NiTi Alloy Research Articles

Comparing Cyclic Fatigue Resistance and Free Recovery Transformation Temperature of NiTi Endodontic Single-File Systems Using a Novel Testing Setup.

The aim of this study was to assess the effect of body temperature (37 °C) on the cyclic fatigue resistance of three endodontic single-file systems using a new testing setup. One Shape® new generation (OS), WaveOne™ (WO) and WaveOne® GOLD (WOG), which are made from different NiTi alloys and operated in different motions (rotation/reciprocation), were evaluated. The study design included four groups. Each group comprised 30 files, 10 files of each of the three file systems, tested at 20 ± 2 °C (group 1 and 3) and at 37 ± 1 °C (group 2 and 4). All files were tested in a custom-made metal block with artificial canals of 60° angle, and a 5 mm and 3 mm radius of curvature, respectively. A heating element was attached to replicate a temperature of 37 °C. Files were introduced 18 mm into the canals and operated until failure. Transformation temperatures of five samples of each of the tested file systems were determined via the bend and free recovery (BFR) method. With the exception of WOG in canals with a 3 mm radius of curvature (p = 0.075), all the tested file systems showed statistically significantly less time needed to fracture when operated at 37 ± 1 °C compared to at 20 ± 2 °C in canals with a 5 mm and 3 mm radius of curvature using Mann-Whitney U test (p < 0.05). All file systems showed transformation temperatures below the body temperature. We concluded that body temperature directly affects the cyclic fatigue resistance of all tested file systems. Bend and free recovery can be suitable for the determination of austenite finish temperatures (Af) of endodontic instruments as it allows testing a longer portion of the instrument.

Corrosion behavior of laser powder bed fusion additive manufacturing produced TiNi alloy by micro-arc oxidation

To improve the corrosion resistance of TiNi alloy fabricated by laser powder bed fusion (LPBF), a porous oxidation layer was synthesized by micro-arc oxidation in a sodium aluminate and sodium silicate electrolyte. The influences of the applied voltage and the processing time on the morphology of oxidation layer were investigated, and the corrosion behavior of the oxidation layer in artificial saliva was evaluated and compared with that of the as-fabricated LPBF alloy. The results indicate that, as increasing the applied voltage and the processing time, the oxidation layer becomes uniform and integrated. The optimum parameters are with an applied voltage of 450 V and processing time of 40 min. The oxidation layer primarily contains α-Al2O3 and consists of two layers, i.e., a thin, compact and uniform inner layer and a porous outer layer. The formation of stable α-Al2O3 phase in the coating and its almost non-porous dense structure reduce the channels for corrosion ions to penetrate into the substrate through coating, thereby improving the corrosion resistance of TiNi alloy.

Effect of milling time on structural, mechanical and tribological behavior of a newly developed Ti-Ni alloy for biomedical applications

Ti-Ni alloy is the most attractive material in the medical field for orthopedic implants like total hip replacement due to its special properties. The purpose of this study was to examine the impact of grinding/milling times on the characteristics of the nanostructured Ti50-Ni50 alloys produced via the mechanical alloying process (MA), utilizing a planetary high-energy ball mill at different grinding times (2, 6, 12, and 18 h). The grinding time causes grain refinement and a decrease in the porosity of materials, which allows for the improvement of a material’s mechanical properties and wears resistance. The structural, physical and tribological behavior of Ti50-Ni50 alloy was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation test, and ball-on-disk tribometer operated under varying applied loads of 2, 10, and 20 N. The findings demonstrated that the crystallite size was reduced and the microstrain increased reaching values of 29 nm, and 0.99%, respectively, at higher grinding times. The percentage of porosity in the Ti50-Ni50 alloy was 11%, while the density attained was 89% after 18 h of grinding. Moreover, as the grinding duration increased up to 18 h, the hardness increased (341 HV) owing to the refinement of the grains. It was also noted that the Ti50-Ni50 alloy that was milled at 2 h exhibited the lowest elastic modulus (89 GPa). In addition, it was found that wear rates and the coefficient of friction were decreased reaching the values of 29.67 µm3/N.μm, and 0.31, respectively, for the samples milled at 18 h and examined under 2 N. Enhanced structural and mechanical qualities are responsible for this improvement in tribological properties. These properties make Ti50-Ni50 alloys clinically relevant for human biomedical implantation.

