An investigation of in situ observation and phase transformation mechanism in cyclic heat treatment of post-rolling Ti-44Al-4Nb-1.5Mo-0.1B alloys

• A novel cyclic heat treatment enables synergistic β-phase elimination and lamellar refinement in TNM alloy plates, simultaneously achieving two key microstructural control objectives and yielding a β-free fine lamellar microstructure. • Reduced Nb/Mo segregation lowers β-phase content, while lamellar spacing is tuned via dynamic nucleation-growth—initial refinement from activated nucleation, followed by coarsening due to depleted nucleation and dominant growth—enabling precise lamellar microstructure control. • For the first time, lamellar colony orientation is found unchanged during cyclic heat treatment of TiAl alloys, attributed to residual L12 phases (from differential transformation rates) acting as “guide structures” to stabilize orientation, which advancing controlled microstructural design in TiAl alloys. TiAl alloys are promising for aerospace structures but limited by poor high-temperature deformability. β-phase solidified TiAl alloys exhibit improved HT formability but suffer β-induced brittleness and coarse lamellae, affecting serviceability. We developed a cyclic heat treatment protocol for Ti-44Al-4Nb-1.5Mo-0.1B alloy sheets that concurrently eliminates β-phase and refines lamellar colonies, achieving β-free full lamellar structure. The cycling process led to β-phase reduction, lamellar colonies coarsening, lamellae orientation fixing, and lamellar spacing was first densified then sparsened. The above phenomena are attributed to two phase transition processes: β → α and α → α 2 + γ. In the β → α stage, only a diffusive phase transformation occurred. Extended cycling alleviated Nb/Mo segregation, raising β-phase lattice constant and lowering its content. The α → α 2 + γ transformation involves martensitic (α → L1 2 ) and diffusion (L1 2 → γ) transformation stages. The difference in transformation rates between these two processes during the cyclic heat treatment leads to the residual presence of L1 2 phases within the lamellar colonies. During lamellae formation, the L1 2 phase acts as a “guide”, influencing the extension direction of the lamellae and playing a role in fixing lamellae orientation. This heat treatment resolves the processability-service performance trade-off, advancing TiAl alloys for aerospace engine components.