On the massive transformation in TiAl-based alloys

As one of the important phase transformation modes in TiAl-based alloys, massive transformation (MT) is of great importance for microstructure control. It has been successfully ultilized for refining a coarse lamellar structure with a colony size ∼1000 μm to a fine one of ∼10 μm without any thermomechanical process. In this paper, various controlling factors and the underlying mechanisms of MT in Ti–46Al–2Cr–2Nb and Ti–48Al–2Cr alloys are studied. It is found that high cooling rate is needed for implementation of this transformation, but further increasing cooling rate may induce the competing transformation of α→α2 and result in a decrease in the volume fraction of the MT product (γm). Solution treating temperature and time before quenching should be high and long enough for the formation of γm, but too long time holding at high temperature would result in fewer γm. The MT takes place more easily in those alloys with a fine initial microstructure than in those with a coarse one. The γm can be observed in the present alloy with only 46Al but alloyed with Nb and Cr even after oil quenching. It is emphasized that the controlling factors including cooling rate, solution treating temperature and time, initial microstructure, and chemical composition are not independent from each other, but mutually related.