First-principle Study Of Cr Doping To Improve Oxidation Resistance Of TiAl Alloys

TiAl alloys are widely used in aerospace,automobile manufacturing and other industrial fields because of its excellent properties:light weight,high specific strength,wear resistance,combustion resistance and oxidation resistance.Although TiAl alloys have many good properties,there are two prominent defects:poor plasticity at room temperature and poor oxidation resistance at high temperature.With the continuous development of alloy performance improvement technology,the application of TiAl alloys has been more in-depth research and understanding.However,the experimental results do not reflect the interaction between atoms at the microscopic level.This paper discussed the properties of the basic phase of TiAl alloys,the oxidation mechanism of ?-TiAl,and the methods to improve the oxidation resistance through the method of first-principle calculation,which provides theoretical guidance for the experimental and practical application research.Firstly,we selected four basic phases of TiAl alloys based on the TiAl binary phase diagram.The calculation results showed that the four phases had thermodynamic stabilities and mechanical stabilities.The calculation results of Poisson's ratio and BIG showed that they were brittle materials.The bonding condition and electronic structures demonstrated that TiAl and Ti₃Al had some covalency,which is related to the hybridization of orbital electrons of Al-s,Al-p and Ti-d.The covalency of TiAl is stronger,which is also the internal micro reason for the poor plasticity of TiAl alloy at room temperature.In comparison,the heat transfer performance of Ti₃Al is better.Secondly,y-TiAl is the most stable phase in TiAl alloys,with the lowest energy and the lowest equilibrium volume.The surface energy of(110)surface in low index surfaces is lower and the surface is more stable.Therefore,the adsorption and diffusion of O atom on the surface of ?-TiAl(110)were discussed.O atom tended to occupy adsorption sites with more Ti atoms because of the strong interaction between O and Ti.The whole process of O atom diffusion was endothermic.The key step of diffusion occurred in the diffusion from surface layer to sub-surface with higher activation energy.Once the diffusion occurs,it is very easy to further diffuse into the bulk phase.The diffusion of O from surface to subsurface can be effectively inhibited by Cr surface alloying.The doping of Cr atom weakened the covalent interaction between O atom and alloy atoms.Finally,the relationship between the doping concentration of Cr atoms and the oxidation resistance of the alloys was demonstrated by first-principle calculation.The oxidation resistance of the alloys was characterized by atomic formation energy.It was concluded that when the doping concentration of Cr atom is about 7%?9%,the alloys not only have energy stability,could exist stably and are easy to prepare,but also improve the diffusion ability of Al atom,inhibit the diffusion ability of Ti atom,and make it difficult for O atom to diffuse into the alloy,which is conducive to forming a dense protective film of Al₂O₃ on the surface.The doping system T₄A₀ is the most ideal one in theory.At the end of this paper,we summarized the micro mechanism of the effect of alloying on the oxidation resistance of ?-TiAl alloy,and prospected the systematic calculation of the effect of Cr atom doping in the surface layer on the oxidation resistance of the alloy.