Improved mechanical properties of porous nitinol by aluminum alloying

Aluminum alloying effects (up to 2 at %) on the macrostructure, microstructure, and mechanical properties of porous nitinol (NiTi) obtained by self-propagating high-temperature synthesis (SHS) were studied. It has been established that Ni and Ti interactions with liquid Al (0.5–1 at % Al) in the SHS process significantly change macrostructure, decrease the size of the interpore bridges, and increase their number, resulting in a larger effective cross-sectional area. An increase in the aluminum content above 1 at % leads to larger interpore bridges in the SHS product. The microhardness of TiNi(Al) increases from 305 HV50 g to 422 HV50 g with aluminum concentration, while the fraction of the TiNi(Al) (B2 + B19′) phases decreases from 75% to 50%. The Ti2Ni(Al) phase fraction increases from 25% to 50% with Al concentration. The 64 MPa tensile strength and 2.9% fracture strain of porous Ti50Ni49Al1 alloy are higher than without Al. The increase in strength is due to the formation of a more homogeneous macrostructure and solid solution strengthening of the alloy-forming phases

Biocompatibility assessment of coatings obtained in argon and nitrogen atmospheres for TiNi materials

This work aims to study the cytocompatibility of protective coatings obtained in argon and nitrogen atmospheres on a TiNi surface. Particular attention is paid to comparing the interaction of cell culture with coatings and an uncoated TiNi sample, using for comparison the number of viable cells on the surface, the phase composition, structure, wettability, surface charge and topography. The Ti/Ni/Ti nanolaminate was deposited on a TiNi substrate by magnetron sputtering. Reaction annealing of Ti/Ni/Ti nanolaminate on a TiNi substrate, when heated to 900 ◦C in argon, leads to the formation of a dense two-layer coating 2.0–2.1 µm thick: layer I (TiO + Ti2N), layer II (Ti4Ni2 O(N)). Reaction annealing in nitrogen leads to the formation of a thin three-layer nanocoating 250 nm thick: I (TiO2 + TiN), II (Ti4Ni2N(O) + Ti3Ni4), III (TiN). The coating synthesized in nitrogen is more favorable for cell attachment and proliferation because of the moderately hydrophilic rough surface and mixed phase composition of titanium nitrides and oxides

Hyperelastic behavior of knitted TiNi mesh under uniaxial tension

TiNi-based alloys belong to the class of materials with shape memory effects and superelasticity, which are currently being actively studied and successfully used in engineering and medicine. In these alloys, their natural ability to undergo large inelastic deformations and return to their original shape by increasing temperature or relieving stress takes place. The key characteristic of these phenomena is thermoelastic martensitic transformations (MT). The problem of biocompatibility of implants is very relevant, as the number of operations using implants in various fields of medicine is growing rapidly. Currently, a large number of studies are underway on the deformation behavior of biological tissues and various implant materials. Wires made of TiNi are one of the most important metal biomedical materials used in endovascular surgery, orthodontics, soft tissue plastics in the form of stents, catheters, orthodontic archwires, metal-knitted materials [1-3]. Textile implants should be singled out from a wide range of structures made of thin TiNi wire, with the help of which complex surgical problems are solved. A variety of mesh structures made of titanium nickelide are characterized by a particular complexity of deformation characteristics, the manifestation of which in the implant-bio-tissue interface is difficult to predict. To create the appropriate mechanical behaviour of an implant in the form of mesh structures, it is necessary to study their deformation behaviour. Therefore, to describe the functioning of a superelastic implant in the interface with a biological tissue, the aim of this work is to study the deformation behavior of wire samples 40, 60, and 90 µm thick from the TiNi alloy and metal knit made from them by the method of uniaxial tension

Anisotropy of elastic properties of SHS-synthesized porous titanium nickelide

Samples of porous NiTi were obtained by the method of self-propagating high-temperature synthesis. The mechanical characteristics of the porous ones were studied by quasi-static compression. When the samples of porous titanium nickelide were subject to quasi-static compression, the deformation was of an elastic-plastic nature. Three characteristic types of the fracture surface under quasi-static compression of the porous SHS – TiNi alloy were identified: 1) ductile fracture of the austenite phase in the form of a cup relief, 2) brittle fracture accompanied by the formation of cleavage steps, 3) large areas of plastic shear deformation, on which cups and cleavage facets were nucleated. To determine the anisotropy of the porous TiNi alloy properties, the volume of the porous sample was simulated, and estimated calculations were carried out. Based on the results of reconstructing 3D neutron high resolution tomography of the porous volume of titanium nickelide and the numerical parameters of the model porous medium, an algorithm was developed for obtaining a solid-state 3D model of the porous framework for using in finite element calculations. The studied porous titanium nickelide alloy, as well as spongy bone tissues, was shown to have orthotropic elastic properties conditioned by the geometric features of the porous framework. The effective moduli of elasticity and shear for the porous volume of the material were determined. The calculation results of the elastic moduli for the studied model of porous titanium nickelide numerically agree with the results obtained by compressing the samples of porous TiNi. The porous TiNi alloy under uniaxial compression was established to be destroyed under the action of tangential shear stresses at an angle of 45 degrees to the direction of uniaxial compression

