Ab-initio Investigation of Hydrogen Trap State by Cementite in bcc-Fe

概要

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Hydrogen trap state by cementite in ferrite was investigated by ab-initio calculations. The calculated trap energy at cementite interstitial was 33 kJ/mol and that at the interface of cementite/ferrite was 39 kJ/mol. The calculated activation energy of the migration over the stable interstitial sites is 55 kJ/mol. Considering calculated zero point vibration energy, the trap energy at cementite interstitial becomes 41 kJ/mol and the activation energy becomes at least 59 kJ/mol. The low temperature peak at about 400 K in Thermal Desorption Spectrum (TDS) of cementite reported by other researchers is considered to correspond to the trap site at the cementite/ferrite interface. Since the interstitial trap site in cementite is not effective because of the high diffusion barrier at low temperature, the higher temperature peak at about 500 K in the TDS is considered to correspond to the migration energy from the defects such as grain boundaries in cementite introduced by deformation.