First-principles Study On Structural Stability And Plastic Properties Of Nb/V Doped ?-TiAl Alloys

Due to the advantages of low density,high modulus of elasticity,good high temperature strength,creep resistance and oxidation resistance,?-TiAl alloys are widely used in high temperature parts of automobile or aeroengine Such as turbine discs,blades and valves such as high temperature and pressure and other extreme environments.But at room temperature,the brittleness of the material restricts its wide application.Through the people continue to explore the discovery,the use of a small amount of other impurity elements is not only an effective way to improve room temperature brittleness,but also improve the high temperature oxidation resistance of the main methods.It can be seen from the literature that Ti-47A1-lCr-1V-2.5Nb can be used to improve the plasticity effect by multi-element doping,and its elongation is close to 4%.However,the study of their electronic structure properties,the computer simulation method is an effective way to explore the microscopic mechanism of its material science.The method of alloying to improve the ductility of ?-TiAl alloy has been achieved by theoretical and experimental methods.However,there are still many problems to be studied.There are few reports on the influence of element doping concentration on the properties of ?-TiAl alloy.However,there are few reports on the comparison of single element doping and double doped ?-TiAl properties.Therefore,it is necessary to further explore the alloying method to enhance the ductility of ?-TiAl alloy,to provide the basis for material research and development.In this paper,V and Nb were chosen as dopant elements to study the effect of single element and double element doping on the ductility of ?-phase TiAl alloy,and the mechanism of intrinsic electronic structure was analyzed.According to the third generation of ?-phase TiAl alloy mixed element atomic percentage content is generally less than 10%,we choosed ?-phase TiAl alloys with Nb or V single concentration of 1.85 at.%,2.78 at.%,4.17 at.% and 6.25 at.% as single-mixed model and Nb and V double concentration of 1.85at(Nb)+ 1.78 at.%(V),2.78 at.%(Nb)+ 2.78 at.%(V),4.17 at.%(Nb)+ 4.17 at.%(V),6.25 at.%(Nb)+ 6.25 at.%(V)?-phase TiAl alloy as a double-mixed model.Based on the density functional theory(DFT),the ?-phase TiAl alloy is calculated and analyzed by using the CASTEP module of Nb-The formation energy,geometric structure parameters,axial ratio R,bulk modulus,shear modulus,differential charge density before and after Nb and V.(1)from the atomic average formation can be contrasted,Nb or V single doping system,Nb and V atoms to replace the Ti atomic energy than the substitution of Al atomic energy lower,indicating that Nb and V atoms are easy to replace the ?-phase TiAl alloy Ti position.In Nb and V atoms double doped system,Nb atoms are easy to replace Ti,V atoms easily replace the Al position.(2)In the Nb or V single doping system,the Nb and V atoms replace the Ti atom to decrease the ductility of the ?-TiAl alloy,and replace the Al atom from the unit cell geometry,the axial ratio R and the elastic modulus.The anisotropy of ?-TiAl alloy is reduced and the ductility of ?-TiAl alloy is improved.The anisotropy of Nb-V atom double-doped system is decreased and the ductility of ?-TiAl alloy is improved.V or Nb atomic single doping anisotropy and ductility improvement is obvious.(3)From the differential charge density and the state density analysis,when the V atom or Nb atom is doped with Ti,the counter bond between the impurity atom and the Al atom is stronger than that between the undoped Ti and Al atoms,while not having much influence on the surrounding Ti-Ti bonds,Ti-Al bonds and Al-Al bonds.Therefore,the Ti or Nb atoms are doped with Ti to improve the brittleness of the TiAl alloy.When the Al sites are doped,the metal bonds are formed around the impurity atoms,and the p-p covalent bonds are weakened.The brittleness of the Ti Al alloys is inhibited,the free electron density increases,the number of free electrons involved in the formation of metal bonds increases,improving the ductility of TiAl alloys.In the Nb and V atom double doped TiAl alloy model,the position of the Ti atom is replaced with the Nb atom,and the free electron state density of the alloy matrix is increased with the V atom,and the free electron number The addition of impurity atoms around the formation of metal bonds,while weakening the pp covalent bond,inhibited the TiAl alloy brittleness,improved TiAl alloy ductility.