## 1 Superalloy
### Types of Casting Superalloys and Their Properties
#### 1.2.3 Single crystal superalloys  
Similar to DS superalloys, single crystal superalloys were developed to increase the creep resistance and temperature acceptability by minimizing grain boundaries. Therefore, additive elements such as C, B, Zr, and Hf were removed from most single crystal superalloys to suppress the formation of grain boundaries. In addition, as heat treatment was possible at a higher temperature by removing the above added elements, it was possible to increase the fraction of the \(\gamma^{\prime}\) phase and improved microstructure uniformity. * First-generation alloys Single crystal alloys did not receive much attention initially because the creep, thermal fatigue, and oxidation resistance of conventionally heat-treated single crystal Mar-M200 are not significantly better than those of the DS alloy. Subsequently, it was found that the incipient melting temperature increased considerably when the grain boundary strengthening elements such as B, C, and Zr were removed. In addition, \(\gamma^{\prime}\) can precipitate finely and uniformly after complete dissolution without local melting when the incipient melting temperature increased, improving the creep properties of single crystals [12]. Based on these results, some single crystal superalloys have been developed. Rene N4 was developed by GE. Cr and C contents in Rene N4 are increased to improve the hot corrosion resistance of Rene N, the first single crystal alloy developed by GE. The Al/Ti ratio was increased to enhance oxidation resistance, and Ta content was increased to compensate for the decrease in \(\gamma^{\prime}\) volume fraction due to Ti reduction. In addition, 0.15% Hf was added to improve the oxidation resistance, and B was added to prevent degradation of mechanical properties due to the creation of a low angle boundary [13]. Figure 2: Mechanical properties of directional solidified (DS) superalloys, (**a**) strength, (**b**) Larson-Miller plot [10; 11]. PWA1480 developed by Pratt and Whitney was the first commercialized single crystal alloy among the PWA series in 1982 [14]. Increased Ta content improves the oxidation resistance and inhibits freckle formation, and sufficient Cr and Al are added for oxidation resistance. The alloying elements, Mo and V, were removed while maintaining a sufficiently high Cr content to obtain good hot corrosion resistance [15]. Yield strength and creep properties of some first-generation single crystal superalloys are shown in Figure 3.  
* Second-generation alloys  
CMSX-4 is a second-generation single crystal alloy developed by Canon-Muskegon in the early 1980s and has been used in gas turbines since it was first commercialized in 1991. The alloy was modified to CMSX-4 [SLS] with enhanced oxidation resistance in the 2000s. Rene N5 is a second-generation single crystal alloy developed by GE. GE has also developed Rene N515 with 1.5% Re and a Re-free alloy, Rene N500, based on this alloy. Rene N5 is used not only for turbine blades but also in aircraft engines and power generation gas turbines. PWA1484 was designed by Pratt and Whitney to exhibit approximately 50 \({}^{\circ}\)F (28 \({}^{\circ}\)C) higher temperature stability than PWA1480. In addition to the creep properties, various properties such as stability of the microstructure, castability, oxidation resistance, thermal fatigue resistance, and high cycle fatigue (HCF) properties were considered during development [14]. Then, 5.5% Al and 8.7% Ta were added to increase the solvus temperature of \(\gamma^{\prime}\) and also to suppresses coarsening of \(\gamma^{\prime}\) at high temperature and to maintain the volume fraction of \(\gamma^{\prime}\)[14]. The mechanical properties of second-generation single crystal alloys, CMSX-4, Rene N5, and PWA1484, are compared and shown in Figure 4.  
* Third-generation single-crystal superalloys  
Figure 4: Mechanical properties of second-generation single crystal superalloys, (**a**) strength, (**b**) Larson-Miller plot [3; 19].