Abstract: This study comprehensively studied the precipitation of αlaths and formation of induced γnanotwins in a cast Ti-48Al-3Nb-1.5Ta (at.%) alloy tempered at the α+ γphase region. The evolution of the microstruc-ture and its corresponding atomic mechanisms were investigated by transmission electron microscopy. The results show that αphase precipitates as intersected laths, nucleates at stacking faults, and grows by a diffusion-controlled ledge-kink-terrace mechanism. The precipitation of αlath introduces high stress concentrations at the γ/ αinterface, produces dislocations and stacking faults, and induces the formation of γnanotwins. The γnanotwins nucleate at stacking faults emitted from the γ/ αinterface 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 γnanotwins are limited to sub-microns due to the lack of driving force. High temperatures promote the formation of γnanotwins by activating non-planar reactions of dislocations and atom diffusion. In addition, Nb and Ta can promote αlath precipitation and γnanotwin formation by reducing the stacking fault energy and preferring stacking fault emission. In summary, these findings provide new insights into αlath precipitation and γtwinning mechanisms, which are relevant as theoretical bases for microstructure control and refinement of cast γ-TiAl alloys.

Key words: Cast γ-TiAl alloys, Transmission electronic microscopy, Phase transformation, Nanotwinning Tantalum