Diffusion Of Atoms Research Articles

Pore engineering of Porous Materials: Effects and Applications.

Porous materials, characterized by their controllable pore size, high specific surface area, and controlled space functionality, have become cross-scale structures with microenvironment effects and multiple functions and have gained tremendous attention in the fields of catalysis, energy storage, and biomedicine. They have evolved from initial nanopores to multiscale pore-cavity designs with yolk-shell, multishells, or asymmetric structures, such as bottle-shaped, multichambered, and branching architectures. Various synthesis strategies have been developed for the interfacial engineering of porous structures, including bottom-up approaches by using liquid-liquid or liquid-solid interfaces "templating" and top-down approaches toward chemical tailoring of polymers with different cross-linking degrees, as well as interface transformation using the Oswald ripening, Kirkendall effect, or atomic diffusion and rearrangement methods. These techniques permit the design of functional porous materials with diverse microenvironment effects, such as the pore size effect, pore enrichment effect, pore isolation and synergistic effect, and pore local field enhancement effect, for enhanced applications. In this review, we delve into the bottom-up and top-down interfacial-oriented synthesis approaches of porous structures with advanced structures and microenvironment effects. We also discuss the recent progress in the applications of these collaborative effects and structure-activity relationships in the areas of catalysis, energy storage, electrochemical conversion, and biomedicine. Finally, we outline the persisting obstacles and prospective avenues in terms of controlled synthesis and functionalization of porous engineering. The perspectives proposed in this paper may contribute to promote wider applications in various interdisciplinary fields within the confined dimensions of porous structures.

Effect of Material Extrusion Method on the Microstructure and Mechanical Properties of Copper Parts

In the present study, three extrusion-based Additive Manufacturing (AM) technologies were considered: Fused Filament Fabrication (FFF), Pellet Extrusion Process (PEP) and Atomic Diffusion Additive Manufacturing (ADAM). In order to compare these technologies, the same initial material was employed: a copper filament commercialized by Markforged® (Waltham, MA, USA). The copper filament was employed as received for ADAM and FFF technologies and shredded for PEP technology. Different printing parameters were studied for each technology (except for ADAM, which does not allow it) and the manufactured disc-shaped and tensile test parts were debindered and sintered under the same conditions. Part density, micrography and mechanical properties were analyzed. The density was observed to change with geometry, showing a relative density of around 95% for the tensile test parts through all the technologies but lower relative densities for the disc-shaped parts: around 90% for ADAM, between 85–88% for PEP and between 90–94% for optimized FFF printing parameters. The micrographies present big cavities between infill and contour for ADAM, whereas such cavities were not observed in either PEP or FFF parts. On the other hand, the parts made with PEP showed less and smaller porosity, but they had poor surface finishing, indicating that some printing parameters should be readjusted. Finally, the FFF parts had a better finishing but exhibited a non-uniform pore distribution. Concerning the mechanical properties, all the printed parts show similar properties.

Synergistic optimization of microstructure and mechanical properties of AISI 430 ferritic stainless steel via dual-phase zone annealing process

This study systematically evaluated the impact of dual-phase zone annealing on the microstructure and mechanical properties of cold-rolled 430 Ferritic Stainless Steel (FSS), focusing on the phase transformation process from ferrite to austenite and the dissolution and precipitation behavior of the M23C6 phase. The annealing process significantly affects the microstructural characteristics of ferrite and martensite, including grain size, the distribution of grains of various sizes, aspect ratio, and phase content. The amount of martensite first increased and then decreased with rising annealing temperatures. The sample annealed at 1050 °C exhibited the highest martensite content of 29.5%. The variation in martensite content reflects the complex interplay between phase transformation kinetics and thermodynamic equilibrium. As the annealing time increased or temperature rose, the M23C6 phase at ferrite grain boundaries gradually dissolved and re-precipitated at higher temperatures. At lower temperatures, the M23C6 phase precipitated within martensite, but as the temperature increased, the precipitated phase diminished until it disappeared. Temperature changes affect the solubility of precipitates, atomic diffusion rates, and the uniform distribution of atoms across different phases. Appropriately increasing annealing temperature or extending annealing duration enhances the strength and hardness of 430 FSS. However, excessively high annealing temperatures lead to a decline in mechanical properties. The sample annealed at 1000 °C demonstrated the best mechanical properties, achieving a tensile strength of 783 MPa and an elongation of 16.0%. Furthermore, the analysis of fracture morphology and yield behavior in stress-strain curves further elucidated the macroscopic mechanical performance of the material.

