Electron microscope characterization of low cycle fatigue in a high-strength multiphase titanium aluminide alloy

The micromechanisms controlling low cycle fatigue of a Nb-bearing TiAl alloy (TNB-V2) have been characterized by conventional and high-resolution transmission electron microscopy. Fully reversed isothermal tests were performed under strain control at temperatures of 25, 550 and 850 °C. Samples fatigued at 25 and 550 °C exhibited dense structures of ordinary dislocations, dipoles and debris that were accumulated in tangles. The dipole defects apparently serve as additional glide obstacles but may also contribute to dislocation multiplication if the local stress rises. Another important low temperature deformation mechanism is the stress-induced transformation of an orthorhombic phase, which is a significant constituent of the microstructure. The orthorhombic phase is apparently unstable under tetragonal distortion and transforms into γ(TiAl) phase. Under high-temperature fatigue the lamellar microstructure degrades by phase transformation combined with dynamic recrystallization.