Interactions between γ-TiAl melt and Y 2O 3 ceramic material during directional solidification process

Abstract

A γ-TiAl alloy with nominal composition of Ti-47%Al (molar fraction) was directionally solidified in an alumina mould with an Y 2O 3 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 Y 2O 3 protective coating exhibits an effective barrier capability to avoid direct contact between the mould base material and the TiAl melt, although the Y 2O 3 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 Y 2O 3 inclusions were dispersed in the metal matrix. The reason for this metal contamination is the Y 2O 3 coating dissolution by the TiAl melt. One mode of the interaction between Y 2O 3 and the TiAl melt is dissolution of yttrium and atomic oxygen in the melt by reaction Y 2O 3(s)=2Y (in TiAl melt)+3O (in TiAl melt). Both the extent of alloy contamination and the volume fractions of Y 2O 3 inclusions depend on the melting temperature and the interaction time.