High Temperature Oxidation And Hot Corrosion Resistances Of Silicon Modified Aluminide Coatings On TiAl Alloy

TiAl alloy has low density,high elastic modulus and excellent high temperature performance.It is a kind of light-weight high temperature structural material with promising application in aviation and aerospace fields.However,the mixed oxide film of TiO2 and Al2O3 is formed on TiAl alloy above 800℃.The mixed oxide film is loose,porous and easily spalling,which cannot prevent the erosion of oxygen.Therefore,protective coatings should be applied to improve the oxidation resistance of TiAl alloy at high temperature.Al2O3 oxide film can be formed on the surface of the diffusion Al coating at high temperature,which can prevent further erosion of oxygen and improve the hightemperature oxidation resistance of TiAl alloy to a certain extent.The addition of Si to the diffusion Al coating can stabilize Ti,reduce the activity of Ti,promote the selective oxidation of Al,and further improve the high temperature oxidation resistance of the coating.In this thesis,based on the beneficial effects of Si mentioned above,a siliconaluminizing diffusion coating was prepared on the surface of γ-TiAl substrate by cold spraying Al-40Si alloy coating with subsequent thermal diffusion treatment,and its oxidation and hot corrosion behavior at high temperature was systematically studied.A dense Al-40Si alloy coating was prepared on the surface of γ-TiAl substrate by cold spray.The microstructure of the coating was basically the same as that of Al-40Si powder,and it was composed of Si phase and Al phase.The effects of thermal diffusion temperature and time on the microstructure of diffusion coatings were investigated.It was found that the diffusion coatings were composed of τ2 Si-rich layer and Ti(Al,Si)3 layer at different thermal diffusion temperatures.With the extension of diffusion time,the τ2 Si-rich layer became thinner and the Ti(Al,Si)3 layer became thicker.With the increase of diffusion temperature,the diffusion velocity of elements increased significantly,and the thickness of the diffusion coating increased obviously after the same time treatment.A silicon-aluminizing diffusion coating composed of Ti(Al,Si)3 phase was obtained by thermal diffusion treatment at 600 ℃ for 2 h using an Al-40Si alloy coating with thickness of about 30 μm.The high temperature oxidation behavior of the silicon-aluminizing diffusion coating was tested at 950℃ for 1000h.It was found that an oxidation film dominated by α-Al2O3 was formed on the surface of the silicon-aluminizing diffusion coating,which significantly improved the high temperature oxidation resistance of γ-TiAl alloy.Due to the interdiffusion of elements at high temperature,the silicon-aluminizing diffusion coating degenerated from a single layer of Ti(Al,Si)3 to a mixed outer layer of Ti(Al,Si)3 and Ti5Si3 with reticular structure,a continuous and dense diffusion barrier Ti5Si3 layer and an inner layer of TiAl2.With the increase of oxidation time,the thickness of the outer mixed layer decreased gradually,the thickness of the inner TiAl2 layer increased,and the middle Ti5Si3 layer remained relatively stable.The reticular Ti5Si3 phase and Ti5Si3 diffusion barrier are very stable,which would effectively inhibit the interdiffusion of elements between the coating and the substrate,thus improving the long-term high-temperature oxidation resistance of the silicon-aluminizing diffusion coating.The influence of different oxidation temperatures on the oxidation behavior of the silicon-aluminizing diffusion coating was studied.It was found that the coating effectively improved the high temperature oxidation resistance of substrate at 900℃,950℃ and 1000℃.With the increase of oxidation temperature,the thickness of oxide film on the surface of the silicon-aluminizing diffusion coating increased,and the microstructure degradation of the silicon-aluminizing diffusion coating accelerated obviously.After 300 h of oxidation at 1000℃,the Al-rich phases in the coating almost completely degraded.However,the reticular Ti5Si3 phase and Ti5Si3 diffusion barrier maintained good stability and would prevent further invasion of oxygen to a certain extent.The thermal corrosion behavior of the silicon-aluminizing diffusion coating in pure Na2SO4 salt and 3:1(wt.%)Na2SO4:NaCl mixed salts at 900℃ was studied.It was found that the silicon-aluminizing diffusion coating effectively improved the hot corrosion resistance of TiAl alloy in pure Na2SO4 salt,but failed to prevent the corrosion ofNa2SO4:NaCl mixture salts.This was due to the chlorination and oxidation autocatalytic reactions between Cl-and metal elements in the silicon-aluminizing diffusion coating,which greatly accelerated the failure of the coating.