This article presents the results of studies of the features of the development of thermoelastic martensitic transformations during cooling/heating in the free state and under load of Ti
Ni
Nb
Mo
...alloys (X = 0.5, 1.0 and 1.5 at% Nb) with shape memory effects. Using X-ray diffraction analysis, it was found that all the alloys studied at room temperature contained a multiphase mixture consisting of intermetallic compounds with the TiNi (B2, B19'), Ni
Ti
Nb
, and Ti
Ni compositions. Scanning electron microscopy was used to study the microstructure of TiNi (Nb,Mo) alloys and it was found that the distribution of fine Ni
Ti
Nb
particles in the matrix depends significantly on the concentration of the alloying element. A correlation was established between changes in the structural-phase state in TiNi (Nb,Mo) alloys and the occurrence of the B2↔B19' martensitic transition in the free state and under load. Based on physical and mechanical studies, the temperature ranges of the martensitic transformations (MT) in the free state and under load were established. Based on the thermodynamic description of the MT and the analysis of the characteristic temperatures of the MT, it was found that the MT mechanism is strongly dependent on the concentration of the alloying element.
Coatings based on calcium phosphate with thicknesses of 0.5 and 2 μm were obtained by high-frequency magnetron sputtering on NiTi substrates in an argon atmosphere. The coating was characterized ...using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and in vitro cytocompatibility and bioactivity studies. A biphasic coating of tricalcium phosphate (Ca
(PO
)
) and hydroxyapatite (Ca
(PO
)
(OH)
) with a 100% degree of crystallinity was formed on the surface. The layer enriched in calcium, phosphorus, and oxygen was observed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Scanning electron microscopy showed that the surface structure is homogeneous without visible defects. The 2 µm thick coating obtained by sputtering with a deposition time of 4 h and a deposition rate of 0.43 µm/h is uniform, contains the highest amount of the calcium phosphate phase, and is most suitable for the faster growth of cells and accelerated formation of apatite layers. Samples with calcium phosphate coatings do not cause hemolysis and have a low cytotoxicity index. The results of immersion in a solution simulating body fluid show that NiTi with the biphasic coating promotes apatite growth, which is beneficial for biological activity.
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.
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•Porous NiTi with 0–2 % Al was obtained by self-propagating high-temperature synthesis.•Al doping changed macrostructure and decreased interpore bridge size.•The TiNi(Al) phases fraction decreased from 75 % to 50 % with more Al.•Vickers hardness increased with Al concentration.•The optimal Al content is 0.5–1 % for improved structure and mechanical properties.
Samples of skin, tendons, muscles, and knitwear composed of NiTi wire are studied by uniaxial cyclic tension and stretching to rupture. The metal knitted mesh behaves similar to a superelastic ...material when stretched, similar to soft biological tissues. The superelasticity effect was found in NiTi wire, but not in the mesh composed of it. A softening effect similar to biological tissues is observed during the cyclic stretching of the mesh. The mechanical behavior of the NiTi mesh is similar to the biomechanical behavior of biological tissues. The discovered superelastic effects allow developing criteria for the selection and evaluation of mesh materials composed of titanium nickelide for soft tissue reconstructive surgery.
In order to reduce infections, porous NiTi alloys with 62% porosity were obtained by self-propagating high-temperature synthesis with the addition of 0.2 and 0.5 at. % silver nanoparticles. Silver ...significantly improved the alloys’ antibacterial activity without compromising cytocompatibility. An alloy with 0.5 at. % Ag showed the best antibacterial ability against Staphylococcus epidermidis. All alloys exhibited good biocompatibility with no cellular toxicity against embryonic fibroblast 3T3 cells. Clinical evaluation of the results after implantation showed a complete absence of purulent-inflammatory complications in all animals. Even distribution of silver nanoparticles in the surface layer of the porous NiTi alloy provides a uniform antibacterial effect.
