## 3 Results
### Microstructure Evolution and Reinforcement Distribution  
Figure 3a shows a typical composite microstructure of the as-fabricated Al\({}_{3}\)BC/Al specimen with heterogeneous grain structure comprised of both coarse (CGs) and fine grains (FGs). The microstructure reveals a deformed morphology with the presence of the following phases: a denser and wider size distribution of elongated shaped (60-70 \(\upmu\)m) agglomerates (clusters) of grey Al\({}_{3}\)BC, darker contrast AlB\({}_{2}\) phases, and a string/or network of submicron grey Al\({}_{3}\)BC phases dispersed within a highly deformed (brighter contrast) _fcc_-Al matrix (Figure 3b). The EDS element overlay mapping in Figure 3c identifies the following elements: B, C, and Al. The EDS results confirm the presence of the desired in situ Al\({}_{3}\)BC phase (yellow circle), and the matrix is primarily composed of _fcc_-Al. Notably, network-like Al\({}_{3}\)BC structure comprises both CG (elongated) and FG (equiaxed) grains and  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Phase** & \(\mathbf{V_{f}}\) **(wt.\%)** & **Lattice Parameter (Å)** & **Hardness (GPa)** & **Modulus (GPa)** & \(\mathbf{A_{f}}\) **(\%)** \\ \hline Al\({}_{3}\)BC & 65 \(\pm\) 2 & \
\begin{tabular}{c} (a) 3.488 \(\pm\) 0.003 \\ (c) 11.535 \(\pm\) 0.003 \\ \end{tabular} & 22 \(\pm\) 7 & 326 [20] & 52 \(\pm\) 4 \\ _fcc_-Al & 25 \(\pm\) 2 &
\begin{tabular}{c} (a) 4.098 \(\pm\) 0.001 \\ (a) 2.978 \(\pm\) 0.001 \\ (c) 3.259 \(\pm\) 0.001 \\ \end{tabular} & 3 \(\pm\) 1 & 52 & 31 \(\pm\) 2 \\ \hline \end{tabular}
\end{table}
Table 1: The summary of phase formation along lattice parameters, \(a\); wt./mass fraction (%), \(V_{f}\); area fraction, \(A_{f}\); [PERSON]’s modulus; and hardness of as-evolved phases.  
Figure 2: (**a**) Differential scanning calorimetry traces showing peaks around 660 \({}^{\circ}\)C (exothermic) and 681 \({}^{\circ}\)C (endothermic) indicating Al melting and solid-solid reaction, leading to the formation of Al\({}_{3}\)BC. (**b**) X-ray diffraction patterns of (B+GNPs)/Al powders as a function of milling time showing the presence of single-phase _fcc_-Al peaks. (**c**) X-ray diffraction pattern of the as-fabricated Al\({}_{3}\)BC/Al composite showing the presence of the following phases: Al, Al\({}_{3}\)BC, and AlB\({}_{2}\).  
clusters of black, rod-like, micron-size AlB\({}_{2}\) phases located at the interfaces of Al\({}_{3}\)BC agglomerates. However, obtaining a clear element overlay for the AlB\({}_{2}\) phase was challenging due to its brittle nature.  
The standard metallographic preparation employed likely fractured the clusters, leading to the presence of inclusions. Similar morphology can be found in other relevant studies [38]. Therefore, the overall microstructure reveals a heterogeneous microstructure where in situ Al\({}_{3}\)BC phase with varying grain sizes, from coarse-grained Al\({}_{3}\)BC to sub-micron fine-grained Al\({}_{3}\)BC, dispersed within _fcc_-Al matrix [43]. The solid-solid reaction at 1000 \({}^{\circ}\)C likely accelerated the agglomeration, promoting the formation of micron-scale Al\({}_{3}\)BC/AlB\({}_{2}\) clusters within the composite [44]. The phase area fraction (_Af_) was quantitatively measured (including more than 5000 particles) using the ImageJ software ([[https://imagej.net/ij/](https://imagej.net/ij/)]([https://imagej.net/ij/](https://imagej.net/ij/))). The average _Af_ values were found to be \(A_{f}^{Al_{2}O_{3}}\) = 52 \(\pm\) 4%, \(A_{f}^{Al}\) = 31 \(\pm\) 2%, and \(A_{f}^{AlB_{2}}\) = 17 \(\pm\) 3%, respectively. Such heterogeneity in microstructure is in good agreement with the XRD results [\(V_{f}^{Al_{3}BC}\) = -65 wt.%, \(V_{f}^{Al}\) = -25 wt.%, and \(V_{f}^{AlB_{2}}\) = -10 wt.%] (Table 1). The presence of micro-voids (Figure 3b), cracks (Figure 3a), and fracturing in some CG-Al\({}_{3}\)BC phases can be observed throughout the microstructure, possibly due to the low pressure and high temperature (400 \({}^{\circ}\)C/40 MPa) employed during deformation (hot press).