Ex-situ observation of the short crack initiation and propagation behaviour of nickel-base wrought alloy Inconel 718 for critical rotating components under fatigue conditions at elevated temperatures

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400 °C and 600 °C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50 μm, while cracks larger than 100 μm showed similar growth characteristics.