## 6 Summary  
Mg alloys have advantages of low density and high specific strength, showing unique advantage as structural materials in terms of lightweight applications. Although the effects of the solid solution strengthening and precipitation strengthening of Mg alloys are far inferior to those in Al alloys and steel, as a metal with low stacking fault energy, Mg alloys usually present a higher Hall-Petch slope than that of Al alloys, which is mainly related to the low ability of intergranular deformation due to the limited number of slipping systems. MDF has proved to be an effective process to control the microstructure evolution, modify grain orientation and increase mechanical properties. When the grain size is decreased to the submicrometer scale, the yield strength can reach 350 MPa\(\sim\)400 MPa. In this paper, the grain refinement, second-phase distribution and mechanical properties of MDFed Mg alloys are reviewed. The effect factors and strengthening mechanisms are analyzed systematically. The findings are drawn as follows:  
1. The MDF process is an ideal method for preparing fine-grained Mg alloys by adjusting process parameters, like deformation temperature, accumulative strain and strain rate. Based on the current research, the accumulative strain is the main reason to achieve the decreasing grain sizes, second-phase fragmentation and texture weakening. Based on the HCP structure of Mg alloys, although a high strain rate and low deformation temperature are beneficial to obtain fine grains, they will affect the uniform deformation. Thus, a moderate deformation strain rate and temperature are chosen to prepare high-performance Mg alloys. It is proved that the multiple slip systems can be activated through plastic deformation with reasonable parameters like relatively high temperature and a low strain rate. Meanwhile, the high density of low-angle grain boundaries with low-energy characteristics can be formed via controlling the DRX process. During the MDF process, the dislocation walls constructed in Mg alloys are thermally stable with high migration resistant due to the << + a> dislocation with large CRSS values. Moreover, the low-angle grain boundaries that formed exhibit high lattice coherence, and the relaxed grain boundaries can stably resist migration under thermal activation. The solute segregation of MDFed Mg alloys at grain boundaries can decrease the grain boundary energy and make them stable. Thus, solute precipitation at grain boundaries due to high-temperature deformation should be avoided as much as possible. 2. Different microstructures processed by MDF can have a significant impact on properties. Grain orientation is considered to be an important aspect that needs to be controlled during the MDF process because of its significant effect on the formation of texture. Crystallographic deformation texture plays an important role in the generation of the anisotropy of properties of Mg alloys. Mg alloys processed by MDF exhibit a relatively random texture due to multi-directional rheological stress loading. Meanwhile, the fraction of DRX increases with increasing accumulative strain, further resulting in the weakening of basal texture. Moreover, the deviation of basal texture is also determined by the composition of alloying elements. Mg alloys processed by MDF can have a significantly increasing effect on mechanical properties such as grain refinement, high dislocation density, and numerous dynamic precipitates. Grain refinement and precipitation strengthening are considered the dominate strengthening mechanisms for the enhancement of strength, and solid solution and texture strengthening are secondary. In summary, this review confirms that MDF is considered to be an excellent processing method to prepare Mg alloys with high comprehensive performance. Meanwhile, as an SPD process, it can fabricate large-scale components, which may be widely used in industries.