Intrinsic Two-way Shape Memory Effect Research Articles

Outstanding intrinsic two-way shape memory effect of the Ni54.71Mn22.14Ga23.15 oligocrystalline shape memory microwire

The shape memory effect of the Ni54.71Mn22.14Ga23.15 oligocrystalline microwire was investigated. The microwire was expected to exhibit an outstanding two-way shape memory effect due to the directional internal stress induced in the preparation. The results show that the microstructure of the as-prepared Ni54.71Mn22.14Ga23.15 microwire was mainly oriented martensites at room temperature. An intrinsic two-way shape memory strain of 8.8% (±0.2%) in the as-prepared microwire was acquired in the process of cooling and heating with no applied stress. A shape memory strain of 14.4% (±0.2%) was obtained under the constant stress of 10 MPa. The isothermal tensile tests were carried out on the microwire in the temperature range of 383–463 K and the recoverable tensile strains were over 13%. The microwire after superelastic training maintained a two-way shape memory strain of 7.3% (±0.2%) and a work output of 31 J/cm3. The shape memory effect of the microwire in this work displays prominent advantages in the reported shape memory alloys, mainly in the aspects of larger intrinsic shape memory strain and higher work output under relatively lower stress with the operating window around room temperature.

Ultra-wide-temperature-range superelasticity and intrinsic two-way shape memory effect in Co–Ni–Ga microwires

We demonstrate perfect superelasticity and inherent two-way shape memory effect in Co49Ni21Ga30 microwires fabricated by a Taylor–Ulitovsky method. With the formation of an almost complete [001]A-oriented single crystal along the axis of the wire, the as-drawn microwire displays great superelastic behaviors with a large reversible tensile strain of >8% over an ultra-wide temperature window of 550 K (223–773 K). Simultaneously, an excellent intrinsic two-way shape memory effect with a considerably large strain output (∼6.3%) was also obtained in this Co49Ni21Ga30 microwire. After mechanical training, the two-way shape memory strain can reach up to 6.8% at a low operating temperature. With the combination of above extraordinary functional properties and the low cost of fabrication, the Co49Ni21Ga30 microwire holds a significant potential for applications in miniature sensing and self-actuating devices in the future.

Microstructural study and simulation of intrinsic two-way shape memory behavior of functionally graded Ni-rich/NiTiCu thin film

In this research, the gradient structure and mechanical properties of an austenitic/martensitic (Ni50.8Ti/Ni45TiCu5 (at.%)) bi-layer thin film was investigated. The bi-layer was deposited on a Si substrate using DC magnetron sputtering. After crystallization of the film at 773K for 60min, the microstructure and mechanical properties of the bi-layer thin film were characterized using X-ray diffraction (XRD), transmission electron microscopy (STEM and HRTEM) and nanoindentation, respectively. The diffraction pattern illustrated that the crystallized bi-layer was combined of martensitic and austenitic layers while secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy analysis demonstrated the existence of a compositional gradient through the thickness and residual strain in the interface of the bi-layer, respectively. Furthermore, the compositional gradient in the bi-layer led to gradual variations in the structure, hardness and Young's modulus through the thickness of the bi-layer. The intrinsic two-way shape memory effect due to its functionally graded structure and the induced stress was confirmed by experimental test and finite element simulation.

Phase transformation and structure of functionally graded Ni–Ti bi-layer thin films with two-way shape memory effect

Ni–Ti thin film shape memory alloys (SMAs) have become a promising material for micro-actuators because of their excellent mechanical properties and biocompatibility. Films with composition gradient have the added feature of an intrinsic two-way shape memory effect. In this study two kinds of the Ni–Ti bi-layer thin films (Ni50.8Ti/Ni45TiCu5 and Ni50.8Ti/Ni49Ti) were developed. The thin films were deposited onto Si (111) substrates by RF magnetron sputter deposition from separate alloy targets. The three targets with atomic compositions of Ni45Ti, Ni47Ti and Ni42TiCu5 were prepared by vacuum arc re-melting (VAR). For crystallization, the as-deposited amorphous thin films were annealed at 773K for 1h. The annealed bi-layer thin films formed compositional gradients at the interface of the films through diffusion. The effect of the composition on film structure, surface morphology, transformation temperatures and mechanical properties was studied using grazing incidence X-ray diffraction, transmission electron microscopy, atomic force microscopy, electrical resistivity and nano-indentation. The chemical and stress gradient in the bi-layers affects the structure, phase transformation and shape memory effect. Thin films exhibit multi-step phase transformation. Also, the bi-layer presented pseudo elastic and shape memory effect, simultaneously. The depth recovery ratio due to pseudo elastic in the Ni50.8Ti/Ni45TiCu5 compared with the Ni50.8Ti/Ni49Ti bi-layer is higher while the recovery ratio due to shape memory in the Ni50.8Ti/Ni49Ti bi-layer is higher. The combination of pseudo elastic with shape memory effect produces a two-way shape memory effect with a reduced hysteresis in the bi-layers.

Characterization and properties of NiTi(W) and CuAlNi shape memory alloys

Abstract Shape memory wires were trained under constant stress in order to introduce a uniaxial shape memory effect by a thermomechanical treatment. These investigations were carried out on three different alloy systems (NiTi, NiTiW and CuAlNi) with different microstructures (cold-worked and annealed condition, with and without particles, large and small grain size). Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the deformation behaviour. The influence of the microstructure on development and stability of the intrinsic two-way shape memory effect has been discussed. Furthermore, this work deals with the production of thin specimens of shape memory alloys by melt-spinning and splat-cooling, the training procedure and their characterization with respect to microstructure and functional properties.

Microactuators of free-standing TiNiCu films

Ti50Ni47Cu3 films were prepared using magnetron sputtering deposition, and their martensitictransformation and mechanical properties were characterized. Free-standing TiNiCufilms showed an intrinsic two-way shape memory effect which is attributed toa composition gradient through the film thickness. Different types of TiNiCumicroactuators, including microtweezers and microcages, have been successfullyfabricated which employ this two-way shape memory effect for operation. Uponheating/cooling, the microtweezers showed both horizontal and vertical displacement dueto combined shape memory and thermal effects. The microcage actuators couldbe opened/closed through substrate heating with a maximum temperature of90 °C, or by electrical heating with a power less than 5 mW and a maximum frequency of100–300 Hz. Issues related to the fabrication and applications, such as stability and beambending after release, have been addressed.

Thin film shape memory alloys for optical sensing applications

Based on shape memory effect of the sputtered thin film shape memory alloys, different types of micromirror structures were designed and fabricated for optical sensing application. Using surface micromachining, TiNi membrane mirror structure has been fabricated, which can be actuated based on intrinsic two-way shape memory effect of the free-standing TiNi film. Using bulk micromachining, TiNi/Si and TiNi/Si3N4microcantilever mirror structures were fabricated.

A new type of intrinsic two-way shape-memory effect in hooks of NiTi-wires

Shape-memory hooks were fabricated in a single processing step (one-time procedure) by a combination of tensile and bending deformation of a martensitic NiTi-wire. The hooks reversibly changed curvature on heating and cooling in a reproducible manner over 50 thermal cycles. This new type of intrinsic two-way shape memory effect was studied by subjecting the hooks to a beam of high-energy synchrotron x-rays at room temperature ( A f ). This technique demonstrated that the hooks were martensitic at room temperature but contained both austenite and martensite at 150°C. The presence of martensite above A f is is due to a high dislocation density, which stabilizes martensite and inhibits its transformation. The amount of stabilized martensite scales with the extent of plastic deformation and thus increases from the inner to the outer curvature of the hook. On heating, the inner part of the hook transforms to austenite, while the stabilized martensite in the outer part is deformed pseudo-elastically by twinning. On cooling, the back-transformation of the inner part is biased by the pseudo-elastic recovery of the outer part, thus forcing the hook back into its initial shape.

Einfluss von Gefüge und Wärmebehandlung auf das Ermüdungsverhalten von Formgedächtnislegierungen / Influence of Microstructure and Thermal Treatment on the Fatigue Behavior of Shape Memory Alloys

Development and stability of the intrinsic two-way shape memory effect on three alloys with different microstructure are studied in this work. Wire specimens of a binary NiTi alloy, a CuAlNi alloy and finally a shape memory alloy with W-dispersoids in a NiTi matrix are investigated. After a training procedure the samples are subjected to 4000 two-way cycles. Changes in deformation behavior are discussed. Whereas the influence of work hardening on the magnitude and stability of the effect should be aim of the work with the binary alloy, results of different grain size are investigated in the CuAlNi alloy. The NiTiW alloy should deliver information about the effect of internal stress fields caused by the W-dispersoids on fatigue behavior. It is shown in this work that quite good stability of the two-way effect can be achieved by optimizing the microstructure. Certainly it must be admitted that any measure to improve the stability reduces the magnitude of the effect. Consequently an appropriate compromise has to be realized by combining a suitable microstructure and thermal treatment.

