## 3 Results and Discussion
### Effects of Heat Treatment on the Interfacial Microstructures

Table 4: The qualitative analysis of the intermetallic phase corresponding to the points in Figure 5.  
Figure 4: SEM micrographs of the Al/Fe alloys hybrid structure’s interface under excessive heat treatment for Rh; (**a**) 500 \({}^{\circ}\)C, (**b**) 530 \({}^{\circ}\)C, and (**c**) 550 \({}^{\circ}\)C.  
The different types of nickel-bearing phases produced by nickel diffusion at the interface and Al matrix could be explained by the results of the thermodynamic calculations, which reveal the equilibrium nickel concentration range of each phase. Figure 6a,b show calculated phase diagrams of the A319 (Al-Si-Cu-Mg-Fe) alloy system at different nickel contents (0-10%). The red line indicates the specific nickel concentration that affected the transformation of the nickel-bearing phase. Table 5 presents the nickel-bearing phase transformation stages of this alloy system under equilibrium conditions, at each nickel concentration range. The results show that the types of nickel-bearing phases and nickel concentrations in the alloy system were interdependent, and that based on the nickel content ranges, the equilibrium phases of the system differed. At nickel contents above 5.14%, \(\epsilon\)-Al\({}_{3}\)Ni starts to form in this system, but then transforms to 8-Al\({}_{3}\)CuNi, until the nickel concentration reached 6.16%. In this reaction, the liquids reacted with \(\epsilon\)-Al\({}_{3}\)Ni to produce 8-Al\({}_{3}\)CuNi. Therefore, for \(\epsilon\)-Al\({}_{3}\)Ni to be stable in this system, the nickel concentration must exceed 6.16%; this is the minimum amount of nickel required to diffuse into the Al matrix, and form \(\epsilon\)-Al\({}_{3}\)Ni during heat treatment. At critically low nickel concentrations, the Al and Cu atoms combined with Ni to form 8-Al\({}_{3}\)CuNi, and attained equilibrium in the absence of \(\epsilon\)-Al\({}_{3}\)Ni, until the nickel concentration reached 5.14%. The 8-Al\({}_{3}\)CuNi remained in equilibrium over a wide range of nickel concentrations because the A319 alloy system contained significant amounts of Cu. It should be noted that \(\epsilon\)-Al\({}_{3}\)Ni and 8-Al\({}_{3}\)CuNi attained equilibrium at different ranges of nickel concentration. The analysis of the phase diagram indicated that the Ni:Cu ratio determined which nickel-bearing phase would form in this alloy system. Although 8-Al\({}_{3}\)CuNi attained the equilibrium state at almost every nickel concentration range, \(\epsilon\)-Al\({}_{3}\)Ni was solely formed when the Ni:Cu ratio in the alloy system exceeded 2.2. These results indicate that the formation of the nickel-bearing phases, influenced by the solution temperature and proximity to the interface, can vary. The T-Al\({}_{3}\)FeNi attained the equilibrium state over a wide range of nickel concentrations  
Figure 5: SEM micrographs of the intermetallic phases in the Al matrix according to distance from the interface under as-cast and 550 \({}^{\circ}\)C heat treatment conditions; (**a**) as-cast (near interface), (**b**) as-cast (far from interface), (**c**) 550 \({}^{\circ}\)C heat treatment (near interface), and (**d**) 550 \({}^{\circ}\)C heat treatment (far from interface). in this system; when the nickel concentration surpassed 0.15%, T-Al\({}_{9}\)FeNi started to form; however, it did not attain equilibrium during solidification until the 0.73%. On the range of 0.73% to 5.14% nickel, T-Al\({}_{9}\)FeNi formed through the peritectic reaction between the liquids and \(\beta\)-Al\({}_{4.5}\)FeSi. Especially, \(\beta\)-Al\({}_{4.5}\)FeSi was completely transformed into T-Al\({}_{9}\)FeNi at 2.32% nickel and above. When the nickel concentration exceeded 5.14%, the T-Al\({}_{9}\)FeNi phase was formed without the peritectic reaction, while maintaining its equilibrium state as the nickel concentration continued to increase.