Nickel-titanium Shape Memory Alloy Research Articles

Effect of Coating Method on the Surface Characteristics of NiTi Shape Memory Alloy

Metallic materials including stainless steel, cobalt-chromium alloys, titanium and its alloys, and nickel-titanium (NiTi) shape memory alloys have been used in biomedical applications since the 1940s due to their favorable mechanical properties. Classified as a smart material, NiTi alloy has gained noticeable popularity in biomedical fields such as orthopedic, dental, and cardiovascular applications as it retains reasonable corrosion resistance and biocompatibility along with unique shape memory properties. However, several studies have shown their limits in medical applications due to the risk of Ni ions release from the NiTi implant surface. In order to prevent Ni release, ceramic-based surface coatings such as hydroxyapatite, alumina, and titanium dioxide have been proposed applied by various methods such as electrophoretic deposition, sol-gel, biomimetic and dip coating. In this work, HA coatings on NiTi wire samples were obtained by using biomimetic and dip-coating methods. The biomimetic coating consisted of immersing the NiTi wire samples into simulated body fluid for 24 and 48 h periods, whereas dip coating was accomplished by placing the NiTi wire samples into the HA suspension and retracting the sample with a controlled speed. The effects of the two coating methods on coating quality, surface characteristics and corrosion behavior of NiTi wire samples were investigated. Dip coating method was shown to be a more favorable technique for the NiTi wire samples used throughout this study.

Origins of the transformability of nickel-titanium shape memory alloys

The near equiatomic NiTi alloy is the most successful shape memory alloy by a large margin. It is widely and increasingly used in biomedical devices. Yet, despite having a repeatable superelastic effect and excellent shape-memory, NiTi is very far from satisfying the conditions that characterize the most reversible phase-transforming materials. Thus, the scientific reasons underlying its vast success present an enigma. In this paper, we perform rigorous mathematical derivations and accurate density-functional theory calculations of transformation mechanisms to seek previously unrecognized twinlike defects that we term involution domains, and we observe them in real space in NiTi by aberration-corrected scanning transmission electron microscopy. Involution domains lead to an additional 216 compatible interfaces between phases in NiTi, and we theorize that this feature contributes importantly to its reliability. They are expected to arise in other transformations and to alter the conventional interpretation of the mechanism of the martensitic transformation.

Actuation-Induced stable crack growth in near-equiatomic nickel-titanium shape memory alloys: Experimental and numerical analysis

Shape Memory Alloy (SMA) actuation technology requires a thorough understanding of the failure response of these alloys under loading that involves thermal variations, termed “actuation” loading. In this paper, the experimental observation of stable crack growth in SMA compact tension specimens during temperature changes under constant bias loads is reported for the first time. The intrinsic damage mechanisms that promote crack advance are those reported in literature for nominally isothermal overload fracture, i.e., cleavage assisted by ductile void growth. Moreover, a numerical analysis is employed, and the resulting simulations are compared with the experimental data with the purpose of building confidence in the insight provided on the role of extrinsic mechanisms that further promote or impede crack advance. It is concluded that phase transformation plays a dual role on the crack growth kinetics by promoting crack growth when occurring in a fan in front of the crack tip and providing the toughness enhancement that results in stable crack growth when left in the wake of the advancing crack. While the latter is well known as transformation-induced toughness enhancement, the former has just been recently observed experimentally and is characteristic of SMAs subjected to actuation loading conditions.

Piezoresistive sensing of cementitious composites reinforced with shape memory alloy, steel, and carbon fibres

The piezoresistive behaviour of self-compacting cementitious composites incorporating superelastic nickel-titanium shape memory alloy fibres (SMAFs), steel fibres (SFs), and carbon fibres (CFs) is presented. Piezoresistivity in the cementitious composites allows sensing stress and strain under cyclic compression. The matrix of composites consists of cement, fly ash, ground-granulated blast-furnace slag, and silica fume as the binder and both fine aggregate. Six fibre volume fractions ranging from 0.25% to 1.50% at intervals of 0.25% are considered for SMAFs and SFs while CFs are added at low volume fractions ranging from 0.1% to 0.6% at intervals of 0.1%. First, fresh and mechanical properties of the developed composites are discussed. Then, the piezoresistive sensitivity, repeatability, and gauge factor are analysed under cyclic compressive stress with an amplitude of 10 MPa. Results show noticeable polarisation in composites with SMAFs and SFs, compared to CF reinforced composites. A maximum gauge factor of 935 and the highest correlation coefficient between fractional change in resistivity and compressive strain are obtained for the composite containing 0.6% CFs.

