Multi-scale experimental investigation on microstructure related subsurface fatigue cracking behavior of selective-laser-melted superalloy at elevated temperature

Subsurface fatigue cracking behavior related to microstructure characteristics at 650 °C of a selective-laser-melted as-deposited superalloy was experimentally investigated by using multi-scale testing technologies including axial loading tests with two stress ratios, electron-backscattered diffraction and transmission electron microscope. As a result, the subsurface failures correspond to two cracking modes: defect assisted faceted cracking and non-defect assisted faceted cracking. The presence of a peculiar facetted cracking area with relatively larger plastic deformation becomes the typical failure feature. Due to the grain orientation difference, the microcracks are mainly nucleated from the larger grains with the soft orientation especially under the assistance of defect, and mainly propagate as the mode II microcrack along {111} slip plane in the direction of the maximum shear stress, leading to the formation of crystallographic facets. Moreover, the dislocation structures such as tangled dislocations, dislocation loops and stacking faults reveal that the deformation mechanism with faceting cracking is attributed to the combined effect of anti-phase boundary shearing, precipitates bypassing and stacking fault shearing, especially under the stress concentration effect of crack or defect. Combined with the evaluation of threshold values at the crack front, finally, the subsurface crystallographic faceting cracking mechanism at 650 °C related to the microstructure characteristics is elucidated.