No 1 (2022)

A characteristic feature of high-entropy alloys is high strength at maintaining plasticity, wear and corrosion resistance, and fracture toughness at cryogenic temperatures. Currently, CoCrFeNiMn is the best-investigated high-entropy compound. However, its application is limited in the high-temperature region due to the low values of the deforming stress level at the plasticity breaking point at T>296 K. One of the common ways to improve the material durability is the addition of substitution atoms of larger atomic radius, and Al, Ti, and Mo are some of these atoms. The paper presents the analysis of the mechanical behavior of single crystals of CoCrFeNiMn and CoCrFeNiMо FCC high-entropy alloys (at. %) oriented along the [001] direction: the author studied the temperature dependence of critical shear stresses τ_cr(T) within the temperature range of T=77–973K, the type of dislocation structure, strain hardening coefficient θ_II, plasticity and fracture at Т=296 K under tension. The study shows that the alloying with Mo atoms 4 at. % of the CoCrFeNi system (at. %) causes the solid solution hardening, and critical shear stresses τ_cr increase within the entire studied temperature range. The onset of plastic deformation is associated with slip at all temperature tests. At T=296 K, the author identified a planar dislocation structure with flat dislocation pile-ups and dislocation networks in CoCrFeNiMo while in equiatomic CoCrFeNiMn, at such test temperature, a uniform distribution of dislocations was observed in several systems without flat pile-ups. Work hardening coefficient, plasticity, and the level of stresses before fracture turn out to be similar in [001]-crystals of CoCrFeNiMo and CoCrFeNiMn high-entropy alloys, which are determined by the development of slip deformation simultaneously in several systems. Crystals are destroyed viscously at 296 K at the same level of stress.

Friction stir welding (FSW) is an innovative technology for the solid-phase joining of metal materials. It allows producing permanent joints of materials conventionally considered to be nonweldable, in particular aluminum alloys. However, an essential drawback of FSW is the relatively low stability of the stir zone microstructure. In particular, during post-weld heat treatment, seams frequently demonstrate abnormal grain growth. Such an undesirable phenomenon is often interpreted in terms of the so-called Humphrey’s cellular model, according to which the abnormal behavior is attributed to the essential microstructure refinement and the dissolution of the second-phase particles occurring during FSW. Since these two processes significantly depend on the temperature, the authors suggested that the thermal stability of the produced FSW seams should also be associated with the FSW heat conditions. To test this hypothesis, the authors obtained two welded seams at different FSW conditions and then studied their microstructural behavior during T6 mode thermal treatment (involving solution heat treatment followed by artificial aging). The authors used the advanced electron backscatter diffraction technique (EBSD) to investigate microstructure. In full accordance with the initial idea, the investigation showed that microstructural evolution in both studied microstructure states varied wildly. Specifically, the study identified that the reduction in the FSW temperature causes the suppression of abnormal grain growth. The authors suggested that the enhanced thermal stability of the material is associated with the conservation of the second-phase particles during the low-temperature FSW.

Due to the intensive development of spectroscopic techniques for detecting acoustic emission signals, the problem of providing the best time-frequency resolution through the application of specific time-frequency transformation algorithms comes to the fore. The Short-Time Fourier Transform, the Wavelet Transform, the Smoothed Pseudo Wigner Distribution, the Choi-Williams Distribution, and the Hilbert-Huang Transform are currently the main time-frequency transformations used or integrated into the acoustic emission method. However, today in the literature, there is not enough information that allows evaluating time-frequency transformations regarding the effectiveness of their application to specify the features of discrete and continuous acoustic emission signals. On this basis, the authors carried out an experimental comparison of synthetic and actual model signals to determine the efficiency of specified time-frequency transformations. The synthetic model signals were a chirp signal, ideal sinusoids, and a Dirac delta function. The actual signals were a discrete acoustic emission signal from the Hsu Nelson source decomposed into dispersion modes in the acoustic channel and a continuous acoustic emission signal from the air outflow through a calibrated hole. The analysis shows that only the Fourier transform and the Wavelet transform can define all control features of model signals at the frequency components’ energy gap of about 25 dB. Wigner Distribution, Choi-Williams Distribution, and Hilbert-Huang Transform demonstrated higher time-frequency resolution did not identify frequency components of low energy. Therefore, the authors recommend using them to identify spectral changes in the resonance and discrete signals but in the narrow energy range. The Fourier transform and the Wavelet transform demonstrated the best result to analyze continuous acoustic emission. However, to use the latter, the procedure of selection of the optimal basis function is necessary. The study determined that the Hilbert-Huang transform allows identifying the frequency fluctuations, but it is necessary to develop ways to increase sensitivity and extract basic information from the spectrograms to enhance the validity of its results.

