Manufacturing of long-length rods of ultrafine-grained Ti-Ni shape memory alloys

Abstract

Ti-Ni shape memory alloys were subjected to a rotary forging (RF) from the diameter of 20 or 12 mm to 5 mm at relatively low temperatures of 450 and 350 °C. The structure was studied using X-ray diffraction analysis and transmission electron microscopy. Mechanical properties were determined by uniaxial tensile and Vickers hardness tests, and the maximum completely recoverable strain by a thermomechanical method. The obtained results showed that RF at low temperatures allows manufacturing 3 m-long Ti-Ni rods. A mixed submicrocrystalline and nanosubgrained structure with an average size of structural elements of 118 ± 7 nm after forging from 20 mm, and 126 ± 7 nm after forging from 12 mm at 350 °C provides the highest mechanical (sB = 1113 MPa after forging from 20 mm, and 1065 MPa after forging from 12 mm) and functional (maximum completely recoverable strain of 8.0 %, after post-deformation annealing at 400 °C, 1 h) properties as compared to the initial state before RF (700 MPa, and 4.4 %, respectively).