Grain refinement and property assessment in low and high Nb-content cast tial-based alloys

The problems associated with producing pore-free castings of TiAl-based alloys are reviewed and recent progress in improving the quality of castings discussed. The influence of the plastic anisotropy of fully lamellar TiAl alloys on their yielding behaviour is highlighted since castings generally have structures which are near fully lamellar. Similarly the influence of borides on the ductility of castings is discussed since these are formed in castings which are grain refined by addition of boron. On the basis of these considerations it is suggested that a new approach is required if fine-grained cast components are to be manufactured which have adequate and reproducible properties. Such an approach requires improved casting technologies and improved TiAl alloys with low Al contents (about 46at%) for high strength and high alloying addition (such as 8at%Nb) to provide adequate oxidation resistance and high temperature strength. Recent work has shown that it is possible to produce pore-free castings. In addition it has been shown that the microstructure of B-free cast alloys such as Ti48Al2Cr2Nb can be refined by tempering massively transformed samples in the alpha plus gamma two phase region so th t the brittle borides present in samples grain refined through addition of B are eliminated. TiAl-based alloys are being used for the manufacture of turbochargers in top-of-the-range automobiles and exhaust valves for formula 1 cars. (AU)