Latest Progress of High Formability in Mg Alloy Sheets：A Review

As the most widely used lightweight metal structural materials，Mg alloy has the advantages of high specific strength and stiffness，excellent damping performance and recyclability. Fabrication of high-performance Mg alloy sheet is one of the main technological innovations to achieve the carbon peak and neutrality targets. Moreover，China is rich in magnesium resources，whose application and promotion can alleviate the shortage of traditional energy materials such as iron and aluminum and reduce pollution. The critical shear stress（CRSS）of most Mg alloys is much larger than that of the base-plane slip system when the non-basal-plane slip system is operated. The relative magnitude relation of the critical shear stress in various deformation modes is as follows：τbasal<a><τtwin<τprismatic<a>< τpyramidal<a><τpyramidal<c+a>. For example，in polycrystalline magnesium alloys with an average grain size of 10 μm，the critical shear stress required for pyramidal <c+a> dislocation slip is about 20 times that of basal plane slip. The critical shear stress difference required by the operation of the basal-surface and non-basal-surface displacement system of magnesium alloy is large，leading to the basal-surface slip as the main factor in the plastic deformation process. Therefore，the conventionally processed deformed magnesium alloys have strong basal texture and anisotropy. At the same time，the forming ability of conventionally processed deformed magnesium alloys at room temperature is low and the forming process is difficult，which greatly limits the large-scale application and development of magnesium alloys. Wrought Mg alloys are widely used in aerospace，transportation and biomedical scaffolds. However，most of the wrought Mg alloys have hexagonal closepacked（hcp）structure，and there are few independent slip systems that can be operated at room temperature，so it is easy to form the strong basal texture during the plastic deformation，resulting in poor plastic forming ability at room temperature. The further development and application of Mg alloy sheets are limited by the low forming capacity at room temperature，which is restricted due to the strong basal plane texture，anisotropy and stress concentration. In the traditional plastic deformation of wrought magnesium alloys，the orientation relationship between the stress-strain and material deformation flow are not optimized and controlled，which results in the easy generation of a strong basal texture，strong anisotropy，and stress concentration. This results in a poor room-temperature formability of magnesium alloy sheets. How to improve the room-temperature formability of Mg alloy sheet is one of the main problems to be solved in expanding the application of Mg alloy. In recent years，a lot of research were focused on the poor formability of magnesium alloys at room temperature and texture regulation was a major research field. It could be seen that weakening basal texture could effectively improve the formability of magnesium alloy sheet. At present，most of the control techniques mainly focused on adding trace alloying elements and plastic deformation processing. On the one hand，the addition of Nd，Ce，Gd，Y and other rare earth elements could weaken the texture of the basal plane，and the content of rare earth elements needed to be strictly controlled. If excessive，some second phase particles would be formed，which was not conducive to the subsequent plastic forming process，and the cost of rare earth metals was high. On the other hand，magnesium alloy sheet was usually processed by traditional plastic processing methods，such as hot extrusion，warm rolling and cold rolling，so that the c-axis of most grains was almost parallel to the normal（ND）of the sheet，showing low plasticity and formability. A great deal of work and new research progress in improving the formability of Mg alloy sheet in recent years were summarized，mainly focusing on adding alloying elements and plastic pre-deformation to melt strong texture and low forming barrier. The development of the weak basal texture to prepare magnesium alloys and to develop microstructure orientation control theory was essential to promote Mg-alloy applications. The deformation mode could be activated by grain orientation and additional applied load direction. For hcp magnesium with a c/a ratio of ~1.622，｛10-12｝twinning could be activated by a compressive stress parallel to the basal plane or a tensile stress perpendicular to the basal plane. The control mechanism on microstructure，crystal orientation and formability of Mg alloy sheet by means of adding rare earth elements，micro-alloying，new rolling and extrusion processing，plastic pre-deformation and so on were introduced respectively，which could provide reference for the preparation of highly formable magnesium alloy sheet. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.