Microstructure evolution and fracture mechanism of a TiAl–Nb alloy during high-temperature tensile testing

Abstract

A TiAl–Nb alloy was prepared by casting, and its tensile properties and microstructure evolution at different temperatures were investigated. The results show that the brittle-ductile transition temperature (BDTT) of the alloy is 880–920 °C. At low temperatures, deformation mainly occurs in the γ lamellae and coarsening γ phase between the lamellar colonies. As the temperature increase in the transitory stage, a small number of dislocations may pass through the interface from the coarsening γ phase into the γ lamellae. During the brittle stage, the dislocation and dislocation array bow out from the phase interface, and twins are emitted from the phase interface, which may contribute to the accommodation of stress concentration. During the ductile stage, the γ lamellae coarsen due to the thinner and smaller α2 lamellae dissolving and phase boundary transfer. The dislocations interact with the dislocation walls to form subboundaries in the γ lamellae and transform into equiaxed crystals by dynamic recrystallization (DRX), which increases the plasticity of the alloy. With the transition from the brittle model to the ductile model, the fracture mode of the alloy changes from the mixed fracture mode of transgranular fracture and translaminar fracture to intergranular fracture.