Two-way Shape Memory Research Articles

One-way and two-way shape memory effects of a high-strain cis-1,4-polybutadiene–polyethylene copolymer based dynamic network via self-complementary quadruple hydrogen bonding

A polymer network based on a cis-1,4-polybutadiene–polyethylene copolymer exhibits multi- and two-way shape memory effects as well as a high-strain capacity.

Initiation of the shape memory effect by temperature variation or ultrasonic vibrations in the NiTi shape memory alloy after different preliminary deformation

The strain variations induced by conventional heating or ultrasonic vibrations in the Ni – 49.8 at.% Ti shape memory alloy after different preliminary deformation (from 5 to 30%) in the martensite state were studied. It was found that if the residual strain was 16% or less, then the shape memory effect value did not depend on the initiation method. The strain recovery was induced by ultrasonic vibrations during 20 s and occurred at lower temperatures than during conventional heating, due to partially suppression of the martensite stabilisation effect. The ultrasonic vibrations did not influence the plastic strain but they decreased the value of the two-way shape memory effect due to the dislocation rearrangement. The results of the study showed that the USVs might be successfully used for the initiation of the shape memory effect in actuators without heat sources.

Giant rubber-like behavior induced by martensite aging in Ni51Fe18Ga27Co4 single crystals

Stress-induced martensite aging along the [110]B2||[100]L10-direction in compression was employed to enhance the functional properties of Ni51Fe18Ga27Co4 single crystals. Stress-induced martensite aging induces two effects. Firstly, a two-way shape memory effect with a reversible compressive strain of −4.3% along the [110]В2 direction and a reversible tensile strain of +9.0% along the normal [001]B2-direction. Secondly, rubber-like behavior with giant reversible compression strain up to −15.0% along the [100]B2-direction caused by L10-martensite variant reorientation was observed. This exceeds the maximum compressive transformation strain (−6.3%) during the B2-L10 martensitic transformation by a factor of two.

Two-Way Shape Memory Effect Induced by High-Temperature Isothermal Training in [001]-Oriented Heterophase Single Crystals of Ni49Fe18Ga27Co6 Alloy

Results of investigations of the two-way shape memory effect (TWSME) and its stability during stress-free thermal cycling of [001]-oriented Ni49Fe18Ga27Co6 single crystals after high-temperature isothermal training (100 loading/unloading cycles at a temperature of 373 K) in superelasticity (SE) conditions during L21–L10 martensitic transformations (MT) are presented. A maximal reversible strain of single crystals aged at 673 K for 4 h with 3.8% TWSME has been obtained. For crystals after stress-free aging at 673 K for 4 h, the TWSME is 2.2%. It has been established that on the one hand, the high-temperature training at 373 K, in comparison with analogous training at a low temperature of 295 K of stress-assisted aged crystals causes the maximal TWSME to decrease by 1.7%, and on the other hand, it promotes the increase of the cyclic stability of TWSME during reversible strain. The influence of the isothermal training temperature on the TWSME mechanism and on its degradation during stress-free cooling/heating cycles is discussed.

Facile fabrication of foamed natural Eucommia ulmoides gum composites with heat‐triggered shape memory behavior

Shape memory polymers (SMPs) are a type of smart material with potential application in aerospace engineering, biomedical devices, and sensors. In this article, we propose a new technique for the preparation of foamed Eucommia ulmoides gum (EUG) composites, which have excellent one‐way and two‐way shape memory performances. The advantages of these foamed composites are not only light weight but also good shape memory performance. For their remarkably light weight, large deformation, and efficient cost, these composites are expected to be a kind of popular smart materials in the future. We studied the properties of foamed composites via mechanical measurements, swelling experiments, thermal analysis and shape memory analysis, hoping to establish the relationship between the microstructure and macroscopic properties of composites. The results showed that the composites with 0.4 phr sulfur had superior one‐way shape memory properties and moderate mechanical properties. In the two‐way shape memory tests, the composites performed excellent two‐way shape memory properties with the applied stress ranging from 200 to 350 KPa. POLYM. COMPOS., 40:3075–3083, 2019. © 2018 Society of Plastics Engineers

Two-Way Shape Memory Effect Induced by Tensile Deformation in Columnar-Grained Cu71.7Al18.1Mn10.2 Alloy.

