Interactions between γ-TiAl melt and Y2O3 ceramic material during directional solidification process

A gamma-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.