Aleksandr I. Lotkov

Interaction of human endothelial cells and nickel-titanium materials modified with silicon ions

The paper studies the influence of chemical and phase compositions of NiTi surface layers modified with Si ions by plasma immersion implantation on their interaction with endothelial cells. It is shown that certain technological modes of Si ion implantation enhance the adhesion, proliferation, and viability of endothelial cells. It is found that the Si-modified NiTi surface is capable of stimulating the formation of capillary-like structures in the cell culture.

Structural phase states in nickel-titanium surface layers doped with silicon by plasma immersion ion implantation

The paper reports on a study of NiTi-based alloys used for manufacturing self-expanding intravascular stents to elucidate how the technological modes of plasma immersion ion implantation with silicon influence the chemical and phase composition of their surface layers. It is shown that two types of surface structure can be obtained depending on the mode of plasma immersion implantation: quasi-amorphous Si coating and Si-doped surface layer. The Si-doped surface layer contains new phases: a phase structured as the main B2 phase of NiTi but with a lower lattice parameter, R phase, and phase of highly dispersed SiO2 precipitates.

Temperature dependence of inelastic strain recovery in TiNi-based alloys under torsion

The paper presents research results on the temperature dependence of inelastic strain recovery due to superelasticity and shape memory effects under torsion in coarse-grained and microcrystalline TiNi specimens containing 50.8 at % of Ni (Ti49.2Ni50.8 (at %)). It is found that under certain temperature and deformation conditions, the recovery strain exceeds the theoretical limit of recoverable strains or maximum lattice strain in B2↔R↔B19′ transformations. Using a special algorithm, the torsional strain is converted to its equivalent (by Mises) tensile strain. The results are compared with data obtained by other researchers.

Effect of hydrogen on superelasticity of the titanium nickelide-based alloy

The hydrogen effect on the inelastic properties and plastic strain development after electrolythical hydrogenation in physiological solution was investigated. This effect virtually results in a failure under torsion of Ti49.1Ni50.9 (atom per cent) alloy specimens with coarse-grained (CG) and submicrocrystalline (SMC) structures. It is shown that hydrogen embrittlement (HE) phenomenon occurs irrespective of the grain size in the studied specimens at approximately equal strain values. However, compared to the specimens with CG structure, those with SMC structure accumulate two to three times more hydrogen for the same hydrogenation time. It is found that hydrogen has a much smaller effect on the inelastic properties of specimens with SMC structure as compared to those with CG structure.

Effect of heat treatment on superelasticity of NiTi-based intravascular implants

The paper analyzes the conditions for realization of superelasticity in nickel-titanium (NiTi) alloys due to martensite transformations. The degree of superelasticity, its temperature range, and related shape recovery necessary for NiTi alloys as intravascular implant materials are determined. It is shown that the desired characteristics of medical NiTi implants are attained by heat treatment at 773±10 K for 15–30 min.

THE INCREASE OF THE MARTENSITIC DEFORMATION DURING SHAPE MEMORY EFFECT IN DEFORMED TiNi

The evolution of the ultrafine structure, obtained at thermal treatment below recrystallization, and its effect onto shape memory characteristics in NiTi alloy was under study. It was shown that low temperature annealing (< 0.5 Tmelt) of the hot rolled NiTi leads to the structure refinement accompanied by the growth in accumulated martensite deformation. It was suggested that this is possible due to the increase of the volume fraction of martensite crystals properly oriented in respect to external stress.

Mechanical behavior of deformed intravascular NiTi stents differing in design. Numerical simulation

Self-expanding intravascular NiTi stents serve to recover the lumen of vessels suffered from atherosclerotic stenosis. During their manufacturing or functioning in blood vessels, the stents experience different strains and local stresses that may result in dangerous defects or fracture. Here, using the method of movable cellular automata, we analyze how the design of a stent influences its stress state during shaping to a desired diameter on a mandrel. We consider repeated segments of different stents under two loads: uniform diametric expansion of their crown and expansion with relative displacements. The simulation data agree well with experiments, revealing critical strain, stress, and their localization sites at the shaping stage, and provide the way toward optimum stent designs to minimize the critical stress during shaping.Self-expanding intravascular NiTi stents serve to recover the lumen of vessels suffered from atherosclerotic stenosis. During their manufacturing or functioning in blood vessels, the stents experience different strains and local stresses that may result in dangerous defects or fracture. Here, using the method of movable cellular automata, we analyze how the design of a stent influences its stress state during shaping to a desired diameter on a mandrel. We consider repeated segments of different stents under two loads: uniform diametric expansion of their crown and expansion with relative displacements. The simulation data agree well with experiments, revealing critical strain, stress, and their localization sites at the shaping stage, and provide the way toward optimum stent designs to minimize the critical stress during shaping.

Effect of plasma immersion ion implantation in TiNi implants on its interaction with animal subcutaneous tissues

Here we investigated in vivo interaction of Si-modified titanium nickelide (TiNi) samples with adjacent tissues in a rat subcutaneous implant model to assess the impact of the modification on the biocompatibility of the implant. Modification was performed by plasma immersion ion processing, which allows doping of different elements into surface layers of complex-shaped articles. The aim of modification was to reduce the level of toxic Ni ions on the implant surface for increasing biocompatibility. We identified a thin connective tissue capsule, endothelial cells, and capillary-like structures around the Si-modified implants both 30 and 90 days postimplantation. No signs of inflammation were found. In conclusion, modification of TiNi samples with Si ions increases biocompatibility of the implant.

Phase and structural states in the NiTi-based alloy surface layer formed by electron-ion-plasma methods using tantalum

The paper reports on a study of regularities of formation gradient nano-, submicron and microstructural conditions in the surface layers of the samples after pulsed electron-beam melting of tantalum coating on the substrate NiTi alloy. Experimentally revealed the presence of submicron columnar structure in the upper layers of the tantalum coating. After irradiation modified NiTi surface takes on a layered structure in which each layer differs in phase composition and structural phase state.

In-situ X-ray diffraction studies of the phase transformations and structural states of B2, R and B19′ phases in Ti49.5Ni50.5 alloy

The martensitic transformation, Debye–Waller factor, mean-square atomic displacements and the coefficient of thermal expansion on cooling of the Ti49.5Ni50.5 shape memory alloy were examined using in-situ X-ray diffraction. It was revealed B2→R (TR ≡ T = 273 ± 10 K) along with B2→B19’ (Ms ≡ T = 273 ± 10 K) transitions occur. It was found that Debye–Waller factor and mean-square displacement of B2 phase undergo significant increase as functions of temperature when phase transition B2→R and B2→B19’ take place. The analysis of the thermal expansion coefficient of the B2 phase indicates that the value of a increases almost linearly while cooling.