Superelastic NiTi Alloy Research Articles

Continuous rotating bending NiTi sheets for elastocaloric cooling: Model and experiments

Elastocaloric refrigeration using superelastic NiTi alloys has attracted much attention recently because it has a large energy saving potential, no environmental effects, and a low cost. Achieving the continuous operating of elastocaloric devices, i.e., separating the cold and hot areas on the NiTi alloys physically, benefits the efficient release and absorption of heat and avoids the reciprocal parts and intervals of outputs in the system. In this article, an analytical model including the temperature difference on the NiTi alloys and temperature span between the heat sink and heat source for continuous operating (elasto)caloric devices is presented. In order to validate the model, an experimental proof of concept was developed based on a set of rotating bending NiTi sheets with which the copper heat sink and heat source contact cyclically. Operations at different rotation speeds, applied strains, and heat transfer fluid velocities were performed to study the effects of operation variables on the cooling performance of the rotating bending NiTi sheets. The comparison between the model and experiments was given. The results in this article are significant for improving the cooling performance of continuous operating (elasto)caloric cooling and heat pumping devices.

The role of texture and restoration mechanisms in defining the tension-compression asymmetry behavior of aged NiTi alloys fabricated by laser powder bed fusion

Tension-compression asymmetry is an inherent property of superelastic NiTi alloys linked to the microstructural evolutions and the stress induced martensite transformation during different deformation modes. Deep understanding of NiTi's asymmetry, a topic that is often overlooked in additive manufacturing, is imperative in designing functional parts that experience multiple deformation modes. Accordingly, in this paper, the asymmetry parameters, namely the critical stress for martensite transformation, transformation strain, and strain recoverability were measured for an aged material with optimum superelasticity, which was fabricated by two different scanning strategies. The acquired results were correlated to the changes in the developed texture components during laser powder bed fusion and subsequent annealing processes, highlighting the role of dynamic and static restoration mechanisms, respectively. The deviation of texture from <001>||building direction, caused by the occurrence of the restoration mechanisms during fabrication and subsequent aging, was found to be dissimilar for samples fabricated by 67° or 90° rotation angles, effectively impacting the behavior of the material in different deformation modes. For a sample with enhanced superelasticity, aged at 550°C for 6 h, 90° scanning strategy resulted in lower asymmetry and superior superelasticity in compression, while 67° rotations led to highest strain recoverability in tension, but an intensified asymmetrical behavior.

Design and Modeling of Multi-vertebrae Continuum Robot for Minimally Invasive Surgery

The constant curvature assumption is normally utilized in kinematic modeling of continuum manipulators due to its simplicity. Nevertheless, in practical applications, factors such as frictional disturbances may cause the curvature of a continuum manipulator to deviate from being constant. In this article, we design a novel multi-vertebrae continuum manipulator that utilizes the uniform joint deformation curvature of a superelastic NITI alloy rod. We propose kinematic modeling approach for a multi-vertebrae continuum manipulator. To improve the unevenness of joint angles and increase motion accuracy, we inserted elastic beams into the multi-vertebrae continuum manipulator. The design takes into account frictional forces, and we established geometric constraint equations and force equilibrium equations for the hinged continuum manipulator. We employed an identification method to establish the physical equations of the deformation section. By solving these equations and analyzing the geometric relationships, we obtained the mapping relationship between the driving screw force and the end displacement of the continuum manipulator. To prove the effectiveness of the proposed method, we performed MATLAB simulations. This paper serves as a reference for the design and modeling of future minimally invasive surgical manipulators.

Enhanced antibacterial response in Zn-modified additively manufactured NiTi alloy

In this work a superelastic NiTi alloy with improved antibacterial behavior was manufactured via Laser Powder Bed Fusion (LPBF) process. In details, NiTi and Zn powders were premixed for printing fully dense samples. It was found that the presence of Zn (2 % wt.) alloyed onto the surface of the as-built NiTi samples promoted a promising antibacterial response against Staphylococcus aureus bacterium, showing an improved behaviour with respect to the undoped NiTi samples. Additionally, Zn did not supress the martensitic transformation of the NiTi alloy and allowed to maintain the superelastic recovery.

