NiTi Ribbons Research Articles

Heat Source Reconstruction and Its Relationship with Functional Fatigue of Pseudoelastic NiTi Ribbons

Heat Source Reconstruction and Its Relationship with Functional Fatigue of Pseudoelastic NiTi Ribbons

Magnetron Sputtering of Au-Based Alloys on NiTi Elements: Surface Investigation for New Products in SMA-Based Fashion and Luxury Accessories and Watchmaking

A novel approach for the deposition of Au-based coatings on NiTi components was proposed to give rise to innovative SMA-based products for the fashion, luxury, and watchmaking fields. Different Au-Cu and Au-Ag-Cu alloys (with confidential compositions within the color designations 2N, 4N, and 5N) were deposited by magnetron sputtering on superelastic and shape-memory NiTi ribbons. After preliminary morphological and microstructural characterizations, the influence of the film deposition on the functional, mechanical, and tribological behavior was deeply investigated. The macroscopic mechanical properties, including the damping, superelastic, and shape recovery characteristics, were not affected since the behavior upon both small and severe deformations was unchanged and the coatings were not damaged. Indeed, both the film adhesion and the precious aspect were maintained. Furthermore, a more detailed surface characterization, through nanoindentation, tribocorrosion, and scratch and wear tests, was performed. This experimental investigation evidenced the ductile behavior of the Au-based films and their good adhesion on NiTi substrates. Moreover, the coatings exhibited a good wear resistance, both in dry conditions and simulated body fluids, which proved to be suitable for fashion and watchmaking fields. Despite slight differences being observed within the films’ responses, all of them could be considered suitable and interesting for the design of smart luxury accessories, proving that the chosen deposition process is sound and reliable for these applications.

Damping properties of innovative NiTi elements: development of proof of concept and demonstrators

Thanks to their functional properties, NiTi and NiTi-based shape memory alloys (SMAs) exhibit both pseudo-elastic and intrinsic damping ability finding applications in several fields. The use of these alloys in damping devices or systems allows the attenuation of different kinds of vibration such as noise or more intense oscillations. In this work, the damping behaviour of different NiTi semifinished products has been evaluated by means of dynamic thermomechanical measurements in order to have a preliminary indication for the integration of these SMA elements in damper demonstrators. The evolution of the internal friction parameter with respect to temperature, frequency and strain extent is evaluated. Moreover, some possible demonstrators’ concepts, i.e. a cladding panel, some dampers and a shock absorber, are proposed and characterized. The investigated SMA elements are a plain-woven mesh made of NiTi and steel thin wires, wave-shaped NiTi ribbons and NiTi single-turn wave springs. The elements have been thermally treated in order to achieve the required shapes and thermomechanical conditions. The perspectives and potential of these semifinished products in the field of damper devices are investigated by a preliminary characterization and discussed.

Grain-size gradient NiTi ribbons with multiple-step shape transition prepared by melt-spinning

A grain-size gradient NiTi ribbon with multiple-step shape transition was papered by means of melt-spinning. The ribbons contain coarse and fine grains in the free surface side and copper roller surface side, respectively. The grain-size gradient microstructure induces a two-stage phase transformation behavior in the ribbons during heating or cooling. After tensile deformation pre-treatment, the ribbons exhibit a back-and-forth shape change (shape A→B→A) upon a single heating or cooling process, resulting from the sequential phase transformation through the thickness of the ribbon as dictated by gradient grain size. The activating performance of the ribbons, i.e. shape transition amplitude and speed, can be customized by controlling the pre-deformation strain. This work offers a new opportunity for innovative designs to reach a novel shape memory behavior in NiTi alloys conveniently and efficiently.

