Powder Metallurgy and Metal Ceramics最新文献

The thermokinetics of recrystallization processes that occur during the heating of porous compacts, produced by cold pressing a mixture of ultrapure aluminum and iron powders in a 50: 50 ratio in a steel die, was experimentally studied. Over the 130–190°C temperature range, the aluminum component of the mixture undergoes relaxation, exhibiting wave-like behavior with a period of 0.2– 0.3 sec. Complete recrystallization occurs within the 165–235°C range. Subsequently, the relaxation process begins in the iron component of the mixture, with the initial stage characterized by nonlinear oscillations, transitioning to the next stage involving wave propagation of thermal energy. There are several periods of changes in wave propagation. The nonlinear wave-like rise in temperature during relaxation typically ends with another surge of energy release when the temperature rise period shortens, indicating more intense heat release. At its initial stage, the recrystallization process shows stationary linear behavior, which later transitions to the emergence of nonlinear waves. Changes in wave frequency are observed, along with intermittent wave behavior, suggesting the turbulence of thermal flows. Following this regime, the temperature increases up to the melting point of aluminum. However, complete melting does not occur because of crystallization within lower-temperature regions. All transitions marked by changes in thermokinetic paths at both relaxation and recrystallization stages are accompanied by bifurcation changes in the amplitude of the thermal waves.

The influence of heat treatment temperature of the starting powders, ranging from 400 to 1450°C, on the consolidation of zirconia-toughened alumina (ZTA) composites was studied. In these composites, particles of a ZrO₂ (Y₂O₃, CeO₂) solid solution with high fracture toughness are dispersed in an Al₂O₃ matrix. The powders for developing the ZTA composites (wt.%)—90 Al₂O₃–10 (ZrO₂ (Y₂O₃,CeO₂) (90 AZK) and 70 Al₂O₃–30 (ZrO₂ (Y₂O₃,CeO₂) (70 AZK)—were produced using a combination of hydrothermal synthesis and mechanical mixing. Cold uniaxial pressing and sintering in air at 1600°C for 1.5 h were employed for the consolidation. The sintered materials were examined by X-ray diffraction and scanning electron microscopy. The compaction of the composites was associated with the brittle fracture of hard sintered agglomerates (for 90 AZK) and with the plastic deformation effect (for 70 AZK). The sintering of the 90 AZK and 70 AZK composites was accompanied with ZrO₂ phase transformations, abnormal growth of the Al₂O₃ grains, and zonal segregation effect. The Zener pinning effect in the 70 AZK composites inhibited the abnormal growth of Al₂O₃ grains. The increased porosity of the 90 AZK composites was due to the bimodal distribution of the starting powder agglomerates in accordance with the shape factor. The Vickers hardness of the 90 AZK samples varied from 6.4 to 4.4 GPa and that of the 70 AZK samples from 10.3 to 7.2 GPa. The decreased hardness of the 90 AZK composites was attributed to the formation of a two-scale porous microstructure. The topological memory of the ceramics was demonstrated to determine the conditions for microstructural design of advanced ZTA composites for tool, structural, and functional purposes with the required properties.

Powder metallurgy (PM) is a manufacturing approach that enables the production of complex shapes with minimal waste. This approach allows for creating intricate and readily deployable components, often surpassing the capabilities of traditional casting and metal forming techniques. The present study explores the feasibility of employing friction stir spot welding (FSSW) to join aluminum alloy 6061 (AA6061) components fabricated using the PM route. The research involves fabricating AA6061 samples employing the PM process. The sintering process involved induction heating at 530–550°C, followed by hot forging, ensuring optimal densification. FSSW was applied to both PM-processed and conventionally wrought AA6061 samples using three welding parameters. Comparative evaluations focused on the resulting welds’ microstructural characteristics and mechanical properties. Microstructural analysis revealed that PM-fabricated samples exhibited finer grain structures and superior hardness within the weld zones than their wrought counterparts. Furthermore, the thermomechanically affected zone (TMAZ) was narrower in PM-processed joints, indicating enhanced mechanical properties and a more uniform microstructure. The hardness values witnessed in the FSSW of the PM-processed Al 6061 point to the formation of superior-quality joints when compared to FSSW-welded sections of sheets produced through conventional means. This study highlights the potential of PM-fabricated AA6061 for advanced welding applications, demonstrating notable improvements in microstructural refinement and joint quality, integrating PM techniques with advanced welding processes to overcome traditional manufacturing limitations.

