Effect of stress axis orientation on the creep deformation behavior of Ti–48Al polysynthetically twinned (PST) crystals

The creep deformation behavior of PST crystals Ti–48Al was investigated at 1150 K under 100–400 MPa. Two hard orientations with the lamellar plates parallel or perpendicular to the compression axis and a soft orientation with the lamellar plates oriented 35° to compression axis were compared to investigate the effect of the orientation of lamellar interface on the creep behavior. The soft PST orientation exhibited the highest creep rates and larger primary creep strain than those of the hard PST orientations. The stress exponents were evaluated as 7.0 in the hard PST orientations for the stress dependence of the minimum creep rates. In the soft PST orientations, the stress exponent increased with decreasing stress from 4 at high stress regime to 8 at low stress regime. The primary creep strain increased with decreasing stress. The rotation of lamellar played an important role to accommodate the creep deformation in the soft PST orientation. The acceleration of creep rate after the minimum strain rates were attributed to the microstructural softening at strain concentrated regions. Those are kinking and dynamic recrystallization along the shear band in the hard PST orientations and recrystallization along the region at which angle of lamellar boundaries abruptly change in soft PST orientation.