Microstructure Evolution Of Heterogeneous Materials Based On Strain Gradient Plasticity Theory

Heterostructured materials(HS),are novel materials consisting of heterogeneous regions with significantly different mechanical or physical properties.The mutual coupling between these non-homogeneous structures produces a strong ductile synergistic effect that exceeds the properties predicted by the mixing rule.HS materials have excellent mechanical or physical properties that cannot be achieved by conventional homogeneous materials.They can be produced on a large scale and at low cost using conventional industrial technologies and facilities.The excellent properties,new material science and great potential for applications are driving the rapid development in the field of HS materials.Accurate prediction of complex mechanical behavior in plastic deformation of HS materials is an emerging area of research.In the numerical simulation of heterogeneous structural composites,the finite element method is the most widely used numerical simulation tool in this research area.In this thesis,Al/7055 aluminum alloy with gradient periodic structure and graphene-reinforced GNS/AI-C u-Mg alloy with bimodal grain structure are used to study the plastic deformation behavior of HS materials in combination with Strain Gradient Plasticity Theory(SGP)and explore the relationship between microstructure and The intrinsic connection between the microstructure and the mechanical properties of the material was investigated.The main research contents of this thesis are as follows:(1)Based on the Taylor dislocation model and discrete gradient calculation algorithm,the SGP of strain gradient plasticity theory is constructed,and on this basis,a strain gradient plasticity model is established by considering mixed enhancement mechanisms such as grain refinement strengthening,dislocation strengthening and back stress strengthening.This constitutive model is implemented in Abaqus finite element software by writing subroutines in Fortran language.(2)In this thesis,Al/7055 heterostructured aluminum alloy is selected as the research object.The mechanical parameters such as Young’s modulus,yield strength and hardening index of pure Al and 7055 aluminum alloys prepared by powder metallurgy method were obtained by microcolumn compression experiments.Meanwhile,the macroscopic mechanical response of Al/7055 heterostructured aluminum alloys was determined by three-point free bending experiments.The mechanical properties of Al/7055 heterostructured aluminum alloys with different numbers of gradient cycles were compared and analyzed.The microstructure and morphology of Al/7055 heterostructured aluminum alloys were investigated by electron backscattered diffraction(EBSD)and backscatter electron imaging(BSE).(3)Based on the grain morphology obtained by EBSD,the RVE model with similar micro structure to that of Al/7055 heterostructured aluminum alloy and GNS/Al-Cu-Mg bimodal aluminum alloy was constructed using Python script parameterization A hybrid experimental-numerical inversion method was used to fit and optimize the material parameters with the experimental results as the target.(4)The effect of the number of gradient cycles on the mechanical properties and micro structure evolution of Al/7055 heterostructured aluminum alloy composites was investigated by combining experiments and simulations.The accuracy of the strain gradient plasticity model was verified by comparing the results of numerical simulations and experiments.Simulations of the RVE model of GNS/Al-Cu-Mg bimodal structural aluminum alloy under tensile loading conditions were carried out to investigate the efifects of coarse crystal integration number and coarse grain size on the mechanical properties of GNS/Al-Cu-Mg bimodal structural aluminum alloy.