Multi-scale study of subsurface fatigue cracking behavior of laser-powder bed fused Inconel 718 at stress ratios and temperatures

Multi-scale subsurface fatigue cracking is a significant lifetime-limiting failure mode, not yet fully understood, of additively manufactured superalloys in mechanical applications. Here, combined with some technologies of electron-backscattered diffraction, focused Ion beam, and transmission electron microscope, a series of axial fatigue tests at different stress ratios and temperatures are conducted to investigate the fatigue failure behavior of as-built superalloy manufactured by Laser-Powder Bed Fusion. The main results show that especially in the assistance of defect (e.g., pore or inclusion) and elevated temperature, the faceted cracking related to the grain feature is a typical failure mode. It is mainly controlled by shear stress and is more popular under a positive stress ratio. Based on the analysis of dislocation structure with faceted cracking, the localized plastic deformation at 25 °C is controlled by anti-phase boundary (APB) shearing, whereas that at 650 °C is caused by a combination of APB shearing, precipitation bypassing, and stacking fault shearing mechanisms. Combined with the definition of threshold value at the crack tip, a crack nucleation life prediction approach associated with the faceted cracking characteristics is proposed, and the predicted results show a good agreement with the experimental results.