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The effects of cyclic frequency, hold time, and stress-intensity factor range (AK) on rates of fatigue crack growth in air at 500°C and 700°C have been studied for the direct-aged version of the nickel-based superalloy 718. The main effects were similar to those observed for Waspaloy (Part I), namely: ( i ) small effects of cyclic frequency and hold time at 500"C, (ii) higher rates of crack growth at lower cyclic frequencies at high AK at 700"C, and (iii) lower rates of crack growth at low Aii (and higher AK thresholds) for longer hold times at 700°C. For DA 718, there was no increase in crack growth rates with increasing hold times (0-60 s) at high AK at 700°C (unlike the large increases reported for standard processed Alloy 718). Metallographic and fractographic observations showed that crack growth was predominantly transgranular at 500"C, and predominantly intergranular at 700°C except at 2 Hz at high AK. The precise intergranular crack path, extent of branching, and fracture-surface appearance depended on AK, wave-form and cycle-period. The mechanisms responsible for the observed effects, and possible explanations for the different behaviour of DA 718 and standard processed Alloy 718, are discussed. NOMENCLATURE K = stress intensity factor da/dt = crack growth rate per unit time AK = stress intensity factor range da/dN = crack growth rate per cycle AKeN = effective stress intensity factor range

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The effects of cyclic frequency, hold time, and stress-intensity-factor range ( A K ) 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 AK at 600 and 700°C. and (ii) lower rates of crack growth at low AK (and higher AK thresholds) for longer hold times at 7WC, 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. NOMENCLATURE K = stress intensity factor dn/dt = crack growth rate per unit time AK =stress intensity factor range da/dN = crack growth rate per cycle AK, = threshold stress intensity factor range