Superelastic NiTi Alloy Research Articles

Fatigue Modeling for Superelastic NiTi Considering Cyclic Deformation and Load Ratio Effects

A cumulative energy-based damage model, called total fatigue toughness, is proposed for fatigue life prediction of superelastic NiTi alloys with various deformation responses (i.e., transformation stresses), which also accounts for the effects of mean strain and stress. Mechanical response of superelastic NiTi is highly sensitive to chemical composition, material processing, as well as operating temperature; therefore, significantly different deformation responses may be obtained for seemingly identical NiTi specimens. In this paper, a fatigue damage parameter is proposed that can be used for fatigue life prediction of superelastic NiTi alloys with different mechanical properties such as loading and unloading transformation stresses, modulus of elasticity, and austenite-to-martensite start and finish strains. Moreover, the model is capable of capturing the effects of tensile mean strain and stress on the fatigue behavior. Fatigue life predictions using the proposed damage parameter for specimens with different cyclic stress responses, tested at various strain ratios (R e = e min /e max) are shown to be in very good agreement with the experimentally observed fatigue lives.

Tunable Mechanochemistry of Lithium Battery Electrodes.

The interplay between mechanical strains and battery electrochemistry, or the tunable mechanochemistry of batteries, remains an emerging research area with limited experimental progress. In this report, we demonstrate how elastic strains applied to vanadium pentoxide (V2O5), a widely studied cathode material for Li-ion batteries, can modulate the kinetics and energetics of lithium-ion intercalation. We utilize atomic layer deposition to coat V2O5 materials onto the surface of a shapememory superelastic NiTi alloy, which allows electrochemical assessment at a fixed and measurable level of elastic strain imposed on the V2O5, with strain state assessed through Raman spectroscopy and X-ray diffraction. Our results indicate modulation of electrochemical intercalation potentials by ∼40 mV and an increase of the diffusion coefficient of lithium ions by up to 2.5-times with elastic prestrains of <2% imposed on the V2O5. These results are supported by density functional theory calculations and demonstrate how mechanics of nanomaterials can be used as a precise tool to strain engineer the electrochemical energy storage performance of battery materials.

A coupled model between hydrogen diffusion and mechanical behavior of superelastic NiTi alloys

The undesirable effects of hydrogen show significant alterations to the thermomechanical behavior of superelastic NiTi shape memory alloys. Through experimental results, the presence of hydrogen induces a delay of forward transformation. Added to that, hydrogen-induced expansion is clearly noticed. We also remark a loss of superelasticity. These effects occur according to the hydrogen absorption by the NiTi alloy. The aim of this paper is to develop a coupled diffusion-mechanical model of shape memory alloys, which regards the aforesaid effects of hydrogen on the thermomechanical behavior and the transformation mechanism of NiTi alloys. The model is derived from the relationship between the chemical potential of hydrogen and the thermodynamics laws. Furthermore, we introduce a special transformation hardening function that predicts stress–strain behavior well during the transformation plateau. The model is implemented in ABAQUS finite element analysis software through the UMAT and UMATHT subroutines. The simulation results present good concordance with the experiments.

Influence of clinical use on physical-structural surface properties and electrochemical potential of NiTi endodontic instruments.

