FATIGUE CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700°C. II: DIRECT-AGED ALLOY 718

Abstract— The effects of cyclic frequency, hold time, and stress-intensity factor range (Δ K ) 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 Δ K at 700°C, and (iii) lower rates of crack growth at low Δ K (and higher Δ K 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 Δ K 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 Δ K. The precise intergranular crack path, extent of branching, and fracture-surface appearance depended on Δ K , 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.