Recovery, recrystallization and phase transformations during thermomechanical processing and treatment of TiAl-based alloys Original Research Article

The conversion of the cast microstructure of a Ti–45Al–10Nb (at.%) ingot due to extrusion and subsequent heat treatments was investigated using scanning and transmission electron microscopy. The starting lamellar microstructure was broken down by dynamic recrystallization into a fine-grained duplex microstructure after extrusion, however, a small amount of remnant lamellae was retained. It was found that the fine substructures in the remnant lamellae varied from one area to another. Recrystallization and phase boundary bulging were found to be the major mechanisms responsible for lamellar globularization. The existence of remnant lamellae was ascribed to the anisotropic flow behavior of the starting lamellar colonies. Microstructural and process variables that influence the scale of remnant lamellae will be elucidated. Annealing in the (α+γ) two-phase field cannot eliminate the remnant lamellar structures. However, the detrimental effect of remnant lamellae can be minimized after alpha treatments provided that the remnant lamellae are smaller than about 80 μm.