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 (alpha+beta) structure. After the heat treatment, a combination of primary a 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 degrees C with an initial strain rate of 2x10(-3) s(-1) on specimens with different combinations of heat treatment and HPT straining. The test in the asreceived condition presented a maximum elongation to failure of similar to 400% after HPT processing for N=5 revolutions. The highest elongation to failure in the heat-treated samples was similar to 580% in the sample quenched in liquid nitrogen and processed for N=5 revolutions.