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

Abstract

In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (α2 + γ) microstructure, Ti–43.5Al–8Nb–0.2W–0.2B (at.%), has been examined and studied by means of hot tension from 850 °C to 1050 °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 °C with a strain-rate sensitivity exponent (m) of about 0.5 and an apparent activation energy (Qapp) value of about 390 kJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 °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 °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 γ grains was the rate-controlling step.