• Fatigue of LPBF M789 is governed by discontinuity-microstructure interaction. • Intrinsic discontinuities behave as long cracks, favouring LEFM-based models. • Pits as extrinsic discontinuities follow notch behaviour. • Limited interaction of closely spaced discontinuities controlling fatigue strength. • Extreme value analysis enables conservative fatigue strength prediction. Laser Powder Bed Fusion (LPBF) M789 is a recently developed alloy for fatigue‑critical tooling applications, yet its fatigue behaviour in the presence of intrinsic and extrinsic discontinuities remains largely uncharacterised. This study evaluates the discontinuity‑controlled fatigue response of the alloy using polished, as‑printed, and corrosion‑pitted specimens, fatigue tested and complemented by non‑destructive characterisation, fractography, and extreme value analysis. A Kitagawa–Takahashi diagram incorporating the experimentally measured long‑crack threshold was employed to assess the applicability of LEFM, El‑Haddad, and Murakami models. The experiments highlighted that failure initiated at discontinuities with contributions from the microstructure. Fatigue strength ranged from 93 to 282 MPa depending on surface condition, making the idealised peak fatigue strength unattainable. Process‑induced discontinuities exhibited crack‑like behaviour accurately captured by LEFM and El‑Haddad models, whereas corrosion pits have a notch‑controlled behaviour driven by geometry, with minimal interaction effect between neighbouring discontinuities. The geometric effects account for deviations from fatigue‑strength predictions and demonstrate that discontinuity size alone is insufficient to rank severity when morphologies differ. Overall, the study establishes a discontinuity‑informed basis for predicting the fatigue behaviour of LPBF M789 and supports the development of reliable design methodologies for fatigue‑critical tooling components manufactured from the alloy. .
Sanni et al. (Sun,) studied this question.