## 4 Conclusions  
The results regarding the evolution of the microstructure and thermal transfer process in the Inconel 718 alloy subjected to short, repeated heating at high temperatures, up to 1000 \({}^{\circ}\)C, have been presented. The reproduction of accidental, short-term increases in temperature was achieved by applying thermal shocks with solar energy for a history of 3 to 20 cycles. The microstructural analysis of the samples treated by thermal shocks highlighted the distribution and size of the precipitates located at the grain boundaries and the large intragranular precipitates. The SEM EDS characterization of the acicular \(\gamma^{\prime\prime}\) precipitates showed a high niobium content, even up to 78.91% Nb, in accordance with studies conducted by [PERSON] et al. [39,40]. The increase in the number of thermal cycles and the temperature of the shocks led to a decrease in the number of intragranular precipitates, an increase in the size of the remaining ones, and their arrangement in strings oriented perpendicular to the surface where the thermal shock was applied. At high temperatures, the acicular phases disappeared, and the structure remained polyhedral with fine intergranular precipitates. The morphology and elemental chemical composition of the oxide layers were characterized. The layer formed at 800 \({}^{\circ}\)C was compact and exhibited a stratified distribution of the alloying elements, with a significant area rich in chromium near the oxide-metal interface. At 1000 \({}^{\circ}\)C, the oxide layer showed signs of degradation, with a less distinct distribution of elements, but maintained a thinner chromium-rich layer near the interface with the metal [41]. The oxide layer formed on the surface of the samples exhibited a stratified distribution of alloying elements in layers parallel to the metal-oxide interface. Notably, there was a chromium-rich layer in the immediate vicinity of the metal-oxide interface, with continuous and substantial thickness, which accounts for the high corrosion resistance. Increasing the thermal shock application temperature for samples subjected to the same number of shocks highlighted an increase in diffusivity. For the same thermal shock temperature, increasing the number of applied shocks led to an increase in diffusivity. The diffusivity values of the samples after being subjected to thermal shocks were close to and permanently lower than those recorded for the reference sample, except for one sample (1000 \({}^{\circ}\)C, six cycles). The variation curve for the reference sample showed a maximum in the range of 600-750 \({}^{\circ}\)C (in accordance with studies conducted by [PERSON] and [PERSON]) and a  
Figure 16: The decrease in hardness relative to the thermal shock surface as a function of the number of cycles applied at the same temperature: (**a**) 700 \({}^{\circ}\)C; (**b**) 800 \({}^{\circ}\)C; (**c**) 900 \({}^{\circ}\)C; and (**d**) 1000 \({}^{\circ}\)C; \({}^{\prime}\)d—the distance from the opposite surface to the thermal shocks’ application surface. small maximum below 850 \({}^{\circ}\)C. For the samples previously subjected to thermal shocks, these maxima were small. Increasing the temperature for the same number of thermal shocks and increasing the number of shocks applied at the same temperature led to an increase in the depth of the heat-affected zone below the surface of shock application, reaching the full length of the sample for 20 cycles applied at 1000 \({}^{\circ}\)C.  
The experimental results contribute to expanding the database for the Inconel 718 alloy. They are useful for evaluating the heating/cooling rates during heat treatment, assessing the mechanical properties of surfaces subjected to cyclic thermal stresses, and evaluating the depth affected by structural transformations. Future research aims to further study the variation with the maximum diffusivity curves and to conduct a similar study for other candidate alloys for Generation IV reactors. Conceptualization, M.A., E.R.U.A. and D.A.N. ; methodology, E.R.U.A., A.G., D.A.N.