Columnar-grained Cu71.7Al18.1Mn10.2 shape memory alloy (SMA) was prepared by a directional solidification method and exhibited a high superelasticity of 8.18% and excellent ductility at room temperature, which provided the possibility of obtaining high shape memory. However, proper pre-deformation is an essential part of repeatedly obtaining large and stable shape change. In this paper, one-time uniaxial tensile pre-deformation was carried out at the temperature range −70–−80 °C. Then, the two-way shape memory effect (TWSME) of the alloy was evaluated by the martensitic transformation strain (εM) which was measured by a thermal expansion test to investigate the relationship between the pre-deformation strain (εT) and the TWSME. The results showed that εM of the columnar-grained Cu71.7Al18.1Mn10.2 alloy increased at first and then decreased with the increase of εT. The maximum value 2.91% of the εM could be reached when εT was 6%. The effects of the εT on transformation temperatures were also measured by differential scanning calorimetry. Based on the variations of transformation temperatures, the relationship between the internal stress induced by the pre-deformation process and the εM, and the influence mechanism of the pre-deformation strain on the TWSME in columnar-grained Cu71.7Al18.1Mn10.2 alloy, were discussed. The results obtained from this work may provide reference for potential applications of Cu-based SMAs, such as self-control components, fasteners, etc.

Tuning of reversible actuation via ROMP-based copolymerization semicrystalline polymers

There exists a need for reversible actuation involving two-way shape memory polymers that can respond to changes in ambient temperature in the 20 °C–40 °C range, especially for soft actuators in applications requiring near-room-temperature environmental responsiveness. Herein, we report on the fabrication and analysis of a family of soft, semicrystalline copolymers based upon cis-cyclooctene and various melting point reducing comonomers via ring-opening metathesis polymerization (ROMP). Reversible actuation behavior of such copolymers was tuned by varying not only the species and the incorporation level of a second comonomer but also the concentration of an incorporated thermal initiator for crosslinking. Results revealed that actuation was tuned to occur best and near room temperature for a copolymer consisted of 97 mol% cis-cyclooctene and 3 mol% 1,5-cyclooctadiene and 0.50 wt% dicumyl peroxide initiator. Also, all crosslinked copolymers feature actuation magnitude greater than 50% and recovery magnitude greater than 94% at a single stress of 0.8 MPa, while thermal strain hysteresis was 17.5 °C in breadth, on average.

Repeated Intrinsic Self-Healing of Wider Cracks in Polymer via Dynamic Reversible Covalent Bonding Molecularly Combined with a Two-Way Shape Memory Effect

To enable repeated intrinsic self-healing of wider cracks in polymers, a proof-of-concept approach is verified in the present work. It operates through two-way shape memory effect (SME)-aided intrinsic self-healing. Accordingly, a reversible C-ON bond is introduced into the main chain of crosslinked polyurethane (PU) containing an elastomeric dispersed phase (styrene-butadiene-styrene block copolymer, SBS). The PU/SBS blend was developed by the authors recently, and proved to possess an external stress-free two-way SME after programming. As a result, the thermal retractility offered by the SME coupled with the reversible C-ON bonds can be used for successive crack closure and remending based on synchronous fission/radical recombination of C-ON bonds. Moreover, multiwalled carbon nanotubes are incorporated to impart electrical conductivity to the insulating polymer. Repeated autonomic healing of wider cracks is thus achieved through narrowing of cracks followed by chemical rebonding under self-regulating Joule heating. No additional programming is needed after each healing event, which is superior to one-way SME-assisted self-healing. The outcomes set an example of integrating different stimuli-responsivities into single materials.

Two way shape memory effect in NiTiHf high temperature shape memory alloy tubes

Two-way shape memory effect (TWSME) in nano-precipitation hardened, Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) thin walled tubes and its thermal stability were investigated. Torsional TWSME was induced by repeated thermal cycling across their martensitic transformation under applied shear stress. The effects of training parameters and geometric factors, such as the number of training cycles, shear stress levels, and thickness of the tube walls, on the resulting TWSME were evaluated. Thermal stability of TWSME was characterized as a function of annealing treatments at elevated temperatures. It was found that under 200 MPa, 600 thermal cycles were sufficient to reach a two-way shape memory strain (TWSMS) as high as 2.95%, which was shown to be stable upon annealing up to 400 °C for 30 min. This TWSMS was 85% of the maximum measured actuation strain under 200 MPa. The microstructure after thermo-mechanical training was investigated using transmission electron microscopy (TEM), which did not indicate a significant change in precipitate structure and size after the training. However, small amount of remnant austenite was revealed at 100 °C below the martensite finish temperature, with notable amount of dislocations. Overall, it was found that nano-precipitation hardened Ni50.3Ti29.7Hf20 shows relatively high TWSMS and stable actuation response after much less number of training cycles as compared to binary NiTi and nickel lean NiTiHf compositions. Tube wall thickness and training stress levels have been found to have negligible effect on shape memory strains and number of cycles to reach the desired training level, for the ranges studied.

Preparation of self-lubricating NiTi alloy and its self-adaptive behavior

NiTi alloys with different porosities were obtained by the Spark Plasma Sintering method. The two-way shape memory effect of NiTi alloys with good integrated performance was obtained by the thermal-mechanical cycling training, and it was used to endow NiTi alloy with the self-adaptive behavior. The results show that the porous NiTi alloys with excellent tribological performances and functional properties were obtained. With the storage of oil, the friction coefficient can decrease from 0.391 to 0.041. The self-lubricating mechanism is the formation of a fractionated thin film, and the main wear form is abrasive wear. The lubricating oil inside the specimens with self-adaptive behavior can be squeezed out or stored back, so as to accommodate the lubricating film for better lubrication.

Two-Way Shape Memory Effect in Rapidly Quenched Highly Doped Alloys of TiNi–TiCu System upon Laser Treatment

Using an original approach, a pronounced two-way shape memory effect is created for the first time in rapidly quenched alloys of the quasibinary TiNi–TiCu system with copper contents of up to 38 at %. The technique includes the combined effect of the dynamic crystallization of the amorphous state by application of a single pulse of electric current of 10 ms duration and pulsed laser radiation (λ = 248 nm, τ = 20 ns). The obtained composite structure materials can be used to create different micromechanical devices, especially microswitches and microtweezers for gripping micro-objects.

The feasibility design and modeling for thermo-mechanical ocean energy harvester using shape memory alloys

In this paper a feasibility design of thermal energy harvester is proposed base on martensitic phase transformations in shape memory alloys (SMAs). Energy conversion from the thermal form to mechanical form via the martensite phase transformation in SMAs spring is modeled and analyzed. The phase transformation is induced thermally by the sea water temperature difference. In the proposed design, a macroscopic differential model of the two-way shape memory effects in one-dimensional SMA structure is employed to estimate the restoring force of the SMA spring. Power output is measured and found reasonable enough to be used. Thus the feasibility of the proposed design for ocean thermal energy harvesting is validated.

Development of Micromechanical Device on the Base of Two-Way Shape Memory Alloy Ribbon

Development of Micromechanical Device on the Base of Two-Way Shape Memory Alloy Ribbon

Two-way shape memory effect in a Ti–Zr–Nb–Ta high-temperature shape memory alloy

The two-way shape memory effect in a Ti–18.5Zr–10Nb–3.5Ta high-temperature shape memory alloy was investigated. X-ray diffraction measurement shows that the alloy is composed of orthorhombic α″-martensite. ω phase is not found in Ti–18.5Zr–10Nb–3.5Ta alloy due to the suppressing effect of Ta element. The α″-martensite laths are found in the transmission electron microscope observation; after the bending deformation, there appear a lot of dislocations. The alloy exhibits a shape memory strain of 3.8% and a high reverse martensite transformation start temperature of 464 K. The maximum two-way shape memory strain of 1.2% is obtained in the alloy with the pre-bending training strain of 10%. The mechanism can be ascribed to the effect of internal stress field caused by dislocations.

Influence of the Cross-Link Density on the Rate of Crystallization of Poly(ε-Caprolactone).

Cross-linked poly(ε-caprolactone) (PCL) is a smart biocompatible polymer exhibiting two-way shape memory effect. PCL samples with different cross-link density were synthesized by heating the polymer with various amounts of radical initiator benzoyl peroxide (BPO). Non-isothermal crystallization kinetics was characterized by means of conventional differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). The latter technique was used to obtain the dependence of the degree of crystallinity on the preceding cooling rate by following the enthalpies of melting for each sample. It is shown that the cooling rate required to keep the cooled sample amorphous decreases with increasing cross-link density, i.e., crystallization process slows down monotonically. Covalent bonds between polymer chains impede the crystallization process. Consequently, FSC can be used as a rather quick and low sample consuming method to estimate the degree of cross-linking of PCL samples.