Modulation of characteristic zones in NiTi alloys fabricated via wire arc additive manufacturing

The application of additive manufacturing (AM) is gradually extending to different industrial sectors. Most additively manufactured metallic components typically exhibit an inherent columnar grain structure, the control of which has been a challenging issue. In this study, the microstructures of the characteristic zones (heat-affected zone and transition zone) were controlled by adjusting the deposition strategy and arc mode during wire arc additive manufacturing (WAAM) of NiTi alloy. Compared to the sample fabricated by cold metal transfer (CMT) mode, the oscillating (O) sample shows a higher tensile strength of 793.6 MPa, while the pulsed (P) sample shows a better elongation of 5.8%. The high strength of the O samples is mainly due to the significantly lower aspect ratio of its columnar grains, while fine equiaxed grain structure in the transition zone of the P samples is responsible for the better plasticity. Different microstructures in the characteristic zones can be tailored by combining the forced melt flow, pulse frequency and undercooling, which provides vast freedom for the associated mechanical properties control. Our work provides a new strategy to regulate the microstructure and thus mechanical properties of additively manufactured NiTi alloys.

Well-Adhered Ti Alloying Layer on NiTi Alloy: Surface Ni Content, Corrosion Resistance, and Cytocompatibility

Well-Adhered Ti Alloying Layer on NiTi Alloy: Surface Ni Content, Corrosion Resistance, and Cytocompatibility

Analysis of Cutting Forces and Geometric Surface Structures in the Milling of NiTi Alloy.

This paper presents a study of the total cutting force used and selected parameters of the geometric structure of the surface (e.g., Sa, Sz) during the end milling process of NiTi alloy. The input parameters included are cutting speed (vc), feed per tooth (fz), and radial depth of cut (ae). A Box-Behnken experimental design was employed to conduct the research. The obtained experimental results were utilized within the framework of a response surface methodology (RSM) to develop mathematical and statistical models capable of predicting cutting force components and selected 3D surface parameters. These models provide valuable insights into the relationships between the cutting parameters and the output variables, facilitating the optimization of the NiTi alloy milling process. The findings of this study contribute to a better understanding of the behavior of NiTi alloy during the milling process and offer information for process optimization. By employing a Box-Behnken experimental design, it was possible to investigate the effects of different parameter combinations on the components of total cutting force and selected 3D surface parameters according to ISO 25178, thus aiding in the identification of optimal milling conditions to achieve desired outcomes in the machining of NiTi alloy.

Surface Characterization of Metallic Materials in the Case of the Turning Process of NiTi Alloy.

A study was made of the machinability of NiTi alloy in turning, under conditions resulting in a small cutting layer. The experiment involved cutting with variable feeds ranging from 0.01 to 0.1 mm/rev. The cutting conditions were carefully chosen, considering the rounding radius of the cutting edge. The machined surface was examined and measured in 3D using a confocal microscope and in 2D with a contact profilometer. These measurements were used to estimate hmin, leading to the development of a surface formation model that considers both the lateral material flow due to hmin and the lateral material flow due to altered thermodynamic conditions from the previous blade pass. A method for evaluating the surface and selecting its characteristics was proposed based on analyses derived directly from surface features: PCA (Principal Component Analysis) and EMD (Empirical Mode Decomposition) with the Hilbert transform (Hilbert-Huang transform). PCA analysis facilitated the examination of individual surface component variances, while analysis of the IMF components enabled the assessment of surface component energy combined with instantaneous frequencies.

An atomistic study of effects of temperature and Ni element on the phase transition and wear behavior of NiTi shape memory alloy

Phase transition and tribological behavior of NiTi alloys have drawn great attention. Molecular dynamics simulation was adopted to investigate the effects of temperature on the mechanical behavior and variant formation of NiTi, as well as its wear behavior. Thermally and stress-induced phase transition behavior of NiTi alloys were studied. It is found that residual austenite phase boundaries during martensitic phase transformation could inhibit martensitic phase transition, changing the phase transition temperatures, and the mechanical properties of NiTi. Wear behavior of single-crystal NiTi with a wide temperature domain and content was studied. The optimal wear resistance of NiTi was at 300 K during cooling. Appropriate improvement in Ni content is beneficial to the formation of martensite variants and enhancement of the wear resistance.

