# The Influence of Accidental Overheating on the Microstructure and Hardness of the Inconel 718 Alloy
## 3 Experimental Results and Discussion
### The Microstructural Analysis
The increase in temperature, along with the growth in oxide layer thickness, leads to a spatial distribution of chromium within the layer and the formation of a less chromium-rich layer at the metal-oxide interface. At 900 \({}^{\circ}\)C, the growth of the oxide layer is associated with the occurrence of degradation processes (Figure 9). The chromium layer at the metal-oxide interface is dense but thinner. At 1000 \({}^{\circ}\)C, the layer begins to show signs of degradation. The chromium oxide layer at the metal-oxide interface becomes thinner, and Cr oxide is dispersed toward the upper regions (Figure 8). In the literature, reference is made to the essential role of Cr and Al in corrosion resistance [2]. The characterization of oxide layers has highlighted the role of chromium at the metal-oxide interface. The aluminum present in the alloy is distributed in precipitates that enhance hardness and stability at high temperatures. No significant presence was observed at the interface region or in the surface layer. The Inconel 718 alloy is widely utilized in aerospace and energy applications due to its exceptional high-temperature strength, corrosion resistance, and overall mechanical performance. The behavior of these alloys is critically influenced by the diffusion of alloying elements, which governs the formation, growth, and stability of strengthening precipitates, as well as other microstructural features. Figure 10: SEM EDS chemical composition mapping for the layer formed at 1000 \({}^{\circ}\)C (\(\times\)2k). We can see the degradation of the oxide layer; in severely degraded areas, Nb- and Mo-containing compounds are distributed. Chromium is essential in corrosion resistance, as it forms a protective Cr\({}_{2}\)O\({}_{3}\) layer on the alloy's surface, enhancing its durability in high-temperature environments. It also plays a role in the stabilization and dissolution of carbides. Chromium diffusion is relatively slow compared to other elements. For Inconel 718, the chromium-rich layer is ensured by the alloy's high chromium concentration. The thickness of the chromium-rich layer increases with rising temperature, but the structure degrades. At 700-800 \({}^{\circ}\)C, the layer remains compact, perfectly covering the metal-oxide interface. However, starting at 900 \({}^{\circ}\)C, degradation processes appear in the upper region (Figures 9 and 10), although a compact layer remains near the interface, becoming increasingly thinner. The concentration of other elements in the surface layer increases with rising temperature. Their presence in the surface layer must be discussed in relation to their role in the alloy. Niobium is a principal strengthening element in Inconel 718. The \(\gamma^{\prime\prime}\) (Ni\({}_{3}\)Nb) phase significantly enhances tensile strength and creep resistance. The diffusion behavior of niobium is key in controlling the size, distribution, and stability of these precipitates. At lower temperatures, limited diffusion ensures a fine dispersion of the \(\gamma^{\prime\prime}\) phase. Higher temperatures accelerate coarsening and phase instability, leading to a decline in mechanical properties. As the temperature increases, the chromium-rich layer becomes enriched in niobium, forming a niobium-rich layer on top of the chromium-containing layer (Figure 8). This results in the depletion of niobium in the alloy near the surface. In the oxide layer, niobium tends to accumulate in degraded regions of the chromium-rich layer (Figures 9 and 10). Its role in maintaining alloy stability at 900-1000 \({}^{\circ}\)C is less significant than that of titanium. Titanium contributes to alloy strengthening through the formation of the \(\gamma^{\prime}\) (Ni\({}_{3}\)(Al,Ti)) phase.