Electronic bonding characteristics of hydrogen in bcc iron: Part I. Interstitials
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Electronic bonding characteristics of hydrogen in bcc iron: Part I. Interstitials Yoshio Itsumi Material Research Laboratory, Kobe Steel, Ltd.,
D. E. Ellis Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 (Received 19 June 1995; accepted 26 May 1996)
Electronic structure calculations were carried out for bcc iron (Fe) clusters with or without hydrogen (H), and also involving a vacancy, using the self-consistent Discrete Variational method (DV-Xa) within the local density functional formalism. Bonding characteristics investigated show the following: (i) Interstitial H notably decreases interatomic Fe–Fe bond strengths, but acts over a small distance (within 0.3 nm). (ii) In the perfect Fe lattice field, interstitial H feels a repulsive force at any site. As a result of lattice relaxation, volume expansion may be expected. (iii) H in combination with a vacancy prefers a position shifted from the octahedral site toward the vacancy. This is fairly consistent with an experimental result.
I. INTRODUCTION
H is well known as an embrittlement inducer in metals such as Fe, aluminum alloys, and intermetallic compounds.1–3 This is especially troublesome in high strength steels, since high strength tends to correlate with much higher sensitivity to fracture. The observed delayed fracture phenomenon has caused their structural use to be restricted. In spite of many efforts over several decades, the failure mechanisms still have not yet been adequately understood. H does not have much solubility in Fe (less than 10 wppm) nor does it form any compound with Fe atoms exposed in the atmosphere. Moreover, H mobility is much higher than that of other impurities, so direct observations are extremely difficult. Thus, it has been hard to obtain reliable experimental data. Theoretical calculations of the electronic structure of H in transition metals have been carried out intensively.4–7 However, H in Fe has not been studied as well as others,8,9 perhaps because of the limited experimental data as mentioned above. It is important to discuss the local electronic structure in the neighborhood of hydrogen, especially its bonding nature, in order to understand the embrittlement process. Band calculations are well suited for periodic alloys, but require very large supercells to treat the low H density relevant here. Embedded cluster methods such as are used in the present work give a very convenient way to treat localized impurities self-consistently, using first principles theory. LCAO expansions make it easy to extract local bonding information, charge transfer, and related properties. Molecular Orbital (MO) cluster methods permit a detailed description of effective atomic configurations, and critical orbital interactions, making use 2206
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 9, Sep 1996
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of Mulliken population analysis. Using this approach, Adachi and Imoto8 gave a systematic interpretation of electronic prop
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