GRAIN-BOUNDARY FRACTURE AND BORON EFFECT IN NI3SI ALLOYS

Intergranular fracture and boron effect in Ni3Si (22.5 at% Si) were st udied by tensile testing in various environments at room temperature. Our results indicate that brittle grain-boundary fracture in Ni3Si is caused by two major factors: (1) moisture-induced hydrogen embrittleme nt, and (2) poor grain-boundary cohesion. Boron is found to segregate strongly to grain boundaries and suppress environmental embrittlement in Ni3Si. Boron, however, does not enhance the cohesive strength of th e boundaries. As a result, Ni3Si showed a moderate increase iri room-t emperature tensile ductility in moist air when doped with boron. Auger analyses indicate that the silicon concentration at the grain boundar ies is significantly lower than that in the bulk; however, there is no indication of strong co-segregation of nicker and boron atoms to the boundaries.