Fracture of Polymer-Coated Nitinol During Gamma Sterilization

After gamma sterilization of a packaged medical device, fractures were discovered in the superelastic nitinol wire used as part of the assembly. The nitinol wire was encased in fluorinated ethylene propylene (FEP) shrink tube. The only fractures occurred where the encased wire was held under strain during gamma sterilization. A study was conducted to determine the susceptibility of nitinol to this type of failure. The variables studied included wire diameter, wire surface finish, wire oxide layer, quantity of wires encased, type of tubing, and strain level during gamma sterilization. The greatest susceptibility to fracture occurred to single wire samples with a light oxide layer held under high strain in FEP shrink tube. Gamma sterilization experiments were conducted to isolate and confirm this failure mechanism. Scanning electron microscopy was used to analyze the fractured samples. Chemical analysis was performed in an attempt to detect trace elements to determine the root cause of the failures. Stress corrosion cracking caused by the liberation of fluorine due to the degradation of the polymer during gamma sterilization is suspected.     

Static and Cyclic Load-Deflection Characteristics of NiTi Orthodontic Archwires Using Modified Bending Tests

Near-equiatomic nickel-titanium (nitinol) has the ability to return to a former shape when subjected to an appropriate thermomechanical procedure. One of the most successful applications of nitinol is orthodontic archwire. One of the suitable characteristics of these wires is superelasticity, a phenomenon that allows better-tolerated loading conditions during clinical therapy. Superelastic nitinol wires deliver clinically desired light continuous force enabling effective tooth movement with minimal damage for periodontal tissues. In this research, a special three-point bending fixture was invented and designed to determine the superelastic property in simulated clinical conditions, where the wire samples were held in the fixture similar to an oral cavity. In this experimental study, the load-deflection characteristics of superelastic NiTi commercial wires were studied through three-point bending test. The superelastic behavior was investigated by focusing on bending time, temperature, and number of cycles which affects the energy dissipating capacity. Experimental results show that the NiTi archwires are well suited for cyclic load-unload dental applications. Results show reduction in superelastic property for used archwires after long-time static bending.     

Preparation and Characterization of Nitinol Bone Staples for Cranio-Maxillofacial Surgery

The aim of this work was to form NiTi and TiNiCo body temperature activated and superelastic staples for clinical joining of mandible and face bone fractures. The alloys were obtained by VIM technique. Hot and cold processing was applied to obtain wires of required diameters. The martensitic transformation was studied by DSC, XRD, and TEM. The shape memory effects were measured by a bend and free recovery ASTM F2082-06 test. The superelasticity was recorded in the tension stress-strain and by the three-point bending cycles in an instrument equipped with a Hottinger force transducer and LVDT. Excellent superelastic behavior of TiNiCo wires was obtained after cold working and annealing at 400-500 °C. The body temperature activated shape memory staples were applied for fixation of mandibular condyle fractures. In experiments on the skull models, fixation of the facial fractures by using shape memory and superelastic staples were compared. The superelastic staples were used in osteosynthesis of zygomatico-maxillo-orbital fractures.     

Acceptance Criteria for Corrosion Resistance of Medical Devices: Statistical Analysis of Nitinol Pitting in In Vivo Environments

ASTM F 2129 test method nor the FDA provides any guidance as to what constitutes an acceptance criterion for the corrosion resistance of implantable medical devices. Neither provide any guidance on how many samples to test or how to handle censored data, i.e. datasets where there are only a few tests that breakdown. The development of both a statistically valid acceptance criterion for corrosion resistance and a method of evaluation would be of significant benefit to the medical device community. This study of 420 nitinol cyclic polarization tests, which builds on previous research that was presented at SMST 2007, investigates the effect of long-term exposure to simulated in vivo environments with differing degrees of aeration. This was accomplished by pre-exposing electropolished (EP) nitinol to phosphate buffered saline (PBS) at 37 °C that had been sparged with either ultra high purity nitrogen or laboratory air. Immersion times ranged from 1 h up to 30 days. A total of 290 EP samples were tested in order to obtain a reasonable number of samples with breakdown, i.e. pitted. In addition, a total of 130 mechanical polished (MP) samples were also analyzed. This data allow us to test our statistical model that was presented at SMST 2007. This model takes into account the probability of breakdown per unit of exposed surface area and, if breakdown occurs, predicts the probability that E _b ? E _r is greater than some threshold value. Aerated PBS environments were found to have a large influence on the margin of safety against pitting in vivo. Statistical methods for treating highly right censored pitting data are presented.     