Pull-out resistance of shape memory alloy nickel-titanium ribbons embedded in silicone matrix for development of flexible composites

Active flexible composites can be developed through the coupling of silicone matrices and smart materials such as NiTi shape memory alloys (SMA). This work evaluates the mechanical behavior and adhesion of NiTi SMA ribbons embedded in a silicone matrix for the development of active flexible composites. Ribbons with cross-section of 0.15 × 0.8 mm2 were cold-rolled from superelastic NiTi wires with 0.4 mm in diameter. These ribbons were heat treated to either obtain characteristics of superelasticity (SE) or shape memory effect (SME) at room temperature. Silicone, NiTi wire, and ribbons were characterized by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), and tensile tests. Adhesion between SMA ribbons and silicone matrix was evaluated with pull-out tests. The use of an adhesion promoter proved to be indispensable to ensure a good pull-out strength and shear stress at the NiTi ribbon and silicone interface. A silicone rubber cylinder with an embedded SME ribbon was built to evaluate the functionality of the NiTi ribbon in the development of flexible structures, which was demonstrated by the resistive heating and deformation of the SMA/silicone rubber composite.

Thermomechanical model for NiTi-based shape memory alloys covering macroscopic localization of martensitic transformation

The work presents a thermomechanical model for polycrystalline NiTi-based shape memory alloys developed within the framework of generalized standard solids, which is able to cover loading-mode dependent localization of the martensitic transformation. The key point is the introduction of a novel austenite–martensite interaction term responsible for the strain-softening of the material. Mathematical properties of the model are analyzed, and a suitable regularization and a time-discrete approximation for numerical implementation to the finite-element method are proposed. Model performance is illustrated on two numerical simulations: the tension of a superelastic NiTi ribbon and bending of a superelastic NiTi tube.

Finite element analysis on the effect of martensitic transformation and plastic deformation on the stress concentration factor in a thin notched superelastic NiTi ribbon

The severe nonlinear behavior caused by the martensitic transformation (MT) and subsequent plastic deformation (PD) of detwinned martensite leads to a complex local stress redistribution at the location of stress risers of superelastic shape memory alloy (SMA) components. Nevertheless, in the literature, the simple linear elastic fracture mechanics (LEFM) equations are widely used in the evaluation of the fracture response of superelastic components which has resulted in obvious conflicts between the conclusions regarding the effect of MT on the fracture parameters, i.e. stress intensity factor (SIF) and material toughness. Furthermore, the linear elasticity method is frequently used in the literature to calculate the stress intensity range ([Formula: see text]) when the fatigue crack growth rate dependence on [Formula: see text] ([Formula: see text]) is being evaluated. Moreover, the PD followed by MT is poorly considered in the fracture mechanics of SMAs. This paper presents a numerical investigation on the role of both MT and PD, as well as the notch acuity, on the evolution of notch-tip stresses and strains and stress concentration factor ([Formula: see text]) upon the incremental application of the macroscopic tensile load on a thin NiTi notched superelastic ribbon, to mimic the effects of MT and PD on the SIF of superelastic parts. It is revealed that MT results in drastic deviations of the notch-tip stress, as well as the stress concentration factor ([Formula: see text]), from that obtained in LEFM. Due to the heterogeneous evolution of MT, the trend of the deviations is not regular and unique upon monotonic external loading. Accordingly, the results represent the ineffectiveness of the LEFM method in the evolution of the stress concentration factor (hence, the SIF) and toughness in monotonic loading, as well as the stress intensity range ([Formula: see text]) under fatigue loading in SMA components.

Resin Injection Process in the Manufacture of a Polymer Composite Reinforced with NiTi Ribbons: A CFD Analysis

This work aims to numerically simulating the resin injection manufacturing process of a polymer composite,reinforced with ribbons of NiTishape memory alloy, using the software Ansys CFX®. The multiphase flow mathematical modeling was used to describe the transient and isothermal resin-air flow during the process. Results of the pressure fields, velocity andvolume fractionsof the involved phases are presented. The fluid flow inside the mold was compared withthe flow between parallel flat plates and showed to be consistent. Process parameters, such as resin volumetric flow rate, resin inlet and air outlet positions have a large influence in the mold filling time, volume and position of voids fractions inside de mold and final product quality.

Study of Interfacial Adhesion between Nickel-Titanium Shape Memory Alloy and a Polymer Matrix by Laser Surface Pattern

The aim of this article is to investigate the interfacial adhesion of Ni-Ti shape memory alloy with a polymer matrix of Poly (methyl methacrylate) (PMMA). The surface pattern on Ni-Ti plates was channeled by a solid state laser machine. The laser machine allows for creating channels on the Ni-Ti surface for infiltration of the PMMA matrix, which could be attached as an intra-surface locking pattern to the Ni-Ti surface. The influence of the PMMA matrix on the surface of the NiTi plate was evaluated by thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA). The surface characterization was carried out by an optical microscope on the PMMA/NiTi composite after mechanical testing. During mechanical testing, the polymer displays the multiple cracks in the longitudinal direction that result in slipping and fracture. TMA and DMA analyses were performed on the Ni-Ti- and PMMA-coated Ni-Ti ribbon to observe elasticity and the storage modulus for both samples. Better adhesion than 80 % was observed in the Ni-Ti surface, in the laser surface pattern, in comparison to the free plain surface. However, the polymer acts as mechanical backing that caused a reduction in the shape-memory properties of the composite material.

The Effect of Isochronous Annealing on the Surface Characteristics of Ni50Ti50 Metal Glass Ribbons

Crystallization of NiTi ribbon metallic glass of equiatomic composition has been studied by x-ray structural analysis and atomic force microscopy. The surface of the original ribbon obtained by spinning contained only crystallites of impurity phases. As a result of exposure to annealing for 10 min at a temperature of 500°C, a phase transition has been observed in the form of Ni50Ti50 phase ordering. The parameters of inhomogeneities, roughness, fractal dimension, and values of multifractal formalism have been calculated in the analysis of the ribbon surface geometry. It has been found that all the above characteristics had local extremes after heat treatment at 500°C for 10 min. In particular, the minimum of the values of the widths of the spectra of singularities has been observed, indicating system monofractalization.

Thermomechanically transforming Notched NiTi Thin ribbon: Effect of Martensitic Transformation on Stress Gradients

In this work, we used numerical simulations to investigate the effect of thermally induced forward martensitic transformation (MT) under constant load on the stress redistribution around stress risers. Finite element simulations of stress redistribution around two-sided notches in a thin NiTi ribbon subjected to constant tensile force and homogeneous cooling from austenite to martensite were performed. Changes in stress gradients around the notches were analyzed with regard to heterogeneity of thermomechanically driven MT. A parametric study was performed in order to understand the effect of transformation strains, Young’s modulus of martensite and notch radius on the magnitude of notch-tip stresses after the completion of MT. We found that forward MT critically amplifies the initial elastic stress gradients around the notch. The increase in magnitude of notch-tip stress is closely related to elastic deformation in martensite needed to accommodate the transformation strain in the surrounding notch unaffected zone. This approximation holds for notch radii being small compared to the ribbon width. For large radii MT is more homogeneous, the notch-tip is more shielded from transforming surroundings and, hence, the notch-tip stress approach the nominal stress.

Shape Setting in super-elastic NiTi ribbon

The various application of shape memory alloys requires setting of NiTi ribbon in particular form. Shape setting is accomplished by constraining the NiTi ribbon on a mandrel that undergoes heat treatment. Surface annealing of super-elastic NiTi ribbon was performed shape inside the furnace. The duration of furnace temperature was kept fixed for 2 h. The sample was held in bend position for annealing the martensitic phase created on the surface of NiTi ribbon. The aim is to anneal the martensitic phase on the upper surface region and introduce the compressive stress on the edge of the alloy. The induced surface strain determines the volume fraction of martensitic formed on the sample in the flat position at room temperature. The shifting of the peak and evolution of new peaks as function of the temperature on the annealed NiTi ribbon at the flat position was observed. X-ray diffraction was carried out on the shape set bended sample at room temperature and constrained flat position (both outer and inner surface). The bended NiTi ribbon annealed at 350° C shows a mixture of austenitic phase and plastic deformation region that confirms from the broadening of some peaks. The residual stress arises from the plastically induced periphery region and the elastic core region that allows the NiTi ribbon returns partially to original flat shape. However, fully shape set was achieved on super-elastic NiTi ribbon annealed at 400 and 450 °C.

Superelastic Behavior in NiTi Shape Memory Alloy Wires and Ribbons

Shape memory alloy ribbons in austenitic state were studied in a tensile testing machine in order to assess their superelastic behavior. They were compared with conventional materials and hair wire. The shape memoy alloy ribbon shows a particular behavior, with an ultimate tensile stress of about 1450 MPa reached at 9.5 % strain. The superelastic plateau was recorded around 590 MPa on loading and around 350 MPa on the unloading branch. Following multiple loading and unloading cycles, the superelastic behavior was not affected, nor was affected the ultimate tensile strength, that remained in the same range as for the one tested before cycling. The advantages of the superelastic ribbon compared to the ones of conventional materials are discussed. The superelastic properties of the ribbon are in the range of single crystals on what concerns the recoverable strains. This is attributed to the particular fine microstructure of the NiTi ribbon.

A Smart Polymer Composite Based on a NiTi Ribbon and a Magnetic Hybrid Material for Actuators with Multiphysic Transduction

A smart composite material constituted of a magnetic hybrid film and a NiTi shape memory alloy (SMA) ribbon was obtained and characterized. The magnetic hybrid film was joined to the NiTi ribbon in order to combine the properties of both materials. This new composite material combines magnetic properties of the hybrid film, (Fe2O3-CMC)/(polyvinyl butyral), and the shape memory properties of the NiTi ribbon, which has a chemical composition of Ti-50.13 at. % Ni. This smart composite material has a mass of 18.3% NiTi ribbon and 81.7% magnetic hybrid film. Results obtained by DSC show that the smart composite material presents a small delay of transformation during warming and cooling because the magnetic hybrid film acts like a thermal insulator. Thermomechanical results indicate that the hybrid material also acts as a mechanical reinforcement, since it is observed that the Stress-Assisted Two-Way Memory Effect (SATWME) of the smart composite is lower than the SATWME of the SMA ribbon. The density current values of phase transformations were clearly identified with a thermomechanical apparatus developed in our laboratory. Finally, displacements of the smart composite material in cantilever configuration are obtained by applying an external magnetic field. All these results demonstrate that the smart composite material can be activated by temperature, electrical current, stress, and/or magnetic field, offering good expectations for actuating applications with multiphysic transduction.

Thermomechanical Behavior of a Composite Based on a NiTi Ribbon with a Magnetic Hybrid Polymer

A SMA ribbon is joined with a magnetic hybrid polymer in order to obtain a new composite material. The hybrid polymer is composed by nanoparticles of Fe2O3 coated by carboxymethyl cellulose (CMC), and embedded in a polymer matrix of polyvinyl butyral (PVB). In this work, the characterizations by DMA of the NiTi ribbon, the (Fe2O3-CMC)/PVB hybrid polymer and the composite material, are presented. Besides, the thermomechanical cycling at constant stress of the NiTi ribbon and the composite material are evaluated and compared; polymer acts as mechanical backing causing a reduction in shape memory properties of the composite material.

Microstructural Investigation of Melt-Spun NiTi Based Ribbons

Abstract In order to get small-dimensioned shape memory alloys (SMAs) with good functional and mechanical properties, a rapid solidification technique was employed. This paper discusses the use of different melt-spinning procedures for the preparation of proper thin NiTi ribbons. Metallographic characterization of SMAs is an important way to evaluate material properties, therefore the effect of alloy composition and different conditions of melt-spinning process (crucible, melting time and temperature) on the various inclusions and microstructure of NiTi alloys was investigated. Specimens for metallographic studies were cut from the longitudinal cross sections of melt-spun ribbons. The microstructures and different phases were examined by optical, scanning and transmission electron microscopy and the influence of the processing methods on microstructures of ribbons will be discussed.

Analyzing mechanical properties and nondestructive characteristics of brazed joints of NiTi shape memory alloys to carbon steel rods

Research is being conducted on the use of shape memory alloys, in particular NiTi, in civil engineering, due to the superelastic behavior of NiTi which can be used for damping. In this particular application, NiTi has to be joined to steel, which constitutes a major difficulty due to the considerably different properties of materials involved and the poor weldability of NiTi. Brazing was investigated for this application, since it is an economic and efficient process to joint dissimilar materials. This paper presents a study on the mechanical behavior in the superelastic regime of dissimilar NiTi/steel joints and the feasibility of a nondestructive method based on eddy currents to characterize the weld metal. Brazed joints in lap configuration were produced with a 20 % Ag braze alloy and tested under fatigue conditions in the superelastic regime. Lap joints of steel rods to NiTi ribbons have undergone up to 60 cycles of load/unload without rupture at a maximum load of the superelastic plateau close to the pull-out load of the joint. Measurements of the electrical impedance, with a helicoidal cylindrical coil along the lap joint, allowed the identification of the location of the joint, with a good spatial resolution characterizing the morphology of the brazed joints.

Characterisation of melt spun Ni-Ti shape memory Ribbons’ microstructure

NiTi alloys are the most technologically important medical Shape Memory Alloys in a wide range of applications used in Orthopaedics, Neurology, Cardiology and interventional Radiology as guide-wires, self-expandable stents, stent grafts, inferior vena cava filters and clinical instruments. This paper discusses the use of rapid solidification by the melt spinning method for the preparation of thin NiTi ribbons for medical uses. Generally, the application of rapid solidification via melt-spinning can change the microstructure drastically, which improves ductility and shape memory characteristics and leads to samples with small dimensions. As the increase in the wheel speed led to a reduced ribbon thickness, the cooling rate increased and, therefore, the martensitic substructure became finer. Furthermore, no transition from the crystalline phase to the amorphous phase was obtained by increasing the cooling rate, even at a wheel speed of 30 m/s. Specimens for our metallographic investigation were cut from the longitudinal cross sections of melt-spun ribbons. Conventional TEM studies were carried out with an acceleration voltage of 120 kV. Additionally, the chemical composition of the samples was examined with a TEM equipped with an EDX analyser. The crystallographic structure was determined using Bragg-Brentano x-ray diffraction with Cu-Kα radiation at room temperature.

Effect of the linear velocity during the melt spinning process on shape memory transformation of Ni-Ti ribbons

AbstractThe usual process to produce NiTi shape memory alloys is vacuum induction melting (VIM). Currently a new alternative process to produce NiTi shape memory alloys by rapid solidification structures called Melt Spinning has been studied. In this work, results of ribbons with a chemical composition Ti-55.2 Ni (wt %) alloy prepared by this method are presented. The ribbons are prepared at two different linear velocities: 30 m/s and 50 m/s. After that, samples are heat treated at 350 °C during 1 hour. The alloys are characterized by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction. According to the cycled DSC test, transformation peaks are associated with the B2→R→B19´ transformation during cooling and B19´→R→B2 during heating, showing transformation in multi-peaks. The martensite B19´ start (Ms) is varying from 35 to 39°C and the martensite finish (Mf) from 15 to 21°C, 42-47°C for austenite B2 start (As) and 65-69°C for austenite finish (Af) approximately. All analyzed ribbons show very similar values of transformation hysteresis temperatures at 50% of transformation of around 28°C. In order to change solidification rate, linear velocity is varied during the melt spinning process. Results indicate that linear velocity affects directly the temperature of transformation. When the linear velocity is increasing, crystallographic Ti-rich precipitates are developed, but dendritic growth disappears, changing the microstructure and decreasing these transformation temperatures. Then changes in linear velocity can dramatically affect shape memory properties, and in this case a velocity of 50 m/s produces a more homogenous alloy.

Smart Switch Metamaterials for Multiband Radio Frequency Antennas

We investigate metal–matrix composite metamaterials with embedded electrical switches made of shape memory nickel–titanium (Ni–Ti) for use in broadband radio frequency (RF) antennas. Experiments show that a Ni–Ti ribbon can form an electrical contact that opens and closes depending on the Ni–Ti phase being austenite or martensite. Finite element modeling of thermal gradients illustrates the phase change within the ribbon. Ultrasonic additive manufacturing (UAM), a solid-state additive manufacturing process, was utilized for embedding a Ni–Ti switch in an aluminum matrix. The aluminum matrix must have structural-grade strength for use in load-carrying antennas; thus, mechanical testing was conducted to quantify the longitudinal tensile, transverse tensile, and shear strength of the UAM matrix. Reconfiguration using a Ni–Ti switch was proven using a shape memory switch on a monopole RF antenna producing an operating frequency shift from 270 to 185 MHz when the switch is connected. A planar microstrip line was used to demonstrate signal transmission and reflection efficiency in a smaller, second switch. Transmission tests yielded less than −10 dB signal reflection proving the feasibility of reconfigurable planar antenna arrays using smart switches.