Investigation of microstructural evolution and mechanical properties for in-service nickel-based superalloy

In this study, the microstructural evolutions are systematically investigated in the IN718 alloy that has been inservice for eight years. Correspondingly, the effect of in-service process on mechanical properties are explored in terms of tensile properties, fatigue performance and crack growth behavior. Results show that the grain sizes at different sampling locations vary from 9.8 to 15.5 mu m, exhibiting the inhomogeneous microstructures for the inservice IN718 alloy. The volume fraction of delta phase and kernel average misorientation (KAM) at outermost layer are the maximum ones, followed by the middle and innermost layers. In addition, the high magnitude of delta phase is caused by the transformation from strengthening phase gamma ''. From the perspective of mechanical properties, the in-service IN718 alloy can maintain excellent yield strength and elongation at the high temperature of 650 degrees C, while the tensile performance is degenerated at room temperature (RT) compared with those of new IN718 alloy. The fatigue life at 650 degrees C is similar to that at RT under the stress levels lower than 600 MPa. As the stress level increases to 800 MPa, an obvious decrease in fatigue life at 650 degrees C compared with that at RT. This special fatigue performance is revealed based on the fractography examinations under different loading conditions. Moreover, the investigation of fatigue crack growth for the in-service IN718 alloy is carried out under different stress ratio and temperatures.