TiAl intermetallics are typical metallic materials involving complex solid-state phase transformations, with crystal orientations that are difficult to control due to multi-transformation variants. However, lamellar orientation control is crucial to the development of a polysynthetic twinned single crystal structure in TiAl-based alloys for jet engines or other high-temperature systems. In this study, β-solidifying TiAl alloys were used to study the relationships between the lamellar structure and the phase transformation process under directional solidification (referred to as the directional phase transformation, DPT). It was found that the β → α phase transition affects the lamellar orientations and that the subsequent process of α → α₂ + γ leads to the final formation of the polysynthetic lamellar structure. Detailed analyses based on crystallography show that the β/α phase interface is responsible for the different oriented lamellar structures with the 0° or 45° orientation. With a lower interfacial energy, the 0° oriented α phase nucleates more easily but grows much more slowly than the 45° oriented α phases during DPT, which makes it feasible to control the lamellar orientations for TiAl-based alloys. The crystallographic origin for the control of lamellar orientations was then studied and confirmed by using EBSD in a β-solidifying Ti–Al–Nb alloy.

Although lamellar structures widely exist in materials, the existing nucleation knowledge has failed to describe the nucleation process of lamellae. This is because it involves the simultaneous formation of two different phases whose early-stage nuclei are always anisotropic and ordered. We report here a coupled nucleation of lamellae in a eutectoid TiAl system, demonstrating the coupled formation of a pair of anisotropic and ordered α2 + γ lamellae from the matrix, and each pair nucleates independently and heterogeneously. The coupled pair has been identified by alternative segregation of Nb in the interface using atom probe tomography. The nucleation kinetics have been confirmed by atomistic simulations and in situ high-energy synchrotron X-ray diffraction. A theoretical model has been proposed for this heterogeneously coupled nucleation, offering potential applications in various systems or phase transitions involving anisotropic, ordered, or lamellar structures. This coupled nucleation model represents a significant enhancement to the existing nucleation theory.