On the oxidation of the third-generation single-crystal superalloy CMSX-10

The oxidation behavior of the third-generation nickel-base single-crystal superalloy CMSX-10 is examined. Since the in-service performance of the alloy is of the greatest practical significance, a detailed study is made of the microstructural degradation of a turbine blade that had been removed prematurely from a commercial gas turbine engine. The results are augmented with isothermal oxidation tests conducted in the laboratory for 100 hours, at temperatures of 800 degrees C, 900 degrees C, and 1000 degrees C. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, microhardness testing, and X-ray diffraction (XRD) were employed. It was found that the oxidation of CMSX-10 at temperatures below 1000 degrees C does not produce either Al2O3 or the spinel Ni(Cr,Al)(2)O-4, both of which are found in the internal oxidation zone of the earlier generations of superalloys. Surprisingly, it is demonstrated conclusively that the oxidation of CMSX-10 generates the beta phase (NiAl). This reaction, which to the authors' knowledge has not yet been reported, is termed self-aluminization. The XRD studies demonstrate that the internal oxidation of CMSX-10 produces (Ni,Co)Ta2O6, (Ni,CO)WO4, CrTaO4, and Cr(W,MO)O-4. There is indication that the formation of the delta phase (Ni2Al3) slows the oxidation rate at 1000 degrees C.