To investigate the surface morphology and electrochemical potential of superelastic (SE), M-Wire (MW) and shape memory technology (SMT) NiTi instruments before and after single clinical use invivo. A total of 60 ProTaper Universal F2 (PTU-SE), ProTaper Next X2 (PTN-MW), Typhoon (TYP), Hyflex (HF) and Vortex Blue (VB), the last three SMT, and size 25, .06 taper (n=6 of each type) files were examined. Scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) and electrochemical potential analysis were employed before and after clinical use. Statistical analysis was performed with one-way analysis of variance and Bonferroni's post hoc test. Significance was determined at the 95% confidence level for both tests. SEM observations of new instruments indicated the presence of marks left by the machining process during manufacturing and EDS revealed the existence of an oxide coating on shape memory instruments. After clinical use, the five types were associated with propagation of transverse cracks 3mm from the tip. The surface oxide layer of TYP, HF and VB instruments had microcracks in multiple directions, whilst TYP and HF had fragmentation in chip form of the oxide layer. EDS analysis demonstrated a significant reduction of the oxide layer in shape memory instruments, except for VB. Electrochemical potentials were higher for shape memory instruments than for M-Wire and superelastic NiTi instruments, respectively (P<0.05). It appears that shape memory technology NiTi instruments have a dysfunctional oxide layer after clinical use. Additionally, they featured higher electrochemical potential relative to NiTi instruments manufactured from M-Wire, and conventional superelastic NiTi alloy.

Design, Analysis and Testing of a Novel Mitral Valve for Transcatheter Implantation

Mitral regurgitation is a common mitral valve dysfunction which may lead to heart failure. Because of the rapid aging of the population, conventional surgical repair and replacement of the pathological valve are often unsuitable for about half of symptomatic patients, who are judged high-risk. Transcatheter valve implantation could represent an effective solution. However, currently available aortic valve devices are inapt for the mitral position. This paper presents the design, development and hydrodynamic assessment of a novel bi-leaflet mitral valve suitable for transcatheter implantation. The device consists of two leaflets and a sealing component made from bovine pericardium, supported by a self-expanding wireframe made from superelastic NiTi alloy. A parametric design procedure based on numerical simulations was implemented to identify design parameters providing acceptable stress levels and maximum coaptation area for the leaflets. The wireframe was designed to host the leaflets and was optimised numerically to minimise the stresses for crimping in an 8 mm sheath for percutaneous delivery. Prototypes were built and their hydrodynamic performances were tested on a cardiac pulse duplicator, in compliance with the ISO5840-3:2013 standard. The numerical results and hydrodynamic tests show the feasibility of the device to be adopted as a transcatheter valve implant for treating mitral regurgitation.

Grain size effect on mechanical performance of nanostructured superelastic NiTi alloy

The mechanical performance of superelastic NiTi with various grain sizes (GSs) in nanocrystalline regime (GS < 30 nm) are investigated. With the help of digital image correlation, both global and local mechanical responses of NiTi during quasi-static test and fatigue cycling are recorded. If GS is below 14 nm, NiTi deforms homogenously; if GS is above 14 nm, NiTi deforms in a heterogeneous manner. The mechanical response, the fatigue life, the dissipation energy and the resistance to the dissipation energy degradation of nanostructured NiTi are addressed and analyzed. The results indicate that the mechanical performance of NiTi can be designed and optimized by controlling GS in a moderate regime.

陰極チャージおよび浸漬試験により水素チャージしたNi-Ti超弾性合金の水素放出挙動解析

陰極チャージおよび浸漬試験により水素チャージしたNi-Ti超弾性合金の水素放出挙動解析

Strong interactions between hydrogen in solid solution and stress-induced martensite transformation of Ni–Ti superelastic alloy

The role of the dynamic interactions between hydrogen in a solid solution and the stress-induced martensite transformation in hydrogen embrittlement has been investigated using trained Ni–Ti superelastic alloy. In a cyclic tensile test in the stress plateau region caused by stress-induced martensite and reverse transformations after hydrogen charging, a further decrease in the critical stress for the martensite transformation is observed. In addition, the number of cycles to fracture for a trained specimen is significantly larger than that for a non-trained specimen. Since most of the charged hydrogen is preferentially trapped in defects induced by training, the hydrogen embrittlement is considerably suppressed as a result of decreasing interactions between the hydrogen and the transformation. The present results indicate that hydrogen in a solid solution more strongly interacts with the stress-induced martensite transformation than hydrogen trapped in defects, thereby further enhancing the hydrogen embrittlement related to phase transformations.

