Physical Mesomechanics最新文献

The paper presents the history of the autowave model for localized plastic flow developed at the ISPMS SB RAS and describes the fundamentals of the autowave approach to the problem of inhomogeneous plastic flow based on the idea of plastic flow localization. Attention is drawn to the historical aspect of the development of new ideas about the phenomenon of plasticity. The main relations of the autowave plasticity model (conformity principle, elastoplastic and mass invariants of plastic strain) are given, and its technical applications are considered. The two-component model is used to develop the theory of inhomogeneous plastic flow, taking into account the correlated (collective) properties of ensembles of deformation carriers. It is shown that solutions of the obtained equations describe patterns observed at all stages of strain hardening.

The paper reports on experiments and numerical simulation of high-speed impact loading of additively manufactured heterogeneous specimens. It is shown that the proposed model of direct numerical simulation of a heterogeneous material based on random distribution of given materials over difference cells accurately reproduces the processes of deformation, fracture, and cratering under impact loading. A series of calculations are made to determine the ballistic efficiency of additively manufactured heterogeneous metal-ceramic specimens using the DoP (depth-of-penetration) method. A procedure is proposed for estimating the efficiency of heterogeneous metal-ceramic specimens, revealing local maxima in it.

Diffusion of oxygen and nitrogen in titanium nitride was studied using the projector augmented wave method in combination with transition state theory. Atomic migration energies were calculated for two diffusion mechanisms (interstitial and vacancy ones). It was found that the oxygen migration energy by the interstitial mechanism is ~0.3 eV lower than that by the nitrogen vacancy mechanism. However, the indirect mechanism of diffusion through the body-centered position of the cubic lattice formed of titanium and nitrogen atoms is more preferable. The estimation of the temperature-dependent coefficient of oxygen and nitrogen diffusion in titanium nitride by the two mechanisms showed their strong dependence on the concentration of thermal vacancies. It was shown that the interstitial diffusion of nitrogen occurs at temperatures below 1500°C, and the vacancy diffusion mechanism prevails at high temperatures. The calculated activation energies and diffusion coefficients showed good agreement with the experimental values. At high concentrations of constitutional vacancies, the coefficients of oxygen diffusion by both mechanisms are comparable with the experimental values for TiO₂, and the values obtained at low concentrations remain several orders of magnitude higher than those for Al₂O₃.

Wave propagation analysis can be employed in various fields, such as nondestructive testing and structural health monitoring, which makes it so interesting and attractive. In the present investigation, an analytical method based on an exponential function was used to solve the wave propagation problem in functionally graded (FG) beams with consideration for imperfection via refined higher-order shear deformation theory. The recently developed porosity-dependent homogenization model was used to analyze the influence of imperfection on the wave dispersion behavior of porous beams. Material properties of FG beams were assumed to change across the thickness. The conventional porosity model illustrates a linear relationship between the porosity coefficient and material properties. However, the influence of porosity is actually characterized by a nonlinear relationship. This statement rose from some experimental investigations. To examine the interchange between the porous beam and foundation, Winkler–Pasternak two-parameter models were used as the elastic foundation. Uniform temperature change is taken into account to study the thermal environment effect. The principle of Hamilton is implemented to derive equations of motion for imperfect FG beams. The obtained governing equations were analytically solved. The influence of the wave number, porosity coefficient, temperature change, gradient index, length-to-thickness ratio, Winkler and Pasternak coefficients on the wave propagation in porous FG beams was studied.

The paper reports on physical experiments on disintegration of reinforced concrete by the electric pulse method based on the Vorobiev effect. Concrete is fractured under the action of a compression wave propagating from the discharge channel between the electrode on the concrete surface and the reinforcement. Disintegration experiments are conducted on pebble concrete. It is shown that a single pulse results in separate cracks in the material on retention of its integrity. Disintegration of concrete and cavitation at the point of application of the electrode begin after the second or third pulse. Computer simulation is made for the action of an expanding discharge channel on reinforced concrete. A structural model of reinforced concrete is plotted, explicitly taking into account its main constituents, namely, cement, stone inclusions, and reinforcement. The inelastic behavior of cement is described within the modified Drucker–Prager–Nikolaevsky model with the nonassociated flow rule for quasi-brittle media. Cracking is simulated using the fracture criterion based on tensile stresses. The performed numerical simulation confirms the conclusions of the physical experiment: a single pulse causes the formation of separate cracks parallel to the free surface, and the network of horizontal, vertical and inclined cracks appears in the cement after 2–3 pulses, resulting in a cavity at the point of application of the electrode. Cavitation in reinforced concrete is governed by the presence of stone inclusions, whose boundaries serve as sites of redistribution of maximum tensile stresses and formation of vertical and inclined cracks, as well as of accumulation of irreversible strains and stresses retained in the cement after the first pulse.

