Phase transformation and corrosion behavior of stainless steel bombarded by pulsed energetic ion beams

The treatment of stainless steel by pulsed energetic nitrogen ion irradiation in the millisecond regime may cause phase transformations close to the surface. The characterization of the phases was done by measurements of Mößbauer and grazing incidence X-ray diffraction. The results indicate transformations of the austenitic phase into Cr2N, Fe2N and the γN-phase (paramagnetic and magnetic) in dependence on the bombardment conditions and the nitrogen depth profiles measured by nuclear reaction analysis. Some samples were studied by depth selective conversion electron Mößbauer spectroscopy. One of these samples contained three new phases: Fe2N, γN2-phase (paramagnetic) and γN1-phase (magnetic). The magnetic phase is found predominantly in the surface region, whereas the Fe2N and the γN2-phase (paramagnetic) reach their maximum concentration at a depth of approximately 100 nm. The other sample contained four new phases: the three phases mentioned above and additionally α-(Fe,Ni)—martensite, indicating the formation of Cr2N. This phase reaches its maximum concentration at a depth of approximately 40 nm. Thus, by interaction of different mechanisms, i.e. segregation of Cr and formation of Fe–Ni rich clusters by radiation enhanced diffusion, the α-(Fe,Ni)-phase is formed on the expense of the other phases. Hardness was measured as Vickers hardness and current-density/potential-curves in a 5 N H2SO4-solution were performed as corrosion tests. The influence of the different surface phases on mechanical and chemical properties are discussed.