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 understoo d. In fact, the literature on this subject appears to be fragmented an d contradictory. A major obstacle is the interpretation of results obt ained from plane-view transmission electron microscopy (TEM) samples. Another difficulty is the similarity of the crystal structures and d-s pacings of different nanocrystalline precipitates that can form in thi s system, leading to extremely complex and confusing selected area dif fraction patterns. In the present work, cross-sectional transmission e lectron microscopy (XTEM), selected area diffraction (SAD) and microbe am diffraction (MED) techniques were used to investigate the surface m icrostructure of AISI 316 stainless steel, implanted with high doses o f nitrogen ions at 450 and 520 degrees C using the plasma immersion io n implantation technique. It was found that the implantation temperatu re strongly affects the evolution of microstructure. For instance, an amorphous layer up to 3 mu m thick was formed on the surface of specim ens implanted at 450 degrees C whereas at 520 degrees C cellular preci pitation of chromium nitride and ferrite dominated the microstructure. The sequence of events leading to the formation of the amorphous laye r and other microstructural features are discussed in detail.