Multi-scale study of very-high-cycle fatigue cracking behavior of PBF-LB/GH4169 superalloy at elevated temperature

Very-high-cycle fatigue (VHCF) failure behavior of GH4169 superalloy fabricated via powder bed fusion using laser beam (PBF-LB/GH4169) at elevated temperatures remains incompletely understood, thus delaying its practical application in industrial fields. To address this critical gap, we apply a multiscale characterization framework integrating ultrasonic fatigue testing at 650 degrees C, fracture analysis, and high-resolution transmission electron microscopy to directly uncover the elevated temperature fatigue failure mechanism. Notably, we demonstrate a fundamental shift from an internal-defect dominant mode to a highly competitive crack initiation mechanism involving both surface indentations and internal pores. Furthermore, a critical oxidation-fatigue interaction is clarified: high-temperature oxidation alters the early damage evolution process of surface crack initiation, enabling surface-governed failures to reach the VHCF regime. Furthermore, this study provides direct evidence for a consistent local microstructural evolution process in the internal pore-induced VHCF crackinitiation region of PBF-LB/GH4169 at both room temperature and 650 degrees C. Driven by highly constrained cyclic plasticity associated with internal pore defects, dislocation proliferation occurs; subsequently, the shearing of precipitate phases by mobile dislocations leads to their dissolution, triggering continuous dynamic recrystallization (CDRX) and the formation of fine grains, within whose boundaries microcracks ultimately nucleate. Crucially, this localized behavior accelerates nanoscale (Ti,Nb)C precipitation, as well as the detrimental transformation of the strengthening gamma '' phase into the brittle delta phase. Collectively, these findings establish a unified mechanistic understanding of elevated temperature VHCF failure in PBF-LB/GH4169, directly informing synergistic processing strategies, specifically eliminating internal pores and enhancing surface oxidation resistance to mitigate fatigue in extreme environments.