Microstructure evolution and grain refinement during hot deformation of Ti-44Al-4Nb-1.5Cr-0.1Mo-0.1B alloy

Abstract

The principle of the forging and rolling combined process (FRCP), along with the major microstructure evolution features and grain refinement mechanism of Ti–44Al–4Nb–1.5Cr–0.1Mo–0.1B alloy, was investigated thoroughly in this study. A crack-free rolled sheet characterized by a bimodal microstructure with an average grain size of approximately 3.52 μm was produced after FRCP. The microstructure per deformation stage was further examined via transmission electron microscope (TEM) observation and electron backscatter diffraction (EBSD) analysis. The grain size of the material gradually decreased from 120 μm in the initial structure to 3.52 μm during hot deformation. Dynamic recrystallization (DRX) and deformation twinning should account for continuous grain refinement during FRCP. The refinement effect of the microstructure was quantitatively evaluated by explaining the mechanism of the grain boundary distribution. The fraction of the high-angle grain boundaries induced by DRX exceeded 80%, thereby indicating that the nucleation and growth of DRX were the dominant mechanisms for grain refinement. Annealing at 1150 °C after forging stimulated the activity of 89 ± 3° misorientation angles mostly located within the matrix grains. Dislocation slipping and deformation twinning played synergistic roles in the refinement process. Several possible mechanisms for microstructure refinement via deformation twinning were identified, including twin intersection, formation of secondary twins, and parallel twinning.