Latent Heat of Martensitic Transformation in Nickel Titanium Shape Memory Alloys Depending on Composition and Thermo-Mechanical Processing

The amounts of latent heat of the martensitic transformation in Nickel Titanium shape memory alloys with different compositions between 48.0 at.% and 51.0 at.% of Ni were measured with a differential scanning calorimeter. The results indicate that the solution-treated samples show larger amount of latent heat than those after thermo-mechanical processing. The largest amount of 37.5 J/g was obtained in the equiatomic alloy annealed at 1,100 °C followed by quenching. It was suggested from the present result together with literature survey that NiTi shape memory alloys may be novel solid-state heat-storage materials.

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.

Reconstruction of Heat Sources Induced in Superelastically Loaded Ni-Ti Wire By Localized Deformation Processes

Shape memory alloys (SMAs) are phase transforming materials featuring strong thermomechanical couplings. Infrared thermography and heat source reconstruction (HSR) enable to track the calorific signature of deformation processes. The objective was to characterize the transformation processes in a superelastic nickel-titanium SMA wire subjected to a force-controlled superelastic tensile cycle. In-situ recorded thermographs were converted into spatiotemporal maps of heat sources using an in-house developed post-processing method based on the heat diffusion equation resolved numerically for unknown heat sources. Sequentially appearing patterns of localized transformation events of four types were identified and associated with martensite bands nucleations and their subsequent merging upon tensile loading. Analogically, the events associated with austenite bands nucleations and their subsequent merging were identified upon unloading. In addition, weak heat sources observed before and after the localized transformation events were associated with the homogeneous martensitic transformation. The intrinsic dissipation heat associated with the nucleation and merging events is estimated to be ~ 25% of the released/absorbed latent heat.

Parametric evaluation of electrical discharge coatings on nickel-titanium shape memory alloy in deionized water

Nickel-titanium shape memory alloy (NiTi) has a unique capacity to restore its initial shape after deformation, which is highly applicable to orthopaedic implantations, especially for the minimization of invasive surgeries. The high nickel content of this alloy can lead to unfavourable effects on the human body upon dissolution; thus, a reliable barrier of coatings on the NiTi surface is required to alleviate the nickel migration and increase its biocompatibility. In this paper, analyses of a titanium oxide layer development on NiTi surface using electrical discharge coating (EDC) process is presented. The recast layer thickness, crater sizes, and surface roughness were characterized based on five parameters; polarity, discharge duration, pulse interval, peak current, and gap voltage. The results show that the discharge duration is the most significant parameter to influence all responses, followed by peak current. The surface characteristics of the EDC substrate is depending on the crater formations and is highly correlated with the discharge energy intensity. As a result, appropriate parametric conditions of the electrical discharge coating process can enhance the NiTi surface for future medical applications, without compromising the shape memory effect.

Effect of Fibre Material and Fibre Roughness on the Pullout Behaviour of Metallic Micro Fibres Embedded in UHPC

The use of micro fibres in Ultra-High-Performance Concrete (UHPC) as reinforcement increases tensile strength and especially improves the post-cracking behaviour. Without using fibres, the dense structure of the concrete matrix results in a brittle failure upon loading. To counteract this behaviour by fibre reinforcement, an optimal bond between fibre and cementitious matrix is essential. For the composite properties not only the initial surfaces of the materials are important, but also the bonding characteristics at the interfacial transition zone (ITZ), which changes upon the joining of both materials. These changes are mainly induced by the bond of cementitious phases on the fibre. In the present work, three fibre types were used: steel fibres with brass coating, stainless-steel fibres as well as nickel-titanium shape memory alloys (SMA). SMA fibres have the ability of “remembering” an imprinted shape (referred to as shape memory effect), triggered by thermal activation or stress, principally providing for superior performance of the fibre-reinforced UHPC. However, previous studies have shown that NiTi-fibres have a much lower bond strength to the concrete matrix than steel fibres, eventually leading to a deterioration of the mechanical properties of the composite. Accordingly, the bond between both materials has to be improved. A possible strategy is to roughen the fibre surfaces to varying degrees by laser treatment. As a result, it can be shown that laser treated fibres are characterised by improved bonding behaviour. In order to determine the bond strength of straight, smooth fibres of different metal alloy compositions, the present study characterized multiple fibres in series with a Compact-Tension-Shear (CTS) device. For critical evaluation, results obtained by these tests are compared with the results of conventional testing procedures, i.e., bending tests employing concrete prisms with fibre reinforcements. The bond behaviour is compared with the results of the flexural strength of prisms (4 × 4 × 16 cm3) with fibre reinforcements.

Heterogeneity and inelasticity of deformation in a notched martensitic NiTi shape memory alloy specimen

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.

Design and behaviour of steel braced frame reinforced with NiTi SMA wires

Shape memory alloys offer unique characteristics that can be utilized in enhancing the seismic performance of various types of structures. An innovative steel frame braced with tension-only pseudoelastic nickel-titanium (NiTi) shape memory alloy (SMA) wires was developed in this research for seismic retrofit applications. The structural performance of the developed system was experimentally compared with standard steel frame braced with tension-only plates. Three main parameters were investigated, namely, the ability of the system to achieve a self-centering response, the amount of energy dissipated and the ability to reach a target drift of 4%. Test results showed that the SMA braced frame had significant re-centering capabilities (i.e. recovered large portions of the post-elastic drifts), and a potential for moderate energy dissipation. Failure involved SMA wires slipping out of their anchorage which did not allow for their full utilization. This paper focuses on the design and experimental test of a control braced frame and an SMA braced frame.

Optimization of Wire EDM Process Parameters for Medical Grade Nickel Titanium Shape Memory Alloy

Abstract Nickel Titanium (NiTi) alloys are the class of smart materials classified under shape memory alloys. The traditional machining of these alloys is hard because of various inherent mechanical characteristics of these alloys. Therefore, non-traditional machining process such as wire electro discharge machining (WEDM) has been employed for machining of such alloys. The present study is focused on multi-performance characteristic simultaneous optimization of WEDM process parameters, in which three system control factors, namely, pulse on time (TON), pulse off time (TOFF) and wire feed (WF) are considered for simultaneously maximizing material removal rate (MRR), while minimizing surface roughness (SR) and tool wear rate (TWR). The simultaneous optimization is performed using Taguchi’s Quality Loss Function. Analysis of means and analysis of variance have been carried out to identify the significance level of each system control factor. The different levels of settings and the optimized setting have been analysed using scanning electron microscope images for surface morphological studies. The multi-response optimization investigations revealed that TON is the major contributing factor and optimal performance values were obtained at TON of 125μs, TOFF of 25μs and at WF of 4 m/min.

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.

Strengthening bonding strength in NiTi SMA fiber-reinforced polymer composites through acid immersion and Nanosilica coating

Extensive applications of shape-memory alloy fiber-reinforced polymer composites are limited by the weak bonding conditions between alloy fibers and their surrounding matrix. In this work, we aim to investigate an effective means for elevating the interfacial bonding between nickel-titanium shape-memory alloy fibers and epoxy matrix by the combination of nitric acid immersion and nanosilica particles coating. Three experimental tests were carefully designed and conducted for verifying and validating the proposal. They include the uniaxial tensile test of chemically etched fibers, the pull-out test against a single fiber embedded inside a cylindrical epoxy matrix, and the three-point flexural test on fiber-reinforced polymer composites. Extensive parametric tests were performed in order to both qualify and quantify the effects of chemical etching, nanoparticle coating, and their combination. Experimental tests were followed a comprehensive analysis on both the mechanical behavior and the microscopic and macroscopic morphology of the specimens. Experimental results suggest the possibility of effectively elevating the interfacial bonding strength between nickel-titanium shape-memory alloy and epoxy matrix by the appropriate coupling of acid immersion and nanoparticle coating. Microscopical optimization mechanisms were also proposed and validated in terms of both intergranular and transgranular cracking along fiber/matrix interface subjected to shear loading.

Loading influence on the corrosion assessment during stress-induced martensite reorientation in nickel-titanium SMA

The effect of stress-induced martensite reorientation on the corrosion behavior of a nickel-titanium (NiTi) shape memory alloy (SMA) was characterized by mechanical-electrochemical techniques. This martensite reorientation influenced the passive state of NiTi SMA. These passive to active state transitions could be sensed by the global interfacial electrochemical measurements, such as electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and linear polarization resistance (LPR) methods. The increase in the loading when exposed to a corrosive environment resulted in the breakdown of the passive layer. A later repassivation stage was reached due to the formation rate becoming more dominant than the breakdown rate during loading conditions. The integration of the mechanical loading by tensile testing, strain distribution by digital image correlation (DIC) and electrochemical methods by OCP and EIS, characterized and detected the influence of the mechanical effect in the corrosion assessment and interfacial mechanisms.

