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Ni–Ti alloys are widely applied in various fields due to their good biocompatibility and special shape memory function. To date, to make the Ni–Ti alloy ferromagnetic, the commonly used method is to add Fe or Ni particles to the Ni–Ti alloy. Within the knowledge, no reports have reported that the Ni–Ti alloys can possess ferromagnetism without adding magnetic particles. In the first half of this article, a process that can produce ferromagnetism in Ni–Ti alloys and explain the principle of the new process according to the Ni–Ti binary phase diagram is reported. On the other hand, magnetic micro‐robots have received widespread attention recent years and laser powder bed fusion (LPBF) technology makes it possible to fabricate Ni–Ti micro‐robots. The remainder of this article introduces a type of thermally responsive micro‐robot, which is fabricated by the LPBF technology and combined with this new Ni–Ti alloy phenomenon. Experiments are performed to confirm that Ni–Ti micro‐robots have the ability to transport cargo and target delivery under the control of an external magnetic field. The combination of this new Ni–Ti alloy phenomenon and LPBF technology can provide a new avenue for manufacturing magnetic micro‐robots. [ABSTRACT FROM AUTHOR]

Shape memory alloy (SMA) helical springs are special mechanical parts that require a previous evaluation of its behavior for application. Therefore, in this paper thermal and mechanical behaviour of superelastic Ni-Ti SMA helical extension springs manufactured by investment casting (IC) are evaluated. Phase transformation temperatures were measured by Electrical Resistance as a function of Temperature (ERT) and Differential Scanning Calorimetry (DSC). Tensile tests were carried out within strain and temperatures ranges. The pitch angle and stiffness of each spring were determined. Results demonstrated that Ni-Ti SMA helical springs produced by IC presented phase transformation corresponding to the superelastic effect (SE). The reversible deformations under tensile test were of the order of 70%. The mechanical behavior as function of temperature revealed a linear relationship between maximum force and spring temperature.

Ni-rich Ni-Ti alloys, such as 60NiTi, show a higher hardness and dimensional stability than equiatomic or near-equiatomic NiTinol ones. These make them suitable to be employed in structural applications. Laser powder bed fusion technique (LPBF) was used in this research to print parts from a Ni-rich Ni-Ti powder mixture consisting of about 63 wt.% Ni and 37 wt.% Ti. The microstructure of the obtained parts, processed using different parameters, showed inhomogeneity, contained undesirable Ni-rich and Ti-rich regions and a significant amount of cracks. To eliminate these defects, homogenize the microstructure and to obtain the required phases, we applied hot isostatic pressing (HIP) to the printed samples with a selected set of parameters. After HIP at 180 MPa and 1050 °C for 4 h, we observed the formation of a homogeneous microstructure containing mainly NiTi and Ni _3 Ti phases in the printed samples. However, cracks still persisted in the microstructure of these HIP treated samples. Applying another round of HIP treatment at a temperature just slightly above the melting start temperature of the samples, treated by the elementary HIP procedure, could successfully eliminate the cracks in the microstructure of samples and also increase the crystallinity of the existing phases.

The work addresses two main questions that have baffled the shape memory research community. Firstly, the superb ductility of B2-NiTi cannot be solely attributed to slip on {0 1 1} planes, because there are not a sufficient number of independent slip systems under arbitrary deformations. We show unequivocally, upon diffraction measurements and local strain field traces, that deformation twinning on {1 1 4} planes that can provide additional systems to accommodate plastic flow is activated. Secondly, the slip direction on the {0 1 1} planes has not been established in NiTi with certainty. It is proved precisely to be in 0 0 1 direction based on crystallographic shear analysis producing the specific strain tensor components (measured at mesoscale with digital image correlation, DIC). Based on the single-crystal experiments, the CRSSs (critical resolved shear stress) are established as 250 and 330 MPa for slip and twinning, respectively. The results have implications in devising correct crystal plasticity formulations for shape memory alloys. [ABSTRACT FROM AUTHOR]

