Physical Mesomechanics最新文献

Shock wave loading of heterogeneous materials was numerically investigated using three models: a homogeneous alloy model with experimental parameters, an additive approximation model with parameters calculated from the constants and concentrations of the components, and a discrete numerical model constructed based on a random concentration distribution of components over the sample volume. The verification of the computational schemes was done by calculating the shock wave loading of homogeneous materials. Hugoniot curves were plotted and compared with experimental data to show a less than 5% deviation of the numerical results. A series of numerical simulations of spall fracture in homogeneous plates revealed that the free surface velocity profile resulting from spall fracture corresponds to the experimental profile. A relationship was derived to determine the ultimate spall strength for a heterogeneous medium based on the fracture parameters of its homogeneous components. The found homogeneous material parameters were used to simulate the shock wave loading of plates made of nickel titanium and tungsten carbide/cobalt cermet constructed with heterogeneous models. It was shown that the heterogeneous models can be effectively applied to problems of shock wave loading with spall fracture, and the deviation between the calculated free surface velocity of a heterogeneous plate and the experimental data does not exceed 10%.

This paper considers the application of two known models for the evaluation of distortion in high entropy shape memory alloys with the B2 structure. Distortion was calculated using the hard sphere and soft sphere models for binary TiNi and senary Ti-Hf-Zr-Ni-Cu-Co alloys with different chemical composition. It was shown that both models could not be used because they gave a high distortion value even for binary Ti₄₉Ni₅₁ alloy. Distortion was so large for all alloys that they must be amorphous. However, this contradicted the experimental data according to which all alloys were crystalline with the B2 structure. A modification of both models was proposed taking into account that the TiNi alloy is an intermetallic compound. The formation of intermetallic compound was accompanied by a change in the spatial distribution of electron density around the nuclei of interacting atoms, which led to a change in the atomic sizes. The proposed modification gave the distortion value that was consistent with the lattice stability criterion for alloys where the concentration of each alloying element did not exceed 5 at %.

Technological metal forming processes involving hot and superplastic deformation are sensitive to temperature and strain rate. This is because inelasticity mechanisms operate in different ways under different conditions, leading to the formation of different structures and therefore different effective physical and mechanical properties of the material. Optimal temperature and strain rate conditions for the forming process which provide improved performance of the resulting products with acceptable energy consumption (or, conversely, minimum energy consumption with acceptable performance characteristics) can be most effectively determined by mathematical modeling. The key elements of the latter are constitutive models for describing the behavior of the material (physical equations), which account for the influence of temperature and strain rate on various mechanisms of inelastic deformation. Such constitutive models can be most effectively developed using a multilevel approach based on the introduction of internal variables, crystal plasticity, and an explicit description of the material structure and physical deformation mechanisms. There are many works that propose multilevel mathematical models of metals that somehow explicitly account for the temperature and strain rate effects on inelastic deformation. Based on physical considerations, this analytical review defines the most promising approach to constructing multilevel constitutive models with comprehensive consideration of the temperature and strain rate effects.

Nanoparticles are mesoobjects that occupy an intermediate position in size between ordinary molecules and macroscopic particles. Suspensions with nanoparticles, called nanofluids, are also specific mesoscopic suspensions. Today it is known that their thermophysical and mechanical properties are not described by classical theories. The unusual properties of these dispersed fluids make them extremely popular in a wide variety of applications. However, successful use of nanofluids involves the prediction of their properties, which in turn requires systematic studies. This paper presents an experimental study on the thermophysical properties of water- and ethylene glycol-based nanofluids with aluminum and copper particles. The thermal conductivity, rheology and electrical conductivity of the nanofluids were systematically studied. The weight concentration of nanoparticles varied from 2.5 to 20%. It was shown that the thermal conductivity of the nanofluids significantly exceeds that of nanofluids with oxide particles. Its higher values are determined by the size of the nanoparticles and the thermal conductivity of the base fluid. The nanofluids studied are either pseudoplastic or viscoplastic. Their rheology is determined by the concentration and size of nanoparticles. The smaller the size of nanoparticles and the higher their concentration, the more likely the change in rheology is. Correlation curves were constructed for the rheological parameters of nanofluids versus the concentration and size of nanoparticles. It was found that the electrical conductivity of nanofluids increases almost linearly with increasing nanoparticle concentration and strongly depends on the nanoparticle size. The electrical conductivity mechanisms of nanofluids were discussed. The speed of sound in the nanofluid and its dependence on particle size were measured.

