Microstructural and oxidation behavior of NiCrAl super alloy containing hafnium at high temperature

In this research, Ni12Cr4.5AlxHf alloys containing 0.1, 0.2 and 0.4 wt% Hf were produced and their isothermal oxidation behavior has been studied in air for 100 h at the temperature of 1000 °C. Microstructures of the oxidized samples were examined using scanning electron microscopy (SEM). The samples were observed to exhibit different microstructures, and oxidation behavior depending on their chemical composition. The results indicated that the sample containing 0.4 wt% Hf had the lowest weight gain, while the weight gain of the sample without Hf was the highest. Layer investigations show that an external scale of spinel overlying a region of internal oxides precipitates was formed on Ni12Cr4.5Al. On the other hand, an external Al2O3 formed on the samples containing Hf additions. The improvement in the oxidation resistance is believed to be due to the transition between the internal and external oxidation of aluminum and also due to the formation of the relatively continuous, adherent and protective Al2O3 oxide layer on the surface of the Ni12Cr4.5Al0.4Hf alloy. Microstructural investigations using SEM show that the layer adhesion can be affected by mechanical keying at the alloy/scale interface resulting from the pegs' formation during oxidation.