Phase transformations in some TiAl-based alloys

Samples of a range of TiAl-based alloys have been cooled directly to room temperature at rates between 0.1 and 500 degreesC s(-1) in order to define the transformation behaviour during continuous cooling (CCT). In addition other samples have been cooled rapidly to predetermined temperatures where they have been held for times up to 18,000 s before cooling rapidly to room temperature in order to determine their time-temperature-transformation (TTT) behaviour. It has been found that the massive transformation occurs at the highest cooling rates used (500 degreesC s(-1)) in all the alloys studied apart from Ti-44Al-4Nb-4Zr-0.2Si-1B. In this alloy the high-temperature beta phase partially transformed during rapid cooling to lenticular alpha which, together with the remaining beta, was retained at room temperature. The effects of holding at selected temperatures were as anticipated from the CCT curves and the equilibrium diagrams. In all cases the room temperature tensile properties were improved for the finest microstructures- i.e. for the fastest cooling rates used, although with alloys with B addition (i.e. grain-refined alloys) the effect of cooling rate was less important. The changes in microstructure and changes in the tensile properties and hardness of samples which have been tempered after quenching have also been determined. Appropriate tempering of samples which had been cooled at a rate which caused them to transform massively gives rise to fine microstructures of intimately mixed equilibrium phases. In the case of Ti-48Al-2Nb-2Cr this leads to a mixture of convoluted alpha and gamma grains of about 50 mum (even although it contains no B and is therefore not grain-refined) and to a plastic elongation of 1.3% which is significantly better than the 0.5% found in coarse-grained air-cooled or furnace-cooled samples of this alloy. (C) 2002 Elsevier Science Ltd. All rights reserved.