Dislocation density-based model for describing the mechanical behavior of gradient microstructure generated by SMAT for TWIP/TRIP steels
Résumé
Surface Mechanical Attrition Treatment (SMAT) is a surface treatment based on ball impact boosted by an ultrasonic generator. This technology is able to generate a gradient microstructure in the near surface region of a structure. In this region, the microstructure varies from ultrafine grains at the top treated surface to grains with a micrometric size in the central region. This difference in grain size consequently leads to the presence of a gradient mechanical properties in the SMAT affected region of the structure. Methods: In this work, a dislocation density-based model is developed to describe the gradient properties of a 316L steel treated by SMAT. For this purpose, some variables are introduced to take into account the gradient features of the microstructure including grain size, twinning and martensite transformation. To identify the parameters of the model, micro-pillar compression technique is used to investigate local mechanical properties of a cylindrical structure at different depth from the treated surface. These mechanical tests with micro-pillars, even though of compression nature, could provide qualitative information about the gradient mechanical features of the material. Results: The developed model is then implemented in a subroutine of ABAQUS Explicit to perform 3D simulations of tensile tests in order to investigate the properties of a gradient cylindrical structure. The obtained global mechanical behavior is in good agreement with the stress-strain curve obtained by experimental tensile tests in terms of strength and work hardening. At the local scale, various information can be obtained like the stress and the strain fields of the simulated structure. Conclusions: It has been found that the strain-induced twining and martensite transformation, the grain size and the thickness of the ultrafine grained layer all contribute to the global behaviour of the structure presenting a gradient microstructure generated by SMAT.