## 3 Results and Discussion
### Property Profile  
#### 3.2.1 Mechanical Properties  
The compressive results of the developed Mg alloys are shown in Figure 4, where the representative engineering stress-strain curves of the E21 alloy and its nanocomposites under compression are depicted. E21 alloy is known for its good mechanical properties exhibiting a good strength and moderate ductility.  
Figure 3: Scanning electron micrographs with energy dispersive spectroscopy (EDS) analysis of (**a**) E21 alloy; (**b**) microstructure of E21-1.5B\({}_{4}\)C nanocomposite with EDS point analysis of boron carbide nanoparticle and Mg-RE (Mg-Nd-Gd) phase; and (**c**) X-ray diffraction spectra of the materials taken on the longitudinal section of the samples.  
From Figure 4, it is noticeable that the E21-B4C nanocomposites outperform the E21 alloy in the compressive yield strengths while maintaining the same strain to failure. The YS and UCS of E21-1.5B4C increases by 12.8% and 1.25%, respectively, with respect to the E21 alloy, while the YS and UCS of E21-2.5B4C increases by 17.7% and 8.25%, respectively, with the E21 alloy. Further, the total energy absorbed values are seen to be the highest in E21-2.5B4C with 6.3%, as compared to that of the E21 alloy.  
It is widely known that, at most low-temperatures, permanent deformation of metal comes from the movement of crystalline imperfections, known as dislocations, through the grains in the metal [23]. The addition of boron carbide nanoparticles reduced the grain sizes in E21 alloy; a change in grain size affects the yield strength due to the dislocations interacting with the grain boundary as they move. These boundaries act as obstacles, hindering the dislocation glide along the slip planes. As subsequent dislocations move along the same slip plane, the dislocations pile-up at the grain boundaries. On the contrary, a larger grain would result in more dislocations within the grain, resulting in more dislocations in the pile-up. Therefore, a lower applied stress is required to produce a stress great enough to cause the grain boundary to collapse. Furthermore, due to the presence of higher volume fraction of nanoparticles present in E21-2.5B4C, the contribution to strengthening by boron carbide nanoparticles is significant compared to E21 and E21-1.5B4C. In addition, the further increase in YS and UCS in  
Figure 4: (**a**) Engineering stress-strain curves of all materials tested under uniaxial compressive loading; (**b**) fractography of the Elektron21 (E21) sample fractured under compression; (**c**) fractography of E21-1.5B4C sample fractured under compression; and (**d**) fractograph of E21-2.5B4C sample fractured under compression.  
E21-2.5B4C could also be due to the increased dislocation density formed due to coefficient of thermal expansion mismatch between reinforcements and matrix. This creates an excellent improvement in strength while maintaining the ductility, which makes it a viable option to be used in the aerospace sector. The samples had a very large strain-to-failure of greater than 30% and the same is observed in the fractographs given in Figure 4b-d. From the figures, it is confirmed that the amount of plastic deformation underwent by all the materials is very high and the samples typically deformed by shear. Further, there is no significant difference in the mode of deformation observed among the materials which reinforces the fact that the nanoparticles did not hamper the compressive ductility in the nanocomposites.