*3.1.2 Microstructure evolution of AZ91 Mg alloy*

**Figure 9** shows the microstructure of the as-received and homogenized AZ91 Mg alloy. The microstructure of the as-received AZ91 Mg alloy shows a coarse α-Mg phase and β-Mg17Al12 secondary phase along the grain boundaries which is confirmed through XRD analysis as shown in **Figure 8**.

The mean grain size of as-received Mg alloy is ~58.69 μm as shown in **Figure 9(a)**, measured by the linear intercept method (ASTM E 112). From **Figure 9(b)**, it could be found that after homogenization treatment at 673 K for 24 h the slight increase of mean grain size of Mg alloy of ~59.82 μm was observed this is due to grain growth

#### **Figure 8.**

*XRD analysis on AZ80 Mg alloys (a) as-received (b) homogenized at 673 K-24 h (c) die A: 2P at 598 K (d) die A: 4P at 598 K. XRD analysis on AZ91 Mg alloys (e) as-received (f) homogenized at 673 K-24 h (g) die A: 2P at 598 K and (h) die A: 4P at 598 K.*

**Figure 9.** *Optical images of (a) as-received (b) homogenized at 673 K-24 h.*

*Effect of ECAE Die Angle on Microstructure Mechanical Properties and Corrosion Behavior… DOI: http://dx.doi.org/10.5772/intechopen.94150*

effect during the homogenization process. Similar observations are made by Nikulin et al. [15]. After ECAP, the microstructure of the alloy is effectively refined by dynamic recrystallization process (DRX) [16]. From **Figure 10**, it was observed that two-pass pressing through die A and die B exhibited bimodal grain structure and more fine grains appeared for the processing in the 900 die than the 1100 die. The effectiveness of the grain refinement was observed after four passes of pressing in the 900 die as shown in **Figure 10(b)**. Also, it is noticed that with the increase in the number of ECAP passes, the amount of fine grains is increased greatly.
