Phase transformations of the L12-Ti3Al phase in γ-TiAl alloy

A new phase L1(2)-Ti3Al was found in the Ti-46Al-6Nb alloy after rapid-cooling and then annealing at 850 degrees C for 5 h. The transformation kinetics involving the L1(2) phase were observed by transmission electron microscopy (TEM): The plate-like L1(2) phase precipitates in the massive gamma (gamma(m)) phase and then partially transforms into alpha(2) or B2 phase; the transformations of bulk alpha(2) (alpha(b)) -> L1(2)/L1(2) -> gamma were also observed to form a fine lamellar structure with an average interlamellar spacing <20 nm. The orientation relationships between gamma, alpha(2) and L1(2) phases were confirmed by TEM observation and crystallographic calculation, which are (001)gamma // (001)L1(2),[010]gamma // [010]L1(2),[100])gamma // [100]L1(2) and (0001)alpha(2) // {11}L1(2), < 11<(2)over bar>0 > alpha(2) // < 1 (1) over bar0 > L1(2). The L1(2) phase was also observed in Ti-the 47AI alloy after the quenching-annealing process. Thermodynamic analysis of the transformations indicates that the gamma -> L1(2) transformation is extremely susceptible to temperature changes while transformations between the alpha(2) and L1(2) phases are not. (C) 2017 Elsevier Ltd. All rights reserved.