A quantitative role of rafting on low cycle fatigue behaviour of a directionally solidified Ni-based superalloy through a cross-correlated image processing method

•Rafting extent of the superalloy was quantitatively characterized by a cross-correlated image processing method.•Rafting behaviour decreases the resistance to plastic deformation and the hardening effect of the γ' precipitates, which promote the accumulation of the fatigue damage and reduce the fatigue life.•A fatigue life prediction model considering the microstructural rafting was proposed, which is of good consistence with the experimental data. Operating under elevated temperature and high stress for long terms, the regularly cubic γ' precipitates of the Ni-based superalloy degrades to plate-like morphology perpendicular to the external stress. This process is called rafting and plays a deteriorated role on the mechanical properties of the alloy. In this study, the quantitative effect of rafting on the low cycle fatigue behaviour of a directionally solidified Ni-based superalloy was investigated. Fatigue tests were conducted at 850 °C with a 1.1% total strain range with six different rafting states. The fatigue life decreased considerably over 90% as the rafting state aggravated. The rafting process increased the width of the matrix channel, which weakened the plastic deformation resistance and the mean stress relaxation of the superalloy. To build a quantitative relationship between the life reduction and rafting state, a life prediction model was proposed based on the strengthening mechanism of Ni-based superalloys by assuming that the rafted alloy suffered a higher loading level compared with the virgin state. Moreover, the scanning electron microscopy was employed to observe fatigue fracture surfaces and the rafted microstructures. The micro-parameters such as the width of the γ' precipitates and the matrix channel were obtained through a cross-correlated image processing method.