Mechanical Response Of Crystal Plasticity For Pure Titanium

The metal titanium has a highly anisotropic mechanical property,due to the hexagonal close-packed structure.The plastic deformation of titanium is quite complex,in which the slip and twins play an important role.The crystal plasticity model is combined with the finite element method.The pure titanium crystal is selected as the object of this study.The plastic deformation process of the crystal structure is simulated in micro grain scale.The relationship between the microscopic plastic deformation mechanism and the macroscopic mechanical response is established.Compared with the traditional isotropic constitutive theory,the theory of crystal plasticity attributes the plastic deformation of metals to the slip of dislocation and deformation twins.The theory of crystal plasticity is the most effective method to simulate the heterogeneity of deformation inside the crystal,which is more close to the real physical essence of plastic deformation of metals.In this paper,the plasticity constitutive model and hardening model of slip and twin crystal are established with the mechanism of slip and twin deformation being considered.The main research work is as follows:(1)The relationship between the microscopic plastic deformation mechanism and the macroscopic mechanical response was established by researching the relationship between the crystallographic characteristics of the slip and twins and the shear strain and the gradient tensor.Based on the classical Taylor model,the deformation twin is considered into the plasticity theory of the crystal,and the plastic constitutive relation of the crystal is deduced by the interaction of slip,twin and their interaction.For different deformation modes and hardening characteristics of tensile twin and compressive twin,different hardening models are used to describe them.(2)Through the development of the user-defined constitutive relation subroutine UMAT,the crystal plastic finite element simulation is achieved in ABAQUS.The uniaxial compression of monocrystalline titanium was established,and three kinds of uniaxial compressions with different orientations were used respectively.Six kinds of deformation mechanisms,i.e.the basal,prismatic,pyramidal <a> and pyramidal <c+a> slips,tensile twinning and compressive twinning,are used to accommodate the plastic deformation of titanium,and a uniform material parameter was used to simulate the microstructural deformation of hexagonal closely-packed metal titanium.The results show that the plasticity theory of the crystal is reasonable in describing the mechanics response and texture evolution of the material.The stress-strain curve of the three kinds of orientation is obtained,which is in good agreement with the corresponding simulation results.(3)Based on the visco-plastic self-consistent model,the transition of the plastic behavior of crystals from microscopic physical quantities to macroscopic physical quantities is analyzed.The constitutive laws of the materials are described from two aspects: grains and aggregates.And the visco-plastic self-consistent model was modified using the Voce hardening model.Three uniaxial quasistatic compressive deformation,two different kinds of dynamic compressive deformation of monocrystalline titanium and uniaxial deformation of polycrystalline titanium were simulated.The corresponding stress-strain curves,relative activities and texture pole diagram were obtained by simulating these modes,which is more consistent with the experimental data.