Abstract: TiAl alloys with the (α₂ + γ) lamellar structure are highly valued for their excellent high-temperature strength and creep resistance. Understanding the formation mechanism of the lamellar structure is crucial for tuning the microstructure and properties. This work investigates the formation of lamellar structure in Ti-48Al-7Nb-2.5V-1Cr alloy, revealing the presence of hcp-based long-period superstructure (hcp-LPS) as a metastable phase during lamellar formation. The identification of hcp-LPS demonstrates that the necessary solute enrichment for the formation of γ lamellae occurs on the hexagonal α matrix, implying that phase separation of α → Al-rich α lamellae + Al-depleted α lamellae is the first step of lamellar formation. Once phase separation is completed, all subsequent phase transitions occur within the Al-rich α lamellae. Additionally, the formation of twin lamellae is further discussed. The formation of the twin lamellae occurs sequentially. Pre-existing lamella promotes the formation of later lamella by inducing solute enrichment in its surrounding region, and then the successive slip of Shockley partial dislocations with opposite Burgers vectors ensures special stacking of later lamellae. These findings not only contribute to the fundamental understanding of spinodal mechanisms in hexagonal crystals, but also provide novel insights into the formation of twin lamellae.

Key words: Titanium aluminides, Phase separation, Spinodal decomposition, Metastable phases, Twin