Résumé
The study of nanomaterials, which are characterized by a typical grain size below 100 nm, is nowadays a very active and dynamic branch of materials science research. The starting point of this great interest, which led to numerous studies and the development of new technologies, is the famous publication of H. Gleiter in 1989. It has been established that metallic materials with a grain size in the ultrafine (<500 nm) or in the nanocrystalline (<100 nm) regimes, exhibit remarkable mechanical and physical properties. This is due of the small grain size of these materials, or more precisely to the very large volume fraction of grain boundaries induced by these narrow grains. From a mechanical point of view, these materials present higher yield strength and hardness, as well as greater wear resistance. A major challenge today is to quantify the real mechanical response of nanomaterials. Actually, most of the elaboration processes of nanomaterials have none negligible effect on their mechanical response, and appropriate tests have to be developed to characterize the mechanical properties of these materials. The aim of this paper is to characterize the mechanical response of 316L stainless steel treated by SMAT (Surface Mechanical Attrition Treatment) using nanoindentation. After SMAT, this stainless steel looks like a multilayer structure characterized by a grain size gradient from the top treated surface to the bulk of the materials. Whereas the global mechanical response of the SMATed material can be analysed by classical tensile tests, precise nanoindentation tests have to be performed in order to reach local information on the nanocrystalline layer formed at the top surface of the material during the treatment. To this end, cyclic nanoindentation experiences have been performed over the cross section of a SMATed sample. A finite element model was then developed to simulate indentation tests, and to determine afterwards the constitutive law for each sublayer. It is thus possible to find a correlation between the microstructure evolution and the mechanical response of the material.