Enhanced Strength in Cu-Ag-Zr Alloy by Combination of Cold Working and Aging

In this investigation, the effect of different degree of cold rolling and post-aging treatment on the microstructure and mechanical properties of a Cu-3wt.%Ag-0.5wt.%Zr alloy was studied by means of hardness measurement, tensile tests, optical and electron microscopy. The alloy was subjected to cold rolling up to 80% followed by aging in the temperature range of 400-500 °C. The yield strength, ultimate tensile strength and hardness were found to increase as degree of cold rolling increased, but at the expense of ductility. Aging of cold rolled samples in the studied temperature range has resulted in different combinations of strength and ductility. However, aging of cold rolled samples at 400 °C for 1 h has resulted in a combination of high strength and moderate ductility. A yield strength and ultimate tensile strength of 511 and 560 MPa, respectively with a ductility of 12% were achieved for 80% cold rolled and aged (400 °C for 1 h) sample. The high strength achieved after 80% cold rolling and aging is mainly attributed to precipitation of fine silver precipitates.     

Dynamic Embrittlement in Cu-Cr-Zr-Ti Alloy: Evidence of Intergranular Segregation of Sulphur

In the present investigation, Cu-0.6Cr-0.005Zr-0.0045Ti alloy was subjected to different heat treatment and thermomechanical treatment (TMT) to simulate the conditions experienced during brazing and forming, respectively. Grain coarsening was observed in the samples subjected to heat treatment, and grain refinement was observed in the samples subjected to TMT. Tensile tests conducted with these samples at room temperature and 600 °C have shown that Cu-Cr-Zr-Ti alloy was susceptible to dynamic embrittlement (DE). However, the observation was limited to coarse-grained samples (280-350 μm) at 600 °C. On the other hand, the fine-grained samples (20-40 μm) showed good ductility. Electron microscopy studies conducted on the tensile-tested specimens prone to DE indicated the presence of sulfur on the fractured surface and intergranular segregation of sulfur. Therefore, it can be inferred from the results that DE due to sulfur can occur in Cu-Cr-Zr-Ti alloy at elevated temperature for coarse-grained samples.     

Transformational behaviour of constrained shape memory alloys

The transformational behaviour of shape memory alloy (SMA) wires embedded into a fibre reinforced epoxy composite was investigated and is discussed in this article. The effects on the transformational temperatures, and heats of the embedded SMA wires and the generation of recovery stresses within the composites on heating are shown to be related to the reversible martensitic transformation of the SMA wires. This article details the effects of the constraining matrix on the transformations of self-accommodating and preferentially oriented martensite. It was found that there is little change in the transformation temperatures of the constrained SMA wires with increasing pre-strain, but that the measurable transformation heats decrease significantly with increasing pre-strain. It is concluded that the transformation of self-accommodating martensite is nearly not affected by the constrained matrix, whereas the transformation of the preferentially oriented martensite is suppressed.     

The Mechanical and Thermal Behaviors of Heat-Treated Ni-Rich NiTi Orthodontic Archwires

The near equiatomic nickel-titanium alloy is an outstanding intermetallic compound exhibiting distinctive properties associated with characteristic thermal and stress-induced martensitic transformations. The process of producing orthodontic wires has been modified to obtain the optimal shape memory behaviors. Phase transformation temperatures and load-deflection characteristics of this binary alloy are very significant variables in the performance of this alloy and can be manipulated by different thermomechanical treatments via inducing precipitation or dislocation networks in the matrix. In this study, one brand of commercial heat-activated nickel-titanium archwire (3 M Unitek) was selected and solution treated. Then, the wires annealed at 400 °C for 10, 30, and 60 min. Thermal transformation temperatures were determined using differential scanning calorimeter. It was showed that these temperatures increased with increasing the time of heat treatment and multistage transformation occurred as the result of inhomogeneities. In order to evaluate mechanical parameters of heat-treated archwires, they were placed on an arch-form fixture simulating maxillary dentition and load-deflection curves were obtained by three-point bending test at 37 °C. The results compared to as-received archwires and the best superelasticity was observed after 30 min aging.     

