Superelastic NiTi Alloy Research Articles

Improved Super-Elastic Ti-Ni Alloy Wire for Nonsurgical Treatment of Skeletal Class II Malocclusion Combined with Large Overjet and Gummy Smile Case

Improved Super-Elastic Ti-Ni Alloy Wire for Nonsurgical Treatment of Skeletal Class II Malocclusion Combined with Large Overjet and Gummy Smile Case

Hydrogen states and contents in superelastic TiNi alloy after cathodic polarization

The superelastic TiNi alloy sometimes undergo failure under wet environments owing to hydrogen absorption and/or hydride formation; however, the effects of electrochemical conditions such as potential and pH and the hydrogen state and content on the susceptibility of the material to such degradation processes remain unveiled. In this study, the hydrogen content in the solid-solution state and hydride state in superelastic TiNi alloy were determined upon applying a constant cathodic potential in sulfate solution, and the dependence of the contents on the potential and solution pH was investigated. The formation of hydride was detected via X-ray diffraction (XRD), and the hydrogen content in each state was determined by means of thermal gas desorption spectroscopy (TDS). The specimen receiving a smaller cathodic charge density showed no hydride in the XRD pattern and one obvious peak in hydrogen desorption rate at higher temperature in the TDS profile. In contrast, larger charge density induced hydride formation and two peaks in the hydrogen desorption rate at two different temperatures. The peaks at higher and lower temperatures in the TDS profile can be attributed to hydrogen in solid-solution and hydride states, respectively. The logarithmic content of hydrogen in each state increased almost linearly with increasing logarithmic charge density. Linear relationships were obtained irrespective of potential and pH, with slopes of 0.57 and 0.99 for the solid-solution and hydride states, respectively. The susceptibility to environment-assisted cracking suddenly increased at a charge density of 0.025 MC·m−2, at which the hydrogen contents in the solid-solution state and hydride states were determined to be 30 and 5 mass ppm, respectively.

Phase field study on the grain size dependent electrical resistivity-strain response in superelastic nanocrystalline NiTi alloys

The superelastic NiTi alloy's electrical resistivity exhibits significant variations in response to deformation, making it a highly attractive material for sensing applications. However, there exists a lack of comprehensive knowledge of the effects of grain size (GS) on the electrical properties of NiTi alloy. To address this gap, a novel electro-mechanical coupled phase field simulation model is developed, with crucial parameters determined through experimental investigations. The results demonstrate a notable change in the relationship between electrical resistivity and strain, transitioning from a quasi-linear pattern to a piecewise nonlinear one as the GS increases. The observed modification is found to be primarily influenced by the martensitic transformation, characterized by a transition from a uniform mode to a localized mode. As the GS increases, there is an initial drop in the total change of electrical resistivity at peak strain, followed by a subsequent stabilization. This is because the GS, through the stress and the degree of phase transformation, exerts two opposing effects on the variation of electrical resistivity. Furthermore, the combined influence of hysteresis and hysteresis in the martensite fraction contributes to a consistent decrease in resistivity hysteresis as the GS diminishes. This study improves the understanding of how GS affects NiTi alloy's electrical resistivity-strain response and lays the foundation for future sensing performance regulation, expanding its multi-functionality as an intelligent material.

Hydrogen penetration into the NiTi superelastic alloy investigated in-situ by synchrotron diffraction experiments

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm.

Insight into the cracking mechanism of super-elastic NiTi alloy fabricated by laser powder bed fusion

ABSTRACT Laser powder bed fusion is considered to be an effective method for forming high-precision NiTi alloys. NiTi alloys formed from Ti-rich or low-Ni content NiTi powders as raw materials exhibit shape memory or trained to be superelastic at room temperature. To fabricate super-elastic NiTi alloys without training, it is often to increase the Ni content of the powder. However, Ni-rich NiTi powders are highly susceptible to cracking during fabrication. Therefore, this study has conducted a systematic investigation and proposed a corresponding process optimisation strategy. The results show that cracking is the result of the combined effect from residual thermal stresses and microstructure during the printing process. The melt pool morphology influenced by the thermal history as well as elemental evaporation determines the direction of crack formation and propagation. We have successfully fabricated room-temperature super-elastic NiTi samples without cracking using high laser power and high scanning speed.

