*3.1.3 Variation of grain size with two different die after 2 and 4 ECAP passes for AZ80/91 Mg alloys*

**Figure 11** displays the variation of average grain size of processed and unprocessed AZ80 and AZ91 Mg alloys of 2 and 4 ECAP passes through die A and die B. As it could be observed from **Figures 6** and **9** as-received and homogenized (0P) Mg alloy has moderately large grain size approximately ~50.20 μm and ~50.70 μm for AZ80 and 58.69 μm, 59.82 μm for AZ91 alloy respectively. The increased average grain size of Mg alloy after homogenization treatment AZ80/91 Mg alloy at 673 K-24 h is due to the phenomenon of grain growth effect [8, 17, 18]. Further, it can be shown that after 2P and 4P ECAP volume fraction of grains increases compared to as-received and homogenized Mg alloy. Mean grain size of AZ80 Mg alloy after ECAP-2P and 4P were ~28.87 μm and ~6.35 μm respectively for die A. Similarly, the average grain size of same Mg alloy processed through die B is ~36.14 μm and ~9.77 μm for ECAP-2P and 4P respectively. Further, an average grain size of AZ91 Mg alloy after 2P and 4P of ECAP were ~30.86 μm, ~7.58 μm for die A and ~36.14 μm, ~9.7 μm for die B respectively. It is apparent that the obtained grain refinement is due to DRX during ECAP and they increase in many ECAP passes which result in much smaller grain structure. However, from this it is noticed that the alloy processed with 900 die shows smaller grain sizes than 1100 die for both alloy, this is due to

**Figure 10.** *Optical images for die A: (a) 2P (b) 4P and die B: (c) 2P and (d) 4P ECAP passes.*

**Figure 11.** *Variation of average grain size with two different die.*

the accumulation of very large plastic strain while processing with a low angle die [19, 20]. The calculated equivalent plastic strain for 1100 to be ~0.742 and ~1.015 for 900 indeed, the strain developed by 4P ECAP through 900 die is higher than that of 1100 die. Therefore, large strain in the material exhibited more dislocation density lead to the formation of fine grains during this process. Therefore, undoubtedly it is evident that ECAP die angles significantly affect the deformation homogeneity and this influences the variation in microstructure [21–23].

Also, the microstructural change contributes towards improved mechanical properties and corrosion resistance. Finally, in general, AZ Mg alloy processed through die A and die B showed the same trend of decreasing grain size from the homogenized condition. By extruding in the die A, the mean grain size of AZ80 and AZ91 Mg alloy decreased by 35% and 22% when compared with material processed through die B [19, 20]. Also, from the result, it was observed that AZ80 Mg alloy processed through die A at 598 K exhibited fine grain structure of about ~6.35 μm after four ECAP passes, which is lower when compared to ECAPed AZ91 Mg alloy processed at same processing temperature.

#### **3.2 X-ray diffraction analysis**

The X-ray diffraction patterns of AZ80/91 Mg alloy before and after ECAP processes as shown in **Figure 8**. The XRD patterns of the as-received, homogenized at 673 K and ECAPed AZ Mg alloys revealed two sets of peaks, one for the α-Mg primary phase and another one for the β-Mg17Al12 secondary phase. But as-received alloy of AZ80 has shown new peaks corresponding to the formation of the ternary phase appeared at 41.4° as shown in **Figure 8(a)** which is disappeared after homogenization treatment and ECAP depicts in **Figure 8(b)**–**(d)** due to diffusion annealing treatment and dynamic precipitation during the ECAP process. Further, **Figure 8(c)** and **(d)** presents the XRD patterns for ECAPed AZ80 Mg alloys for 2P and 4P processed with

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

die A at processing temperature 598 K. It was observed that the peak intensities were increased after 4P ECAP when compared to the ECAP-2P sample. This is due to an increased volume fraction of secondary phases and more homogenous microstructure. But 2P ECAP processed sample presented lower peak intensity this is mainly due to non-homogeneity in the microstructure and crystal defects.

Furthermore, **Figure 8(e)**–**(h)** shows the XRD spectra of AZ91 Mg alloy (e) as-received (f) the homogenized at 673 K for 24 h (g) the two-passed AZ91 Mg alloy ECAPed with die A at 598 K and (h) the four-passed AZ91 Mg alloy ECAPed with die A at 598 K. Regardless of the number of ECAP pass, the as-received and processed samples contained α-Mg and β-Mg17Al12 phase. The intensity of the peak in the ECAP processed specimens at 598 K is lower than that of the as-received specimen. Also, it can be seen that there exists great difference on the magnitude of the peak intensity of ECAP processed specimen at 598 K for two and four passes this is mainly due to induced plastic strain during ECAP similar results has been observed by Avvari et al. [24–28].

## **3.3 Mechanical behavior**

This section explains the effect of ECAP die channel angle on mechanical properties of as-received and ECAPed AZ80/91 Mg alloys.

### *3.3.1 Effect of die parameters and processing temperature on microhardness*

**Figure 12** shows the impact of channel angle on microhardness during ECAP of AZ80/91 Mg alloys. From the results, it was observed that AZ80/91 Mg alloy processed through lower channel angle of 90° (die A) exhibited enhanced microhardness when compared to material processed through die B at 598 K after 4 Passes of ECAP. The improved microhardness is mainly due to the accumulation of large plastic strain while processing at 90° channel angle and obtained more equiaxed microstructure.

Hence, die A which has 90° channel angle is considered as an optimal die parameter to get the highest Microhardness for both AZ80/91 Mg alloys. Also, from **Figure 12** it was established that there is a significant increase in Microhardness after a four pass of ECAP in AZ91 Mg alloy after processing using a die A compared to AZ80 Mg alloy processed through the same die and this is anticipated from measurements of the effective refinement of grain size.
