The effect of Fe on the moisture-induced embrittlement in (Ni,Fe)Ti alloys

Iron addition beyond 9 a/o in NiTi leads to embrittlement by water or hydrogen. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption shows that decomposition of water to produce atomic hydrogen occurs on both surfaces. X-ray photoelectron spectroscopy further demonstrates that the decomposition occurs around 190 K in both cases. A statistical model was developed to explore the effect of dopant atoms on hydrogen diffusion, assuming that diffusion depends on local composition. Dopant atoms such as boron that bind strongly to hydrogen slow down diffusion and therefore should suppress the environmental effect, consistent with atomistic simulations. Fe binds to hydrogen less strongly and therefore should result in faster diffusion. However, the present model shows that this increase in diffusivity is minimal. Therefore, this result indicates that nonchemical effects must be at work in moisture-induced embrittlement of (Ni,Fe)Ti alloys when the Fe concentration exceeds 9 a/o.