On the massive transformation in TiAl-based alloys

As one of the important phase transformation modes in TiAl-based alloys. ma ssive transformation (MT) is of great importance for microstructure control . It has been successfully utilized for refining a coarse lamellar structur e with a colony size similar to 1000 mum to a fine one of similar to 10 mum without any thermomechanical process. In this paper, various controlling f actors and the underlying mechanisms of MT in Ti-46Al-2Cr-2Nb and Ti-48Al-2 Cr alloys are studied. It is found that high cooling rate is needed for imp lementation of this transformation, but further increasing cooling rate may induce the competing transformation of alpha --> alpha (2) and result in a decrease in the volume fraction of the MT product (gamma (m)). Solution tr eating temperature and time before quenching should be high and long enough for the formation of gamma (m), but too long time holding at high temperat ure would result in fewer gamma (m). The MT takes place more easily in thos e alloys with a fine initial microstructure than in those with a coarse one . The gamma (m) can be observed in the present alloy with only 46Al but all oyed with Nb and Cr even after oil quenching. It is emphasized that the con trolling factors including cooling rate, solution treating temperature and time, initial microstructure, and chemical composition are not independent from each other, but mutually related. (C) 2001 Elsevier Science Ltd. All r ights reserved.