The Improved Superelastic Ni-Ti Alloy Wire in Orthodontic Treatment: Characteristics and Clinical Application

Orthodontists usually use Ni-Ti alloy wires for leveling and aligning of teeth without bending the wire. However, Japanese Ni-Ti alloy wires, which are bendable and retain their unique characteristics even after bending, have been developed. After several decades, improved superelastic Ni-Ti alloy wires (ISW) were introduced and have been found to be useful in the leveling stage owing to their superelasticity, shape-memory effect, and low hysteresis. Moreover, despite its low stiffness, the ISW has been used in all treatment stages and in many cases, except in patients with a strong occlusal force and immediately before and after orthognathic surgery. The ISW can be used to upright molars and close spaces in extraction cases with proper bends to control force and moment. Clinicians can bend wires with two pliers connected to a heat treatment machine, which increases the stiffness of the wires and the elastic modulus of the bent region while maintaining its original characteristics. Furthermore, through combined use long hooks, the lower molars can be moved mesially without tipping by moment control, which has been proven using the new sensing device we invented. In this chapter, we introduce this attractive archwire and original force systems in some clinical cases. (199/200 words)

Inhibition of hydrogen absorption due to wear between Ni–Ti superelastic alloys in NaCl solution by H2O2 addition

• Ni–Ti superelastic alloy absorbs hydrogen due to wear in NaCl solutions without marked corrosion. • The amount of hydrogen absorbed in NaCl solutions is almost as much as that in purified water. • The hydrogen absorption due to wear is inhibited by adding a small amount of H 2 O 2 into solutions. The inhibition of hydrogen absorption due to wear between Ni–Ti superelastic alloys in 0.15 M NaCl solution has been attempted by adding H 2 O 2 . For comparison with previous results in purified water, the present wear test has been conducted under the same conditions as in the previous test except for the test solutions. The amounts of wear loss and hydrogen absorbed during the wear test in the NaCl solution without H 2 O 2 are almost as much as those in water, suggesting hydrogen embrittlement. Upon the addition of 0.01 M H 2 O 2 into the NaCl solution, the corrosion potential during the wear test shifts in the noble direction; the amount of absorbed hydrogen substantially decreases. The present results indicate that the addition of a small amount of H 2 O 2 into solutions is effective for the inhibition of hydrogen absorption due to wear.

Long-term corrosion resistance of Cu-Al-Mn superelastic alloys and steel rebar for use in bridges

• Long-term corrosion behavior of Cu-Al-Mn superelastic alloys has been investigated for the first time. • Both single and polycrystal Cu-Al-Mn bars were evaluated. • Results are benchmarked against NiTi SEA composition and common steel rebar. • Potentiodynamic tests were performed to determine corrosion rates in comparison to common construction steels. Cu-Al-Mn (CAM) superelastic alloys (SEAs) have been shown to improve the seismic performance of bridges and buildings by replacing the plastic hinge steel reinforcement. Despite research on mechanical performance at the material and structural scales, there is limited data on the degradation in the properties of CAM SEAs under harsh environmental conditions; particularly, under long-term corrosion that are relevant for civil engineering structures. This research aims to fill this knowledge gap by studying the long-term corrosion resistance of CAM SEAs in comparison with different types of steel reinforcing bars (rebar) used in construction. Long-term accelerated corrosion testing of CAM SEAs and four types of commonly used steel rebar, namely, mild steel (MS), high chromium steel (XS), epoxy coated steel (ES), and stainless steel (SS), was conducted up to 1,051 days. Three different diameters (U.S. #3, Φ = 9.53 mm; U.S. #5, Φ = 15.88 mm, and U.S. #10, Φ = 32.26 mm) of steel rebar were considered to determine the effect of bar size on the corrosion rate. Mechanical tests were conducted after specimens reached predetermined corrosion levels. The critical mechanical properties of CAM SEAs and four steel rebar were extracted and analyzed. In addition, potentiodynamic polarization tests were performed and the Tafel curves were employed to determine the corrosion rates of CAM SEAs, steel rebar and NiTi SEAs. NiTi alloy was used as a reference for being the more traditional composition used as SEAs. It was found that the corrosion resistance of CAM SEAs is higher than MS and XS, but lower than NiTi SEA, SS and ES. The superelasticity, particularly the strain recovery, of CAM SEAs showed almost no degradation after over three years of corrosion. The experimental data and control groups in this research provide detailed evaluation of the corrosion resistance of CAM SEAs and guides the application of CAM SEAs in harsh environmental conditions.

Microstructure of NiTi superelastic alloy manufactured by selective laser melting

Nitinol is used in dental tools for its superelasticity, shape memory and biocompatibility. Selective laser melting (SLM) on a powder bed is a potential process for manufacturing such as dental tools. Microstructural studies are conducted on NiTi-SLM using a parallel strip strategy and 90° laser beam rotation. The multi-scale (nano-, micro- and mesoscopic) microstructure is revealed. Similar “cast” type microstructure features are found. The SLM melt is casted in a “micro-mold”. Fine micro-grains with random crystallographic orientation are formed near the mold walls and served as seats for the rapid growth of columnar dendritic grains along the 〈0 0 1〉 direction with epitaxial crystallographic relationships. The growth of the columnar grains curves towards the center of the “micro-mold”, coinciding with the center of the laser beam. It is proposed that SLM processing resembles “Metallic Self Micro-Mold Casting” (MSµMC) and/or “Self Micro-Molding” (S2M).

