Effect Of Pre-strain And Subsequent Annealing On Mechanical Behavior Of Rolled AZ31 Mg Alloys

Due to the poor room formability, strong tension-compression yield asymmetry and mechanical anisotropy, the application of magnesium and its alloy was limited to some extent. Many kinds of methods were applied to improve the properties of magnesium alloy, in order to meet different engineering requirements. And the improvement of yield strength, anisotropy and the tension-compression asymmetry of magnesium alloy through deformation twin introduced by pre-strain, has gradually become one of research highlights. Besides, annealing is one of the efficient and economic methods to change the formability of magnesium alloy. The final texture of magnesium alloy can be improved by annealing. Therefore, it is important to combine the strength of pre-strain and annealing to change the properties of magnesium alloy.In this paper, the rolled AZ31 magnesium alloy was selected as the subject of this study. Different pre-compressions were conducted. Then the pre-compressed samples were annealed. The effects of pre-compression and annealing on the re-compression behavior of rolled AZ31 magnesium alloy at room temperature were investigated, by means of optical microscopy(OM), X-ray diffraction(XRD) and electron backscatter diffraction(EBSD). The main conclusions can be summarized as follows:Pre-compression along TD was conducted on rolled AZ31 magnesium alloy. It was found that optimal distribution appear on the(0002) basal plane and(1010) cylindrical plane after pre-compression. The {10 1 2} extension twinning introduces a crystallographic lattice re-orientation of ~86° in twinned regions, resulting in the formation of the twin texture with c-axis aligned parallel to the compressive load. However, the macro-texture has no obvious change when pre-compression along ND. For the pre-compressed samples, when re-compression was conducted along ND, the phenomenon of de-twin will occur.Annealing for different time was carried out on pre-compressed samples. The results revealed that a lot of non-recrystallized extension twin remained in samples pre-compressed along TD after annealing for 4h. The grains were not refined and the increase of yield strength after annealing was mainly attributed to annealing hardening(i.e., annealing retards the detwinning activity and consequently increase the yield strength). However, no twin was observed in the sample pre-compressed along ND after annealing for 4h and the grains were greatly refined. The increase of yield strength was due to the grain refinement caused by static recrystallization.Two steps of pre-compression(pre-compression along TD and subsequent re-compression along RD) and annealing were conducted on the hot-rolled AZ31 Mg alloy. The results revealed that the generation of the {1012} extension twins in the matrix and the {1012}-{1012} double twins in the previous {1012} twins dominated the re-compression along RD. The {1012}-{1012} double twins formed in the previous {1012} twins, which significantly modified the compression behavior along the normal direction(ND). The changes occurred in macro-texture with different pre-compression and re-crystallization texture was observed after annealing. The intensity of the texture was decreased after annealing and its distribution is much more evenly distributed. The grain refinement by twin lamellas after pre-strain, together with the crystallographic orientation redistribution after pre-strain and annealing, greatly reduce the mechanical anisotropy along RD, TD and ND.