Defect-dependent nitride surface layer development upon nitriding of Fe–1 at.% Mo alloy

Abstract

Upon nitriding of binary Fe–1 at.% Mo alloy in a NH3/H2 gas mixture under conditions (thermodynamically) allowing γ′-Fe4N1– x compound layer growth (nitriding potential: 0.7 atm−1/2 at 753 K (480 °C) – 823 K (550 °C)), a strong dependency of the morphology of the formed compound layer on the defect density of the specimen was observed. Nitriding of cold-rolled Fe–1 at.% Mo specimens leads to the formation of a closed compound layer of approximately constant thickness, comparable to nitriding of pure iron. Within the compound layer, that is, in the near-surface region, Mo nitrides are present. The growth of the compound layer could be described by a modified parabolic growth law leading to an activation energy comparable to literature data for the activation energy of growth of a γ′-Fe4N1− x layer on pure iron. Upon low temperature nitriding (i.e. ⩽793 K (520 °C)) of recrystallized Fe–1 at.% Mo specimens, an irregular, ‘needle-like’ morphology of γ′-Fe4N1− x nucleated at the surface occurs. This γ′ iron nitride has an orientation relationship (OR) with the matrix close to the Nishiyama–Wassermann OR. The different morphologies of the formed compound layer can be interpreted as consequences of the ease or difficulty of precipitation of Mo as nitride as function of the defect density.