Nickel-titanium Shape Memory Alloy Research Articles

PVA/gelatin-based hydrogel coating of nickel-titanium alloy for improved tissue-implant interface

Nickel-titanium shape memory alloys have attracted notable interest for biomedical applications due to their unique properties. However, there are still some concerns on the potential release of nickel ions causing adverse reactions in-vivo at high concentrations. In this work, samples were coated with a polyvinyl alcohol/gelatin hydrogel. Coating stability, corrosion resistance and biocompatibility were investigated. Coated samples exhibited similar shape memory response to that of the uncoated substrates in wire form. Additionally, the hydrogel coating led to a decrease in the surface temperature and significantly improved the corrosion resistance of the shape memory substrate. The presence of the hydrogel layer does not affect the biocompatibility of the alloy while providing a semi-degradable, hydrophilic surface that forms a cushion at the tissue-implant interfaces.

A Differential Evaporation Model to Predict Chemistry Change of Additively Manufactured Metals

The desire for increased performance and functionality has introduced additional complexities to the design and fabrication of additively manufactured (AM) parts. However, addressing these needs would require improved control over local properties during the fabrication process. In this regard, differential evaporation is an inherent characteristic in metal AM processes, directly influencing local chemistry, material properties, functionality, and performance. In the present work, a differential evaporation model (DEM) is presented for laser powder bed fusion (LPBF) AM to predict and control the effect of evaporation on chemistry and properties on local and part-wide scales. The DEM model is coupled with an analytical thermal model that is calibrated against 51.2 Ni [at.%] nickel titanium SMA single-track experiments and a multi-layer model that accounts for the AM part’s multi-layer design and the inherent melt pool overlap and chemistry propagation. The combined hierarchical model, consisting of the thermal, evaporation, and multi-layer components, is used to predict location-specific chemistry for LBPF AM fabrication of 50.8 Ni [at.%] nickel titanium shape memory alloys(NiTi SMAs). Model predictions are validated with values obtained from multi-layer experiments on a commercial LPBF system, resulting in a root mean square error (RMSE) of 0.25 Ni [at.%] for predicted Ni content. Additionally, martensitic transformation temperature, Ms, is calculated and compared with empirical data, resulting in an RMSE of 18.6 K. A practical account of the cumulative and propagative thermal-induced evaporation effect on location-specific chemistry is made through this linkage of models. Fundamentally, this model chain has also provided a solution to the forward modeling problem, enabling steps to be taken towards resolving the inverse design problem of deter-mining processing parameters based on desired location-specific properties.

Evaluation of the mechanical properties and clinical application of nickel-titanium shape memory alloy scaphoid arc nail.

To investigate the mechanical and biomechanical properties of nickel–titanium (Ni–Ti) shape memory alloy scaphoid arc nail (NT‐SAN) fixator as well as study the surgical method of treating carpal scaphoid fractures and evaluate its clinical efficacy. (1) Static and dynamic bending tests with embedded axial bending fixture were conducted to study the mechanical properties. (2) To evaluate biomechanical strength and fatigue, 32 scaphoid samples were classified into four groups to perform the fixation rigidity test: intramedullary Kirschner fixation (group A), Kirschner straddle nail fixation (group B), screw nail fixation (group C), and NT‐SAN fixation (group D). Next, 24 scaphoid waist fracture models were classified to conduct fatigue experiments as follows: Kirschner straddle nail fixation (group E), screw nail fixation (group F), and NT‐SAN fixation (group G). (3) The Krimmer score chart was used for clinical evaluations. (1) NT‐SAN showed excellent mechanical performance and a long lifespan. (2) NT‐SAN was fixated with a strong intensity and an anti‐fatigue outcome. (3) Ninety‐eight interviewed patients were satisfied with the therapeutic effects of the arc nail (satisfaction rate: 95.92%). The designed strength and hardness of NT‐SAN corresponded with the anatomical characteristics of the scaphoid, and the designed mechanical properties met the biomechanical requirements of a scaphoid fracture. The fatigue strength can meet the requirements of bone healing after the scaphoid fracture. Clinical trials on NT‐SAN scaphoid fracture treatment have shown that the surgery is simple and the clinical results are satisfactory. The therapeutic level of NT‐SAN is III; thus, it is worth promoting.

