Microstructure and texture evolution of high-temperature α phase in TiAl alloy during thermomechanical processing

Increasing demands on modern turbines require (α2+γ) lamellar-structured TiAl alloys with fine colony size and properly aligned lamellae. According to the α → α2+γ phase transformation, the lamellar structure depends directly on the high-temperature α phase. Thus, the lamellar structure optimization could be realized by the modification of high-temperature α phase through thermomechanical processing. In this work, a thorough investigation was conducted on the high-temperature α phase in two TiAl alloys in terms of the deformation behavior, dynamic recrystallization (DRX), grain growth and texture evolution.Under uniaxial compression, the DRX of the α phase is a continuous fragmentation process (CDRX) in three characteristic stages: ⅰ) serrations of grain boundary and formation of symmetrical-tilt boundaries with disorientation axis near boundary bulging regions; ⅱ) formation of subgrains by evolving symmetrical-tilt boundaries into asymmetrical-tilt boundaries with disorientation axis by absorbing basal dislocations, or tilt-twist boundaries with disorientation axis by rotational movements of the bulged parts; ⅲ) detachment of subgrains from the parent grain with gradually increased misorientation, and mixture with the ones fragmented from other grains by grain boundary sliding. These three processes happened repeatedly from grain boundary regions toward grain interiors until the whole initial microstructure was replaced by the DRXed one. The plastic deformation of the α phase is closely related to the crystallographic orientations of the initial α grains that play an important role in the deformation mechanism and the CDRX progress (softening). For the soft α grains, CDRX was completed quickly at a relatively small macroscopic strain by intragranular dislocation slip. The hard α grains demonstrated two ways: ⅰ) grains with //LD, in which dislocation accumulation was only assisted by the local strain accommodation with the neighboring α grains from boundary regions; ⅱ) grains with //LD, in which dislocation accumulation was achieved by kinking through basal slip and dislocation slip in the boundary regions from incompatible local strain. These grains required large strain to accumulate sufficient dislocations for CDRX. The strain-resolved contribution of the deformation (hardening) and CDRX (softening) result in the specific flow stress states. The texture evolution is mainly induced by dislocation slip. The orientations of the DRXed grains were largely inherited from those of the parents. With the deformation, the tilted basal fiber typed orientations developed in both the retained coarse α and the formed DRXed α grains. The hot compression produces refined α grains but not expected texture to align (α2+γ) lamellae. The microstructure and texture of the α phase during hot extrusion in the (α+β) phase region exhibit different features. The extruded microstructure was very heterogenous, comprising a large population of unRXed α grains and fine primary RXed α grains with //ED and a small population of grown α grains with //ED. The two texture components are beneficial for lamella alignment. Besides, the thermally-induced α → β phase transformation interweaving with the abnormal α grain growth happened during the transition from extrusion to water-quenching, producing two types of β particles: i) intergranular β particles with the Burger OR with their neighboring α grain; ii) intragranular β particles without the OR with the hosts from the intergranular β ones after being swallowed by the abnormally grown α grains.