Effect of the Dwell Time at High Temperature on Crack Propagation in a Single Crystal Superalloy Under Counter-Clockwise Diamond Typed Thermomechanical Fatigue Loading

Single crystal Ni-based superalloys have been used as a material for turbine blades and vanes in gas turbine power generation because of their superior high temperature strength. The turbine blades are exposed to the thermomechanical fatigue (TMF) loading, which is superposition of temperature and mechanical loading. The wall thickness of the superalloy blade with the cooling flow path is thin as some millimeters. Therefore, understanding the small fatigue crack propagation by the TMF loading is essential to prevent the failure. This study investigated the small crack propagation behavior in a single crystal Ni-based superalloy under the counter-clockwise diamond (CCD) typed TMF loading cycle and with the holding at the maximum temperature for 3 min and 10 min. The experimental results indicated that the introducing hold time at the maximum temperature accelerated the crack growth rates even if the mechanical loading was almost zero. The oxide film and γ’ depleted zone at the crack tip might dominate the small crack propagation behavior under CCD TMF loading with the high temperature holdings. In addition, observations of microstructure around the crack tip under the out-of-phase (OP), the In-phase (IP), and the CCD conditions revealed that the composition of the oxide film and the degree of influence on crack propagation behavior depends on the phase angle.