The effect of Fe on the moisture-induced embrittlement in (Ni,Fe)Ti alloys Yanfeng Chen a , Yip-Wah Chung a, * , Dongmei Wu b,1 a Department of Materials Science and Engineering, Northwestern University, 2220 Campus Dr., Evanston, IL 60208, USA b Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA Available online 1 April 2004 Abstract 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. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Environmental embrittlement; B. Diffusion; E. Simulations, atomistic 1. Introduction In the past few decades, environmental embrittlement has been one of the major issues in the fabrication and application of many ordered intermetallics [1,2]. Liu et al. [3] discovered that moisture embrittlement was caused by atomic hydrogen produced by the reaction between water vapor in the environment and reactive elements in the alloy (Al, Ti, Mo, Mn, Mo, etc.). Many studies have been performed on moisture-induced embrittlement in different intermetallics, such as Ni 3 Al, FeAl, TiAl, Co 3 Ti, Ni 3 (Al,Ti), Ni 3 (Si,Ti) and Ni 4 Mo [4–8]. Understanding the mechanism of such embrittlement is critical to the development of ductile intermetallic alloys for structural applications. Recent experiments by Zhu and Liu [9,10] showed that equiatomic NiTi alloy is not embrittled by water or hydrogen at room temperature. This is quite unexpected, since Ni and Ti are both known to dissociate water and hydrogen. However, iron addition beyond 9 a/o leads to embrittlement by water or hydrogen. Fe occupies Ni sites and stabilizes the B2 (CsCl) structure at room temperature when the Fe concentration exceeds 1 at.% [11,12]. Therefore, this environmental sensitivity induced by Fe is not directly due to the change of crystal structure. To explore the origin of the high ductility of equiatomic NiTi and the effect of Fe addition in a moist environment, we investigate the chemical interactions between water vapor and (Ni,Fe)Ti surfaces with different Fe concen- trations, using a combination of surface analytical tech- niques, statistical analysis and atomistic simulations. 2. Experimental procedures All experiments were carried out in a stainless steel ultrahigh vacuum chamber with a base pressure of 1.5– 3 £ 10 210 Torr equipped with sputter-ion gun, gas hand- ling, Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS) and residual gas analyzer (RGA). The single crystals were grown at 2 cm/h at 1870 K with Bridgman technique. Two single crystals with different Fe concentrations were used in this experiment. One was of 45 at.% Ni, 5 at.% Fe and 50 at.% Ti. The other was of 40 at.% Ni, 10 at.% Fe and 50 at.% Ti. Each crystal was oriented using Laue backscattering and then cut by electric- discharge machining along the (110) plane to produce single-crystal slices about 10 mm in diameter and 2 mm in thickness. Each crystal was mechanically polished using 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.02.031 Intermetallics 12 (2004) 815–819 www.elsevier.com/locate/intermet 1 Present address: Ames Laboratory, Ames, IA 50011, USA. * Corresponding author. Tel.: þ 1-847-491-3112; fax: þ1-847-491-7820. E-mail address: ywchung@northwestern.edu (Y.-W. Chung).