The Influence of Hot Corrosion Damage on the Low Cycle Fatigue Fracture Modes of a Disk Superalloy

Abstract

Fatigue cracking of nickel disk superalloy fatigue specimens subjected to hot corrosion exposures prior to and during fatigue testing at 704 °C has been investigated. Pre-corroded notched fatigue specimens were exposed to a mixture of sulfates at 677 °C prior to fatigue testing, while in situ treated specimens were only exposed to the sulfate mixture during the elevated temperature fatigue test. Both the pre-corroded and in situ corrosion treatments resulted in a significant reduction in fatigue life compared to uncorroded specimens tested at the same temperature and stress conditions. Investigation of fracture surfaces by scanning electron microscopy and electron microprobe equipped with an energy-dispersive spectrometer allowed for the determination of fatigue crack initiation sites, as well as morphological and chemical evaluation of the alloy microstructure damaged by hot corrosion. Fatigue crack initiations originated from pit features and microstructural degradation caused by hot corrosion. For pre-corroded specimens, cracks initiated along grain boundaries near the pit depth and propagated intergranularly into the alloy. For in situ treated specimens, cracks formed near the surface oxide above growing pits and propagated into the pit as it grew, eventually propagating in a transgranular nature further into the alloy. Sulfur-rich precipitates were detected along grain boundaries below the hot corrosion pits of both pre-corroded and in situ treated specimens. Sulfur was also detected along crack surfaces and grain boundaries that were 50–100 μm below crack initiation sites in the in situ treated specimens.