Characteristic variation of pulsed‐anodized NiTi surface by the adjustment of voltage‐unapplied state

Pulsed anodization of superelastic NiTi alloy in nitric acid electrolyte is a novel surface modification process that enables the formation of an almost Ni‐free TiO2 layer on their surface. The core technology is the modulation of voltage‐unapplied state that is necessary for the chemical reaction with an electrolyte. Here, the effect of the adjustment of the voltage‐unapplied state on the layer characteristics was investigated. Prolongation of the voltage‐unapplied state led to the formation of a thicker TiO2 layer. This result was attributed to the elimination of Ni by nitric acid. As a result, nanometer‐sized pores appearing as vestiges of Ni elimination increased and enlarged concomitantly. Furthermore, by prolonging the voltage‐unapplied state, the amount of Ni ions released into a phosphate‐buffered saline (PBS) solution diminished significantly. Such prolongation enabled significant suppression of the dissolution of the layer itself; thus, Ni ion release from the surface region was considered to be reduced.

Neutron diffraction study on strain rate dependent mechanical response in superelastic NiTi alloy: Bulk structural evolution and constitutive model

As expanded service environments, it is necessary to investigate the deformation behavior of NiTi shape memory alloy under dynamic and impact loadings. In this paper, we study bulk structural responses of a superelastic NiTi alloy in strain rates from 10−5 s−1 to 3 × 103 s−1 by neutron diffraction method. The objective is to quantify structural parameters of NiTi alloy at various strain rates, and compare the differences in deformation mechanisms. Further, we derive a phenomenological model based on experimental cognition to describe the strain rate dependent mechanical responses. The results demonstrate that impact loading obviously prevents stress-induced martensitic transformation in NiTi alloy. The austenite parent phase undergoes more significant slip deformation during high-speed deformation, which is hardly observed in quasi-static compression. Meanwhile, the austenitic crystal orientation during impact loading is not essentially different from that of quasi-static condition. Through an estimation of temperature effect, the strain rate effect is caused by adiabatic temperature rise. In validation of the phenomenological model, it is proved that the model well reproduces not only stress-strain curves but also structural parameters obtained from the neutron diffraction experiments.

Hydrogen absorption due to wear between Ni–Ti superelastic alloys in water

Hydrogen absorption behavior has been investigated during wear between Ni–Ti superelastic alloys in purified water at room temperature by a hydrogen thermal desorption analysis. For the present wear test under a rotation speed of 100 rpm and an applied load of 2.0 N, the amounts of mass and thickness losses of a specimen increase without marked corrosion with increasing wear time. Even after 0.5 h, there is sufficient hydrogen absorption (approximately 12 mass ppm) to be detected. The amount of desorbed hydrogen (absorbed hydrogen) increases with increasing wear time. The hydrogen desorption behavior of wear specimens is not necessarily the same as those of cathodically hydrogen-charged specimens and specimens that absorbed hydrogen in acid solutions, as reported previously. The residual martensite phase and hydride are observed after the wear test. The present results indicate that Ni–Ti superelastic alloy absorbs a substantial amount of hydrogen, which causes hydrogen embrittlement, during wear in water within a short time.

Kinematics analysis and trajectory planning of a continuum manipulator

Continuum manipulators with flexibility and compliant characteristics have attracted the attention of more and more researchers due to its wide application prospect. This paper introduces the design of a continuum manipulator made with super-elastic material NiTi alloy that can increase the motion posture and obtain wide workspace of a continuum manipulator. The design adopts a novel co-radial configuration and uses NiTi alloy rods as the segmented skeleton of the manipulator, which is driven by the stepper motors installed on the base of the manipulator. Under the assumption of piecewise constant curvature bending, the homogenous transformation matrix is proposed for analyzing forward kinematics and an efficient closed-form solution based on the displacement compensation method is proposed to investigate the inverse kinematics. The end position of each segment of the manipulator is obtained through numerical simulations, and the results reveal the reachable workspace of the designed manipulator. In addition, the circular trajectory planning of the manipulator is analyzed and discussed based on its inverse kinematics. Finally, a series of experiments are conducted using the manipulator to verify the effectiveness of the proposed model. The experimental results for the space posture and the trajectory planning are consistent with the simulation results, which demonstrates that the designed continuum manipulator has good motion performance.

