## 3 Results and Discussion
### Effects of Heat Treatment on the Interfacial Microstructures
Second, the Kirkendall effect [31] generated by the difference in the diffusion ratio of the metals, causes the detachment of the intermetallic layer and Al substrate from the interface [32]. This phenomenon can trigger the formation of voids and pores at the interface, which assists detachment of the interface [28]. Figure 5 shows that the Ni elements diffused in the interface during heat treatment at 550 \({}^{\circ}\)C also contributed to the precipitation of the nickel-bearing phase in the Al matrix, in accordance with the distance from the interface. The EDS results (Table 4; Figure 5a,c) indicated that the type of intermetallic phases generated in the Al matrix near the interface differed between the as-cast and 550 \({}^{\circ}\)C heat treatment conditions. However, as indicated in Figure 5b,d, the identical types of intermetallic phases in the Al matrix away from interface were formed, regardless of the heat treatment. This implies that the nickel diffusion during  
Figure 3: **(a)** The quantitative analysis of intermetallic layer thickness under different solution temperature and annealing times and (b) electron probe X-ray microanalyzer (EPMA) line mapping of the Al/Fe alloys hybrid structure’s interface under 550 \({}^{\circ}\)C heat treatment, followed by furnace cooling. the heat treatment process also affected the type of intermetallic phases in the Al matrix near the interface. The nickel reacted with the Al, Cu, and Fe atoms to precipitate the nickel-bearing phase, which was stable in this system. In particular, the types of phases containing Fe were different in the as-cast and 550 \({}^{\circ}\)C heat treatment conditions. Generally, the \(\beta\)-Al\({}_{4.5}\)FeSi formed by the presence of Fe in Al-Si-Cu alloys was presented as needle-shaped particles that causes stress concentration, and was weakly bonded to the Al matrix. This phase therefore compromises the mechanical properties of the alloys [33; 34]. Accordingly, the presence of this phase at the interface could reduce the bonding strength of the Al/Fe alloys hybrid structure. However, the nickel coating at the interface prevented the diffusion of Fe into the Al matrix, and the diffused Ni atoms can also act as Fe absorbers. Thus, the formation of the \(\beta\)-Al\({}_{4.5}\)FeSi phase was suppressed, and instead the formation of Al\({}_{9}\)FeNi was promoted [27; 35]. Due to the inability of nickel to diffuse into the Al matrix far from the interface during heat treatment, Al\({}_{2}\)Cu and \(\beta\)-Al\({}_{4.5}\)FeSi compounds were instead formed. (Figure 5d). The nickel-bearing phase (which possesses good thermal stability at elevated temperatures) that was formed at the \(\alpha\)-Al grain boundary, improved the mechanical properties of the aluminum alloy at elevated temperatures. As mentioned above, among the nickel-bearing phases, the strip-like \(\delta\)-Al\({}_{3}\)CuNi particularly enhance the mechanical properties of aluminum alloys at elevated temperatures [36]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & \multicolumn{5}{c}{**Compositions (at.\%)**} & \multirow{2}{*}{**Inference Component**} \\ \cline{2-2} \cline{4-6}  & **Al** & & & & & \\ \hline
1 & 65.73 & - & - & 34.27 & - & Al\({}_{2}\)Cu \\
2 & 66.36 & 16.3 & - & - & 17.34 & Al\({}_{4.5}\)FeSi \\
3 & 70.45 & 13.14 & - & 1.32 & 15.08 & Al\({}_{4.5}\)FeSi \\
4 & 66.64 & - & - & 33.36 & - & Al\({}_{2}\)Cu \\
5 & 70.98 & 14.81 & - & - & 14.21 & Al\({}_{4.5}\)FeSi \\
6 & 70.71 & 4.94 & 18.88 & - & 5.47 & Al\({}_{9}\)FeNi \\
7 & 58.28 & - & 20.55 & 21.18 & - & Al\({}_{3}\)CuNi \\
8 & 70.04 & 5.29 & 18.93 & - & 5.75 & Al\({}_{9}\)FeNi \\
9 & - & - & - & - & 100 & Si particle \\
10 & 57.63 & - & 22.28 & 20.08 & - & Al\({}_{3}\)CuNi \\
11 & 68.46 & - & - & 30.35 & 1.19 & Al\({}_{2}\)Cu \\
12 & - & - & - & - & 100 & Si particle \\
13 & 67.42 & 17.06 & - & - & 15.53 & Al\({}_{4.5}\)FeSi \\
14 & 70.2 & 13.32 & - & - & 16.48 & Al\({}_{4.5}\)FeSi \\ \hline \hline \end{tabular}
\end{table}