The effect of interfaces on mechanical and superplastic properties of titanium alloys

The effect of volume fraction of the β-phase on the mechanical and superplastic properties of ultrafine-grained titanium alloys with grain size d of ~0.2 µm was investigated by transmission and scanning electron microscopy, X-ray diffraction analysis, and tensile test experiments. The ultrafine-grained structure of the materials was formed by the multi-directional pressing technique. The structure in question is shown to improve the mechanical properties by 30–50 % and to lower down to 823 K, the temperature at which superplastic flow starts as compared to coarse-grained analogs, no matter what the phase composition and concentration of the alloying elements used. The reduced temperature is attributable to the activation of diffusion-controlled grain boundary sliding in the case of nonequilibrium interfaces of materials produced by severe plastic deformation. The fraction of the β-phase and its precipitation pattern are found to have significant influence on the temperature range in which superplastic flow occurs and on the maximum elongation to failure. A near-β Ti-5Al-5Mo-5V-1Cr-1Fe alloy with a large fraction of the β-phase (>34 %) under superplastic conditions exhibits record-breaking strains (>1300 %) that do not cause fracture of the material and extremely low flow stresses. This is associated with the activation of the grain boundary sliding due to an increase in the diffusivity along the phase boundaries in a case of microduplex structure.