Effect of Severe Torsion Deformation on Structure and Properties of NiTi Shape Memory Alloys

In the present work, the possibility of applying severe torsion deformation (STD) to a bulk near-equiatomic NiTi shape memory alloy in order to accumulate super-high strain and improve mechanical and functional properties was studied. STD was performed using the multidirectional test system “BÄHR MDS-830” at a temperature of 300-600 °C (the upper border temperature for the development of dynamic polygonization) in 14 and 30 turns with accumulated true strain values of 4.3 and 9.1, respectively. Structural phase state and properties were studied using differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, hardness measurements, and thermomechanical bending tests. STD at 350 °C allowed for the accumulation of high strain without failure. As a result of STD in 14 turns, a submicrocrystalline structure with an average grain/subgrain size of about 250 nm was formed. This structure ensured the achievement of high maximum completely recoverable strain values of 7 %. The results obtained show the prospects of applying severe torsion straining deformation to titanium nickelide in terms of forming an ultrafine-grained structure and high properties.