Drift diffusion model of hydrogen atoms for hydrogen embrittlement in polycrystalline Ni3Al

It is well known that single crystals of Ni[sub 3]Al are ductile, whereas polycrystalline Ni[sub 3]Al has very low ductility at room temperature due to the intrinsic brittleness of its grain boundaries, which results in premature intergranular fracture. Since Aoki and Izumi found that adding a small amount of boron to polycrystalline Ni[sub 3]Al could suppress intergranular fracture and improve its ambient temperature ductility dramatically, many studies have been carried out on the effects of boron on the structure and chemical composition of grain boundaries in polycrystalline Ni[sub 3]Al. At present, there have been several explanations for the beneficial effect of boron, but its origin has not yet been clearly established. Recently, environmental embrittlement of grain boundaries has been observed in polycrystalline Ni[sub 3]Al with and without boron in air involving moisture and in hydrogen gas. The initiation of intergranular crack in polycrystalline Ni[sub 3]Al relates to the local stress concentration induced by a piled up of edge super dislocation [111] at grain boundary, while hydrogen atoms in the Ni[sub 3]Al superlattice would diffuse toward the end of the edge super dislocation pile-up under the action of the local stress concentration. Considering the relationships of intergranular crack initiation withmore » both of the edge super dislocation pile-up at the grain boundary and the drift diffusion of hydrogen atoms in the superlattice toward the grain boundary, a drift diffusion model of hydrogen atoms for hydrogen embrittlement in polycrystalline Ni[sub 3]Al is proposed to explain the effects of boron content, grain size in polycrystalline Ni[sub 3]Al and test environment, tensile rate as well as test temperature on the degree of hydrogen embrittlement in polycrystalline Ni[sub 3]Al.« less