Powder Metallurgy and Metal Ceramics最新文献

The structure of suspensions used to apply films by colloidal methods determines their key properties: thickness, surface roughness, and density. Direct structural studies of thin suspensions are significantly complicated, especially when the task is to determine changes in the structure induced by mechanical loads present in the film development process. This problem can be addressed through rheological studies. For this purpose, a method for normalizing the degree of thixotropy/rheopexy was devised to serve as a quantitative parameter for evaluating the structure of fluids based on their rheological properties. The trapezoidal integration method for calculating the flow curve area was demonstrated. The developed normalization method relies on a modified standard score equation that accommodates the peculiarities of flow curves. The normalized degree of thixotropy/rheopexy was employed to assess the structures of suspensions with identical compositions but subjected to varying maximum shear rates (200, 500, and 800 sec^–1) to plot the flow curves. The nonnormalized degrees of thixotropy for these suspensions differed by 11 to 12 times. The developed parameter allowed the deviation to be reduced to 16–19%. The normalized degree of thixotropy/rheopexy, along with the flow behavior index and effective viscosity, was used for the indirect evaluation of structural changes in suspensions with higher nanopowder content based on the rheological properties. This approach enabled the identification of four structural states of suspensions: isolated agglomerates, enlargement of the agglomerates accompanied by rheopectic flow, transition to Newtonian flow after the agglomerates deformed in the flow direction, and evolution of a regular network structure signified by thixotropic flow.

Mass spectra of molecular ions in the cathode sputtering of hydroxyapatite within argon glow discharges were studied experimentally and through mathematical simulation. The study was aimed at developing a highly sensitive technique for the determination of toxic and physiologically active elements in hydroxyapatite, used for medical purposes, by glow discharge mass spectrometry. Mass spectra were simulated employing the method developed previously for the calculation of molecular ion concentrations in glow discharge plasma and the computer program for its implementation. The effective equilibrium constants were refined for the formation–dissociation reactions of molecular ions during cathode sputtering of hydroxyapatite on a tantalum substrate. Comparison between the experimental and calculated mass spectra confirmed that the model was accurate. The study revealed molecular interferences in the mass range from ¹⁹F to ²³⁸U that were not adequately separated from the isotopes under study, thus reducing the analysis detection limit. The isotopes that were minimally affected by molecular interferences were chosen, and the resolution needed to achieve a detection limit of around 1 ppm for monoisotopic elements was calculated. To maintain a sufficiently high ionic current for nonconductive matrix isotopes (⁴⁴Ca, ³¹P), a previously improved design of the analytical cell with high-purity tantalum as a substrate was employed. Most of the studied elements can be determined within ppm-ppb limits employing mass spectrometers with a high resolution (≥9000) at half the peak height.

Powder metallurgy (PM) Fe (steel) products still suffer from abrasion and fatigue, thus reducing their life expectancy. The physical vapor deposition (PVD) technique can significantly enhance surface wear and fatigue resistance. On the other hand, ultrasonic shot peening (USSP) treatment is an emerging and effective technique that can enhance surface strength and improve surface density, particularly for PM products. Duplexed PM Fe (steel) samples are expected to have excellent mechanical properties and long service life. In this work, PM Fe (steel)-based samples (ρ = 6.9) were prepared as substrates for CrN coatings with and without USSP treatment. Holes sized about 35 μm can be observed on CrN coatings deposited on as-sintered substrates, while the CrN coatings on USSP-treated substrates were very compact. According to the results of scanning electron microscopy (SEM), X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), the crystal structures and chemical bonds of CrN coatings on different substrates are the same. Nevertheless, CrN coatings were found to be discontinued on sintered substrates because of holes in the surface. In friction tests, such specific structures created plenty of “steps” in the coating, increasing residual stress, which crushed the coatings into fragments and led to severe abrasive wear. Additionally, wear rates would increase with load. Nevertheless, USSP treatment can generate a compact layer, about 700 μm thick, to offer strong support to CrN coatings. Therefore, CrN coatings on the USSP-treated substrates exhibited lower and more stable coefficients of friction and wear rates. This generally describes a simple method to enhance the surface strength and densities of PM Fe (steel) products. Besides, it offers a new way of prolonging the service lives of PM Fe (steel) products by USSP and PVD duplex treatment.

