## 1 Superalloy  
### Evolution of Superalloys  
Gas turbines used in aircraft engines are fabricated using various materials such as aluminum alloys, titanium alloys, steels, composite materials, and nickel-based superalloys. For turbine parts exposed to extreme operating conditions such as high temperature and stress, materials and component manufacturing techniques must be selected to improve engine performance and lifetime. Initially, the gas turbine engines (W1) designed by [PERSON] used various types of stainless steels, which were later replaced by nickel-based superalloys such as Nimonic or Inconel that exhibit better heat resistance [1; 2]. Nickel-based superalloys used in the initial gas turbines included wrought alloys. However, with the development of precision casting technology in the 1950s, thin part of blades or internal cooling flow paths could be applied to increase the operation temperature of the turbine blade. In addition, as the constraints on processing such as forging and rolling were reduced, the composition of the superalloy became more diverse, and the stability of engine parts also improved. Directional solidification by the Bridgman method marked the breakthrough in the casting of turbine blades. Initially, precision parts were cast as polycrystals with many grain boundaries. After solidification slowly proceeded in one direction in the melting furnace to remove grain boundaries perpendicular to the stress axis, directional solidified (DS) blades were produced as the usable temperature was increased. In addition, a single crystal (SC) casting technology has been introduced to produce single crystal blades by selectively growing only one grain to eliminate all grain boundaries, resulting in a higher maximum operating temperature compared to that of DS blades. Single crystal blade technology was first applied to the JT9D-7R4 engine from Pratt and Whitney in 1982. As the turbine entry temperature (TET) of air engines improved, manufacturing technologies for engine parts were developed. Superalloys were developed from the viewpoint of material science such as the fraction and shape of the main phase, and when the new process was developed, the composition of the alloy was optimized for the process. Table 1 shows the representative alloys and their chemical compositions for casting superalloys. Unlike polycrystalline alloys and directionally solidified alloys, single crystal superalloys do not contain grain boundary hardening elements such as C, B, and Zr. In addition, the content of Cr decreases in the order of polycrystalline alloys, directionally solidified alloys, single crystal alloys, the contents of refractory elements such as Mo, W, Ta, and Re increase in the same order. In the case of single crystals, alloys have been developed with increased temperature stability by adding expensive elements such as Re and Ru. Therefore, single crystal superalloys are categorized into generations according to the Re and Ru contents. Single crystal alloys containing no Re are classified as the first generation, alloys containing two-three percent Re are classified as the second generation, and alloys containing five-six percent Re are classified as the third generation. The fourth and fifth generations refer to Ru-added alloys.