15 Cr ferritic oxide dispersion strengthened (ODS) steels are considered prime fuel cladding materials in nuclear reactors due to their excellent creep, swelling, and oxidation resistance. In the present study, the nominal compositions Fe–15Cr–2W–xY₂O₃ (x = 0, 0.3, 0.7, and 1.0) of ferritic ODS steels were prepared by mechanical alloying followed by spark plasma sintering. The sintered samples were annealed at different temperatures of 950, 1100, and 1250°C with a holding time of 60 min at respective temperatures. Further, the samples were also annealed at 1100°C for various durations of 0, 60, and 120 min. The role of varying yttria dispersoids and annealing temperatures on the grain growth kinetics, as well as their mechanical properties (hardness and compressive strength), were analyzed. The compressive strength of the sintered samples with varying yttria contents and at elevated temperatures of 600 and 700°C was determined. Modeling of compressive yield strength at room and elevated temperatures, as well as a correlation with the experimental values, were established for all the compositions. The grain growth exponent (n) and activation energy (Q) rose with the increase in yttria content and were estimated to be 11.52 and 612.91 kJ/mol, respectively, with 1.0 wt.% yttria. The grain size was nearly stable at the annealing temperature of 1100°C. A significant rise in compressive strength at room temperature and elevated temperatures was observed with a yttria reinforcement content of 0.7 wt.%. According to the strength model at different conditions, the role of ultrafine grains and dispersoids seemed to be predominant at room temperature and high temperatures, respectively.

The detonation spraying of coatings from fine composite materials is analyzed in the paper. The use of detonation coatings was found to improve the properties of machines and mechanisms and extend their life, while their functional performances are maintained over long-term operation. The structural features, strength, and fracture toughness of the coatings produced by multichamber detonation spraying from 75 wt.% Cr₃C₂ + 25 wt.% NiCr and Ni–Cr–Fe–B–Si (77–81.5 wt.% Ni, 10–14 wt.% Cr, 5–7 wt.% Fe, 2.0–2.3 wt.% B, 2.0–3.2 wt.% Si, 0.5 wt.% C) powder materials were examined. Changes in the detonation spraying parameters were proved to significantly influence the structure of the coatings: microhardness, phase composition, volume content of lamellae, sizes of grains and subgrains, phase formation, and dislocation density. The structural and phase state of the coatings was studied at all structural levels using a comprehensive approach, involving light and scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The prospects of the multichamber detonation spraying method, ensuring the necessary combination of structural and phase parameters of the coating material with a simultaneous increase in their physical, mechanical, and operational properties, were demonstrated. A high level of strengthening and fracture toughness of the coatings was promoted by optimal structural and phase constituents: fine grain and subgrain structure, uniform distribution of nanosized strengthening particles, and uniform dislocation density. The improved fracture toughness of the coatings is due to the absence of extended structural areas of dislocation clusters. The gradient-free distribution of dislocation density prevents the formation of local internal stress concentrators in the resulting coatings.

