Multiscale Model For Crack Propagation Of Two Phase TiAl Alloy Based On Cohesive Zone Model

TiAl alloy has the advantages of high specific strength,high temperature resistance and low density,so it has a broad development prospect in the fields of aerospace,medical devices and automotive parts.Crack is an important factor leading to the failure of TiAl alloy,which is related to the safety of people's life and property.Therefore,it is of great significance to reveal the law of crack propagation for effectively restraining crack and predicting fatigue life.In the microscale,the defects will cause damage to the mesoscopic structure under the action of external conditions,and further evolve into macro cracks,leading to the failure and destruction of the material and affecting its service properties.For this reason,it is necessary to combine micro defects,mesoscopic crystal damage and macro fracture organically through a method,and then explain the fracture mechanism from both quantitative and qualitative aspects,so as to predict the relationship between micro properties and macro properties more effectively.Therefore,a multiscale model of coupling between two scales of TiAl alloy is established in this paper.The crack growth behavior from microscale to mesoscale and macroscale is analyzed,and the stress intensity factors at macroscale are compared with the experimental results.The specific work is as follows:1.The grain distribution of TiAl alloy was counted from the experiment,and the mesoscopic model of polycrystalline TiAl alloy was generated by Voronoi tessellation method.The periodic boundary conditions in the finite element model are set by using the representative volume element theory,and the theory is successfully applied to the Voronoi tessellation model through the secondary development of Python.The differences among three different fracture modes are discussed,and the type of open crack(type I)propagation required for the research is selected,and the calibration model of compact tensile(CT)specimen is established.In addition,based on the cohesive force theory,the fracture simulation of cohesive force region in the finite element model is realized.2.The atomic packing of two-phase cell in TiAl alloy is discussed,and the orientation of two interfaces is determined.Molecular dynamics(MD)was used to establish the true twin(TT)?/?interface and the?₂/?interface model.The open crack loading method was applied to both models,the?/?interface at the same temperature(300K)under the conditions of no initial defects,blunt cracks and blunt cracks+central hole,and the?₂/?and?/?interfaces without initial defect at 1K,300K and 500K were simulated,respectively.The corresponding crack growth behavior was obtained and its fracture mechanism was analyzed.According to the T-S relationship in the cohesion theory,the constitutive parameters of the cohesion on the interface are extracted,and the differences between the constitutive parameters are discussed.3.The mesoscopic polycrystalline finite element model was obtained by geometric simi-larity,and the cohesive force constitutive parameters under different conditions were coupled into the model.The critical stress fracture nephogram and tensile fracture relation curve under different defects and temperatures were obtained;The effects of interface parameters under different conditions on the intergranular fracture of single-phase polycrystalline and dual-phase polycrystalline are described,and the difference between the fracture behavior of single and dual phase polycrystalline is analyzed;A macroscopic finite element model(FEM)was established based on continuum mechanics.The force-displacement response curve was obtained by simu-lating the compact tensile specimen,and the fracture toughness of the material was obtained to verify the validity of the model.The effects of different defects and temperature on the fracture toughness of the crack were also elucidated.