Modeling the Nitrogen Diffusion Enhancement Resulting from a NanoPeening® Treatment on a Pure Iron – Influence of the Grain Morphology

Abstract

It is well known that nanocrystalline materials have enhanced diffusion properties due to their high grain boundary density which act as fast diffusion channels compared to the lattice. In this paper, we aim at simulating the nitriding process of a pure iron nanostructured by NanoPeening® process. We use a simple diffusional approach taking into account the grain size and the grain morphology resulting from the NanoPeening® treatment. EBSD measurements are carried out to extract morphological parameters which are used in the homogenization method to extract the effective diffusivity distribution. Then a 1D diffusion simulation is performed with this distribution and shows that the grain morphology resulting from the NanoPeening® treatment does not deteriorate the diffusion properties of the material but in fact, improves the nitrogen penetration depth and the diffusion kinetics in addition to the effect of the grain size reduction.

Diffusion induced decomposition of cementite layers during gas nitriding of 4140 steel: thermodynamical and kinetical aspects

“…Hence, the grain boundary diffusion occurs faster than the volume diffusion, and the penetration depth of atoms along the grain boundaries is much higher than intragranular diffusion. Also, increasing the concentration of solute atoms (such as nitrogen) at grain boundaries leads to their diffusion inside the grains [37]. About the diffusion of elements like nitrogen (capable of reacting chemically with the matrix atoms), it seems that in the early stage of nitriding, rapid diffusion of nitrogen atoms along grain boundaries causes an increase in the penetration depth.…”

“…For unstable states, Fick's second law is used. Under this law, diffusion depth (case depth) is obtained as x≈√Dt, where x is case depth, D is diffusion coefficient, and t is treatment time [37]. In a study conducted by Lakhtin et al [39], the diffusion coefficient of nitrogen in ferrite has been reported as:…”