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 degreesC and crack propagation behaviour during cyclic thermal fatigue test between 900 and 15 degreesC. 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 mum. (C) 2002 Elsevier Science B.V. All rights reserved.