Quantifying the Effects of Type II Hot Corrosion on the High Temperature Fatigue Cracking Behavior of a Ni-based Superalloy

This study investigates type II hot corrosion effects on the fatigue behavior of the nickel-based superalloy, ME3, also known as René 104. The primary application of ME3 is for turbine disks in the aerospace industry. Hot corrosion pitting caused by molten salt environments has been found to drastically reduce the fatigue life of ME3 components. A better understanding of the corrosion damage features/metrics and mechanisms that govern crack initiation is necessary to inform modeling of the safe remaining fatigue life of a corroded component.

High temperature fatigue testing was performed at two stress levels on samples that had two conditions of pre-corrosion which differed in salt concentration. Loading sequence induced markers on the fracture surface enabled the quantification of initiation life and small crack propagation life. Post-test fractography determined the crack initiation location which was analyzed in the context of the full distribution of damage on the surface obtained from pre-test topographic characterization. Specifically, corroded surfaces were characterized for various pitting metrics on both bulk and regional scales using white light interferometry. As expected, crack formation occurs at corrosion damage features. At low stress, the corrosion damage modestly reduced the crack initiation life, but the reduction was more significant at high stress. Furthermore, for all conditions, the primary fatigue cracks initiated at one of the largest 1% of pits, typically in regions of high pitting density. Such results provide novel insights into fatigue cracking evolution from corroded surfaces; furthermore, they support the common assumptions of using the deepest corrosion damage features as the initial damage feature in fracture mechanics based modeling of corroded components.