Structure and Properties of Ti-19.7Nb-5.8Ta Shape Memory Alloy Subjected to Thermomechanical Processing Including Aging

In this work, the ternary Ti-19.7Nb-5.8Ta (at.%) alloy for biomedical applications was studied. The ingot was manufactured by vacuum arc melting with a consumable electrode and then subjected to hot forging. Specimens were cut from the ingot and processed by cold rolling with e = 0.37 of logarithmic thickness reduction and post-deformation annealing (PDA) between 400 and 750 °C (1 h). Selected samples were subjected to aging at 300 °C (10 min to 3 h). The influence of the thermomechanical processing on the alloy’s structure, phase composition, and mechanical and functional properties was studied. It was shown that thermomechanical processing leads to the formation of a nanosubgrained structure (polygonized with subgrains below 100 nm) in the 500-600 °C PDA range, which transforms to a recrystallized structure of β-phase when PDA temperature increases. Simultaneously, the phase composition and the β → α″ transformation kinetics vary. It was found that after conventional cold rolling and PDA, Ti-Nb-Ta alloy manifests superelastic and shape memory behaviors. During aging at 300 °C (1 h), an important quantity of randomly scattered equiaxed ω-precipitates forms, which results in improved superelastic cyclic properties. On the other hand, aging at 300 °C (3 h) changes the ω-precipitates’ particle morphology from equiaxed to elongated and leads to their coarsening, which negatively affects the superelastic and shape memory functional properties of Ti-Nb-Ta alloy.     

The Effect of Exposure to Simulated Body Fluids on Breakdown Potentials

Current methods to evaluate the corrosion resistance of small medical implants are typically based on short-term in vitro tests. While the duration of these tests is kept to a minimum to make it feasible to evaluate a large number of samples in a reasonable time, these methods do not account for the long-term changes that can occur in the oxides of metals exposed to biological fluids. Given other electrochemical changes to these materials with time in solution, it is a reasonable question to consider whether breakdown potentials are a fundamental parameter of a material and surface condition. Data on 316L and nitinol wire samples, and nitinol stents, show that breakdown potentials increase with time in solution up to 28 days. The difference between the breakdown potential and rest potentials either increased or exhibited no change.     

Microstructure and Phase Composition of Ni-Based Alloy Obtained by High-Speed Direct Laser Deposition

Phase composition and structural features of nickel-based alloy EP741 obtained by high-speed direct laser deposition were discussed in the paper. The technology of high-speed direct laser deposition has been successfully applied during the formation of samples with the various operating parameters of the experimental device. The laser power was between 450 and 1200 W, the scanning speed was 1.2 mm/s, the powder feed rate was 45 g/min, and the laser beam diameter was 1.2 mm. The structure and phase composition of the initial material and as-deposited samples were studied using optical and scanning electron microscopy, x-ray analysis and transmission electron microscopy. The investigation has shown that spherical particles of powder (EP741) can be used to form products by additive manufacturing with the presence of additional heat treatment, since almost all the as-deposited samples obtained do not contain cracks, and large volume of pores was observed only in the sample obtained with the power of 450 W. In addition, the phase composition of the as-deposited samples showed a high content of precipitating phase Ni₃(Al, Ti) in matrix, which is coherent to the solid solution based on nickel. In theory, the presence of Ni₃(Al, Ti) phase corresponds to the heat-treated nickel-base alloy obtained by standard methods of processing.     

Fabrication of Entangled Tough Titanium Wires Materials and Influence on Three-Dimensional Structure and Properties

Pure titanium (Ti) TA1 fibers/wires with 0.08 and 0.15 mm diameters were processed by a novel method that combined press forming, vacuum sintering (≥10^?2 Pa), and heat treatment to fabricate entangled Ti wire materials (ETWMs). The ETWMs exhibited a total porosity ranging from 44.2 ± 0.1 to 81.2 ± 0.1% and an open porosity ranging from 43.5 ± 0.1 to 80.9 ± 0.1%. The processing parameters of fiber diameter, formation pressure, sintering temperature, and sintering time were applied to examine porous ETWM morphology, porosity, pore size, and mechanical properties. The importance of primary factors controlling porous structure and porosity in ETWMs were found to be fiber/wire diameter > formation pressure > sintering temperature > sintering time. Furthermore, Ti fiber diameter was shown to directly impact pore size. High formation pressure resulted in a fine, uniform porous structure with low porosity. Sintering at high temperature for long-time periods promoted sintering point formation, resulting in neck coarsening. This effect contributed to the characteristic mechanical properties observed in these ETWMs. If the sintering effect is considered in isolation, ETWMs fabricated with 0.08 mm diameter Ti fibers/wires and sintered at 1300 °C for 90 min achieved smaller, more uniform porous structures that further exhibited improved connections among fibers/wires and excellent mechanical properties.     

