FATIGUE CRACK GROWTH IN NICKEL‐BASED SUPERALLOYS AT 500‐700°C. I: WASPALOY

Abstract

Abstract— The effects of cyclic frequency, hold time, and stress‐intensity‐factor range (δK) on rates of fatigue crack growth in air at 500‐700°C have been studied for Waspaloy—a nickel‐based superalloy used for gas‐turbine engine discs. The main effects observed were: (i) higher rates of crack growth for lower cyclic frequencies at high δK at 600 and 700°C. and (ii) lower rates of crack growth at low δK (and higher δK thresholds) for longer hold times at 700°C, compared with those at a baseline frequency of 2 Hz. Metallographic and fractographic observations suggested that the effects of cyclic frequency and hold time could be rationalised in terms of the competing effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide‐induced crack closure, fracture‐surface‐roughness induced crack closure, and crack‐branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at low cyclic frequencies or long hold times are discussed.