Martensitic transformations in ion implanted stainless steels

Abstract

ABSTRACTUsing ion implantation it is possible to induce a variety of phase transformations in the surfaces of stainless steels. The implanted layer can be either amorphized, undergo an fee (γ) => bec (α') martensitic transformation, or compounds can be formed between implanted atoms and target atoms. Stress provides a dominant contribution to the driving forces for amorphization and martensitic transformations. Implantation-induced alloying is important for obtaining the right composition in the implanted layer, whereby the transformation conditions can be optimized. Amorphization is best achieved by implantation of metalloids or other conventionally known glass forming elements. Martensitic transformations in austenitic steels are most easily induced after implantations with noble gases — including helium, where highly pressurized noble gas inclusions are formed. Implantations at high fluences with compound forming species such as nitrogen, boron or phosphorus, will often produce an implanted layer with a structure resembling a ceramic glazing which, depending on the implant species, will be amorphous or crystalline.In order to understand and predict changes in the surface properties of implanted stainless steels, a detailed knowledge of the implantation induced microstructures is essential. The implanted surface will often have a multiphase structure giving only moderate improvements in corrosion properties, and in particular, resistance to pitting corrosion will often be reduced or only slightly improved. Surface hardness will, on the other hand, in many cases be increased, and large improvements will often be observed in wear and friction properties of the implanted surfaces. Under conditions where formation of a homogeneous surface layer is facilitated, the fatique life of implanted samples may also be extended.