Environment-Assisted Cracking of Super-Elastic TiNi Alloy Depending on Solution pH and Electrochemical Potential

The behavior of environment-assisted cracking (EAC) for super-elastic TiNi alloy was investigated as functions of solution pH and electrochemical potential. The specimen was immersed in sulfate solutions at various pHs, and subjected to a potentiostatic slow strain rate tensile test. As a result, the EAC map for the TiNi alloy as functions of the solution pH and the applied potential was successfully produced, and it was revealed that larger EAC susceptibility was obtained in the region of lower potential and lower pH. The tendency was quite typical for hydrogen embrittlement, but not similar to that of TiAl. The EAC susceptibility was classified by cathodic charge density, irrespective to pH nor potential: The charge density larger than 0.025 MC.m-2 induced a maximum EAC susceptibility.

Origin of zero and negative thermal expansion in severely-deformed superelastic NiTi alloy

We have investigated the physical origin of anomalous in-plane thermal expansion (TE) anisotropy leading to invar-like behavior and negative TE in nanostructured NiTi sheets manufactured via severe cold-rolling. The roles of grain size (GS), crystallographic texture, thermally-induced phase transformation, and intrinsic (lattice level) TE of austenite (B2) and martensite (B19′) phases on the macroscopic TE behavior are addressed. It is shown that by controlling the cold-rolling thickness reduction and heat-treatment temperature the coefficient of thermal expansion (CTE) can be controlled in a wide range from positive (α ∼ 2.1 × 10−5 K−1) to negative (α ∼ −1.1 × 10−5 K−1) via in-plane anisotropy of TE. A very small CTE of α ∼ −5.3 × 10−7 K−1 (invar-like behavior) in a wide temperature window of 230 K (353–123 K) is obtained at an angle of 33.5° to the rolling direction (RD) of the severely cold-rolled sheet. TEM and XRD studies show that the microstructure underlying such anomalous TE behavior consists of a mixture of B2 nano-grains and retained/residual deformation-induced martensite and that the observed anomalous TE anisotropy is due to the intrinsic anisotropic TE of residual martensite. The invar-like behavior is the result of the cancellation of the positive TE of austenite phase with the negative TE of residual martensite along 33.5° to the RD. A simple rule of mixture model incorporating the intrinsic TE of B2 and B19′ lattices and the texture coefficients of the sample is proposed which successfully captures the anomalous in-plane TE anisotropy. The discovery of high dimensional stability over a wide temperature window along with temperature insensitive non-hysteretic linear superelasticity of the severely-deformed NiTi opens up a new route for designing stable SMAs for applications in ragged environments.

Stress serration and arch-shaped Lüders stress plateau behaviour of Ti–50.8 at% Ni wire prepared by selective electrical resistance over-aging

Joule heating of NiTi shape memory alloy wires is a commonly applied technique for heat treatment and shape setting in many applications. Another innovative use of this method is to produce functionally graded NiTi. In this study, NiTi wires with spatially varied shape memory characteristics along the length were created by electrical resistance over-aging of a Ni-rich superelastic NiTi alloy. The stress–strain behaviour of such wires exhibited some new and unique characteristics during the stress-induced martensitic transformation, including two discrete stress plateaus, stress serration during transition between the two stress plateaus and an arch-shaped stress plateau in the over-aged section. These unique features have direct implications to design using NiTi alloys and the underlying mechanisms are explained in this study.

Strain Engineering to Modify the Electrochemistry of Energy Storage Electrodes.

