Microstructure formation and interface characteristics of directionally solidified TiAl-Si alloys in alumina crucibles with a new Y(2)O(3) skull-aided technology

The microstructure evolution and interface characteristics of a directionally solidified Ti-43Al-3Si (at.%) alloy in an alumina (Al(2)O(3)) crucible with new Y(2)O(3) skull-aided technology were investigated. The Y(2)O(3)-skull that is in contact with the TiAl-melt is relatively stable, which results in a more controlled reaction between the skull and the melt than in the case of an Al(2)O(3) crucible is used. A thin reaction layer was formed between the mould and the melt through mutual diffusion. The layer thickness increased with increasing reaction time. The thickness of this layer was less than 80 μm for reaction times up to 5800 s. Y(2)O(3) particles were not found in the specimen because the mould coating was prepared with fine Y(2)O(3) powder without a binder, which prevented the Y(2)O(3) particles splitting from the coating as a consequence of thermal physical erosion. The oxygen content of the TiAl-alloy increased with increasing reaction time. The total oxygen content of the solidified specimen was less than that of the specimen solidified in the Al(2)O(3) crucibles. This new Y(2)O(3) skull-aided technology is expected to improve the surface quality of TiAl-alloys and reduce the reaction between the crucible/mould and molten TiAl alloys during directional solidification processing with longer contact times.