*3.2. Microstructure of ECAP Alloys*

Figure 4 shows the SEM images of three Mg-3.7Al-1.8Ca-0.4Mn ECAP alloys processed at different temperatures and passes. Seen from the low-magnification images of Figure 4a,c,e, the continuous network eutectic structure became distorted after ECAP, and more ECAP numbers and a lower processing temperature resulted in a finer microstructure and narrower dendrite spacing. Moreover, as can be seen from the enlarged SEM images of Figure 4b,d,f, the distorted Al2Ca dendritic phases were broken into ultrafine particles (with particle size lower than 1 μm). However, the average sizes of broken Al2Ca particles are different in three ECAP alloys. High ECAP numbers refined the Al2Ca particles from 0.6 μm to 0.5 μm for 623K-4p and 623K-12p alloys, while lower ECAP temperature was more effective to refine these particles, and the average size of Al2Ca particles was approximately 0.3 μm in 573 K-12p alloy.

**Figure 4.** SEM images of ECAP alloys processed at (**a**,**b**) 623 K-4P, (**c**,**d**) 623 K-12P and (**e**,**f**) 573 K-12P with (**a**,**c**,**e**) low and (**b**,**d**,**f**) high magnifications.

To characterize the grain size evolution of studied alloy during ECAP at different processing parameters, Figure 5 shows the inverse pole figure maps and grain size distribution histograms of ECAP alloys along the extrusion direction. The 623 K-4p alloy showed an uneven grain size distribution. Both coarse and fine grains were identified, suggesting an incomplete dynamic recrystallized (DRX) microstructure. After 12p ECAP at either 623 K or 573 K, homogeneous grain size distribution was observed (Figure 5c,d), indicating a high degree of DRX. Furthermore, Figure 6 shows the area fraction of DRX grains and un-DRX grains in three ECAP alloys. It is apparent that the fraction of DRX grains in two 12p alloys were much higher than the 4p alloy.

**Figure 5.** Electron back-scatted diffraction (EBSD) maps and its corresponding grains distributions of ECAP alloys processed from (**a**,**b**) 623 K-4P, (**c**,**d**) 623 K-12P and (**e**,**f**) 573 K-12P.

**Figure 6.** Area fraction of the recrystallized and un-recrystallized grains in three ECAP alloys.

Owing to the high DRX ratios, the average grain sizes of two 12p alloys were smaller than 4p alloy. The average grain sizes were estimated to be 9.2 μm, 4.3 μm and 3.7 μm for 623 K-4p, 623 K-12p and 573 K-12p alloys, respectively. It can be seen that higher ECAP numbers and lower processing temperature contributed to refinement and uniformity of α-Mg grains. Two factors resulted in finer grains for 573 K-12p alloy. For one thing, lower processing temperature could restrain the migrations of grain boundaries. For another, fine and dispersedly distributed Al2Ca particles are more effective to hinder the growth of DRX grains. Figure 7 shows the TEM images of DRX regions in three ECAP alloys. High density of dislocations and sub-structures were observed in 623 K-4p alloy (Figure 7a). When the ECAP number increased to 12, the density of dislocations declined obviously, and the boundaries of DRX grains became distinct, either in 623 K and 573 K processed alloys. Overall, the grain sizes of DRX grains in these alloys are in good agreement with EBSD results.

**Figure 7.** Ddynamic recrystallized (DRX) grains and sub-structured in (**a**) 623 K-4P, (**b**) 623 K-12P, (**c**) 573 K-12P alloys.

As can be seen from the Kernel Average Misorientation (KAM) maps shown in Figure 8, the distribution of dislocation density varied for three ECAP alloys (shown in green and yellow colors). It is shown in Figure 8a that fine DRX grains and coarse deformation grains were mixed in 623 K-4p alloy. Plenty of dislocations existed within coarse deformed grains, while fine DRX grains had little dislocations within, because the operation of DRX consumed dislocations. After increasing ECAP number to 12, more DRX fine grains and less deformation grains suggested a lower density of dislocations (Figure 8b). Moreover, compared with Figure 8b,c, it can be concluded that lower temperature processing led to stronger strain accumulation for ECAP alloys at the same pass (shown in green color).