Since the 1980s, different devices based on superelastic alloys have been developed to fulfill orthodontic applications. Particularly in the last decades several researches have been carried out to evaluate the mechanical behavior of Ni-Ti alloys, including their tensile, torsion and fatigue properties. However, studies regarding the dependence of elastic properties on residence time of Ni-Ti wires in the oral cavity are scarce. Such approach is essential since metallic alloys are submitted to mechanical stresses during orthodontic treatment as well as pH and temperature fluctuations. The goal of the present contribution is to provide elastic stress-strain results to guide the orthodontic choice between martensitic thermal activated and austenitic superelastic Ni-Ti alloys. From the point of view of an orthodontist, the selection of appropriate materials and the correct maintenance of the orthodontic apparatus are essential needs during clinical treatment. The present work evaluated the elastic behavior of Ni-Ti alloy wires with diameters varying from 0.014 to 0.020 inches, submitted to hysteresis tensile tests with 8% strain. Tensile tests were performed after periods of use of 1, 2 and 3 months in the oral cavity of patients submitted to orthodontic treatment. The results from the hysteresis tests allowed to exam the strain range covered by isostress lines upon loading and unloading, as well as the residual strain after unloading for both superelastic and thermal activated Ni-Ti wires. Superelastic Ni-Ti wires exhibited higher load isostress values compared to thermal activated wires. It was found that such differences in the load isostress values can increase with increasing residence time.

The assessment of heat treatment on dissimilar Ni-Ti and Ti-Mo welding has been evaluated in this work. An experimental design 3 2 was used as optimization tool to evaluate the effect of time and temperature after the welding process on the welding quality. Welded joints without significant difference in the thickness of the welded wires were obtained successfully. After the heat treatment welded joints with shape memory were obtained. After the welding process a change in the chemical composition of the melting zone has been observed. There was a decrease of nickel, molybdenum and zirconium content and an increase of the titanium content. The temperature and time of heat treatment affected the corrosion resistance performance. It was observed that the short time and lower heat treatment temperature favors the formation of Ni 4 Ti 3 precipitate, which is responsible for the formation of stable passivation film. The longer the time and temperature of heat treatment more the decrease in the corrosion resistance and increase in the fragility of the obtained joints, and consequently the amount of the precipitated Ni 4 Ti 3 . [ABSTRACT FROM AUTHOR]

Gaseous nitriding of ferritic Fe–Cr and austenitic Ni–Ti solid solutions reveals that the extent of the uptake of dissolved nitrogen depends on the crystallographic orientation of the surface grains of the substrate. In both ferritic and austenitic substrates, the surface nitrogen concentration and the nitriding depth decrease upon increasing the smallest angle between the surface normal and the normal of a {1 0 0} plane of the surface grain considered. This phenomenon could be ascribed to the residual compressive macrostress developed during nitriding which varies as a function of crystallographic orientation of the (surface) grains due to the elastically anisotropic nature of ferrite and austenite solid solutions investigated in this study. [ABSTRACT FROM AUTHOR]

In order to analyze the relationship between the shape memory behavior of Ti-rich NiTiCo thin films and the microstructures obtained after different annealing treatments, a batch of Ni 46.2 Ti 51.1 Co 2.7 (at.%) thin films were produced by magnetron co-sputtering and annealed at 773 K, 873 K and 973 K for 60 min. The microstructure of the thin films was studied by X-ray diffraction and transmission electron microscopy. Electrical resistivity measurements were performed to determine martensitic transformation temperatures. Additionally, a home-made device was specially designed for studying the mechanical behavior associated with the martensitic transformations of the thin films at different constant external stresses. The sample annealed at 773 K presented Guinier-Preston (GP) zones within the NiTiCo grains, while the samples annealed at 873 K and 973 K showed Ti 2 Ni precipitates inside the grains with semi-coherent interface with the matrix. The appearance of GP zones can explain why the sample annealed at 773 K reaches higher values of recoverable strain than the other samples, with negligible irreversible strain. The maximum recoverable strain of 5.7 % for this specimen represents a noticeable mechanical behavior, similar or even better than the one observed in the binary alloy with similar Ti concentration. The maximum recoverable strains were analyzed in terms of the Sachs bound, which represents a theoretical upper limit. [Display omitted] • Ti-rich NiTiCo thin films show an excellent mechanical behavior. • The addition of Co offers a controlled lowering of martensitic transformation temperatures. • Specimens annealed at 773 K present GP zones within the NiTiCo grains. • Specimens annealed at 773 K reach a recoverable strain of 5.7 % with negligible irreversible strain. • The recoverable strain of 5.7 % represents a promising response, in comparison with the Sachs bound. [ABSTRACT FROM AUTHOR]

The microstructure and internal friction of Ni-Ti alloys after hydrogen absorption have been investigated by means of optical microscopy, X-ray diffraction, differential scanning calorimetry and low frequency torsional internal friction apparatus. The results show that, after hydrogen absorption, the grains tend to be elongated, and the nucleation and growth of hydrides are mainly concentrated at the grain boundaries. In addition, a new phase ultimately identified as Ti2NiH0·5 phase forms after absorption of hydrogen. The hydrogen induced martensite promotes the emergence of a two-stage transformation. However, the growth of hydrides causes a reduction of the hydrogen induced martensite. The hydrides act as strong pinning points, resulting in a dramatic increase in the internal friction. In addition, the marked change of the internal stress, caused by the microscopic strain and the mismatching of the volumes, also improves the internal friction. [ABSTRACT FROM AUTHOR]

