The role of coarsening on LCF behaviour using small coupons of a DS Ni-based superalloy

Turbine blades made of Ni-based superalloys always experience microstructural coarsening during service, inevitably causing a corresponding reduction in the mechanical properties. Thus, it is important to reveal the relationship between coarsening degradation and the reduction in mechanical properties to maintain the safety and economical overhaul. The present work began with a series of coarsening experiments to obtain the different coarsened states of a directionally solidified (DS) superalloy. Then Micro Vickers hardness was tested to investigate the microstructural degradation before fatigue experiments. Low cycle fatigue (LCF) experiments under the condition of 810 MPa/850 °C were employed to measure the reduction in the LCF resistance using small coupons. The results showed that the LCF lifespan considerably decreased with aggravation of coarsening, and a failure mode transformation from the Type I fracture mode to shear fracture along the slip planes also occurred. A logarithmic relationship between coarsening degradation and LCF life reduction was proposed based on the strengthening mechanisms in Ni-based superalloys. In addition, cracks initiating from the grain boundaries having a lager angle to the applied stress, which is related to the coarsened precipitates and the variation of grain boundaries, were observed via scanning electron microscopy (SEM).