Fracture in Ni3Al : Environmental and dopant effects

Environmental and dopant effects are by far the most significant of the extrinsic factors that influence intergranular fracture in Ni₃Al. Environmental embrittlement is a result principally of the atomic hydrogen generated in the reaction with water vapor in ordinary ambient air. Dry hydrogen gas, in contrast, is relatively benign, presumably because it does not dissociate readily into atomic H on the Ni₃Al surfaces. The effect of boron doping is primarily to suppress moisture-induced embrittlement, and secondarily to improve grain-boundary cohesion. In gaseous hydrogen atmospheres, boron has a third (unexpected) embrittling effect, which produces brittle intergranular fracture in otherwise ductile Ni₃Al, presumably by enhancing the dissociation of H₂ on the grain boundaries. These and other extrinsic factors that affect the ductility and fracture behavior of Ni₃Al are reviewed.