## 3 Results and Discussion
### Mechanical Behavior
#### 3.2.3 Fractographic Analysis  
The fracture surfaces of Al-Si-(Cu)-Mg alloys (Alloy A and Alloy B) following the tensile testing at room temperature (RT), 150 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C were examined via SEM and are presented in Figure 5. The images provide visual evidence of the fundamental differences in the fracture behavior between the two alloys under various temperature conditions.  
Alloy A exhibited a predominantly ductile fracture mode across all temperatures, characterized by a uniform distribution of deep and densely populated dimples on the fracture surfaces (Figure 5a,c,e). This ductile behavior is attributed to a synergistic effect of the homogeneous Cu solid solution within the Al matrix, which induces lattice distortion, and the presence of finely dispersed \(\alpha\)-Al(FeMn)Si phases that assist in stress delocalization. These features act as crack deflection and bridging mechanisms, delaying crack initiation and propagation and thereby enhancing both the ductility and high-temperature fracture resistance.  
In contrast, Alloy B consistently exhibited features of brittle fractures under all tested conditions (Figure 5b,d,f). The fracture surfaces were dominated by cleavage facets, numerous microcracks, and irregular crack propagation paths. These characteristics suggest that acicular \(\beta\)-AlFeSi phases and agglomerated Mg\({}_{2}\)Si precipitates acted as stress concentrators, promoting interfacial debonding and early failure. Notably, the \(\beta\)-AlFeSi phase remained thermally stable and persisted along grain boundaries, serving as a preferential crack path even at elevated temperatures.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Alloy A**} & \multicolumn{3}{c}{**Alloy B**} \\ \hline \multirow{2}{*}{**Temperature** **(\({}^{\circ}\)C)**} & **Tensile Strength** & **Yield Strength** & **Elongation (\%)** & **Temperature (\({}^{\circ}\)C)** & **Tensile Strength** & **Yield Strength** (MPa)** & **Elongation (\%)** \\ \hline RT & 347.01 \(\pm\) 7.56 & 227.74 \(\pm\) 3.12 & 5.03 \(\pm\) 0.16 & RT & 277.85 \(\pm\) 4.26 & 123.98 \(\pm\) 2.55 & 2.12 \(\pm\) 0.11 \\ \(150\) \({}^{\circ}\)C & 343.27 \(\pm\) 5.28 & 227.62 \(\pm\) 2.87 & 4.26 \(\pm\) 0.21 & 150 \({}^{\circ}\)C & 249.45 \(\pm\) 7.81 & 112.27 \(\pm\) 3.21 & 3.12 \(\pm\) 0.12 \\ \(250\) \({}^{\circ}\)C & 306.68 \(\pm\) 4.45 & 176.87 \(\pm\) 3.41 & 6.55 \(\pm\) 0.20 & 250 \({}^{\circ}\)C & 229.92 \(\pm\) 4.98 & 106.30 \(\pm\) 2.71 & 3.43 \(\pm\) 0.18 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Average tensile properties of Alloy A and Alloy B after T6 heat treatment: UTS, YS, and EL measured at RT, 150 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C.  
With the increasing temperature, Alloy A showed minor evidence of localized cleavage and crater formation; however, the prevalence of dimples indicated that the ductile behavior was retained. In contrast, Alloy B underwent severe thermal embrittlement, with cracks propagating rapidly along phase boundaries and through brittle constituents.  
The stark contrast in the fracture behavior is attributed to differences in the type, distribution, and thermal stability of strengthening phases. The \(\alpha\)-Al(FeMn)Si phase contributes to ductility by dispersing stress and bridging cracks, whereas the \(\beta\)-AlFeSi and Mg\({}_{2}\)Si phases promote brittleness by concentrating stress and disrupting grain boundary cohesion. Particularly, the spheroidized \(\alpha\)-Al(FeMn)Si structure in Alloy A provides an effective crack-bridging capability, significantly improving the fracture toughness.