Hydrogen diffusivity in boron-doped polycrystalline Ni3Al

Abstract

Recent studies have shown that Ni[sub 3]Al, like many other ordered intermetallics, exhibit severe environmental embrittlement in air at ambient temperatures. Polycrystalline Ni[sub 3]Al shows a tensile ductility as high as 23% when tested in ultra-high vacuum at room temperature, whereas the same alloy exhibits a low ductility of only 3% in laboratory air. The loss in ductility is due to air moisture, which reacts with aluminum and generates atomic hydrogen at crack tips. These results clearly indicate that the grain boundaries in Ni[sub 3]Al are not brittle at all, and that moisture-induced hydrogen causes severe embrittlement. It has been known for 15 years that additions of boron in ppm levels suppress brittle grain-boundary fracture and dramatically improve the tensile ductility of polycrystalline Ni[sub 3]Al. The ductilizing effect of boron, however, is still not well understood at present. Several researchers have suggested that boron strongly segregates to Ni[sub 3]Al grain boundaries and reduces hydrogen penetration along these boundaries. Nevertheless, there is a lack of critical experimental data to support this mechanism. The objective of this work is to provide an experimental evidence that the doping of boron in Ni[sub 3]Al substantially reduces the hydrogen diffusivity at the grain boundaries, therebymore » suppressing moisture-induced hydrogen embrittlement in Ni[sub 3]Al doped with sufficient amounts of boron.« less