Experiment and modelling on the effect of microstructural morphology on fatigue life of a Ni-based superalloy

Abstract

A directionally solidified Ni-based superalloy was pre-treated under high temperature with/without the applied stress to obtain different microstructural morphology. Then, strain-controlled low cycle fatigue experiments were carried out under 850°C with a 0 strain ratio to measure the reduction in the fatigue resistance. The scanning electron microscopy (SEM) was employed to characterize the microstructures of the alloy with different treatments. The micro-deformation and dislocation movement were investigated via transmission electron microscopy (TEM). The results show that both coarsening and rafting morphology have negative effects on the fatigue properties, which is closely related to the deformation mechanism of the alloys with different γ' morphology. The coarsening morphology decreases the shear resistance of the γ' precipitates, resulting in shearing of γ' particles. More plastic deformation introduces during the cyclic process, leading in the reduction of fatigue life. On the other hand, the wide γ channel promoted the piling up and movement of dislocations in the matrix, also resulting in reduction of plastic deformation resistance and lifespan. Finally, a fatigue life prediction model was proposed based on the continuum damage mechanics (CDM) by introducing the variation of matrix channels’ width, showing a good competency compared with the experimental results.