The influence of rapid-diffusive β-stabilizers on the microstructure formation and superplasticity of titanium-based alloys

Abstract

The superplasticity of Ti-based alloys is improved due to grain refinement, an increase in grain size stability, and an increase in the β-phase fraction up to ∼0.5. Alloying with β-stabilizers reduces the β-transus temperature and increases the β-phase fraction at low forming temperatures, thus improving the superplasticity and lowering its temperature. Both grain growth and superplastic deformation mechanisms are diffusion-controlled phenomena, and, therefore, the diffusivity of the alloying elements should have a significant effect on superplasticity. This work deals with the influence of the high-diffusive elements Fe, Co, and Ni on the grain structure and superplasticity of titanium alloys. For this purpose, the Ti-Al-Mo-V alloy, and alloys with proportional replacement of low-diffusive Mo by Fe, Co, or Ni, exhibiting an interstitial diffusion mechanism, were studied. The alloying elements content was selected to ensure the same β-phase fraction at a deformation temperature of ∼875 °C. The microstructure evolution after thermomechanical processing, annealing, and superplastic deformation, as well as post-deformation room-temperature mechanical properties of the studied alloys, were compared. The actual phase ratio was reconstructed by comparing scanning electron microscopy images in backscattered electrons and electron backscatter diffraction data for the same fragments. It was found that Mo replacement for highly diffusive elements accelerated dynamic grain growth during superplastic deformation at an elevated temperature of 875 °C but improved superplasticity at a lower temperature of 775 °C. The results confirmed that alloying with high-diffusive elements is a promising strategy for the design of low-temperature superplastic forming alloys.