No 1 (2020)

Most of the advanced construction and functional materials are elastically nonuniform, moreover, for many of them, the elongated holes and inclusions are typical, which are similar to a cylinder in form. The strength and physicochemical properties of a material, to a great extent, depend on the peculiarities of the strain-stress state of the near-surface and boundary layers of the materials in the heterogeneous systems. The development of the processes of elastic deformation and fracture in these areas, to a large extent, determines the mechanical behavior of a material in general and arouses much interest. The authors study the influence of interfacial stresses on the strain-stress state of elastic bimaterial with smooth waveform interface; consider the 2-D solid mechanics problem of an elastic body with nanoscale boundary surface texture, which appears between the nearly circular inclusion and the matrix. It is expected that a body is situated within a uniform stress field. To solve the problem, the authors used the simplified Gurtin-Murdoch’s surface/interface elasticity model, where the interfacial boundary is the negligibly thin layer exactly bordered on the bulk phases. It is acknowledged that there are no displacement discontinuities on the interfacial boundary, and the stress jump is determined by the effect of surface/interfacial stress according to the generalized Laplace-Young law. Using the boundary perturbation method, the problem solution for each approximation is limited to a singular integrodifferential equation against the unknown surface/interfacial stress. The paper gives the numerical results for the problem to a first approximation. As a result, the authors carry out the comparative analysis of the strain-stress state using the finite-element method and analytical boundary perturbation method.

Currently, the selective laser melting technique (SLM) with the use of powder metallic materials is a promising area in the aircraft and engine technology. Due to this technique, it is possible to produce parts with any complexity configuration at fewer expenses for tooling and mechanical processing and the prototyping of goods becomes simpler as well. The issue of application of powder materials of heat-resisting alloys in the additive production is particularly topical, which is due to the problems caused by their complex chemical composition, insufficient thermal conductivity, and shrinkage tendency. The paper studies the influence of laser output power on the microstructure and properties of specimens of Inconel 738 heat-resisting nickel alloy produced with the help of a commercial 3-D printer using the SLM printing technology. Moreover, the authors considered the way of improvement of the specimens’ mechanical properties through the improvement of microstructure after SLM and further heat treatment. The authors carried out the metallographic and electron microscopic study of the initial material and the specimens grown using the SLM technology at the laser output power of 75, 100, 125, and 325 W; analyzed the microstructure evolution in the result of heating caused by the growth of supply energy. Further heat treatment made it possible to study the influence of step quenching on the microstructure and mechanical properties of specimens. Further heat treatment made it possible to study the influence of step quenching on the microstructure and mechanical properties of specimens. The authors determined the optimum technological parameters of laser emission to produce parts from Inconel 738 heat-resisting alloy using the SLM technique and produced parts with the minimum quantity of the defects. The study identified that heat treatment, including step quenching, improves the mechanical properties - ultimate resistance, yield limit, and percent elongation - through the “healing” and the defects’ size reduction.

Shape memory alloys belong to the class of functional materials with unique properties that make them useful in many engineering applications. Since in the austenitic state, due to the pseudoelasticity effect, such alloys have a significant damping capacity, and one of the possible applications is vibro-protection devices. Working elements of damping devices made of shape memory alloys are used in the conditions of cyclically varying stresses and/or temperature. The theoretical models adequately describing such behavior make it possible to advance the efficiency of damping devices. The paper aims at the microstructural modeling of alternating deformation of the sample of TiNi shape memory alloy. Phase transformations in the materials with the martensite channel of inelasticity take place with the release and absorption of heat, which can lead to a shift in the working temperatures of the element and the change in its functional properties. Consequently, when theoretically describing the mechanical behavior of the material, the authors considered heat release at the direct transformation and heat absorption at the reverse transformation. Within this study, the authors implemented such consideration for the adiabatic regime of alternating deformation and compared the obtained data with the results of modeling of isothermal alternating deformation. When calculating, the authors took into account the irreversible strain accumulation at cycling, which, in the real device, can cause the change in its working characteristics and operational life loss. The study showed that taking into account the latent transformation heat during cycling in the strain-controlled regime increases the maximum stresses in the cycle and reduces the volume fraction of the resulting martensite. When taking into account the microplastic strain, the deformation loop evolves. In this case, in the adiabatic regime in the first cycles, the temperature increases, later on, about by the seventh cycle, the temperature increase slows down, and the average temperature ceases to change markedly.

Currently, many technical problems require a comprehensive study of the properties of materials operating in hydrogen-containing environments. The authors investigated the effect of age-hardening on the hydrogen embrittlement and fracture micromechanisms of high-nitrogen austenitic Fe-23Cr-17Mn-0.1C-0.6N (wt. %) steel. For this purpose, using heat treatments, the authors formed in specimens of Fe-23Cr-17Mn-0.1C-0.6N steel the structural phase states characterized by different distribution and content of dispersed phases. The experiment determined that the accumulation of hydrogen atoms occurs predominantly in the grains in solution-treated specimens without dispersed phases. This causes the effects of solid solution hardening and leads to a change in the micromechanism of steel fracture from a ductile dimple fracture in the absence of hydrogen to a transgranular fracture by the quasi-cleavage mechanism in hydrogen-charged specimens. It was established that the discontinuous decomposition of austenite with the formation of Cr₂N cells and austenite depleted in nitrogen, predominantly along the grain boundaries causes the formation of a large fraction of interphase (austenite/Cr₂N particles) boundaries. Cells of discontinuous decomposition promote hydrogen accumulation along the grain boundaries and cause brittle intergranular fracture of hydrogen-charged specimens during plastic deformation. The study showed that in specimens with the discontinuous decomposition of austenite both along the grain boundaries and spreading into the grain body, plenty of intragranular interphase boundaries (Cr₂N plates in austenite) are formed, which causes the formation of a transgranular brittle fracture in the hydrogen-charged specimens.

Due to the combination of lightness, high specific strength and corrosion resistance, titanium and its alloys are highly interesting for applying in many areas of industry (mechanical engineering, shipbuilding, and aircraft manufacturing). Technically pure titanium is the first choice to be used in medicine because of its high biocompatibility and lack of toxic elements. Pure titanium has high ductility and corrosion resistance but it is inferior to titanium alloys in other mechanical characteristics, such as tensile strength, yield strength, and hardness. Megaplastic deformation (MPD) is a promising method for increasing the strength of titanium to the level of highly alloyed alloys. The paper deals with the study of the influence of MPD in the Bridgman chamber on the structure (phase transformations occurring in technically pure VT1-00 and VT1-0 titanium), corrosion resistance, and microhardness. Using the high-pressure torsion (HPT), the authors obtained samples with different degrees of deformation: from 0.25 to 4 revolutions of the movable anvil. The authors carried out the X-ray diffraction analysis and electrochemical tests of samples and studied the phase composition of titanium samples of two grades containing 0.1 and 0.3 % of impurities before and after MPD. The study identified that the HPT led to the formation of a two-phase mixture a+ɷ. The results showed the positive effect of MPD on the mechanical properties of titanium. The microhardness of the deformed material increases in comparison with the initial state, while there is no deterioration in the corrosion resistance in the studied environment. Under all deformation modes, titanium stays in a passive state. For the VT1-0 alloy, the stationary corrosion potentials of samples after HPT have a more positive value compared to the original undeformed material.