Plastic anisotropy and fracture behavior of cyclically deformed TiAl polysynthetically twinned crystals

The cyclic deformation and fracture behavior of TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the effects of total strain amplitude, lamellar structure and the angle φ between the loading axis and the lamellar planes. Fatigue tests were performed in cyclic tension-compression mode at room temperature at total strain controlled amplitudes between ± 0.2% and ± 1.0% using Ti-49.1at.%Al and Ti-50.8at.%Al PST crystals containing fine and coarse lamellae respectively. Strong anisotropy in plastic behavior was observed at φ = 0° and φ = 45°. At γ = 0° the stress amplitude increased with the number of cycles and the cyclic hardening became significant as the strain amplitude increased. Numerous dislocations and twins were activated depending on the types of γ domain, and their motion was interrupted at the γ/γ domain and α2/γ lamellar boundaries resulting in an inhomogeneous deformation substructure. Specimens with φ = 45° showed weak cyclic hardening and broke without significant hardening. Fracture often occurred on the basal plane in the α2 plates. Dislocations glided simply on (111) planes parallel to the lamellar planes and travelled a long distance since the γ/γ domain boundaries did not act as strong barriers to the motion of dislocations. Refinement of the lamellar structure is effective in achieving an increased fatigue life at both φ = 0° and φ = 45°.