## 3 Results and Discussion
### Effects of Heat Treatment on the Interfacial Microstructures
The intermetallic layer formed at 530 \({}^{\circ}\)C continuously increased until 8 h, but remained thinner than the layer formed at 550 \({}^{\circ}\)C. After heat treatment at 550 \({}^{\circ}\)C for 1 h, the intermetallic layer at the interface was continuous and notably thicker than that for the other temperatures. At this heat treatment temperature, and compared with other temperatures, the thickness of the intermetallic layer increased sharply with heat treatment time. Hence, the solution temperature was determined to significantly promote the formation of thick and continuous intermetallic layers. The optimal solution temperature for the chemical bonding of the Al/Fe alloys hybrid structure was 550 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multirow{2}{*}{**Number**} & \multicolumn{6}{c}{**Compositions (at.\%)**} & \multirow{2}{*}{**Inference Component**} \\ \cline{3-4} \cline{6-9}  & & **Al** & & & & **Fe** & **Ni** & **Cu** & **Si** \\ \hline \multirow{4}{*}{530} & 1 & 53.3 & - & 25.72 & 20.32 & - & Al\({}_{3}\)CuNi \\  & 2 & 72.28 & - & 27.72 & - & - & Al\({}_{3}\)Ni \\  & 3 & 74.36 & - & 25.64 & - & - & Al\({}_{3}\)Ni \\  & 4 & 58.72 & - & 21.33 & 18.98 & - & Al\({}_{3}\)CuNi \\ \hline \multirow{2}{*}{530} & 5 & 47.77 & - & 48.47 & - & 3.76 & NiAl \\  & 6 & 73.84 & - & 26.16 & - & - & Al\({}_{3}\)Ni \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The qualitative analysis of the intermetallic layer corresponding to the points in Figure 2c,e. Figure 3b shows the EPMA line mapping results of the Al/Fe alloys hybrid structure's interface, after heat treatment at 550 \({}^{\circ}\)C. The diffusion of Ni and Al elements across the interface was evident, implying successful chemical bonding between the nickel and the aluminum following heat treatment. Furthermore, these results confirmed that the Fe elements from the steel did not actively diffuse into the Al matrix during the heat treatment process. This was due to the nickel coating. Figure 2: SEM micrographs of the Al/Fe alloys hybrid structure’s interface under different solution temperatures followed by furnace cooling; **(a)** 500 \({}^{\circ}\)C, **(b,c)** 530 \({}^{\circ}\)C, and **(d,e)** 550 \({}^{\circ}\)C.  
Although the intermetallic layer thickness increased with increasing the solution treatment time and temperature, excessively long heat treatment times decreased the bonding strength of the Al/Fe alloys hybrid structure. Long-term heat treatment for 8 h caused the formation of cracks between the intermetallic layer and the aluminum substrate (Figure 4). We believe that there are two main reasons for this. First, the large difference in the coefficients of thermal expansion (CTE) between the aluminum alloy and the intermetallic layer induces excessive thermal stress, sufficient to form cracks at the interface [26; 28; 29]. The condition under which the crack is generated at the interface between the intermetallic layer and the aluminum alloy, is described by Equation (3) [30]:  
\[\Delta T=\frac{16}{3(1+\
u)}\frac{x^{2}}{a^{2}}\frac{1}{\Delta\alpha} \tag{3}\]  
where \(\Delta T\) is the temperature change at which cracking and detachment are initiated at the interface between the aluminum alloy and the intermetallic phase, \(x\) is the thickness of the intermetallic layer, \(\Delta\alpha\) is the difference in the coefficients of thermal expansion between the aluminum alloy and intermetallic phase, \(\
u\) is Poisson's ratio, and a is the length of the crack created at the interface as the temperature decreases during cooling. Among these parameters, \(\
u\) and \(\Delta\alpha\) are intrinsic properties of the material and \(\Delta T\) is a constant determined by heat treatment. Resulting from Equation (3), an increase in the intermetallic layer thickness induces a growth in the crack length [28]. Excessive heat treatment, including extreme solution temperatures and/or times, can trigger cracks at the interface, resulting in a decline in the bonding strength.