Preparation and Characterization of Ethylene Vinyl-Acetate Copolymer/Silicone Blends with Excellent Two-Way Shape Memory Properties

This study investigated the preparation of ethylene-vinyl acetate copolymer (EVA) and silicone rubber blends using a melt-blending method. EVA/Silicone-D blends were prepared by using peroxide (Dicumyl peroxide, DCP) as a crosslinking agent. The microstructure, thermal properties, and shape memory behavior were investigated for the EVA/Silicone-D blends. DCP not only could assist the crosslinking of EVA and silicone, but also increase the compatibility of blends. Silicone tended to endow the prepared EVA/silicone shape memory blends with high thermal stability, hydrophobicity, and recovery magnitude. The crystal orientation and crosslinking effects of EVA/Silicone-D blends played key roles in the shape memory behavior. Two-dimensional X-ray diffraction analysis revealed the increased crystal orientation with increasing loads. The one-way shape memory results demonstrated that both Rf (shape fixing ratio) and Rr (shape recovery ratio) values were above 97% after three consecutive shape memory cycles. From two-way shape memory results, it was observed that Ract, actuation magnitudes, contributed from the cooling-induced elongation increased with increasing loads and EVA contents. In addition, owing to this increased molecular orientation with increasing loads, the entropy-driven contribution relative to the overall actuation magnitudes increased accordingly. However, the shape memory response was hindered, when the load exceeded a critical value. With increasing silicone contents, the entropy-driven contribution decreased due to the reduced orientation degree at the tighter networks or increased crystalline EVA contents in the blends.

A latent crosslinkable PCL-based polyurethane: Synthesis, shape memory, and enzymatic degradation

Abstract

Effects of cold and warm rolling on the shape memory response of Ni50Ti30Hf20 high-temperature shape memory alloy

The present work reveals the effects of cold and warm rolling on the shape memory response and thermomechanical cyclic stability of Ni50Ti30Hf20 high-temperature shape memory alloy (HTSMA). Cold rolling without intermediate annealing was performed up to the maximum possible thickness reduction before surface cracks appeared. These samples were subsequently annealed at various temperatures. 550 °C was determined to be the optimum annealing temperature for 30 min durations based on the differential scanning calorimetry (DSC) and microhardness test results. Isobaric thermal cycling experiments were conducted under different constant tensile stresses. The effects of warm rolling at different temperatures and thickness reductions on the resulting recoverable transformation strains, residual strains, transformation temperatures, and thermal hysteresis were also evaluated. It was shown that the rolling led to an increase in the resistance against defect generation accompanying martensitic transformation in the present HTSMA, resulting in a significant improvement in dimensional stability during thermal cycling. The results revealed that 15% cold rolling followed by the 550 °C 30 min annealing condition showed the best actuation response, exhibiting comparable recoverable transformation levels to the hot extruded samples, with much lower residual strains. All warm rolled samples exhibited notable two-way shape memory effect (TWSME) with compressive two-way shape memory (TWSM) strains, pointing out the existence of compressive internal stress storage following warm rolling. While transformation temperatures of all rolled samples were lower than those of the starting hot extruded sample, thermal hysteresis was notably higher in the rolled samples. This was attributed to the increase in dislocation density and the change in the martensite microstructure with rolling. The present study demonstrates that NiTiHf HTSMAs that have attracted recent interest in high-temperature applications can be processed by using conventional rolling methods while preserving desired cyclic shape memory response.

The Influence of Long-Term Storage on the Functional Properties of Shape Memory Alloys

The influence of long-term storage of TiNi-based and CuZnAl alloys on their functional properties was investigated. It was established that during 30 years’ storage of TiNiFe thermomechanical couplings, the stresses in them practically did not decrease. Couplings with CuZnAl alloy sleeves during storage weakened slightly. The one-way shape memory effect in equiatomic TiNi alloy did not change after 25 years. It was found that the value of two-way shape memory effect in the first cycle after storage at room temperature in martensitic state for more than 15 years was higher than before storage.

The effect of stress-induced martensite ageing on the two-way shape memory effect in Ni53Mn25Ga22 single crystals

The influence of stress-induced martensite ageing (SIM-ageing) on the two-way shape memory effect (TWSME) in Ni53Mn25Ga22 (at.%) single crystals was studied. The most effective regime of SIM-ageing (423 K, 175 MPa, 2–3 h) inducing the compressive TWSME with the strain of 3.6 (±0.2)% was established. The stability of the TWSME was investigated at thermal cycling through the temperature interval of martensitic transformations with and without overheating above the SIM-ageing temperature of 423 K. The TWSME strain does not decrease considerably during stress-free thermal cycling. In contrast, overheating above the SIM-ageing temperature on 30 K results in decreasing the TWSME strain down to 1.2%.