Sp. Lynch et al., FATIGUE-CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700-DEGREES-C.2. DIRECT-AGED ALLOY-718, Fatigue & fracture of engineering materials & structures, 17(3), 1994, pp. 313-325
The effects of cyclic frequency, hold time, and stress-intensity facto
r range (DELTAK) on rates of fatigue crack growth in air at 500-degree
s-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 tho
se 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, an
d (iii) lower rates of crack growth at low DELTAK (and higher DELTAK t
hresholds) for longer hold times at 700-degrees-C. For DA 718, there w
as no increase in crack growth rates with increasing hold times (0-60
s) at high DELTAK at 700-degrees-C (unlike the large increases reporte
d 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 exc
ept at 2 Hz at high DELTAK. The precise intergranular crack path, exte
nt of branching, and fracture-surface appearance depended on DELTAK, w
ave-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.