No 3-2 (2022)
- Year: 2022
- Articles: 10
- URL: https://vektornaukitech.ru/jour/issue/view/49
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Description:
Published 30.09.2022
Full Issue
Impact strength of VT6 titanium alloy with the ultra-fine grain structure produced by the equal-channel angular pressing method
Abstract
The wide use of two-phase titanium alloys in aircraft engine building, as well as the intense development of this industry, stipulate more and more stringent requirements to structural materials and the enhancement of their reliability, strength and performance characteristics. The formation of an ultrafine-grained (UFG) state in metals and alloys using severe plastic deformation (SPD) processing enables achieving high strength properties. However, an important aspect of UFG materials is their structural and textural effects which may lead to a strong anisotropy of their properties. In this respect, the authors studied the effect of microstructural features on the mechanical properties and impact toughness of the VT6 alloy after equal-channel angular pressing (ECAP) and subsequent deformation by upsetting, imitating die forging. The study showed that the formation of a UFG structure in the VT6 titanium alloy with a grain size of about 0.4 µm allows increasing the ultimate tensile strength up to 1250 MPa. The additional upsetting of the UFG alloy at T=750 °C leads to grain growth up to 0.5–1 µm and a decline in strength to 1090 MPa as a result of the recovery and recrystallization processes. Impact toughness tests were conducted on specimens with a V-shaped stress raiser at room temperature, showing that the impact toughness of the UFG VT6 alloy was 0.41 MJ/m2. The tests revealed the anisotropy of impact toughness in the UFG VT6 alloy after equal-channel angular pressing and additional upsetting due to the metallographic and crystallographic texture formed as the result of deformation treatment. In test direction No. 1, the impact toughness value is the lowest and equals 0.31 MJ/m2.
7-15
The selection of interatomic potentials for simulation of extreme actions within the tungsten lattice
Abstract
Simulation of crystal lattices under conditions far from equilibrium is an increasingly important subject of research and requires confidence in the validity of the applied interatomic potentials in a wide range of atom deviations from the balanced condition. To make such an assessment for modeling tungsten as an advanced material for various nuclear applications, the authors analyzed the nonlinear behavior of the lattice using several interatomic potentials. In a BCC tungsten crystal, oscillations were simulated according to the laws of several delocalized nonlinear vibrational modes – exact solutions to the equations of motion of atoms, the geometry of which is determined by the lattice symmetry at any amplitudes and does not depend on the type of interaction between the nodes. The authors considered two-dimensional cases of oscillations in one of the close-packed planes and three-dimensional cases when the motions of atoms have three components in space for a tungsten cell consisting of 2000 atoms and 31.6×31.6×31.6 Å in size. The amplitude-frequency characteristics of these modes were calculated for several interatomic potentials available in the LAMMPS library. The study identified that several interatomic potentials, namely eam.fs, set, Olsson, and Zhou show practically identical results, which is an indirect confirmation of their validity and the possibility of their use for modeling extreme impacts in the considered lattice. The authors calculated such characteristics of the system as kinetic energy, heat capacity, and pressure. Based on the results obtained, one can assume that mode 15, due to the modulation instability, will lead to the energy localization on individual atoms.
16-24
The influence of severe plastic deformation on mechanical properties of pure zinc
Abstract
Biodegradable materials, which have the ability to resorb in the body, are new and promising materials for medical implants. Currently, scientists carry out the investigations according to three directions: Mg, Fe, and Zn alloys. Zinc-based alloys and zinc have good solubility in the body, which meets the clinical requirements of implants. However, pure zinc has low mechanical properties, including hardness and tensile strength. Therefore, at present, the world scientific community is seeking ways to improve the properties of pure zinc by alloying. Another known approach is the ultrafine-grained (UFG) structure formation by the severe plastic deformation (SPD) methods, which are based on the large plastic deformations under high pressure and relatively low homologous temperatures. In this work, the authors studied the influence of high pressure torsion of pure zinc with various numbers of revolutions. The paper presents calculations of shear deformation after SPD. The authors investigated the dependence of mechanical properties and microstructure on the deformation degree. Tension tests at room temperature were carried out, and microhardness was measured. The authors studied the structure using scanning electron microscopy and optics. The study identified that the use of high pressure torsion leads to an increase in the tensile strength of pure zinc up to 140 MPa and ductility up to 40 % resulting from dynamic recrystallization.