The powders produced by plasma-arc wire atomization in an argon atmosphere or air were studied for their use in 3D printing of complex-shaped metal parts and in granular metallurgy. The dependence of the morphology, structure, phase composition, and microhardness of the powders on the current and atomization conditions was established. In all studied operating modes of the plasma torch (180, 220, and 270 A), the atomized particles are predominantly spherical. The number of nonspherical particles increases with particle size. The powders atomized in an argon atmosphere exhibit a stable phase composition. The main component is iron aluminide Fe₃Al (or a mixture of Fe₃Al and AlFe). The α-Fe, Fe₃O₄, and Fe₂O₃ phases were also found. At currents of 220 and 270 A, the powder in –200+100 μm fraction contains the highest amount of aluminides, 83.88 and 86.30 wt.%, and the lowest content of oxides, 6.61–10.18 wt.%. In fine powders (–100+75 μm), the content of aluminides is 70.38– 28.3 wt.%), but the amount of oxides increases to 23.32–29.62 wt.%. The microhardness of oxide particles (5320–8150 MPa) is higher than that of metal particles (3070–4590 MPa). In atomization in air, the key components are Fe₂O₃, Fe₃O₄, FeO, and Al₃O₄. The total amount of oxides reaches 57.19–90.34%. The percentage of iron aluminides decreases significantly, and their maximum content (28.3 wt.%) is shown by the –315+200 μm powder at a plasma torch current of 270 A. In the finest powder fraction of –100+75 μm, the content of aluminides ranges from 6.2 to 15.36 wt.%. The average microhardness of metal particles is much lower (2750–4940 MPa) than that of oxide particles (4500–7460 MPa). It was found that the best material in terms of phase composition, structure, hardness, and shape factor was produced by atomization of a flux-cored wire in an argon atmosphere. In atomization in air, intense oxidation processes occur.

Pressed porous glass-ceramic carbon fiber biocomposites were produced from hydroxyapatite/glass and nanostructured carbon fibers. The specific features of the production process, as well as the composition, macrostructure, microstructure, and porosity of these biocomposites, were studied. The prospects for their medical applications, particularly in surgical osteoplasty, were identified. The starting materials included calcium phosphate glass ceramics derived from biogenic hydroxyapatite, featuring both bound and migrating glass phases, and activated nanostructured carbon fibers. The glass ceramics with a bound glass phase were produced by sintering powder mixtures of biogenic hydroxyapatite and sodium borosilicate glass, while those with a migrating glass phase were produced through mechanical mixing of biogenic hydroxyapatite and sodium borosilicate glass powders. The fine activated nanostructured carbon fibers used in the biocomposites were obtained by the mechanical grinding of a woven material from activated nanostructured carbon fibers. This material resulted from the controlled stepwise pyrolysis of hydrocellulose fabrics, followed by high-temperature vapor activation of the nanostructured fiber surface. To make cylindrical biocomposite samples, the fine activated nanostructured carbon fibers were blended with moistened mixtures of biogenic hydroxyapatite glass ceramics with bound and migrating glass phases and subjected to semidry pressing and incremental sintering with holding at 800°C. The selected process parameters enabled the production of pressed carbon fiber biocomposites with the desired composition and showed the ability to control their porous structure, achieving a relative density of 0.36–0.41, by regulating the behavior of the glass phases and the sintering of the reinforcing component. The biocomposite structures were examined by scanning electron microscopy. Energy-dispersive X-ray analysis was conducted to determine the chemical composition of the samples. The structures of the composites were analyzed and compared on the basis of their sorption capacities, determined from benzene adsorption–desoprtion isotherms using the gravimetric method. Analysis of the macrostructure, microstructure, and surface morphology of transverse and longitudinal sections of the biocomposites revealed a multiporous amorphous-crystalline microstructure, arising from the varying behavior of the glass phases, the presence of chaotically oriented short fine nanostructured carbon monofibers with diameters of several microns and a developed system of micro- and macropores on their surface, and spatial multidirectional hollow channels formed through the complete or partial combustion of the fibers.

