NiTi SMA Research Articles

Effect of Hydrogen on the Transformation Ratcheting of NiTi SMA Wires: Experiment and Modeling

Effect of Hydrogen on the Transformation Ratcheting of NiTi SMA Wires: Experiment and Modeling

Comparison of Key Properties of Ag-TiO2 and Hydroxyapatite-Ag-TiO2 Coatings on NiTi SMA.

To functionalize the NiTi alloy, multifunctional innovative nanocoatings of Ag-TiO2 and Ag-TiO2 doped with hydroxyapatite were engineered on its surface. The coatings were thoroughly characterized, focusing on surface topography and key functional properties, including adhesion, surface wettability, biocompatibility, antibacterial activity, and corrosion resistance. The electrochemical corrosion kinetics in a simulated body fluid and the mechanisms were analyzed. The coatings exhibited hydrophilic properties and were biocompatible with fibroblast and osteoblast cells while also demonstrating antibacterial activity against E. coli and S. epidermidis. The coatings adhered strongly to the NiTi substrate, with superior adhesion observed in the hydroxyapatite-doped layers. Conversely, the Ag-TiO2 layers showed enhanced corrosion resistance.

Investigation of laser-assisted micromachining of NiTi SMA bimorph based actuators toward developing optical shutters

Abstract This work explores laser-assisted micromachining for the precision cutting of NiTi shape memory alloy (SMA)-based bimorph in a single-pass laser scan. SMA thin film based optical shutters were developed with this method, which can generate the opening and closing via Joule’s heating rather than mechanical components. NiTi SMA bimorph was fabricated using the e-beam evaporation technique followed by micromachining using a 1064 nm fiber laser. The influence of laser power (LP), laser speed (LS), spot diameter (SP), and laser travel direction on kerf width and heat-affected zone after micromachining was studied using the design of experiments. The optimized parameter for micromachining was at LP 5 W with 5 mm s−1 LS and 50 μm SP. Various shapes were cut at the optimized parameter, including an optical shutter of a diameter of 30 mm. A LS of 6 mm s−1 has produced microchannels in the bimorph due to NiTi film removal only. The SEM analysis of the shutter reveals the formation of refined grains at micromachined edges compared to the center of the actuator. Fabricated optical shutter shows negligible optical transmittance in the 450–700 nm wavelength range.

Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Active interlocking metasurfaces enabled by shape memory alloys

Interlocking metasurfaces (ILMs) are a newly developed joining technology that relies on arrays of interlocking features that transmit force and constrain motion between adjoining bodies in one or more directions. This study explores harnessing the shape memory effect (SME) in Nickel-Titanium shape memory alloys (NiTi SMAs) in structures fabricated using additive manufacturing (AM) to advance the development of active ILMs by creating unit cells that open or close at specific temperatures. The study encompasses designing and fabricating two distinct interlocking array configurations using near-equiatomic NiTi powder and the laser powder bed fusion (L-PBF) AM technique, following a previously developed AM process optimization framework to manufacture defect-free parts. To guide the design process, finite element analysis (FEA) was employed to predict strain values during engage-disengage cycles. The martensitic transformation characteristics of the ILMs were characterized. Thermomechanical testing revealed that the ILMs demonstrate high locking force once engaged, coupled with complete shape recovery and good cyclic stability. Digital image correlation (DIC) was also employed to validate the FEA predictions during the engage-disengage cycles. The results indicate that NiTi SMA-based ILMs can be designed and fabricated into complex shapes using L-PBF. By leveraging the SME, the functionality of an ILM can be improved upon. The combination of computational modeling, additive manufacturing, and thermomechanical and physical property characterization provides a framework for designing future ILMs out of active materials.

