Search Results (43)

The plastic deformation during equal-channel angular pressing (ECAP) can be affected by various material- and processing-related factors. For instance, the initial crystal orientation and grain size play an important role in determining the material flow, which may cause localized deformation in terms of macroscopic deformation banding. In this study, we use a continuous cast AA1080 aluminum alloy with coarse columnar grains to analyze the influence of casting texture on the local material flow during ECAP. Billets are extracted with their columnar grains inclined either in the same direction as the ECAP shear plane or opposite to it. Visio-plastic analysis is performed on split billets. The pass is interrupted halfway through the ECAP tool to accurately capture steady-state deformation conditions. Flow lines at several positions within the billet are identified based on the positions of deformed and undeformed marker points and fitted to a phenomenological model based on a super-ellipse function. For further characterization, hardness measurements, optical and electron microscopy are carried out on the ECAP-deformed samples. Significant differences in terms of local material flow and microstructure evolution regarding the resulting crystal orientation and deformation banding are observed. Our results confirm and emphasize the importance of initial grain size and texture effects for ECAP processing. Full article

Green hydrogen production from water electrolysis (WE) is one of the most promising technologies to realize a decarbonized future and efficiently utilize intermittent renewable energy. Among the various WE technologies, the emerging anion exchange membrane (AEMWE) technology shows the greatest potential for producing green hydrogen at a competitive price. To achieve this goal, simple methods for the large-scale synthesis of efficient and low-cost electrocatalysts are needed. This paper proposes a very simple and scalable process for the synthesis of nanostructured NiCo- and NiFe-based electrode materials for a zero-gap AEMWE full cell. For the preparation of the cell anode, oxides with different Ni molar fractions (0.50 or 0.85) are synthesized by the sol–gel method, followed by calcination in air at different temperatures (400 or 800 °C). To fabricate the cell cathode, the oxides are reduced in a H₂/Ar atmosphere. Electrochemical testing reveals that phase purity and average crystal size significantly influence cell performance. Highly pure and finely grained electrocatalysts yield higher current densities at lower overpotentials. The best performing membrane electrode assembly exhibits a current density of 1 A cm⁻² at 2.15 V during a steady-state 150 h long stability test with 1 M KOH recirculating through the cell, the lowest series resistance at any cell potential (1.8 or 2.0 V), and the highest current density at the cut-off voltage (2.2 V) both at the beginning (1 A cm⁻²) and end of tests (1.78 A cm⁻²). The presented results pave the way to obtain, via simple and scalable techniques, cost-effective catalysts for the production of green hydrogen aimed at a wider market penetration by AEMWE. Full article

In this work, arc ion plating (AIP) was employed to deposit AlCrMoN coatings on cemented carbide substrates, and the effects of substrate bias voltages (−80 V, −100 V, −120 V, and −140 V) on the microstructures, mechanical properties, and tribological behaviors of the coatings were investigated. The results showed that all AlCrMoN coatings exhibited a single-phase face-centered cubic (FCC) structure with columnar crystal growth and excellent adhesion to the substrate. As the negative bias voltage increased, the grain size of the coatings first decreased and then increased, while the hardness and elastic modulus showed a trend of first increasing and then decreasing, with the maximum hardness reaching 36.2 ± 1.33 GPa. Room-temperature ball-on-disk wear tests revealed that all four coatings demonstrated favorable wear resistance. The coating deposited at −100 V exhibited the lowest average friction coefficient of 0.47 ± 0.02 and wear rate ((3.27 ± 0.10) × 10⁻⁸ mm³/(N∙m)), featuring a smooth wear track with minimal oxide debris. During the steady-state wear stage, the dominant wear mechanisms of the AlCrMoN coatings were identified as oxidative wear combined with abrasive wear. Full article

