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چكيده انگليسي :

Modeling of Grain and Grain Boundary at Nano and Micro Crystalline with Finite Element Method Mohammad Jafari Email mohammad jafari@me iut ac ir Date of submission January 15 2013 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language Farsi Supervisor Saeed Ziaei Rad Email szrad@cc iut ac irAbstractRecent experiments on polycrystalline materials show that nanocrystalline materials have a strongdependency to the strain rate and grain size in contrast to the microcrystalline materials In this study mechanical properties of polycrystalline materials in micro and nano level were studied and a unifiednotation for them was presented To completely understand the rate dependent stress strain behavior andsize dependency of polycrystalline materials a dislocation density based model was presented that canpredict the experimentally observed stress strain relations for these materials In nanocrystalline materials crystalline and grain boundary were considered as two separate phases The mechanical properties of thecrystalline phase were modeled using viscoplastic constitutive equations which take dislocation densityevolution and diffusion creep into account while an elasto viscoplastic model based on diffusion mechanismwas used for the grain boundary phase For microcrystalline materials the surface to volume ratio of thegrain boundaries is low enough to ignore its contribution to the plastic deformation Therefore the grainboundary phase was not considered in microcrystalline materials and the mechanical properties of thecrystalline phase were modeled using an appropriate dislocation density based constitutive equation Finally the constitutive equations for polycrystalline materials were implemented into a finite element code and theresults obtained from the proposed constitutive equations were compared with the experimental data forpolycrystalline copper and good agreement was observed Dislocation density based constitutive modeling had a good capability and reliability for predicting themechanical behavior of coarse grained or fine grained metallic materials so these equations were used fordual phase steels While the grain boundary phase was separated of ferrite and martensite phases thegeometry of the dual phase steels microstructure was modeled with voronoi method The Gurson Tevergaard Needleman damage model the dislocation density based model and elastic perfect plastic modelwere used for grain boundary ferrite and martensite phases respectively and the mechanical behavior of dualphase steels were investigated by finite element method in uniaxial tension test The obtained results showthat the grain level inhomogeneity plays a main rule in plastic deformation of these materials Finally crystal plasticity constitutive equations were used to investigate the crystalline direction effect andmaterial texture Voronoi method was used for simulating the non homogeneity of the microstructure inplastic deformation In addition the elastic modulus parameters for the model were obtained by moleculardynamic simulations The plastic deformation of Fe metal was simulated with the finite element method andgood agreement was observed with the available experimental data Keywords Polycrystalline material Dislocation density based moadel Dual phase steel Voronoi method crystalline plasticity

جعفري، محمد

مدل سازي دانه و مرزدانه در نانو و ميكرو كريستال ها به روش المان محدود

پلي كريستال , چگالي نابجايي , فولاد دوفازي , مدل وروني , كريستال پلاستيسيته