Molecular dynamics simulation for nanometric cutting of NiTi shape memory alloys at elevated temperatures

In nanometric cutting, the deformation mechanism is closely related to the cutting temperature. This study investigates the cutting mechanism at elevated temperatures via molecular dynamics simulation of nanometric cutting on equiatomic NiTi SMAs. Mutual transformation between martensite and austenite during the indentation and cutting process, and effects of the cutting temperature are analyzed. The results indicate that the NiTi workpiece atoms experience martensite phase transformation and reverse phase transformation during the indentation and cutting process, with different orientation of martensite facilitating the transformation. Besides, the average resistance coefficient decreases with increasing cutting temperature due to the elevated temperature lubrication effect, and the cutting forces and phase transformation in the processing area also experience a gradual reduction. Some of the dislocations inside the NiTi workpiece show plugging and tangling phenomena at temperatures of 400 and 600 K. Cutting simulations reveal that fewer dislocation slips and less residual stress occur at elevated temperatures and lead to significantly reduce of residual martensite phase after cutting. From an atomic perspective, this investigation explains the cutting mechanism of difficult-to-machine materials NiTi SMAs at elevated temperatures and gives guidance for improving its machinability.

R-Phase Transformation Evolution in NiTi SMA Wires Studied via the Internal Friction Technique

The specific damping capacity variation of heat-treated NiTi was observed during a pseudoelasticity test. The detailed B2 → R-phase transformation process in cold-drawn NiTi wires undergoing middle-temperature aging was studied via X-ray diffraction, transmission electron microscope, and internal friction technique. Results show that, as aging time increased at 450 °C, the dynamic phase transition splitting from B2 → R to B2 → R1 and B2 → R2 became evident. However, such a splitting process was not observed for the sample after aging at 400 °C. The reason for R-phase generation is attributed to non-uniformly distributed stress fields. The splitting of the internal friction peak, in conjunction with high-resolution transmission electron microscope and mechanic results, suggests a substantial occurrence of short-range segregation of Ni atoms in the B2-NiTi matrix. Furthermore, the specific damping capacity (SDC) exhibits a gradual increase with prolonged annealing time. Specifically, the sample with significant dynamic phase transition splitting reaches an SDC value of 0.60.

Microstructure and elastocaloric effect of NiTi shape memory alloy in-situ synthesized by laser directed energy deposition additive manufacturing

The NiTi SMAs were successfully in-situ synthesized by LDED additive manufacturing, and the microstructural evolution and elastocaloric effect of the NiTi SMAs along parallel and perpendicular to the build direction (BD) were investigated. The NiTi SMAs at room temperature consisted of B2 austenite, B19’ martensite and the precipitation phase Ti2Ni, which was diffusely distributed within the B2 grains and at grain boundaries. Grain morphology that crosses the molten pool boundary and elongates along the thermal gradient is found in the plane parallel to the BD direction. NiTi SMAs loaded perpendicular to the BD direction have less energy loss and fewer stress mechanical training times to obtain fully recoverable stress-strain behavior, which shows better stress-strain response behavior than that loaded parallel to the BD direction. The maximum strain and adiabatic temperature changes follow a monotonically increasing trend from the maximum stress when the compressive stress was applied along parallel and perpendicular to the BD direction and rapidly removed. When the 900 MPa stress was removed, the adiabatic temperature changes reached −5.2 K and −5.9 K, respectively. The degradation of the elastocaloric effect of loading parallel to the BD direction reaches 15% after 100 stress-strain cycles (800 MPa). In contrast, the degradation of loading perpendicular to the BD direction is negligible. This work provides a new idea to realize the preparation of NiTi SMAs and to study the elastocaloric effect of NiTi SMAs.

Understanding the asymmetric orientations and stress states in polycrystalline NiTi SMA by in-situ synchrotron-based high-energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Lüders-type mechanism. The asymmetrical SIMT starts with the forward Lüder band and disappears along with the backward Lüder band. The austenite with the favored orientation of ⟨110⟩A//loading direction (LD) transformed and recovered back at a higher rate compared to other orientations during both loading and unloading.

