Overload-induced fatigue crack growth retardation in LPBF Inconel 718: role of anisotropic microstructure and crack closure mechanisms

Kurzfassung

Additive manufacturing (AM) via laser powder bed fusion (LPBF) induces a severe crystallographic anisotropy to Inconel 718 (IN718). Its influence on fatigue crack growth under variable amplitude loading requires high-resolution micromechanical analysis. This study provides a mechanistic evaluation of overload-induced crack growth retardation in as-built LPBF IN718 across two build orientations (0° and 90°), compared against a wrought baseline. Employing a single-specimen, continuum-to-microstructural scale methodology to deeply profile localised interactions, crack growth was tracked under constant load amplitude interrupted by a single tensile overload. In-situ digital image correlation quantified local crack opening behaviours, coupled with EBSD, XRD, and fractography to characterise active microstructural deformation and residual stress states. While the wrought condition exhibited lower nominal crack growth rates than the LPBF ones, normalising the driving force to the effective stress intensity factor (dKeff) caused all responses to converge in the stable Paris regime, indicating comparable intrinsic propagation resistance. Following the overload, AM builds exhibited a severely truncated retardation zone compared to the wrought alloy. Fractography revealed that AM closure is driven by a combination of roughness-induced - characterised by tortuous deflections against columnar grains - and plasticity-induced crack closure. Crucially, despite diverging macroscopic yield strengths between AM orientations, their retardation spatial extents were identical. This implies a disconnect in LPBF IN718: the localised transient retardation behaviour does not simply follow the plastic zone dimensions predicted by the bulk macroscopic yield strength.