The effects of solute and vacancy segregation on APB migration in Ti3Al, and their dependence on composition, have been investigated by using a phase-field simulation in which vacancy distribution is taken into account. Al-atoms are depleted and vacancies segregate at APB in stoichiometric Ti3Al (Ti-25Al), whereas Al-atoms segregate and vacancies are depleted in Alrich one (Ti-28Al). The simulation indicates that APB in Ti3Al migrates much faster in Ti-25Al than in Ti-28Al with the effect of vacancy segregation whereas it migrates slightly faster in Ti-28Al than in Ti-25Al in the absence of the effect of vacancy segregation.

A mixed microstructure of antiphase domains (APD) and fine lamellar structure were introduced in a Ti-39at%Al single crystal and it was examined whether the APD hardening works even in nano-scaled lamellar structures. The hardness increases with decreasing APD size even where the L is smaller than 100 nm below which the hardening by lamellar refining saturates. The mechanism of the additivity of strengthening by APD and lamellar structure is discussed in the context of the geometries of slip direction, lamellar boundaries and APD boundaries (APDBs). For {1100}<1120> prism slip (the easiest slip system of α2-Ti3Al), the lamellar boundaries are parallel to the slip direction, and therefore they interrupt the motion of screw dislocations effectively. On the other hand, APDBs inclined from lamellar boundaries can effectively obstruct the dislocation motion regardless of the dislocation character because the shear of such APDBs results in the formation of step-like APDBs on the slip-plane and requires additional stress for dislocation motion whereas APDBs parallel to the slip direction can be sheared without forming such a step-like APDB. Accordingly, APDs and lamellar structure can contribute to the strengthening complementarily.

We have studied the effects of straining prior to annealing for lamellar structure formation in a Ti-39at%Al single crystal intending to accelerate the lamellar structure formation and decrease the resultant lamellar spacing via preferential nucleation of γ-TiAl phase at dislocations in α2-Ti3Al phase. In a crystal cold-rolled to 10% reduction before annealing, a fine lamellar structure with 88 nm average spacing was formed by annealing at 1073 K for 1×104 s whereas no lamella were observed in the crystal subjected to the same annealing without cold-rolling. In addition, it was also demonstrated that when the crystal is locally strained by indentation and annealed at 1073 K for 1×104 s, a lamellar structure with 43 nm average lamellar spacing is formed in areas near the indentation and there were no lamella beyond these areas. This means that distributions of lamellae can be controlled by controlling the distribution of strain before annealing.

We studied antiphase domain (APD) growth and lamellar structure formation during isothermal annealing of Ti-39at%Al single crystals at α2+γ dual phase temperature after quenching from α single phase state, intending to obtain a APD/lamellae mixed microstructure and to examine whether such a microstructure provides a strength higher than that obtained only by refining lamellar structure. The effect of plastic deformation prior to the annealing was also examined expecting a acceleration of γ lamellae formation through a preferential nucleation of γ-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 γ 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 1×104 s, while no γ plate was obtained only by such an annealing without deformation.