Conglomerated (Ti, Cr)B₂–NiAlCrWCoMoTi composite powders for thermal spraying and deposition through sintering followed by milling were produced. The main processes stages in the production of powders, such as mixing, grinding, sintering, milling, and classification, were optimized. The effect of briquette sintering temperature on the milling efficiency and finished powder yield was determined. Solid-phase sintering was found to be feasible at temperatures 200–300ºC lower than the liquid-phase formation temperature. The influence exerted by the ratio of the refractory to metal components on the technological properties of the powders was studied. The flowability of the powders increased nonlinearly with a higher content of the metal component and greater particle sizes. The influence exerted by the ratio of the refractory to metal components on the morphology of powder particles was analyzed. The powder particles had a fragmentary elongated shape with a higher content of the refractory (Ti, Cr)B₂ component, while they acquired a more oval shape with a greater amount of the metal component. The composition and microstructure of individual powder particles were examined. They showed a heterophase microstructure. The uniform distribution of structural components in individual powder particles was found to depend on the mixing and grinding modes for powder mixtures. The uniform distribution of structural components in individual particles requires that particles of the starting powder components be significantly smaller (one-fifth or even less) than particles of the finished composite powder. The difference in the starting particle sizes for refractory and metal components should be no more than 2–3 times.

The thermal processes involved in the recrystallization within porous ultrapure iron compacts heated after cold pressing in a steel die were experimentally studied. The temperatures of recrystallization stages were shown to correspond to the heating behavior of metals and alloys. Depending on the heating rate of compacted powdered iron, various types of thermokinetic behavior were identified, indicating the nonlinear nature of interactions within the deformed powder body. Stages with both nonlinear and linear thermal behavior were present, as evidenced by the competition between several processes leading to the emergence of wave-like transfer of energy, accumulated while powdered iron deformed during cold pressing. The thermokinetic patterns observed in the heating of compacts, especially those made of pure iron, allowed all stages of relaxation in the deformed metal to be identified. The temperature profile in the heating process generally reflects the thermal state of the powdered metal, influenced by the additional release of energy accumulated in deformation. At some stages, heat release led to nonlinear processes, resulting in the occurrence of thermal waves. Both asynchronous and synchronous temperature changes were observed. In asynchronous behavior, nonlinear waves emerged. The superposition of relaxation processes in the generation of thermal waves is possible. At specific heating rates, the maximum amount of energy is simultaneously released in the relaxation and recrystallization processes, as testified by an increase in temperature to 550°C. The relaxation processes involve less energy compared to the recrystallization processes and the transition to the annealed state. In addition, after complete recrystallization, the onset of the sintering process was observed under temperature oscillations with damping.

The potential for amplifying tunnel magnetoresistance and magnetoimpedance in island nanofilms without energy consumption or the use of amplifying devices was studied. Such amplification was observed for the films deposited on a Gd₂O₃ layer instead of a glass substrate. The enhancement is due to the f–d exchange interaction established between atoms with unfilled f- and d-electron shells, present in both Fe and Gd₂O₃. The f–d exchange interaction enhances the ordering of the magnetic structure within the Fe ferromagnetic layer, increases its magnetization, and subsequently improves the properties that depend on this magnetization. Iron and Gd₂O₃ were selected because the magnetic moments of Fe in the iron group and Gd in the lanthanide series are among the highest effective ones: μ_Fe = 7.13 μ_B and μ_Gd = 7.95 μ_B. This, according to theory, determines the high energy of the f–d exchange interaction. The island morphology of Fe films deposited on a Gd₂O₃ layer enabled electron tunneling under the influence of the f–d exchange interaction. This allowed the study of tunnel magnetoresistance under direct current and magnetoimpedance under alternating current influenced by the f–d exchange interaction. The frequency dependence of the active component Z', the reactive component Z'', and the total impedance Z of Fe films on glass and Gd₂O₃ substrates, without and under a constant magnetic field of 7500 Oe, was analyzed. These characteristics are used to determine the frequency dependence of tunnel magnetoimpedance and estimate tunnel magnetoresistance for Fe island films on glass and Gd₂O₃ substrates. These characteristics were found to be 55 % higher, on average, for Fe deposited on Gd₂O₃ than for Fe on glass, both at a low frequency of 0.1 Hz for tunnel magnetoresistance and at higher frequencies of 1–10⁵ Hz for tunnel magnetoimpedance. This results from the influence of f–d exchange interaction on electron tunneling between the iron islands.