Consideration of softening near free surface in the direct model of crystal plasticity

Current industrial demands require the improvement of existing technologies and the creation of new ones that allow the production of parts and structures with advanced performance properties. Especially important are the issues of design and analysis of thermomechanical treatment of metals and alloys by intensive inelastic deformation methods. The study of manufacturing processes for miniaturized parts deserves special attention. The boundary value problems arising in this case belong to the class of physically and geometrically nonlinear problems of the mechanics of deformable solids, for the solution of which it is necessary to develop appropriate mathematical models. Most classical models are based on the macrophenomenological theory of elastoplasticity. However, in the processes under consideration, significant structural changes occur at the meso- and microscale that have a significant influence on the physical and mechanical characteristics of the processed materials and the performance characteristics of the products. These changes are not described by the mentioned theories. An effective approach to describe these processes is to use multilevel models of crystal plasticity. Despite the tendency to miniaturize products, the parameters of such models are usually determined from the results of experiments on macrosamples. The question arises whether such parameters are valid for the analysis and modeling of real structures with typical length scales below 500 gm, where the internal and external boundaries of crystallites play a special role. In this study, the influence of the free surface on the mechanical properties of single crystalline and polycrystalline materials is considered. Modification of the basic direct crystal plasticity model of elastoviscoplasticity to account for the softening of slip systems near free boundaries due to easier exit of dislocations to the surface is proposed.