Evolution of inclusions in DH36 grade ship plate steel during high heat input welding

In this paper, the solid solution and precipitation behavior of inclusions on the surface and 1/2 thickness of the tested steel plate under the condition of welding heat input of 400 kJ/cm is investigated by using laser confocal experiments with hot-rolled state DH36 ship plate steel as the research object, and the mechanism of the effect of inclusions on the phase transformation of an acicular ferrite is revealed. The results show that the inclusions of the tested steel are mainly composed of Oxide-MnS, MnS, Oxide, TiN, Spinel, etc. The amount of inclusions on the surface of the tested steel plate is significantly higher than that at the 1/2 thickness position. During the heating stage, the small inclusions on the surface immediately disappeared, and the large inclusions gradually solidified in the matrix; atomic diffusion occurred at the bond between the inclusions and the matrix; while the small inclusions at the 1/2 thickness position gradually disappeared at the beginning of the heating stage, and the inclusions began to precipitate and grow when the temperature was increased to 990 °C. The acicular ferrite preferentially nucleates and grows near the boundary of the inclusions during the post-weld cooling stage, and its growth ends when two acicular ferrites cross.

Fully resolved simulations of micro-unit composite fuel in hydroxyl-terminated polybutadiene (HTPB): Al@AP

Micro-unit composite fuel (Al@AP) is a promising strategy for achieving stable and efficient combustion, due to its reduced pressure dependency and ability to prevent agglomeration. This research focuses on the heat and mass transfer as well as the reaction mechanism within the Al@AP structure through molecular modeling. The direct contact between the metal fuel and oxidizer leads to an immediate redox reaction at the corresponding interface, facilitating the diffusion of oxygen atoms from surrounding AP molecules into the Al particles. Heat is transferred from the Al particles to the surrounding oxidizer, instead of inward heat transfer from the AP-HTPB interface as observed in the conventional Al/AP composite. This novel assembly method significantly accelerates the reaction rate, more than doubling that of the conventional Al/AP composite. Moreover, the research explores combustion behavior at diverse pressure conditions (condensed phase vs. vacuum), revealing pressure’s minimal impact on the rapid burning stage primarily driven by diffusion mechanism. Under vacuum, reaction products propel burning Al within gas flow, transitioning to surface reactions on Al particles with gaseous products. Regarding agglomerations, the coalescence of two burning Al droplets is observed to form a larger product owing to the reduced distances within AP particle. The Al droplets are almost consumed before coalescence due to the rapid combustion, resulting in no negative impact on the Al combustion. The above findings highlight the unique features of micro-unit composite fuel in the application of solid propellants, and serve as an atomistic guide for propellant manufacturing and design.

Scalable and Non-Destructive Analysis of Battery Degradation and Performance

Electrochemical processes occurring during battery operation are highly complex. Thermodynamic, kinetic and transport-based phenomena all play a significant role in the dynamic evolution of the time-based electrochemical response of any battery system. Researchers have long sought to quantify these various interconnected phenomena, but to-date elucidating the role these mechanisms play in the performance of battery systems has been challenging or impossible. To aid researchers in uncovering the mechanistic behavior of their systems, we have developed a novel testing protocol and corresponding data analysis method that is scalable and non-destructive (SAND). This protocol and analysis can be applied to any battery system, in any form factor, at any point in the battery life cycle; including R&D, manufacturing, and second life battery screening to name a few.The testing protocol is based on a series of galvanostatic current pulses at a variety of currents and/or times. By utilizing the natural differences in time-constant of the various electrochemical phenomena inside a battery, this technique can probe each mechanism to glean novel insight using standard battery cycling equipment. Specifically, the insights this technique can provide include: the electrochemical potential of each phase during lithiation and delithiation quantification of electrode kinetics at various states-of-charge (including exchange current density estimation from the Butler-Volmer model)quantitative estimation of transport-based polarization losses from both the electrodes and the electrolyte. This analysis provides an un-parallelled level of mechanistic insight into the inner workings of any battery, at any point, spanning small-scale R&D cells to large format cells in mass production.To demonstrate the capabilities of this technique this presentation will focus on the analysis of multiple cells (same model) from a Tier 1 supplier containing a lithium transition metal oxide cathode material cycled at various temperatures. These results have led to an extensive set of conclusions. For brevity a few of these conclusions are: The initial exchange current density of these cells is calculated to be 25.5 A/m2. The H2→H3 phase transition during charging is the least efficient phase, exhibiting the largest power loss during chargingThe inefficiencies in the H2→H3 phase transition are dominated by solid-state lithium atom diffusion within the electrodes. At low current densities the overpotential associated with transport is the dominant source of losses, while at higher currents kinetic overpotentials become the dominant source. In summary, we have developed a novel characterization technique requiring only standard battery cycling equipment that can be easily and quickly performed by researchers around the world. Whether on small-scale coin cells or large-format pouch cells, the analysis from this technique will give the battery community insight into the thermodynamic, kinetic, and transport properties of their system. Moreover, this technique can be applied in a variety of contexts related to battery development, including understanding degradation mechanisms, performing root-cause analysis of manufacturing defects, or providing more accurate parameters for computational modeling.

