Physical Mesomechanics最新文献

The paper analyzes a numerical model of an “active medium” with linear elasticity and a negative initial dissipation constant dynamically renormalized under deformation. The analysis shows that such a system, being seemingly unstable over a wide range of geometries and origins of deformation, can spontaneously reach stable dynamic modes in which its time- and space-alternating dissipation forms complex quasiperiodic patterns and its total volume (length, area) oscillates on a large scale. The results presented in the paper are of interest in academic terms and in terms of mechanical and biological application.

The effect of tantalum on the impact toughness of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from –40 to +120°C with determining the ductile-brittle transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the total fracture toughness versus temperature curve is 30–50 J/cm² higher across the entire temperature range, including the upper and lower shelf energies, and the ductile-brittle transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller prior austenite grain size, a larger average size but lower density of M₂₃C₆ carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

Refractory high-entropy alloys (HEAs) are a new class of metallic materials based on group 4–6 elements of the periodic table with possible additions of Al, Si, Re, C, or B. Some single-phase refractory HEAs can maintain high strength up to 1600°C, while multiphase compositions have more attractive specific properties at temperatures up to 1200°C. Here we examine the structure and mechanical properties of refractory HEAs Nb₃₀Mo₃₀Co₂₀Hf₂₀, Nb₃₀Mo₃₀Co₂₀Zr₂₀, and Nb₃₀Mo₃₀Co₂₀Ti₂₀ (at %). The alloys consisted of an intermetallic B2 matrix and particles of a disordered bcc phase, as well as a minor volume fraction of additional bcc (Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀) or fcc (Nb₃₀Mo₃₀Co₂₀Ti₂₀) phases. When tested for uniaxial compression, Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy showed a higher yield strength in the temperature range of 22–1000°C than Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloys. Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloy did not fail at temperatures of 22–800°C to a given 50% strain, while Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy turned out to be brittle. All alloys demonstrated high strain hardening in the temperature range of 22–800°C, and they can compete in terms of specific strength with commercial nickel and cobalt superalloys.

The study discusses the effect of the local stress state at the crack tip on the fracture behavior of coarse- and ultrafine-grained bcc, fcc and hcp materials under single impact and static loads. The local stress state of the materials at the crack tip under impact and static loading was evaluated by the h_max/t ratio, where h_max is the maximum depth of the plastic zone under the fracture surface and t is the specimen thickness. The depth of plastic zones under the fracture surface was determined using layer-by-layer etching of the surface with subsequent X-ray diffraction analysis. The study results showed that it is not always possible to establish an unambiguous relationship between the fracture mechanisms of metallic materials and the local stress state of a material at the crack tip. Nevertheless, some particular features were found: (i) the cleavage, quasi-cleavage or intergranular brittle fracture of materials, regardless of the lattice type, is indicative of plane strain, (ii) under plane stress, all materials, regardless of the lattice type, exhibit ductile fracture with the formation of a microdimple pattern, and (iii) most fcc materials fail by a mixed mechanism in the transition region from plane strain to plane stress.

This paper presents the numerical and experimental results of hardening of an annular zone on the flat surface of an X20Cr13 steel specimen by friction stir processing (FSP) with a WC-Co hard alloy tool moving along circular and fan-shaped paths. A finite element model of the process is proposed for predicting the temperature distribution through the width and depth of the annular zone for the considered tool paths and for detecting the reverse tempering regions. The influence of the paths of a cylindrical friction stir tool with a flat end on microhardness distribution in the surface layer of the hardened zone was studied experimentally. It was shown that FSP along the fan-shaped path provides uniform hardening of the annular zone, while processing along the circular trajectory leads to softening of the material in the regions where the friction tracks overlap. The uniformity of surface hardness in the friction stir processed annular zone of X20Cr13 steel was evaluated by calculating the “covering uniformity” (CU) index proposed by Campana. The hardening behavior is in full agreement with the results of finite element simulation of the FSP process. Hardness measurements and microstructural studies showed that the fan-shaped tool path provides surface layer hardening to a depth of 400 μm with the CU index ranging from 0.78 to 1.00. In the case of the circular path, the CU index ranges from 0.48 to 0.72 at the same depth. The proposed research methods can be applied to evaluate the FSP efficiency when using other workpiece and tool materials.

The microstructure and phase composition of electron beam welded Ti-6Al-4V titanium alloy were studied by optical and scanning electron microscopy, backscattered electron diffraction, and X-ray diffraction analysis. Deep-penetration electron beam welds were made in a single pass on rectangular Ti-6Al-4V samples obtained by rolling and wire-feed electron beam additive manufacturing. It was found that the weld width in the 3D printed Ti-6Al-4V samples is greater than in the rolled material. The influence of the vapor capillary on the size, shape and structure of primary β grains formed in the fusion zone was shown. The variation of the penetration coefficient, volume fraction of the residual β phase, and residual stresses along the weld length was studied for Ti-6Al-4V samples obtained by both rolling and 3D printing.