Prediction of white layer formation in μ-WEDM process of NiTi shape memory superalloy: FEM with experimental verification

Microscopic changes in the surface of nickel-titanium (nitinol) shape memory alloys (SMAs) in micro-wire electro-discharge machining (μ-WEDM) due to the formation of a resolidified layer on the machined surface, called white layer, are one of the main drawbacks in the processing of such alloys. Since these changes significantly affect the shape memory and elastic recovery characteristics of these alloys, reduction of the white layer thickness (WLT) based on the selection of optimum process parameters is essential to raise the quality of the machined parts. In this regard, a finite element model (FEM) has been developed to simulate the effects of μ-WEDM process parameters, including discharge current, pulse on-time, pulse off-time, and servo voltage, on the heat distributing in Ni55.8Ti SMA to predict the WLT. The flushing efficiency of electric discharges and the effect of flow regime of the dielectric fluid on the heat distribution in the workpiece and the formation of the WLT are analyzed. Experimental data are used to verify the accuracy of the FEM. The results show that the developed model can predict the WLT in μ-WEDM process of Ni55.8Ti SMA with an average error of 14%. The effects of discharge parameters on the formation of the WLT are discussed in details based on the results of the FEM.

EFFECTS OF PARTICLE SIZE AND SINTERING TEMPERATURE ON SUPERELASTICITY BEHAVIOR OF NiTi SHAPE MEMORY ALLOY USING NANOINDENTATION

The present study investigates the superelasticity properties of spark plasma sintered (SPS) nickel titanium shape memory alloy (NiTi SMA) with the influence of sintering temperature and particle size. The nanoindentation is conducted on the surface of the NiTi SMA at various loads such as 100, 300 and 500[Formula: see text]mN. The nanoindentation technique determines the quantitative results of elasto-plastic properties such as depth recovery in the form of superelasticity, stiffness, hardness and work recovery ratio from load–depth ([Formula: see text]–[Formula: see text]) data during loading and unloading of the indenter. Experimental findings show that the depth and work recovery ratio increases with the decrease of indentation load and particle size. In contrast, increasing the sintering temperature exhibited a better depth and work recovery due to the removal of pores which could enhance the reverse transformation. The contact stiffness is influenced by [Formula: see text] which leads to attain a maximum stiffness at the highest load (500[Formula: see text]mN) and particle size (45[Formula: see text][Formula: see text]m) along with the lowest sintering temperature (700∘C). NiTi alloy exhibited a maximum hardness of 9.46[Formula: see text]GPa when subjected to indent at the lowest load and particle size sintered at 800∘C. The present study reveals a better superelastic behavior in NiTi SMA by reducing the particle size and indentation load associated with the enhancement of sintering temperature.

The Effects of Chemical Oxidation on Corrosion Behavior of Ni-Ti Alloy

Nickel Titanium intermetallic (NiTi) has been used in several fields (mostly corrosion issues were not a concern) since it was discovered about sixteen years ago. In the last decade, alloys made from NiTi have been widely used in the internal and external biomedical tools and equipment such as nails, fixation plates, bone fracture, self-expanding cardiovascular and urological stents, and orthodontic wires. The effects of chemical oxidation on corrosion behavior of Nickel-Titanium shape memory alloy have been investigated in this study. Technique of powder metallurgy was used to prepare the alloy from the elemental powders of Titanium and Nickel with 600 MPa of compacting pressure. The sintering process was achieved in a 10−4 torr vacuum atmosphere at 950 °C . XRD analysis exhibited that Ni and Ti were fully transformed into NiTi (both monoclinic and cubic phase) and Ni3Ti phase. After sintering, the samples are grinded and polished, then samples were surface modified by chemical oxidation. Optical microscopy, scanning and x-ray diffraction techniques were utilized for characterizing the surface samples. The corrosion rate has been studied in vitro by using polarization curves technique in NaF solution at different concentration (0.5, 1,1.5, 2) wt,%. Results of corrosion test indicated that the corrosion current density after chemical oxidation decreased from 0.303 for bare sample to 0.074 in 0.5% NaF solution, which indicates that TiO2 layer on the NiTi SMA surface are protective after chemical oxidation.