While fluvial features are plentiful on Mars and offer valuable insights into past surface conditions, the climatic conditions inferred from these valleys, like precipitation and surface runoff discharges, remain the subject of debate. Model-based estimations have already been applied to several Martian valleys, but exploration of the related numerical estimations has been limited. This work applies an improved precipitation-based, steady-state erosion/accumulation model to a Martian valley and compares it to a terrestrial Mars analogue dessert catchment area. The simulations are based on a previously observed precipitation event and estimate the fluvial-related hydrological parameters, like flow depth, velocity, and erosion/accumulation processes in two different but morphologically similar watersheds. Moderate differences were observed in the erosion/accumulation results (0.13/−0.06 kg/m²/s for Zafit (Earth) and 0.01/−0.007 for Tinto B (Mars)). The difference is probably related to the lower areal ratio of surface on Mars where the shield factor is enough to trigger sediment movement, while in the Zafit basin, there is a larger area of undulating surface. The model could be applied to the whole surface of Mars. Using grain size estimation from the global THEMIS dataset, the grain size value artificially increased above that observed, and decreased hypothetic target rock density tests demonstrated that the model works according to theoretical expectations and is useful for further development. The findings of this work indicate the necessity of further testing of similar models on Mars and a better general analysis of the background geomorphological understanding of surface evolution regarding slope angles. Full article

The results of a study of the kinetics of oxygen sorption from water by silver-containing nanocomposites synthesized on the base of macroporous ion exchangers with different pore sizes are presented. In the case of the Lewatit K 2620 ion exchanger, the pore size was fixed (41 nm), and for KU-23, it varied in the range from 10 to 100 nm. The nanocomposite materials Ag⁰⸱KU-23 and Ag⁰⸱Lewatit K 2620 were prepared by chemical precipitation. Using the different physicochemical methods, it was found that due to the monoporosity of the ion exchanger, the average size of the silver particles in the Ag⁰⸱Lewatit K 2620 nanocomposite is smaller than for KU-23. This effect contributes to the intensification of oxygen absorption and is proved by the results of studying the rate and degree of oxygen sorption by nanocomposites in the entire studied range of their capacity on metal. On the other hand, the polyporosity of the KU-23 ion exchanger, due to its better diffusion permeability, contributes to the more uniform distribution of silver over the volume of nanocomposite grains and ensures the steady state of the sorption process. Based on the presented experimental results, the synthesized silver-containing nanocomposites can be recommended as multifunctional materials with bactericidal action and catalytic effect for different industrial applications, including the deep removal of dissolved oxygen in the production of ultrapure water for energetics and microelectronics. Full article

Hydrogen atoms can enter into metallic materials through penetration and diffusion, leading to the degradation of the mechanical properties of the materials, and the application of hydrogen barrier coatings is an effective means to alleviate this problem. Zirconia coatings (ZrO₂) have been widely studied as a common hydrogen barrier coating, but zirconia undergoes a crystalline transition with temperature change, which can lead to volumetric changes in the coating and thus cause problems such as cracking and peeling of the coating. In this work, ZrO₂ coating was prepared on a Q235 matrix using a sol-gel method, while yttria-stabilized zirconia (YSZ) coatings with different contents of rare earth elements were prepared in order to alleviate a series of problems caused by the crystal form transformation of ZrO₂. The coating performances were evaluated by the electrochemical hydrogen penetration test, pencil hardness test, scratch test, and high-temperature oxidation test. The results show that yttrium can improve the stability of the high-temperature phase of ZrO₂, alleviating the cracking problem of the coating due to the volume change triggered by the crystalline transition; improve the consistency of the coating; and refine the grain size of the oxide. The performance of YSZ coating was strongly influenced by the yttria doping mass, and the coating with 10 wt% yttria doping had the best hydrogen barrier performance, the best antioxidant performance, and the largest adhesion. Compared with the matrix, the steady-state hydrogen current density of the YSZ coating decreased by 72.3%, the antioxidant performance was improved by 65.8%, and the ZrO₂ coating hardness and adhesion levels were B and 4B, respectively, while YSZ coating hardness and adhesion were upgraded to 2H and 5B. With the further increase in yttrium doping mass, the hardness of the coating continued to improve, but the defects of the coating increased, resulting in a decrease in the hydrogen barrier performance, antioxidant performance, and adhesion. In this work, the various performances of ZrO₂ coating were significantly improved by doping with the rare earth element, which provides a reference for further development and application of oxide coatings. Full article

