Phase transformation induced by classical ion implantation in austenitic stainless steel

Structural modifications induced by Cr, Ar, N, Xe, Mo, Ag and Pb ion implantations in a 316LVM austenitic stainless steel within a 70-nm thicksurface layerwere studied by grazing incidence X-ray diffraction. In theimplanted layer, austenite lattice expansion was first observed, then ferrite formation and in some cases amorphization. Atomic force microscopy (AFM) was used to study changes in surface topography under ion implantation: roughening, grain boundaries revelation, holes formation. Magnetic force microscopy (MFM) was used to distinguish on the surface ferrite domains when they were formed. Ferrite started to grow at grain boundaries and covered all the surface of the sample in some cases. A theoretical damage level was estimated using SRIM-2003 computer program. The amount of ferrite and the amorphization rate were considered with respect to the damage level brought in the implanted layer by ion introduction. Existence of damage level thresholds for ferrite formation (200 displacements per atom (dpa)) and amorphization (400 dpa) were considered as evidence. These thresholds were independent of the nature of implanted element and the fluence of implantation.