Microscopic observation of superplastic deformation in a 2-phase Ti3Al–Nb alloy

Microscopic aspects of superplastic deformation in a 2-phase Ti3Al–Nb alloy were studied using transmission electron microscopy. A 5-kW radiant heating system capable of heating up 200°C per min was used to minimize possible microstructural changes during heating and cooling stages. A Ti–24Al–11Nb alloy with the grain size of 3.6 μm showed the maximum elongation of 1280% at 970°C under an initial strain rate of 10−3/s, which is the largest elongation ever reported in open literature. Localized dislocation activity was observed mainly along α2/α2 grain boundaries and/or α2/β phase boundaries without noticeable dislocation activity inside the α2 grains of the superplastically deformed specimen. Severe deformation was also observed in the soft β phase. Adjacent α2 grains, however, only contained some dislocations that were piled up at the region near the triple junctions. The observations on dislocation activities suggest that boundary sliding plays a major role in superplastic deformation of this alloy, together with a dominant accommodation mechanism via dislocation motion inside β and α2 grains. The effects of strain rate and grain size on deformed microstructure were also investigated, and finally, a possible superplastic deformation mechanism of 2-phase Ti3Al–Nb alloy is proposed.