Superplasticity of nanostructured Ti-6Al-7Nb alloy with equiaxed and lamellar initial microstructures processed by High-Pressure Torsion

Microstructural modifications of a biomedical Ti-6Al-7Nb alloy were accomplished via heat treatment in 3 different quenching mediums and then processed by High-Pressure Torsion (HPT) at room temperature. The microstructure of the as-received condition is composed of an equiaxed duplex (α+β) structure. After the heat treatment, a combination of primary α phase and lamellar structures was obtained with an increasing fraction of the martensitic lamellar with increasing cooling rate. After HPT processing, refinement of the microstructures was observed for N=5 revolutions. Transmission electron microscopy (TEM) of the sample quenched in liquid nitrogen confirmed the nanostructure with grain sizes below 100 nm and high density of lattice defects after HPT processing for N=5 revolutions. High-temperature tensile tests were carried out at 800 °C with an initial strain rate of 2×10^-3 s^-1 on specimens with different combinations of heat treatment and HPT straining. The test in the as-received condition presented a maximum elongation to failure of ∼400% after HPT processing for N=5 revolutions. The highest elongation to failure in the heat-treated samples was ∼580% in the sample quenched in liquid nitrogen and processed for N=5 revolutions.