In order to enhance the high-temperature mechanical performance to meet service requirements, the microstructures and tensile properties of heat-treated Al-8.4Cu-2.3Ce-1.0Mn-0.2Zr-xNi (x = 0, 0.5, 1.0, 2.0, 4.0 wt.%) were investigated. The metallographic analysis techniques have been used to examine the microstructural changes with different Ni contents. Results show that after adding 0.5% Ni to the Al-8.4Cu-2.3Ce-1.0Mn-0.2Zr alloy, the spheroidized Al₇Cu₄Ni phase is formed. With Ni content further increasing, the Al₈CeCu₄ and Al₂₄MnCu₈Ce₃ phases disappear, and the nano-sized Al₂₀Cu₂Mn₃ and Al₂Cu phases decrease gradually. When Ni content reaches 4.0%, the Al₃CuNi phase appears. It turns out that the addition of 0.5% Ni has significantly improved the tensile properties at 400 °C. The ultimate tensile strength, yield strength, and elongation of Al-8.4Cu-2.3Ce-1.0Mn-0.2Zr-0.5Ni alloy at 400 °C reach 103 MPa, 93 Mpa, and 18.0%, respectively, which makes the alloy possible to be employed at 400 °C. The intermetallic micro-skeleton, composed of thermostable Al₈CeCu₄, Al₂₄MnCu₈Ce₃, Al₁₆Cu₄Mn₂Ce, and Al₇Cu₄Ni phases at the grain boundaries as well as nano-sized Al₂₀Cu₂Mn₃ and Al₂Cu precipitates in the grains, contributes to the good elevated-temperature tensile strength. The fracture mechanism is changed from quasi-cleavage at ambient temperature to coexistence of quasi-cleavage and dimple at 400 °C.

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.

Intermetallic compounds including TiAl and NiAl suffer from an abrupt brittle-ductile transition at a critical temperature, limiting their higher temperature applications. Conventional viewpoints attribute this transition to thermally activated dislocation actions or dynamic recrystallization. Here, we discover a novel brittle-ductile transition mechanism in polysynthetic twinned TiAl, driven by lamellar degradation, which significantly deviates from the conventional mechanisms. The results demonstrate that the lamellar degradation is primarily caused by the \( \alpha_2 \rightarrow \gamma \) phase transformation at the brittle-ductile transition temperature, which stems from a non-monotonic increase in the stacking fault energy of \( \alpha_2 \) phase with temperature. This unconventional transition can be described efficiently by an energy density-based strength model, offering new insights into the brittle-ductile transition behavior of intermetallic compounds at elevated temperatures.