Production of superplastic Ti-6Al-7Nb alloy using high-pressure sliding process

A Ti-6Al-7Nb alloy was processed by severe plastic deformation through high-pressure sliding (HPS) at room temperature for grain refinement. The microstructure consists of grains with sizes of 100-200 nm in diameter having high and low angles boundaries. Superplasticity appeared with the total elongation of more than 400% and this was more likely when the tensile specimen is deformed in the direction parallel than perpendicular to the sliding direction. However, the superplastic elongation is almost the same irrespective of whether the sliding was made in the single direction or in the reversible directions as far as the total sliding distance is the same. The total elongation is invariably higher for the tensile testing at 1123 K than at the other temperatures, reaching the highest elongation of 790% at the initial strain rate of 1×10^-3 s^-1. The strain rate sensitivity and the activation energy for the deformation were determined to be more than ∼0.3 and 200 kJ/mol, respectively. The microstructural observation reveals that the α phase region covers more than 85% of the tensile specimens after deformation and their grains are equiaxed in shape. It is concluded that the superplastic deformation is mainly controlled by grain boundary sliding through thermally activation by lattice diffusion.