Pseudoelastic Nitinol-Based Device for Relaxation of Spastic Elbow in Stroke Patients

A compliant brace (EDGES) promoting spastic elbow relaxation was designed to investigate the potentialities of pseudoelastic NiTi in orthotics. By exploiting its peculiar characteristics, EDGES could improve elbow posture without constraining movements and thus avoiding any pain to the patient. A commercial Ni_50.7-Ti_49.3 alloy heat treated at 400 °C 1 h + WQ was selected for this application. A prototype of EDGES was assembled with two thermoplastic shells connected by polycentric hinges. Four 2-mm-diameter NiTi bars were encastred in the upper-arm shell and let slide along tubular fixtures on the forearm. Specially designed bending tests demonstrated suitable moment-angle characteristics. Two post-stroke subjects (aged 62 and 64, mild elbow flexors spasticity) wore EDGES for 1 week, at least 10 h a day. No additional treatment was applied during this period or the following week. A great improvement (20° ± 5°) of the resting position was observed in both patients as early as 3 h after starting the treatment. Acceptability was very good. A slight decrease in spasticity was also observed in both subjects. All the effects disappeared 1 week after discontinuation. EDGES appears to be a good alternative to traditional orthoses in terms of acceptability and effectiveness in improving posture, especially whenever short-term splinting is planned.     

Influence of Post-Weld Annealing on Transformation Behavior and Mechanical Properties of Laser-Welded NiTi Alloy Wires

The influence of annealing on the transformation behavior, mechanical, and functional properties of laser-welded NiTi wires was investigated. The results show that Ti₃Ni₄ precipitates occur after post-weld annealing and coarsen with increasing annealing temperature. The as-welded specimen exhibits one-step B2 → B19′ transformation, while the annealed ones show two-step B2 → R → B19′ transformation. Annealing at 400 °C for 1 h can improve the tensile strength and superelasticity of the welded joints. However, these properties decrease when annealing at 500 °C for 1 h. The change of mechanical and functional properties after annealing is attributed to the different size of Ti₃Ni₄ precipitates. Annealing to produce smaller coherent precipitates (10 nm) improves the mechanical and functional properties of the welded joints. As the Ti₃Ni₄ precipitates coarsen, the mechanical and functional properties decrease.     

Analysis of microstructure and wear strength of hard facing used by the sugar and alcohol industry

The sugar and alcohol sector has seen huge growth in recent years in Brazil; however, maintenance in this industry comes at a high cost due to loss of metal from the equipment via wear mechanisms. The objective of this work is to evaluate the abrasive wear resistance and the microstructure of hard claddings deposited in a single layer. Four types of consumables are used in the sugar and alcohol industry: a clad electrode of FeCrC alloy, 4.0 mm of diameter and three self-shielded tubular wires, all 1.6 mm in diameter, of FeCrC, FeCrCNb and FeCrCTiMo alloys. The base metal used was SAE1020 steel. The welds with the tubular wires were produced in short circuit transfer mode, with the same welding current and voltage values. For the wear test a rubber wheel was used, in accordance with ASTM standard G65-91. The wear test pieces were removed from the central region of the test plates, and from the same region two groups of samples were removed for microstructural analysis (under an optical microscope). The results of the tests with the rubber wheel showed that the FeCrCNb alloy had the highest wear resistance, followed by the clad electrode, and the FeCrCMoTi alloy and FeCrC alloy had the worst performance. The FeCrC alloy (for both the clad electrode and for the tubular wire) showed a microstructure formed by M7C3 primary carbonates distributed in a matrix of lower hardness; the alloy containing Nb showed a similar microstructure as well as the presence of NbC carbonates; in turn, the alloy with Ti and Mo added revealed the presence of large primary titanium carbonates.     

