Effect of Stress Ratio on Very High Cycle Fatigue Properties of PM-HIPed Inconel 625

Abstract

This study investigated the impact of stress ratio on the fatigue performance and fatigue crack initiation characteristics of PM-HIPed Inconel 625 in the very high cycle regimes. Ultrasonic fatigue tests, operating at a frequency of 20 kHz, were conducted on PM-HIPed Inconel 625 samples under stress ratios of R = −1 and 0.1 up to the ultimate fatigue life of 10⁹ cycles. Detailed fractographic and microstructural analyses were conducted to identify the mechanisms of crack initiation. The results revealed that stress ratio played a critical role in the crack initiation process. At R = 0.1, cracks predominantly initiated at carbonitrides and non-metallic inclusions, with neighboring crystallographic facets assisting in the formation of microcracks. Conversely, at R = −1, crack initiation was driven by large gains and triple junctions. Microstructural characteristics resulting from the HIP process significantly influenced fatigue crack initiation. Prior particle boundaries were found to affect fatigue crack initiation behavior through the presence of large grains within the boundaries, as well as carbonitrides and non-metallic inclusions networks along the rim. The discussion explored fracture mechanics, fracture surface analyses, and associated microstructural properties to elucidate the observed phenomenon.