Diffusion Kinetics Of Nitrogen In Nitrogen Surface-modified Austenitic Stainless Steels

At a moderate temperature (<723K), Fe-Cr-Ni austenitic stainless steels can be nitrogen-modified by the processes including gas nitriding, plasma nitriding, and ion implantation, forming a nitrided layer of single fcc (γN) phase containing high content of nitrogen (20-30at.%) with high hardness and a combined wear and corrosion resistance. In this paper, the nitrogen diffusion kinetics in austenitic stainless steels is explored by experimental and theoretical studies to reveal the nitriding mechanism of austenitic stainless steels, improve the nitriding efficiency and optimize processing parameterizes. First, the nitrided layers on the AISI304austenitic stainless steel was obtained by the plasma-based low-energy nitrogen ion implantation to find the relationship between the concentration of nitrogen and the diffusion depth from the surface. The nonlinear diffusion behavior of nitrogen in the modified layer was analyzed. Second, nonlinear interstitial diffusion model for nitrogen in austenitic stainless steel was established according to the ab initio calculation analysis on the interaction between nitrogen atoms. Third, according to the experimental results of plasma-based low energy nitrogen ion implanted austenitic stainless steel, the characteristic parameters of nonlinear interstitial diffusion model for nitrogen in austenitic stainless steel were determined and used to discuss the experimental results of gas nitriding, plasma nitriding and ion implantation for confirming the rationality of the model and mechanism of diffusion enhancement was revealed. Finally, considering the anisotropy elastic of nitrogen in single crystalline austenitic stainless steel, the anisotropy interaction between nitrogen atoms was further studied, by which the nonlinear kinetics of nitrogen diffusion in the single crystalline austenitic stainless steel and the corresponding controlling factors were determined.The nitrided layers composed of a single phase (γN) on AISI304austenitic stainless steel were obtained by the plasma-based low-energy nitrogen ion implantation at673K with plasma power source of400W, at an average ion current density of nitrogen in the range from0.3to0.6mA/cm2, respectively. Along with increasing the average ion current densities, concentration of nitrogen at the outmost surfaces increased from19at.%to31at.%, while the corresponding thickness of nitriding layers increased from6μm to13μm. The concentration-depth profiles of nitrogen exhibited the non error functional plateau-type shapes slowly decreasing from the surface, followed by a rather sharp leading edge. This suggests that the diffusion kinetic of nitrogen in the austenitic stainless steel was of a nonlinear charateristic.The equilibrium states of nitrogen in the nitrogen-modified austenitic stainless steel supercells were analyzed through the energy, structure, electron density distribution, and differential electron density distribution based on density function theory. The energy of the γN phase supercell decreased with increasing the number of Cr atoms in the octahedron containing a nitrogen atom. The energies were0.380,0.045,-0.427,-0.816eV, respectively, with the number of Cr increasing from0to3. The potential energy of the nitrogen atoms in the nearest neighboring sites were in the range from2.3to2.6eV, suggesting a strong repulsive interaction. Based on the nonlinear kinetic theory, the nonlinear interstitial diffusion model of nitrogen in austenitic stainless steel was established by incorporating the results from ab initio calculation. The nonlinear interstitial diffusion model of nitrogen in austenitic stainless steel containing four characteristic parameters:the interaction energy between nitrogen atoms VNN, diffusion activation energy for an isolated nitrogen atom EA, prefactor Do, and saturated concentration cc. According to the experimental results of plasma-based low energy nitrogen ion implanted AISI304stainless steel, the values of characteristic parameters were deterimined, including VNN,EA, Do and cc were0.041eV,1.01eV,4.7叁10-9m2/s, and50at.%, respectively. The nonlinear interstitial diffusion model of nitrogen in austenitic stainless steels was then employed to simulated the reported experimental results, including the concentration diffusivity profile of AISI316by gas nitriding reported by Christiansen et al.,concentration-depth profiles of AISI316by plasma nitriding reported by Templier et al., and that of DIN1.4571by low energy, high flux ion implantation reported by Parascandola et al. The simulated results were in agreement with the experimental results.Considering the anisotropy elastic energy of nitrogen in single crystalline austenitic stainless steel, the concentration-depth profiles of nitrided AISI316L single crystalline austenitic stainless steel along (100),(110),(111) at temperature643-718K, were obtained by the the nonlinear interstitial diffusion model of nitrogen in austenitic stainless steel with unchanged orientation independent parameters of EA, D0and cc, and modified parameter, VNN. The calculation results were in good agreement with the experimental results reported by Martinavicius et al. The binary interstitial alloy lattice statics model was used according to the binary substitution alloy lattice statics theory by simplifying the matrix sublattice as continuous media, through which the interstitial atoms interact. It was found that the ratio of interaction of N atoms along different crystal orientations (001),(110),(111) was1.42:1.19:1, which was in agreement with the modified VNN. The agreement further proved the rationality of the nonlinear interstitial diffusion model of nitrogen in austenitic stainless steel.