EFFECT OF LOW-PRESSURE HYDROGEN ON THE ROOM-TEMPERATURE TENSILE DUCTILITY AND FRACTURE-BEHAVIOR OF NI3AL

The effect of low-pressure (less than or equal to 1 . 3 x 10(3) Pa) hy drogen gas on the ductility and fracture behavior of polycrystalline N i3Al (23 . 4 at% Al) was investigated. Room-temperature tensile ductil ities remained high over the entire pressure range tested: from 41% el ongation to fracture at 5 . 7 x 10(-8) Pa pressure to 31% at 1 . 3 x 1 0(-3) Pa. Over this pressure range, the amount of transgranular fractu re also remained quite high and scaled with the tensile ductility, inc reasing from similar to 60% in the samples with 31% ductility to simil ar to 70% in the specimen with 41% ductility. The ionization gage-used to measure hydrogen pressure-had a dramatic (deleterious) effect on t he ductility of Ni3Al: at any given hydrogen pressure, the ductility m easured with the ion gage on was about half to a quarter of that measu red with the ion gage turned off. Accompanying this decrease in ductil ity was a change in fracture mode from predominantly transgranular to predominantly intergranular. The role of the ion gage is believed to b e hot-filament-assisted dissociation of molecular H-2 into atomic H, w hich is quickly absorbed and embrittles the crack-tip regions. In the absence of any Al-induced embrittlement (either by filament-assisted d issociation of H-2 or by Al-induced reduction of H2O), polycrystalline Ni3Al is found to be quite ductile, with tensile elongations exceedin g 40% and predominantly (>70%) transgranular fracture. Since these duc tilities are similar to those of the most ductile B-doped alloys, the main role of boron is to suppress environmental embrittlement. Our res ults indicate that, at room temperature, low-pressure H-2 does not dis sociate very efficiently into atomic H on the surfaces of Ni3Al and th at, at comparable pressures, hydrogen is not as harmful to ductility a s moisture (H2O). Copyright (C) 1996 Elsevier Science Ltd