A study of refractories as crucible and mould materials for melting and casting γ-TiAl alloys

An assessment has been made of the potential use of mullite (3Al(2)O(3). 2SiO(2)), zirconia (ZrO2), alumina (Al2O3) and yttria (Y2O3) for making crucibles and ceramic moulds for gamma -TiAl alloys. Small refractory crucibles have been made by slip casting or pressing techniques and used to melt a gamma -TiAl (Ti-48at%Al-2at%Nb-2at% Mn) alloy in a carbon resistance furnace. The effects of different refractories, volume fractions of open porosity and melting time on interactions and microstructures have been established. The width of the interaction region versus the fielding time at 1550 degreesC can be described by a linear law for the mullite crucible and by parabolic laws for the zirconia, alumina and yttria crucibles. Based on these small-scale tests, it can be concluded that mullite and zirconia are unsuitable as crucible and mould materials for melting and casting gamma -TiAl alloys, whereas both pure alumina and yttria show promise.