Corrosion Behavior of Nickel–Titanium Continuous-Casted Alloys

Variations in the corrosion behavior of biomedical NiTi alloys in Cl− containing and acidic environments present a problem with their biological implantation. The objective of this research was to evaluate the synergy of the microstructure, the corrosion behavior, and the biocompatibility of novel continuous-cast NiTi alloys and to compare them with commercial NiTi alloys. The two alloys have a practically identical nominal chemical composition, but they differ in production technology. The continuous casting technology involved vacuum induction melting of the basic components and vertical continuous casting, while the commercial NiTi alloy was produced through a process of casting, hot rolling, and forming into square shapes. The microstructure was revealed to determine the surface area and size of grains. The corrosion of a commercial nickel–titanium alloy and one prepared by a novel continuous casting method in acidic and chloride-containing solutions was studied via analytical and electrochemical tests. Localized corrosion characteristics related to oxide properties, when exposed to 9 g L−1 NaCl solution, were examined with focused ion beam analysis and subsequent microchemical analysis of the formed corrosive products. Corrosion potential over time and the oxide film resistance were analyzed using linear polarization measurements. To obtain a preliminary estimate of biocompatibility, human fibroblast cells were used in indirect contact, i.e., alloy conditioning medium. The continuous casting method resulted in a reduction in the average grain size in comparison to the commercial sample and better corrosion stability of the sample in an acidic environment. Also, in a solution of 9 g L−1 NaCl the commercial sample showed high values for the corrosion current density (jcorr = 6 μA cm−2), which indicated low corrosion resistance, while the continuous casting sample possessed much better corrosion stability and lower values for the corrosion current density (jcorr = 0.2 μA cm−2). In line with that, elemental analysis of the corroded surfaces showed higher Cl− ion deposition over the surface layer of the commercial sample, suggesting local oxide breakdown. Moreover, NiTicc reached a value three times higher for polarization resistance (Rp = 270 kΩ cm2) over time in comparison to the commercial sample (Rp~100 kΩ cm2). Biocompatibility evaluation showed that human fibroblast cells exhibited altered metabolic activity. An MTT assay showed that cells’ mitochondrial activity dropped below that of control cells in the presence of both materials’ supernatants.

Effects of grain size and dislocation density on thermally-induced martensitic transformation of nanocrystalline NiTi alloys

Nanoscale grain and high dislocation density are the most common microstructures of nanocrystalline NiTi alloy. Although grain size and dislocation density influence the functional properties of NiTi alloy significantly, their effects on martensitic transformation have not been investigated quantitatively. In this work, the effects of grain size and dislocation density on thermally-induced martensitic transformation of nanocrystalline NiTi alloys are studied based on experiment, molecular dynamics simulation and theoretical analysis. Firstly, the drop of martensite-start-temperature and austenite-finish-temperature with decreasing grain size is determined, which is attributed to the increased fraction of grain boundary and the enhanced disordering of austenite near grain boundary via molecular dynamics simulation. Secondly, the positive correlation between dislocation density and transformation temperatures is verified by experiments and molecular dynamics simulations, which is ascribed to the enhanced stability of correspondence variant pair with habit plane of (−0.4138 0.8684 0.2688) and invariant shear direction of [−0.4432 −0.458 0.77706] under the dislocation-induced stress field. At last, a novel thermodynamic model of nanocrystalline NiTi considering grain size and dislocation density is proposed to predict martensite-start-temperature and austenite-finish-temperature. Prediction results accord to experimental data reasonably with the error generally less than 15 K. This study provides a basis for regulating the functional properties of nanocrystalline NiTi alloys through microstructural manipulation.

Near-spherical micron-porous NiTi alloys with high performances fabricated via metal injection molding

In this study, NiTi alloys with different porosity were prepared using metal injection molding combined with the powder space-holder method (MIM-PSH). The microstructures, transformation temperatures, compression and its recovery properties were investigated. The porous structure consisted of near-spherical pores (40.8 and 44.9 μm). The pore size was tied to the size of space-holder but not porosity, which was beneficial to controllability of pore diameter. The matrix was dominated by B2 austenite, with small amounts of B19′ martensite and Ti2Ni inclusions. Ni4Ti3 precipitates were also observed, not only in micrometer scale, but also as nanodomains. Highly broadened R peaks were observed in the differential scanning calorimetry thermograms, which were attributed to extensive, non-uniform, multi-scale precipitation. Overall, pores at this scale shown no influence on the transformation behaviors. Due to the low final oxygen content (0.19 % and 0.23 %) and homogeneous near-spherical pores, high compressive properties had been exhibited. The compressive recovery was almost complete at 3 %, 6 % and 8 % pre-strains. Irreversible strain increased with increasing pre-strain. The average volume variation of the pores under different deformations was analyzed in detail. In the future, MIM-PSH should be used to further enhance the customized fabrication of porous NiTi alloys.

