EVOLUTION OF THE MICROSTRUCTURE OF AUSTENITIC STAINLESS-STEEL NITROGEN-IMPLANTED AT ELEVATED-TEMPERATURES

The exact nature of microstructural changes brought about by nitrogen implantation of austenitic stainless steels is not very well understood. In fact, the literature on this subject appears to be fragmented and contradictory. A major obstacle is the interpretation of results obtained from plane-view transmission electron microscopy (TEM) samples. Another difficulty is the similarity of the crystal structures and d-spacings of different nanocrystalline precipitates that can form in this system, leading to extremely complex and confusing selected area diffraction patterns. In the present work, cross-sectional transmission electron microscopy (XTEM), selected area diffraction (SAD) and microbeam diffraction (MED) techniques were used to investigate the surface microstructure of AISI 316 stainless steel, implanted with high doses of nitrogen ions at 450 and 520 degrees C using the plasma immersion ion implantation technique. It was found that the implantation temperature strongly affects the evolution of microstructure. For instance, an amorphous layer up to 3 mu m thick was formed on the surface of specimens implanted at 450 degrees C whereas at 520 degrees C cellular precipitation of chromium nitride and ferrite dominated the microstructure. The sequence of events leading to the formation of the amorphous layer and other microstructural features are discussed in detail.