Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Abstract

Hot deformation behavior, microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach. The true stress–strain curves exhibited typical work hardening and flow softening, in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon (Z) parameter, and the activation energy was calculated to be 446.9 kJ/mol. The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation. The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy. The coarse recrystallized γ and several massive γ phase were observed at grain boundaries. While in the case of Nb segregation, β/B2 phase harmonized bending of lamellae, combined with the growth of recrystallized γ grains and α + β + γ → α + γ transition under conditions of temperature and stress, leading to the breakdown of α2/γ lamellar colony. During the hot compression process, gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism, leading to extensive γ twins and cross twins in α/γ lamellae and at grain boundaries. In general, homogeneous microstructure during the hot deformation process can be obtained in TiAl alloy with high Nb addition and low Al segregation. The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.