A Study Of The Influence Of Multi-component Alloying On Oxidation Resistance Of γ-TiAl Based Alloy

Compared with traditional titanium alloys, y-TiAl have many advantages, such as: low density, high specific strength and specific stiffness, rich raw materials, good workable property, and it can be used at a high temperature up to 750～900℃. Based on these superior properties, y-TiAl alloys are predicted to have a more promising perspective of application in the field of aviation, as a high temperature material in parts of engine. However these alloys have poor high temperature property and corrosion resistance which restrict their application and development. In this thesis, for further improve high temperature property and corrosion resistance of y-TiAl based alloy, the influence of multi-component alloying on oxidation resistance of y-TiAl based alloy are investigated by using a first-principles plane-wave pseudo potential method based on Density Functional Theory (DFT). We found that first, the alloying element addition would reduce the vacancy formation energy of aluminum, while increase that of titanium's, which makes the diffusion of aluminum to the surface easily. Alloying elements strengthen the interaction between titanium and other atom, but weaken the interaction between aluminum and other atom, which also facilitates the diffusion of aluminum to the surface. The diffusion of aluminum to the surface leads to the formation of a Al2O3 dense oxide layer, so that enhance the oxidation resistance of y-TiAl based alloy.In addition, there are both internal oxidation and external oxidation in y-TiAl based alloy. Alloying elements reduced the concentration of Oion, thus it lowers the speed of internal oxidation. The doped alloying elements strengthen the interaction between oxygen and titanium, aluminum and other alloying element, so to some extent the diffusion of 0 is restrained, which would decrease the speed of internal oxidation in y-TiAl based alloy, hence, the high temperature oxidation resistance of y-TiAl is improved。Furthermore, the absorption of oxygen on the surface of y-TiAl based alloy is chemical absorption. Nb and Si co doping will make the absorption much easier. There are both covalent and ionic bond between oxygen and Al, Ti. And there is little difference in total effect, which enable the generation of TiO2 and Al2O3 simultaneously, when the intermetallic compound oxidizes. Meanwhile, silicon would react with oxygen directly to form SiO2 which can prevent the internal oxidation together with Al2O3. Niobium play an important role in the bulk, while silicon is effective on the surface, if doping them at the same time, these two effects would combine together, that is, doping both niobium and silicon will make the oxidation resistance of y-TiAl based alloy much better than doping those elements alone.