Intergranular fracture of polycrystalline Ni3Al at 77K

Abstract

Environmental embrittlement of grain boundaries in polycrystalline Ni[sub 3]Al, involving moisture in air, has recently been observed in alloys with and without boron. This environmental embrittlement phenomenon is reported to be similar to that observed in many other ordered intermetallics, except that the fracture mode in Ni[sub 3]Al is intergranular while it is transgranular in many other intermetallics. The proposed mechanism for this moisture-induced embrittlement is similar to that proposed for moisture induced embrittlement of aluminum and its alloys, which involves the reaction of aluminum with water to generate hydrogen, which then embrittles the alloy. The environmental component of the brittleness is evidenced by comparing tensile ductility in moist air, vacuum, and dry oxygen. In general, testing in dry oxygen has been shown to increase the ductility of alloys susceptible to water vapor induced embrittlement, presumably due to formation of an oxide film that inhibits the formation of hydrogen and its transport into the alloy. Room temperature tensile elongation of Zr doped substoichiometric Ni[sub 3]Al was shown to increase from 6-9% in water, and 11-13% in air, to 48-51% in oxygen environment. This observation raises questions about whether grain boundaries in Ni[sub 3]Al are really intrinsically brittle, and suggests amore » new explanation for the boron effect.'' Based on these observations, the ductilizing role of boron has been proposed to involve the suppression of the environmental embrittlement, by blocking the diffusion of hydrogen at grain boundaries.« less