Molecular dynamics study on homogeneous and heterogeneous nanostructuring in massive nitrogen supersaturation into FCC-structured iron

Abstract

High nitrogen concentration supersaturation to the austenitic stainless steels, transforms their original structure into an expanded austenite (γ_N) layer with nanostructures and improves their mechanical properties. Molecular Dynamics simulation via MEAM potential was utilized to theoretically describe this massive nitrogen supersaturation process. Nitrogen atoms with 2 eV kinetic energy were impinged into pure FCC-structured iron grains with specific crystal orientations. High nitrogen concentration up to 35 at% is detected at the surface of the expanded austenite (γ_N) phase. This new structure contains high lattice strains against the original FCC structure. The high nitrogen content at the surface decreased homogeneously to a plateau with constant nitrogen content, where the octahedral vacancy sites were occupied with diffusing nitrogen atoms. Toward the nitriding front end, the nitrogen content decreases heterogeneously. This unique phenomenon takes place due to various atomic configurations of iron and nitrogen as a result of nitrogen occupation to both tetrahedral and octahedral vacancy lattice sites. The nitrogen depth profile is categorized into three different regions with dependence on nitrogen concentration, plastic straining, and atomic configurations in each region: nitrogen-impinging dependent region, stationary region, and monotonously decaying region.