The decomposition behavior of Ni-rich Ni-Ti was reassessed using Tomographic Atom Probe (TAP) and Laser Assisted Wide Angle Tomographic Atom Probe. Single crystalline specimens of Ni-11.3 at.% Ti were investigated, the states selected from the decomposition path were the metastable y" and y' states introduced on the basis of small-angle neutron scattering (SANS) and the two-phase model for evaluation. The composition values of the precipitates in these states could not be confirmed by APT data as the interface of the ordered precipitates may not be neglected. The present results rather suggest to apply a three-phase model for the interpretation of SANS measurements, in which the width of the interface remains nearly unchanged and the L12 structure close to 3:1 stoichiometry is maintained in the core of the precipitates from the y" to the y' state. [ABSTRACT FROM AUTHOR]

The transformation behaviour of the multistage martensitic transformation in aged Ti-50.8 at.% Ni alloys was investigated by differential scanning calorimetry (DSC) andin situscanning electron microscopy (SEM). The specimens aged from 673 to 748 K for 3.6 ks under an unregulated heat treatment atmosphere exhibited the double-stage transformation during cooling. The quadruple-stage transformation was observed in the specimens aged at 773 and 798 K, and the triple-stage transformation appeared in the specimen aged at 823 K under an unregulated heat treatment atmosphere. The distribution and size of Ti3Ni4precipitates were heterogeneous in these specimens. The single-stage transformation in the specimen aged at 848 K was similar to that of the solution-treated specimen. In the forward quadruple-stage transformation, the R-phase transformation occurred in the intermediate region and around the grain boundary. The first martensitic transformation, which corresponded to the M1 peak in the DSC cooling curve, took place in the intermediate region of grains via the R phase. The second transformation, which corresponded to the M2 peak, occurred around the grain boundary via the R phase. The final transformation, which corresponded to the M3 peak, arose directly from the B2 parent phase at the grain centre. The transformation sequence and areas described above were quantitatively verified by comparing the SEM observations with the DSC measurements. The transformation sequence of the triple-stage transformation was also discussed. [ABSTRACT FROM PUBLISHER]

Four characteristic interface microstructures between habit plane variants (HPVs) in the self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by scanning transmission electron microscopy (STEM). The straight interface of a B19′ type I twin is present at interface I. The relaxation of the transformation strain at interface II is achieved by a volume reduction of the minor correspondence variants (CVs) in the relevant habit plane variants (HPVs). The relaxation of the transformation strain at interface III is mainly due to the formation of a B19′ type I twin between the two major CVs. Subsequently, local strain around the tips of the minor CVs perpendicular to the interface is released by the formation of micro-twins with the ⟨011⟩B19′ type II and/or B19′ type I relation. The major and minor CVs in each HPV are alternately connected through fine variants with the B19′ type I twin relation parallel to interface IV. The results are compared with macroscopic observations and the predictions of PTMC analysis. [ABSTRACT FROM PUBLISHER]

Competition between the invariant plane (IP) condition at the habit plane, the twin orientation relation (OR) and the kinematic compatibility (KC) at the junction plane (JP) of self-accommodated B19′ martensite in Ti–Ni was investigated via the geometrically nonlinear theory to understand the habit plane variant (HPV) clusters presented in Parts I and II of this work. As the IP condition cannot be satisfied simultaneously with KC, an additional rotation Q is necessary to form compatible JPs for all HPV pairs. The rotation J necessary to form the exact twin OR between the major correspondence variants (CVs) in each HPV was also examined. The observed HPV cluster was not the cluster with the smallest Q but the one satisfying Q = J with a { 1}B19′ type I twin at JP. Both Q and J are crucial to understanding the various HPV clusters in realistic transformations. Finally, a scheme for the ideal HPV cluster composed of six HPVs is also proposed. [ABSTRACT FROM PUBLISHER]