Latent heat storage capacity of NiTi shape memory alloy

The largest amount of latent heat of the martensitic transformation in nickel titanium shape memory alloy was explored. The measured amounts of heat in the alloys with different compositions between 48.0 at.% Ni and 51.0 at.% Ni were compared. The largest amounts of \(-\) 37.8 J/g in absorption and 34.8 J/g in emission were obtained as the averaged values of several samples with equiatomic composition. These magnitudes are close to those of novel heat storage ceramics, VO2 (51 J/g) and Ti3O5 (60 J/g), suggesting the NiTi alloy is potential candidate for heat storage material.Graphical abstract

Innovative hybrid damper with structural steel and nickel–titanium shape memory alloy bars

An innovative structural fuse was developed based on the axial combination of structural steel and nickel–titanium (Nitinol) shape memory alloy (SMA) bars. To investigate the performance of the proposed system, six dampers with different steel and SMA configurations were evaluated experimentally under cyclic loads and the stress–strain curves of the samples were obtained. The results showed that as the amount of structural steel was increased with respect to the SMA, the energy dissipation capacity increased while the re-centring capability decreased and vice versa. An SMA/steel ratio of 1.0–1.5 produced energy dissipation and re-centring in a balanced amount. The proposed device thus has uses the advantages of both structural steel and SMA while limiting their disadvantages, with cyclic energy dissipation and strain recovery being the two contrary characteristics of the proposed dampers. Finite-element (FE) simulations were also conducted and the results were compared with the experimental results in order to verify the FE model. The FE model was able to accurately simulate the proposed hybrid damper.

Finite element heat transfer analysis of the aluminum-composite solid state cooling device using latent heat storage effect of nickel titanium shape memory alloy

Finite element heat transfer analysis of the aluminum-composite solid state cooling device using latent heat storage effect of nickel titanium shape memory alloy

The effect of short time aging on the function fatigue property of super-elastic nickel titanium shape memory alloy

The effect of short time aging on the function fatigue property of super-elastic nickel titanium shape memory alloy

Optimization of WEDM parameters on surface integrity characteristics of NiTi shape memory alloy

The extensively adopted non-traditional approach of wire electrical discharge machining (WEDM) process can be formulated the complicated shapes and difficult-to-cut products with cost effective solution. The surface integrities and mechanical characteristics are sternly affected in the nickel titanium shape memory alloy (NiTi SMA) during WEDM process. In this study, an attempt has been made to investigate the effects of machining parameters on the surface integrity characteristics of NiTi SMA. The machining parameters such as servo voltage, input power, wire speed, pulse-on and pulse-off time were considered as inputs and their effects on surface integrity characteristics such as surface roughness, material removal rate, microstructure and hardness were studied. The machining process has been carried out with three levels of input parameters in the full factorial design. The TOPSIS analysis was used to evaluate the optimum machining parameters in order to exhibit the better surface integrity characteristics for NiTi alloy. The results of the optimization technique revealed that the better surface integrity properties (minimizing the roughness and maximizing the material removal rate) were achieved with the combination of 3 A input power, 9 µs pulse on time, 8 µs pulse off time, 20 V servo voltage and 3 m/min wire speed.

Synthesis of Ni/Ti thin film by magnetron sputtering to study the effect of annealing time on microstructure and mechanical properties

Thin films composed of a single layer of nickel and titanium were fabricated using magnetron sputtering at room temperature on a Si substrate and the as-deposited films were subsequently annealed to achieve nickel-titanium (NiTi) shape memory alloys. In this paper, we have reported a simplistic way that can be applied to achieve the near equiatomic Ni-Ti alloy thin films with precise controllability of chemical compositions. The compositions were prepared by taking the weight of individual elements in the proper stoichiometric proportions maintaining near equiatomic ratio of Ni and Ti. The surface-interface studies have been performed focusing on interlayer diffusion over the nano-level structure at 500°C for different holding times. The X-ray diffraction patterns of the films annealed at 500°C for different annealing times revealed various kinds of intermetallic compounds and second phase particles along with silicides. Nanoindentation tests on the single bi-layer (two alternative layers) of Ni-Ti thin films were conducted to evaluate the mechanical properties. Subsequent investigation revealed diffusion and migration of atoms between Ni and Ti layers by post-deposition heat treatment consequently have a profound impact on mechanical properties of these films.

Creep behavior of Nickel–Titanium shape-memory alloys under static and dynamic loadings: an FEM approach

The shape-memory effect and superelasticity properties of NiTinol alloys make them a versatile candidate for various engineering and biomedical applications. The use of these alloys as biomedical implants necessitates the study of any long-term creep failure possibility. The present study utilizes the finite element method to investigate this creep behavior of NiTinol alloys under static and dynamic loading conditions over a long period. The Maxwell creep model has been implemented, as well as validated with the polypropylene and low-density polyethylene. Further, the same model has been utilized for the creep analysis of NiTinol with suitable material properties and assumptions, for a time of 10,000 h. The results demonstrated the application of sinusoidal varying loads, the creep strain observed was lower as compared to other loading conditions, and in the case of static load, higher creep strain values were experienced than that of the dynamic smooth step loading case.

