Effects of high energy nitrogen implantation on stainless steel microstructure

Low energy ion implantation is known to improve chemical and mechanical sur face properties of metals. This treatment is often used to enhance wear and corrosion resistance or mechanical life-time of fatigue test of stainless steel or titanium alloys. The aim of this work is to investigate these effe cts at higher energy, for which deeper (and still not well understood) modi fications occur. High fluence (10(18) cm(-2)) N-15 and N-14 implantations a t 1 MeV have been performed in the 316LL stainless steel and some specimen have been annealed in the 200-500 degrees C temperature range. Nitrogen con centration distribution, structure, morphology and microhardness have been examined with Nuclear Resonance Analysis. Grazing Incidence X-Ray Diffracti on and Nanoindentation, respectively. Precipitates of steel and chromium ni tride phases and a superficial martensitic transformation can be observed, leading to a significant increase of hardness. The best result is obtained after one hour annealing at 425 degrees C, due to a larger and more homogen eous repartition of nitride species. In this case, a near surface accumulat ion is observed and explained in terms of diffusion and precipitation mecha nisms. (C) 1999 Elsevier Science B.V. All rights reserved.