Advances in phase relationship for high Nb-containing TiAl alloys

γ-TiAl alloys, including two categories (the conventional TiAl and the high Nb-containing TiAl (high Nb–TiAl)), are technologically intriguing because of their applications at high temperatures. Specifically, the service temperature of the high Nb–TiAl alloys is 60–100 °C higher than that of conventional TiAl alloys. Recently developed TiAl alloys, for example TNB, TNM, β-γ alloys, belong to the high Nb–TiAl alloys, displaying similar behavior in phase transformation, strengthening, oxidation at high temperatures, and relationships between composition, microstructure, and mechanical properties. This work presents an in-depth review of the high Nb–TiAl alloys regarding the advances in phase diagram, formation mechanism of the new γ1 phase, microsegregation induced by adding a high content of alloying element Nb, and the mechanism of the B2/ω phase formation. Some challenges in developing the high Nb–TiAl alloys are also discussed.