## 3 Results  
### Microstructure  
Figure 4 shows the surface microstructure and cross-section characteristics of MIMC composites. From Figure 4a, the matrix and reinforcement are interpenetrating, anisotropic, and interwoven. In addition, in the longitudinal direction, a portion of the structure of the interpenetrating network of reinforcements is exposed from the matrix (white circles), and another portion of the reinforcement is wrapped and buried by magnesium matrix. Only a few are exposed from the matrix in the transverse direction (black arrow). In addition, the matrix and reinforcement form a good interface combination, as shown as in Figure 4b. Furthermore, the distribution of the chemical elements of the samples are examined by EDS, and subjected to line scanning, as shown as in Figure 5. The reinforcements of the circular area are stainless steel (cyan line in Figure 5a), titanium alloy (blue line in Figure 5b), and aluminum alloy (cyan line in Figure 5c), respectively. The magnesium matrix is represented as a red line in the MISC and MIAC composites, however, it is described as a green line in MITC composite. Figure 6 shows the OM microstructure morphology and grain size distribution of magnesium matrix in three kinds of composite as-cast states. In MIMC composites, their grain distribution and grain size have the same characteristics, that is, the grain distribution does not obey a normal  
Figure 4: SEM micrograph of MIMC composites: (**a**) the surface microstructure; (**b**) cross-section of reinforcement with matrix. Figure 5: The energy dispersive spectroscopy (EDS) analysis of MIMC composites: (**a**) magnesium matrix composites interpenetrated by stainless steel reinforcement (MISC); (**b**) magnesium matrix composites interpenetrated by titanium alloy reinforcement (MITC); (**c**) magnesium matrix composites interpenetrated by aluminum alloy reinforcement (MIAC). distribution and is randomly distributed. In addition, the grain size is mainly concentrated in the range 200-300 \(\upmu\)m. Therefore, the results show that the grain and its area fraction are not affected by any of the reinforcement materials. However, compared with the as-cast state (Figure 6a,c,e), the micromorphology of the as-extruded samples has greater changes, which are shown in Figure 7a,c,e. The relatively small grain size of the as-extruded samples is owing to the accumulation of shear strain and dynamic recrystallization during extrusion. However, the grain refinement degree is different in three kinds of composites. In MISC composites, it can be seen, from Figure 7a, that the grain size follows a normal distribution. Moreover, most of the grain size is 10-20 \(\upmu\)m, so the average grain size is 14 \(\upmu\)m. In MITC composites, although the grain size does not conform to the normal distribution (Figure 7d), most of them are concentrated between 15-30 \(\upmu\)m. Therefore, the average grain size is 20 \(\upmu\)m. In MIAC composites, the grain size adheres to the Weibull distribution (Figure 7f), and most of them are concentrated between 20-30 \(\upmu\)m. The average grain size is 24 \(\upmu\)m. In short, the different reinforcement materials result in different microstructure characterization of magnesium matrix during deformation. The reason is that, due to the different strengths of reinforcement, the magnesium matrix bears a different stress from the reinforcement during extrusion. As we know, magnesium matrix alloys usually form a strong texture, resulting in the decreases of ductility at room temperature because of their intrinsic characterization of the hexagonal close-packed structure [29]. Since the room temperature-forming properties of the magnesium alloys are mainly affected by the initial crystal structure of the basal plane, the improvement of the crystal grains will significantly improve the formability of the MIMC composites [30].