Powder Metallurgy and Metal Ceramics最新文献

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.

The paper examines three microlayer Ti/TiAl₃ materials of initial Ti–Al composition, which were produced through reactive sintering and rolling of packets consisting of alternating titanium and aluminum ribbons of varying thickness at 600, 700, and 770°C. Young’s modulus of these materials was determined under longitudinal vibrations at room temperature with a frequency of about 45 kHz and under resonant bending vibrations at high temperatures ranging from 20 to 820°C with a frequency a hundred times lower. The temperature dependences of the elastic modulus E for the microlayer materials exhibited slopes between those of the dependences for titanium and the well- known VT25U alloy. The Ti/TiAl₃ materials were heated and held at 700°C to result in a material with stable E values, surpassing those of the VT25U alloy at temperatures up to 700°C. The dependences of stresses in the samples on the relative power of the test installation were determined at constant temperatures of 650 and 700°C for the microlayer Ti/TiAl₃ and VT25U materials. The microlayer materials dissipated a significantly larger portion of mechanical vibration energy than the heat-resistant VT25U material. The difference in the fatigue resistance mechanisms for the microlayer and isotropic materials at high temperatures is not solely attributed to their distinct temperature dependences of Young’s modulus at atomic interaction levels. The difference primarily arises from the variation in temperature-dependent cyclic strains associated with dislocations at microstructural and macrostructural levels. A fatigue crack is shown to delaminate the material in the middle of the intermetallic layers.

The use of the hydrogenation and disproportionation (HD) and desorption and recombination (DR) route (HDDR) for sintering Nd₂Fe₁₄B-based ferromagnetic alloys, such as Nd_11.7Fe_81.1Zr_1.2B₆ and Nd₁₆Fe_73.9Zr_2.1B₈, was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The dependence between the production conditions—grinding of the alloys into powders, compaction pressure of the powders, hydrogen pressure and temperature at the first stage of sintering in hydrogen (HD), and temperature at the second stage of sintering in vacuum (DR)—and the porosity and microstructural particle size of the sintered materials was evaluated. The powders were ground in hydrogen in a planetary-ball mill at 200 rpm for 1 h and compacted at 2, 5, and 6 t/cm². The first sintering stage was carried out at a hydrogen pressure of 0.05 MPa and a temperature of 760°C, and the second stage at 850 and 950°C. The powders were found to sinter at the first stage. The porosity of the sintered materials decreased with increasing compaction pressure. The grain size of the ferromagnetic Nd_11.7Fe_81.1Zr_1.2B₆ phase in the sintered materials ranged from 100 to 300 nm. The physical mechanism behind the reduction in the sintering temperature was attributed to an increase in the diffusion rate of alloy components resulting from hydrogen-induced phase transformations, such as disproportionation and recombination, and to the presence of a hydrogen solid solution at both stages of the process, HD and DR. A very important aspect of this research is that the powders were sintered under low hydrogen pressure required to produce magnetically anisotropic materials. Problematic aspects of the properties shown by the sintered materials, particularly microstructural heterogeneity, were analyzed, and approaches to their solution, through homogenizing the particle size of the powders and optimizing the HDDR parameters (hydrogen pressure, temperature, reaction time), were proposed. The process advantages of the new sintering method compared to similar techniques included the temperature lower by more than 100°C, the potential for producing nanostructured anisotropic materials, and the use of technically simpler and cheaper sintering furnaces.

The densification of a fine-grained tungsten carbide cermet containing 36 wt.% copper binder during impact assisted sintering in a vacuum at thermodynamic temperatures of 1023, 1123, and 1223 K with an initial impact velocity of 6.4 m/s was studied. Based on the experimental data and calculated elastic properties of the samples and the impact machine, computational modeling of the densification dynamics with a trial and error method was carried out using a third-order dynamic system in combination with the rheological model of Maxwell’s viscoelastic body, and as a result previously unknown values of shear viscosity for material cermet matrices were obtained. The time dependences of force, compression, velocity, and acceleration of the system, as well as shrinkage, root-mean-square stress, and strain rate, of the cermet samples during impact assisted sintering were determined. The calculated phase trajectory of the dynamic system movement showed that the initial kinetic energy of the impact was not completely exhausted for the irreversible densification of the cermet samples. Part of the energy dissipated in the environment after the rebound of the machine’s impact parts. At the initial stage, the system exhibited nonperiodic (atemporal) damping during its movement at high ratios between the system’s stiffness and the cermet samples’ viscous resistance. As the ratio decreased, the movement transformed into damping oscillations. The work of densification and the thermomechanical effect, which significantly increased the temperature of porous samples, were evaluated. The estimated activation energy of the viscous flow for the porous cermet matrix was 0.34 eV or 33 kJ/mol that indicated the dislocation mechanism of sintering. The samples produced by impact assisted sintering showed significantly higher strength values compared to pressureless sintered samples at a higher temperature.