Relevant Aspects in the Mechanical and Aging Degradation of NiTi Alloy with R-Phase in Endodontic Files.

One of the most important challenges in endodontics is to have files that have excellent flexibility, toughness, and high fatigue life. Superelastic NiTi alloys have been a breakthrough and the new R-phase NiTi alloys promise to further optimize the good properties of NiTi alloys. In this work, two austenitic phase endodontic files with superelastic properties (Protaper and F6) and two austenitic phase files with the R-phase (M-wire and Reciproc) have been studied. The transformation temperatures were studied by calorimetry. Molds reproducing root canals at different angles (30, 45, and 70°) were obtained with cooling and loads simulating those used in the clinic. Mechanical cycles of different files were realized to fracture. Transformation temperatures were determined at different number of cycles. The different files were heat treated at 300 and 500 °C as the aging process, and the transformation temperatures were also determined. Scanning and transmission electron microscopy was used to observe the fractography and precipitates of the files. The results show that files with the R-phase have higher fracture cycles than files with only the austenitic phase. The fracture cycles depend on the angle of insertion in the root canal, with the angle of 70° being the one with the lowest fracture cycles in all cases. The R-Phase transformation increases the energy absorbed by the NiTi to produce the austenitic to R-phase and to produce the martensitic transformation causing the increase in the fracture cycles. Mechanical cycling leads to significant increases in the transformation temperatures Ms and Af as well as Rs and Rf. No changes in the transformation temperatures were observed for aging at 300 °C, but the appearance of Ni4Ti3 precipitates was observed in the aging treatments to the Nickel-rich files that correspond to those with the R transition. These results should be considered by endodontists to optimize the type of files for clinical therapy.

Electrochemical degradation of the hydrogen-absorption-induced passive film on an Ni–Ti superelastic alloy in an NaCl solution

This study was conducted to investigate the effects of electrochemically generated hydrogen on the passivation behaviour of Ni–Ti alloy surfaces. Hydrogen charging via galvanostatic polarisation increased the passive current density during cyclic potentiodynamic polarisation experiments. Electrochemical impedance spectroscopy and Mott–Schottky analyses revealed an increase in the electrochemical conductivity of the film, indicating that hydrogen charging increased the donor density. Changes in the compositions were detected by Auger electron spectroscopy and X-ray photoelectron spectroscopy. Scanning transmission electron microscopy revealed film degradation after hydrogen charging. These results demonstrate that the electrochemical conductivity of the passive film increased owing to hydrogen absorption.

Enhanced superelasticity and reversible elastocaloric effect in nano-grained NiTi alloys with low stress hysteresis

Solid-state cooling technologies have been considered as potential alternatives for vapor compression cooling systems. The search for refrigeration materials displaying a unique combination of pronounced caloric effect, low hysteresis, and high reversibility on phase transformation was very active in recent years. Here, we achieved increase in the elastocaloric reversibility and decrease in the friction dissipation of martensite transformations in the superelastic nano-grained NiTi alloys obtained by cold rolling and annealing treatment, with very low stress hysteresis (6.3 MPa) under a large applied strain (5%). Large adiabatic temperature changes (ΔT max = 16.3 K at ε = 5%) and moderate COPmater values (maximum COPmater = 11.8 at ε = 2%) were achieved. The present nano-grained NiTi alloys exhibited great potential for applications as a highly efficient elastocaloric material.

