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

Abstract

The precipitation behaviors of βo(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 βo(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 βo(ω) phases. Four different ωo variants are identified at the βo regions of the deformed samples, indicating the residual stress induced by deformation cannot change the preferential formation of ωo variants during exposure. Diffusion during the parallel decomposition promotes the formation of ωo phase by making Nb enriched in the remaining α2 laths. During the growth of ωo grains, W is excluded and enriched in local regions of the α2 laths, leading to the formation of βo 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 βo(ω) phases.