FATIGUE-CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700-DEGREES-C.1. WASPALOY

The effects of cyclic frequency, hold time, and stress-intensity-facto r range (DELTAK) on rates of fatigue crack growth in air at 500-700-de grees-C have been studied for Waspaloy-a nickel-based superalloy used for gas-turbine engine discs. The main effects observed were: (i) high er rates of crack growth for lower cyclic frequencies at high DELTAK a t 600 and 700-degrees-C, and (ii) lower rates of crack growth at low D ELTAK (and higher DELTAK thresholds) for longer hold times at 700-degr ees-C, compared with those at a baseline frequency of 2 Hz. Metallogra phic and fractographic observations suggested that the effects of cycl ic frequency and hold time could be rationalised in terms of the compe ting effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide-induced crack closure, fracture-surface-roug hness induced crack closure, and crack-branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at l ow cyclic frequencies or long hold times are discussed.