25-31
The study of influence of the reaction gases ratio at the Ti–Al–C–N coating deposition on the cutting tool wear resistance
Abstract
The paper presents the results of the study of the component composition of the reaction gases mixture when synthesizing carbonitride coatings of the Ti–Al–C–N system influencing the cutting tool durability. The coating was applied using the updated unit NNV-6.6-I1 by spraying from two one-component cathodes assisted by the incandescent cathode plasma source. During applying the coating, the mixture of reaction gases of N2 nitrogen and C2H2 acetylene in the ratio of 1:4, 2:3, 3:2, and 4:1 was delivered to the chamber. The paper presents the results of measuring the microhardness of studied specimens, which show that a sample with the coating deposited at the reaction gases ratio of N2:C2H2=2:3 had the largest microhardness value (4870 HV0.05). The paper presents the results of field tests of carbide-tipped tools with the studied coatings. Durability tests identified that a cutter with the coating deposited at the gas ratio of N2:C2H2=4:1 increases the tool durability ten times compared to a cutting tool without coating. Using the electron microscopy method, the authors investigated the chemical composition of the tool cutting face after tests. The analysis of the chemical composition of the surface after cutting showed that the content of coating elements on the surface of the sample with a coating deposited at the 4:1 ratio of the reaction gases of nitrogen and acetylene was considerably higher than that of other studied coatings, which indicates the less coating wear. However, ferrum is present in some areas of the cutting face, which says about the adhesion of treated material to the tool.
32-43
Structural-phase transformations in the Zn–Li–Mg alloy exposed to the high pressure torsion
Abstract
In this paper, using the X-ray scattering method, the authors found the similaritues and differences in the structural-phase transformations in a Zn–Li–Mg alloy under the artificial and dynamic aging. The artificial aging (AA) of the alloy was implemented at a temperature of 300 ºС for 24 h, while the dynamic aging (DA) was performed through high-pressure torsion at room temperature for a few minutes. For the first time, using X-ray phase analysis, the authors identified the type and parameters of the LiZn2 phase crystal lattice (Pmmm, a=0.48635 nm, b=1.11021 nm, c=0.43719 nm, α=β=γ=90º) and the β-LiZn4 phase (P63/mmc, a=b=0.279868 nm, c=0.438598 nm, α=β=90º, γ=120º) to the eutectics in specified conditions. The study found that SPD leads to intensive precipitation of Zn particles in the primary β-LiZn4 phase, and β-LiZn4 particles precipitation in the Zn eutectics phase. While analyzing the diffraction patterns, the authors estimated the lattice parameter, the size distribution of coherent scattering regions, the averaged dislocation density, and the fraction of edge and screw dislocations after AA and DA. For the first time, by small-angle X-ray scattering, the authors identified the quantitative characteristics of the size, shape, and nature of the bimodal precipitate distribution in the above-mentioned conditions. In particular, it was found that fine Zn precipitates in the form of needles of 8 nm in diameter and up to 27 nm in length and coarse Zn precipitates in the form of rods of 460 nm in diameter and up to 1000 nm in length are produced in the alloy after AA. In the case of DA, fine Zn precipitates of a primarily spherical shape with an average diameter of 20 nm and coarse Zn precipitates, which formed in the primary β-LiZn4 phase a network with a cell diameter of 200–300 nm and wall thickness of 62 nm are produced in the Zn–Li–Mg alloy.
