Research On Effect Of Nb Substitution And Concentration On Microcrack Propagation Of ?-TiAl Alloy

The application of TiAl alloys in the aerospace field is restricted by its poor plasticity at room temperature,lack of oxidation resistance above 850°C and high crack growth rate severely.The problem would be solved by adding Nb in ?-TiAl alloy.The main trend of development of TiAl alloy would be the Ti-Al-Nb ternary system,and extensive attention has been on the effects of alloying elements on the properties of TiAl alloy in the field of materials engineering.In this thesis,the crack propagation of TiAl alloys with Nb substitution mode and atomic ratio concentration under uniaxial tensile loading was simulated from the microscopic scale by using molecular dynamics method.The results of the study are as follows:(1)The effect of different substitution modes of Nb on the crack propagation of ?-TiAl alloy was studied.The crack propagation and dislocation evolution process during the simulation was analyzed by applying uniaxial tensile loading on Nb-free system,Nb-substitution Al system and Nb-substitution Ti system models.In addition,stress-strain curve,energy evolution,atomic structure number,radial distribution function change curve was also studied.The results show that no matter what substitution is,the interaction between Nb and the neighboring matrix atoms is stronger than that of Ti-Al atoms before the substitution.Bonding and bonding strength between doped Nb atoms and matrix atoms,the bond strength of the unit cell,the bonding force and cohesion between the surrounding atoms would be enhanced.These lead to the atoms in the crack tip bond tightly,crack tip blunting,crack propagation slowly,crack propagation path changes,and tensile strength and fracture toughness of TiAl alloy increase.The yield strength of the substitutional Ti system is higher than that of the substitutional Al system.The lattice distortion degree of the substitutional Al system is larger than that of the substitutional Ti system.The dislocation density and fracture toughness of the substitutional Al system are higher than those of the substitutional Ti system.(2)The effect of different Nb concentration on the crack growth of single-crystal ?-TiAl alloys was investigated.The results show that the stress at the plastic stage decreases while cohesion between atoms increases with the increasing of Nb concentration.As a result,the stress field around Nb increases and the peak stress value increases,while the crack propagation rate decreases.Ranging from 0% to 6% of the atomic ratio concentration of Nb is helpful to increase the yield strength and ductility of ?-TiAl alloy.(3)The crack propagation process can be divided into elastic stage and plastic stage.The elastic stage is mainly the breakage of the atomic bond in the dislocation-free zone.The plastic stage is mainly caused by the nucleation and emission of dislocation and dislocation movement near the crack tip;In the ?-TiAl alloy,the density of 1/6<112> Shockley dislocation is the largest dislocations,and it increases with the stress.The evolution of dislocations is related to the distorted hcp structure and the change trend of bcc structural.The dislocation density peak region is mostly concentrated in the stress peak region,and the dislocation density increases with increasing stress.When the stress is highly concentrated,a part of the concentrated stress would be released by dislocation emission form crack tip.