Abstract:A γ-TiAl alloy with nominal composition of Ti−47%Al (molar fraction) was directionally solidified in an alumina mould with an Y₂O₃ protective coating. The effects of processing parameters (melting temperature and interaction time) on the metal-coating interface, microstructure and chemical composition of the alloy were evaluated. The result shows that the Y₂O₃ protective coating exhibits an effective barrier capability to avoid direct contact between the mould base material and the TiAl melt, although the Y₂O₃ coating is found to suffer some erosion and be slightly dissolved by the molten TiAl due to the coating-metal interactions. The directionally solidified alloys were contaminated with Y and O, and Y₂O₃ inclusions were dispersed in the metal matrix. The reason for this metal contamination is the Y₂O₃ coating dissolution by the TiAl melt. One mode of the interaction between Y₂O₃ and the TiAl melt is dissolution of yttrium and atomic oxygen in the melt by reaction Y₂O₃(s)=2Y (in TiAl melt)+3O (in TiAl melt). Both the extent of alloy contamination and the volume fractions of Y₂O₃ inclusions depend on the melting temperature and the interaction time.

Key words: TiAl alloys; Y₂O₃ protective coatings; interactions; directional solidification