Numerical Analysis of Hydrogen Trap State by TiC and V₄C₃ in bcc-Fe

Abstract

Hydrogen trap states by TiC and V₄C₃ precipitates in bcc-Fe are investigated by numerical calculations. The trap states at interstitial site and, carbon vacancy in metal carbide and bcc-Fe/metal-carbide interface were studied by ab-initio calculation. The calculated trap energies of these sites for TiC compared with the energy at interstitial site in bcc-Fe were respectively –58 kJ/mol, 125 kJ/mol and 48 kJ/mol and those for V₄C₃ were respectively –106 kJ/mol, 116 kJ/mol and –6 kJ/mol. The activation energy of detrapping from an isolated carbon vacancy is estimated at 183 kJ/mol for TiC and at 222 kJ/mol for V₄C₃ from the difference of the calculated energy at carbon vacancy and that at interstitial site in metal carbide. Hydrogen trap energy in coherent strain field around of TiC and V₄C₃ coherent precipitates in bcc-Fe are also calculated by Finite Element Method (FEM). The calculated energies are respectively less than 29 kJ/mol and less than 15 kJ/mol. These results indicate the main trap site of TiC is TiC/bcc-Fe interface, because TiC contains few carbon vacancies and has large activation energy of detrapping at the sites. That of V₄C₃ is carbon vacancies because V₄C₃ contains abundant carbon vacancies and the activation energy of migration between the neighbored carbon vacancy sites is expected to be lower than the calculated value . The estimated main trap sites of TiC is in good agreement with 3 Dimensional Atom Prove (3D-AP) observation results which reported that hydrogen atoms observed at TiC/bcc-Fe interface of TiC precipitate in bcc-Fe.