The mechanical and tribological properties of cast monocarbides and multicomponent high-entropy carbides produced by vacuum arc melting using starting monocarbide powders were examined. The cast monocarbides demonstrated a hardness of 20–30 GPa and an elastic modulus of 400–600 GPa. Among the studied monocarbides, ZrC showed the highest hardness (29–32 GPa), while MoC exhibited the lowest hardness (16–18 GPa). The friction coefficient for monocarbides was determined by pin-on-disk testing with diamond in dry friction conditions and in the presence of water. The friction coefficient was found to increase for WC and TiC carbides and decrease for MoC in the presence of water. Based on the studies of monocarbides, cast single-phase multicomponent high-entropy carbides with a NaCl-type cubic lattice and a homogeneous microstructure without any phase separation by chemical composition were developed and produced. The hardness of the cast multicomponent high-entropy carbides was determined, and their normalized hardness was calculated. The high-entropy carbides exhibited higher hardness (33–40 GPa) and normalized hardness (0.072–0.105) but a slightly lower elastic modulus than the monocarbides. The elastic modulus and lattice parameter were theoretically calculated, and the relationship between the size mismatch and hardness of the cast multicomponent high-entropy carbides was shown. The friction coefficient of the multicomponent high-entropy carbides determined by tribological tests was lower than that of the monocarbides both in dry friction conditions and in the presence of water. The friction coefficient was not either found to be dependent on hardness or elastic modulus.

The development of analytical methods for predicting the strength characteristics of pellets produced by the compaction of fine-grained materials remains a significant and relevant area. To advance this area, the mechanisms of phase interactions in bulk media were analyzed. The analysis was then used to develop local models of adhesion processes for two basic particle interaction schemes: ‘particle + particle’ and ‘particle + liquid phase + particle’. For each local model, the types, nature, and combination of adhesion processes that occurred simultaneously were theoretically established, and the factors and indicators that determined the occurrence and intensity of adhesive bonding were justified. The experimental studies conducted in the laboratory premises of the Nekrasov Iron and Steel Institute of the National Academy of Sciences of Ukraine were analyzed to evaluate the nature and extent of influence exerted by the selected factors, determining the adhesion processes, on changes in the strength characteristics of compacts. Considering the results obtained and the analytical dependences established for the ‘particle + particle’ interaction scheme, a method for predicting the strength characteristics of pellets produced from fine-grained materials with zero moisture was developed. This paper justifies the methodological conditions for experiments intended to create strong bonds within the compacts under the ‘particle + liquid phase + particle’ interaction scheme, taking into account the mechanical, physical, and physicochemical interaction processes between individual particles of the pelletized material and between the charge components (liquid phase). A generalized analysis of the experimental findings was carried out to estimate the range of potential adhesion processes inherent in the ‘particle + liquid phase + particle’ interaction scheme, identify their manifestation, study the nature of their interaction, and evaluate the effect of introducing the liquid phase into the pelletized charge, considering the applied compaction pressures. The collected array of experimental data will enable a detailed cross-correlation analysis to determine the influence of various factors on the adhesion processes and the formation of strong bonds within the compacts. The analysis will also help establish the dependence of strength characteristics of compacts on integral indicators contributing to the formation of adhesive bonds and describe the dependence by analytical methods. The results will be used to develop a method to determine the moisture content in the charge required to produce compacts with maximum strength from materials that belong to the first of the four groups of systematization. The classification of materials into specific groups of systematization is determined by their pycnometric density.

