Investigation on the Hot Deformation Behaviors and Globularization Mechanisms of Lamellar Ti-6Al-4V Alloy within a Wide Range of Deformation Temperatures

Abstract

In this study, the hot deformation behavior and microstructural evolution of Ti-6Al-4V alloy having a lamellar initial microstructure were systematically investigated within a wide range of deformation temperatures (700~950°C). Flow softening was commonly observed in the true stress-strain curves and the amount of flow softening gradually decreased with increasing deformation temperature. Based on the microstructure characterization and quantitative analysis of the deformed microstructures, the deformation temperatures were roughly divided into two regions, namely region I (900~950°C) and region II (700~900°C). By a detailed transmission electron microscopy (TEM) analysis, different globularization mechanisms of α phase were found for the two regions. At relatively higher deformation temperatures (region I), α lamellae sub-division and grain growth was determined to be the main globularization mechanism. At relatively lower deformation temperatures (region II), on the other hand, dynamic recrystallization (DRX) of α lamellae was confirmed to be the dominant globularization mechanism. The DRX of α lamellae firstly occurred either along the colony boundaries or at the lamellae kinking areas, and then expanded to the interiors of α lamellae. In addition, nano-twinning also acted as a secondary globularization mechanism of α lamellae in region II. Finally, it was found that the presence of α lamellae enhanced the DRX of β phase, by providing a strain partitioning between soft β phase and hard α phase as well as reducing the size of β phase.