New insights into the improved low-temperature hot corrosion fatigue mechanisms of GH4169 superalloy by USRP treatment strengthening-induced gradient microstructural stabilization

Ultrasonic surface rolling processing (USRP) is emerging as a critical methodology to tailor the surface mechanical integrity of superalloys, yet its fundamental mitigation mechanisms against coupled mechano-chemical degradation remain elusive. In this study, we systematically elucidate the low-temperature hot corrosion fatigue (LT-HCF) behavior of a GH4169 alloy subjected to varying USRP intensities at 650 °C in a mixed-salt environment. Integrating multi-scale microstructural characterizations with thermodynamic analyses, we demonstrate that USRP orchestrates a depth-dependent microstructural hierarchy, forming a gradient microstructure composed of high-density dislocations, stacking faults (SFs), Lomer-Cottrell (L-C) locks, and deformation twins. Additionally, USRP introduced a high-intensity, deeply-distributed compressive residual stress (CRS) field, significantly increasing surface hardness and LT-HCF performance. Among the treated specimens, the sample subjected to six passes of USRP exhibited the best performance, with optimal CRS field stability and an LT-HCF limit 18.2% higher than that of untreated specimens. In contrast, the one-pass-treated sample exhibited insufficient strengthening, while the twelve-pass-treated sample showed surface damage caused by over-rolling, reducing the strengthening effect. Thermodynamic calculations and microstructural observations indicated that the synergistic effects of corrosion and cyclic loading accelerated fatigue crack initiation and propagation. The strengthening mechanism of USRP was attributed to the synergistic interaction of gradient microstructures and CRS fields, as well as the interplay of L-C locks and γ“ precipitate phases, effectively inhibiting fatigue crack initiation and propagation.