Experimental investigation of mechanical properties of NiTi superelastic shape memory alloy cables

As a self-centering element in a recoverable functional structure, shape memory alloys (SMAs) usually require a large diameter to provide sufficient bearing capacity, and pre-strain is applied to obtain good self-centering performance and initial stiffness. This study aims to explore the mechanical properties of large-diameter NiTi SMA cables and their potential applications in recoverable functional structures. Through the uniaxial tensile test of 11 SMA cables with 7 × 7 × 1.0 mm, the mechanical properties of SMA cables under different strain amplitudes, cyclic loading effects, loading rates, and different pre-strain modes were evaluated. The results indicate that the energy dissipation capacity of SMA cable increases with the increase of strain amplitude, but at the same time, it will reduce its self-centering capabilities. The equivalent viscous damping ratio decreases after the SMA cable enters the martensitic hardening stage. The residual strain of SMA cable depends on the combined effect of strain amplitude and number of cycles. The larger the strain amplitude and the number of cycles, the more significant the degradation of self-centering performance. After 20 times of constant amplitude loading and unloading, the mechanical properties of SMA cable are relatively stable. The reverse phase transformation stress of the pre-strained SMA cable gradually decreases with the increase of the tensile strain amplitude. The pre-strained SMA cable exhibits a stable restoring force strength in the sub-cycle, and the residual strain is zero. The test results support experimental data for applying large-diameter SMA cables in recoverable functional structures.

Imparting high elastocaloric cooling potential to NiTi alloy by two-step enhancements

The large adiabatic temperature change during the stress-induced phase transformation in the superelastic NiTi alloy makes it a leading candidate material for solid-state elastocaloric cooling applications. However, NiTi with a uniform grain structure generally offers either large cooling capacity or low energy dissipation, but not both. In an earlier study, we demonstrated that the alloy with graded grain sizes, obtained using differential annealing of heavily cold-worked sheets, can overcome this intrinsic limitation. However, non-transformable components (residual martensite and amorphous phases), which are introduced into the microstructure during cold rolling, impair the full exploitation of the cooling potential of the graded NiTi. Here, we introduce an intermediate processing step, namely post-deformation annealing, which eliminates the undesirable phases while maintaining nanocrystallinity, before imparting a microstructural gradient. Results show that the synergy between various microstructural elements results in a significant enhancement in the cooling capacity while keeping the energy dissipation low. Moreover, the functional fatigue performance of the graded alloy is also preferable to that of the conventional coarse-grained NiTi. The micromechanical reasons behind these improvements are elucidated by recourse to detailed experiments.

Changes in hydrogen states after interactions between hydrogen and stress-induced martensite transformation for Ni–Ti superelastic alloy

ABSTRACTChanges in hydrogen states after dynamic interactions between hydrogen and a stress-induced martensite transformation have been investigated for Ni–Ti superelastic alloy. After homogenised hydrogen concentration by aging, the interactions change the hydrogen thermal desorption behaviour. Hydrogen in a solid solution probably changes to hydrogen trapped in damage regions induced by the interactions, as reported previously. With time after the interactions, the amount of hydrogen desorbed at low temperatures (200–400°C) markedly increases, clearly indicating that hydrogen states further change from the states immediately after the interactions. In cyclic tensile deformation in the stress plateau region caused by stress-induced martensite and reverse transformations, the number of cycles to fracture substantially increases with time after the interactions. The present results indicate that hydrogen states further change to a nominally interacting state after the interactions despite the absence of dynamic structural changes and a hydrogen concentration gradient, thereby suppressing hydrogen embrittlement related to the martensite transformation of the alloy.

Improved Super-elastic Ti-Ni alloy wire (ISW) for Treatment of Adult Angle Class III with Anterior Crossbite

Improved Super-elastic Ti-Ni alloy wire (ISW) for Treatment of Adult Angle Class III with Anterior Crossbite

Improved Super-elastic Ti-Ni Alloy Wire for the Treatment of Skeletal Class III with Unilateral Buccally Positioned High Canine

Improved Super-elastic Ti-Ni Alloy Wire for the Treatment of Skeletal Class III with Unilateral Buccally Positioned High Canine

Improved super-elastic Ti–Ni alloy wire intrusion arch for skeletal class II malocclusion combined with deep overbite: A case report

