Microstructure stability and micro-mechanical behavior of as-cast gamma-TiAl alloy during high-temperature low cycle fatigue

This study systematically investigated the low cycle fatigue deformation of a high Nb-containing TiAl alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 °C by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and synchrotron-based high-energy X-ray diffraction (HE-XRD) techniques. Cyclic stress-strain (CSS) behavior, lattice strain, and peak broadening of {100}_α2, {201}_γ, and {202}_ωo planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Δε_t/2 = ±0.25%, Δε_t/2 = ±0.28%, and Δε_t/2 = ±0.30%. At early deformation stages, α_2lamellae transformed into ωo phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, γ particles precipitated within the single ω_o area. In addition, according to the peak intensity and peak broadening results, the ω_o → B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the ω_o phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the γ phase and α₂ phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the α₂/γ lamellar interface. This study provides a better understanding of the low cycle fatigue deformation of TiAl alloys.