44-55
The formation of PEO coatings on the superelastic Ti–18Zr–15Nb alloy in calcium-containing electrolytes
Abstract
The paper discusses the influence of the electrolyte composition on the characteristics of a biocompatible coating produced by plasma electrolytic oxidation (PEO) on titanium superelastic shape memory alloy Ti–18Zr–15Nb. The scientific novelty of the work is in the identification of the most effective electrolyte composition to form a PEO coating with improved functional properties for advanced metal implants. Having important scientific and social significance, the scientific results of the work will serve as the basis for the development of modern technologies for the production of new-generation implants for orthopedy and neurosurgery. To identify the most effective electrolyte composition, the authors studied the morphology and microstructure of the coatings, phase and elemental composition, adhesive properties, and surface wear resistance, and also they carried out electrochemical corrosion tests. The resulting coatings have a thickness in the range of ~15.5–17 µm, and porosity of ~12–18 %. The additive of sodium silicate significantly smooths the surface and increases the wear resistance, but, at the same time, it reduces the adhesive properties of the coatings. The coatings contain biocompatible calcium phosphate compounds, which presence is confirmed by an amorphous halo between ~25° and ~40° in the results of X-ray phase analysis and by the identified elements Ca and P in the elemental analysis. The electrochemical impedance spectroscopy results identified the difference in the structure of the PEO coatings and the corrosion processes occurring in them. Coatings formed in the phosphate electrolytes have two layers: the external porous and internal compact, and in the phosphate-silicate electrolytes – a single layer. The study identified that the plasma-electrolytic oxidation reduces the corrosion currents by 1–3 orders compared to a specimen without the PEO treatment. The coating formed in a phosphate electrolyte with the addition of boric acid and calcium acetate has the best corrosion characteristics and the highest roughness, which could positively affects the biocompatibility. This electrolyte can be recommended for further research as the most effective one.
56-67
The study of influence of temperature and speed conditions on the mechanical properties of bioresorbable Zn–4Ag–Cu zinc alloy during equal-channel angular pressing
Abstract
Recently, innovative medical techniques for restoring lost functions of patients have been actively developed, in which the use of bio-soluble (bioresorbable) materials is of particular importance. Such materials include alloys based on Mg, Fe, and Zn, and can significantly reduce the cost of surgical operations and shorten the duration of treatment. However, these metals have such disadvantages as insufficient strength and increased fragility to be used in medical implants. Therefore, increasing the mechanical characteristics of bioresorbable alloys is an urgent problem. In this work, the authors solve this problem using an advanced method of plastic treatment – severe plastic deformation (SPD), which, due to active initial structure refinement to nano- and ultrafine state, allows effective improvement of the mechanical strength of metal materials. The authors used the most effective and well-spread SPD method –equal-channel angular pressing (ECAP). The paper presents the results of computer ECAP research of Zn–4Ag–Cu zinc alloy at different deformation rates (0.4 and 7.8 mm/sec) and temperature conditions (150, 200 °C) chosen based on equipment performance potential and conditions to ensure thermal stability of the structure. The patterns of distribution of accumulated deformation degree, deformation rate, average stress values, and temperature-force conditions are obtained. According to the results of computer modeling, the authors recommended carrying out ECAP processing at the temperature of 150, 200 °C and a speed of 0.4 mm/s, which ensures a uniform thermal field at the deformation zone. During the experimental work according to the selected modes, the authors obtained samples after four ECAP cycles, which had advanced mechanical properties improving performance characteristics. The increased strength will allow minimizing the implants’ sizes ensuring less trauma during their installation and faster dissolution in the physiological environment of the body when retaining functionality.