Hydroxyapatite coatings were deposited for 1, 2, and 3 h on NiTi substrates using plasma-assisted radio frequency sputtering. The matrix consisted of NiTi B2 and NiTi B19’ phases and Ti2Ni, Ni3Ti, ...and Ni4Ti3 intermetallic compounds. The surface coating was monoclinic hydroxyapatite. Increasing the deposition time to 3 h made it possible to form a dense hydroxyapatite layer without visible defects. The phase contrast maps showed that the coating consisted of round grains of different fractions, with the smallest grains in the sample deposited for 3 h. The wettability tests showed that the coating deposited for 3 h had the highest surface energy, reflected in the proliferation density of the MCF-7 cell line.
This paper studied the features of the martensitic transformations and mechanical properties of 40, 60, and 90 µm thick NiTi wires with nanocrystalline B2 structures. It was established that the ...wires were composites and consisted of a TiNi matrix and a TiO
+ TiNi
surface layer. Structural methods showed that the wire matrix was formed by grains of up to 20 nm in size. The method of measuring the electrical resistivity during cooling and heating revealed a two-stage nature of the martensitic transformation. Cyclic loading-unloading demonstrated that all the samples exhibited superelasticity effects and completely restored their shape when unloaded from a 4-8% relative strain at room temperature. An increase in mechanical characteristics with respect to the wire thickness was experimentally established. This was due to the change in the composition of the TiNi matrix during drawing.
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.
NiTi alloys are actively used in medicine as implants because of their mechanical properties. At the same time, however, the NiTi surface is not biologically active, which has a negative effect on ...osteogenesis. In addition, Ni is capable of causing various toxic reactions in the human body. It is therefore necessary to create a composite coating that is both bioactive and prevents the release of Ni ions. It has been shown that by changing the modes of plasma assisted RF sputtering of a calcium phosphate target, it is possible to obtain coatings with different contents of the amorphous phase. The coatings formed consist of hydroxyapatite and amorphous calcium phosphate. On the surface of a sample not treated with argon plasma, there are no phases containing calcium or phosphorus, although these elements are present on the surface. The different ways in which the CaP coatings were applied also affected their structure. It was found that the CaP coatings of samples M2 and M3 have mediocre corrosion resistance, dissolving quickly. This has a positive effect on the ability to osteogenesis, which was discovered as a result of in vitro tests. These tests have shown that the sample obtained at Idischarge = 30 A and Ubias = 30 V has the highest bioactivity and produces the lowest percentage of dead cells.
The problem of using magnesium in medicine is biodegradation, which leads to the loss of the implant mechanical integrity before the bone tissue formation. Doping and subsequent coating deposition is ...an effective solution to this problem. This paper investigates the effects of electrolyte exposure time during micro-arc oxidation of the Mg-Ca-Zn alloy on its phase composition, biodegradation, and biocompatibility. As a result of micro-arc oxidation, a dense coating with a gradient structure is formed on the surface of the Mg-Ca-Zn alloy, consisting of amorphous and amorphous-nanocrystalline parts. The amorphous layers of the coating mainly consist of O, P, and Mg. The nanocrystalline layer is enriched in F, while the diffusion zone is enriched in O, F, and Mg. The coating has a porous structure typical for microarc oxidation. The nonlinear behavior of the diffraction pattern at small diffraction angles indicates the amorphous state of the coating. The amount of amorphous component is increasing with exposure time, indicating an increase in the coating thickness. Scratch testing with a diamond indenter tip indicates good coating adhesion. All coated Mg-Ca-Zn samples show a significant reduction in weight loss compared to the uncoated sample after 21 days in the culture medium. Surgical treatment of rabbit femurs with implants made from coated Mg-Ca-Zn samples demonstrated high biocompatibility. Clinical evaluation of the results showed a complete absence of purulent-inflammatory complications in all animals during the first 28 days after implantation.
•Microarc oxidation results in a dense gradient coating on the Mg-Ca-Zn surface.•The coating consists of an amorphous and an amorphous-nanocrystalline layers.•The coating of Mg-Ca-Zn alloys prevented biodegradation in the culture medium.•In vivo testing of the coated implants demonstrated their high biocompatibility.
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