High strength and superconductivity in nanostructured niobium–titanium alloy by high-pressure torsion and annealing: Significance of elemental decomposition and supersaturation

A powder mixture of Nb–47wt.% Ti (a well-known composition for superconducting magnets) was subjected to severe plastic deformation using high-pressure torsion (HPT) and subsequently annealed at 573K. Ti gradually dissolved in Nb with increasing shear strain, with a fast kinetics comparable to lattice diffusion at 700–1200K. At large strains, a complete transition to a nanostructured β phase occurred at room temperature, which is far below the equilibrium temperature of 690K. Nanoclusters of Ti with a body-centered cubic structure were also detected at large strains. Subsequent annealing led to elemental decomposition, formation of a nanoscale lamellar structure and segregation of Nb at grain boundaries. Superconductivity occurred at temperatures below 9K, while the transition temperature decreased with increasing shear strain because of supersaturation of Ti in Nb and increased with annealing because of elemental decomposition. The Nb–Ti alloy after HPT exhibited hardness/strength peaks followed by softening at large strains, while hardening occurred after annealing. The maximum hardness, tensile and bending strengths were 4, 1.7 and 2.7GPa, respectively.