The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

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Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature.