Factors influencing the development of two-way shape memory in NiTi

Achieving Superior Two-Way Actuation by the Stress-Coupling of Nanoribbons and Nanocrystalline Shape Memory Alloy

Inspired by the driving principle of traditional bias-type two-way actuators, we developed a novel two-way actuation nanocomposite wire in which a massive number of Nb nanoribbons with ultralarge elastic strains are loaded inside a shape memory alloy (SMA) matrix to form a continuous array of nanobias actuation pairs for two-way actuation. The composite exhibits a two-way actuation strain of 3.2% during a thermal cycle and an actuation stress of 934 MPa upon heating, which is about twice as high as that (∼500 MPa) found in reported two-way SMAs. Upon cooling, the composite shows an actuation stress of 134 MPa and a mechanical work output of 1.08 × 10(6) J/m(3), which are about three and five times higher than those of reported two-way SMAs, respectively. It was revealed that the massive number of Nb nanoribbons in the compressive state provides the high actuation stress and high work output upon cooling, and the SMA matrix with high yield strength offers the high actuation stress upon heating. Compared to traditional bias-type two-way actuators, the two-way actuation composite with a small volume and simple construct works well with the miniaturization and simplification of actuators.

Functionally Graded Ti-Ni Shape Memory Alloys

Shape memory characteristics and superelasticity of an temperature gradient annealing(TGA) treated equiatomic Ti-Ni alloy have been investigated by means of differential scanning calorimetry(DSC), thermal cycling tests under constant load and tensile tests. By annealing 25% cold worked alloy under the temperature gradient from 658 K to 466 K, 7 K variation in TR*and 19 K variation in Ms* were obtained along the length of sample(150mm). Temperature dependence of transformation elongation(dε/dT) of TGA treated Ti-Ni alloy wires was in the range of 0.05 %/K and 0.01 %/K depending on annealing temperature ranges. The dε/dT obtained from TGA treated sample under the temperature gradient from 658 K to 466 K was 0.03 %/K. TGA treated alloy showed the clear superelastic recovery.