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

The conversion of the cast microstructure of a Ti-45Al-10Nb (at.%) ingot du e to extrusion and subsequent heat treatments was investigated using scanni ng and transmission electron microscopy. The starting lamellar microstructu re 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 lame llae varied from one area to another. Recrystallization and phase boundary bulging were found to be the major mechanisms responsible for lamellar glob ularization. The existence of remnant lamellae was ascribed to the anisotro pic how behavior of the starting lamellar colonies. Microstructural and pro cess variables that influence the scale of remnant lamellae will be elucida ted. Annealing in the (alpha + gamma) two-phase held cannot eliminate the r emnant lamellar structures. However, the detrimental effect of remnant lame llae can be minimized after alpha treatments provided that the remnant lame llae are smaller than about 80 mu m. (C) 2000 Acta Metallurgica Inc. Publis hed by Elsevier Science Ltd. All rights reserved.