Characterization of fatigue crack growth, damage mechanisms and damage evolution of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC under thermomechanical fatigue loading using in situ optical microscopy

In this paper, the influence of different phase angles, load ratios and dwell times on the thermomechanical fatigue (TMF) crack growth of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC is studied. The thermomechanical fatigue crack growth tests are performed under in-phase (IP) and out-of-phase (OP) TMF loading between 300 °C and 950 °C. It is shown, that the applied load ratio plays a crucial role for the resulting fatigue crack growth rates and whether IP or OP TMF loading is more detrimental. A digital optical microscope is installed, which allows in situ observation of the fatigue crack growth, the active damage mechanisms and the damage evolution even at high temperatures. The in situ observations are accompanied by fractographic investigations, which confirm, that IP TMF loading preferentially leads to interdentritic fracture. Finally, first steps towards a quantification of the active damage mechanisms are taken using digital image processing.