68-78
Thermal stability of the ЭИ-961Ш steel structure after combined processing
Abstract
A crucial aspect in the development of materials with improved functional properties is ensuring their ability to withstand the operating temperatures of a finished product. To increase the service life and efficiency of products made of ferrite-martensite steels, various types of deformation and thermal treatments are used. The authors studied the influence of different temperature regimes on the structure and thermal stability of ЭИ-961Ш ferrite-martensite steel subjected to rolling and additional hardening. As a method of deformation and heat treatment, the authors used cold rolling followed by re-quenching from a temperature above the ferrite/austenite phase transition. The samples were rolled during several passes on a laboratory rolling mill with the deformation of 6 % per pass for a final thickness of 4.3 mm to a reduction degree of 70 %. The authors carried out structural studies by transmission electron microscopy and scanning electron microscopy. The study showed that as a rolling result, a bimodal band structure forms with the distribution of Cr23C6 carbide particles along the grain boundaries. When using additional hardening, an increase in the globular carbides proportion is observed, and during the study by transmission electron microscopy, nano-twins were found in the structure. The bands’ width after the reduction by 50 % was 0.5 microns and after cold rolling and additional heat treatment – 0.4 microns. The authors carried out short annealing in the operating temperature range to study the thermal stability of ferrite/martensite steel structure after cold rolling and additional heat treatment. The thermal stability study showed that many structural features formed during previous deformation and heat treatment are preserved, however, after annealing at 600 °C, there are no visually observable nano-twins in the structure.
79-89
Planar superstructural defects in the alloys with L10 superstructure
Abstract
Planar superstructural defects have a great influence on the mechanical, functional properties of binary ordered alloys of the L10 superstructure based on the fcc lattice, but there is no complete analysis of their structure and energy in the literature. For the L10 superstructure alloys of the stoichiometric composition AB, the paper gives the expressions for calculating the sublimation energy and the energy of a planar superstructural defect in the model of hard coordination spheres and pair interatomic interactions. The crystal lattice tetragonality was not taken into account. The authors presented the ordered alloy structure as a union of four monoatomic simple cubic lattices, two of which are occupied by A atoms, and the other two by B atoms. This approach allows calculating the sublimation energy required for crystal evaporation. The first eight coordination spheres were taken into account in the work. The paper shows an algorithm for determining all possible geometrically different representations of the L10 superstructure with the same sublimation energy, gives an expression for finding the planes of occurrence of all possible conservative antiphase boundaries. The study identified that the conservative and nonconservative antiphase boundaries, as well as conservative and nonconservative boundaries of C-domains are observed in the binary ordered alloys of the L10 superstructure based on the fcc lattice. The algorithms described in this work make it possible to carry out a crystal-geometric analysis of planar defects in both binary and multicomponent ordered alloys with various superstructures.
90-98
The structure and mechanical properties of the AK12D (Al–Si–Cu–Ni–Mg) aluminum alloy subjected to friction stir processing
Abstract
The application of friction stir processing (FSP) to modify the structure of the Al–Si alloys, in particular the fragmentation of large silicon particles, can lead to an increase in the level of mechanical properties. This work is aimed to study features of local surface hardening of AK12D aluminum alloy (Al–Si–Cu–Ni–Mg system) during FSP and subsequent T6 hardening heat treatment. The authors investigated the influence of FSP and subsequent heat treatment parameters on the structure, microhardness, and hardness of the AK12D alloy. FSP was carried out at speeds of processing tool rotation and traverse of 2000 rpm and 8, 16 mm/min, respectively. The subsequent hardening T6 heat treatment was carried out according to the standard regime for the AK12D alloy. The paper shows that the FSP mode at a rotation speed of 2000 rpm and a traverse speed of 8 mm/min contributed to the formation of a monolithic and defect-free treatment zone. The study revealed that the formed microstructure is heterogeneous due to the influence of various thermomechanical effects. The most intense structural changes occurred in the stir zone. Friction stir processing and subsequent heat treatment led to fragmentation and partial dissolution of intermetallide particles in the α-Al solid solution followed by its decomposition and formation of secondary hardening phases. Moreover, the FSP and T6 heat treatment led to the formation of quasi-equiaxed fine-grained structure. The AK12D alloy microhardness after treatment under the study varied nonmonotonically and depended on the structure in different zones. At the same time, the Brinell hardness values after FSP and subsequent T6 heat treatment increased compared to the initial heat-treated state.
99-108