Formation and temporal evolution of modulated structure in high Nb-containing lamellar γ-TiAl alloy

The formation and temporal evolution of the modulated structure in a lamellar γ-based Ti−45Al−8.5Nb alloy have been investigated by transmission electron microscopy (TEM) in combination with first-principles theory in this work. The results show that the Nb-rich O phase as a constituent of the modulated structure is thermodynamically stable below 650 °C in the α ₂ lamellae. The morphology of the O phase variants changes from thin plate-like shape with a low volume fraction at initial annealing to rectangle/square shape with a high volume fraction after a prolonged annealing, and the retransformed α ₂, named as α _{2 - ІІ} hereafter, emerges at intersections of the variants with two orthogonal habit planes due to their elastic interactions. The partitioning coefficient of Nb between the O phase and α ₂ is about 2 at 600 °C. The diffusion coefficient of Nb derived from growth kinetics of the O phase is about (1.3 ± 0.2) × 10 ⁻²² m ²s ⁻¹ in the α ₂ lamellae. Significant precipitation hardening effect of the O phase has been revealed for the α ₂ lamellae and γ/(α ₂+O) lamellar microstructure, which is supposed to be attributed to refining the α ₂ lamellae associated with elastic strain energy from the α ₂→O phase transformation and introducing the interface between the modulated lamella and adjacent γ phase.