Nitinol

In the current investigation, the mechanical properties evaluation and optimization of process parameters of plasma-sprayed NITINOL coating on mild steel substrate has been performed. The relation between the mechanical properties and process parameters of the coating was established by proposing a nonlinear regression equation. The obtained coefficient of determination (R 2) and the mean relative error for microhardness (99.7 and 4.85%) and adhesion strength (99.2 and 6.88%) confirmed the statistical validity of the proposed nonlinear regression equation. The Genetic Algorithm optimization technique was implemented to obtain the optimized parametric setting for the plasma spray coating process. To check the quality of the coating fabricated by the optimized parametric combination (plasma arc current of 550 A and primary gas flow rate of 45 lpm), the characterizations (x-ray diffraction, scanning electron microscope, energy dispersive spectroscopy, and 3D surface roughness measurement) of the coating were performed. The SEM morphology of the surface and the interface of the coating revealed the better flattening behavior of the splat and the lamellar structure of the coating, respectively. The obtained phases (Ni, Ti, NiTi, Ti 2 Ni, Ni 3 Ti, Ni 4 Ti 3 , TiO, NiO) found in the XRD pattern were confirmed from the EDS analysis at various regions of the specimen surface, and the obtained coating surface roughness (roughness average (S a)) is 12.35 lm.

In the current investigation, an elemental blending of equiatomic Ni and Ti powder was considered to spray on mild steel by the atmospheric plasma spray process. The results revealed that the coating developed with 120 mm stand-off distance (SOD) has better mechanical properties such as microhardness and adhesion strength. Again, SOD predominantly influences the formation of intermetallics (NiTi, Ni 3 Ti, Ti 2 Ni, and TiO) that helps to enhance the microhardness (683.85 HV) as well as the mechanical interlocking and chemical bonding that is solely responsible for the high adhesion strength (43.17 MPa) of the coating. The failure analysis of the coating developed at too high and too low SOD revealed that rapid expansion of gas stream, reduction in enthalpy of particles, improper heat transfer, burning of splat, agglomeration of particles during flight, and oxidation are the key factors responsible for the reduction in mechanical properties of plasma-sprayed Ni-Ti alloy (NITINOL) coatings. In addition to the above, the solid particle erosion analysis revealed the increase in brittleness of the coating with increasing in SOD. The surface morphologies of the eroded surface depict various erosion mechanisms at both 45° and 90° impingement angles such as chip formation, lip formation, plastic deformation, scratches, etc.

A most used structural material, mild steel which is prone to catastrophic failure is studied to enhance the surface properties with the application of NiTi using atmospheric plasma spray
(APS) technique. Near equiatomic Ni-Ti composition is popular for its shape memory effect and superelastic behaviour. In this investigation, atmospheric plasma spray is a cost effective as well as the
user friendly process in which a homogenized elemental mixture of Ni and Ti powder has been taken as feeder materials. Physical and mechanical properties have been revealed for the coating materials
using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), x-ray diffraction (XRD) and Vickers hardness testing machine. Particle erosion wear test has been studied to find out
the principle of material removal and brittle/ductility behaviour on the coating surface. It is clear that the mechanical properties of the substrate (mild steel) enhanced fivefold with the application of NiTi
alloy. A detail microstructural behaviour has been explained before erosion and after erosion. There is an increase in hardness is due to the formation of intermetallic of NiTi and oxide phases on the
surface of the coating.

This clinical report describes a treatment protocol for completely edentulous patients using digital implant planning for an all-on-4 treatment of both the maxilla and mandible as well as the use of a shape-memory alloy retention system to secure a complete-arch restoration to the mandible.