(Invited) High-Density, Flip-Chip Alkali Doping of Graphene and Observation of the Lifshitz Transition

The physical properties of 2D materials can be tuned by doping to extreme charge carrier density, enabling the investigation of phenomena such as superconductivity, charge density waves, band inversion and Lifshitz transitions. For example, it is variably estimated that an electron density of 3.7 − 5.1 × 1014 cm−2 is required in order to induce a Lifshitz transition in monolayer graphene [1,2]. At this charge density, the Fermi surface passes through the hyperbolic saddle point of graphene at the M-pointin the Brillouin zone, where the Fermi surface geometry changes and the density of states diverges at a van Hove singularity. Achieving charge density sufficient to reach the Lifshitz transition has been limited to chemical doping in ultra-high vacuum (UHV) conditions and observation by angle resolved photoemission spectroscopy (ARPES). Integrating experimental methods for charge transport measurements with UHV chemical doping conditions is typically difficult. Field-effect methods including dielectric gating and ionic liquid gating are readily integrated with charge transport measurements, but achieve lower charge carrier density than UHV chemical doping methods.We report here an integrated flip-chip method to dope graphene by alkali vapour in the diffusive regime, suitable for charge transport measurements at ultra-high charge carrier density. We introduce a liquid cesium droplet source into a sealed cavity filled with inert gas (argon) to dope a monolayer graphene Hall bar on a quartz substrate by the process of cesium atom diffusion, adsorption and ionization at the graphene surface (Fig. a). Doping can be monitored by operando ac Hall measurement of longitudinal Rxx and transverse Rxy resistance of graphene (Fig. b). An optical image of a representative graphene Hall bar is shown in Fig. c (scale bar = 100 μm). The measured time dependent Rxx and Rxy during Cs exposure can be used to determine the time dependent electron density n and electron mobility μ.Upon sealing, the flip-chip assembly containing doped graphene is stable in ambient conditions, enabling sample characterization by various means, including non-resonant Raman scattering measurement through the transparent quartz window. Charge transport versus temperature and magnetic field in a variable temperature insert was performed, including Hall measurements to B = 7 T (Fig. d). By these methods, we confirm that flip-chip doping with Cs can be used to achieve charge density exceeding 4 × 1014 cm−2 . The Lifshitz transition is observed via the inversion of cyclotron effective mass, corresponding to sign inversion of the Hall coefficient RH (Fig. d). Employing a third-nearest-neighbour tight binding (3NNTB) calculation, we estimate the Fermi energies and Fermi surfaces corresponding to the charge densities observed by high field magnetotransport (Fig. e). At the Lifshitz transition, the Fermi surface transforms abruptly from that of electron pockets around the K and K' points, to a hole pocket around Γ. Further experimental observations will be discussed.In summary, our findings show that chemical doping, hitherto restricted to ultra-high vacuum conditions, can be applied in a diffusive regime at ambient pressure in an inert gas environment. We anticipate that our flip-chip doping method can be applied to a variety of 2D materials and dopant speciies beyond monolayer graphene and Cs.[1] P. Rosenzweig et al., "Overdoping graphene beyond the van Hove singularity", Physical Review Letters 125, 176403 (2020).[2] A. Zaarour et al., "Flat band and Lifshitz transition in long-range-ordered supergraphene obtained by Erbium intercalation. Physical Review Research 5, 013099 (2023). Figure 1