In the present article, the application of an artificial neural network (ANN) model whose function is the development of plastic instability maps of a medium carbon microalloyed steel during the hot forming process is studied. Secondly, we proceed to create another ANN capable of providing the recrystallized grain size in the steady state resulting from forming deformation. We start from the experimental data of a medium carbon microalloyed steel obtained by hot compression tests with strain rates that vary between 10⁻⁴ s⁻¹ and 3 s⁻¹ and in a range of temperatures between 900 °C and 1150 °C. These experimental data are used to train the proposed ANN and obtain flow curves. Finally, the processing maps are developed by applying the dynamic materials model (DMM), according to which the safe hot forming domains and the plastic instability domains of the studied material are delineated. The comparison between the ANN and the experimental maps is carried out. It is ascertained that the optimal regions of forging in the ANN maps coincide with those obtained in the experimental maps. In addition, a study of the influence of the microstructure on the behavior of the studied steel during hot forming is carried out. Full article

This work is aimed at the analysis of the development of flotation technology by applying carrier minerals. Based on the concepts of continuum mechanics, a theoretical analysis of the influence of the carrier minerals (wall) on the motion of a single solid particle is provided, taking into account their hydrodynamic interaction (in the case of low Reynolds numbers). A correction was obtained in the form of a ratio of the particle size to its distance from the wall to take into account the influence of the wall on the hydrodynamic force acting on the particle. The influence of the wall is manifested through a rapid approximation of the liquid vortex flow in the gap between the solid wall and the particle to the steady-state mode, accompanied by the suppression of the transverse movement of particles. When the liquid slides along a wall-mounted gas–liquid layer with a reduced viscosity, the liquid flow increases in the interfacial gap, which can be analyzed by a dimensionless correction that includes values describing the properties of a continuous medium (dynamic viscosity) and a disperse phase (geometric particle size). The reason for the decrease in the induction time when gold grains adhere to each other is assumed to be due to the forces of hydrophobic attraction (when the grains have a mirror-smooth surface) and the sliding of the flow along the hydrophobic surface of the particles along the gas layer (when the grains have a rough surface). When polydisperse particles are aggregated, the threshold energy of the fast coagulation was established to be lower than that arising during the interaction of monodisperse particles, whose aggregation requires a large depth of the potential pit. Performing natural experiments on the ore using a rougher concentrate as a carrier material showed that the concentrate yield decreases by 20.52% rel. In the second case, the gold extraction was higher by 4.69% abs. While maintaining the achieved level of gold extraction, the double mixing of the rougher concentrate and the initial feed increased the gold content in the rougher concentrate from 4.97 to 6.29 g/t. Full article

Background: It is adverse for the safety of a tailings dam to use fine-grained tailings as the materials for a high tailings dam because of the low penetration coefficient, the slow consolidating velocity, and the bad physical mechanical property. Furthermore, with the influence of complicated geography conditions, the phreatic line will be increased enormously when encountering special conditions, which directly affect the safe operation of the tailings dam. Methods: In this study, based on the engineering, geological, and hydrogeological conditions and survey results of a tailings dam, a 210 m fine-grained tailings dam located in three gullies was selected and used to simulate the three-dimensional seepage field of a tailings dam under a steady saturated state by using the finite element software MIDAS GTS. The permeability coefficient was inverted, the seepage field of the project under different working conditions was simulated, and the position of the phreatic line was obtained. The controlled position of phreatic lines was determined by combining the seepage field with the stability requirements. Results: Back analysis could accurately reflect the actual permeability coefficient of each partition of tailings dams. Due to the multiple areas of seepage accumulation, large valley corners, and narrowing of the dam axis, the phreatic line of the shoulder region was elevated by 2~3 m compared to the surrounding area and was thereby the most critical region of the tailings dam seepage control. The stability requirements and minimum controlled position of the phreatic line requirements could be met when the controlled position of the phreatic line was 23 m. Conclusion: This study revealed the key areas and reasons why the tailings dam’s phreatic line is prone to be uplifted under complicated geography conditions. It was very critical to control the local phreatic line by adopting local horizontal seepage drainage measures or radiation wells in the key areas of the tailings dam to ensure the safety of the tailings dam. In addition to strengthening the daily monitoring of the key areas and the exfiltration facilities of the tailings dam, it is recommended to carry out determination tests of the permeability coefficient and particle size at regular intervals. The findings could provide countermeasures for seepage control. Full article