Strain engineering has been a critical aspect of device design in semiconductor manufacturing for the past decade, but remains relatively unexplored for other applications, such as energy storage. Using mechanical strain as an input parameter to modulate electrochemical potentials of metal oxides opens new opportunities intersecting fields of electrochemistry and mechanics. Here we demonstrate that less than 0.1% strain on a Ni-Ti-O based metal-oxide formed on superelastic shape memory NiTi alloys leads to anodic and cathodic peak potential shifts by up to ~30 mV in an electrochemical cell. Moreover, using the superelastic properties of NiTi to enable strain recovery also recovers the electrochemical potential of the metal oxide, providing mechanistic evidence of strain-modified electrochemistry. These results indicate that mechanical energy can be coupled with electrochemical systems to efficiently design and optimize a new class of strain-modulated energy storage materials.

Effects of temperature, phase, elastic deformation and transformation on inhibition of localized corrosion of hydrogen-charged Ni–Ti superelastic alloy in NaCl solution

The effects of the solution temperature, phase, elastic deformation and the phase transformation on the inhibition of localized corrosion of a small amount of hydrogen-charged Ni–Ti superelastic alloy in 0.9% NaCl solution have been investigated by electrochemical measurements. No pitting potentials are observed in the anodic polarization curves obtained above room temperature and under static applied stress in the presence of parent or stress-induced martensite phases. In a dynamic cyclic tensile test in the elastic deformation region of the parent phase under various constant applied anodic potentials, the critical applied potential for an increase in the current density, which corresponds to the occurrence of localized corrosion, considerably shifts in the noble direction. At the stress plateau, i.e., in the superelastic region, caused by stress-induced martensite and reverse transformations, the critical applied potential for a marked increase in the current density slightly shifts in the noble direction. In the elastic deformation region of the stress-induced martensite phase, the critical applied potential for a marked increase in the current density shifts in the noble direction. The present results indicate that hydrogen charging is effective for inhibiting the localized corrosion of Ni–Ti superelastic alloy in NaCl solution at various temperatures, irrespective of the phase itself, under dynamic elastic deformation and the phase transformation.

Effects of grain size on tensile fatigue life of nanostructured NiTi shape memory alloy

The effects of grain size (GS) on tensile fatigue life of nanostructured NiTi superelastic shape memory alloys (SMAs) with GS=10nm, 42nm and 80nm are investigated. Macroscopic stress-controlled tensile fatigue tests, acoustic energy measurements and fracture surface observations were performed. It is shown that low-cycle fatigue life (under σmax=450MPa) of nanostructured NiTi polycrystalline SMA increases significantly when GS decreases from 80nm to 10nm. However, there is no significant effect of GS on the intermediate-cycle fatigue life (under σmax=300MPa). It is found that accumulated acoustic energy can be used to distinguish the three stages of fatigue: slow crack propagation, fast crack propagation and final fracture. Micro cracks were found on fracture surfaces of all GS specimens under intermediate-cycle fatigue and on fracture surfaces of 10nm GS specimen under low-cycle fatigue, while micro voids were found in 42nm and 80nm GS specimens under low-cycle fatigue. The results of the paper indicate that grain refinement down to nanoscale has potential in developing high fatigue resistance SMAs.

Development of a flexible nanocomposite TiO2 film as a protective coating for bioapplications of superelastic NiTi alloys

An experimental procedure to coat superelastic NiTi alloys with flexible TiO2 protective nanocomposite films using sol–gel technology was developed in this work to improve the metal biocompatibility without deteriorating its superelastic mechanical properties. The coatings were characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and glazing incidence X-ray diffraction. The elasticity of the film was tested in coated specimens submitted to three-point bending tests. A short densification by thermal treatment at 500°C for 10min yielded a bilayer film consisting of a 50 nm-thick crystallized TiO2 at the inner interface with another 50-nm-thick amorphous oxide film at the outer interface. This bilayer could sustain over 6.4% strain without cracking and could thus be used to coat biomedical instruments as well as other devices made with superelastic NiTi alloys.

