Study On High Temperature Resistance And Surface Coating Of Ti-Al Intermetallic Compound Based Alloys

Ti-Al intermetallic compound based alloys are new type high temperaturestructural materials applied for aerospace and aircraft industry because of their lowdensity, high strength and good creep resistance. The problem of poor ductility ofthese materials at room temperature has been mostly solved by alloying andmicrostructure controlling. Thus the problem of farther improving high temperatureoxidation resistance has become one of the most important research subjects. Firstly, the high temperature oxidation resistance of the Ti-Al intermetalliccompound based alloys at 650℃, 750℃ and 850℃ were evaluated respectively. Theprocess and mechanism of the oxidation of the alloys were studied. The effect ofalloying elements on high temperature oxidation resistance of the alloys wasdiscussed. Then the special enamel coating on the surfaces of the γ-TiAl and Ti2AlNbbased alloys was performed. The effect of such coating on high temperature oxidationresistance was investigated. The results obtained in the present study are as follows: γ-TiAl based alloy with composition of Ti-46.5Al-2.5V-1.0Cr(at%) showedexcellent oxidation resistance at 650℃. The mass gain became obviously at 750℃,and the oxidation layer lost its protection function while the temperature reached to850℃. Polyhedral and regular crystals piled on the surfaces of the oxidation samplesafter high temperature oxidation. The reaction products mainly consisted of TiO2 andAl2O3 oxides. The oxidation scale was composed of a series of thin layers which haddifferent structural characteristics, and the oxidation process could be classified intothree steps approximately. V element had disadvantageous effect on the hightemperature oxidation resistance of theγ-TiAl based alloy. The most important reasonwas the melting point of V2O5 only being 690℃, which could be volatilized as liquidstate if the temperature exceed this value and led to disastrous oxidation. The contentof Cr element in the alloy was too low to forming continuous and compact Al2O3 film,and thus decreased the oxidation resistance by impurity function. Ti-Al-Nb based alloys with compositions of Ti-23Al-17Nb(at%) and Ti-22Al-25Nb(at%) showed good oxidation resistance at 650℃, 750℃ and 850℃. The high —Ⅲ—北京工业大学工学硕士学位论文temperature oxidation products mainly consisted of TiO2 and Al2O3 oxides, but theoxides of NbO2, Nb2O5 and AlNbO4 were fewer. The addition of Nb can increase theactivity of Al element, thereby reduce the required concentration of Al that formedAl2O3 on the surfaces of the alloys. The existence of Al-riched in outer layer couldlead to the Al-depleted phenomena in the inner part of the oxidation scale. The contentof Nb affected the oxidation resistance greatly in Ti-Al-Nb intermetallics based alloys,because a moderate content of Nb could decrease the mobility of Ti atoms, increasedthe oxygen partial pressure for the formation of titanium oxides, and also reduced themetallic ion-excess structural defects of TiO2, thus Al2O3 could be selectivelyoxidized much adequately. However, the excessive addition of Nb resulted in theoccurrence of metallic ion-vacancy and interstitial ions, which could lead to the oxidesof Nb formed more easily, and thus accelerated greatly the speed of oxidation. The oxidation mass gains were dropped for Ti-46.5Al-2.5V-1.0Cr(at%) andTi-22Al-25Nb(at%) alloys after the special enamel coating performed. This enamelcoating improved the oxidation resistance remarkably, especially for γ-TiAl basedTi-46.5Al-2.5V-1.0Cr(at%) alloy. Therefore, the enamel coating should to have protectivefunction.