## 3 Results and Discussion
### Micro-Hardness Distribution in the Al/Fe Alloys Hybrid Structure  
Under optimized heat treatment conditions, multiple layers were formed at the Al/Fe alloys hybrid structure's interface. The effects of heat treatment and elemental diffusion on the hardness of each layer and matrix were analyzed using the Vickers hardness test. Figure 9 illustrates the microhardness distribution across the Al/Fe alloys hybrid structure interface for as-cast and optimized heat treatment conditions. The hardness of the steel matrix and the nickel coating layer did not change after the heat treatment. The intermetallic layer, composed of the hard and brittle nickel-bearing phase, exhibited the highest hardness in the Al/Fe alloys hybrid structure. Furthermore, it was confirmed that heat treatment significantly reduced the hardness of the Al matrix (40%, from 100 HV to 60 HV). We  
Figure 8: Schematic illustration of the chemical bonding process of the Al/Fe alloys hybrid structure under optimized heat treatment condition.  
Figure 7: (**a–c**) SEM micrographs of the Al/Fe alloys hybrid structure’s interface under different solution temperature, followed by air cooling; (**a**) 500 \({}^{\circ}\)C, (**b**) 530 \({}^{\circ}\)C, and (**c**) 550 \({}^{\circ}\)C. (**d**) The precipitated Al\({}_{3}\)Ni at the matrix after 550\({}^{\circ}\) C heat treatment, followed by air cooling.  
believe that the heat treatment parameters, such as 550 \({}^{\circ}\)C solution temperature and furnace cooling, were sufficient for grains to develop in the Al matrix, thus impacting its hardness [37]. Furthermore, the hardness of the Al matrix remained similar, regardless of the distance from the interface. In the microstructural analysis, the intermetallic phases that formed differed depending on the distance from the interface, implying that the nickel-bearing phase created by the diffusion of nickel did not affect the hardness of the Al matrix at room temperature. The effect of artificial aging (AG) was investigated for alleviating the hardness reduction of the Al matrix. The artificial aging process only affected the hardness of the Al matrix in the Al/Fe alloys hybrid structure; however, the effect was dependent on the distance from the interface. The hardness of the Al matrix far from the interface increased by approximately 35% (from 60 HV to 93 HV), whereas that of the Al matrix near the interface did not increase. During artificial aging, the Al-Si-Cu aluminum alloy precipitates nanosized 0-Al\({}_{2}\)Cu in the Al matrix. The precipitation hardening effect from these precipitates increased the hardness of the Al matrix far from the interface [38]; however, the hardness of the Al matrix near the interface did not increase. Thus, precipitation hardening did not occur near the interface, even after artificial aging. The explanation for this could involve diffusion of nickel. The nickel moved into the Al matrix and combined with Al and Cu atoms to form the nickel-bearing phase. During this reaction, the Cu atoms in the Al matrix near the interface were depleted. Therefore, the 0-Al\({}_{2}\)Cu phase could not form and precipitation hardening did not occur [27]. To summarize, the nickel that diffused into the Al matrix contributed to the formation of the nickel-bearing phase but interfered with the precipitation hardening process.