Phase transformations in TiAl based alloys

Microstructural characteristics of a fully lamellar Ti49Al47Cr2Nb2 alloy have been investigated in different annealed conditions by quantitative transmission electron microscopy. Statistical analyses have yielded clear information about the γ–γ interfaces, the respective orientation groups of the γ phase, and the distribution of orientational variants. From the results, three sequences of lamellar transformation have been identified with decreasing temperature: (1) a high-temperature heterogeneous transformation characterized by the nucleation of isolated primary γ lamellae mostly belonging to the same orientation group and having locally the same order; (2) a low-temperature homogeneous transformation in the ordered α2 phase characterized by the formation of a fine lamellar structure with an even distribution of the orientation groups and a random ordering of γ lamellae; and (3) a coherent interfacial transformation at the α2/γ interfaces characterized by the nucleation of ultra-fine twin related lamellae. Finally, the driving forces for these various transformations as well as the nucleation mechanisms of γ lamellae involved in these transformations are discussed.