TEM and X-ray diffraction studies of structure of TiNi SMA treated by low-energy high-current electron beam

Advanced scientific and technological achievements in the surface engineering of TiNi shape memory alloys are closely related with their mechanical performance. Electron-beam treatments could be applied in order to modify surface properties of smart materials and promote stress-induced martensitic transformations. The fundamental issue of electron-beam processing is residual elastic stresses arising from the ultrafast quenching (∼108 K/s) of thin (∼2–4 μm) surface layers. We report the experimental results on phase composition, structure and residual elastic stresses formed in the binary TiNi SMA irradiated by a low-energy (< 25 keV) high-current (∼ 25 kA) electron beam. Microstructure, fracture mechanisms and phase analysis of the as-cast and deformed tensile specimens have been studied by X-ray diffraction and electron microscopy. After electron-beam treatment, both martensitic B19′ and R phases are found in the near surface layer. The X-ray diffraction studies indicate that compressive stresses in the B2 phase of the irradiated TiNi alloy reach –460 MPa. In the fractured specimen (after tensile test) the residual stresses decrease to –160 MPa and release through the B2→B19′→B2 martensitic transformation. In-depth gradient distribution of the defect substructures responsible for the mechanical behavior of TiNi alloys was revealed.

Multifunctional coatings of nickel-titanium implant toward promote osseointegration after operation of bone tumor and clinical application: a review.

Metal implants, especially Ni-Ti shape memory alloy (Ni-Ti SMA) implants, have increasingly become the first choice for fracture and massive bone defects after orthopedic bone tumor surgery. In this paper, the internal composition and shape memory properties of Ni-Ti shape memory alloy were studied. In addition, the effects of porous Ni-Ti SMA on osseointegration, and the effects of surface hydrophobicity and hydrophilicity on the osseointegration of Ni-Ti implants were also investigated. In addition, the effect of surface coating modification technology of Ni-Ti shape memory alloy on bone bonding was also studied. Several kinds of Ni-Ti alloy implants commonly used in orthopedic clinic and their advantages and disadvantages were introduced. The surface changes of Ni-Ti alloy implants promote bone fusion, enhance the adhesion of red blood cells and platelets, promote local tissue regeneration and fracture healing. In the field of orthopaedics, the use of Ni-Ti shape memory alloy implants significantly promoted clinical development. Due to the introduction of the coating, the osseointegration and biocompatibility of the implant surface have been enhanced, and the success rate of the implant has been greatly improved.

A study on Analysing the Selection of Materials for Knee Implant Femoral Components using the TOPSIS method

Specialists are creating more sophisticated and useful materials daily as technology advances. For orthopaedic implants such as "knee replacements, hip replacements, and orthopaedic accessories", biomaterials are employed to produce prosthetic organs. In patients with severe osteoarthritis (OA) of the knee, "total knee replacement (TKR)" is one of the most successful surgical procedures for pain management and functioning rehabilitation. Humans continue to experience issues with the kneecap, such as aseptic dislocation brought on by excessive wear across articular surfaces, stress-shielding of the bone by prostheses, and soft tissue formation at the junction of implanted bone. due to improper “TKR material selection”. Since selecting the best materials for the femoral component of TKR requires careful consideration, the "technique for order of preference by similarity to ideal solution (TOPSIS)" is used in this research paper. This technique uses an order of preference based on how closely the preferred option is to the ideal solution. The "equal weights method (EWM)" assigns various criteria distinct weights of importance. The rank of alternatives using the TOPSIS method for “Co-Cr-Mo is fourth, Co-Ni-Cr-Mo is third, NiTi SMA is first, Porous NiTi SMA is second, pure Ti is sixth and Ti-5Al-2.5Fe is fifth”. The result indicated that FC material using TKR Nickel Titanium Shape Memory Alloy (NiTi SMA) is at rank 1 with properties such as Tensile Strength 960 MPa, Density 6.45 g/cc, extremely high Corrosion resistance, and exceptionally high Wear resistance.

Finite Element Method of Functionally Graded Shape Memory Alloy Based on UMAT

Functionally graded shape memory alloy (FG-SMA) is widely used in practical engineering regions due to it possessing the excellent properties of both FG material and SMA material. In this paper, the incremental constitutive equation of SMA was established by using the concept of a shape memory factor. On this basis, the secondary development function of the ABAQUS software 2023 was used to write the user-defined material subroutine (UMAT). The phase transformation and mechanical behavior of transverse and axial FG NiTi SMA cantilever beams under concentrated load at free ends were numerically simulated by discrete modeling. Numerical results show that the stress and shape memory factor were distributed asymmetrically along the thickness direction of the transverse FG-SMA cantilever beam, while the stress and the shape memory factor distributed symmetrically along the thickness direction of the cross section of the axial FG-SMA cantilever beam. The bearing capacity of the axial FG-SMA cantilever beam is stronger than the SMA homogeneous cantilever beam, but weaker than the transverse FG-SMA cantilever beam. The load-bearing capacity of the transverse FG-SMA cantilever beam is twice that of the axial FG-SMA cantilever beam under the same functionally graded parameters and deflection conditions. The discrete modeling method of FG-SMA beams proposed in this paper can simulate the phase transformation and mechanical behavior of an FG-SMA beam well, which provides a reference for the practical application and numerical calculation of FG-SMA structures.

