Effect of low-pressure hydrogen on the room-temperature tensile ductility and fracture behavior of Ni3Al

The effect of low-pressure (≤1.3 × 10³ Pa) hydrogen gas on the ductility and fracture behavior of polycrystalline Ni₃Al (23.4 at% Al) was investigated. Room-temperature tensile ductilities remained high over the entire pressure range tested: from 41% elongation to fracture at 5.7 × 10^-8 Pa pressure to 31% at 1.3 × 10³ Pa. Over this pressure range, the amount of transgranular fracture also remained quite high and scaled with the tensile ductility, increasing from ∼60% in the samples with 31% ductility to ∼70% in the specimen with 41% ductility. The ionization gage - used to measure hydrogen pressure - had a dramatic (deleterious) effect on the ductility of Ni₃Al: at any given hydrogen pressure, the ductility measured with the ion gage on was about half to a quarter of that measured with the ion gage turned off. Accompanying this decrease in ductility was a change in fracture mode from predominantly transgranular to predominantly intergranular. The role of the ion gage is believed to be hot-filament-assisted dissociation of molecular H₂ into atomic H, which is quickly absorbed and embrittles the crack-tip regions. In the absence of any H-induced embrittlement (either by filament-assisted dissociation of H₂ or by Al-induced reduction of H₂O), polycrystalline Ni₃Al is found to be quite ductile, with tensile elongations exceeding 40% and predominantly (>70%) transgranular fracture. Since these ductilities are similar to those of the most ductile B-doped alloys, the main role of boron is to suppress environmental embrittlement. Our results indicate that, at room temperature, low-pressure H₂ does not dissociate very efficiently into atomic H on the surfaces of Ni₃Al and that, at comparable pressures, hydrogen is not as harmful to ductility as moisture (H₂O). Copyright © 1996 Elsevier Science Ltd.