X-ray diffraction studies were conducted to examine changes in the structural-phase state and dislocation density of Ti_49.8Ni_50.2 alloy depending on the isochronal annealing temperature after severe plastic deformation by abc pressing at 573 K. The total true strain achieved in the alloy specimens during abc pressing was e = 9.55. Isochronal annealing was carried out for 1 h at 573, 673, 773, 873 and 973 K. Analysis of all studied specimens at room temperature revealed the coexistence of R and B19′ phases, whose relative fractions varied with annealing temperature. The high-temperature B2 phase was not detected. It was found that the most rapid decrease in the dislocation density, which was measured at 393 K (in the B2 state), occurred after annealing at 673 and 773 K. Specimens annealed at 773 K had the minimum dislocation density, which is more than an order of magnitude lower than the dislocation density immediately after abc pressing. In the same temperature range, there is a significant decrease in the root-mean-square B2 lattice microdistortions <ε²>^1/2 and a slight increase in the average size of coherently diffracting domains (crystallities). After abc pressing and isochronal annealing, the main contribution to the intrinsic X-ray line broadening is made by B2 lattice microdistortions, while the contribution from crystallite size is insignificant. The obtained results show that intense recrystallization in Ti_49.8Ni_50.2 alloy after abc pressing at 573 K begins at T ≥ 773 K.

This study examines the influence of hot isostatic pressing and heat treatment on the microstructure and mechanical properties of specimens manufactured by selective electron beam melting (SEBM) of the metal powder composition (MPC fraction 40–100 μm) of a new six-component intermetallic beta-solidifying TiAl alloy Ti–44.5Al–2V–1Nb–2Cr–0.1Gd, at % (Ti–31.0Al–2.5V–2.5Nb–2.5Cr–0.4Gd, wt %). It is shown that SEBM with a high line energy input (E_L = 285 J/m) produces a fine-grained microstructure in the as-built material with a grain size of 5–14 μm and residual porosity of less than 0.5 vol %. An increase in the electron beam current (I) from 9.5 to 19.0 mA intensifies Al evaporation, as a result, the fraction of large columnar grains (d = 30–100 μm in width, h = 150–400 µm in height) formed mainly in Al-depleted regions (layers) increases. Heat treatment of the as-built SEBM specimens by two-stage annealing in the (α + γ)- and (α₂ + γ + β)-phase fields or by thermal cycling in the (α + γ)-phase field leads to complete or partial fragmentation of columnar grains. Combined postprocessing of the specimens produced at lower I by hot isostatic pressing in the α-phase field and two-stage annealing completely eliminates residual porosity and transforms the columnar structure into a fine-grained one with the grain size less than 150 μm. As a result, the achieved short-term mechanical characteristics at 20°С (UTS = 525 ± 5 MPa, δ = 1.1%) and 750°С (UTS = 405 ± 10 MPa, δ = 3.8%) are comparable to those of the studied TiAl alloy in the as-cast state.

High-entropy alloys (HEAs) consisting of five or more components in an equimolar ratio are attracting increasing attention due to a unique combination of various properties. Doping HEAs with small amounts of certain elements (most often rare earth, trace or noble metals) is a promising way to improve the characteristics of such alloys and to control their properties. This paper reports the results on the microstructure, phase composition, and microhardness of as-cast Ag_xAl_0.5CoCrCu_yFeNi HEAs (x = 0, 0.1; y = 0.5, 1.0). The effect of silver addition on the oxidation behavior of the studied HEAs at 700°C was determined. The morphology, phase and chemical composition of the resulting oxide film were studied. It was shown that the introduction of silver improves the mechanical characteristics of the alloys, but deteriorates the oxidation resistance due to the formation of copper-silver eutectic in the alloy microstructure, leading to a change in the morphology and phase composition of the formed oxide layer. Along with the solid solution of (Al, Cr)₂O₃ oxides and CuCr₂O₄, NiCr₂O₄ spinels, the addition of silver leads to the formation of copper oxide CuO and a small amount of silver oxide Ag₂O in the surface film.

The binding energy, equilibrium geometry, and vibrational frequencies of small free Ni_n clusters (n ≤ 20) are calculated using interatomic interaction potentials found within the embedded atom method. Calculations of the energy parameter of stability ΔE₂ and dissociation energy show that the most energetically stable clusters are those with the magic numbers of atoms n = 4, 6, 13, and 19. Calculations of atomic vibrations reveal that the dynamic contribution to the stability of clusters is determined by the minimum vibrational frequency, whose extreme values fall on clusters with the magic numbers of atoms n = 4, 6, 13, and 19. The maximum vibrational frequency varies nonmonotonically, and it has unclear extreme values for clusters with n < 19. This result is consistent with the available experimental data on stable structures of small and medium-sized metal clusters.

The effect of aging temperature in the range of 300–500°C on the structure, R martensitic transformations and mechanical characteristics of nanocrystalline Ti–50.9 at % Ni alloy with a grain/subgrain structure was studied. It was found that variation in the spatial distribution of coherent Ti₃Ni₄ particles in the nanostructure from location on dislocations during low-temperature aging to precipitation at dislocation boundaries under accelerated aging is accompanied by a change in the morphology of the R phase from a nanodomain to a self-accommodating lamellar structure. The nanodomain structure of the R phase contributes to homogeneous deformation of the alloy during loading/unloading and stabilization of superelasticity. When loading the alloy with a lamellar R-phase morphology, localized deformation bands are formed by the R-phase reorientation in a Lüders deformation manner.