Fig. 1 Dimension of the ex situ fatigue test specimen

Fig. 2 XCT acquisition datasets from the specimen at as-cast, 8400 cycles, 9400 cycles and final fracture states during the fatigue test

Fig. 3 Segmentation process of carbides by using deep learning network: a the raw 2D transverse slices; b adjusting the brightness and contrast of the slices; c extracting the carbides by the trained conventional neural network; d 3D reconstruction of the extracted carbides

Fig. 4 a Blocky carbides and script carbides from SEM characterization; b carbides from the XCT acquisition dataset; EDS analyses of the blocky carbide c and script carbide d

Fig. 5 Shrinkage pores from: a SEM, b XCT, c 3D morphology of carbides and shrinkage pores

Fig. 6 Crack initiation process near the carbides: a-d 2D transverse slices and e-h 2D vertical slices of the crack initiation site; i-l 3D volume rendering of the carbides and pores

Fig. 7 Microcracks close to carbides in the bulk region of the specimen: a 2D transverse slices in the middle section at the 9400 cycles, b, c the magnified images of the cracks near the carbides in a

Fig. 8 Clustering pores in crack initiation zone: a, b 2D transverse slices and c, d 2D vertical slices; e, f 3D volume rendering of the clustering pores induced cracks at as-cast and 9400 cycles state

Fig. 9 Quantitatively analysis of the relationship between fatigue loading cycles and pore number density, averaged pore equivalent diameter and averaged pore sphericity

Fig. 10 SEM observation of a overall fracture surface, b carbide-induced crack initiation zone, c crack propagation path, d pore-induced crack initiation zone in the sample

Fig. 11 a 3D rendering of the final fracture surface. The as-cast pores (red objects) mapped to the b crack propagation zone and c crack initiation zone

Fig. 12 3D rendering of a the final fracture surface and b the as-cast carbide distribution of the sample

Fig. 13 Illustration of how to calculate fracture surface roughness factor

Fig. 14 Comparing fracture surface roughness of the carbide-induced crack initiation zone, pore-induced crack initiation zone, carbide-affected crack propagation zone and pore-affected crack propagation zone

Fig. 15 Quantitatively comparing the influences of different pore characteristics on crack initiation zone, crack propagation zone