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The reasonable design and application of phase transformation is an efficient approach to coordinate the microstructure and properties of alloys. The sequence of compositional and structural transformation determines the type and mechanism of phase transformation. Because of their low density and excellent high-temperature properties, TiAl alloys that contain fully lamellar microstructure have been applied to low-pressure turbine engine blades. However, there is no consensus on the mechanism of lamellar formation. Here, we propose a new mechanism that is dominated by spinodal decomposition, in which compositional transformation occurs prior to structural transformation during lamellar formation. Transitional lamellae with fcc-based long-period superstructures are introduced to prove the mechanism of sequentially spinodal decomposition, shear transformation, compositional modulation and ordering.