Effect of Severe Torsion Deformation on Structure and Properties of Titanium–Nickel Shape Memory Alloy

Abstract

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 500 °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 500 °C allowed for the accumulation of high strain without failure. As a result of STD in 30 turns, a submicrocrystalline structure with an average grain/subgrain size of about 500 nm was formed. This structure ensured the achievement of high maximum completely recoverable strain values of 6.1–6.8%. 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.

Structure and Properties of TiNi Shape Memory Alloy after Quasi-Continuous Equal-Channel Angular Pressing in Various Aged States

“…The deformation of samples after pre-aging for 1 h allows the formation of a somewhat more finely dispersed structure with an average size of its elements (grain and subgrain) of 92 nm, as compared to that of samples after pre-aging for 5 h (115 nm). It should be noted that, in itself, obtaining a mixed nanograined and nanosubgrained structure in titanium nickelide bulk samples is an important scientific and technological result [27][28][29]. During pre-aging for 1 h, the growth of Ti 3 Ni 4 phase particles proceeds less intensively than that after pre-aging for 5 h [30][31][32].…”