Superplastic deformation mechanisms of high Nb containing TiAl alloy with (α2 + γ) microstructure

In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (alpha 2 + gamma) microstructure, Ti-43.5Al-8Nb-0.2W-0.2B (at.%), has been examined and studied by means of hot tension from 850 degrees C to 1050 degrees C under an initial strain rate of 10(-4) s(-1). The mechanical behaviour and microstructure evolution have been characterized and analyzed. Besides, to gain insight into deformation mechanisms, the texture evolution during deformation at ordinary (non-superplastic) and superplastic conditions has been systematically studied. The results showed that, the alloy exhibited impressive superplastic elongation at 1000 degrees C with a strain-rate sensitivity exponent (m) of about 0.5 and an apparent activation energy (Q(app)) value of about 390 KJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 degrees C), severe grain refinement occurred and the fraction of low-angle grain boundary notably increased. Meanwhile, the textures were characterized by <100> and <111> double-fiber components parallel to the tensile direction. All these observations suggested a dislocation slip and twinning mechanism. However, if deformed at the superplastic condition (1000 degrees C), it was found that the microstructure was fairly stable in terms of grain size, morphology and grain boundary characteristics during tension, but a continuous weakening of the initial <110> fiber texture (resulted from canned-forging) was observed. This was believed to be an indication of grain boundary sliding mechanism. Moreover, the deformation texture (<100> + <111>)-though is very weak-was simultaneously appeared. According to a detailed discussion on the deformation kinetics and microstructure evolution, it was believed that the slip/twinning-accommodated grain boundary sliding was responsible for superplastic deformation and the dislocation climb inside of gamma grains was the rate-controlling step. (C) 2016 Published by Elsevier Ltd.