This study considered the effect of pavement aggregate grain size on tyre–pavement contact interaction during the late stages of pavement skid resistance. First, hemispherical shells 7, 9, and 13 mm in diameter were used to simulate coarse pavement aggregates. Subsequently, a three-dimensional finite element tyre–pavement contact model developed using ABAQUS was employed to analyse the contact interaction between each simplified pavement type and the tyre under steady–state rolling and braking conditions. Finally, the concept of occlusal depth was proposed and applied to characterise pavement skid resistance. The results showed that under steady–state rolling conditions, the peak contact stress of the simplified pavement increased with the pavement mean texture depth, whereas the contact area decreased. Under steady–state braking conditions, the effect of the contact interaction between the tyre and simplified pavement aggregates was ranked in order of superiority as aggregate grain sizes of 9, 7, and 13 mm, indicating that aggregate grain size did not exhibit any correlation with tyre–pavement contact interaction. Additionally, the squares of linear correlation coefficients between the pavement cumulative occlusal depth and horizontal braking force reached 0.921, 0.941, 0.889, and 0.894 for vehicle speeds of 30, 60, 90, and 120 km/h, respectively, indicating that they could be used to assess pavement skid resistance. Full article

The effects of climate change on animals are typically viewed in terms of survivability and wellbeing. In this study, we broaden that purview to include climate impacts on reproductive capability. There are not only climate spaces for daily function, but climate cliffs that represent reproductive failures in the face of climate warming. This alternative focus suggests that climate warming challenges may be more immediate and profound than initially imagined. This research describes a state-of-the-art mechanistic model, Dairy Niche Mapper (DNM), and independent validation tests. Where test data are absent, the calculated results are consistent with expected responses. Simulations of metabolic chamber conditions reveal the local steady-state impacts of climate and animal variables on milk production capacity, metabolic rate, food consumption and water needs. Simulations of a temperature humidity index (THI) show strengths and limitations of that approach. Broader time- and spatial-scale calculations applied in the western and eastern halves of the northern hemisphere identify current and future monthly latitudinal climate change impacts on milk production potential, feed and water needs in dairy cows of different sizes. Dairy Niche Mapper (DNM) was developed from a broadly tested mechanistic microclimate-animal model, Niche Mapper (NM). DNM provides an improved quantitative understanding of the complex nonlinear interactions of climate variation and dairy bovine properties’ effects on current and future milk production, feed and water needs for grazing and confinement dairy operations. DNM outputs include feasible activity times, milk production and water and feed needs of different-sized Holstein cows on high-grain (confinement feeding) versus high-forage (grazing feeding) diets at three arbitrary north latitudes, 12°, 30° and 60°, for North and Central America and for Asia. These three latitudes encompass current northern hemisphere bovine production environments and possible future production locations. The greatest impacts of climate change will be in the low elevations in tropical and subtropical regions. Global regions above 30° and below 60° latitude with reliable rainfall will be least affected by current projected levels of climate change. This work provides the basis for computational animal design for guiding agricultural development via breeding programs, genetic engineering, management options including siting or the manipulation of other relevant environmental and animal variables. Full article

Fission gas plays a significant role in fuel rod performance following accidents. The amount of fission gas increases dramatically under accidental conditions. This leads to a subsequent rise in the fuel rod internal pressure and temperature due to aggravated gap conductance between the fuel pellet and cladding. As a result, fuel rod performance degrades. Therefore, studying fission gas behavior is crucial for accident assessment and evaluating fuel rod performance. Minimizing the Impact of fission gas on fuel rods is essential for maintaining their integrity and safety within nuclear reactors. One important aspect of ensuring safety is predicting fission gas release (FGR). In this study, we presented an extended model to be used in light water reactors (LWRs). The FGR can be modeled using COMSOL Multiphysics with the finite element method. This modeling approach considers both normal and abnormal conditions, with the latter categorized as Class-II type incidents. The model assumes that the gas diffusion inside a spherical grain varies over time. By examining perfect sinks with gas production, perfect sinks without gas production, and imperfect sinks under steady-state conditions, different initial and boundary conditions are set. To validate the accuracy and universality of expressions used in the model, input parameters from other models and experiments are utilized. By comparing the model’s results with these inputs, the accuracy and applicability of the expressions can be confirmed. This validation process ensures that the model provides reliable predictions for fission gas behavior in fuel rods under both normal and abnormal operating conditions. Based on our findings, it is evident that the FGR fraction displays an upward trend as diffusion coefficients and temperatures rise. Conversely, larger grain sizes and higher linear heat generation rates are associated with a reduction in the FGR fraction. Notably, enhanced resolution leads to a postponed onset of FGR. Furthermore, the influence of the diffusion coefficient on the FGR fraction primarily stems from the interconnected effects of temperature and linear heat generation rate. Full article

