Molecular Dynamics Study On Tensile Properties Of γ-Tial Alloy With Vacancies Along Different Crystal Directions

γ-TiAl alloys display attractive properties such as low density,high strength,high stiffness and good oxidation resistance.Thus,it has been becoming a new high temperature structural materials with great development potential and application prospect.However,during the manufacturing process of γ-TiAl alloys,due to the change of external environment,all kinds of micro defects may be produced.Vacancies are one of the most universal defect in metal materials.Under external conditions,vacancies will evolve into dislocations and other defects,which have an important impact on the mechanical properties of metallic materials.Therefore,it is necessary to study the effect of vacancy on mechanical properties of γ-TiAl alloys,analyze the evolution process of vacancy,and reveal the relationship between vacancy evolution and the change of macro-mechanical property.In this thesis,the lattice resistance of γ-TiAl alloys on different crystal planes was obtained by theoretical calculation.In addition,the effect of vacancy and crystal direction on the mechanical properties of γ-TiAl alloys were studied by molecular dynamics method,the results as following.1.The dislocation widths on different crystal planes were calculated by the method of density ratio.Then,the Peierls-Nabarro formula also adopted to obtain the Peierls stress in(100),(110)and(111)planes and the relationship between critical shear stress and dislocation strengthening are obtained by theoretical derivation.The results show that the dislocations are most likely to cause strengthening in the(111)plane for γ-TiAl alloys.Under constant strain rate loading,when the value of the critical shear stress is smaller,the effect of dislocation strengthening will be more obvious.2.Molecular dynamic simulations were performed to study the effects of Ti and Al vacancies on the mechanical properties of γ-TiAl alloys at different temperature.The results show ultimate stress non-linear decrease with the increase of Ti and Al vacancies concentration in the absence of thermal effect.But the ultimate stress of the model with Al vacancy is smaller than the model containing Ti vacancy.This is mainly due to the difference of the vacancy formation energy between Ti vacancy and Al vacancy in γ-TiAl alloys,which leads to the difference of diffusion coefficient.When the temperature is increased from 300 K to 1100 K,the ultimate stress gradually decreases.The potential energy of the system increases with the increase of temperature.3.The tensile behavior of γ-TiAl alloys with vacancy defect was studied by molecular dynamics simulation in different crystal direction.A succession of simulations were performed to analyze the effect of vacancy and lattice orientation on mechanical properties and micro-structure evolution.In addition,The influence of temperature on yield strength is also discussed.The results indicate that the orientation have evidently impacts on critical stress of the γ-TiAl alloys.In the[100],[110] and [111] crystal directions,the yield strength of the γ-TiAl alloys with Ti and Al vacancies increase sequentially.However,the yield stress of the model with Ti vacancies is higher than containing Al vacancy model along three crystal directions.Besides,during the deformation of single crystal γ-TiAl alloys,it is found that the dislocation density have the same changeable trend as the number of stacking faults.