FATIGUE-CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700-DEGREES-C .2. DIRECT-AGED ALLOY-718

The effects of cyclic frequency, hold time, and stress-intensity factor range (DELTAK) on rates of fatigue crack growth in air at 500-degrees-C and 700-degrees-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-degrees-C, (ii) higher rates of crack growth at lower cyclic frequencies at high DELTAK at 700-degrees-C, and (iii) lower rates of crack growth at low DELTAK (and higher DELTAK thresholds) for longer hold times at 700-degrees-C. For DA 718, there was no increase in crack growth rates with increasing hold times (0-60 s) at high DELTAK at 700-degrees-C (unlike the large increases reported for standard processed Alloy 718). Metallographic and fractographic observations showed that crack growth was predominantly transgranular at 500-degrees-C, and predominantly intergranular at 700-degrees-C except at 2 Hz at high DELTAK. The precise intergranular crack path, extent of branching, and fracture-surface appearance depended on DELTAK, 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.