First-principles Calculations Of The Effect Of Nb On Oxidation Resistance And Friction And Wear Of TiAl Alloys

TiAl alloy is an ideal high-temperature structural material,exhibiting low density,high elastic modulus,excellent high-temperature strength and creep resistance.However,the oxidation resistance above 800 ? is weak,which limits its practical application.High Nb alloying is one of the most effective ways to improve the oxidation resistance of TiAl alloys.A large number of experimental studies on the oxidation resistance of high Nb-TiAl alloys provide many empirical phenomena and data,but the theoretical work on the oxidation resistance of TiAl alloys is relatively insufficient,e.g.the diffusion path of O element in TiAl alloys,the mechanism of the effect of alloying elements,like Nb,on the O diffusion and surface-oxide formation.On the other hand,wear resistance is also very important for the hot end parts of aerospace engines,and the insufficient wear resistance of the TiAl surface has become an increasingly prominent problem for its application.The research on the friction and wear resistance of TiAl alloys still lacks in experiments and theoretical calculations.The main work of this thesis is to study the diffusion of O element in TiAl alloy,to understand the diffusion mechanism of O in TiAl alloy,and to study the effect of alloying elements on the oxidation resistance of TiAl alloys by first-principles calculations.These calculation results were compared with the existing experimental data.On the other hand,friction and wear properties of TiAl alloys were studied.The mechanism of Nb on the friction and wear resistance of TiAl was analyzed by experiments,and the mechanical properties of the TiAl were analyzed by first-principles calculations.The following conclusions were obtained:(1)First-principles calculations and Debye model were utilized to investigate the preference of site occupation of Nb,Y,Sc,Zr,Hf,V,Ta,Cr,Mo,W in y-TiAl phase.It was observed that Nb,Y,Sc,Zr,Hf,Ta,Mo,W tend to occupy the Ti site at 0 K,while Cr and V occupy both the Ti and Al sites,as the difference of the reaction energy for Cr and V to occupy the Ti site and the A1 site is relatively low.With increasing temperature,the difference in the formation energy for Nb to occupy the Ti site and the Al site gets smaller.In the temperature range of 600-800?,the reaction energy for Nb to occupy the Ti site is still lower,and the Nb still tends to occupy the Ti site.(2)Based on the first-principles calculations,the diffusion path of O in the y-TiAl and ?2-Ti3Al phases and the corresponding diffusion barrier were simulated by CINEB method.The diffusion barrier of O atom was also calculated in dependence of Nb content.The calculation results show that the addition of Nb has significant effect on increasing the diffusion barrier of O in TiAl alloy,and the ability to hinder the diffusion of oxygen atoms increases with increase of Nb content.With increasing temperature,the effect of the Nb on the oxygen diffusion in the alloy is further confirmed.This calculation results agree well with the experimental results.(3)For the formation ability of oxides on the surface of TiAl alloys,the stretching force constants(SFC)were studied to describe bond strength of metal-oxygen bonds in different oxides.The effect of alloying element on the bond strength of Al-O and Ti-O bonds was calculated,and the effect of Nb on the growth of Al2O3 and TiO2 was analyzed specifically.The alloying elements Cr,Ti,Zr,Hf,Y,Nb can increase the SFC of the Al-O bond in Al2O3.The SFC increased by about 10 eV/A2 for Nb and Y.On the other hand,the addition of Nb decreases the SFC of the Ti-O bond in TiO2.Therefore,Nb can promote the formation of Al2O3 while inhibit the formation of TiO2.Thus a dense and stable Al2O3 oxide film can be formed on the surface of TiAl alloy,which improves the oxidation resistance of the alloy.The calculation results provide a theoretical basis of the previous experimental results.(4)The addition of Nb can also improve the high-temperature wear resistance of the TiAl alloy.With increasing the Nb content in TiAl,the friction and wear resistance is increased revealed by wear experiments at high temperatures.By using first-principles calculations,it was revealed that the hardness of the TiAl alloy increases by addition of Nb,thus the wear resistance is improved.The addition of Nb can improve the formation of dense and firm Al2O3 oxide layer on the surface of the TiAl alloy,thus improve the oxidation resistance of the matrix.In the meanwhile,the layer can avoid the direct contact between the TiAl matrix and the friction pair,which is the main reason for improving the friction and wear resistance of the TiAl alloy.