Continuous polyamorphic transition in high-entropy metallic glass

Polyamorphic transition (PT) is a compelling and pivotal physical phenomenon in the field of glass and materials science. Understanding this transition is of scientific and technological significance, as it offers an important pathway for effectively tuning the structure and property of glasses. In contrast to the PT observed in conventional metallic glasses (MGs), which typically exhibit a pronounced first-order nature, herein we report a continuous PT (CPT) without first-order characteristics in high-entropy MGs (HEMGs) upon heating. This CPT behavior is featured by the continuous structural evolution at the atomic level and an increasing chemical concentration gradient with temperature, but no abrupt reduction in volume and energy. The continuous transformation is associated with the absence of local favorable structures and chemical heterogeneity caused by the high configurational entropy, which limits the distance and frequency of atomic diffusion. As a result of the CPT, numerous glass states can be generated, which provides an opportunity to understand the nature, atomic packing, formability, and properties of MGs. Moreover, this discovery highlights the implication of configurational entropy in exploring polyamorphic glasses with an identical composition but highly tunable structures and properties.

Study on dynamic behavior of atomic diffusion at interface between TiAlN coating and WC matrix

The molecular dynamics method was used to simulate the dynamic behavior change of atomic diffusion at interface between TiAlN coating and WC matrix in the carbide tool with TiAlN coating. It was determined that the one-way diffusion of C atom to TiAlN coating resulted in serious lattice distortion on the surface. Meanwhile, the whole diffusion process was accompanied by the continuous increase of stress, resulting in gradual deformation of the material. The interface bonding performance also decreases. Then, the surface energy and atomic layer convergence are calculated by the first-principles method. The TiAlN crystal surface is stable at 8 atomic layers, and the WC crystal surface is stable at 6 atomic layers, and then a stable microinterface model can be built. The relaxation processing for the established model was performed to determine the diffusion path of C atom. Then, the energy barrier, adhesion work, interface energy and fracture toughness of different transition state structures in the diffusion path of C atom are calculated, and it is determined that with the deepening of the diffusion of C atom in the coating, the binding ability of the interface of the coating base and the performance of the coating tool are more and more serious damage. Finally, the cutting experiment of TiAlN coated tool is designed. According to the surface topography of TiAlN coated tool in different periods, the simulation and theoretical analysis mentioned above are verified, which provides a theoretical basis for the subsequent research on improving the performance of coated tool.

Intermittent cluster dynamics and temporal fractional diffusion in a bulk metallic glass

Glassy solids evolve towards lower-energy structural states by physical aging. This can be characterized by structural relaxation times, the assessment of which is essential for understanding the glass’ time-dependent property changes. Conducted over short times, a continuous increase of relaxation times with time is seen, suggesting a time-dependent dissipative transport mechanism. By focusing on micro-structural rearrangements at the atomic-scale, we demonstrate the emergence of sub-diffusive anomalous transport and therefore temporal fractional diffusion in a metallic glass, which we track via coherent x-ray scattering conducted over more than 300,000 s. At the longest probed decorrelation times, a transition from classical stretched exponential to a power-law behavior occurs, which in concert with atomistic simulations reveals collective and intermittent atomic motion. Our observations give a physical basis for classical stretched exponential relaxation behavior, uncover a new power-law governed collective transport regime for metallic glasses at long and practically relevant time-scales, and demonstrate a rich and highly non-monotonous aging response in a glassy solid, thereby challenging the common framework of homogeneous aging and atomic scale diffusion.

