Formation Of Siliconized Coating On TiAl-Based Alloy And Mechanism For Its Resisting High Temperature Oxidation

TiAl-based alloy was taken as an ideal candidate material for high temperature structural application in aerospace and automobile indursity, owing to its low density, high strength at elevate temperatures and good creep resistance. With the improvement of room temperature ductility, insufficient oxidation resistance above 750℃has become one of the main barriers to its wider applications at elevated temperatures. Pack siliconization is one of effective methods to improve the high temperature oxidation resistance of TiAl-based alloy.This paper discussed the effect of siliconization medium, time and temperature on the structure of siliconization coating and high temperature oxidation resistance, formation mechanism of siliconization coating and its evolution during high temperature oxidation, and high temperature oxidation behavior of siliconized alloy. The results are as follows.The high temperature oxidation resistance of TiAl-based alloy was greatly improved by pack siliconization. The one, siliconized at 1250℃for 2 h with the cementation of 85%Al₂O₃+15%Si, exhibits superior oxidation resistance. The weight gain is only 0.3125 mg·cm^-2 after oxidization at 900℃for 1000 h.The structures of siliconized coating are similar when siliconized at different temperatures from 1050 to 1250℃and for different hours from 0.5 to 8 h. The siliconized coating is composed of two layers. The outer is a continious and compact Al₂O₃ layer and the inner is a Ti₅Si₃-based layer in which some Al₂O₃ particles dispersed. Between Ti₅Si₃-based layer and TiAl substrate, an Al-riched layer formed. With increasing siliconized temperature and prolonging the time, the Al₂O₃ layer becomes more compact and Ti₅Si₃-based layer thicker and more adherent to TiAl substrate.During siliconization at high temperature, Si atoms in cementation will diffuse into TiAl substrate. On the concentration of Si reaching its solubility in TiAl substrate, Ti₅Si₃ will precipitate. At the same time, the Al atoms will be substitued by Si atoms from TiAl substrate. Some of the Al atoms will diffuse inward into TiAl substrate to form Al-rich layer, some of them diffuse outward to the surface to react with the O in furface to form Al₂O₃, which will be combined with the Al₂O₃ in the cementation to form the outer Al₂O₃ layer, and the others will react with residual O in alloy matrix to form Al₂O₃ particles in Ti₅Si₃-based layer.The coefficients of thermal expansion (CTE) of Al₂O₃, Ti₅Si₃ and TiAl were measured by high temperature X-ray diffraction. The results showed that the CTE of alloyed Ti₅Si₃ is between that of Al₂O₃ and TiAl substrate. Such coating structure reduces cracking tendency of the coating during high temperature cyclic oxidation.The high temperature oxidation curves of siliconized TiAl alloy were fitted by the least square method. The results showed that oxidation curve of siliconized alloy consist of three regions, the parabolic, the linear and the quadratic. For the siliconized specimen which exihibits superior oxidation resistance, only parabolic region appeared during the whole oxdation test up to 1000 h with an oxidation rate Constance K_p = 6.03×10^-5 mg²Â·cm^-4·h^-1.The siliconized coating exihibits some ductility and self-curability during the oxidation test with imposing of stress and strain. To some extent, the siliconized coating can relax the stress induced by slight deformation. Furthermore, even though crack occured in the siliconized coating during larger deformation, the inner Al-rich layer would form protective Al₂O₃, which prohibit the further oxidation of TiAl substrate.