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

The relaxation behavior under tensile loading of a superelastic NiTi alloy was investigated after hydrogen charging with respect to aging from one to 77 days in air at room temperature. The specimens were immersed for 3 h in a 0.9 % NaCl aqueous solution and then relaxed with an imposed strain of 4.8 %—which results in half of the martensite transformation—for different strain rates of 10−4, 10−3, and 5 × 10−3 s−1. For the non-charged specimens, the relaxed stress at the beginning exhibited a temporary dependence on the strain rates and then reached the same equilibrium stress after 2.5 h. After hydrogen charging, this equilibrium stress did not vary for the as-charged specimen. Nevertheless, the greater the aging period is the greater the equilibrium stress is. This behavior can be attributed to the diffusion of hydrogen into the entire specimen, which hinders the relaxation mechanism of the martensite bands.

Ni-Ti超弾性合金の水素放出挙動に及ぼす陰極水素チャージ条件の影響

Ni-Ti超弾性合金の水素放出挙動に及ぼす水素吸収条件の影響について調査した．陰極水素チャージ時の電流密度の増大に伴い，低温 （～200 ℃） における放出水素量は増加した．この低温における放出水素量の増加は，水素化物の形成量の増加に起因していると考えられた．また，陰極水素チャージ時の電流密度の増大に伴い，試料表層の硬さは上昇した．これは，試料表層に形成された水素化物に起因していると推察された．したがって，陰極水素チャージ時の電流密度，つまり単位時間の発生水素量の増加に伴い，試料表層の水素濃度が高くなり，水素化物の形成が促進されたと考えられた．

Role of the Electrochemical Potential and Solution pH to Environment-Assisted Cracking of Super-Elastic TiNi Alloy

Role of the Electrochemical Potential and Solution pH to Environment-Assisted Cracking of Super-Elastic TiNi Alloy

Improved Superelastic NiTi wire for the Treatment of Adult Skeletal Class III Malocclusion in a Surgery-first Case

ABSTRACT A 19-year-old female came to our department with the chief complaint of facial asymmetry. Clinical examination showed skeletal class III relationship with mandibular prognathism, large reverse overjet, lower right mild crowding, and left hemimandibular elongation. After a thorough discussion with the patient, she accepted the proposal of orthognathic surgery and mandibular setback by bilateral sagittal split osteotomy (BSSRO), and therefore, tooth extraction was performed for 18, 28, 38, and 48 before the surgery. An improved superelastic Ti-Ni alloy wire (ISW wire), developed by Tokyo Medical and Dental University, was then used to facilitate the correction of lower crowding. Involved mechanism included intermaxillary elastics for a better interdigitation and jaw relationships. Adequate overbite and overjet were achieved after treatment completion. How to cite this article Fang C-Y, Tsai Y-Y, Yu J-H. Improved Superelastic NiTi wire for the Treatment of Adult Skeletal Class III Malocclusion in a Surgery-first Case. Int J Experiment Dent Sci 2016;5(2):133-138.

Uniaxial Pre-strain and Free Recovery (UPFR) as a Flexible Technique for Nitinol Characterization

The measurement of phase transformation temperatures of superelastic (SE) and shape memory (SM) NiTi alloy products and components was studied in this work. The transformation temperatures of a set of twenty different 300 μm NiTi superelastic wires were measured by two well-established and standardized techniques, namely differential scanning calorimetry (DSC) and bend and free recovery (BFR) and then compared with the results from the Uniaxial Pre-Strain and Free Recovery (UPFR) test. UPFR is a tension-based test, whose aim is to overcome the limitations associated with BFR testing. Within this work, a test procedure has been set up and validated. UPFR is found to be the only method showing a very strong correlation with the mechanical properties measured using the standard uniaxial tensile test method for superelastic NiTi alloy. Further, UPFR has been validated as a robust technique for measuring the R-phase and austenitic transformation temperatures in specimens of various sizes, composition, and of different geometries. This technique overcomes the limitations of BFR and DSC which cannot be used for testing products such as 25 μm SM wire, a 50 μm SE strip, and different springs and microsprings for actuation.