Cold Gas Spraying of Nickel-Titanium Coatings for Protection Against Cavitation

Cavitation erosion is a sever wear mechanism that takes place in hydrodynamic systems. Examples are turbine vanes of hydropower plants or components of valves and pumps in hydraulic systems. Nickel-titanium shape memory alloys (NiTi) are attractive materials for cavitation-resistant coatings because of their pronounced intrinsic damping mitigating cavitation-induced erosion. In this work, NiTi coatings were produced by cold gas spraying. The phase transformation behaviors of the powder feedstock and the as-sprayed coatings were investigated. Regarding the obtained transformation temperatures, the measured substrate temperatures during spraying rule out that either the shape memory effect or the pseudoelasticity of NiTi could affect the deposition efficiency under the applied conditions of cold gas spraying. Another potential effect is stress-induced amorphization which could occur at the particle–substrate interfaces and impair particle bonding by stress relaxation. Moreover, also oxide formation can be significant. Thus, the presence of amorphous phases and oxides in the near-surface zone of particles bounced off after impact was investigated. Oxidation could be confirmed, but no indication of amorphous phase was found. Besides, also the evolution of local microstrains implies that the substrate temperatures affect the deposition efficiency. These temperatures were significantly influenced by the spray gun travel speed.

Mechanical behavior of a NiTi superelastic bone plate obtained by investment casting assisted by additive manufacturing

The mechanical behavior of nickel-titanium shape memory alloys (NiTi SMA) presents unique features that benefit the bone fracture healing process: matching elastic modulus ranges, the ability to recover large deformations, and mechanical dissipation capacity. In this sense, this work proposes a new manufacturing process for NiTi SMA bone implants, particularly bone plates: additive manufacturing assisted investment casting (IC). This process delivers near-net NiTi SMA parts with either simple or complex shapes, also allowing a high degree of customization. Four identical NiTi SMA bone plates were manufactured through IC and characterized. Four-point bending tests were performed on the bone plate prototypes to verify their mechanical and fatigue behaviors. The NiTi bone plates have an elastic modulus of the order of 49 GPa at 37 °C and recovered all deformations upon unloading when submitted to four-point bending deflections up to 4 mm. The fatigue life was between 103 and >105, depending on the deflection amplitude. Our next work with this technology will address aspects of cytotoxicity and implantation of the part in animals.

Analysis of Surface Roughness and Flank Wear Using the Taguchi Method in Milling of NiTi Shape Memory Alloy with Uncoated Tools

The aim of this study was to optimize machining parameters to obtain the smallest average surface roughness (Ra) and flank wear (Vb) values as a result of the surface milling of a nickel-titanium (NiTi) shape memory alloy (SMA) with uncoated cutting tools with different nose radius (rε) under dry cutting conditions. Tungsten carbide cutting tools with different rε (0.4 mm and 0.8 mm) were used in milling operations. The milling process was performed as lateral/surface cutting at three different cutting speeds (Vc) (20, 35 and 50 m/min), feed rates (fz) (0.03, 0.07 and 0.14 mm/tooth) and a constant axial cutting depth (0.7 mm). The effects of machining parameters in milling experiments were investigated based on the Taguchi L18 (21 × 32) orthogonal sequence, and the data obtained were analyzed using the Minitab 17 software. To determine the effects of processing parameters on Ra and Vb, analysis of variance (ANOVA) was used. The analysis results reveal that the dominant factor affecting the Ra is the cutting tool rε, while the main factor affecting Vb is the fz. Since the predicted values and measured values are very close to each other, it can be said that optimization is correct according to the validation test results.

Deployment of Solar Sails by Joule Effect: Thermal Analysis and Experimental Results

Space vehicles may be propelled by solar sails exploiting the radiation pressure coming from the sun and applied on their surfaces. This work deals with the adoption of Nickel-Titanium Shape Memory Alloy (SMA) elements in the sail deployment mechanism activated by the Joule Effect, i.e., using the same SMA elements as a resistance within suitable designed electrical circuits. Mathematical models were analyzed for the thermal analysis by implementing algorithms for the evaluation of the temperature trend depending on the design parameters. Several solar sail prototypes were built up and tested with different number, size, and arrangement of the SMA elements, as well as the type of the selected electrical circuit. The main parameters were discussed in the tested configurations and advantages discussed as well.

