## 3 Effect of Deformation Parameters on Microstructure Evolution
### Strain Rate  
Mg alloys have a structure of HCP, and a poor deformation ability is observed at low temperatures. Meanwhile, Mg alloys are extremely sensitive to the strain rate during hot plastic deformation. Many studies [48; 49; 50] have shown that a high deformation strain rate may not only increase the resistance but also produce shear bands or even cracks. Thus, strain rate is an import process parameter of Mg alloys prepared by MDF to obtain a homogenous microstructure and high strength and toughness.  
Currently, Mg alloys usually take plastic deformation at a low strain rate to ensure the homogeneous deformation. [PERSON] et al. [51] investigated the grain refinement mechanism of Mg-8.59 Gd-3.85Y-1.14 Zn-0.49 Zr alloys by MDF at 350 \({}^{\circ}\)C with strain rates of 0.01 s\({}^{-1}\) and 0.1 s\({}^{-1}\), as shown in Figure 5. It can be seen that the number fraction of spherical phases at a strain rate of 0.01 s\({}^{-1}\) is lower than 0.1 s\({}^{-1}\). The second phase marked by B in Figure 4c is determined to be \(\beta\)-MgSGd by TEM analysis and be refined to 1 \(\upmu\)m during MDF, which means that it can create regions of high dislocation density by hindering the dislocation movement. By the observation of EBSD analysis, it can be seen that the DRX fraction at a strain rate of 0.01s\({}^{-1}\) is lower than at 0.1 s\({}^{-1}\) with a deformation temperature of 400 \({}^{\circ}\)C. However, the higher strain rate can cause a strong basal texture. [PERSON] et al. [52] carried out the  
Figure 4: The influence of accumulative strain on microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys processed by MDF: (**a**) 3 passes, (**b**) 6 passes, (**c**) 9 passes, (**d**) 12 passes and (**e**) stress–strain curves [34].  
five passes of MDF on AZ61 alloys with an increasing strain rate from 0.0125 s\({}^{-1}\) to 0.4 s\({}^{-1}\). The results showed that the influence of strain rate on microstructure can be negligible, and the grain refinement mainly attributed to the accumulative strain. [PERSON] et al. [53] investigated the grain refinement behavior of AZ80 alloys processed by MDF with a strain rate of 3.0 \(\times\) 10\({}^{-3}\)\(\sim\)3.0 \(\times\) 10\({}^{-1}\). They found that the grain sizes were refined to 0.8 \(\upmu\)m, and a high dislocation density was observed with the increasing strain rate, which could be attributed to the DRX occuring more easily at a high strain rate. Similar results were reported by [PERSON] et al. [54], who found that a high strain rate can produce numerous twins and dislocation in the deformed grains, which can serve as nucleation particles to promote the generation of DRX grains.  
In summary, Mg alloys have a severe strain rate sensitivity, which is closely related to their HCP crystal structure. Specifically, a high strain rate will obtain the inhomogeneous microstructure with rheological phases, and it even may have cracks, while a low strain rate can effectively release the stress and dislocation concentration. Meanwhile, a low strain rate can achieve sufficient DRX. Thus, a relatively low strain rate is usually selected to prevent an inhomogeneous microstructure and the damage of Mg alloys.