# A New Method to Lightweight Magnesium Using Syntactic Composite Core
## 3 Results and Discussion
### Compressive Behavior  
The room temperature compression properties of the as-cast pure Mg and Mg/Mg-20 GMB hybrid composite are shown in Figure 7, and corresponding data are summarized in Table 4. The obtained yield strength (YS) and ultimate compressive strength (UCS) of pure Mg are 88 \(\pm\) 3 and 215 \(\pm\) 5 MPa, respectively whereas the Mg/Mg-20 GMB hybrid composite shows a YS and UCS of 151 \(\pm\) 4 and 165 \(\pm\) 3 MPa, respectively. The increment in YS of the Mg/Mg-20 GMB hybrid composite was by 171.6% and the drop in the UCS was by 123.25% when compared to as-cast pure Mg. It can also be observed from Table 4 that the yield strength of Mg/Mg-20 GMB hybrid composite is superior or even comparable with other commercial cast Mg alloys. This can be attributed to the change in work hardening behavior and the competing load sharing between the shell and core material [33]. Further, it was interesting to note that the developed as-cast Mg/Mg-20 GMB hybrid  
Figure 6: Energy dispersive X-ray spectroscopy (EDX) spectra of as-cast (**a**) pure Mg and (**b**) Mg/Mg-20 GMB hybrid composite (the inset shows the element content). \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Materials**} & \multicolumn{3}{c}{**Microhardness (Hv)**} \\ \cline{2-4}  & **Shell Region** & **Core Region** & **Interface Region** \\ \hline as-cast pure Mg & \(44\pm 5\) & — & — \\ as-cast Mg/Mg-20 GMB & \(53\pm 3\) & \(80\pm 4\) & \(74\pm 6\) \\ \hline \end{tabular}
\end{table} 
Table 3: Microhardness values for as-cast Mg/Mg-20 GMB hybrid composite. composite exhibited a second work hardening phase after the failure of the Mg shell. For as-cast Mg/Mg-20 GMB hybrid composite, the stress was transferred to the Mg-20 GMB core when the Mg shell fractured. It was seen that the stress decreased after reaching a peak at the end of the linear elastic region and was followed by a plateau region. Thereafter, the load is experienced by the Mg-20 GMB core. The compression behavior of the Mg-20 GMB core is similar to the behaviors commonly observed for metal and polymer syntactic foams [20]. The plateau region appears at a constant stress level and slowly decreases. Incorporating porosity in the Mg-20 GMB syntactic foam core through the addition of hollow GMB particles leads to a stress plateau at constant rate. Plastic deformation regime as observed in this study. The pores which are surrounded by stiff, strong GMB walls, impede the deformation and collapse during compressive loading; with further loadings deformed material occupies the porosity opened up due to fracture. Loading further makes progressive densification of the microstructure. Further, embedding porosity with the addition of 20 wt.% GMB in the core results in a decrease of the UCS value in the hybrid composite from 215 MPa to 165 MPa, as compared to as-cast pure Mg, due to reduction in load bearing area. The ductility of the hybrid composite is a combination of the ductility of the core and the shell. The ductility of the as-cast Mg/Mg-20 GMB was \(\sim\)186% higher than that of as-cast pure Mg.  
Mg/Mg-20 GMB hybrid composite shows the maximum energy absorption (EA) value of 51 MJ/m\({}^{3}\), which is \(\sim\)41.66% greater than that of as-cast pure Mg. The core (Mg-20 GMB) material resulted in a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **YS (MPa)** & **UCS (MPa)** & **Ductility (\%)** & **Energy Absorption (MJ/m\({}^{3}\))** \\ \hline as-cast pure Mg & \(88\pm 3\) & \(215\pm 5\) & \(14.8\pm 0.3\) & \(36\pm 2.3\) \\ as-cast Mg/Mg-20 GMB & \(151\pm 4\) & \(165\pm 3\) & \(42.4\pm 0.3\) & \(51\pm 1.6\) \\  & (71.6\%) & (\(\downarrow\)23.25\%) & (\(\uparrow\)186.48\%) & (\(\uparrow\)41.66\%) \\ AZ91E * & 130 & 400 & – & – \\ RZ5(ZE41) * & 130–150 & 330–365 & – & – \\ ZREI(EZ33) * & 85–120 & 275–340 & – & – \\ \hline \hline \end{tabular}  
* denoted the data from the ASM metal alloy handbook. Note: YS = yield strength, UCS = ultimate compressive strength.  
\end{table}