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 degrees 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 mu m showed the maximum elongation of 1280% at 970 degrees 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 alpha(2)/alpha(2) grain boundaries and/or alpha(2)/beta phase boundaries without noticeable dislocation activity inside the alpha(2) grains of the superplastically deformed specimen. Severe deformation was also observed in the soft beta phase. Adjacent alpha(2) 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 beta and alpha(2) 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. (C) 1999 Elsevier Science S.A. All rights reserved.