Energies of multiple hydrogen trapping by vacancies in bcc iron


A. Mirzoev1, D. Mirzaev1 and K. Okishev1

1South Ural State University, Chelyabinsk, Russia

Keywords: vacancy, hydrogen, trapping
property: trapping energy
material: Fe

Energies of hydrogen atom trapping by vacancies in bcc iron were calculated ab initio by the LAPW method using GGA'96 in WIEN-2k package. A supercell of 54 Fe atoms was used. Absorption of the first, second and third hydrogen atom by a vacancy decreases system energy by 0.603, 0.607 and 0.396 eV correspondingly; this is very close to the results[1]. Calculated vacancy formation energy is 2.15 eV, which is greater than experimental values (~1.6 eV).On the basis of a general approach proposed by A. A. Smirnov a thermodynamic theory of various vacancy - hydrogen complexes was developed. In a solution containing 6 ppm H (a typical H content in steels) equilibrium concentration of vacancies free from H atoms becomes smaller than that of vacancies bound with one or more H atoms at temperatures below 1100 K. Dominating type of complexes at rather high temperatures should be VH1, VH2 and VH3, while VH5 dominates below 400 K (for VH4, VH5 and VH6 Tateyama and Ohno's binding energies were used in calculation). Strong interaction between vacancies and H atoms makes equilibrium vacancy concentration to decrease slower with decreasing temperature than in pure iron, and below 500 K it begins even to grow. Most H atoms are not bound with vacancies at all temperatures; however, if H concentration is increased to 20 ppm, trapping may become almost complete at 300 K. This means that vacancies may be effective traps for hydrogen atoms in bcc iron and affect significantly their behaviour. This research is supported by a grant from the federal target program "Scientific and educational research staff for Innovative Russia "for 2009-2013.

References
  1. Y. Tateyama , T. Ohno , Phys. Rev. B, 67, 174105, (2003)

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