Microstructure-property relationship in Zr-alloyed Ni-rich NiTi alloys: Enhancements in high-temperature stability and superelasticity

This study delves into the influence of the Zr element on the thermal stability and superelasticity of Ni-rich NiTi(Zr) alloys. The inclusion of Zr heightened the hardness, reshaped the morphology of the Ni4Ti3 phase, and promoted the formation of nanodomains. The Zr-containing alloys demonstrate remarkable thermal stability, as evidenced by the hardness of 3Zr alloy remaining at 540.6 HV after 100 h of exposure to 600 °C. This marks a substantial 63.9% improvement compared to the 0Zr alloy. After treatments at 400 °C and 500 °C, the 3Zr alloy precipitated the H-phase. Following heat treatment at 600 °C for 100 h, the microstructure of the 0Zr alloy primarily consisted of Ni3Ti2 and Ni3Ti phases, whereas the 3Zr alloy was mainly composed of Ni4Ti3, Ni3Ti2, and Ni3(Ti, Zr)2 phases. The 3Zr alloy showcases exceptional superelastic properties, enduring a cyclic load of 1200 MPa for 5 times while maintaining a recovery rate exceeding 90%. The microstructure of the loaded 3Zr alloy consisted of Ni4Ti3 and nanodomains, exhibiting stable structural characteristics. Relevant mechanisms are discussed, laying the foundation for the application of Ni-rich NiTi alloys.

Enhanced photocatalytic antibacterial activity of Au-coated Ni–Ti–O nanotubes for biomedical applications

Nickel-titanium (NiTi) alloy is a smart material exhibiting superelasticity, contributing to low stiffness and facilitating integration with bone structures. However, NiTi alloy's bioactivity and antibacterial activity limit its dental applications. In this study, we fabricated nickel titanium oxide (Ni–Ti–O) nanotubes with the potential for bioactivity and photocatalytic properties on the surface of NiTi alloy. Specifically, we applied a coating of gold (Au) nanoparticles to the surface of Ni–Ti–O nanotubes, in order to leverage the antibacterial activity through the photocatalytic effect under 470 nm visible light. Field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) observations revealed that Au was deposited mainly at the top surface of Ni–Ti–O nanotubes (diameter: 12.76 ± 2.91 nm, height: 721.12 ± 50.25 nm). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS) analyses revealed that the Ni–Ti–O nanotubes were NiTiO3 crystalline phases with low crystallinity. Therefore, based on the methylene blue (MB) degradation test, the photocatalytic effect was only achieved under visible light at 470 nm, facilitated by the presence of Au. The antibacterial colony forming unit test and enzymatic activity tests demonstrated that Au-coated Ni–Ti–O nanotubes displayed excellent antibacterial activity under 470 nm visible light irradiation, which was also attributed to photocatalytic effect. Consequently, the combination of Au-coated Ni–Ti–O nanotubes and visible light-based photofunctionalization holds promise for expressing antibacterial properties in dental applications.

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.

Effects of adsorption of O2 and H2O molecules on the corrosion behavior of the NiTi alloy surface: a DFT investigation.

The influence of O2 and H2O adsorption significantly affects the electrochemical corrosion of the NiTi alloy, with unresolved corrosion disparities between the NiTi-B2 and NiTi-B19' phases. Density functional theory (DFT) calculations are utilized in this investigation to explore the adsorption of O atoms at varying coverages on the NiTi-B2(110) and NiTi-B19'(010) surfaces. The goal is to elucidate their oxidation behavior differences. Subsequently, the effect of O adsorption on the dissolution trends of these phases is assessed by inducing Ni/Ti vacancies to simulate alloy dissolution thermodynamically. Additionally, interactions between H2O molecules and O-pre-adsorbed NiTi alloy surfaces are examined to simulate the atomic evolution of the oxidized surface under exposure to humid air and corrosive solutions. The findings indicate a propensity of the NiTi-B19' phase to react with O, forming an oxide film more readily than the NiTi-B2 phase. O adsorption facilitates Ni dissolution and retards Ti dissolution on the alloy surface. Higher O coverage promotes easier dissolution of Ni and Ti atoms on the NiTi-B2(110) surface compared to the NiTi-B19'(010) surface, underscoring the greater corrosion resistance of the NiTi-B19' phase. Both clean and O-pre-adsorbed NiTi alloy surfaces physically adsorb H2O molecules. Notably, an O monolayer substantially mitigates the detrimental effects of H2O molecules on the corrosion resistance of alloy surfaces. This research contributes to a deeper comprehension of the corrosion mechanisms in NiTi alloys.

