Microstructural evolution and formation of fine grains during fatigue crack initiation process of laser powder bed fusion Ni-based superalloy

This study reports the fatigue failure mechanism at very-high-cycle fatigue (VHCF) regime of laser powder bed fusion (LPBF) GH4169 superalloy through a series of detailed microstructural characterizations, including scanning electron microscopy (SEM), Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and energy dispersive spectrometer (EDS). Microstructural characterization of the solution and double-aging post-treated initial material exhibits process-induced imperfections (mainly pores), as well as γ′, γ′′ and δ precipitates, and carbide. The fatigue tests were performed using an ultrasonic fatigue tester (20kHz). Fatigue fracture analysis suggests there exists a competitive fatigue failure mechanism with surface flaw initiation and internal pore initiation, corresponding to high-cycle fatigue (HCF) and VHCF life, respectively. Focused ion beam (FIB) samples taken within the fatigue initiation area (FIA) revealed grain refinement and precipitation dissolution. Based on the characterization results, the fatigue crack initiation mechanism was hypothesized: During numerous cyclic loading, the mobile dislocations shear γ′ and γ′′ precipitates, causing their dissolution and local chemical and mechanical alterations near internal pores. This enables twinning and promotes sub-grain formation. Sub-grains refine via localized continuous dynamic recrystallization (CDRX), forming a fine-grained layer that leads to crack initiation. This work reveals how precipitate dissolution contributes to VHCF crack initiation in LPBF GH4169 superalloy, highlighting the potential for extending alloy lifespan by adjusting precipitates and LPBF defects and incorporating these factors into fatigue life predictions for enhanced accuracy.