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 (gamma)-(gamma) interfaces, the respective orientation groups of the (gamma) phase, and the distribution of orientational variants. From the results, three sequences of lamellar transformation have been identified with decreasing temperature: (1) a hightemperature heterogeneous transformation characterized by the nucleation of isolated primary (gamma) lamellae mostly belonging to the same orientation group and having locally the same order; (2) a low-temperature homogeneous transformation in the ordered (alpha)2 phase characterized by the formation of a fine lamellar structure with an even distribution of the orientation groups and a random ordering of (gamma) lamellae; and (3) a coherent interfacial transformation at the (alpha)2/ (gamma) 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 (gamma) lamellae involved in these transformations are discussed.