Improved super-elastic Ti–Ni alloy wire intrusion arch for skeletal class II malocclusion combined with deep overbite: A case report

Inhibition of Localized Corrosion of Ni–Ti Superelastic Alloy in Sodium Hypochlorite Solution by Sodium Carbonate Addition

Inhibition of Localized Corrosion of Ni–Ti Superelastic Alloy in Sodium Hypochlorite Solution by Sodium Carbonate Addition

Revealing the microstructure evolution of the deformed superelastic NiTi wire by EBSD

The generation of irreversible macroscopic strains in superelastic NiTi alloys is an important issue closely related to the crystallography of the transformation from B2 austenite to B19’ martensite under stress. Although widely investigated at micro and nano scales by transmission electron microscopy (TEM), the observation of this phase transformation at the mesoscale is still lacking. In the present study, a superelastic NiTi wire was maintained in bending condition and investigated by electron backscattered diffraction (EBSD) technique. A strong variant selection and a formation of martensite texture were observed and quantified using the interaction work (IW) based on the distortion matrix of a single variant. A new habit plane between martensite and parent B2, {114}B2 || (201¯)M, was experimentally measured. It was explained by the Phenomenological Theory of Martensite Crystallography (PTMC) using dislocation slip {110}<001>B2 as lattice invariant shear (LIS). Some new B2 domains were also found in {114}B2 twinning relation with primary B2 grain. Two scenarios are proposed to explain them: a) the direct formation of {114}B2 deformation twin in B2, and b) a more complex one in which a primary B2 grain is transformed into a martensite variant, a (201¯)M deformation twin is formed inside this variant; then the (201¯)M twinned martensite reversely transforms into a new {114}B2 twinned B2 domain. The scenario b) permits to explain some uncommon (130)M, (201)M, and (101¯)M twins observed by EBSD between the martensite variants of the two {114}B2 twin-related B2 phases.

Characterization of Hydrogen Thermal Desorption Behavior and Enhancement of Hydrogen Embrittlement in Ni–Ti Superelastic Alloy Induced by Cathodic Hydrogen Charging in the Presence of Chloride Ions

Characterization of Hydrogen Thermal Desorption Behavior and Enhancement of Hydrogen Embrittlement in Ni–Ti Superelastic Alloy Induced by Cathodic Hydrogen Charging in the Presence of Chloride Ions

The roles of residual martensite and plastic deformation in thermomechanically coupled functional degradation of nanocrystalline superelastic NiTi alloys

Residual martensite and plastic deformation are the culprits of functional degradation in NiTi alloys. However, their roles in thermomechanically coupled functional degradation of nanocrystalline superelastic NiTi alloys have not been investigated systematically and quantitatively. In this paper, the origins of thermomechanically coupled functional degradation of nanocrystalline superelastic NiTi alloys are subdivided into (i) plastic deformation including dislocation slipping in nanocrystals and shearing of amorphous phase, (ii) Type A residual martensite (RMA) originating from the incomplete reverse transformation during non-isothermal deformation, and (iii) Type B residual martensite (RMB) stabilized by the internal stress owing to inconsistent plastic strains among neighboring grains. For the first time, the inverse relation between fRMA and strain rate in NiTi with GS of 6 nm (a crystalline-amorphous nanocomposite) is determined experimentally. We find that the residual strain is mainly caused by plastic deformation except when RMA accumulates drastically. It is substantiated that plastic deformation lowers both forward and reverse transformation stresses significantly. Residual martensite plays an important role in the decrement of forward transformation stress, while has little impact on reverse transformation stress.

