Oxidation of niobium silicide-based in-situ composites

12 Oxidation of niobium silicide-based in-situ composites 

To achieve higher efficiency and performance of gas turbine engines, the inlet gas temperature should be inaeased. Currently, the operating temperature on high pressure turbine engines is approaching 1150°C, which is very close to the maximum melting point of the most advanced nickel-based superalloys ( 1350°C). The service temperature of intermetallic compounds based on Ni-Al and Ti-Al cannot meet the requirements for use in the hot part of 

The effects of alloying additions on the oxidation of Nb silicide-based in-situ composites have also been investigated and could be summarised as follows  

Al and Cr are commonly required for high-temperature alloys since they can form stable and slow-growing oxide scales, AI2O3 and Cr203. In these composites, Al and Cr could improve the oxidation resistance of Nb solid solutions (Nbss) and silicide [286]. Cr can also stabilise the Cr-rich Laves phase. It was believed that introduction of this phase could enhance the oxidation resistance of the composites [289,291]. However, at low to moderate temperatures, e.g. 800°C, higher oxidation rates and lower adherence of oxide scales were observed with increase of the Cr concentration in the Al-containing Nb-Ti-Si composites [298]. 

Addition of Ti can improve the oxidation resistance [286,291,292,299]. It is supposed that formation and distribution of fine Ti oxide particles in Nbss and formation/segregation of Ti oxide along the phase boundary between the Nbss and the intermetallics are responsible for the decreased oxidation mass gain [302]. Other researchers have also found that Ti could reduce the area fraction of Nbss and then bring a reduction in the oxidation rate [303,304]. 

---