Biocompatibility and antibacterial behavior of electrochemically deposited Hydroxyapatite/ZnO porous nanocomposite on NiTi biomedical alloy

In this article, Hydroxyapatite (HA) and its composite with ZnO nanoparticles were electrochemically deposited on NiTi superelastic alloy to evaluate the effect of nanoparticles on its various properties. Morphological observations revealed that the addition of 400 mg/L ZnO nanoparticles created an HA layer with a porous network structure. FTIR and GIXRD results pointed out that the nanoparticles contributed to pure HA synthesis. Given AFM topography parameters, a significant rise was detected in roughness values as ZnO entered the HA matrix. The rough surface with a larger volume of porosity improved the wettability of the composite coating by reducing the contact angle to approximately half of its initial value (14.57°). Furthermore, ZnO incorporation proved to be quite helpful in contact with living species. Cell culture assessments disclosed the considerable impact of ZnO nanoparticles on MG-63 osteoblast-like cells’ attachment and proliferation on the composite coating. They also improved cell viability reported by the MTT assay. The antibacterial resistance of HA and HA/ZnO coatings was investigated against E. Coli and S. Aureus bacteria colonies by the disk diffusion method. For the first time, the pure HA matrix showed antibacterial activity, though insignificant. The addition of ZnO nanoparticles promoted this behavior which was more vigorous in the case of S. Aureus bacteria.

Numerical Simulation of Ultrasonic Spot Welding of Superelastic NiTi Alloys: Temperature Distribution and Deformation Behavior

Abstract Ultrasonic spot welding (USW) has attracted increasing attention due to its high-throughput solid-state bonding mechanism, which shows great potential in the semiconductor and automotive industries for joining of metal sheets. However, the short welding cycle makes it challenging to effectively monitor the temperature history and deformation of the workpieces during the process. In this study, a three-dimensional (3D) finite element analysis model for USW of superelastic NiTi shape memory alloy (SMA) with Cu interlayer was developed using ansysworkbench. The thermal-stress coupled phenomena including the heat generation and stress distribution during the welding process were simulated and analyzed. First, a superelastic constitutive model for NiTi SMAs was constructed. The distribution of temperature and stress fields was then obtained by thermal-stress analysis using the direct coupling method, and the superelasticity of SMAs was observed. The simulation results showed that the highest temperature occurred in the center of the welding area during USW, which is proportional to the welding time and inversely proportional to the clamping pressure. In addition, the maximum stress occurred at the center of the contact surface between upper NiTi and Cu interlayer. After that, the validity of the simulation results was verified by setting up a thermocouple temperature measurement platform to collect the temperature data, which exhibited a good agreement with the simulated results. The simulation procedure demonstrates its potential to predict temperature and stress distributions during the USW process.

Superelastic alloy based electrical interconnects for highly stretchable electronics

To achieve stretchable inorganic electronics, improving elastic stretchability of the electrical interconnects becomes a bottleneck needed to be addressed. Here, we propose a material of Ni-Ti superelastic alloy for the design and fabrication of deformable interconnects, whose intrinsic elastic property overcomes the low intrinsic elastic strain limit of conventional metals. The serpentine interconnect made by Ni-Ti alloy with an intrinsic elastic strain limit of ~7.5% represents a much higher elastic stretchability than conventional Cu interconnect. The deformation behavior of the interconnect is systematically investigated through finite element analysis (FEA) simulations and experiments. The results reveal that the interconnect exhibits an elastic stretchability up to 196%, and its resistance only changes by 0.4% with 100% strain. Moreover, the potentials and challenges of other superelastic alloys as electrical interconnects are discussed. The proposed superelastic alloys fundamentally boost the stretchable properties of electrical interconnects, which would open up opportunities for flexible and stretchable electronics.

Corrosion resistance of a superelastic NiTi alloy coated with graphene–based coatings

The mechanical responses under superelastic loading cycles and the corrosion protection capacity of two graphene–based coatings applied to a NiTi alloy were evaluated. Graphene oxide (GO) and a composite of reduced graphene oxide (rGO) incorporated in a block copolymer of styrene–ethylene–butylene–styrene (SEBS) were deposited on the substrate via dip–coating. The morphology of the coatings was characterized by scanning electron microscopy (SEM) before and after uniaxial loading cycles up to 6%. Atomic force microscopy (AFM) was also used to evaluate the roughness of the coatings, which was expressed in terms of average roughness (Ra). The corrosion behavior was studied by potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis (MSA). SEM images suggested that both GO and rGO + SEBS create uniform coatings on the substrate. The measured Ra was 25 ± 3 nm for the bare NiTi, 24 ± 3 nm for the GO coated NiTi, and 8 ± 2 nm for the rGO + SEBS coated NiTi. Both coatings presented the capacity of following several superelastic cycles, maintaining their morphological integrity. PP and EIS showed that only the rGO + SEBS coating results in a significant improvement in terms of corrosion resistance. Furthermore, rGO + SEBS decreased the oxygen vacancies in the passive film when compared with the bare NiTi and GO coating.

