Isothermal oxidation comparison of three Ni-based superalloys
Heggadadevanapura Thammaiah, Mallikarjuna
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Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behaviour of three nickel-based superalloys; namely, polycrystalline cast IN738LC, single-crystal N5 and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900°C in the static air up to 1000h and the specific aspects studied were the oxidation behaviour of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behaviour of oxide scale growth and subsurface changes were analysed in detail using various techniques such as SEM, EDS and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, kp was calculated for each alloy and found to be; kp = 2.79 x 10-6 mg2.cm-4.s-1 for IN738LC, kp = 1.42 x 10-7 mg2.cm-4.s-1 for N5 and kp = 1.64 x 10-7 mg2.cm-4.s-1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr2O3 and discontinuous inner scale of Al2O3, whereas N5 and TAS showed a dense outer scale of Al2O3 alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions.