Metastable transformation behavior in a Ta-containing TiAl-Nb alloy during continuous cooling

This study experimentally investigates the metastable transformation behavior in a cast Ti-48Al-3Nb1.5Ta (at。%) alloy during continuous cooling by in-situ and ex-situ methods. A CCT diagram of the alloy was constructed. A small amount of Ta significantly promotes metastable transformations, e.g., Widmanstätten, feathery, and massive γ transformations during continuous cooling at different rates (50–1200 °C min⁻¹). In particular, Ta can reduce the cooling rates required for metastable transformations, decrease the undercooling required and enhance the volume fraction of metastable microstructures. Crystallographic and kinetics studies reveal that the Widmanstätten transformation is a direct α→γ phase transformation that obeys the Σ11 CSL γ nucleation mechanism rather than {112̅2}α twinning. Furthermore, the effect of Ta on metastable transformations is detailed by thermodynamic analyses. Ta may potentially reduce the energy of the CSL boundaries of the γ phase to promote the nucleation of metastable structures. Consequently, uniform refinement of the microstructure can be achieved in low-Ta containing TiAl-Nb alloys by heat treatment alone.