Hydrogen-boron interaction and its effect on the ductility and fracture of Ni sub 3 Al

Ni sub 3 Al 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 H sub 2 gas at pressures ranging from approx6x10 exp -8 to 7x10 exp 3 Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: approx36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H sub 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 sub 2 pressures employed, B-doped Ni sub 3 Al (50 or 100 wppm) is brittle and fractures predominantly intergranularly, whereas B-free Ni sub 3 Al is ductile and fractures predominantly transgranularly. This result indicates that B, by possibly promoting the dissocation of molecular H sub 2 into atomic H, embrittles Ni sub 3 Al in a dry H sub 2 environment, unlike in H sub 2 O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni sub 3 Al.