Effect of local stresses of primary twinning on double twinning, twin intersection and their competition

Abstract

Taking into account the local stresses resulting from the primary twinning with transformation strains, the competition between double twinning {10–11}–{10–12} and (10–12)–(–1012) (CDTW and EDTW) is theoretically modeled. Using electron backscatter diffraction technique, the statistical fractions, thickness-length ratios and their critical values of CDTW and EDTW within grains of as-cast AZ31 magnesium alloy sheets at the low strain stage of hot rolling processing are obtained to validate the theoretical results. In addition, the competition between the DTW and twin intersection are also discussed in detail. The results show that: (1) The secondary twin growth complies with the classical Hall-Petch relationship. Due to the difference in the driving forces resulting from different transformation strains between the primary extension and contraction twinning, the secondary extension twin of EDTW is more difficult to grow compared to that of CDTW for primary twin bands with small thickness-length ratios, which is contrary to the regulation for primary twin bands with large thickness-length ratios; (2) The greater the thickness-length ratio of the primary twinning is, the smaller the critical shear stress of the secondary twinning is. This is opposite to the effect of thickness-length ratio on the critical shear stress of the incident twinning in the twin intersection. It is indicated that the thickening of the primary twinning provides the higher driving force for the formation of DTW, however hinders the twin intersection more evidently; (3) Within coarse grains, at the low strain stage, the formation of DTW can dominantly induce dynamic recrystallization and continue plastic deformation, while the stress concentration resulting from the twin intersection is inclined to cause brittle fracture. Therefore, based on the effect of the local stress, the selection mechanism of the thickness-length ratio on the DTW and twin intersection is of great significance to determining the plastic performance of magnesium alloys during hot working. In the current study, massive control factors of the thickness-length ratio of the primary twinning are concluded, which provides some corresponding schemes of material design.