*3.3. Microstructure of ECAP Alloys*

Figure 12 shows the tensile mechanical properties of as-cast and ECAP alloys. The as-cast alloy possessed low mechanical properties with tensile yield strength (TYS) of 70 MPa, ultimate tensile strength (UTS) of 161 MPa and poor ductility (elongation of 5.1%). After the ECAP process, all samples exhibited a remarkable improvement in both strength and ductility. Generally, an increase in the ECAP numbers improved the strength and ductility simultaneously, while decreases in ECAP temperatures further enhanced the alloy, but impaired the ductility slightly. The 573 K-12p alloy showed the highest strength, with TYS of 304 MPa and UTS of 354 MPa, together with a moderate ductility. In addition, by comparing with other commercial Mg alloys and Mg-RE based alloys prepared by ECAP (Figure 12c) [27–32,37–39], this 573 K-12p alloy exhibits a relatively high specific yield strength, which is comparable to those Mg-RE ECAP alloys with high RE contents. Overall, this non-RE low-alloying ECAP alloy with refined and dispersed Al2Ca particles exhibits great potential as high performance magnesium alloys.

Figure 13 shows the SEM fractograph of as-cast and ECAP alloys after tensile tests. Seen from Figure 13a, many laminated cleavage steps appear on the fracture surface of the as-cast alloy, suggesting a brittle fracture. Owing to the large eutectic structures at grain boundaries, microcracks tend to be formed around the eutectic, which leads to early break and poor ductility of the as-cast alloy. After ECAP, a few dimples and laminated cleavage steps are observed on fracture surface of 623 K-4p alloy (Figure 13b), indicating a mixed fracture mechanism. As for two 12p alloys (Figure 13c,d), although the fracture surfaces become fine and flat, some cleavage steps still exist.

**Figure 12.** Mechanical properties of Mg-Al-Ca-Mn alloys. (**a**) Typical tensile curves of as-cast and ECAP alloys; (**b**) The summary of yield strength (YS), ultimate tensile strength (UTS) and elongation of various alloys; (**c**) Comparison of specific yield strength and elongation between this work and other high performance ECAP Mg alloys [27–32,37–39].

**Figure 13.** SEM images of fracture surfaces of (**a**) As-cast, (**b**) 623 K-4P, (**c**) 623 K-12P and (**d**) 573 K-12P alloys. Insets of the images are the corresponding enlarged micrographs.
