Research On High Temperature Oxidation And Hot Corrosion Resistance Properties Of The Al-Y Gradient Coating On The Surface Of ?-TiAl Alloy

?-TiAl alloy has its own unique advantages,like high specific strength,high specific modulus and excellent creep resistance.Therefore it is expected to replace the nickel-based and iron-based super alloy to be high temperature structural materials in aerospace industry,weapon industry and civil industry.However,the application of this alloy is limited due to the lack of high temperature oxidation resistance above 800 ? and insufficient hot corrosion performance in molten salt environment.By using the radio frequency?RF?magnetron sputtering technology and vacuum diffusion technology,the Al-Y gradient coating on the ?-TiAl alloy surface is prepared to improve the high temperature oxidation resistance and hot corrosion resistance of ?-TiAl alloy.The optimal parameters of RF magnetron sputtering to prepare Al-Y deposition layer are listed as follows: sputtering power is 200 W,working pressure is 4 Pa,time is 4 h,distance between target and work-piece is 25 mm,and then vacuum diffusion process at 600 ? for 10 h is conducted to prepare Al-Y gradient coating.SEM was used to detect the morphology of the oxidation film,EDS was used to detect the element distribution and XRD was used to identify the phase.It turns out that the Al-Y gradient coating is uniform and compact without any hole and crack.The surface phases consist of Al,YAl3,Al2O3 and Y2O3.The thickness of coating layer including an outer layer and an Al rich diffusion layer is about 60 ?m.Moreover,a metallurgy bonding is achieved between the substrate and the coating layer,and the bonding force is around 41.8 N.High temperature oxidation tests were carried out at 750 ?,850 ?,950 ? for 100 h.The results show that the Al-Y gradient coating forms a three layer structure including a surface dense oxide layer,an intermediate oxide layer,a gradient diffusion layer of an aluminum rich phase at temperature of 750 ? and 850 ?.This structure can not only provide an Al source for the formation of protective Al2O3 film on the surface,but also enhance the bonding between the coating and the substrate.Furthermore,the presence of Y can also slow down the oxidation rate and increase the adhesion of the oxide layer,which makes the Al-Y gradient coating exhibit excellent oxidation resistance.At 950 ?,a Al2O3 based oxide layer is consumed.It can be attributed to the structure of the gradient diffusion layer and the presence of the active element Y.Large area spalling is not occurred at the entire oxide layer and substrate and a dense Al2O3 layer forms at the interface between coating and substrate.Therefore,Al-Y gradient coating exhibits excellent oxidation resistance at high temperature.Hot corrosion tests in Na₂SO₄ salt of Al-Y gradient coatings were conducted at different temperature?750 ?,850 ?,950 ??holding for various time?5 h,10 h,20 h,50 h?.The results indicate the different corrosion mechanism at different temperature.For low temperature hot corrosion,the presence of Na₂SO₄ salt accelerates the oxidation of ?-TiAl alloy.The dense protective oxide layer on the surface of Al-Y gradient coating can effectively block the diffusion of O and S atoms into the matrix,resulting in the oxidation protection for ?-TiAl alloy.For high temperature hot corrosion,sulfide and oxidation occur on ?-TiAl alloys,accompanied by alkaline melting in the Al2O3 layer.It leads to the catastrophic corrosion failure of matrix alloys.More importantly,the Y elements play a key role in preventing matrix and coating corrosion.It can not only preferentially react with S,reducing the consumption of Al element in the early corrosion stage,but also capture the S element diffusing into the matrix.And also the existence of gradient structure in Al-Y gradient coating effectively slow down the occurrence of hot corrosion.Therefore,Al-Y gradient coating exhibits excellent thermal corrosion resistance.