Preparation of a Zn/hydroxyapatite/chitosan composite coating with an antibacterial ability and cytocompatibility on a NiTi alloy

Zn has been extensively used to prevent microbial infections in orthopedic implants; however, high amounts of Zn inhibit cell proliferation. This study investigated the ability to decrease the cytotoxicity of Zn-doped coatings on NiTi shape memory alloys (NiTi SMAs) while maintaining their antibacterial activity. The Zn3(PO4)2 distributed in the chitosan coating, annotated as ZHAP, was obtained by electrophoretic deposition in a chitosan solution with Zn acetate acid and hydroxyapatite particles. The ZHAP inhibited bacterial growth and exhibited a good antibacterial activity (86 %) against Staphylococcus aureus. In addition, the MG63-cell adhesion and cytocompatibility with ZHAP were better than those with the Zn-doped coating due to the precipitation of Zn3(PO4)2 which decreased the release of Zn2+ ions from the chitosan matrix. The coatings did not affect neither the superelasticity (< 0.5 % residual strain) nor the shape memory ability (100 % recovery rate) of the NiTi SMAs.

Nanobubbles and physical-mechanical properties of the Ti-Ni-Ta-Si-based metallic glass surface alloy fabricated on a TiNi SMA substrate by additive thin-film electron-beam method

An additive thin-film electron-beam method for the synthesis of micron-thick surface alloys is a promising method for modifying the surface layers of metallic materials. This method allows to functionalize the surface properties (corrosion resistance, radiopacity, biocompatibility), as well as to improve the physical-mechanical properties. The amorphous Ti-Ni-Ta-Si-based surface alloy (~1.6 μm thick) was synthesized by 10-fold alternation of operations of the deposition of an alloying film (Ti60Ta30Si10 (at. %), ~100 nm thick) and the liquid-phase mixing of the [film/substrate] system using a pulsed low-energy high-current electron beam. Spherical nanobubbles (diameter varies from ~2.5 up to ~50 nm) were observed within the surface alloy. The additional electron-beam treatments were used to reduce the concentration of the nanobubbles. The paper presents the results of the influence of the electron-beam treatments on the concentration and size of the nanobubbles, and their in-depth distribution. It was found that the single-step electron-beam treatment effectively reduced the concentration of the nanobubbles and affected on their size and distribution. The two-step electron-beam treatment almost restored the characteristics of the nanobubbles. The influence of the characteristics of the nanobubbles on the strength and elastic-plastic parameters of the amorphous Ti-Ni-Ta-Si-based surface alloy was revealed. The characteristics of the nanobubbles had a major effect on the elastic-plastic parameters of the surface alloy while the strength parameters remained practically the same. It was found that a purposeful change of the characteristics of the nanobubbles is an effective tool for controlling physical-mechanical properties of the amorphous Ti-Ni-Ta-Si-based surface alloy fabricated on a TiNi SMA substrate.

Order of magnitude increase in actuation fatigue lifetime through partial austenitic transformation of NiTiHf high-temperature shape memory alloys