The paper analyzes the structure, and the mechanical and tribological properties of Ti–Cr–C–Ni–Fe coatings formed on R6M5 high-speed steel in four modes of electrospark deposition (ESD) using TiC–NiCr electrodes manufactured by extrusion in combination with self-propagating high-temperature synthesis (SHS). The analysis shows that the coatings formed in the four ESD modes at a discharge energy of 0.2, 0.3, 0.6, and 1.0 J are composed mainly of Cr–Ni–Fe–C_solid and Fe_0.7Ni_0.3 matrix phases strengthened with Ti_0.8Cr_0.2C particles the size of which decreases to less than 100 nm in going from the coating surface to the substrate. In the SHS electrode during the deposition, most carbide particles are melted. Increasing the discharge energy increases the fraction of solid grains transferred to the substrate by a factor of up to 9. The dependences of the total anode mass loss and total cathode mass gain on the ESD time in the four modes have a classical form. Also considered are the discharge energy dependences of the SHS electrode transfer coefficient, coating run-in length, and wear of the coating and counterbody. The coating hardness measures 10.6–13.5 GPa.

Experimental and numerical studies were conducted on AlSi12 alloy fabricated by wire-feed electron beam additive manufacturing to examine the structure, thermomechanical behavior and fracture of a eutectic microvolume at the scale of several microns. Dynamic boundary value problems were solved under plane strain conditions. The composite structure of the eutectic phase consisting of an aluminum matrix and silicon particles was taken into account explicitly in the calculations. Isotropic models of the thermoelastoplastic matrix and elastic-brittle particles were implemented in ABAQUS/Explicit. Composite deformation was calculated both with and without allowance for residual stresses caused by cooling of the composite after its fabrication. It was shown that after the cooling of the eutectic, silicon particles are compressed, and the aluminum matrix is under both bulk compressive and tensile as well as under pure shear stresses. It was found that residual stresses play a negative role at the stages of intense deformation of the composite. The fracture strain of the eutectic strongly depends on the yield point of the matrix, while the ultimate fracture stress varies but only slightly. Favorable morphology of silicon particles was determined which prevents early fracture of the eutectic.

This paper explores the structure and changes in the mechanical properties, chemical composition and surface morphology of aluminum alloys AA5056, AA2024 and Grade 2 titanium alloy after high energy impact during plasma cutting. The studies show that plasma cutting causes the formation of a subsurface layer with a dendritic structure typical of cast material and with a partially altered chemical composition. The subsurface layer material is significantly softened when cutting heat treated alloy AA2024, but changes slightly when cutting AA5056 alloy. During plasma cutting of Grade 2 titanium alloy in shielding atmosphere, the presence of even a small amount of atmospheric oxygen leads to the formation of oxides in the layer closest to the surface, which have microhardness values more than 5–7 times higher than the base metal hardness. Below the surface layer with a molten structure, a heat-affected zone is formed where the structure of the base metal is changed as a result of thermal influence. Significant changes in this zone are characteristic only for heat treated alloy AA2024. Metal flow in the cutting zone initiated by the plasma jet and shielding gas flow occurs in both the laminar and vortex modes. Nonuniform metal flow in the cutting zone and nonoptimal process parameters lead to the formation of structural heterogeneities and defects of different structural and scale levels on the surface of the samples.

A systematic analytical study was conducted on the mathematical properties of a previously proposed prototype of a nonlinear Maxwell-type constitutive equation for describing the shear flow of thixotropic substances (viscous liquid polymers, viscoelastic melts, concentrated solutions, pastes, emulsions). The equation takes into account the mutual influence of deformation and structural evolution (the kinetics of intermolecular bond formation and breaking) on viscosity and shear modulus and the effect of deformation on this kinetics. In the uniaxial case, the constitutive equation is governed by a nondecreasing material function and six positive parameters. The equation is reduced to a set of two nonlinear autonomous differential equations for the stress and the crosslinking degree. It is proved that the equilibrium point of this set is unique. The dependences of the point coordinates on all material parameters and on the shear rate for an arbitrary nondecreasing material function are investigated in general form, and all the dependences are proved to be monotonic. Equations for the flow and viscosity curves are derived and investigated. It is proved that the model leads to an increasing shear rate dependence of the equilibrium stress and to a decreasing apparent viscosity curve, which reflect the typical properties of the experimental flow curves of pseudoplastic materials. Using six arbitrary governing material parameters and the governing material function, we analytically study the phase portrait of the nonlinear set of two differential equations, to which the model is reduced, for dimensionless stress and crosslinking degree near its only equilibrium point. It is proved that the equilibrium point is always stable and can be of three types only: a stable node, a degenerate node, or a stable focus. The existence criteria for each type are found in the form of explicit constraints on the material function, model parameters, and shear rate. A stable focus indicates the nonmonotonicity of the set solutions and the existence of deformation modes with damped oscillations of stress and crosslinking degree upon reaching steady-state values. The influence of the material parameters and material function on the type of equilibrium point and on the behavior of the model integral curves is analyzed.