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Fracture Mechanisms in Iron and Nickel Aluminides

Abstract : The high cycle fatigue (HCF) resistance of several boron-doped Nickel Aluminide alloys has been determined over a range of test temperatures. Fatigue and tensile properties of two Ni-rich ternary alloys were much superior to those of the cast 26%A1 alloy or a P/M alloy with 9.3% Manganese. Crack paths were transgranular in the Ni-rich alloys and intergranular or interdendritic in Ni- 26%A1. HCF lives decreased sharply at temperatures above 500C. Crack growth rates increased with temperatures to 600C, in spite of a rising yield stress over the same temperature range, perhaps due to oxygen-induced embrittlement. Single crystals of Ni3A1+B displayed a marked flow stress assymetry in tension and compression. Point defects were observed in large numbers; these condense into voids, thereby contributing to each crack initiation. The high cycle fatigue (HCF) and crack growth resistance of several Iron Aluminide-type alloys was determined in the temperature range 25-600C. Long range order (DO3 type) was effective in prolonging high cycle fatigue lives in Fe-28.1%A1, but not in FE- 23.7%A1 at 25C. Crack growth rates were higher in the DO3 condition. Fatigue results on both Ni3A1 and Fe3A1 alloys are discussed on the basis of microstructure, surface slip band development and internal dislocation substructures revealed by transmission electron microscopy.