Formation of equiaxed O bands at the interface of TiAl/Ti₂AlNb heterostructures: Abnormal growth of O variants and novel annealed twins

This paper investigates the formation conditions of equiaxed O bands at the interface of TiAl/Ti₂AlNb layered composites and explores their nucleation and growth mechanisms. Annealing heat treatment at 750–900 °C can form a stable O band with a thickness of 40 μm, and its nucleation is closely associated with the abnormal growth of O variants laths. During the growth of the O band, particular attention was focused on the orientation relationship between it and the β_o/B2 matrix. The lack of the orientation relationship between them is attributed to the change in orientation of the β_o/B2 matrix during the annealing process, as well as the orientation diversity arising from the continuous generation of [00 1‾] 65° and [00 1‾] 55° annealing twins during the growth of the O band. The novel discoveries of two types of annealing O twins also provide significant reference values for Ti₂AlNb alloys.