## 3 Results and Discussion  
### Structural Characterization  
The addition of nanoparticles changes the density of the materials. The results of density and porosity measurements of as-extruded E21 magnesium alloy and E21-B4C (1.5% and 2.5%) magnesium nanocomposites are shown in Figure 1. It is observed that with the addition of denser B4C (density: 2.52 g/cc) to E21 alloy (density: 1.8 g/cc), the density of the E21 alloy increases. However, since the density of boron carbide is low compared to other ceramic nanoparticles, the overall density is maintained to be less than 1.81 g/cc for the nanocomposites, which is far less than aluminum-based bulk materials and only 4% higher than that of pure Mg. It can also be seen from the Figure 1 that the porosity (%) of the nanocomposites is higher than that of the alloy. However, since all the materials exhibited porosity less than 0.6%, it is implied that near dense materials have been fabricated using the DMD casting technique and the effects of porosity on the properties of the alloy and nanocomposites can be considered negligible. Figure 1: Density and porosity results of the as-extruded materials studied in this work. The addition of B\({}_{4}\)C nanoparticles to the E21 alloy disturbs the microstructural homogeneity of the alloy. This is because, in conjunction to the change in density, the intrinsic structure of the nanocomposites differs in terms of the grain characteristics, secondary phase distribution, and nanoparticle dispersion as compared to that of the alloy. Figure 2a-c shows the grain characteristics of the alloy and nanocomposites. Based on visual inspection and careful image analysis and quantification, it is estimated that the mean grain sizes of the nanocomposites were smaller than that of the E21 alloy. The average grain sizes of the materials are given in the insets in Figure 2, with E21 alloy exhibiting the highest average grain size. The average grain sizes are 29% and 27% lower in E21-B\({}_{4}\)C and E21-2.5B\({}_{4}\)C nanocomposites, respectively, compared to that of the parent material E21 alloy. Further, an interesting observation is that the difference in grain size between the two nanocomposites is very minimal. Figure 2: _Cont._  
Apart from the grain sizes, other significant changes in the microstructure with the incorporation of B\({}_{4}\)C nanoparticles are the distribution of secondary phases and nanoparticles. Secondary phases of Mg-RE (Mg-Nd and Mg-Nd-Gd) and Zr-rich phases are seen in the SEM image of the E21 alloy, as given in Figure 3a and as confirmed by EDS point analysis. Zinc is found to be dissolved in the matrix owing to its good solid solubility in Mg. From Figure 2 it is evident that the secondary phases are found predominantly at the grain boundaries. With the addition of B\({}_{4}\)C nanoparticles, apart from the previously discussed Mg-RE phases and Zr phases, the presence of B\({}_{4}\)C particles were also observed. Although, the presence of B\({}_{4}\)C nanoparticles cannot be assertively confirmed by SEM + EDS, due to the limitation of the EDS to detect elements with lower atomic numbers [17], a few boron carbide nanoparticles were identified based on the EDS point analysis as seen in Figure 3b. Based on Figure 3b, it is observed that the nanoparticles are found in the matrix. However, due to the large volume fraction of the secondary phases, the nanoparticles can also segregate towards the secondary phases [18]; however, this needs further confirmation with the use of high resolution TEM. Further, XRD was used to confirm the presence of B\({}_{4}\)C nanoparticles as well as to identify the type of secondary phases. From the XRD spectra in Figure 3c, the presence of B\({}_{4}\)C is evident in the nanocomposites with peaks corresponding to B\({}_{4}\)C at 26\({}^{\circ}\) diffraction angles. In addition, the intensity of this peak is seen to increase from E21-1.5B\({}_{4}\)C to E21-2.5B\({}_{4}\)C due to the increased content of B\({}_{4}\)C particles.