyclic tension-compression loading of the Mg-wrought alloy AZ31

Cyclic strain controlled tension-compression tests were performed on samples made of the Mg-base alloy AZ31, which were taken from warm rolled sheets in rolling direction. Due to the strong preferred orientation of the grains, a pronounced strength asymmetry was observed with a higher yield strength in tension compared to compression. This can be attributed to the formation of twins as the dominant deformation mechanism, which occurs only under compression and not under tensile loading. Twinning during compression is associated with the formation of macroscopic twin bands which pass through the sample. Thus a strong localization of compressive strains exists. This leads to a complex inhomogeneous strain distribution during cyclic strain controlled deformation. The local strain distribution was analyzed using digital image correlation. Compressive strain is almost exclusively realized by the strongly localized formation and growing of a macroscopic twin band while tensile deformation is achieved by the dissolution of twin bands as well as by homogeneous deformation based on dislocation glide. In following loading cycles, the areas where during the preceding loading cycle twins had been formed and dissolved by detwinning are again areas of preferred formation of new macroscopic twin bands. Local strain distributions also depend on whether the cyclic tests were started in tension or compression and on the related strain hardened volumes.