## 1 Superalloy
### Manufacturing Process for Casting Superalloys and High-Temperature Components  
In the early stages of gas turbine blade manufacturing, many forging methods using nickel-based superalloys were used. However, as the \(\gamma^{\prime}\) volume fraction increased to improve high temperature strength, the high temperature formability decreased, and the manufacturing method was replaced by the casting method. Until recently, most turbine blades were manufactured using a casting method, which allows more precise and complex surface shaping. Casting alloys are used to fabricate complex parts such as blades and vanes. They are generally cast under vacuum because they have some well-oxidized alloying elements such as Al and Ti. Based on the casting method, different microstructures of superalloys are formed. The microstructure shape is closely related to the thermal properties as well as the mechanical properties such as high temperature phase stability, tensile and yield strength, and creep resistance. Therefore, several casting methods have been developed to improve the mechanical and thermodynamic properties of superalloys. Conventional cast superalloys have a polycrystalline grain structure. However, the grain boundary perpendicular to the major stress axis can degrade the creep properties at high temperatures. To improve the creep life of the casted alloys, directional solidification casting was developed to remove grain boundaries perpendicular to the stress axis, and a single crystal casting process was developed to remove all grain boundaries. Figure 6a shows blades with different grain shapes depending on the casting method. A schematic diagram of Bridgman casting is shown in Figure 6b. Very fine grains are formed when the molten superalloy is poured into a mold preheated to approximately 1500 \({}^{\circ}\)C placed on a water-cooled Cu chills. A temperature gradient is generated in one direction between the upper part of the mold with the molten superalloy and the lower part which has already solidified. As a result, the crystal grows in one direction when the mold is gradually lowered. Nickel-based superalloys with FCC structure prefer to grow in the [001] direction during solidification. Dendrites aligned in the direction of the temperature gradient tend to grow rapidly in the [001] direction when the superheated liquid at the interface with the dendritic phase prevents the formation of new grains. Accordingly, columnar grains parallel to the solidification direction are formed after the casting process. In the case of single crystal superalloy casting, as shown in Figure 7, solidification begins from the starter block in contact with the chill plate and proceeds to the selector. The number of grains decreases while passing through the starter block, as can be seen in Figure 7. The mechanism of competitive growth of grains in the starter block is the same as that of directional solidification. Among the many polycrystalline grains produced on chill plates, only some of the grains arranged in favor of the temperature gradient and growth survive to form a columnar structure as solidification progresses. A single grain is selected and grown through the selector among the several columnar grains entering the selector. Another way to cast single crystals is via the use of a seed. A single crystal seed is inserted underneath the mold when the mold is being fabricated. Unlike the selector method in which the single crystal grows in the [001] direction, this method has the advantage that the growth direction of the single crystal can be controlled because the crystal grows in the same direction as the seed. In the conventional single crystal casting method, a temperature gradient is generated using a water-cooled Cu plate. However, a liquid metal cooling (LMC) method using a liquid metal like Sn as a coolant was introduced to increase the temperature gradient, because the larger the temperature gradient, the better the quality of casting.