Physical Mesomechanics最新文献

The elastic properties of a number of binary titanium alloys Ti–Me (Me = V, Nb, Mo, Ta) with a body-centered structure were calculated using the exact muffin-tin orbital method in the coherent potential approximation. It is shown that the elastic constants C₁₁ and C₁₂ increase with concentration of the second component in β-Ti–Me alloys, although the latter weakly depends on the concentration. However, C₄₄ decreases in the presence of V and Nb and increases in the presence of Mo and Ta. According to the calculated densities of electronic states, the concentration behavior of C₁₁ is due to an increase in chemical bonding with the second neighbors, which is most pronounced with an increase in the number of d electrons of the alloying element. It is found that all the studied binary alloys have the lowest Young’s moduli near the β-phase instability region and in the ‹100› direction. With growing tantalum concentration, the anisotropy of Young’s modulus decreases, but its pattern remains unchanged. However, V-, Nb- and Mo-containing alloys become practically isotropic at a certain concentration of the second component, and their anisotropy pattern changes. In general, the obtained elastic characteristics of binary titanium alloys are in good agreement with the available experimental and theoretical data.

This paper examines the nature of h- and S-type plastic flow instabilities within the concept of localized plasticity au-towaves. It is shown that both of these instabilities can be observed in the same ARMCO iron material in the form of switching (h-type instability) or excitation (S-type instability) autowaves. The switching autowave represents the localized deformation front uniformly moving under a constant stress, and the excitation autowave represents the same front, but moving with a constantly decreasing velocity with reducing stress. The switching autowave passes continuously through the object, but the excitation autowave propagates intermittently. The manifestation of one or the other wave is deter-mined by the temperature-strain rate conditions. There is an interval of low temperatures where, regardless of the strain rate, only a switching autowave is generated, and the deformation front velocity increases exponentially with increasing stress. An excitation autowave can generate at high temperatures, when the deformation front moves abruptly during stress drops. This phenomenon can be interpreted in terms of dynamic strain aging. Under such conditions, the front velocity depends linearly on the stress. It is shown that the deformation front velocity is always determined by local strain rates at the front. Using the dislocation approach to dynamic strain aging and by analyzing the dependences of local strain rates on the effective stress, it is established that the switching autowave (h-type instability) is controlled by thermally activated motion of dislocations, and the excitation autowave (S-type instability) is governed by their viscous overbarrier motion.

Superplastic deformation is ensured by grain boundary sliding, which is accommodated by dislocation and diffusion creep. The contributions of these mechanisms depend on the microstructural parameters of alloys and temperature-strain rate conditions of deformation. In this work, the deformation behavior, grain structure parameters, and the contributions of the superplastic deformation mechanisms at the temperature 0.87Т_m and constant strain rate 1 × 10^–2 s^–1 were compared between the rolled Al-5Mg-0.9Fe-0.83Ni-0.14Zr-0.08Sc-0.72Zn alloy with the initial nonrecrystallized grain structure and the recrystallized alloy treated by multidirectional isothermal forging (MDF). The strain rate sensitivity coefficient was ~0.5, and the elongation reached 400–520% under the specified conditions. Superplastic deformation of the rolled alloy resulted in dynamic recrystallization providing strain weakening. In the MDF-treated alloy, deformation was accompanied by strain hardening due to grain growth. Regardless of the treatment mode, at the steady flow stage, the alloy shows similar microstructural parameters and similar contributions of the deformation mechanisms, which were determined from microstructural evolution on the surface with markers. The contribution of grain boundary sliding was ~40%, and intragranular dislocation slip/creep provided was ~20%. The remaining strain localized near transverse grain boundaries with the formation of striated zones, which were associated with grain boundary sliding and diffusion creep mechanisms.

The paper gives a brief review of thermodynamic theories that take into account the structure of materials and phase interfaces. Among them are classical thermodynamics, surface thermodynamics, thermodynamics with internal variables, and extended thermodynamics. It is shown that thermodynamic phase field theory has common features with both extended thermodynamics and thermodynamics with internal variables, but is not identical to them. Coupled phase field equations for binary systems under nonisothermal conditions are deduced based on classical thermodynamics of irreversible processes. Free energy is expanded in a series in temperature gradients, concentration, and phase variable. The obtained general relations are in agreement with the known ones. The new derivation method allowed refining the basic equations. It is shown that not only models of phase transitions with an infinitely narrow interface but also the theory of a two-phase region can be considered as a limiting variant of the theory. The effective transfer enthalpy and the enthalpy due to the evolution of the phase variable are explicitly given in the equations. It is shown that the general relations are equivalent when the basic equations are derived by any of the thermodynamic potentials. Phase field relations are deduced for a regular solution, and the physical meaning of different summands is clarified. The basic equations of phase field theory are derived with consideration for possible chemical interaction of the components.