Mechanical analysis of the improved superelastic Ni-Ti alloy wire using the orthodontic simulator with high-precision sensors

PurposeThe authors have been using improved superelastic Nickel-Titanium alloy wire (ISW) to close and align extraction spaces simultaneously, instead of separately using rigid wires for closing extraction spaces and Ni–Ti alloy wires for leveling and aligning. ISW has a low stiffness, which makes it challenging to generate sufficient moments. This study aimed to demonstrate the forces and moments exerted on adjacent brackets using an orthodontic simulator (OSIM) attached to a high-precision 6-axis sensor. Materials and methodsIn experiment 1, a 0.016 × 0.022-inch ISW, stainless steel (SS) wire, and β-titanium wires were ligatured into the two brackets. The 0.018 × 0.025-inch slot self-ligating brackets were bonded to two simulated teeth at the same height, and the experiment was conducted using the high-precision OSIM. The distance between the brackets was 10 mm, the V-bend angles of the installed wires were 10°, 20°, 30°, and 40°, and the apex position was set at the center of the bracket. In experiment 2, 6.0- and 9.0-mm long elastomeric chains were placed on the same brackets as in Experiment 1 to measure forces and moments. The distance between the brackets was increased by 1.0 mm from 6.0 to 15.0 mm. Both experiments were conducted in a 37 °C thermostatic chamber similar to the oral environment. Results and discussionIn experiment 1, we measured moments on both sides for all the wires. As the V-bend angle increased, the absolute values of the moments also increased. With a V-bend angle of 10°, there was a significant (p < 0.05) difference in the moment generated in the left and right brackets among the three wire types. In the ISW, −1.67 ± 0.38 N・mm was generated in the left bracket, while 0.38 ± 0.26 N・mm was generated in the right bracket at 10°. At 20°, −1.77 ± 0.69 N・mm was generated in the left bracket, while 2.37 ± 0.94 N・mm was generated in the right bracket. At 30°, −2.98 ± 0.49 N・mm was generated in the left bracket, while 3.25 ± 0.32 N・mm was generated in the right bracket. Moreover, at 40°, −3.96 ± 0.58 N・mm was generated in the left bracket, while 3.55 ± 0.53 N・mm was generated in the right bracket. Furthermore, in experiment 2, the moments increased in proportion to the increase in distance between the centers of the two brackets. Absolute values of the moments were approximately equal for the left and right brackets. The 6.0-mm elastomeric chain generated a minimum force of −0.09 ± 0.05 N in the left direction when the distance between brackets was 6.0 mm, while a maximum of 1.24 ± 0.3 N when the distance between brackets was 12 mm in the right bracket. In the left bracket, minimum and maximum forces of −0.09 ± 0.07 and 1.3 ± 0.4 N were generated in the right direction, respectively. The 9.0-mm elastomeric chain generated a minimum force of 0.03 ± 0.07 N in the left direction when the distance between brackets was 9.0 mm, while a maximum of 1.3 ± 0.1 N when the distance between brackets was 15 mm in the right bracket. In the left bracket, minimum and maximum forces of 0.05 ± 0.06 and 0.98 ± 0.2 N were generated in the right direction, respectively. ConclusionMechanical data of the ISW have been collected in the study, which was previously difficult to perform owing to the low stiffness of the wire. It is suggested that the ISW can provide sufficient moments with the addition of V-bends to close the space by bodily movement.

Improved Super-elastic Ti-Ni Alloy Wire for Nonsurgical Treatment of Skeletal Class III Malocclusion Combined with Anterior Crowding and Anterior Crossbite Case

The article reported the use of improved super-elastic Ti-Ni alloy wire (ISW), which was developed by Tokyo Medical and Dental University, to correct a case of skeletal Class III malocclusion combined with anterior crowding and anterior crossbite. An 18-year-old male with a chief complaint of poor dental alignment and protruded lower incisors showed up in our clinic for treatment. Clinical examination revealed skeletal Class III facial profile. Anterior crowding and anterior crossbite could be also noted. Because the patient refused orthognathic surgery, we tried camouflage orthodontic treatment. First, we used ISW crossbite arch to eliminate the anterior interference in order to observe the change of the mandibular position. After that, we use ISW multiloop edgewise arch wire (MEAW) technique and intermaxillary elastics to achieve midline correction and adequate overjet with the bite raising process. The treatment was finished with the improvement of facial appearance and a stable occlusion.