The self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Twelve pairs of minimum units consisting of two habit plane variants (HPVs) with V-shaped morphology connected to a B19′ type I variant accommodation twin were observed. Three types of self-accommodation morphologies, based on the V-shaped minimum unit, developed around one of the {111}B2 traces, which were triangular, rhombic and hexangular and consisted of three, four and six HPVs, respectively. In addition, the variant selection rule and the number of possible HPV combinations in each of these self-accommodation morphologies are discussed. [ABSTRACT FROM PUBLISHER]

high temperature shape memory alloy, Ni–30Pt–50Ti (at.%), with an M near 600 °C, was isothermally oxidized in air for 100 h over the temperature range of 500–900 °C. Nearly parabolic kinetics were observed in log–log and parabolic plots, with no indication of initial fast transient oxidation. On average the rates were about a factor of 4 lower than values measured here for a binary Ni–49Ti commercial SMA. The overall behavior could be best described by the Arrhenius relationships:The activation energy was consistent with literature values for TiO scale growth measured for elemental Ti and some NiTi alloys, at ~210–260 kJ/mol. However, a number of other studies produced activation energies in the range of 135–150 kJ/mol. This divergence may be related to various complex scale layers and depletion zones, however, no specific correlation can be identified at present. [ABSTRACT FROM AUTHOR]

Abstract: Powder metallurgy is an important manufacturing method and developing models that can predict the characteristics of the products is remarkable for researchers. There are many models discussed in the literature for prediction of the product properties however, nonlinear modeling methods including artificial neural networks (ANNs) and fuzzy models have shown better performance. In the present work, a rule-based fuzzy logic model is developed to predict the shear strength of Ni–Ti alloys specimens manufactured by powder metallurgy method. The processing time and temperature are selected as the input variables and a fuzzy model is designed with two inputs and one output variable. Four statistical parameters are used for assessment of the model accuracy. The comparison of this model result and the result of the ANN model that have been reported by previous researchers, shows that the fuzzy model is more accurate and actually better than ANN model for predicting the shear strength of Ni–Ti alloys specimens manufactured by powder metallurgy. [Copyright &y& Elsevier]

Abstract: In this work, the fabrication of shape memory alloys (SMAs) through the use of the plasma skull push–pull (PSPP) process was studied. In this process, small quantities of metallic elements can be quickly melted on a copper crucible using a rotating plasma torch and injected into a metallic mold. In order to validate the utilization of the PSPP process to produce SMA, several Ni–Ti-based and Cu–Al-based binary, ternary and quaternary alloys were tested. It was verified that five melts and re-melts of the SMA before injection are enough to obtain a homogeneous product. The obtained SMAs were characterized by optical microscopy and microhardness measurements. The thermoelastic martensitic transformation at the origin of the shape memory effect (SME) was studied via electrical resistance as a function of temperature and differential scanning calorimetry (DSC). All studied SMAs were shown to undergo a thermoelastic transformation, thus demonstrating the viability of the PSPP process to fabricate a great variety of SMAs. [Copyright &y& Elsevier]

Abstract: Atomistic modeling of segregation in Ni–Ti alloys is performed using the Bozzolo–Ferrante–Smith (BFS) method for alloys. It is found that Ti segregation to the surface decreases rapidly even for small increases in Ni concentration. A competing process is identified in the vicinity of surface defects, leading to excess Ti coverage of the exposed surface and the creation of Ti-covered pits. A simple explanation based on an atom-by-atom energy analysis is found to satisfactorily explain the observed effects. [Copyright &y& Elsevier]

The corrosion of two-phase Ni–10Ti and Ni–15Ti in molten (0.62Li, 0.38K)2CO3 at 650 °C under the atmosphere of air has been studied by electrochemical impedance spectroscopy (EIS). The impedance spectra for both Ni–10Ti and Ni–15Ti are composed of a semi-circle at high-frequency port and a line at low-frequency port indicating a diffusion-controlled reaction. The corrosion of the alloys produces an external scale composed of NiO and TiO2, and a wide internal oxidation region. An equivalent circuit representing the features of the corrosion of the alloys was proposed to fit the impedance spectra, and electrochemical parameters in the equivalent circuit were also calculated. [Copyright &y& Elsevier]

The effect of pre-oxidation treatment at 650°C in air on the corrosion behaviour of binary Ni–Ti alloys containing 5, 10, and 15 wt-%Ti in eutectic (Li, K)2CO3 at 650°C in air has been investigated. The results indicate that the pre-oxidation of the alloys can produce an almost pure TiO2 scale. Nevertheless, this TiO2 scale cannot hinder the formation of NiO outside and beneath the TiO2 scale during subsequent molten salt corrosion. The corrosion rates of the alloys decrease with increasing content of titanium in the alloys. The corrosion of the alloys proceeds mainly by inward diffusion of oxygen through the TiO2 layer, forming large quantities of NiO beneath the TiO2 layer. Compared with Ni–5Ti and Ni–10Ti, the formation of NiO on Ni–15Ti is inhibited significantly, especially in the initial stages. [ABSTRACT FROM AUTHOR]