Submicrometer NiTi alloythin films prepared using biased target ion beam deposition (BTIBD) recently revealed ultranarrow thermal hysteresis and a B2 ⇌ R-phase transformation path. Here, the authors investigate the influence of thermal cycles on the phase transformation characteristics of near-equiatomic NiTi alloy films with 800 nm thicknesses deposited using BTIBD. Evolution of transformation temperatures, thermal hysteresis, and recovery stress over thermal cycles is tracked using the wafer curvature method, and changes in atomic crystal structures are detected using x-ray diffraction. The authors find that the submicrometer thin films exhibit stabilized transformation temperatures, consistent recovery stresses, and reproducible narrow thermal hysteresis over up to 100 cycles although Ni49.7Ti50.3 films undergo two-stage phase transformation B2 → R-phase → B19′ while Ni50.3Ti49.7 films have one-stage transformation between the B2 and R-phases. The inherent deposition mechanism and transformation-path-related lattice distortion can be responsible for the reduced fatigue of functional characteristics in submicrometer NiTi alloythin films.

Statement of problem: Screw- and cement-based retention mechanisms are used to attach prostheses to dental implants; however, each approach can lead to clinical complications such as crown fracturing or peri-implantitis. A novel abutment and prosthesis retention system has been engineered to achieve the esthetics and retention force of cement-based fixation while maintaining the retrievability of screw-retained restorations.

Purpose: The purpose of this pilot clinical study was to evaluate the effectiveness of this innovative retention system on posterior tooth restorations.

Material and methods: This clinical study, with up to 6 months of follow-up, included 8 participants with posterior osseointegrated implants who met the eligibility criteria to receive the abutment and shape memory sleeve. Radiographs were used to evaluate crown seating. Crown stability was measured using the Periotest, and occlusal analysis was performed using the Tekscan system and shimstock. Peri-implant health was evaluated by probing, and the plaque and gingival indices were recorded. In addition, patient-reported outcomes were recorded.

Results: Minimal differences were observed between baseline and endpoint assessment of the plaque and gingival indices, probing depth, and proximal and occlusal contacts. There were no patient-reported problems or complaints about discomfort. The overall peri-implant health remained unchanged from the baseline evaluations for all participants. Proximal contact around the restoration was present at the baseline and at the conclusion of the study for 7 of the participants. Occlusal contact was observed to be either light (5 participants) or holding (3 participants). In addition, visual inspection of retrieved crowns revealed clean surfaces free of macroparticle ingress, and bacterial accumulation at the coping-abutment interface was not detected.

Conclusions: The safety and efficacy findings of this pilot clinical study suggest that this new shape memory alloy-based retention system may provide a suitable alternative for implant prosthodontics. The retention system allowed for easy prosthesis seating and retrieval.

Statement of problem: A nitinol sleeve that uses shape memory to rapidly unlock dental restorations from implant abutments has been developed to allow prosthesis removal for assessment and maintenance, and clinical treatment has been promising. However, objective studies that evaluate the wear and retention performance after short-term clinical use are lacking.

Purpose: The purpose of this clinical study was to evaluate the wear and retention performance of a shape-memory abutment system after 6 months of clinical use.

Material and methods: Shape-memory alloy sleeves on posterior osseointegrated implants were retrieved after 6 months of clinical use. Scanning electron microscopy (SEM) was used to evaluate the surfaces of the retention sleeve's arms for wear. Uniaxial tensile testing was performed to measure the change in retention force after clinical use. Average retention values of the shape-memory abutment system were compared with previously reported in vitro retention values for definitive and interim cements used in titanium abutment and coping assemblies by using the Welch t test.

Results: No evidence of wear, fracture, or chipping was observed during SEM analysis on the shape-memory alloy sleeves. Additionally, no statistically significant difference was found in the median retention force for new (484.5 N) and clinically retrieved (476 N) nitinol sleeve specimens. Compared with a commercially available resin cement, the mean retention force for the control sleeves (480 ±37 N) was higher than that for the freshly cemented specimens (336.3 ±188 N). After 5000 cycles of compressive loads, the mean retention force for cement specimens decreased (209.4 ±83 N), while the clinical sleeves (476 ±50 N) remained unchanged.

Conclusions: According to the results of this study, after 6 months of clinical use, the engaging surfaces of the shape-memory alloy sleeve did not show signs of wear, and the retention force was unchanged.

