Alloy development and shell mould casting of gamma TiAl

Recent studies of the properties and shell mould casting of gamma TiAl-based alloys in the authors’ laboratory are summarised in this paper. As-cast and heat treated ternary alloys containing 44.2–47 at.% Al and 0–8 at.% Nb were investigated for oxidation resistance at 900 °C and crack propagation behaviour during cyclic thermal fatigue test between 900 and 15 °C. Grain refinement through cyclic heat treatment and machinability of the material during grinding were also studied. It was found that alloys with about 5 at.% Nb content show the best balanced properties. An alloy based on Ti–46.5Al–5Nb was then prepared and its mechanical properties measured. A model component was successfully cast using a zirconia-based shell mould. Examination of the castings shows that the thickness of the surface contamination layer is typically less than 80 μm.