Evolution of antiphase domain (APD)/lamella mixed microstructure in Ti–39 at%Al single crystals

Intending to obtain a Ti–Al alloy with a nano-scaled antiphase domain (APD)/lamella mixed microstructure and to achieve a strength higher than that achieved only by lamella refining, we have studied the APD growth and the lamellar structure formation in Ti–39 at%Al single crystals during isothermal annealing at α2 + γ dual phase temperatures after quenching from an α disordered single phase state. The effect of plastic deformation prior to the annealing was also examined expecting an acceleration of γ lamellae formation through a preferential nucleation of γ-plates on dislocations. The lower was the annealing temperature, the smaller both the APD size and the lamellar spacing at the moment of a homogeneous lamellar structure formation tended to be. However, the APD size in the homogeneous lamellar structure was no smaller than 400 nm. A structure with finer APDs and finer lamellar structure was obtained by deforming the crystal before annealing since the lamellar structure formation was accelerated and the time for APD growth before the lamellar structure formation was shortened. A structure with an average lamellar spacing of 88 nm and an average APD size of 214 nm was obtained by deforming the crystals to 10% plastic strain and subsequently annealing at 1073 K for 1 × 104 s, while no γ-plate was obtained only by the identical annealing without deformation. Such a microstructure provided a hardness higher than that obtained only by refining lamellar structure.