## 3 Results and Discussion
### Effects of Heat Treatment on the Interfacial Microstructures
At this concentration range, Ni reacted with Fe to selectively form T-Al\({}_{9}\)FeNi, whereas \(\beta\)-Al\({}_{4.5}\)FeSi was not formed, confirming the theory that nickel can act as an Fe absorber and effectively suppress the formation of \(\beta\)-Al\({}_{4.5}\)FeSi in this system. Since no \(\beta\)-Al\({}_{4.5}\)FeSi were observed near the interface of the Al matrix during the heat treatment process, this theory was confirmed experimentally. Figure 6: (**a,b**) Calculated phase diagram of the Al–Si–Cu–Mg–Fe system for different nickel ranges (0–10%), and (**c**) mole fraction of different phases as a temperature function in the Al-7.98 Si-2.8 Cu–0.64 Mg-0.74 Fe-8.62 Ni system. of temperature in the Al-Si-Cu-Mg-Fe-Ni system containing 8.62% Ni. Due to the considerable amount of nickel required to form the \(\varepsilon\)-Al\({}_{3}\)Ni phase in the A319 alloy system with high Cu, the mole fraction of the \(\varepsilon\)-Al\({}_{3}\)Ni phase started to escalate from above the 6.16% nickel, and was higher than that of \(\delta\)-Al\({}_{3}\)CuNi at 8.62% nickel. Therefore, for the intermetallic layer comprising the \(\varepsilon\)-Al\({}_{3}\)Ni phase to be generated at the interface, more than 8.62% nickel was required in the Al matrix during the heat treatment process. Figure 7 illustrates the interfacial microstructures of the Al/Fe alloys hybrid structures observed at different solution temperatures, followed by air cooling. Cracks occurred between the nickel coating layer and the Al matrix at 500 \({}^{\circ}\)C temperature (Figure 7a). At temperatures of 530 \({}^{\circ}\)C and 550 \({}^{\circ}\)C, cracks formed between the intermetallic layer and the Al matrix, as shown in Figure 7b,c. Hence, cracks occurred at the interface due to air cooling, regardless of the heat treatment temperature. We believe this was due to a large thermal gradient created by air cooling. Thermal stress during heat treatment is created by the difference in CTE between the two materials in contact at the interface [29]. The CTEs in the two materials in the hybrid materials are different, resulting in disparate contraction rates during rapid cooling. This disparity caused severe thermal shock at the interface, which in turn escalated the thermal stress applied to the interface. The results show that furnace cooling was vastly superior to air cooling for promotion of superior chemical bonding at the interface of the Al/Fe alloys hybrid structure. Furthermore, the Al\({}_{3}\)Ni phase was precipitated in the Al matrix close to the interface of the Al/Fe alloys hybrid structure, after heat treatment at 550 \({}^{\circ}\)C followed by air-cooling (Figure 7d). However, as shown in Figure 5c, which shows the furnace cooling conditions after the same heat treatment process, Al\({}_{3}\)Ni was not observed in the Al matrix, indicating that the cooling rate also affects the type of intermetallic phase formed. Under these particular heat treatment conditions, the Al\({}_{3}\)Ni that was formed in the Al matrix, transformed into Al\({}_{3}\)CuNi during the furnace cooling stage. In contrast, under a faster air-cooling rate, the reaction could not be completed and Al\({}_{3}\)Ni was precipitated in the Al matrix instead. Figure 8 summarizes the chemical bonding mechanism of the Al/Fe alloys hybrid structure with nickel coating under optimized heat treatment conditions. During the 550 \({}^{\circ}\)C heat treatment process, inter-diffusion of Al and Ni occurred at the interface of the Al/Fe alloys hybrid structure during chemical bonding, whereas the Fe elements were obstructed by the nickel coating and did not diffuse into the Al matrix (Figure 8a). Figure 8b shows the intermetallics formed by the diffusion of elements at the interface and in the Al matrix during the heat treatment process. The type of intermetallics was dependent on the Ni:Cu ratio and generated a continuous layer at the interface.