## 1 Superalloy
### Types of Casting Superalloys and Their Properties
#### 1.2.3 Single crystal superalloys
Figure 3: Mechanical properties of first-generation single crystal superalloys, (**a**) yield strength, (**b**) Larson-Miller plot [12; 13; 14; 15; 16; 17; 18]. Second-generation single crystal alloys have been successfully applied to gas turbines, but there is a demand for higher temperature stability. Representative third-generation single-crystal alloys include CMSX-10 [20] developed by Cannon Muskegon, GE's Rene N6 [21], and TMS75 and TMS80 alloys [22] developed by NIMS. By increasing the Re content to 6%, CMSX-10, a third-generation single crystal alloy with a low Cr content for thermal stability, was developed by both increasing the content of other refractory elements and significantly changing the Al and Ti contents [20]. Creep strength of the alloy was improved by approximately 30 \({}^{\circ}\)C compared to the second-generation alloy. CMSX-10 has been modified to CMSX-10N with 7% of Re in which the compositions of other elements were slightly adjusted. * Fourth- and fifth-generation single crystal superalloys  
In the case of third-generation single crystal superalloys containing 5-6% of Re, the creep properties are found to be degraded because of the formation of topologically closed-packed (TCP) phases after a long duration under high temperatures. Several studies have found that the addition of ruthenium reduces the formation tendency of TCP [23; 24; 25]. Ru is preferentially distributed to \(\gamma\), while Rh, Pd, and Pt are preferentially distributed to \(\gamma^{\prime}\) and Ir is almost evenly distributed on \(\gamma/\gamma^{\prime}\)[25]. In addition, the high temperature creep characteristics of fourth-generation single crystal alloys are very good. One of them is TMS-138 developed by NIMS in Japan, which has excellent hot corrosion resistance [26]. [PERSON] [27] studied the development of the fourth generation of single crystal alloys by varying the content of Re and Ru and presented an alloy called MC-NG (Mono Cristal-Nouvelle Generation). The results show that the creep properties of MC-NG are nearly equivalent to those of the representative third-generation single crystal alloys, CMSX-10 and Rene N6, and the phase stability of TCP is excellent. Based on TMS-138, fifth-generation single crystal alloys such as TMS-162 and TMS-173 with higher Ru content were developed [26]. They increased the Mo content from 2.9 to 3.9 wt.% and improved the stability of the TCP phase formation by increasing the Ru content. In addition, TEM confirmed that the dislocation network formed at the \(\gamma/\gamma^{\prime}\) interface was much finer and more regular than before. The temperature stability of TMS-162 is approximately 20 \({}^{\circ}\)C higher than that of TMS-138 at 1083 \({}^{\circ}\)C.  
However, the oxidation resistances of TMS-162 and TMS-173 were degraded because of the low Cr content for microstructure stability. Therefore, the TMS-196 alloy was developed to improve creep, thermo-mechanical fatigue, phase stability, and oxidation resistance [28]. Figure 5 shows the creep rupture lives as Larson-Miller curves of three alloys, CMSX-10, TMS-138, and TMS-196 [20; 28; 29]. It can be seen that the creep properties are significantly improved in higher generation alloys, especially in the high-temperature low-stress regions. * Cost effective superalloys  
Figure 5: Larson-Miller plot of three different generations of single crystal superalloys [20; 28; 29]. As mentioned above, Re and Ru are added into single crystal superalloys to increase the temperature stability, but these elements are expensive and their supply is strategically dangerous. Therefore, single crystal superalloys with less Re content that have superior properties to the first-generation single crystal alloys and comparable properties to the second-generation alloys have been developed in recent years. As commercial alloys, CMSX-8 containing 1.5% Re and CMSX-7 without Re were developed by Cannon-Muskegon [3].