No 3-1 (2022)

The paper studies a bundle of oriented carbon nanotubes (CNTs) under the transverse loading under the plane deformation conditions within the framework of a molecular dynamics model with a reduced number of degrees of freedom. The model takes into account CNT wall stretching and bending, as well as van der Waals interactions. Each CNT is represented by a ring of atoms with two degrees of freedom in the plane of the ring. The discrete nature of the model allows describing the large curvature of the CNT wall and the destruction of CNTs at very high pressure. CNT crystal equilibrium structures are obtained under the strain-controlled biaxial loading. Separate CNTs of a sufficiently large diameter have two equilibrium states: with a round and collapsed cross section. Small-diameter CNTs in the free state can only have a circular cross section. The study identified the presence of two phase transitions observed during biaxial compression of a CNT bundle. The first transformation similar to phase transition of the second order leads to ellipticization of CNT cross sections. As a result of the second transition of the first order, bundled CNTs appear in the beam, the proportion of which gradually increases with the increase in compressive strain. The authors calculated beam elasticity constants such as Young’s moduli, shear modulus, and Poisson’s ratios. The study shows that one of the equilibrium structures (with elliptical CNT cross sections) has the property of a partial auxetic, that is, it has a negative Poisson’s ratio under uniaxial loading in a certain direction. The proposed chain model can be effectively applied to analyze physical and mechanical properties of bundles of single-walled or multi-walled CNTs under the plane deformation conditions, and after simple modifications, it can be used to similar structures made of other two-dimensional nanomaterials.

Identifying the peculiarities of the transformation of the structure and properties of bulk metallic glass (BMG) under high-pressure torsion (HPT) is of great interest. It is known that under HPT, the degree of deformation differs from the center to the edge of a disk which leads to the non-uniformity of the structure of obtained specimens. The change in microhardness value indicates the direction of change in BMG structure under the HPT, and the microhardness distribution indicates the HPT-specimen non-uniformity. The aim of the study is to identify the HPT influence on the microhardness value and microhardness distribution over the surface of specimens of amorphous alloys using an example of Vit105Zr-based BMG (Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅). The authors studied the distribution of microhardness over the surface of Vit105 Zr-based bulk metallic glass (BMG) in the initial state, in the state after HPT at n=1 and n=5 rotations, and after relaxing annealing. The study shows that the initial Vit105 BMG is characterized by a small spread in microhardness values, which indicates the material's high homogeneity. By reducing the excessive free volume, relaxing annealing increases microhardness without a significant increase in the spread of its values. HPT leads to a decrease in the zirconium BMG microhardness, which indicates an increase in the excessive free volume, but, at the same time, increases the uneven microhardness distribution over the specimen, while the microhardness values in one half of the HPT sample (n=1) are higher than in the other one. It demonstrates that BMG specimen deformation during HPT is related to the specific loading mechanisms.

Copper is widely used when producing current-conducting parts, basically the electrotechnical power equipment buses. Traditional ways of welding copper become complicated because of high thermal conductivity, fluidity, significant oxidation at fusing temperature, and susceptibility. The application of the solid-phase welding methods, a prominent representative of which is friction stir welding (FSW), is one of the ways to solve problems when welding copper. The paper presents the experimental study of the influence of a tool working part shape and the welding mode parameters: welding rate, tool rotation frequency, and tool dip angle – on the possibility of the appearance of defects in welded joints of M1 copper plates of 5 mm in thickness produced by FSW. The paper contains the results of mechanical tests on static tension and bending of welded joints with a tunnel defect and without it. Welded joints with a tunnel defect showed a decrease in mechanical properties level: the value of ultimate tensile strength at stretching is lower by 33 %, and the specific elongation is lower by 8 % than ones of a joint without defects. The authors specify some factors influencing the appearance of defects at FSW: the welding rate, tool rotation frequency, tool working part construction, tool dip angle, strength and depth of immersion, pin displacement, blank thickness, and grip conditions. The study identified that the application of a tool with a concave surface taper shoulder allows producing welded joints without external and internal defects. Based on data obtained during the experimental research, the authors determined the welding modes, which makes it possible to produce welded joints with the electrical resistance value at the level of a parent metal: tool rotation frequency is 1250 rpm, welding rate is 25 mm/min, and tool immersion depth is no less than 0.41 mm.

