# The Influence of Accidental Overheating on the Microstructure and Hardness of the Inconel 718 Alloy
## 1 Introduction  
The increasing demands for performance, especially in cutting-edge fields such as the aerospace industry, nuclear energy, and the automotive industry, have necessitated the development of new materials with superior mechanical properties, enhanced corrosion resistance, and an optimized specific weight. The pace of scientific progress has significantly accelerated in recent years. In a 2024 report, [PERSON] indicated that theduration of research leading to new discoveries had shortened from years to weeks or months [1]. Recent research by NASA specialists has resulted in the development of alloys with exceptional properties, such as the GRX-810 alloy. This alloy, classified as an oxide dispersion-strengthened (ODS) material, exhibits resistance to extreme conditions up to 1000 times greater than previously used alloys. Nickel-based superalloys, first designed nearly a century ago for high-temperature applications, provide excellent corrosion protection and maintain their properties and strength even at elevated temperatures [2; 3; 4; 5]. The superalloy Inconel 718 demonstrates high resistance to tensile stress, fatigue, creep, and rupture, along with excellent corrosion resistance at high temperatures [6]. Due to its composition and applied heat treatment, INCONEL 718 exhibits high mechanical properties. These are attributed to solid solution strengthening through alloying, carbide precipitation at grain boundaries, and especially the formation of intermetallic compounds between nickel and alloying elements such as Al, Ti, and Nb [7]. The heat treatment applied to strengthen the alloy through the precipitation of secondary phases \(\gamma^{\prime}\) (Ni\({}_{3}\)(Al,Ti)) and \(\gamma^{\prime\prime}\) (Ni\({}_{3}\)Nb) involves solution quenching and tempering. In this alloy, niobium, due to its significant proportion in the composition, is the primary element responsible for precipitation strengthening, forming the largest quantity of secondary intermetallic phases [8]. Temperature influences grain size and the processes of solution/precipitation of secondary phases within grain boundaries [7; 9; 10; 11]. The cooling rate strongly affects the microstructure and mechanical properties [12; 13; 14; 15]. In general, carbide precipitation is an important factor in enhancing the hardness and stability of the alloy at high temperatures. Depending on the shape, size, and distribution of carbide precipitates within the grain boundaries, this process can either have a beneficial or detrimental effect on grain boundary integrity and, consequently, on the mechanical properties of the alloy [16; 17]. Fine, globular carbide precipitates increase the stability of the alloy at high temperatures, prevent grain boundary migration, and enhance mechanical strength. Continuous carbide networks, in the form of chains, reduce the alloy's strength [18; 19]. The morphology of the alloy, influenced by grain size, also affects its mechanical properties. An important effect of maintaining high temperatures is grain growth with the formation of serrated grain boundaries [19; 20; 21]. Inconel 718 is recommended for use at temperatures of up to 650-703 \({}^{\circ}\)C. The stable Ni\({}_{3}\)(Al,Ti) phase is what gives the alloy its stability at higher temperatures [22; 23; 24; 25; 26; 27]. Due to its composition, Inconel 718 contains a small proportion of the \(\gamma^{\prime}\) phase in its structure; the proportion of the \(\gamma^{\prime\prime}\) phase is significantly more substantial, but this phase transforms into the delta phase at high temperatures [28; 29]. Studies on the influence of heating processes on Inconel 718 mainly focus on the effects of heat treatments on microstructural evolution and mechanical properties. These studies include plastically deformed alloys [30; 31; 32; 33; 34; 35] or those obtained through selective laser melting [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52], as well as powder metallurgy [32] and other technologies [53; 54].