B2 ⇒ B19′ ⇒ B2T Martensitic Transformation as a Mechanism of Plastic Deformation of NiTi

Abstract

Deformation of superelastic NiTi wire with tailored microstructure was investigated in tensile loading–unloading tests up to the end of the stress plateau in wide temperature range from room temperature up to 200 °C. Lattice defects left in the microstructure of deformed wires were investigated by transmission electron microscopy. Tensile deformation is localized up to the highest test temperatures, even if practically no martensite phase exists in the wire at the end of the stress plateau. In tensile tests at elevated temperatures around 100 °C, at which the upper plateau stress approaches the yield stress for plastic deformation of martensite, upper plateau strains become unusually long, transformation strains become unrecoverable and deformation bands containing {114} austenite twins appear in the microstructure of deformed wires. These observations were rationalized by assuming activity of B2 ⇒ B19′ ⇒ B2T martensitic transformation into the austenite twins representing a new mechanism of plastic deformation of NiTi, additional to the dislocation slip in austenite and/or martensite. It is claimed that this transformation becomes activated in any thermomechanical load in which the oriented B19′ martensite is exposed to high stress at high temperatures, as e.g., during shape setting or actuator cycling at high applied stress.