Thermal activation approaches to deformation mechanisms for high Nb containing TiAl base alloys

The deformation mechanisms in a wide temperature range from room temperature to 1200 K were investigated by thermal activation approach. Using observed instantaneous stress response to the strain rate jump (Deltasigma(tr)), the activation volume V-a, then the activation enthalpy DeltaH, activation free enthalpy DeltaG and activation entropy DeltaS were calculated. The apparent activation energy of high temperature deformation is estimated to be 3.66 eV, which is larger than the self-diffusion coefficient of binary TiAl (3.01 eV). The dislocations at 1173 K are generally curved or bowed, even helical-shaped dislocations. The climb of ordinary dislocations as well as twinning has greatly contributed to the plastic deformation. The CRSS at 1173 K is estimated to be 180 MPa. The higher resisting stress at both room temperature and elevated temperature might relate to the high Nb content of the alloy.