KCl-Induced Corrosion of a 304-type Austenitic Stainless Steel in O2 and in O2 + H2O Environment: The Influence of Temperature

The oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated in the temperature range 400–600 °C in 5% O₂ and 5% O₂ + 40% H₂O. Exposure time was up to 1 week. Prior to exposure, the polished samples were coated with 0.1 mg/cm² KCl. Uncoated samples were also exposed and used as references. The oxidized samples were analyzed by gravimetry and by ESEM/EDX, XRD, IC and AES. The results show that KCl is strongly corrosive. Corrosion is initiated by the reaction of KCl with the chromia-containing oxide that normally forms a protective layer on the alloy. This reaction produces potassium chromate particles, leaving a chromium-depleted oxide on the alloy surface. At 500 and 600 °C this results in rapid oxidation, resulting in the formation of a thick scale consisting of a mixture of hematite, spinel oxide ((Fe,Cr,Ni)₃O₄) and K₂CrO₄. The thick scale is poorly protective and permeable to e.g. chloride ions. The KCl-induced corrosion of alloy 304L in dry O₂ and in an O₂ + H₂O mixture increases strongly with temperature in the range 400–600 °C. The strong temperature dependence is explained partly by the temperature dependence of the chromate-formation reaction and partly by the ability of the chromium-depleted oxide to protect the alloy at low temperature. At 400 °C, the oxide was still protective after 168 h.     

Swan-like Memory Compressive Connector

Nonunion is a common complication after fractures of the diaphysis of the upper extremity. Conventional internal fixation cannot provide compressive stress at the fracture site, which is critical for fracture repair in nonweight-bearing bones. In order to overcome this problem, we developed a novel nitinol device that provides initial and continuous compression and three dimensional fixation, the swan-like memory compressive connector (SMC). A total of 188 cases (243 bones) of fractures and nonunions were treated by SMC over the course of 16 years. At follow-up, the nonunion sites were bridged by plate-like bone in 92 cases (106 bones) at an average of 3.8 months after surgery. In the fracture group, the fracture sites were bridged by plate-like bone in 93 cases (134 bones) at an average of 2.6 months after surgery. No infection or re-fracture occurred after removal of the SMC. There was no persistent joint dysfunction caused by the SMC.     

Evolution of Microstructure and Texture in AZ31 Alloy Subjected to Gas Forming

In this study, AZ31 sheets were subjected to gas forming using a stepped geometry die and employing two different temperatures (400°C and 450°C) and exposure times (60 s and 120 s). Microstructural examinations revealed that the density of the mechanical twins may be taken as indicative of the progress of recrystallization together with the relative grain size. Although gas forming was successfully applied, the severe residual texture inherited from the initial material was found to persist and was little affected by the operating conditions and the multiaxial stress state of the process.     

Microstructure and Properties of FeCrB Alloy Coatings Prepared by Wire-Arc Spraying

To improve the heat transfer ability and wear resistance of drying cylinders in paper production machines, a series of Fe_87?x Cr₁₃B _x (x = 1 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, and 4 wt.%) cored wires have been produced and used to prepare coatings by wire-arc spraying, in comparison with conventional X30Cr13 solid wire. All coatings presented dense layered structure with porosity of around 4%. The boron content in the cored wires significantly affected the thermal conductivity of the coating, which is attributed to the combined effects of the crystal structure, grain size, and oxide content of the coating. In the investigated range, the coating with 2 wt.% boron content exhibited the highest thermal conductivity, reaching 8.83 W/m-K, greater than that of X30Cr13 coating (5.45 W/m-K). Furthermore, the microhardness and relative wear resistance of the FeCrB coatings obtained from cored wires with boron addition were greatly increased compared with commercial X30Cr13 coating. Therefore, wire-arc-sprayed FeCrB coating has promise as an effective and economic approach to improve the heat transfer behavior and wear resistance of drying cylinders in the paper industry.     

Shot-Peening of Pre-Oxidized Plates of Zirconium: Influence of Residual Stress on Oxidation

The present study deals with oxidation behavior under residual stress of shot-peened plates of “commercially pure” zirconium exposed for 30 min at 650 °C. The influence of the shot-peening on a pre-oxidized material is presented. The results have been used to determine the influences of these chemical (pre-oxidation) and mechanical (shot-peening) treatments on the high temperature oxidation of zirconium. The oxygen profile was revealed using micro-hardness techniques and confirmed by SEM–EDS techniques. After pre-oxidation the samples were first resurfaced then shot-peened in order to introduce residual stress. A significant increase of the hardness of about +400 HV was observed on pre-oxidized shot-peened samples. Thermogravimetric analysis for 30 min at 650 °C under 200 mbar O₂ showed a significantly slower oxidation rate for shot-peened samples. A comparison with our computations points out the role of the residual stresses on the diffusion and, consequently, on the oxidation.