Phase and composition depth distribution analyses of low energy, high flux N implanted stainless steel

The phase and composition depth distributions of a low-energy (0.7 keV), high-flux (2.5 mA/cm2) N implanted fcc AISI 304 stainless steel held at 400 °C have been investigated by step-wise Ar+ beam sputter removal in conjunction with conversion electron Mössbauer spectroscopy and x-ray diffraction (XRD). A metastable, fcc, high-N phase (γN), with both magnetic and paramagnetic characteristics, was found to be distributed in the N implanted layer generated by the low-energy, elevated temperature, implantation conditions. The magnetic γN was found to be ferromagnetic and was distributed in the highest N concentration region of the implanted layer (the top 0.5 μm) while the paramagnetic γN becomes predominant below 0.5 μm, where the N content is only slightly lower. The ferromagnetic state is linked to large lattice expansions due to high N contents (∼30 at.%) as determined by XRD and electron microprobe. The relatively uniform XRD N distribution to a depth of ∼1 μm suggests a sensitive dependence of the magnetic γN phase stability on N concentration and degree of lattice expansion. The XRD results also show that the N contents and depths depend on the polycrystalline grain orientation relative to the ion beam direction. The N was found to diffuse deeper in the (200) oriented polycrystalline grains compared to the (111) oriented grains and the N contents were significantly higher in the (200) planes relative to the (111) planes. The effect of compressive residual stresses (∼2 GPa) is considered. The scanning electron microscopy (SEM) analysis reveals quite clearly the uniform nature of the γN layers with a reasonably well defined interface between the γN layer and the substrate, suggesting uniform N contents with uniform layer thicknesses within a given grain. However, they also show significant variations in the γN layer thickness from one grain to the next along the N implanted layer, clearly supporting the XRD findings of the variation in N diffusivity with grain orientation.