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

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 → α₂ + 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 → α₂ + γ transformation involves martensitic (α → L1₂) and diffusion (L1₂ → γ) transformation stages. The difference in transformation rates between these two processes during the cyclic heat treatment leads to the residual presence of L1₂ phases within the lamellar colonies. During lamellae formation, the L1₂ 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.