Microstructure and mechanical properties of dissimilar NiTi and 304 stainless steel joints produced by ultrasonic welding

Superelastic NiTi alloy and 304 stainless steel (304 SS) were joined with a Cu interlayer by ultrasonic spot welding (USW) using different welding energy inputs. The surface morphology, interfacial microstructure, mechanical properties, and fracture mechanisms of the dissimilar NiTi/304 SS USWed joints were studied. The results showed that the surface oxidation intensified with increasing ultrasonic welding energy due to mutual rubbing between tools and sheets. The weld interface microstructure exhibited voids or unbonded zones at low energy inputs, while an intimate contact was established at the joining interface when applying a higher energy input of 750 J. With increasing energy input to 750 J, the weld interface shows two interfaces due to the behavior of plastic flow of Cu interlayer. The lap-shear load of the joints first increased, achieving a maximum value of ∼690 N at an energy input of 750 J, and then decreased with further increase in welding energy. Interfacial failure was observed at NiTi/Cu interface at all energy inputs, and no intermetallic compounds were found on the fracture surfaces of both the NiTi/Cu and Cu/304 SS interfaces.

The microstructural features and corrosion behavior of Hydroxyapatite/ZnO nanocomposite electrodeposit on NiTi alloy: Effect of current density

In the present study, hydroxyapatite/ZnO nanocomposite coatings were developed on NiTi superelastic alloy via pulse electrodeposition technique under three different current densities. The morphological observations (FESEM) indicated that under 6 mA/cm2, a compact, uniform composite layer could form, whereas lower or higher current densities resulted in non-uniform, porous coatings with uneven distribution of nanoparticles. XRD and FTIR studies revealed that pure hydroxyapatite was not achieved below 6 mA/cm2. Topographic features (AFM) were assessed and demonstrated a continuous rise in roughness parameters as current density increased. The corrosion behavior was investigated through potentiodynamic polarization as well as impedance spectroscopy techniques. According to the extracted data, the porosity and non-uniformity of coatings formed under 3 and 9 mA/cm2 caused a detrimental effect on the corrosion resistance of surfaces. The layer obtained under 6 mA/cm2 showed resistance (Rc) which was almost two times greater than those deposited under 3 and 9 mA/cm2 current densities. Last but not least, the bioactivity of coatings was evaluated in simulated body fluid. It was observed that more compact deposits offered more active sites for apatite nucleation, resulting in refined cauliflower-like grains. Accordingly, it can be asserted that the best composite coating was achieved under 6 mA/cm2 current density.

Neutron Diffraction Study on Strain Rate Dependent Mechanical Response in Superelastic Niti Alloy: Bulk Structural Evolution and Constitutive Model

Neutron Diffraction Study on Strain Rate Dependent Mechanical Response in Superelastic Niti Alloy: Bulk Structural Evolution and Constitutive Model

Degradation caused by self-multiplication of damage induced by an interplay between hydrogen and the martensite transformation in a Ni–Ti superelastic alloy

ABSTRACT The role of damage in the degradation of tensile properties related to a stress-induced martensite transformation in a hydrogen-charged Ni–Ti superelastic alloy has been investigated. By damage, we mean vacancy clusters and dislocation structures induced in advance by a dynamic interplay between hydrogen and the martensite transformation. To homogenise the hydrogen concentration, the specimen was aged at room temperature after charging with a small amount of hydrogen (approximately 30 mass ppm). In this case, no fracture occurs within 2000 cycles during a cyclic tensile deformation test in the stress plateau region generated by stress-induced martensite and reverse transformations. With this hydrogen concentration, cyclic interplay between hydrogen and the martensite transformation scarcely leads to the degradation of tensile properties. Nevertheless, following cyclic martensite transformations before aging that induces pre-damage, fracture occurs after around 1000 cycles under a cyclic tensile deformation test despite the hydrogen concentration being the same. The present results clearly indicate that pre-damage induced by hydrogen affects the subsequent transformations, thereby causing the self-multiplication of damage, which degrades the tensile properties.