Mechanism of boron-induced embrittlement of Ni 3Al in hydrogen

Abstract

Materials that have been exposed to H{sub 2} or hydrogen-containing gases often show embrittlement due to atomic hydrogen. It is now recognized that hydrogen embrittlement is the source of brittle behavior of a number of intermetallics that were previously thought to be intrinsically brittle. A well-known example is polycrystalline Ni{sub 3}Al, which is susceptible to environmental embrittlement in atmospheres containing water vapor, and the mechanism of H production is the dissociation of H{sub 2}O at active Al sites. Further investigations to identify how B additions affect the fracture susceptibility of Ni{sub 3}Al in H{sub 2} atmospheres discovered a surprising phenomenon: boron-induced embrittlement. Insufficient data are available to build more than a speculative atomic-level model to explain this counterintuitive connection of boron with embrittlement. One explanation has been proposed in terms of the chemical participation of boron in the H{sub 2} dissociation step. However, the chemistry of boron does not support a direct dissociation step. And simple electronic structure considerations discussed later suggest boron would not act as a promoter in an indirect dissociation of H{sub 2}. The alternate mechanism proposed here, although tentative until further evidence is attained, is based on well-established atomic-level properties of boron in Ni{sub 3}Al. Inmore » the following, the concept of the activity of interfaces with regard to H{sub 2} molecular dissociation in the region of intersection of the grain boundary with the gas/solid interface is used to explain how boron can affect atomic hydrogen production. Activity will refer exclusively to the H{sub 2} molecular dissociation reaction unless noted otherwise.« less