Coupling effects of deformation and thermal exposure on the precipitation behaviors of β_o(ω) phases in a high Nb-containing TiAl alloy

The precipitation behaviors of β_o(ω) phases are investigated in order to understand the effects of deformation on the microstructural stability during thermal exposure. Deformation can promote the precipitation of the β_o(ω) phases not only at the lamellar boundaries but within the lamellar colonies. The higher exposed temperature and lager engineering strain are in favor of the precipitation of the β_o(ω) phases. Four different ω_o variants are identified at the β_o regions of the deformed samples, indicating the residual stress induced by deformation cannot change the preferential formation of ω_o variants during exposure. Diffusion during the parallel decomposition promotes the formation of ω_o phase by making Nb enriched in the remaining α₂ laths. During the growth of ω_o grains, W is excluded and enriched in local regions of the α₂ laths, leading to the formation of β_o phase. The residual stress of the deformed samples is measured by the hole-drilling method. After deformation, the absolute value of residual stress comes to a peak value and then gradually decreases during exposure. The Vickers hardness shows a similar varying trend with the residual stress. Deformation can significantly decrease the microstructural stability and promote the precipitation of the β_o(ω) phases.