A ribbon of soft-magnetic high-entropy alloy Fe-Co-Ni-Si-B with the nonequiatomic composition and the thickness of ~70 μm was produced by spinning. Its structure, mechanical, tribological and magnetic properties were analyzed by experimental methods of the modern materials science. It was found that the studied material is in an amorphous (X-ray amorphous) state. The microhardness of the ribbon was HV = 8 GPa. Transmission electron microscopy on electrolytically polished foils showed that the size of structural elements of the ribbon did not exceed 10 nm. Ion etching led to partial crystallization of the foil and growth of nanocrystallites to several tens of nanometers. The tensile strength of the ribbon was more than 590 MPa at a low elongation to failure (1%). The distribution patterns of the longitudinal and transverse strain components were constructed, according to which no strain macrolocalization occurred up to fracture. The wear rate in the longitudinal direction of the ribbon was more than 4 times higher than that in the transverse direction. The magnetic properties were characterized by a hysteresis loop, with the maximum value of the specific magnetic moment being ~120 emu/g.

The work is devoted to the theoretical study of the nucleation and propagation of Lüders bands on the yield plateau and of moving Portevin–Le Chatelier bands of type A (solitary waves of plastic flow) at the stage of parabolic hardening during strain aging. The proposed model considers collective deformation modes on the spatiotemporal meso- and macroscales. Strain aging changes the state of a deformable medium on the mesoscale. Deformation of a medium under constant-rate uniaxial tension is described by a system of two coupled nonlinear parabolic equations for dynamic order parameters. The coefficients of these equations depend on the impurity concentration. On the yield plateau, solutions of the equations in the form of a switching wave describe the nucleation (at the yield drop stage) and propagation of Lüders bands. Depending on the temperature and rate of deformation during strain aging, a yield drop may be repeated on the yield plateau. Its formation changes the mode of Lüders band propagation from constant-velocity continuous to discrete one. At the strain hardening stage, the nucleation and propagation of the Portevin–Le Chatelier band are described by solutions in the form of a traveling autosoliton (a solitary wave of plastic flow).

Layered amorphous-crystalline TiNiCu alloy ribbons produced by ultrarapid quenching from the liquid state (melt spinning technique) show the two-way shape memory effect without additional processing, which makes them applicable to various micromechanical devices (microtweezers) for gripping and manipulating microobjects. The present work is devoted to the study of the influence of the rejuvenation process (cryogenic thermal cycling) and the thickness of the crystalline layer on the structure and functional properties of quasi-binary TiNi-TiCu alloy with the copper content 25 at %. It is shown that thickening of the crystalline layer significantly increases not only the enthalpy of martensitic transformation but also its critical temperatures and affects the alloy crystallization pattern and temperatures. Rejuvenation treatment transforms the interface between the amorphous and crystalline layers and changes the ratio between the B19 martensitic phase and the residual B2 austenitic phase in the martensitic state, which affects the martensitic transformation parameters. In addition, cryothermal treatment causes a noticeable increase in reversible strain (magnitude of the two-way shape memory effect) and significantly narrows the temperature hysteresis of shape changing, which can improve the functional properties of microdevices based on rapidly quenched amorphous-crystalline ribbons.

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: the yield stress σ_0.2 = 1140 MPa, ultimate strength σ_В = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain transition-metal carbides Fe₂C. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which appears as the Portevin–Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.

The specimens of Ti–2 wt % Fe alloy were annealed at three different temperatures, in the β-Ti, α-Ti + β-Ti and α-Ti + TiFe fields of the Ti–Fe phase diagram, then water quenched and subjected to high-pressure torsion (HPT). The X-ray diffraction analysis showed that the main phase in all annealed specimens was the α phase (more than 90%), while the main phase after HPT was the ω phase. Hardness H and Young’s modulus E were determined by nanoindentation at the center, in the middle of the radius, and near the edge of each specimen. It was found that the H and E values were different for specimens annealed at different temperatures and depended on the radial coordinate of the indentation region. The maximum H values were obtained in the middle of the radius of the specimens. The E values of all specimens decreased from the center to the edge, reaching very low values. The paper discusses structure transformations during HPT, the behavior of the radial dependences of H and E, and probable causes of a strong decrease in E values.