Hydrogen-boron interaction and its effect on the ductility and fracture of Ni3Al

Ni3Al alloys, of nominal composition Ni-24 at.% Al and three different B concentrations (50, 100 and 500 wppm), were tensile tested at room temperature in high-purity HI gas at pressures ranging from similar to 6 x 10(-8) to 7 x 10(3) Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: similar to 36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H-2 pressure, the ductility of all three alloys dropped precipitously. Accompanying this drop in ductility was a change in the fracture mode from predominantly transgranular to predominantly intergranular. An intriguing result of our present study is that, at the higher H-2 pressures employed, B-doped Ni3Al (50 or 100 wppm) is brittle and fractures predominantly intergranularly, whereas B-free Ni3Al is ductile and fractures predominantly transgranularly. This result indicates that B-by possibly promoting the dissociation of molecular H-2 into atomic H-embrittles Ni3Al in a dry H-2 environment, unlike in H2O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni-3 Al. (C) 1997 Acta Metallurgica Inc.