## 3 Results and Discussion
### Impact of ECAP Processing Factors
#### 3.2.2 Vicker's Microhardness  
Table 11 (Appendix A) depicts the relationship between various passes and the hardness fluctuation of Mg billets processed with dies having \(\phi\) = 90\({}^{\circ}\) and 120\({}^{\circ}\), where the AA-Mg measured a mean hardness of 26 HV on the Vickers scale; additionally, augmenting the number of passes substantially ameliorated the hardness of AA-Mg throughout. As contrasted to the AA equivalents, the HV improved by a whopping 84% during 1P processing with the 90\({}^{\circ}\)-die. Alternatively, when comparing the 1P case of the 90\({}^{\circ}\)-die to that of the 120\({}^{\circ}\)-die, a rise in HV of 14% was found for the Mg-billets processed through the 90\({}^{\circ}\)-die. Accordingly, the increase of the HV values of the Mg-billets that were processed through the 90\({}^{\circ}\)-die compared to the counterparts that were processed through the 120\({}^{\circ}\)-die can be attributed to the increase in the imposed strain when using the 90\({}^{\circ}\)-die compared to the 120\({}^{\circ}\)-die. The higher plastic strain resulted in more grains refinement, as indicated in Figure 3, which led to higher improvement in the HV values. Furthermore, as opposed to its 1P counterpart, the HV was enhanced by 6.25% after being processed via 2 Bc utilizing a 90\({}^{\circ}\)-die. The 120\({}^{\circ}\)-die in the 2 Bc case displayed an identical pattern. HV values for 2A and 2C processed via the 90\({}^{\circ}\)-die were both found to be significantly higher than their AA counterparts, by 76.9% and 84.6%, respectively. In addition, when comparing 2A and 2C samples processed through the 120\({}^{\circ}\)-die to those of the 90\({}^{\circ}\)-die, it was found that the HV outcomes were augmented by 9.5% and 9%, respectively. When adopting a 90\({}^{\circ}\)-die to accumulate the plastic strain up to 4 Bc, the subsequent HV was indeed 111% higher than that of the AA equivalent. Furthermore, the HV values for the 4 Bc case of the 90\({}^{\circ}\)-die were 7.8% higher than those of the 120\({}^{\circ}\)-die, which might be explained by the refined grains produced; Table 11 (Appendix A) shows that the mean \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **Optimized** & \multicolumn{2}{c}{**Training Set**} & \multicolumn{2}{c}{**Testing Set**} \\ \cline{3-6}  & **ML Model** & **RMSE** & **R2** & **RMSE** & **R2** \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Grain size \\ (\(\mu\)m) \\ \end{tabular} } & LR & 0.078 & 0.983 & 0.194 & 0.851 \\  & GPR & 0.024 & 0.998 & 0.175 & 0.878 \\  & SVR & 0.078 & 0.983 & 0.194 & 0.851 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Model evaluation metrics of pure Mg ECAP parameters. Figure 6: (**a**) Predicted grain size against experimentally measured grain size values examined for the training and the testing datasets. (**b**) Predicted grain size values versus sample indices for predicted train, predicted test, and experimental dataset. grain size produced by the 90\({}^{\circ}\)-die was 0.88 \(\upmu\)m and that produced by the 120\({}^{\circ}\)-die was 1.89 \(\upmu\)m at the 4 Bc. The strain hardening that occurred during ECAP processing also contributed to the rise in HV values that was observed as the passes increased. The HV results were then compared to the changes in microstructure shown in Figures 3 and 4. It was the underlying grain refinement that led to the dramatic increase in HV values, with route Bc having the smallest grains. Since the plastic strain placed on the samples is smaller at larger angles, the reduction in hardness when raising \(\upphi\) made sense. Similar findings were reported in previous studies. [PERSON] et al. (2018) reported that ECAP processing for AZ31 resulted in improving the HV values by 132% compared to the AA counterpart.