Nitrogen uptake by an Fe–V alloy: Quantitative analysis of excess nitrogen

Nitrogen absorption isotherms of Fe–2 wt.% V alloys were determined experimentally to study the differences in chemical bonding for various types of absorbed nitrogen. It was observed that the nitrogen uptake was larger than necessary for precipitation of all vanadium as nitride and for equilibrium dissolution of nitrogen in the ferrite matrix: uptake of excess nitrogen. Three types of absorbed nitrogen could be distinguished: (i) nitrogen in the stoichiometric vanadium nitride; (ii) nitrogen adsorbed at the vanadium nitride precipitate/α-Fe matrix interface; (iii) nitrogen dissolved interstitially in the ferrite matrix. Quantitative analysis of the nitrogen absorption isotherms was possible by adopting a model that incorporates (immobile) excess nitrogen, identical to nitrogen adsorbed at the nitride/matrix interface, and (mobile) excess nitrogen, identical to the surplus nitrogen solubility of the ferrite matrix due to the misfit stress field surrounding the nitride precipitates.