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 TiAI alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 degrees 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}(alpha 2), {201}(gamma), and {202}(omega o) planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Delta(epsilon t)/2 = +/- 0.25%, Delta(epsilon t)/2 = +/- 2 = +/- 0.28%, and Delta(epsilon t)/2 = +/- 0.30%. At early deformation stages, alpha(2lamellae) transformed into omega(0) phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, gamma particles precipitated within the single omega(o) area. In addition, according to the peak intensity and peak broadening results, the omega(o) -> B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the omega(o), phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the gamma phase and alpha(2) phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the alpha(2)/gamma lamellar interface. This study provides a better understanding of the low cycle fatigue deformation of TiAI alloys. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.