Circular Economy Opportunity Utilizing Fitness for Service Methodology for Aged HIC Defected Reboiler

The phenomenon of Hydrogen-Induced Cracking (HIC) is a cracking mechanism of carbon steel material as in ASME SA- 516 pressure containing equipment operating in a wet sour H2S environment. Such phenomenon is mainly driven by the diffusion of hydrogen atoms into the carbon steel due to corrosion reaction. The Structural integrity of an in-service equipment that contain a flaw or damage such as HIC could be scrutinised using Fitness For Service (FFS) assessments as quantitative engineering evaluations. The assessments are conducted using methodologies specifically prepared for pressurized equipment. These assessments provide standard procedure that can be used to determine if pressurized equipment containing flaws could continue to operate safely for certain period of time. FFS assessments are recognized and referenced by the API Codes and Standards such as API- 510, 570, & 653, and by National Board-23. Those assessments are considered suitable methodologies for evaluating the structural integrity of pressurized equipment as heat exchangers, pressure vessels, piping systems and storage tanks where inspection has revealed degradation and flaws in the equipment. The objective of this paper isto illustrate API-579 case study of HIC of fifty (50) years operating pressurized equipment, namely reboiler.

Enhanced precision lapping techniques for fused quartz optical elements: Synergistic integration of mechanical and mechanochemical processes

Fused silica is a material of great importance in the field of precision optical equipment due to its excellent optical properties, e.g. photolithography, laser fusion devices, etc. However, the material's inherent hardness and brittleness render it a challenging material to process. Therefore, an enhanced precision lapping technique was proposed, i.e., a synergistic integration of mechanical and mechanochemical lapping processes. Firstly, a synergistic lapping tool with an outer layer of porous diamond abrasive and an inner layer of CeO2 abrasive was fabricated by hot press forming method. The reduction of layer thickness due to abrasion and detachment of the porous diamond abrasive layer was analyzed, and the lapping mechanism of porous diamond on fused silica materials was revealed. The surface creation mechanism of the CeO2 abrasive layer on fused silica material was analyzed by molecular dynamics simulation and the results were validated by enhanced precision lapping experiment. The results demonstrated that the surface atoms of fused silica are removed in a ‘remote disordered’ manner by conventional diamond abrasive grains, whereas the surface atoms of fused silica are removed in a ‘proximally ordered’ manner by the fine porous diamond abrasive grains through the micro-multi-flute cutting. Consequently, the grinding of a porous diamond abrasive layer can significantly reduce the processing damage, with the surface roughness of the lapping Ra reaching 199 nm. The lapping of CeO2 induces the formation of an oxygen bridge on the surface of fused silica, which then produces bending vibration. This is followed by the adsorption and diffusion of oxygen atoms, which form the -Ce-O-Si intermediate with the oxygen atoms shared with the fused silica. This process results in the formation of CeSiO3 and CaSiO3 compounds. The final surface quality with a surface roughness of Ra 21 nm was obtained by mechanochemical lapping of the cerium oxide abrasive layer. The single mechanical-mechanochemical composite ultra-precision lapping process provides technical support for the efficient and low-damage processing of hard and brittle components.

Study on interface diffusion and welding strength of Cu/Sn dissimilar metal electromagnetic pulse welding based on molecular dynamics simulation

The electromagnetic pulse welding (EMPW) method stands out for its rapid welding time, superior precision, and high automation, making it ideal for materials with significant melting point differences, specifically copper and tin, which often struggle to form robust joints. Utilizing molecular dynamics simulations, this study aims to investigate the interface evolution patterns under varying collision velocities and assess the welding strength of joints subjected to distinct tangential velocities. The findings reveal a profound correlation between the diffusion coefficient, system temperature, and the collision velocity during the simulated EMPW vertical collision. Specifically, at a collision velocity of Vz = 500 m/s, the interface tends to be linear, suggesting an unstable bonding state. However, when the collision velocity exceeds 700 m/s, a more intense diffusion of copper and tin atoms is observed, resulting in a wavy interface indicative of enhanced bonding capabilities. Furthermore, the study examines the shear behavior of welded joints subjected to oblique collisions with varying tangential velocities. The analysis reveals a transition in fracture locations from the joint-joint interface to the joint-base material interface and ultimately to the base material itself, as the tangential velocity increases from 50 m/s to 100 m/s and surpasses 200 m/s. This comprehensive evaluation of EMPW process variables provides a theoretical foundation for optimizing the mechanical properties of copper-tin EMPW joints.