The influence of ground R6M5K5 tool steel powder in mixture with gas-atomized powder on the process properties of the powder mixtures was studied. Both powders were sifted through a 50 μm sieve. The ground powder was present in amounts of 0, 10, 20, 30, 40, 50, and 100%. The bulk density, tapped density, flowability, and repose angle of the powder mixtures were determined. Additionally, the Carr index, Hausner ratio, and flow rate were calculated. The bulk density exhibited minimal changes because of a high content of near-spherical particles in the ground powder. The flowability of the mixtures decreased with increasing content of the ground powder (flow time for the standard weighed sample increased). Grinding the powder resulted in reduction of its flowability by nearly 35%. The flowability of the gas-atomized powder was 22.49 sec/50 g. When the mixtures contained 50% ground powder, the flowability became 25.72 sec/50 g, representing a decrease of 14%. The linear fitting of the dependencies relating the bulk density (BD), flowability (τ), and flow rate (V) to the ground powder content (weight percent) in mixture with the gas-atomized powder (X) provided the following results with a high coefficient of determination (R²): BD = 4.52 – 0.0043X (R² = 0.98), τ = 23.48 + 0.07X (R² = 0.95), and V = 36.32 – 0.012X (R² = = 0.97). The linear dependence of the relative bulk density (expressed in percentage) on the ground powder content demonstrated that the effect from the amount of ground powder could be assessed using the slope angle of the dependence on the abscissa axis. The slope angle of the dependence is less than 1 deg, indicating a very weak effect of the ground powder content on the relative bulk density of the powder mixtures.

Phase equilibria and structural transformations in the La₂O₃–Y₂O₃–Gd₂O₃ system at 1600°C were studied by X-ray diffraction, electron microscopy, and petrography in the entire composition range. Fields of solid solutions based on hexagonal (A) modification of La₂O₃, cubic (C) modification of Y₂O₃, and monoclinic (B) modification of La₂O₃ (Gd₂O₃) were identified in the system. The starting materials were La₂O₃, Gd₂O₃, and Y₂O₃ (99.99%) powders. Samples were prepared with concentration steps of 1–5 mol.%. Weighed portions of the oxides were dissolved in HNO₃ (1 : 1) solutions. This was followed by evaporation of the solutions and decomposition of the nitrates at 800°C for 2 h. The samples were heat-treated in three stages: 1100°C (168 h), 1500°C (70 h), and 1600°C (10 h) in air in furnaces with FeCrAl (H23U5T) and molybdenum disilicide (MoSi₂) heating elements. X-ray diffraction analysis was carried out using the powder method with a DRON-3 diffractometer at room temperature (Cu-K_α radiation). The scanning step was 0.05–0.1° at angles 2θ = 15–90°. The isothermal section of the La₂O₃–Y₂O₃–Gd₂O₃ phase diagrams at 1600°C was characterized by three single-phase (A-La₂O₃, B-La₂O₃ (Gd₂O₃), C-Y₂O₃) and two two-phase (A + B, B + C) regions. The solubility limits were determined, and composition dependences of the lattice parameters for the phases formed in the system were plotted. No ordered perovskite-type phase was found in the system at 1600°C. A continuous series of solid solutions based on the monoclinic modification of B-La₂O₃(Gd₂O₃) formed in the system and occupied the largest area of the isothermal section. Yttrium oxide stabilized the total mutual solubility of lanthanum and gadolinium oxides. With the addition of heavier ions, the lattice parameters of the B modification reduced and the lattice volume and, accordingly, density increased. The lattice of solid solutions based on the B modification of rare-earth metal oxides became more densely packed with a higher concentration of yttrium oxide.

