# The Influence of Accidental Overheating on the Microstructure and Hardness of the Inconel 718 Alloy
## 3 Experimental Results and Discussion  
### The Microstructural Analysis  
The evolution of the alloy microstructure along the height of the sample by applying thermal shocks according to the temperature of the shocks is presented in Figure 3.  
Increasing the heat shock temperature shows an increase in grains with temperature, with a more obvious increase near the thermal shock surface for a temperature of 1000 \({}^{\circ}\)C (Figure 3). SEM analysis highlighted the morphology and the distribution of the secondary phases (Figures 4 and 5). The large Ni\({}_{3}\)Nb intragranular precipitates are arranged in parallel string-like formations orientated perpendicular to the surface where the thermal shocks were applied. The Ni\({}_{3}\)Nb, \(\gamma^{\prime\prime}\) phases within the grain boundaries have an acicular structure (Figure 5a), and in some areas, they even have spatial developments (Figure 5b). Figure 3: The microstructure after 20 thermal cycles, determined along the height of the sample in three areas, from left to right: near the surface opposite the shock, in the middle of the sample, and near the surface where the shock was applied, for the temperatures of (**a**) 600 \({}^{\circ}\)C; (**b**) 800 \({}^{\circ}\)C, and (**c**) 1000 \({}^{\circ}\)C.  
The evolution with the temperature of the thermal shock of the thickness of the oxide layers formed is presented in Figure 6.  
Figure 4: Cross-section SEM analysis of the microstructure of the samples treated with 20 thermal cycles: (**a**) large strings of precipitates formed at 700 \({}^{\circ}\)C (\(\times\)700). (**b**) The microstructure of the sample treated at 900 \({}^{\circ}\)C consists of a solid solution matrix; the \(\gamma\) phase appears as mackled polyhedra, with precipitates forming as strings of the molybdenum-rich \(\gamma^{\prime\prime}\) phase and \(\gamma^{\prime}\) phase precipitates located along the grain boundaries (\(\times\)1k). (**c**) The reduction in precipitate volume by dissolution at 1000 \({}^{\circ}\)C (\(\times\)800). Figure 5: The precipitates formed in the Inconel 718 alloy treated with 20 cycles of thermal shocks at 800 \({}^{\circ}\)C: (**a**) \(\gamma^{\prime}\) phase (\(\times\)10k) and (**b**) \(\gamma^{\prime\prime}\) phase (\(\times\)12k). The cross-sectional SEM analysis in Figure 6 highlights the increase in the thickness of the layers with the shock temperature, with the oxide-metal interface marked. The elemental chemical composition maps obtained through SEM-EDS reveal the distribution of alloying elements in the oxide layers (Figures 7-10), emphasizing the protective role of chromium. Figure 7: SEM EDS chemical composition mapping for the layer formed at 700 \({}^{\circ}\)C (\(\times\)10k). We can see the presence of molybdenum together with niobium in the spatially extensive precipitate and the presence of aluminum in a relatively compact precipitate located near the interface. Figure 8: SEM EDS chemical composition mapping for the layer formed at 800 \({}^{\circ}\)C (\(\times\)5k). A continuous, compact layer of chromium oxide, corresponding to the metal–oxide interface, is observed. It is flanked by layers rich in Mo and Nb, with a uniform distribution of Al and Cu throughout the layer. Figure 9: SEM EDS chemical composition mapping for the layer formed at 900 \({}^{\circ}\)C (\(\times\)2.5k). We can see the degradation of the oxide layer and a reduction in the thickness of the Cr-rich substrate; in severely degraded areas, Nb- and Mo-containing compounds are distributed, with a high carbon content. At 700 \({}^{\circ}\)C, the oxide layer formed is uniform (Figure 7). Except for chromium, whose concentration is higher at the metal-oxide interface; the other elements show a distribution similar to that of the alloy. At 800 \({}^{\circ}\)C, the oxide layer formed is continuous and compact. From a chemical composition perspective, it exhibits a stratified structure (Figure 8). The chromium oxide layer located at the metal-oxide interface is uniform and dense.