Effect of the cooling behavior on phase transformation and mechanical property of RAFM steel

The phase transition behavior of RAFM steel during Hot Isostatic Pressing (HIP) diffusion bonding process would extensively impact the service performance of blanket component in the future fusion reactor. In this paper, the influence of cooling behavior on phase transformation and mechanical property of RAFM steel was studied using a combination of Electron Backscatter Diffraction (EBSD) and a thermal dilatometer. The results show that as the cooling rate decreases after austenitizing, the phase category transitions from a single-phase of martensite to the dual-phase of martensite and ferrite, along with an increase in the rate of ferrite, minor martensite packets and high-angle grain boundaries (HAGB). As the cooling rate decreases, the initial temperature for martensite transformation significantly increases, and the transformation driving force slowdown. Moreover, specimens processed with HIP cooling show lower strength, higher elongation, and a greater standard deviation of hardness compared to air cooling. These differences can be attributed to the formation of ferrite and the diffusion of carbon atoms within martensite. The research findings reveal the phase transition and microstructural evolution of martensite under varying cooling conditions, providing a reference for the controlling the cooling process after HIP bonding of blanket steel components in the future fusion reactor.

Microstructure, mechanical performance, thermal stability, and oxidation resistance of AlCrN/AlCrBN nano-multilayer coating

This study synthesized three different coatings using multi-arc ion coating technology: AlCrN, AlCrBN monolayer, and AlCrN/AlCrBN nano-multilayer coatings. A comparative investigation was conducted on the microstructure, mechanical performance, thermal stability, and oxidation properties of coatings. The results indicate the AlCrN, AlCrBN, and AlCrN/AlCrBN nano-multilayer coatings consist of face-centered cubic solid solution fcc-(Cr, Al)N phase. AlCrN/AlCrBN nano-multilayer coatings also exhibit better thermal stability and oxidation resistance than AlCrN and AlCrBN monolayer coatings. The AlCrN/AlCrBN nano-multilayer coatings have a dense, protective, Al2O3 layer formed during high-temperature oxidation. During high-temperature oxidation, the AM (amplitude modulation) decomposition of the AlCrN and AlCrBN monolayers in the AlCrN/AlCrBN nanolayers is inhibited by the structure of the multilayer coatings. The formation of dense oxides on the surface prevents the diffusion of Al and Cr atoms to the outside and prevents the diffusion of oxygen from the outside to the inside of the AlCrN/AlCrBN nanolayers.

High-performance MIM-type aluminum electrolytic capacitors with durable waterproof and wide temperature window

Capacitors are indispensable components of electronic circuits. Filter capacitors, mainly dominated by electrolytic capacitors, are critical for the accurate power supply of integrated circuits for central processors and storage devices, affecting the performance of advanced and sophisticated electronic equipment. However, electrolytic capacitors are restricted in working temperatures (<150 °C) and humidity conditions due to the inherent characteristics of MnO2 or polymer cathodes that tend to deteriorate at high temperatures and moisture. Here, high temperature resistant and conductivity SnO2 cathode and MIM-like (SnO2/AAO/Al) structures are introduced into aluminum electrolytic capacitors via ALD technology. First achieved in a higher temperature window (-60 °C∼330 °C), the capacitor maintains a stable capacity (114.5 ± 3.6 μF/cm2) and phase angles (-89.5 ± 0.2°) at 120 Hz. The tight stacking and conformability of ALD even allows unencapsulated devices to exhibit durable waterproof, showing stable performance for immersion in water within 90 days. Moreover, a thin amorphous Al2O3 (A) as buffer layer is introduced at electrode/dielectric interface, aiming to barrier the interfacial atomic diffusion at the SnO2/AAO. A stable vdW SnO2/A/AAO interfaces is obtained, proven by MD and DFT calculations. Impressively, the capacitor exhibits high reliability with high breakdown field strength (5.5 MV/cm), low leakage current (7.2 × 10–7 A/cm2 at 1 V) and low loss (2% at 120 Hz). Applied in circuits, the capacitor filters 100 kHz signal with a Vripple of only 15 mV and shows excellent charging and discharging behaviors. The work may pave the way for the practical application of electrolytic capacitors in harsh working environments.

