Effect of torsion deformation and electric pulse treatment on transformation behaviour and superelasticity of Ti-50.8 at.% Ni wire

Abstract

The functional stability of Ti-50.8 at.% Ni wires (diameter: 0.5 mm) was improved by torsion deformation coupled with electric pulse treatment. The effects of torsion deformation (T = 5–50 turns) and electric pulse treatment (f = 50–400 Hz and t = 50–1000 µs) on the phase transformation behaviour and superelastic properties of NiTi alloys were comparatively analysed. The results indicate that A → R and R → M transitions occur with broadened transformation peaks during cooling and eventually disappear with the increase in torsion turns. Torsion deformation effectively improves superelasticity via the introduction of dislocations. The superelasticity is initially improved and then reduced under the increase in torsion turns. After electric pulse treatment with various pulse frequencies and switch-on times, the superelasticity is first decreased and then increased. Better superelasticity is obtained in the deformed NiTi alloy after 40 torsion turns, followed by electric pulse treatment of 150 Hz/600 µs. The corresponding accumulative residual strain is reduced, leading to an 85% increase in superelasticity compared to the as-received one.

Keywords

• NiTi shape memory alloy
• Torsion deformation
• Electric pulse treatment
• Phase transformation
• Superelasticity