EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

To strengthen materials and therefore improve the mechanical properties of parts, some of the most widely used techniques are the mechanical surface treatments. Among them, SMAT (Surface Mechanical Attrition Treatment) has great potential for strengthening mechanical components subjected to cyclic loads. This treatment is based on multidirectional impacts of spherical balls set in motion using an ultrasonic generator. SMAT is able to generate a nanocrystalline layer on the extreme surface of the treated part as well as a transition layer characterized by grain size and hardness gradients below the treated surface. In addition, high compressive residual stresses can be generated especially in the near-surface region where the plastic deformation is highly activated. Studies presented in the literature show that SMAT is able to significantly improve the properties of various materials, such as their fatigue resistance.

The main objective of this study is to understand the fundamental damage mechanisms involved in a CoCrMo alloy subjected to fatigue under rotating bending. For this purpose, different load amplitudes in rotating bending are imposed on cylindrical specimens, in the untreated and SMATed states. The results of the fatigue tests are presented in the form of S-N curves according to the JSME S 002 method. A broad materials characterization is done before and after the fatigue tests, in order to gather the data and evidence what features play an important role in fatigue life. Such characterization englobes the use of Scanning Electron Microscopy (SEM), Backscattered Electron Diffraction analyses (EBSD), roughness measurements, X-ray Diffraction analyses (XRD), Micro and Nano hardness tests. Particular attention is paid to the effects of residual stress and work hardening on the fatigue properties of this CoCrMo alloy.