Subsurface faceted cracking behavior of selective laser melting Ni-based superalloy under very high cycle fatigue

The structure-property of a Ni-based superalloy fabricated by selective laser melting (SLM) was investigated under very high cycle fatigue (VHCF), and the subsurface cracking with fisheye pattern becomes a remarkable failure mode. Due to the presence of subsurface failure, the descending S–N curve property is represented. Based on 2D & 3D microscopic observation, a rough region with facets and inclusions, named as “multi-inclusion assisted facetted cracking zone (MAFCZ)”, is present within the fisheye. By combining electron backscatter diffraction analysis, the subsurface failure is related to grain size & orientation, microtexture and crystal defects. The crack is nucleated from the large grain slip with the highest Schmid factor under the assistance of the inclusions. The rough surface morphology with crack deflection reveals ultra-slow crack growth behavior within MAFCZ. Combined with failure modeling and fracture mechanics analysis, the subsurface failure mechanisms of SLM Ni-based superalloy under the VHCF are elucidated.