Microstructural evolution of a hot-rolled Ti–40Al–10Nb alloy

Microstructure of a Ti–40Al–10Nb alloy, which was processed by conventional hot rolling, was investigated using optical metallography, scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of as-rolled Ti–40Al–10Nb alloy consists of α2/γ lamellar grains elongated along rolling direction and γ-phase grains, all dispersing in a B2-phase matrix. During the hot rolling, continuous and discontinuous coarsening of α2/γ lamellar structure occurs and produces a serrated grain boundary with B2-phase matrix. The microstructure of hot-rolled Ti–40Al–10Nb alloy after subsequent heat-treatment at the temperature range from 650 to 1400°C was also examined. The Widmanstätten structure consisting of the α2/γ lamellar laths may be obtained by heat-treating above β transus temperature. By annealing at the temperature of α2+β/B2 two phase region, the microstructure is composed of equiaxed α2/γ lamellar (or α2-phase) grains and B2-phase grains. When the heat-treatment temperature is between 1000 and 1200°C, the elongated α2/γ lamellar grains tend to spheroidize, and the volume of γ-phase increases. When the heat-treatment temperature is below 900°C, the meta-stable B2-phase transforms into the ω-phase of B82 structure, and the γ-phase is also found in B2-phase.