Generation, development and degradation of the intrinsic two-way shape memory effect in different alloy systems

A uniaxial two-way shape memory effect is induced in wire specimens by a thermomechancial treatment. In order to enable a systematic variation of the microstructure with respect to dislocation density, second phase particles and grain size, the investigations are carried out on three different alloy systems. 4000 thermal cycles are performed on the trained shape memory elements, continuously observing the changes in the deformation behavior. The influence of work hardening, grain size and high internal stress-fields on the development and the stability of the intrinsic two-way shape memory effect is discussed, supported by microstructural investigations. The degradation due to cyclic application is related to instabilities in the high- and low-temperature modification. The decrease of the effect size is found to consist of two characteristic stages, where the decay is distinguished by different origin and rate. Definite conclusions about the degradation mechanisms are drawn by analyzing the influence of the preliminary heat treatment on the fatigue behavior. It is shown that the degradation can be minimized and hence the stability of the two-way shape memory effect can be optimized by establishing an appropriate microstructure.

Comparative study of degradation effects in different shape memory alloy systems

The presented investigations aim at clarifying the influence of microstructure and applied training procedure on the stability of the created intrinsic two-way shape memory effect (TWSME). The investigations are carried out on three different alloy systems in order to get an appropriate variation of the microstructure with respect to dislocation density, second phase particles and grain size. A uniaxial TWSME is induced by a thermomechanical treatment in wire specimens of the alloys NiTi, CuAlNi and a two phase alloy composed of a NiTi matrix and dispersoids of tungsten. Subsequently, thermal cycles are carried out, continuously observing the changes in the deformation behaviour. The results suggest that the training must result in both, a large pseudoplastic strain as well as a pronounced irreversible strain in order to obtain a TWSME of high magnitude. The degradation of the TWSME in the NiTi-based alloys was found to consist of two characteristic stages, which can always be recognised, independent of the preliminary heat treatment. The obvious strengthening effect of the particles in the two-phase alloy NiTi-W has a very positive effect on the stability of the created TWSME. This effect can even be improved by preliminary cold working. The stability of the effect in the CuAlNi alloy is far better then in the binary NiTi alloy. No dramatic change in the degradation rate is observed throughout the investigated period of 5000 thermal cycles.

Gefügeänderungen in Cu-Al-Ni-Formgedächtnislegierungen / Microstructural Changes in Cu-Al-Ni Shape Memory Alloys

Microstructures of strong varying grain sizes have been established in wire specimens of a high-temperature CuAlNi shape memory alloy by applying different betatizing heat treatments. In samples suitable for thermomechanical treatment an intrinsic two-way shape memory effect was induced. In the coarse grained samples the development of the reversible strain during training occurs at far smaller external forces compared to the fine grained specimens. While in the microstructure of the untrained samples several martensite variants can be observed per grain, in the trained condition mostly one variant is dominating and the structure is considerably finer. Annealing at appropriate temperatures after betatizing treatment resulted in a positive effect on the magnitude of the obtained two-way shape memory effect.

Influence of the microstructure on the stability of the intrinsic two-way shape memory effect

This work presents a study of the development of the two-way shape memory effect (TWSME) and its degradation due to working cycles. An intrinsic TWSME was induced in wire specimens of two near equiatomic NiTi alloys by thermomechanical training. The development of the effect was analyzed and discussed with respect to the different microstructures obtained by preliminary heat-treatment of the samples. The investigations show that training of cold worked specimens results in a smaller extension of the TWSME, but the effect remains more stable during service cycles compared to annealed samples. Precipitates have a strong negative effect on the trainability of the alloy. They considerably impede the evolution of the recoverable strain, thus limiting the obtainable magnitude of the two-way strain.

Training and stability of the intrinsic two-way shape memory effect in Ni-Ti alloys

In recent years the two-way shape memory effect (TWSME) in Shape Memory Alloys has become a subject of considerable interest due to its ability to generate spontaneous reversible changes of shape during thermal cycling. Trained elements are frequently used as actuators, where they are subjected to several motion cycles. From the application point of view, the functional stability during repeated employment is therefore of considerable importance. So far, the results published on this aspect are rather limited, especially concerning NiTi based alloys. It was therefore the aim of this work to study the influence of training parameters on the degradation characteristics of the intrinsic TWSME for a specific thermomechanical training procedure.