Effect of β/B2 phase on cavitation behavior during superplastic deformation of TiAl alloys

Fine-grained Ti-(42.5-43.5) Al-8Nb-0.2W-0.2B-(0-0.1) Y (in at.%) alloys with two different phase constituents (i.e., (alpha(2)+gamma) alloy and (beta/B2+gamma) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 degrees C under an initial strain-rate of 10(-4) s(-1) to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached similar to 400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (beta/B2+gamma) alloy, while grain boundary sliding was responsible for the large elongation of (alpha(2)+gamma) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of beta/B2 phase on cavitation behavior. It was found that in (alpha(2)+gamma) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (beta/B2+gamma) alloy were located along the beta/gamma interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (beta/B2+gamma) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (alpha(2)+gamma) alloy. Moreover, the large cavities in (beta/B2+gamma) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (alpha(2)+gamma) alloy. The different cavity nucleation-growth-coalescence behavior of the two alloys was clarified based on the divergence in superplastic deformation mechanisms. Besides, theoretical evaluations were performed and revealed that the cavity growth rate of gamma-TiAl was much higher than ordinary Al alloys under similar deformation conditions. This was believed to be the main reason for the relatively lower superplastic elongations of TiAl-based intermetallics. (C) 2016 Elsevier B.V. All rights reserved.