Evolution of antiphase domains and gamma lamellae in Ti-39at%Al single crystals

We studied antiphase domain (APD) growth and lamellar structure formation during isothermal annealing of Ti-39at%Al single crystals at alpha(2)+gamma dual phase temperature after quenching from alpha single phase state, intending to obtain a APD/lamellae mixed microstructure. It was examined whether such a microstructure provides a hardness higher than that obtained only by refining lamellar structure. The effect of plastic deformation prior to the annealing was also examined expecting an acceleration of gamma lamellae formation through a preferential nucleation of gamma-plates at 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, although naturally both the APD growth and the gamma lamellae formation were slower. 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. For instance, 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 1x10(4) s, while no gamma plate was obtained only by such an annealing without deformation.