Influence of laser cycling on electro-structural features of nickel-titanium SMA fabricated by LPBF process

The effect of laser cycling and tempering on structural heterogeneity and electro-structural properties of 2D layers and 3D multilayers of nickel-titanium intermetallide during the laser powder bed fusion (LPBF) process has been studied by the authors and is offered below. Besides, the expediency of the laser melting and solidification parameters tuning, implementation of preliminary and subsequent laser tempering is proved by the final state of the shape memory alloy (SMA). It is shown that specific electrical resistivity (SER) of the studied phases (austenite, R-phase, martensite) abnormally varies with temperature. During a high-speed laser cooling, SER increases in two stages, and the first pre-martensite stage is accompanied by structural heterogeneity growth and corresponds to the intermediate R-phase formation. Electro-structural features of the 3D printed NiTi are almost 2–3 times higher than those of melted 2D monolayers.Varying structural heterogeneity by changing the laser influence (LI) parameters and/or heat treatment allows us to get a material with different functional properties. It can be used both in medicine for approximating SME to the human body temperature, and also in high-temperature applications, for example, as dampers.

Rheological Investigation of Prealloyed NiTi Shape Memory Alloy Powders

Usage areas of Nickel-Titanium shape memory alloys (SMA) have become widespread gradually and new production techniques on these alloys have been improved nowadays. Especially its unique shape memory effect (SME) and employability in many areas have a crucial role in making it widespread. In experimental works, rheological studies have been done for powder injection molding by using prealloyed NiTi powders which are 10 µm, 14 µm and 30 µm sizes. In this study, SEM, EDS, TG, DTA and DSC analyses have been used to determine rheological properties, transformation temperatures and appropriate binder system. Polypropylene, polyethylene glycol 8000 and stearic acid have been used for appropriate prescription system in the work. To determine the optimum production conditions with obtained prescription, different powder-binder rates (60-70 wt% powder) between 160˚-200˚ temperature ranges have been tried. Since PEG series binder has been used in production of parts, two stage debiding process has been carried out. The best molding quantifications have been determined with powders which are 10 µm and 14 µm sizes and 62-64% powder loading rates.

Simulation of three-point bending test of the silicon-coated nitinol bar

Self-expanding intravascular stents made of nickel-titanium shape memory alloy (nitinol) serve to recover the lumen of vessels suffered from atherosclerotic stenosis. A novel drug-eluting stents are usually wrapping by a porous coating containing an antiproliferative drug around the stent struts. From the practical point of view, it is very important to understand the strength of the coating owing to the flexibility of the nitinol struts. In this paper, we study numerically the strength properties of a nitinol bar covered by a silicon coating, as a part of such a stent, under three-point flexural test using the method of movable cellular automata. The results show stages of the coating fracture as well as a periodic pattern of the cracks in the coating, which are confirmed by experimental evidence. The difference in computational and experimental results shows the way for future works on improving the model assumptions.

Investigation on the micro-mechanism of martensitic transformation in nano-polycrystalline NiTi shape memory alloys using molecular dynamics simulations

In this study, the temperature- and stress-induced martensitic transformations in nano-polycrystalline nickel-titanium shape memory alloys were investigated using molecular dynamics simulations, and in turn the micro-mechanism was examined at the atomic scale. The transformation temperature and the stress hysteresis were found to decrease with the decreasing grain size when the grain size was less than 50 nm. The observed martensite variants’ nucleation and growth phenomena during the martensitic transformation and its reverse one clearly demonstrated that the martensitic transformation was attributed to the formation of compound nanotwins structure inside the nano grains, which was a reasonable agreement with available experimental data.

Phase transformation volume amplitude as a low-cycle fatigue indicator in nickel–titanium shape memory alloys

Transformation-induced plasticity due to the motion of martensite fronts is a key mechanism for damage accumulation during cyclic loading of NiTi shape memory alloys (SMAs). Intuitively, a larger motion of martensite fronts should result in a larger accumulation of defects, and lead to a shorter low-cycle fatigue life. We validate this hypothesis through fatigue tests and surface strain-field measurements. This result is agnostic of the tensile or bending loading mode. We also demonstrate that the fatigue lifetime intrinsically depends on a combination of two factors: The transformation volume amplitude and the number of inclusions interacting with the phase transformation domains.