The non-dendritic microstructure plays a crucial role in determining the rheological properties of semi-solid alloys, which are of the utmost importance for the successful industrial application of the thixoforging process. To further understand the impact of the reheating process on the evolution of microstructure and thixotropic deformation behavior in the semi-solid state, a hot extruded and T6 treated 7075 aluminum alloy was reheated to the selected temperature ranges using varying heating rates. Subsequently, thixo-compression tests were performed. The study found that during reheating and isothermal holding, the elongated microstructure of the as-supplied alloy can transform into equiaxed or spherical grains. The presence of recrystallized grains was found to be closely linked to the penetration of the liquid phase into the recrystallized grain boundaries. Furthermore, it was observed that higher heating rates resulted in smaller grain sizes. The thixotropic flow behavior of the alloy with various microstructures was analyzed using the true stress–strain curves obtained by thixo-compression experiments, which exhibited three stages: a rapid increase in true stress to a peak value, followed by a decrease in true stress and a steady stress until the end of compression. The stress fluctuated with strain during the formation of the slurry at a strain rate of 10 s⁻¹, indicating the significant role of strain rate in material flow during semisolid formation. Full article

Considering the ever-increasing interests in natural gas hydrates, a better and more precise knowledge of how host sediments interact with hydrates and affect the formation process is crucial. Yet less is reported for the effects of sediments on structure II hydrate formation with complex guest compositions. In this study, experimental simulations were performed based on the natural reservoir in Qilian Mountain permafrost in China (QMP) due to its unique properties. Mixed gas hydrates containing CH₄, C₂H₆, C₃H₈, and CO₂ were synthesized with the presence of natural sediments from QMP, with quartz sands, and without sediments under identical p–T conditions. The promoting effects of sediments regardless of the grain size and species were confirmed on hydrate formation kinetics. The ice-to-hydrate conversion rate with quartz sand and natural QMP sediments increased by 23.5% and 32.7%, respectively. The compositions of the initial hydrate phase varied, but the difference became smaller in the resulting hydrate phases, having reached a steady state. Beside the structure II hydrate phase, another coexisting solid phase, neither ice nor structure I hydrate, was observed in the system with QMP sediments, which was inferred as an amorphous hydrate phase. These findings are essential to understand the mixed gas hydrates in QMP and may shed light on other natural hydrate reservoirs with complex gas compositions. Full article

Designing a composite, possibly strengthened by a dispersion of (fine) oxides, is a favorable way to improve the mechanical characteristics of Cu while maintaining its advantageous electric conductivity. The aim of this study was to perform mechanical alloying of a Cu powder with a powder of Al₂O₃ oxide, seal the powder mixture into evacuated Cu tubular containers, i.e., cans, and apply gradual direct consolidation via rotary swaging at elevated temperatures, as well as at room temperature (final passes) to find the most convenient way to produce the designed Al₂O₃ particle-strengthened Cu composite. The composites swaged with the total swaging degree of 1.83 to consolidated rods with a diameter of 10 mm were subjected to measurements of electroconductivity, investigations of mechanical behavior via compression testing, and detailed microstructure observations. The results revealed that the applied swaging degree was sufficient to fully consolidate the canned powders, even at moderate and ambient temperatures. In other words, the final structures, featuring ultra-fine grains, did not exhibit voids or remnants of unconsolidated powder particles. The swaged composites featured favorable plasticity regardless of the selected processing route. The flow stress curves exhibited the establishment of steady states with increasing strain, regardless of the applied strain rate. The electroconductivity of the composite swaged at elevated temperatures, featuring homogeneous distribution of strengthening oxide particles and the average grain size of 1.8 µm², reaching 80% IACS (International Annealed Copper Standard). Full article