Study Of Preparation And Oxidation Behavior Of Si-Al Co-deposition Coating On Ti-Al Based Alloy

Ti₂AlNb alloy is one of hot research topic of Ti-Al based alloy and a popular extremely important structural material for aerospace hot end components,due to its high specific strength,specific stiffness and good high temperature mechanical properties.However poor oxidation resistance seriously affects the practical application of the alloy at high temperatures.Therefore,Si-Al co-deposition coatings were prepared on the surface of this alloy to improve the high-temperature oxidation resistance of the alloy.In this paper,the influence of pack powder mixtures composition and temperature was analyzed on the equilibrium partial pressure that was concluded in the pack,and then the Si-Al co-deposition coatings were prepared through pack cementation.XRD,EDS,and SEM were used to analyze the morphology,constituent phase,and composition distribution of the co-deposition coatings.At the same time,the relationship between coatings characteristics and themochemical calculation was analyzed,and the effect of process parameters on the structure of the Si-Al co-deposition coatings was further elucidated.The cyclic oxidation experiments at 850°C and 900°C were performed respectively,comparing the Si-Al co-deposition coating with the aluminide coating and the silicide coating,Surface oxides and oxidation kinetics of these coatings after oxidation were analyzed.The main findings are as follows.The Si-Al co-deposition coatings could be formed by co-depositing Si and Al on the surface of the Ti₂AlNb alloy through pack cementation.By changing experimental parameters,Si-Al co-deposition coatings with silicide as the main constituent phase,or with aluminized as the main constituent phase could be prepared.The Si-Al co-depositon coatings with silicide as the main constituent phase were obtained at 950¹050°C/4h in the packs of2Al-5Si-2NH₄F-bal.Al₂O₃?wt.%?,and the Si-Al co-depositon coatings with aluminized as the main constituent phase were obtained at 950¹050°C/4h in the packs of4Al-?0.5,1?Si-2NH₄F-bal.Al₂O₃?wt.%?.The coatings prepared on the surface of Ti₂AlNb alloy after different processes were subjected to cyclic oxidation at 850°C for 800 h and 900°C for 180 h,and continuous scales was formed on the surfaces of Si-Al co-deposition coatings,and these coatings remained intact without peeling or cracking,etc.The main oxides on the Si-Al co-deposition coating with silicied as the main constituent phase are TiO₂,SiO₂,Al₂O₃;The main oxides on the Si-Al co-deposition coating with aluminide as the main constituent phase are TiO₂,Al₂O₃ and small amout of SiO₂;After oxidation at 850°C and 900°C,weight loss occurs in the Si-Al co-depositon coating with silicide as the main constituent phase,while weight gain occurs in the Si-Al co-depositon coating with aluminized as the main constituent phase though the its weight gain is still higher than aluminide coating's.