Abnormal nitride morphologies upon nitriding iron-based substrates

Abstract

Nitriding of iron-based components is a very well-known surface engineering method for bringing about great improvement of the mechanical and chemical properties. An overview is presented of the strikingly different nitride morphologies developing upon nitriding iron-based alloy substrates. Observed abnormal morphologies are the result of intricate interplay of the thermodynamic and kinetic constraints for the nucleation and growth of both alloying element nitride particles in the matrix and iron nitrides at the surface of the substrate. Alloying elements having strong Me-N interaction, such as Cr, V, Ti, precipitate instantaneously as internal Me-nitrides, thus allowing the subsequent nucleation and growth of "normal" layer-type iron nitride. Alloying elements having weak Me-N interaction, such as Al, Si, Mo, simultaneously having low solubility in iron nitride, obstruct/delay the nucleation and growth of iron nitrides at the surface, thus leading to very high nitrogen supersaturation over an extended depth range from the surface. Eventually, the nucleation and growth of "abnormal" plate-type iron nitride occurs across the depth range of high nitrogen supersaturation. On this basis, strategies can be devised for tuned development of specific nitride morphologies at the surface of nitrided components. © 2013 TMS.