Cross-sectional transmission electron microscopy characterisation of plasma immersion ion implanted austenitic stainless steel

Cross-sectional transmission electron microscopy (XTEM), selected area diffraction (SAD) and nano;beam diffraction (NBD) techniques were used to investigate the surface microstructure of 316 stainless steel, implanted with high doses of nitrogen ions at 150, 250, 350, 450 and 520 degrees C using plasma immersion ion implantation. It has been found that the treatment temperature has a strong influence on the evolution of the microstructure. An amorphous layer of about 1 mu m thick with a heavily stressed substrate underneath was observed on the 150 degrees C implanted sample. Both the 250 and 350 degrees C implanted samples showed a thin nanocrystalline sublayer at the outermost surface and an amorphous sublayer between the nanocrystalline sublayer and the substrate. A thick amorphous layer up to 3 mu m thick was formed on the 450 degrees C implanted sample whereas at 520 degrees C, cellular precipitation of CrN and alpha-ferrite dominated the system. It is suggested that a solid state chemical reaction and the poor mobility of the reactant atoms are the key factors for the solid state amorphisation by nitrogen ion implantation into austenite.