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Microstructure Evaluation and Low Cycle Fatigue Damage due to Degradation of Ni-based Superalloy

Nickel-based superalloys have excellent resistance to extreme environments such as high temperature and high stress and are used as materials for large gas turbines. In this paper, specimens were taken from a blade that had been used for a long time, and their life span was studied by analysis of microstructure. The microstructural analysis of the specimens was carried out using OM and SEM to observe coarsening, carbides and TCP phase on gamma prime. Low-cycle fatigue tests were performed on new material and an airfoil of long time-used blade. The test was conducted under various deformation conditions and temperature conditions of 760 oC and 870 oC. The low cycle fatigue test was carried out using the CoffinManson equation and fatigue life was predicted. After the test, crack paths and fracture surfaces were analyzed using SEM. (Received March 27, 2018; Accepted June 15, 2018)