Role of Ion-Beam Processing Time in the Formation and Growth of the High-Nitrogen Phase in Austenitic Stainless Steel

A systematic series of AISI 316 stainless steel samples has been prepared as a function of exposure time to a nitrogen ion beam. Times from 15 s to 4 h were selected with other conditions maintained as follows: sample temperature = 410oC; accelerating potential = 700 V; beam current density = 2.0 mA/cm2. Compositional, structural, magnetic, and diffusion properties were studied with a combination of x-ray diffraction, backscatter Mössbauer spectroscopy, magnetic force microscopy, surface profilometry, and glow-discharge optical emission spectroscopy. The only N-containing phase detected for all processing times was the high-N-solid-solution phase, γN, and its maximum N content was found to grow rapidly to a saturation value exceeding 30 at.%. A carbon contamination layer, in the form of a C-solid-solution phase, γC, was detected below the γN, and was found to be introduced during the Ar-ion sputter-cleaning/heating step used prior to exposure to the N-ion beam. This C-rich layer is “pushed” ahead of the incoming N. The γN layer thickness growth can be modeled with a simple diffusion-plus-sputtering equation that yields the effective diffusivity and maximum (sputter-limited) γN layer depth for the given processing conditions. The surface roughness increases with processing time. Anisotropy in the lattice expansion for different lattice planes parallel to the sample surface and varying magnetic properties in the different surface grains are observed. This is likely due to a mixture of residual stress and N-composition variation effects. Ferromagnetic maze-like domain structures are observed on the 1 micron size scale.