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

Abstract

Abstract The effect of low-pressure (≤1.3 × 10 3 Pa) hydrogen gas on the ductility and fracture behavior of polycrystalline Ni 3 Al (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 × 10 −3 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 Ni 3 Al: 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 H 2 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 H 2 or by Al-induced reduction of H 2 O), polycrystalline Ni 3 Al 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 H 2 does not dissociate very efficiently into atomic H on the surfaces of Ni 3 Al and that, at comparable pressures, hydrogen is not as harmful to ductility as moisture (H 2 O).