MICROSTRUCTURE AND THERMOMECHANICAL FATIGUE BEHAVIOR OF DIRECTIONALLY SOLIDIFIED NI-BASED SUPERALLOYS IN OP CONDITION

Two directionally solidified (DS) Ni-base superalloys, one with the GTD-111 composition and the other with a modified composition derived by numerical simulation, were produced using the Bridgman method. Solution and aging heat treatments were applied to the DS alloys to produce the desired microstructures. Thermomechanical fatigue (TMF) tests were conducted under fully reversed mechanical strain (R =-1) in the temperature range of 538-927 C in laboratory air. The tests were performed under the out-of-phase (OP) loading condition at the mechanical strain range of 0.8-1.5% with multiple specimens tested at each test condition to confirm the trend in fatigue lives. In general, the OP-TMF lives of the DS alloys depended on the applied mechanical strain range as well as the microstructural features of the alloys. By analyzing the fracture surface and longitudinal section of the tested specimens, the TMF lives of the DS alloys tested at mechanical strain ranges lower than 1.5% were found to be primarily affected by the coarse carbides formed in the interdendritic regions. In this study, the main damage mechanism of each alloy under the OP-TMF condition was elucidated in terms of its microstructural features, and recommendations were made to control the microstructures of the DS alloys to achieve enhanced TMF resistance.