Abnormally high recovery strain in Ti–Ni-based shape memory alloys

This paper focuses on the study of the shape memory effect (SME) and two-way SME (TWSME) in Ti–50.0 at.%Ni and Ti–50.7 at.%Ni alloys (wire with the diameter 0.3 mm) with different types of initial austenite structure: initial polygonisation, nanosubgrained structure, recrystallised structure with grain size of about 5 μm and their mixture (annealing after moderate deformation with true strain e ≈ 0.6). A homogenising annealing at 700 °C, for 20 min served as a reference heat treatment (recrystallised structure of austenite). Post-deformation annealing was performed in the temperature range 250–600 °C for 0.5–1 h. The SME training procedure was carried out in a bending under load through B2 → R → B19′ transformation with the constrained strain ɛt in the range of 11–18%. The initial structure and constrained strain strongly affect all studied SME and TWSME parameters. A combined effect of recrystallised structure with grain size of about 5 μm obtained after 600 °C, 1 h annealing after low-temperature thermomechanical treatment (LTMT) and 18% loading (total) strain brought the highest recovery strain ɛr = 16.6% in Ti–50.7 at.%Ni alloy. The maximum recovery strain in Ti–50.0%Ni alloy (ɛr = 10.4%) was provoked by the mixed structure (polygonised and recrystallised) obtained after 450 °C, 30 min annealing after LTMT. The variation of training parameters enabled additional precise regulation of functional properties. The obtained results can be used for development elements functioning in conditions of the SME and TWSME realisation.

► The recovery strain 16.6% in Ti–Ni alloy with recrystallised structure was realized.
► Two-way shape memory effect up to 4.5% was realized.
► All types of structure after thermomechanical treatment were studied.
► Thermomechanical training modes, creating abnormally high effects are determined.