Diffuse wide-angle neutron scattering experiments have been performed for two states of thermal equilibrium within the γ solid solution, 58Ni–5.8 at.% Ti at 777 K and 58Ni–9.6 at.% Ti at 1103 K. The analysis of the Clapp configurations yields the basic cluster of the L12 structure as the most important local configuration. Neither the static nor the dynamic atomic displacements indicate any precursor of the stable hexagonal phase (D024 structure) containing 25 at.% Ti. Metastable states with precipitates showing non-stoichiometric L12 structure evolve during the decomposition of supersaturated Ni–11.3 at.% Ti polycrystals as seen by in-situ small-angle neutron scattering. From the integrated small-angle scattering intensity, the Ti content of the cuboidally shaped precipitates in the metastable γ′′ state is found to be about 17 at.% between 870 and 950 K. [Copyright &y& Elsevier]

In this paper are presented the design and experimental results of using a shape memory alloy (SMA) wire as an actuator for position control of a composite beam. The composite beam is honeycomb structured, having wires of SMA embedded in one of its face sheets for the purposes of active actuation. Nickel-titanium SMA wires were chosen as actuating elements due to their high recovery stress (>700 MPa) and tolerance to high strain (up to 8%). A simple proportional and derivative controller plus a feed-forward current is designed and implemented for controlling the tip position of the composite beam. Experiments have demonstrated the effectiveness of the SMA wire as an actuator for active position control of a composite beam. [ABSTRACT FROM AUTHOR]

[Display omitted] From a systematic atom probe tomography (APT) characterization of the radiation-induced segregation (RIS) in dilute Fe-Ni and Ni-Ti model alloys, we highlight fluctuations of the solute local concentration up to the scale of the APT specimens. We deduce the RIS at dislocation loops from a solute diffusion equation, that is solved at steady state, within the Voronoi's volume occupied by a single loop. From a statistical sampling of the Voronoi's volume and the dislocation loop radius modeled after the characterization of the microstructure by transmission electron microscopy, we provide the full RIS distribution. The present statistical approach of RIS demonstrates that the fluctuation of local solute concentrations in Fe-Ni and Ni-Ti mainly results from the dispersion in size and density of the dislocation loop population. Besides, we highlight the impact of the post-treatment parameters used in the APT protocol on the extracted RIS profiles. In Ni-Ti alloys, the simulated Ti-depletion profiles are in very good agreement with the measured ones. Furthermore, the dispersion of the loop radius and density is shown to play a critical role on the fluctuations of the Ti local concentration. In Fe-Ni, the identification of discrepancies between the simulated Ni-enrichment profiles and the measured ones provides a signature of additional operating mechanisms of the solute redistribution, such as radiation-induced precipitation. [ABSTRACT FROM AUTHOR]

Developing an efficient stent frame for transcatheter aortic valves (TAV) needs thorough investigation in different design and functional aspects. In recent years, most TAV studies have focused on their clinical performance, leaflet design, and durability. Although several optimization studies on peripheral stents exist, the TAV stents have different functional requirements and need to be explicitly studied. The aim of this study is to develop a cost-effective optimization framework to find the optimal TAV stent design made of Ni-Ti alloy. The proposed framework focuses on minimizing the maximum strain occurring in the stent during crimping, making use of a simplified model of the stent to reduce computational cost. The effect of the strut cross-section of the stent, i.e., width and thickness, and the number and geometry of the repeating units of the stent (both influencing the cell size) on the maximum strain is investigated. Three-dimensional simulations of the crimping process are used to verify the validity of the simplified representation of the stent, and the radial force has been calculated for further evaluation. The results suggest the key role of the number of cells (repeating units) and strut width on the maximum strain and, consequently, on the stent design. The difference in terms of the maximum strain between the simplified and the 3D model was less than 5%, confirming the validity of the adopted modeling strategy and the robustness of the framework to improve the TAV stent designs through a simple, cost-effective, and reliable procedure., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The cytotoxic and antibacterial effects of orthodontic appliances were assessed. Metallic devices used in orthodontics, such as molar bands, brackets, and archwires were tested by the agar overlay cytotoxicity test with mouse fibroblast cells. The same devices were tested for antibacterial effect with Streptococcus mutans and S. sanguis. The multicomponent devices, which are bonded with silver- and copper-based brazing allowys, were more cytotoxic than the single-component devices, probably because copper is more cytotoxic than nickel. The devices had a definite, but low, antibacterial effect, as compared with the 0.05% chlorhexidine positive control. A cytotoxic effect of the devices per se might contribute to a localized singivitis. It is uncertain whether orthodontic devices have nay significant inhibitory effect on dental plaque viability. [ABSTRACT FROM AUTHOR]