The higher transformation temperatures, strength, and thermomechanical stability of NiTiHf high-temperature shape memory alloys (HTSMAs) are attractive for use in solid-state actuation. Maximizing the actuation fatigue lifetime whilst preventing deterioration of actuation stroke during service are the two main challenges facing the widespread application of HTSMAs. Much research has focused on optimizing composition and processing to enhance performance; however, this has proven challenging to implement on a large scale. Simpler solutions involve optimizing the actuation environment to enhance performance. One of the most effective methods to both extend the lifetime and stabilize actuation strain is through partial thermal cycling. This method is more representative of how actuators are employed, as very rarely are they repeatedly cycled to their maximum output. Previous studies have shown partial transformation extends fatigue lifetime in low-temperature NiTi and NiTiCu SMAs, but it has yet to be conclusively demonstrated in the NiTiHf system. The present work is an extensive analysis of the effects of heating limited partial cycling compared to full cycling actuation fatigue of Ni50.3Ti29.7Hf20 HTSMA. The results confirm an order of magnitude increase in actuation fatigue lifetime with less transformation per cycle in partially heated Ni-rich NiTiHf samples. More importantly, keeping the actuation work output almost the same, controlling the actuation strain level is shown to be more effective in increasing the fatigue life than reducing the actuation stress level, yielding 5 to 10 times relative improvement in the fatigue life. The underlying microstructural evolution resulting in the observed enhanced response is provided.

Healing effect of electric current treatment on superelasticity and elastocaloric effect of TiNi shape memory wire

This study investigated the healing effects of electric current treatment on Ni-rich TiNi shape memory wire after every 100 superelastic cycles. The diameter of the TiNi wire was 0.5 mm. The electric current treatment was applied without removing the wire from the tensile machine, aiming for online healing of the shape memory alloy. The electric current treatment showed an obvious healing effect, which restored the superelastic and elastocaloric properties of the functionally degraded TiNi shape memory wire. With treatment conditions of 20.4 A/mm2 and 2 min (the temperature of the wire increased up to 280 °C in 10 s), the elastocaloric temperature rise and drop of the TiNi wire recovered about 14% and 6% after 3–4 treatments, respectively, as compared with the case without any heat treatment. TEM observations indicated that the residual martensite introduced into the TiNi wire during superelastic cycles could be eliminated through the electric current treatment. Experimental results demonstrated that electric current treatment was a promising method to restore the functionality of the TiNi SMA wire after several superelastic cycles without the need to remove the wire from the apparatus, which would thus improve the performance of shape memory alloys during their long-term application.

Effects of Building Directions on Microstructure, Impurity Elements and Mechanical Properties of NiTi Alloys Fabricated by Laser Powder Bed Fusion.

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts.

Understanding Machining Process Parameters and Optimization of High-Speed Turning of NiTi SMA Using Response Surface Method (RSM) and Genetic Algorithm (GA).

This study aimed to optimize machining parameters to obtain better surface roughness and remnant depth ratio values under dry turning of NiTi-shape memory alloy (SMA). During the turning experiments, various machining parameters were used, including three different cutting speeds vc (105, 144, and 200 m/min), three different feed rates f (0.05, 0.1, and 0.15 mm/rev), and three different depths of cut ap (0.1, 0.15, and 0.2 mm). The effects of machining parameters in turning experiments were investigated on the response surface methodology (RSM) with Box-Behnken design (BBD) using the Design Expert 11; how the cutting parameters affect the surface quality is discussed in detail. In this context, the cutting parameters were successfully optimized using a genetic algorithm (GA). The optimized processing parameters are vc = 126 m/min, f = 0.11 mm/rev, ap = 0.14 mm, resulting in surface roughness and remnant depth ratio values of 0.489 μm and 64.13%, respectively.

Reinforcement of cracked aluminum plates using polymeric composite patches embedded with prestressed SMA wires

Many studies have been performed on composite patches as reinforcement of cracked metal sheets. However, in most previous research, composite patches without prestressing have been used to repair cracks. In the present study, a novel method for repairing cracked aluminum sheets is proposed by using polymer composite patches with embedded prestressed Nitinol shape memory alloy (Ni-Ti SMA) wires. Elastic-plastic finite element analysis of an aluminum plate, with a central crack in the pure mode I and the mixed-mode I/II fracture, repaired with SMA wires reinforced composite patch (SMA-CP) was performed. The peel stress on the adhesive layer between the composite patch and the aluminum plate was calculated to evaluate the performance and efficiency of the repair. The effect of the prestress of the embedded Ni-Ti SMA wires on the efficiency of the composite patch was examined. The results show that embedding prestressed SMA wires in the composite patch reduces the stiffness ratio of the patch. Also, the value of the J-integral and the size of the plastic area compared to the patch without SMA wires were decreased.