Structure, biocompatibility and corrosion resistance of the ceramic-metal surface of porous nitinol

One of the key aspects of the biochemical compatibility of medical alloys is the surface corrosion resistance in living organisms. This study discusses the structure of the ceramic-metal surface layer of a porous nickel-titanium alloy (nitinol) and the corrosion resistance in simulated physiological liquids. The structure of the protective layer and glass-ceramic non-metallic inclusions in the surface of the porous alloy have been studied. The formation of the surface ceramic-metal layer and crystallization of various glass-metal-ceramic phases as a result of chemisorption from reaction gases and epitaxial growth from the gas phase during the self-propagating high-temperature synthesis are observed

Reaction synthesis of gradient coatings by annealing of three-layer Ti–Ni–Ti nanolaminate magnetron sputtered on the TiNi substrate

In this work, Ti-Ni-Ti nanolaminate was deposited on a TiNi substrate by magnetron sputtering. Subsequent synthesis in Ar at temperatures of 500, 700 and 900 ◦C was carried out. To investigate the effect of the reaction synthesis temperature for obtaining a dense thin intermetallic biofunctional coating, a comparative analysis of the morphology, structure, phase composition, mechanical properties and wettability of the coatings was carried out. It was found that crystallization processes have a different impact on the amorphous Ti and Ni layers depending on temperature. In all cases, multilayer gradient crystalline coatings were formed that bound to the substrate by the diffusion zone. The reaction synthesis temperatures of 500 and 700 ◦C are insufficient for reaction completion and complete transformation; therefore, the obtained thin loose layers are unstable under stresses caused by the substrate. Reaction synthesis in Ti – Ni – Ti nanolaminate at 900 ◦С formed a dense two- layer gradient coating 2 μm thick, which reliably protects the matrix from further diffusion of interstitial impurities and segregation of Ni atoms onto the surface. An analysis of the nano-indentation deformation diagrams showed that the coatings do not prevent the viscoelastic deformation of the TiNi substrate

Comparative study on the high-temperature oxidation resistance of porous and solid TiNi-based alloys

The present work aims to characterize the surface features of solid and porous (sintered and SHS) TiNi-based alloys subjected to oxidation at 1000 °Cin static air in the context of their resistance to high-temperature atmospheric attack. Clear differences between the intact and oxidated surfaces indicate the complexity of a chemicothermal diffusion process evolving therein. Microscopic and XRDstudies showed that the dominant superficial constituent in all oxidated samples is titanium dioxide in the rutile modification. The phase and structural properties of the surface layers suggest that porous sintered and solid alloys are most susceptible to high-temperature corrosion due to bare reactive surfaces, which negatively affects their overall biocompatibility. Surface morphology analysis revealed microporous and loose superficial layers having a thickness of 8–10 and 50–60 μm, respectively in the solid and sintered alloy. Also, these alloys showed a high content of leaching NiO and free Ni within the surface layer. Conversely, a thin (0.5–0.6 μm), dense, and multifarious layer of oxycarbonitrides Ti4Ni2(O,N,C) concealing the porous SHS-TiNi matrix inhibits the negative effect of high-temperature oxidation

Microstructural characterization, wettability and cytocompatibility of gradient coatings synthesized by gas nitriding of three-layer Ti/Ni/Ti nanolaminates magnetron sputtered on the TiNi substrate

In the study, gradient coatings with a nanometer thickness were successfully synthesized from Ti/Ni/Ti nanolaminates on the TiNi substrate in a nitrogen atmosphere at 900 °C. A technique for producing a gradient coating includes magnetron sputtering of Ti/Ni/Ti nanolayers with a total thickness of 75 nm and 150 nm on a TiNi substrate and further synthesis of the coating by annealing the sample in a nitrogen atmosphere. The developed technique allows to form stable phases of titanium nitride and intermetallic oxynitrides, which are a reliable barrier to nickel diffusion from the substrate to the surface. No nickel was found on the surface of the synthesized coatings. In the synthesized coating from 150 nm thick nanolaminate, these barriers are Ti4Ni2(N,O) and TiN phases. In the synthesized coating from 75 nm thick nanolaminate, these barriers are two continuous TiN layers. The cytocompatibility of the coating synthesized from Ti/Ni/Ti nanolaminates with a total thickness of 150 nm was positively affected by a mixed-phase composition of titanium nitrides and oxides, moderate roughness and hydrophilicity

Structure and phase composition of a coating synthesized from Ti-Nni-Ti laminate on TiNi substrate

A gradient magnetron-sputtered three-layer laminated Ti–Ni–Ti coating is formed by the method of reactio

Phase formation during air annealing of Ti-Ni-Ti laminate

The gradient coating synthesized on a nickelide titanium sample by sputtering a three-layer Ti-Ni-Ti laminat