New Highly Microbial-Induced Corrosion-Resistant Ni-P-Based Coating with Superior Mechanical Properties.

Microbiologically Influenced Corrosion (MIC) has been recognized as a widespread problem in the oil and gas industries because it causes substantial economic losses. Sulfate reducing bacteria (SRB) is one of the essential microorganisms families that have always been linked to MIC mechanisms causing localized corrosion problems. Since the bacterial adhesion on the metal surfaces is a prerequisite condition for the biofilm formation, controlling, and prevention of the colonization/proliferation of the microbial biofilms is of the critical impact regarding the MIC prevention and ensuring the public safety. Titanium-nickel shape memory alloy (TiNi SMA) nanoparticles have several marvelous features, ranging from the physical and chemical properties to the biological performance. Our electroless-plated NiP-TiNi nanocomposite coating (NiP-TiNi NCC) is used to determine the suitability of using it as an inhibition system for SRB on carbon steel (CS). The influence of anaerobic bacteria SRB on the corrosion behavior of API X80 CS, NiP, and NiP-TiNi coatings in simulated seawater was studied by electrochemical impedance spectroscopy (EIS) after different periods of incubation time (7, 10, 14, 21, 28 days). The corrosion products and biofilm formation of the incubated specimens’ surfaces after 7, 10, and 28 days of incubation are checked using the scanning electron microscope (SEM) and X-ray photoelectron spectroscopy. EIS results revealed that the antimicrobial performance of the NiP-TiNi NCC is more efficient compared to the TiNi-free coating that has an inhibition efficiency of 87.5 % after 28 days of incubation time with SRB. Furthermore, SEM, EDS and XPS results confirm that negligible corrosion occurrs for the coated surfaces. Also, The mechanical properties of this coating was proved be superior compared to the Carabon steel and the NiP-based coatings.

Processing Characteristics of Micro Electrical Discharge Machining for Surface Modification of TiNi Shape Memory Alloys Using a TiC Powder Dielectric.

Titanium-nickel shape memory alloy (SMA) has good biomedical application value as an implant. Alloy corrosion will promote the release of toxic nickel ions and cause allergies and poisoning of cells and tissues. With this background, surface modification of TiNi SMAs using TiC-powder-assisted micro-electrical discharge machining (EDM) was proposed. This aims to explore the effect of the electrical discharge machining (EDM) parameters and TiC powder concentration on the machining properties and surface characteristics of the TiNi SMA. It was found that the material removal rate (MRR), surface roughness, and thickness of the recast layer increased with an increase in the discharge energy. TiC powder’s addition had a positive effect on increasing the electro-discharge frequency and MRR, reducing the surface roughness, and the maximum MRR and the minimum surface roughness occurred at a mixed powder concentration of 5 g/L. Moreover, the recast layer had good adhesion and high hardness due to metallurgical bonding. XRD analysis found that the machined surface contains CuO2, TiO2, and TiC phases, contributing to an increase in the surface microhardness from 258.5 to 438.7 HV, which could be beneficial for wear resistance in biomedical orthodontic applications.

Effect of WEDM Process Parameters on Surface Morphology of Nitinol Shape Memory Alloy.

Nickel–titanium shape memory alloys (SMAs) have started becoming popular owing to their unique ability to memorize or regain their original shape from the plastically deformed condition by means of heating or magnetic or mechanical loading. Nickel–titanium alloys, commonly known as nitinol, have been widely used in actuators, microelectromechanical system (MEMS) devices, and many other applications, including in the biomedical, aerospace, and automotive fields. However, nitinol is a difficult-to-cut material because of its versatile specific properties such as the shape memory effect, superelasticity, high specific strength, high wear and corrosion resistance, and severe strain hardening. There are several challenges faced when machining nitinol SMA with conventional machining techniques. Noncontact operation of the wire electrical discharge machining (WEDM) process between the tool (wire) and workpiece significantly eliminates the problems of conventional machining processes. The WEDM process consists of multiple input parameters that should be controlled to obtain great surface quality. In this study, the effect of WEDM process parameters on the surface morphology of nitinol SMA was studied using 3D surface analysis, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. 3D surface analysis results indicated a higher value of surface roughness (SR) on the top of the work surface and a lower SR on the bottom portion of the work surface. The surface morphology of the machined sample obtained at optimized parameters showed a reduction in microcracks, micropores, and globules in comparison with the machined surface obtained at a high discharge energy level. EDX analysis indicated a machined surface free of molybdenum (tool electrode).