The Influence Of Void On Fracture Behaviour Of TiAl-Nb Alloys Using Molecular Dynamics

?-TiAl alloys have attracted much attention because of its low density,good oxidation resistance and high specific strength.It is considered as the most potential high temperature structural material,and has a broad application prospect in automobile,aerospace and other fields.However,in the preparation and use of ?-TiAl alloys,voids are easy to occur in the alloy due to the action of process parameters,temperature,load and other factors.The growth and polymerization of the voids will have a certain effect on the mechanical properties and fracture behavior of the materials.Therefore,it is necessary to study the effect of voids on the fracture behaviour of ?-TiAl alloys,analyze the process of voids growth,and reveal the relationship between voids evolution and fracture behaviour of materials.The molecular dynamics method,as an effective atomic simulation method,can not only capture the evolution process of the position,speed,stress and energy information of each atom or particle,but also can realize the dynamic tracking of the material microstructure.Thus,the molecular dynamics method is used in this paper to study the evolution of the voids under different in different voids radius,number and position and its effect on the fracture behaviour of TiAl-Nb alloys.The main works include the following aspects:(1)The effects of void size,number and position on the fracture behaviour of single crystal ?-TiAl alloys containing Nb have been studied.The relationship between the micro-defect evolution inside the material and the change of the fracture behaviour of the material were analyzed.The results indicate that the yield strength and young modulus of single crystal ?-TiAl alloys are increased with the addition of Nb element.With the increase of void size and number,the yield strength of single crystal TiAl-Nb alloys decreases in sequence.When several voids are distributed parallel to the tensile direction,the yield stress of the material is the largest,and when they are distributed perpendicular to the tensile direction,the yield stress of the material is the smallest.(2)The effect of crystal orientation and temperature on the fracture behaviour of single crystal TiAl-Nb alloys with voids was studied.By analyzing the fracture behaviour of the material under different conditions and the evolution of their internal microscopic defects found that the effect of crystal direction on the fracture behaviour of single crystal TiAl-Nb alloys with voids is small,which is mainly due to the fact that the solid solution strengthening effect due to the addition of Nb element is greaterthan dislocation strengthening.The higher the temperature,the time for the first nucleation of dislocation at the voids is advanced,the number and the types of dislocations are decreased,and the earlier the failure time of the material.(3)The effects of grain boundary and inner void size on fracture behaviour of polycrystal and two-phase polycrystal TiAl-Nb alloys were studied.By analyzing the variation of voids volume fraction and voids growth rate with the initial voids radius,the process of voids growth and the evolution of all kinds of defects in the system,it is found that the void volume fraction with the change of the initial void radius shows a gradual increase,sharp increase,and almost constant rule.The number of dislocations depend on the speed of nucleation and annihilation of dislocations in the voids and the volume fraction of the voids.With the progress of stretching,distorted atoms and internal faults also occur in the system.Most of them appear at the grain boundaries,and some of them appear around the voids or inside the grains.In addition,the number of distorted atoms and internal faults both increased first and then decreased.