α lath precipitation induced γ nanotwin in a cast (Nb,Ta)-containing γ-TiAl alloy

This study comprehensively studied the precipitation of α laths and formation of induced γ nanotwins in a cast Ti-48Al-3Nb-1.5Ta (at.%) alloy tempered at the α+γ phase region. The evolution of the microstructure and its corresponding atomic mechanisms were investigated by transmission electron microscopy. The results show that α phase precipitates as intersected laths, nucleates at stacking faults, and grows by a diffusion-controlled ledge-kink-terrace mechanism. The precipitation of α lath introduces high stress concentrations at the γ/α interface, produces dislocations and stacking faults, and induces the formation of γ nanotwins. The γ nanotwins nucleate at stacking faults emitted from the γ/α interface and grow by Frank partials climbing, Schockley partials gliding, and several new mechanisms related to non-planar reactions of dislocations and annihilation of antiphase boundaries. The γ nanotwins are limited to submicrons due to the lack of driving force. High temperatures promote the formation of γ nanotwins by activating non-planar reactions of dislocations and atom diffusion. In addition, Nb and Ta can promote α lath precipitation and γ nanotwin formation by reducing the stacking fault energy and preferring stacking fault emission. In summary, these findings provide new insights into α lath precipitation and γ twinning mechanisms, which are relevant as theoretical bases for microstructure control and refinement of cast γ-TiAl alloys.