A microscale phase field model developed in Levitas et al. (2004) and Idesman et al. (2005) is slightly advanced for different and anisotropic elastic moduli of phases and is employed for the study of the stress-induced cubic-monoclinic phase transition in NiTi single crystal involving all 12 martensitic variants. The model is scale-independent, without the gradient term, and it is applicable for any scale greater than 100 nm. This model includes strain softening and the corresponding transformation strain localization, and it reproduces a discrete martensitic microstructure. The model only tracks finite-width interfaces between austenite and the mixture of martensitic variants, and does not consider the interfaces between martensitic variants. The model is implemented as a UMAT subroutine in a commercial finite element (FE) package, ABAQUS. Multiple problems for a uniaxial cyclic loading are solved to study the effect of mesh, strain rate, crystal orientation, different numbers of pre-existing nuclei, and the magnitude of the athermal threshold on the stress-strain responses as well as the microstructure evolution. The obtained results exhibit that the microstructure and global stress-strain responses are practically independent of mesh discretization and the applied strain rate for relatively small strain rates. While the presence of the initial nuclei in the sample decreases the nucleation stress, it slightly increases the total energy dis-sipation. The observed microstructure, the sudden drop in the stress-strain curve after initiation of the martensitic transformation, and the absence of a similar jump for the reverse phase transformation are in qualitative agreement with known experiments. Changing the crystallographic orientation of the sample varies the entire behavior, namely, the variants which are involved in the phase transformation, the morphology of the associated microstructure, the stress-strain curve, and the total dissipation. Athermal threshold, in addition to the expected increase in the magnitude of hysteresis, leads to some strain hardening for the direct phase transformation.

Single‐bi‐layer of Ni–Ti thin film was deposited using DC and RF magnetron sputtering technique by layer‐wise deposition of Ni and Ti on Si(100) substrate in the order of Ni as the bottom layer and Ti as the top layer. The deposition of these amorphous as‐deposited thin films was followed by annealing at 300 °C, 400 °C, 500 °C, and 600 °C temperature with 1‐h annealing time for each to achieve crystalline thin films. This paper describes the fabrication processes and the novel characterization techniques of the as‐deposited as well as the annealed thin films. Microstructures were analysed using FESEM and HRTEM. Nano‐indentation and AFM were carried out to characterize the mechanical properties and surface profiles of the films. It was found that, for the annealing temperatures of 300 °C to 600 °C, the increase in annealing temperature resulted in gradual increase in atomic‐cluster coarsening with improved ad‐atom mobility. Phase analyses, performed by GIXRD, showed the development of silicide phases and intermetallic compounds. Cross‐sectional micrographs exhibited the inter‐diffusion between the two‐layer constituents, especially at higher temperatures, which resulted either in amorphization or in crystallization after annealing at temperatures above 400 °C. Copyright © 2015 John Wiley & Sons, Ltd.

The main objective of this study is to hydrophobize nitinol (Ni-Ti alloy) for cardiovascular applications. For this purpose, medical nitinol samples were subjected to sodium hydroxide hydrothermal treatments at various temperatures, followed by hexadecyltrimethoxysilane (HDTMS) functionalization. Then, the structure, wett-ability, corrosion, cytocompatibility and cell adhesion of the prepared samples were evaluated. According to the results, porous blade-shaped layers of sodium titanate were formed on the substrate surface as a result of the alkaline treatment. These nano-rough features offered considerable hydrophobicity after HDTMS processing, where a maximum water contact angle of about 140°was obtained for the sample treated at 120°C, followed by the HDTMS coating. In contrast to the individual application of the alkaline treatments, the subsequent HDTMS processing improved corrosion resistance in the simulated body fluid. Although all the samples presented appropriate cytocompatibility with respect to human umbilical vein endothelial cells, the cells did not show an adhesion tendency to the hydrophobic surfaces. It is concluded that alkaline hydrothermal and HDTMS processed nitinol can be considered for cardiovascular applications demanding hydrophobic surfaces.