Ubiquitous short-range order in multi-principal element alloys

Recent research in multi-principal element alloys (MPEAs) has increasingly focused on the role of short-range order (SRO) on material performance. However, the mechanisms of SRO formation and its precise control remain elusive, limiting the progress of SRO engineering. Here, leveraging advanced additive manufacturing techniques that produce samples with a wide range of cooling rates (up to 107 K s−1) and an enhanced semi-quantitative electron microscopy method, we characterize SRO in three CoCrNi-based face-centered-cubic (FCC) MPEAs. Surprisingly, irrespective of the processing and thermal treatment history, all samples exhibit similar levels of SRO. Atomistic simulations reveal that during solidification, prevalent local chemical order arises in the liquid-solid interface (solidification front) even under the extreme cooling rate of 1011 K s−1. This phenomenon stems from the swift atomic diffusion in the supercooled liquid, which matches or even surpasses the rate of solidification. Therefore, SRO is an inherent characteristic of most FCC MPEAs, insensitive to variations in cooling rates and even annealing treatments typically available in experiments.

Effect of quenching-super intercritical quenching-tempering process on the evolution of microstructure and the yield ratio of Ni–Cr–Mo steel

In the present study, X-ray diffraction, electron probe microanalysis, and transmission electron microscopy, among other advanced characterization methods, were used to analyze the microstructure and mechanical properties of the 9Ni steel treated at different heat treatment conditions. The goal was to reveal the dual-optimal strengthening mechanism of “low yield ratio and ultra-cryogenic toughness” of the investigated steel. The experimental results indicated that the 9Ni steel, designed with a ''Ni–Cr–Mo'' multi-elemental composition, balanced its hardenability and corrosion resistance. After the Q-Q' (710 °C) -T process, the diffusion of Cr and Mo atoms in the steel accelerated, while the Fe displacement diffusion formed a more stable FeNi face-centered cubic structure with a lattice energy of −76.98 eV. At this stage, the microstructure of the investigated steel primarily consisted of tempered sorbite (with an average effective grain size of 13.1 μm), tempered martensite lath clusters, reversed austenite, and precipitated Cr23C6 and Mo2C particles. This composition resulted in a low yield ratio. The concentration of Ni near the lath clusters promoted the formation of the reversed austenite with a volume fraction of 10.2%, ensuring the cryogenic toughness of the investigated steel.

Effect of ARB process on the plane stress fracture toughness, creep properties and forming limit diagram of Aluminum/BN/Copper composite strips

Abstract In the present study, Al/BN/Cu bimetal composite strips were fabricated via the accumulative roll bonding (ARB) process. Then, fracture toughness, mechanical properties, bonding strength, forming limit diagram (FLD), and creep properties have been studied via experimental techniques. The study of creep behavior and mechanical properties showed a formation of a 17 μm atomic diffusion layer at the interface during ARB under three creep loading conditions namely 35 MPa at 225 °C, 35 MPa at 275 °C, and 30 MPa at 225 °C. The tensile strength of samples improved to 144 MPa which was about 234% in comparison with the primary Al monolithic sample. It was observed by the SEM that the fracture types changed to shear kind after higher passes including prominent effects on the fracture morphology. FLD curve dropped severely after the first ARB pass and then began to enhance at higher passes. The fracture toughness value of 30 MPa m1/2 was achieved after ten ARB passes. An intermetallic composition was created near Al. The creep failure time did decline by 44% and the stress level decreased by 13% at a constant temperature. It can be concluded that applied temperature and stress effect on creep properties especially leads to increasing the slope of creep curves with higher stresses. This trend was opposite for the creep temperature at higher temperature levels. Moreover, dynamic recrystallization was observed through the crystalline structure of samples.

Determination of γ/γ' interface free energy for solid state precipitation in Ni-Al alloys from molecular dynamics simulation.

Interface free energy is a fundamental material parameter needed to predict the nucleation and growth of new phases. The high cost of experimentally determining this parameter makes it an ideal target for calculation through a physically informed simulation. Direct determination of interface free energy has many challenges, especially for solid-solid transformations. Indirect determination of the interface free energy from the nucleation data has been done in the case of solidification. However, a slow on molecular dynamics (MD) simulation time scale atomic diffusion makes this method not applicable to the case of nucleation from the solid phase when precipitate composition is different from that in matrix. To address this challenge, we outline the development of a new technique for determining the critical nucleus size from an MD simulation using a recently developed method to accelerate solid-state diffusion. The accuracy of our approach for the Ni-Al system for Ni3Al (γ') precipitates in a Ni-Al (γ) matrix is demonstrated well within experimental accuracy and greatly improves upon previous computational methods [Herrnring et al., Acta Mater. 215(8), 117053 (2021)].