INITIATION AND GROWTH OF SMALL FATIGUE CRACKS IN A NI-BASE SUPERALLOY

This article reports research on the initiation and growth of small fa tigue cracks in a nickel-base superalloy (produced commercially by INC O as INCOLOY 908) at 298 and 77 K. The experimental samples were squa re-bar specimens with polished surfaces, loaded in four-point bending. The crack initiation sites, crack growth rates, and microstructural c rack paths were determined, as was the large-crack growth behavior, bo th at constant load ratio (R) and at constant maximum stress intensity (K-max). Small surface cracks initiated predominantly at (Nb, Ti)(x)C -y inclusion particles, and, less frequently, at grain boundaries. Sma ll cracks grew predominantly along {111} planes in individual grains a nd were perturbed or arrested at grain boundaries. For values of Delta K above the large-crack threshold, Delta K-th, the average rate of sm all-crack growth was reasonably close to that of large cracks tested u nder closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Delta sigma(th)) and the threshold cyclic st ress intensity (Delta K-th) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combinatio n of the fatigue endurance limit and the threshold stress intensity fo r closure-free growth of large cracks can be used to define a fatigue- safe load regime.