**Figure 8.** Kernel Average Misorientation (KAM) maps of alloys processed by (**a**) 623 K-4P, (**b**) 623 K-12P and (**c**) 573 K-12P.

Figure 9 shows the inverse pole figures of three ECAP alloys. A typical fiber texture (i.e., c-axes perpendicular to the extrusion direction) was formed for 623 K-4p alloy, and its maximum texture intensity was 2.40 (Figure 9a). Increasing processing number to 623 K, the orientations (c-axes) of grains exhibited a tendency to be parallel to extrusion direction, and the maximum intensity was 4.89 (Figure 9b). With the decrease of processing temperature to 573 K, the maximum texture intensity declined to 2.80. Similar texture evolutions have also been found in other recrystallized Mg-RE and AZ series magnesium alloys [34,35].

**Figure 9.** Inverse pole figures of ECAP alloys processed from (**a**) 623 K-4P, (**b**) 623 K-12P and (**c**) 573 K-12P.

Figure 10 exhibits the TEM observation of Al2Ca second phase in three deformed alloys. In early ECAP passes at 623 K, lamellar Al2Ca eutectic phase was partially broken into fine particles, as can be seen from the Al2Ca laths and particles shown in Figure 10a,b, respectively. Since Al2Ca is a brittle phase, its refining mechanism is more like a mechanical crushing process. Therefore, the crush of Al2Ca eutectic phase was incomplete and uneven owing to the low deformation strains at low ECAP pass. When the ECAP number was increased to 12, the Al2Ca phase broke thoroughly into submicron particles (Figure 10b,c), though they were still aggregated.

**Figure 10.** TEM images of Al2Ca phase particles in (**a**,**b**) 623 K-4P, (**c**) 623 K-12P and (**d**) 573 K-12P alloys.

Furthermore, TEM observations demonstrated that abundant nano-sized precipitates were observed within un-DRX α-Mg grains (including sub-grains) of all ECAP alloys (Figure 11), while the density of precipitates was relatively low in DRX grains. These precipitates were dynamically precipitated during hot deformation, which was commonly observed in Mg-Al-Ca based alloys. Most of the precipitates had a spherical shape (marked by yellow arrows), and a few exhibited rod shapes (red arrows). Seen from Figure 11a, the diameters of these spherical precipitates were around 5–15 nm in 623 K-4p alloy. With increased ECAP numbers, the density of precipitates increased for 623 K-12p alloy, and both rod-like and spherical precipitates were observed. The rod-like precipitates are usually larger than spherical particles, exhibiting a diameter of 20–30 nm and a length of 30–60 nm. After 12 passes of ECAP at 573 K, precipitates with diameter of 10–20 nm were also detected (Figure 11c), and the precipitation density was the highest for three deformed alloys. Furthermore, Figure 11d shows the corresponding SAED patterns of the precipitates. The diffraction patterns exhibit near-ring characteristic, and index of the diffraction rings demonstrates that the precipitates are Mg17Al12 phases. The Mg17Al12 precipitates are usually reported in AZ91 alloys [36]. As for Mg-Al-Ca alloys, Al2Ca phase served as the main precipitates during hot deformation or aging in most cases [24], and precipitation of Mg17Al12 particles was barely reported. Taking into account the alloy composition and microstructure of this studied alloy, most Ca elements were concentrated within the eutectic phases, and Al elements were more inclined to be enriched in some grains, which caused the precipitation of Mg17Al12 phases under the interaction of heat and strain during multi-pass ECAP.

**Figure 11.** TEM micrographs of the precipitates in (**a**) 623 K-4P, (**b**) 623 K-12P, and (**c**)573 K-12P alloys, as well as (**d**) the corresponding SAED patterns of the precipitates.
