In the present paper, the cyclic stability of the high-temperature two-way shape memory effect was studied in high-strength Ni50.3Ti32.2Hf17.5 polycrystals after various thermomechanical ...treatments—training (thermocycling under stress) and stress-induced martensite aging. The effect of training and stress-induced martensite aging on the microstructure, the two-way shape memory effect, and its cyclic stability was determined. It was found out that both thermomechanical treatments induce the high-temperature two-way shape memory effect at T > 373 K, with a strain of 1.5% in tension. The influence of cyclic tests (up to 100 stress-free cycles of cooling/heating) on the two-way shape memory effect strain, the transformation temperatures, and the microstructure was established. Different degradation mechanisms of the two-way shape memory effect were established after thermocycling and stress-induced martensite aging.
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.
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.
Mo–TiC–La2O3 molybdenum alloys were strengthened and toughened by the synergistic action of nano-carbide particles and rare earth oxides. In this paper, the Mo–TiC–La2O3 alloy system was prepared by ...powder metallurgy. The microstructure was characterized by optical, scanning, and transmission electron microscopy. The mechanical properties were tested using the hardness tester and universal tensile testing machine. The grain size of the Mo–TiC–La2O3 alloy is smaller than the Mo–TiC and Mo–La2O3 alloys. The strength and elongation of annealed Mo–TiC–La2O3 alloy are 1291 MPa and 6.6%, respectively. The strength and ductility of the annealed Mo–TiC–La2O3 alloy are higher than the Mo–TiC and Mo–La2O3 alloys. According to the interfacial mismatch between the secondary phases and the matrix, along with oxygen impurities interactions, the mechanisms of strengthening and toughening of the secondary phases in the Mo–TiC–La2O3 alloy were revealed.
•The strength of alloys increases from Mo–La2O3 to Mo–TiC, to Mo–TiC–La2O3.•Mo–TiC–La2O3 alloy not only has a high hardness, the high tensile strength, but also 6.59% elongation.•Mo–TiC–La2O3 alloy strengthening from nano-TiC, TiO2, and La2Ti2O7 phases and toughening from La2O3.
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.
Display omitted
•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.
Display omitted
•Multiple twins give the alloy higher work hardening ability.•The synergistic effect of multiple mechanisms gives the alloy excellent comprehensive properties.•The ultra-low lattice ...mismatch between the matrix and the precipitates ensures the ductility.
In the present work, the Cu-1.35Ni-1Co-0.55Si alloy with multiple twins and ultra-low interfacial mismatch precipitates was obtained by liquid nitrogen dynamic plastic deformation (DPD) and aging treatment. It has abnormally high work hardening ability, which is more than twice that of the previous DPD treatment without aging. In addition, it has excellent comprehensive properties, and its strength, electrical conductivity, and elongation are 816.4 MPa, 40.2% IACS and 11.3% respectively. The fine multiple twins do not coarsen significantly during peak aging, and the dispersed nano precipitates (∼3 nm) further enhance the strength of the Cu-Ni-Co-Si alloy. In addition, the low interfacial mismatch strain between the precipitates and the matrix also ensures adequate elongation. The Cu-Ni-Co-Si alloy has high strength, electrical conductivity, and ductility under the simultaneous action of various mechanisms (grain boundary strengthening, precipitation strengthening, dislocation strengthening and stacking faults). This strategy paves a new avenue for developing precipitation-strengthened alloys with excellent comprehensive properties.
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.
This study examined the effects of liquid nitrogen vapor on osteogenesis in the rabbit femur. Cryotweezers made of porous nickel titanium alloy (nitinol or NiTi) obtained by self-propagating high ...temperature synthesis were used in this experiment. The porous structure of the cryotweezers allows them to hold up to 10 g of liquid nitrogen after being immersed for 2 min, which completely evaporates after 160 s. To study the effects of liquid nitrogen evaporation on osteogenesis, a rabbit femur was perforated. The formed holes were subjected to cryotherapy with varying exposure times. It was found that a 3 s exposure time stimulates osteogenesis, which was manifested in a greater number of osteoblasts in the regenerate compared to the control sample without liquid nitrogen. It was observed that increasing the exposure to 6, 9 or 12 s had a destructive effect, to varying degrees. The most severe damage was exerted by a 12 s exposure, which resulted in the formation of osteonecrosis areas. In the samples exposed to 6 and 9 s of cryotherapy, destruction of the cytoplasm of osteocytes and osteoclasts was observed.
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.
PDF
