Initiation and growth of small fatigue cracks in a Ni-base superalloy
- Lawrence Berkeley Lab., CA (United States)
- Lawrence Berkeley Lab., CA (United States). Center for Advanced Materials
This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loading in four-point bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (K{sub max}). Small surface cracks initiated predominantly at (Nb, Ti){sub x}C{sub y} inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {l_brace}111{r_brace} planes in individual grains and were perturbed or arrested at grain boundaries. For values of {Delta}K above the large-crack threshold, {Delta}K{sub th}, the average rate of small-crack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress ({Delta}{sigma}{sub th}) and the threshold cyclic stress intensity ({Delta}K{sub th}) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 109786
- Journal Information:
- Metallurgical Transactions, A, Journal Name: Metallurgical Transactions, A Journal Issue: 8 Vol. 26; ISSN 0360-2133; ISSN MTTABN
- Country of Publication:
- United States
- Language:
- English
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