Water dissociation and surface diffusion of atomic hydrogen on single crystal nickel(3)(aluminum, titanium) (110) with and without boron

Polycrystalline Ni₃Al alloys are severely embrittled in a moist environment at room temperature. To date, it is generally accepted that the atomic hydrogen produced by water dissociation causes the embrittlement. Ductility measurements showed that the addition of boron suppresses this moisture-induced embrittlement.; Temperature-programmed desorption and X-ray photoelectron spectroscopy were carried out after D₂O dosing on clean Ni₃(Al,Ti) (110) surface at <130 K. The results indicated that water dissociates on clean surfaces at ∼190 K, resulting in hydrogen evolution at ∼400 K. Aluminum is the active species controlling water dissociation. These observations provide insight into the moisture-induced embrittlement of Ni₃Al in terms of surface chemistry of water vapor.; To explore the effect of boron on water dissociation, we first dosed the surface of clean Ni₃(Al,Ti) (110) with controlled amounts of boron, using a specially designed low-energy negative boron ion source, followed by low-temperature exposure to D₂O. The interactions between water vapor and boron-modified Ni₃(Al,Ti) (110) surfaces were investigated using temperature-programmed desorption, X-ray photoelectron and Auger electron spectroscopy. The hydrogen evolution at ∼400 K from boron-free surface is completely suppressed by ∼0.30 monolayer boron. However, Auger and X-ray photoelectron studies showed that boron also reacts with water to form hydroxyls at <130 K. Further temperature-programmed desorption studies revealed a new hydrogen desorption peak at ∼950 K from boron-modified Ni₃(Al,Ti) (110) surfaces. This suggests that atomic hydrogen is strongly bonded to surface boron, thus suppressing the desorption of atomic hydrogen around room temperature. After water dosing at ∼130 K, electron-stimulated desorption was performed to measure the surface diffusion coefficients of atomic hydrogen on boron-free and boron-modified Ni₃(Al,Ti) (110) surfaces at 270 K. The surface diffusion of atomic hydrogen on 0.05 monolayer boron-modified surface is about 10 times slower than that on the boron-free surface. This leads to lower concentration of atomic hydrogen at the crack tip. We believe this is an important mechanism explaining why boron improves the ductility of polycrystalline Ni₃Al in moist air.