The influence of various rolling methods on the mechanical properties of titanium ribbons was studied. Ribbons produced by asymmetric rolling showed 100% density and higher strength compared to ribbons produced through symmetric rolling. The temperature sensitivity of contact formation and mechanical behavior of ribbons rolled asymmetrically was determined by thermal variations in the plastic deformation mechanisms specific to titanium. In this regard, three temperature ranges were identified: low, intermediate, and high. In the low-temperature range (<100 °C), the elastic modulus and proportionality limit were significantly higher than those from symmetric rolling, although still inferior to the properties of the compact material. In the intermediate-temperature range (100–300°C), the elastic modulus and proportionality limit in the rolling direction matched those of compact titanium but were approximately three times greater than those found for samples tested transversely. In the high-temperature range (>300°C), the elastic modulus in both longitudinal and transverse directions was comparable to that of the compact material, while the proportionality limit surpassed the compact material significantly, owing to the deformation substructure observed in the ribbons. Asymmetric rolling significantly enhanced the mechanical properties of titanium ribbons compared to symmetric rolling. This enhancement was due to the shear strain component that facilitated contact formation at particle boundaries. Under optimal deformation conditions, the ribbons achieved a strength limit of ~800 MPa, comparable to the strength of ribbons produced conventionally. The plasticity of the ribbons did not exceed 1.5% because of their propensity for interparticle fracture.

The densification of a powder mixture containing titanium diboride and 20 wt.% molybdenum disilicide during nonisothermal spark plasma sintering was experimentally studied. The sintering process was assisted with an external pressure of 50.93 MPa in vacuum at a controlled constant heating rate of 1.67 and 2.72 K per second. It was established that sintering occurred when the thermodynamic temperature reached 1155 K, which should be taken as the critical brittle–ductile transition temperature for molybdenum disilicide, a less refractory material. The densification kinetics was analyzed using the continuum theory for bulk viscous flow of a porous body, considering the effect of powder particle shape on the rheological properties of the sintered body. In general, the sintering process was characterized by a decrease in the root-mean-square stress within the porous body matrix to the limiting zero value as it approached the nonporous state and by an increase in the root-mean-square strain rate along the curve with a maximum. Computational modeling of the densification kinetics for the powder composite, involving determination of the activation energy for viscous flow of the composite matrix as a function of temperature and root-mean-square stress, allowed the initial, low-temperature, and medium-temperature stages of spark plasma sintering to be identified. At the initial stage up to 1220 K, the activation energy increased nonlinearly and sharply, which can be caused by active spark flashes with the formation of plasma within the loose random packing of the powder particles, as a similar stage is not observed in conventional pressure assisted sintering. At the next low-temperature temperature stage, the activation energy increased as the root-mean square stress decreased. In the temperature range from 1300 to 1389 K, the activation energy for viscous linear flow of the composite matrix was 223 kJ/mol. In the medium-temperature range from 1414 to 1485 K, the activation energy increased to 255 kJ/mol.

In the framework of the CALPHAD method, the thermodynamic assessment of the Cu–Ti–Hf system has been performed for the first time. This assessment considers the existence of homogeneity regions for Cu₃Ti₂, Cu₄Ti₃, CuTi, Cu₅Hf, Cu₅₁Hf₁₄, and Cu₁₀Hf₇ compounds and the formation of a continuous solid solution of Cu(Ti, Hf)₂ (γ-phase) in the ternary system. The thermodynamic assessments of the boundary binary systems and data on phase transformations and mixing enthalpy of melts in the ternary system became the basis for calculations. The Compound Energy Formalism was used to model the thermodynamic properties of intermetallic compounds with a homogeneity region. The associated solution model was used to describe the complex temperature dependence of the thermodynamic properties of melts from the temperature at which equilibrium melts exist to the glass-formation temperature. Upon the calculations, isothermal sections, vertical sections, projections of the liquidus and solidus surfaces, and reaction scheme of the phase diagram were presented. The liquid phase participates in eleven four-phase invariant reactions occurring in the temperature range 1138–1541 K. The diagrams of metastable phase transformations involving supercooled Cu–Ti–Hf melts and boundary solid solutions based on pure components were calculated. It is shown that supercooled melts in wide concentration ranges are thermodynamically stable in relation to boundary solid solutions based on pure components. The concentration region of glass formation for Cu–Ti–Hf melts by liquid quenching, predicted by the relative position of the ({T}_{0}^{L/phi }) and ({x}_{0}^{L/phi }) lines, is x_Cu ≈ 0.16–0.80.