The strain-induced martensitic transformation in a medical alloy from the ternary Ti–Nb–Mo system was studied. The low-doped Ti_92.5Nb₅Mo_2.5 alloy was produced by arc remelting, followed by annealing, rolling at room temperature, reannealing, and water quenching. X-ray diffraction analysis showed that thermomechanical processing resulted in the alloy primarily consisting of orthorhombic martensite (α?) with a small amount of the β-titanium phase. Hysteresis loops were recorded in loading–unloading cycles with 1% strain increments up to a total strain of 4% under compression testing, employing a precision strain gauge. Young’s modulus under loading varied from 51.2 GPa at the initial section to 39.7 GPa after a 2% residual strain. Young’s modulus remains unchanged, within 74.3 GPa, during unloading. Elastic, pseudoelastic, and plastic strains were found to significantly depend on the previous strain within the first three loading–unloading cycles. To examine the impact of higher strains (up to 23.4%) on structural rearrangements and phase transformations, the samples were compressed without a precision strain gauge. X-ray diffraction analysis revealed that only the crystalline texture of the alloy changed after compression. Strains exceeding 23.4% were achieved by rolling at room temperature. After rolling to a strain of 64%, the diffraction patterns indicated an increased amount of the β-phase, as evidenced by the (200) diffraction peak, not observed previously. The increased amount of the β-phase suggests that strain prompted the reverse martensitic transformation (α? → β).

In this study, nanocrystalline TiAlV alloys were synthesized using the mechanical alloying technique with a high-energy planetary ball mill from pure Ti, Al, and V powders. Various methods, including Vibrating Sample Magnetometry (VSM), Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM-EDS), and X-ray Diffraction (XRD), were employed to characterize the synthesized alloys and study their magnetic behavior, morphology, microstructural and structural properties, respectively. Following the XRD analysis, new phases were confirmed, and a significant reduction in crystallite size from 48.73 to 9.38 nm was observed. Moreover, an increase in lattice strain from 0.15% to approximately 0.81% was noted after 60 h of milling. The EDS analysis gave remarkable results, showing the lack of magnetic iron particles before milling. However, after milling, the EDS spectrum revealed the presence of these magnetic iron particles with varying concentrations. This important observation highlights the profound impact of the mechanical alloying process on the sample composition. It emphasizes the sensitivity of EDS analysis by detecting even subtle changes in the elementary composition of a material. A sensitive approach was employed to monitor the progression of the nanocrystalline alloy and identify any potential defects arising during the mechanical milling. A vibrating sample magnetometer was utilized to achieve this objective. This method is highly effective at capturing even subtle changes that may occur during milling, allowing for an accurate evaluation of the chemical composition and integrity of the alloy. This technique made it possible to detect the presence of magnetic particles whose magnetic properties varied from time to time, indicating a change in magnetic behavior due to the reduction in the size of these particles caused by the collision between the steel balls and the milled powder particles. The results suggest that non-destructive magnetic testing using a VSM can be used to monitor the state of the nanocrystalline alloy.

A scheme for the full cycle of developing 3D products containing carbon nanostructures (CNSs) was developed. The scheme takes into account the state of initial carbon for the synthesis of CNSs and involves the preparation of CNSs for various 3D printing techniques (FDM, CJP, SLA, SLS) with post-processing of the printed 3D products. The developed cycle allows for the transformation of graphite or other carbon-containing materials into functional 3D products using a 3D printer. The 3D development cycle consists of three stages: Stage I is intended to select the starting material and method for CNS synthesis, Stage II involves preparation of CNSs as a consumable for 3D printing, and Stage III includes printing of a 3D product followed by post-processing. Each stage is described in detail and tested for each 3D printing technique (FDM, CJP, SLA, SLS). The entire range of CNSs (fullerenes and fullerene-like nanostructures, graphenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs), nanocomposites, etc.) and their synthesis employing three methods (plasmaassisted chemical synthesis in gaseous and liquid environments and pyrolytic synthesis) in the 3D printing cycle were analyzed. The advantages and disadvantages of the considered 3D printing processes were addressed, and results of the comparison were summarized in a table. Materials for 3D printing and development of associated composites containing soluble and insoluble CNSs were studied. Methods for processing CNSs and preparing CNS-based composites prior to their use in various 3D printing processes were developed. The post-processing results for 3D products prepared with the FDM, CJP, SLA, and SLS 3D printing processes were provided.