Goods made of beryllium bronzes got widespread use in the industry due to the complex of properties: high heat conductivity, strength, hardness, wear resistance, and corrosion resistance. They are not magnesium-based and do not spark on impact; therefore, they are essential for the production of non-sparking tools. The alloys of this system are used in the electrical engineering industry; consequently, it is necessary to pay attention to the improvement of the material’s electrical conductivity. The paper studies the microstructure, microhardness, and electrical conductivity of the Cu–2 wt.% Be alloy exposed to high-pressure torsion (HPT). The authors investigated the microstructure and fine structure of the alloy in various states. The study showed that HPT leads to the formation of an ultrafine-grained nanostructured (UFG NS) state with an average size of grains/subgrains of 22±1 mmn. Additional ageing of samples after HPD led to a slight increase in the grains/subgrains size up to 31±1 mmn. In both states, the authors observed nanosized deformation twins. The authors studied the dependences of microhardness and electrical conductivity of the alloy after HPD on the time of further ageing. The study identified that the microhardness increases from 122±3 HV in the initial state up to 525±8 HV after HPD and ageing. The investigation shows that the electrical conductivity substantially better recovers after ageing of the UFG NS state compared to the initial state. The electrical conductivity of the UFG NS state increased from 14.5±0.1 % IACS up to 27.5±0.6 % IACS in conditions similar to the initial state ageing. Therefore, resulting from such processing, the Cu–2 wt.% Be alloy is characterized by its advanced strength properties and electrical conductivity.

Nowadays, to improve the quality of life, dental implantation is widely used, and ensuring proper functioning and durability of the implantable devices is one of the most crucial tasks for modern-day dentistry. The development of new biomaterials with improved properties, such as nanostructured materials, widens the possibilities of medical goods miniaturization to create new-generation implants. Computer simulation plays a large part when designing these devices, which allows effectively specifying an implant design depending on the materials used and operation conditions. This paper presents the results of modeling using the finite-element method for the comparative analysis of an implant’s deformed behavior within the cyclic load conditions. The authors considered large-grained commercially pure titanium and nanostructured titanium with improved properties as implant material. The authors analyzed various arrangements of an implanted device according to the fatigue testing conditions – considering and not considering the influence of an abutment and the base reaction. The study identified the implant’s characteristics, such as fatigue endurance and safety factor for a specific type of arrangement and material type, as well as the equivalent stress distribution, including taking into account a sign. The research shows that the most realistic results can be achieved when modeling a device in the “abutment – implant – base” arrangement. The study demonstrates that strength characteristics crucial for product destruction are described by the maximum principal stresses, and the studied implant configuration ensures its longstanding proper functioning in the case of its production exceptionally from nanostructured titanium with enhanced properties.

Self-resorbable implants made of magnesium alloys, unlike the traditional implants made of titanium alloys and stainless steels, have the ability to completely dissolve in the human body, which makes it possible to eliminate the need for a recurrent operation to extract them. The issue of the possibility of using magnesium implants in the combination with products made of titanium alloys remains insufficiently studied at the moment. At the same time, it is widely known that the elements such as titanium and iron, with a potential more positive than magnesium, have a disastrous influence on the corrosion of magnesium alloys, since magnesium dissolves much faster due to the galvanic effect. This work is aimed to determine how the distance to a titanium implant affects the corrosion rate of a ZX10 magnesium alloy sample with an ultra-fine grain structure. As it is an issue of medical application, the authors carried out the corrosion tests within the conditions simulating the human body conditions: the corrosion medium circulation and keeping temperature within 37±1 °C. The authors used physiological solution as a corrosion medium. During corrosion testing, a titanium implant was placed in three, six, and twelve centimeters from the magnesium alloy sample; and the control tests were also carried out without a titanium implant. According to the obtained data, at a distance of 3 cm, the galvanic effect between titanium and magnesium manifests itself strongly, increasing the corrosion rate and the size of corrosion damage, but at a distance of 6 cm, the titanium implant does not have a visible effect on the corrosion of a sample.