Effect of iron additions on environmental embrittlement of NiTi-base alloys

The effect of Fe on the room-temperature environmental embrittlement of NiTi-base alloys was investigated and discussed in this paper. Tensile test data and scanning electron microscope fractography have been evaluated to show that there exists a critical Fe content in NiTi, i.e. about 9 at.%, below which no environmental embrittlement was observed and above which hydrogen from the moisture-containing test environments severely embrittled NiTi. Laser desorption mass spectrometric results indicate that the surface reaction to generate atomic hydrogen did not change much with Fe additions. Hydrogen charging experiments show that Fe additions actually reduced the hydrogen diffusivity in NiTi alloys from ∼2×10−14 m2/s for Fe-free NiTi alloy to ∼6×10−16 m2/s for NiTi alloy with 10 at.% Fe, implying that the observed ‘Fe effect’ is not due to hydrogen transport effects in the material. Careful examination of tensile properties suggests that strength level is the key factor in controlling the susceptibility to environmental embrittlement of NiTi alloys with Fe additions.