New strategy for lamellar fragmentation of TiAl alloy: Integrating dynamic and static recrystallization

Obtaining a homogeneous and refined microstructure is the most important prerequisite for the engineering application of TiAl alloys. However, refining the microstructure through thermal deformation is challenging due to the presence of residual lamellar colonies resulting from incomplete dynamic recrystallization. In this paper, a new perspective on refining the coarse lamellar structures of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloys based on the cooperation of static and dynamic recrystallization is proposed. Firstly, incomplete dynamic recrystallization was induced by rapid extrusion. Secondly, the extruded TNM alloy was annealed to stimulate static recrystallization in the deformed region driven by strain energy to achieve complete homogenization of the extruded microstructure. The results show that the recrystallization fraction of the extruded TNM alloy is 66.7 %, with a structure consisting of residual γ lamellae, fragmented α grains and elongated β phase. The inhomogeneous streamlined microstructure demonstrates rapid static recrystallization kinetics early in the annealing process. As annealing progresses, the recrystallization region gradually extends inward from the boundary of the residual lamellae through the strain-induced grain boundary migration mechanism. After 4 h of annealing, the inhomogeneous microstructure was completely transformed into a uniform and fine microstructure consisting of equiaxed α, γ, and β grains. The dependence of the recrystallization behavior on the orientation of the lamellar structure is discussed, and the phase transformations observed during annealing and their influence on the recrystallization process are analyzed.