Mechanical properties and martensitic transformations in nanocrystalline Ti49.4Ni50.6 alloy produced by high-pressure torsion

Nanocrystalline alloy Ti49.4Ni50.6 (numbers indicate at.%) in the shape of a disc 20 mm in diameter has been successfully produced by high-pressure torsion. High-pressure torsion and annealings at temperatures from 400 to 550 °C resulted in formation of nanocrystalline structures with the mean grain size (D) in the range 20–300 nm. The alloys with D = 20 nm possess very high ultimate tensile strength (more than 2000 MPa) but limited ductility. The alloys with D = 300 nm possess high ductility and enhanced ultimate tensile strength. There are areas of strain-induced transformation B2–B19′ on the tensile curves, but their extent depends on the grain size. The relationship between mechanical behavior and martensitic transformation in nanocrystalline TiNi alloys is considered and discussed.