After cold-rolling reduction with the shrinkage of more than 97 % and recrystallization annealing, the edged cubic texture develops in some fcc lattice metals with the high and medium values of stacking fault energy such as Ni, Cu, Al, Pt, and some alloys on their base. The extended bands of metals and fcc lattice alloys can be used to apply multilayer functional compositions. The authors studied the structure and crystallographic texture in bands of three-component copper-nickel-based alloys. The study showed the crucial possibility of creating multi-component alloys based on the Cu+40% Ni binary alloy doped with such elements as Mo or Nb. The paper considers the formation of an edged cubic texture in bands of Cu–Ni–Mn, Cu–Ni–Nb, and Cu–Ni–Мо alloys produced through cold deformation with rolling and recrystallization annealing performed at different temperatures. The study identified that annealing during one hour at 1050 °С was an optimal recrystallization annealing mode when on the surface of bands made of (Cu+40 % Ni)–Me alloys (where Me=Mn, Mo, Nb) deformed at ~99 %, the most perfect cubic texture was realized. According to the data obtained, after such annealing mode, from 94% to 98% of grains with orientation {001}<100> developed in the Cu–40% Ni–1.3% Mn, Cu–40% Ni–0.8% Mo, and Cu–40% Ni–0.5% Nb alloys. It opens the prospect of using these alloys as epitaxial substrates in the technology of high-temperature second-generation superconductors. The evaluation of mechanical characteristics showed that alloying contributed to an increase in the yield strength of Cu–40% Ni–1.3% Mn, Cu–40% Ni–0.8% Mo, and Cu–40% Ni–0.5% Nb alloys by 3–4 times compared with the yield strength value of a textured copper band.

Today, a promising research area is the study of the behavior of the materials’ technological and physical characteristics under the external energy effects, such as constant magnetic fields. It is caused by the emergence of multifactorial scientific and industrial problems arising because of the introduction of high technologies into production. One of the directions is the production of new equipment, devices, and machines that somehow form electromagnetic fields around them. Therefore, an umbrella approach to studying the influence of magnetic field effects on the deformation characteristics of metals and alloys contributes to a deeper understanding of the physical nature of this effect. As an object for the research, the authors selected commercially pure titanium of VT1-0 grade. The work aims to study the influence of a constant magnetic field of 0.3 T on microhardness, creep rate, and fracture surface of commercially pure VT1-0 titanium. The results show that under the influence of a constant magnetic field of 0.3 T, the relative value of VT1-0 titanium microhardness decreases by 2–5 %, followed by relaxation to the initial value. The creep rate of titanium increases by approximately 31 % when applying a field of 0.3 T induction during the test (without field applying, the creep rate is 2.4 %/h, in the magnetic field is 3 %/h). The fracture surface analysis using scanning electron microscopy (SEM) shows that titanium specimens undergo ductile fracture. Numerous equiaxial destruction pits characterize the fracture surface. It should be noted that pits with the stretched areas are present mainly on the samples destroyed under the creep conditions in a constant magnetic field of 0.3 T.