## 3 Effect of Deformation Parameters on Microstructure Evolution
### Deformation Temperature  
Nowadays, most Mg alloys prepared by the MDF method follow a thermodynamic process due to the poor deformation ability of Mg at room temperature. According to the thermodynamic theory, the internal energy of the atoms in Mg alloys becomes active with increasing temperature, and dislocations in grains are enhanced. Dynamic recrystallization (DRX) is an important grain refinement mechanism in wrought Mg alloys, and it is closely related to the deformation temperature [42]. There are many deformed grains full of low grain boundaries or subgrain boundaries in wrought Mg alloys, and increasing temperature can provide the driving force for the nucleation and growth, thereby growing them into DRX grains. However, the DRX process usually happens within a certain temperature range, and grain coarsening occurs when the range is exceeded. Figure 2 shows the influence of deformation temperature on the microstructure of AZ80 alloys investigated by [PERSON] et al. [43]. It can be seen that the grain sizes were not uniform or refining with the decreasing temperature. At 180 \({}^{\circ}\)C, as shown in Figure 2c,f, the microstructure of AZ80 consisted of coarse grain area with an average size of 20.3 \(\upmu\)m and a fine grain area with an average size of 5.6 \(\upmu\)m. The fraction of second phases increased significantly and precipitated near the fine grains. Grain refinement was mainly attributed to DRX, and fine recrystallized grains could be observed near the deformed grains. [PERSON] et al. [34] discussed the effect of deformation on the microstructure homogeneity of Mg-Gd-Y-Zn-Zr alloys processed by MDF. The results showed that the fraction of DRX grains increased with increasing deformation temperature from 400 \({}^{\circ}\)C to 450 \({}^{\circ}\)C. Some characteristics of heterogeneous deformation, such as shear bands, disappeared at 450 \({}^{\circ}\)C. [PERSON] et al. [31] reported the microstructure evolution and mechanical properties of XM11 (Mg-0.59 Ca-0.62 Mn) alloys processed by MDF. The results showed that the grain size increased from 2.0 \(\upmu\)m to 6.7 \(\upmu\)m with temperature increasing from 220 \({}^{\circ}\)C to 340 \({}^{\circ}\)C. The research into tensile strength showed that a higher ultimate strength and yield strength are observed at 220 \({}^{\circ}\)C, and a higher elongation is obtained at 340 \({}^{\circ}\)C. Various slip systems that are provided after the randomization of texture next to second-phase Mg\({}_{2}\)Ca dissolution enhanced the elongation of XM11 alloys. In summary, the changing of deformation temperatures during MDF shows an obvious influence of Mg alloys in microstructure evolution. Specifically, Mg alloys are prone to obtaining a fine microstructure and grains at lower temperatures, and the grains will grow with increasing temperature. This can be attributed to the fact that DRX occurs during MDF and prompts the grain refinement, but the energy provided by the increasing temperature accelerates the grain growth. Thus, deformation with decreasing temperatures pass by pass is a considered method of Mg alloys processed by MDF.