Low-temperature superplasticity of beta-stabilized Ti-43Al-9V-Y alloy sheet with bimodal gamma-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 degrees C was systematically investigated in the temperature range of 750-900 degrees C under an initial strain rate of 10(-4) s(-1). A bimodal gamma grain-distribution microstructure of TiAl alloy sheet, with abundant nano-scale or sub-micron gamma laths embedded inside beta matrix, exhibits an impressive superplastic behaviour. This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m = 0.27 at 800 degrees C, which is the lowest temperature of superplastic deformation for TiAl alloys attained so far. The maximum elongation reaches similar to 360% at 900 degrees C with an initial strain rate of 2.0 x 10(-4) s(-1). To elucidate the softening mechanism of the disordered beta phase during superplastic deformation, the changes of phase composition were investigated up to 1000 degrees C using in situ high-temperature X-ray diffraction (XRD) in this study. The results indicate that beta phase does not undergo the transformation from an ordered L2(0) structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant. Based on the microstructural evolution and occurrence of both gamma and beta dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD), the superplastic deformation mechanism can be explained by the combination of DR and grain boundary slipping (GBS). In the early stage of superplastic deformation, DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature. Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization, which further effectively suppresses apparent grain growth during superplastic deformation. At the late stage of superplastic deformation, these equiaxed grains make GBS prevalent, which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation. This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy. (C) 2021 Published by Elsevier Ltd on behalf of Chinese Society for Metals.

Automated summary

This study systematically investigated the superplastic behavior of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 degrees C in the temperature range of 750-900 degrees C. The inhomogeneous microstructure showed low-temperature superplasticity at 800 degrees C, achieving the lowest temperature of superplastic deformation for TiAl alloys so far. Through a combination of dynamic recrystallization and grain boundary slipping mechanisms, enhanced superplastic deformation and improved elongation were achieved.