PHASE-DECOMPOSITION OF LIQUID-QUENCHED BETA-TYPE TI-CR ALLOYS

Phase decomposition behaviour of liquid-quenched-beta (b c c) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of beta to beta-1 (Ti-rich, b c c) + beta-2 (Ti-lean, b c c) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed beta-1 + beta-2 two-phase field expands towards higher temperatures than the predicted binodal line. The coherent beta-1 + beta-2 two-phase state exhibits the so-called <100> modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the beta-phase. We obtained time-temperature-transformation (TTT) diagrams of beta-type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is beta --> coherent beta-1 + beta-2 --> incoherent beta-1 + beta-2 --> incoherent beta-1 + beta-2 + grain boundary precipitates --> stable state of beta + TiCr2 or alpha + TiCr2. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.