# The Influence of Accidental Overheating on the Microstructure and Hardness of the Inconel 718 Alloy
## 3 Experimental Results and Discussion
### The Microstructural Analysis
Although the proportion of the \(\gamma^{\prime}\) phase in Inconel 718 is much lower than that of the \(\gamma^{\prime\prime}\) phase, it ensures stability at temperatures above 700 \({}^{\circ}\)C. Overheating, through the diffusion of titanium from the alloy into the oxide layer, influences the balance between the \(\gamma^{\prime}\) and \(\gamma^{\prime\prime}\) phases. The increased diffusion of titanium at high temperatures may lead to phase transformations that compromise the alloy's strength and creep resistance. At 800 \({}^{\circ}\)C, titanium is present in the surface layer, with a preference for distribution in the chromium-rich layer at the oxide-metal interface (Figure 8). At 900 \({}^{\circ}\)C, the titanium content in the layer increases and is distributed relatively uniformly, similar to that in the bulk alloy (Figure 9). Carbon, although present in low concentrations, plays an important role in creep resistance and grain boundary stability through the formation of carbides. Carbon diffusion is much faster than that of metallic alloying elements and forms a carbon-rich zone at the surface of the oxide layer, which increases with temperature (Figures 7-9). The decarbarization process near the surface has a negative effect on the alloy's properties by reducing the percentage of hard phases at the grain boundary, as well as the stability of the grain boundary and creep resistance. Molybdenum is an important alloying element in Inconel 718, significantly contributing to the alloy's overall performance, particularly at high temperatures. Mo improves the alloy's strength by dissolving into the nickel matrix and plays a crucial role in maintaining stability and mechanical properties under high-temperature conditions. It also contributes to strengthening through the formation of carbides, which, when present at grain boundaries, enhance the alloy's stability. Additionally, Mo influences the precipitation kinetics of the \(\gamma^{\prime\prime}\) (Ni\({}_{3}\)Nb) phase, promoting the growth of precipitates. By controlling diffusion under high-temperature conditions, molybdenum plays a pivotal role in maintaining the alloy's mechanical integrity and corrosion resistance. Structurally, starting at 800 \({}^{\circ}\)C, the molybdenum-rich diffusion layer, similar to niobium, flanks the chromium-rich layer (Figures 8 and 9). The increasing thickness of the molybdenum layer indicates the surface depletion of the alloy, leading to a decline in its properties. Copper diffusion in the surface layer is comparable to that of niobium and titanium. At 900 \({}^{\circ}\)C, it is uniformly distributed in the corrosion layer, with a concentration much higher than in the metallic mass (Figure 9). Although present in low concentrations, copper influences corrosion resistance, microstructural stability, and grain boundary integrity through small precipitates. Overheating at high temperatures, through the diffusion of elements, affects the alloy's microstructure and properties. The diffusion processes occurring during overheating lead to a reduction in mechanical properties and a decrease in the alloy's lifespan. The increase in temperature leads to the intensification of the degradation processes of the protective chromium oxide (Cr\({}_{2}\)O\({}_{3}\)) layer. These processes are caused by thermal cycling and the diffusion of alloying elements into the layer. Repeated heating and cooling cycles induce differential thermal expansion between the substrate and the oxide layer. This results in the development of microcracks and spallation of the Cr\({}_{2}\)O\({}_{3}\) layer, which reduces its protective function. The phenomenon is clearly visible at 900 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C (Figures 9 and 10). The diffusion of alloying elements from the substrate into the oxide layer can alter its composition and structure, potentially forming complex oxides with inferior protective properties compared to pure Cr\({}_{