NiTi thin films are fabricated using biased target ion beam deposition technique. By design, the technique operates over a broad range of processing pressures; enables control of adatom energies; facilitates low energy bombardment; and promotes uniformity and repeatability. Thus, the technique is advantageous for preparing smooth and dense ultrathin films. Typically NiTi shape memory alloy thin films are deposited using the magnetron-sputtering technique and alloy targets. In this work films are co-sputtered from pure Ti and pure Ni targets and the technique is contrast with magnetron co-sputtering. Approximately 100 nm thick NiTi thin films are prepared with Ni-rich (> 50.5 at.% Ni), near equiatomic, and Ti-rich (< 49.5 at.% Ni) compositions. Atomic force microscopy reveals that films are consistently ultra-smooth over the broad range of compositions. The current findings confirm that biased target ion beam deposition can facilitate the preparation of high quality ultrathin NiTi films. After heat-treatment, the films deposited exhibit B2 and B19′ crystal structures and thus possess potential for martensitic phase transformation, which is the prerequisite for functional shape memory behavior.

The hot compression tests in a wide range of temperature were employed to produce the semi-finished products of equiatomic NiTi shape memory alloy holding enhanced mechanical properties. The precise analyses of the compensated flow curves and the microstructures indicated the occurrence of dynamic recovery at lower temperatures and dynamic recrystallization at higher temperature regime. In addition a significant influence on the critical transformation temperatures was also realized. This could affect the martensite plates' characteristics and the fraction of retained austenite phase. In consequence the room temperature mechanical properties and shape memory response of the alloy were influenced. In an elaborated optimized TMP scheme, the recovery ratio was increased from %30% to %80%.

This work illustrates the design, manufacturing and tensile testing of a novel concept of honeycomb structure made of shape memory alloy (SMA) core material. The honeycomb is manufactured using SMA Nitinol ribbons inserted in a special dye and using cyanoacrilate to bond the longitudinal strips of the unit cells. Analytical and Numerical FE models of the ribbons are developed to predict the homogenized properties and the overall tensile test behavior of the honeycomb sample. Good agreement is observed between numerical nonlinear simulations and experimental results carried out at room temperature (martensite phase).

In this study, effect of heat treatment parameters, which is applied in order to achieve superelasticity and shape memory, on the nitinol microstructure was investigated. Nitinol is a commonly used material in cardiovascular and orthopedic implant production. Nitinol material used in this study has 50.59% Ni, 0.12% C, 0.0005% Cr, 0.003% Cu, 0.001% Co, 0.012% Fe, 0.1% O, <0.26% H andbalanceTi chemical composition (in at.%).Nitinol wires were heat treated at 540 °C, 550 °C, 560 °C and 570 °C for 10 minutes. Then, heat treatment was applied at 550 °C for 8, 9, 10, 11, 12, 13 and 14 minutes. Heat treated nitinol wires were characterized by optical microscope, Energy-dispersive X-ray Spectroscopy (EDS) and mechanical tests.Results show that Ni 4 Ti 3 phase was formed as a result of heat treatment. Heat treatment time and temperature affected amount, distribution and shapes of NiTi and Ni 4 Ti 3 phases. Among heat treatment conditions, suitable microstructures were obtained at 540 °C and 550 °C heat treatment temperature and heat treatment times not exceeding 10 minutes. This finding was verified with mechanical tests.

This paper presents a review of the research on the fatigue performance of superelastic polycrystalline nanocrystalline NiTi shape memory alloys (nc NiTi SMAs). A brief introduction to some focal definitions and basic concepts of fatigue measurement and response in NiTi SMAs were given. Results found in the literature on fatigue in nc NiTi SMAs are discussed. Mechanical behavior and energy dissipation capacity of nc NiTi SMAs are explored and collectively compared with coarse-grained NiTi SMAs, along with an assessment of the influence of grain size refinement and thermomechanical treatments. Several conclusions and suggestions are made, including that nc NiTi SMAs exhibit larger functional fatigue resistance, lower crack tolerance, higher superelasticity, and smaller hysteresis loss.

Many cardiac surgeons are becoming increasingly familiar with the advantages and application of nitinol sternal thermo-reactive clip (Flexigrip; Praesidia, Bologna, Italy) in sternotomy closure. However, we will be describing an alternative technique of flexigrip clip application that is even easier to perform, with additional advantages of a more precise measurement of sternal width and allowed identification of sternal midline before sternotomy.