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 H₂ gas at pressures ranging from approximately 6×10^-8 to 7×10³ Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: approximately 36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H₂ 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 H₂ 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 H₂ into atomic H - embrittles Ni₃Al in a dry H₂ environment, unlike in H₂O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni₃Al.