Superplastic deformation mechanism of a γ-TiAl alloy with coarse and bimodal grain structure

The superplasticity of a Ti-42.5A1-8Nb-0.2W-0.2B-0.1Y (at.%) alloy with similar to 20 vol% metastable beta/B2 phase has been examined at temperature range of 850-1050 degrees C under an initial strain-rate of 10(-4) s(-1). The results showed that though the proposed alloy has a coarse and bimodal grain structure which was unfavourable for fine-structure superplasticity, impressive elongation was obtained at 1000 degrees C (similar to 380%) with a strain-rate sensitivity exponent value of about 0.46. The microstructural characterization showed that remarkable dynamic recrystallization occurred during superplastic tension, and the initial texture-free specimen gradually produced a < 100 > fiber texture whose the main component was cube texture. All these observations seem to be contradictory to the classical theory for fine-structure super plasticity (grain boundary sliding mechanism). Instead, it was suggested that the major deformation mechanism in this alloy was crystallographic slip accompanied by discontinuous dynamic recrystallization. (C) 2017 Elsevier B.V. All rights reserved.