Tim Gabb

Fatigue resistance of a hot corrosion exposed disk superalloy at varied test temperatures

The fatigue resistance of the hot corrosion pitted ME3 disk superalloy was investigated. Low cycle fatigue specimens were subjected to hot corrosion exposures that produced pits on the gage sections. These specimens were tested at varied temperatures and strain ranges. Corrosion pitting influenced fatigue life and failure mode by varying degrees, depending on temperature and strain range. As observed through interrupted tests, fatigue cracks initiated at a smaller fraction of life for high-temperature tests, in comparison to that at low temperatures. Correspondingly, the crack initiation failure mode changed significantly with test temperature. While cracks initiated from the hot corrosion pits for all test conditions, at 704 °C the intergranular initiation failure mode was dominant, whereas at the lower temperatures cracks initiated within the pits from crystallographic facets. Finite element analyses were performed to quantify the effect of varying pit dimensions and spacing on elastic stress concentration. The highest stress concentration was calculated to occur at the narrow ligaments between overlapping hot corrosion pits. Increasing the number of overlapping pits did not further add to the stress concentration. There was good qualitative agreement between the calculated stress concentrations and the location of crack initiations for tests conducted at 704 °C but not for tests conducted at 204 °C.

Effects of Long Term Exposures on Fatigue of PM Disk Superalloys

Turbine disks in some advanced engine applications may be exposed to temperatures above 700°C for extended periods of time, approaching 1000 h. These exposures could affect near-surface composition and microstructure through formation of damaged and often embrittled layers. The creation of such damaged layers could significantly affect local mechanical properties. Powder metal disk superalloys LSHR and ME3 were exposed at temperatures of 704, 760, and 815°C for times up to 2020 h, and the types and depths of environmental attacked were measured. Fatigue tests were performed for selected cases at 704 and 760°C, to determine the impacts of these exposures on properties. Fatigue resistance was reduced up to 98 % in both superalloys for some exposure conditions. The changes in surface composition and phases, depths of these changed layers, failure responses, and failure initiation modes were compared.