Temperature and strain-rate dependence of the superelastic response in EBF3-fabricated and post-heat-treated NiTi shape memory alloy

In the present work, the temperature and strain-rate dependence of the superelastic response behavior of NiTi shape memory alloys prepared via electron beam freeform fabrication (EBF3) technique and post-heat-treated is systematically studied. It is demonstrated that the response hysteresis of the EBF3-fabricated NiTi alloys during superelastic cycling decreases with increasing strain-rate at the same test temperature. However, the superelastic response of material gradually alters to a nearly linear mode with the increase of test temperature under the same strain-rate. The corresponding stable superelasticity recovery ratio decreases from 62.6 % to 47.6 % when the test temperature improves from 100 °C to 150 °C. This is mainly due to the activation of dislocations in the {011}<001> slip system of B2 austenite, in which they become plugged and entangled during plastic deformation, deteriorating the material superelastic recovery ability. These findings provide a selection basis for the service conditions of the rapidly prepared NiTi alloys under EBF3-fabrication and post-heat-treatment technology routes.

Investigation of microstructural, mechanical and tribological properties of NiTi/nB4C metal matrix composites produced by spark plasma sintering

In this paper, it was aimed to improve the microstructure, mechanical and tribological properties of composite samples according to the alloy by adding 0.25%, 0.5% and 1% nano B4C to NiTi shape memory alloy. A 3D ball mixer device was used to prepare composite powders and the samples were produced by Spark Plasma Sintering technique. FE-SEM/EDS and XRD were used for microstructure and phase characterization, nanoindentation to determine mechanical properties, and pin-on disc wear device to determine wear properties. It was determined that with the increase in the reinforcement ratio added to the NiTi alloy, nB4C ceramic particles settled at the grain boundaries and the hardness value of the 1% nB4C (334 HV) sample with the highest hardness increased significantly by 30% unreinforced NiTi alloy (264 HV).

Preparation and characterization of polydopamine and n-butyl methacrylate copolymer coatings on titanium-nickel alloy stents.

Cardiovascular diseases pose a significant global health threat, and stents play a crucial role in managing these diseases. However, challenges exist with respect to the poor adhesion of stent coatings. Cardiac stents are often composed of titanium-nickel (TiNi) alloys as the metallic component and poly(n-butyl methacrylate) (PBMA) as the coating. The poor adhesion of PBMA to TiNi alloy surface may cause detachment and subsequent thrombosis post-implantation. This study utilizes Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization to synthesize a novel block copolymer, PBMA-b-PVP, composed of PBMA and poly(N-vinylpyrrolidone) (PVP). TiNi alloy surfaces are functionalized with polydopamine (PDA) to enhance polymer coating adhesion. PBMA-b-PVP exhibits a remarkable improvement in adhesion from class 5 to class 0 and high coating stability after a 15 days immersion in a phosphate buffer solution. The corrosion current density is reduced by 44% with the incorporation of PDA into PBMA-b-PVP coatings, suggesting high corrosion resistance. PDA-functionalized coatings promote cell viability without cytotoxicity, suggesting high biocompatibility. This study provides a robust strategy for preparing stent coatings with high adhesion, corrosion resistance, and biocompatibility.

Cyclic deformation of Ti-Ni alloy used for endovascular device

Has been studied the possibility of using TiNi alloys with shape memory and pseudoelasticity as an implantation material for correction of functional mitral regurgitation endovascular device. The modes the deformation cycling were developed that allows fulfilness of necessary properties eligible to operate power-unit device for trans-skin implantation method. Preliminary deformation cycling provides mechanical properties range stabilization that guarantees the shape reinstatement of implant after its installation.

First-principle study of Ti50Ni50-xNbx shape memory alloys for biomedical applications

The excellent shape memory effect and biocompatibility of TiNi alloys have revolutionized the scientific and industrial communities, particularly in biomedical applications. These materials can undergo major or quasi-plastic deformation before returning to their original form when subjected to external stimuli like stress or temperature. However, nickel has been shown to be carcinogenic, it may lead to cytotoxicity which can result in infections, follow-up issues, or implant failure. The aim of this study is to improve the biocompatibility of TiNi by substituting Ni with Nb employing the first principle approach. The structural and elastic properties of the TiNi- Nb alloy were investigated using the CASTEP code embedded in Materials Studio. The findings indicated that Ti8Ni4Nb4's formation energy had increased. The poisons ratio decreased slightly, reaching a value of 0.32 in line with the G/B findings. Increasing Nb content also lowered the density of states at the Fermi levels. It was found that Ti8Ni4Nb4 was the most suitable substitute for TiNi alloys.