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., (α₂+γ) alloy and (β/B2+γ) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 °C under an initial strain-rate of 10⁻⁴ s⁻¹to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached ∼400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (β/B2+γ) alloy, while grain boundary sliding was responsible for the large elongation of (α₂+γ) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of β/B2 phase on cavitation behavior. It was found that in (α₂+γ) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (β/B2+γ) alloy were located along the β/γ interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (β/B2+γ) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (α₂+γ) alloy. Moreover, the large cavities in (β/B2+γ) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (α₂+γ) 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 γ-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.