Dedicated to the 40th anniversary of the Institute of Strength Physics and Materials Science SB RAS. The paper reviews the current state of research in the field of sliding friction and wear and the related production technologies with an emphasis on the works of the Institute of Strength Physics and Materials Science SB RAS. These works gave fundamental results on structural and phase transformations of materials below the worn surface, which were later used as a basis for the development of wear-resistant materials, hard coatings, lubricants, and friction processing methods. The main mechanisms of deformation and metal flow in the subsurface layer during adhesive friction and friction stir welding are considered. The development of mathematical modeling of friction and wear processes for various materials is traced. Computer modeling of friction and wear processes allowed solving a number of theoretical and applied problems of plastic flow of materials in the sliding contact zone and the development of surface processing methods. The obtained results were used to explain material flow in friction stir welding. The evolution of industrial technologies based on unlubricated and adhesive friction is reviewed.

Experimental results on the response of solids to shock loading obtained by the original method capable of real-time recording of mesoscopic characteristics contradict the conventional representation of elastic-plastic transition within the local equilibrium concepts of continuum mechanics. Nonlocal mathematical modeling of spatiotemporal correlations in a shock pulse based on nonequilibrium statistical mechanics allowed uniting the obtained experimental data within the new physical representation of shock wave phenomena where the mesoscopic carriers of deformation were interacting wave packets. Microstructural investigations of the studied materials reveal different scales of deformation in three ranges of shock velocities, which are responsible for macroscopic properties of materials.

This work studies how the variable nonlocal parameter is related to the material variations across a functionally graded (FG) nanobeam. Hamilton’s principle is used to derive the governing motion equations for a FG nanobeam within the refined higher-order state-space strain gradient theory. The presented formulation is tested numerically via Navier’s solution for a simply supported FG nanobeam. A comparison with the existing published findings is performed to show the precision of results. Furthermore, the effects of the nonlocal parameters of the ceramic and metal parts, as well as temperature, magnetic potential, and electric voltage on the free vibration response are investigated.

In this work, a model was developed for determining the critical load of a two-layer ceramic-metal composite under three-point loading based on the analysis of the local influence of distributed defects on the stress field. The stressed state is defined as the solution of a boundary value problem for a solid. The object of investigation was a two-layer TiB/Ti-based ceramic-metal composite obtained by free SHS (self-propagating high-temperature synthesis) compression. A method was developed for determining the statistical distribution of defects within a specimen based on metallographic cross-sectional analysis. The critical load was determined by the defect size probability density found from the experimental data. The cases of flat and sinusoidal interfaces between the composite layers were considered. A digital model of a two-layer beam deformed under three-point loading was built in Abaqus finite element software, which was used for numerical simulation of the stress field. Based on the numerical results, the stress field was corrected for the sinusoidal interface. The influence of the corrected stress field on the specimen strength was analyzed when the defect distribution pattern was similar to that in the composite with the flat interface. It was shown that the sinusoidal shape of the interface had no effect on the specimen strength, provided that the period and amplitude of deviation from the rectilinear shape were small relative to the linear size of the specimen.

This work examines the bending and vibration responses of a functionally graded (FG) 2D nanostructure resting on the viscoelastic foundation. The FG structure properties vary gradually in the thickness direction. In this investigation, three porosity patterns are examined. The nonlocal equilibrium equations are derived by Hamilton’s principle using Eringen’s nonlocal elasticity theory, which incorporates the integral plate theory with a reduced number of unknowns. The results computed for the studied simply supported FG nanoplates are compared with those published in the open literature. Several parametric studies are performed to illustrate various influences of the plate geometry, material inhomogeneity, elastic damping coefficient, and nonlocal effect on the stresses, frequency, and central deflection of FG nanoplates.

The work is devoted to magnetic materials based on (Fe₆₂Cr₂₄Co₁₄)_81+xB_18–xSi₁ (at %) alloys, where x = 0, 2, and 4. The materials were prepared in the form of metallic ribbons by melt spinning, i.e. rapid quenching from the melt onto a rotating copper disk in an inert atmosphere. In the as-spun state, ribbons had an amorphous structure. The structure and phase transformations in the alloys upon heating were analyzed using X-ray diffraction methods. The dependences of the magnetic moment and heat flow of the alloys heated at the same heating rate were found. Variation in the phase composition during crystallization of the amorphous structure and its influence on the magnetic properties of the alloys were studied. It was found that, for a highly coercive state to form, the alloy structure should present a mixture of the α and Fe₃B phases, which is also characterized by high thermal stability and microhardness. The alloys demonstrate the coercive force 20.8–43.2 kA/m, saturation magnetization 0.70–1.15 T, residual magnetization 0.36–0.54 T, and microhardness 1350 ± 90 HV after crystallization heat treatment. The (Fe₆₂Cr₂₄Co₁₄)₈₁B₁₈Si₁ alloy has the best set of properties: high thermal stability, a decrease in the saturation magnetization (by 22%) and coercive force (by 13%) in the range from room temperature to 500°C.