Formation Of Short-range Order Research Articles

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.

First-principles and cluster expansion study of the effect of magnetism on short-range order in Fe–Ni–Cr austenitic stainless steels

Short-range order (SRO), the regular and predictable arrangement of atoms over short distances, alters the mechanical properties of technologically relevant structural materials such as medium/high entropy alloys and austenitic stainless steels. In this study, we present a generalized spin cluster expansion (CE) model and show that magnetism is a primary factor influencing the level of SRO present in austenitic Fe–Ni–Cr alloys. The spin CE consists of a chemical cluster expansion combined with an Ising model for Fe–Ni–Cr austenitic alloys. It explicitly accounts for local magnetic exchange interactions, thereby capturing the effects of finite temperature magnetism on SRO. Model parameters are obtained by fitting to a first-principles data set comprising both chemically and magnetically diverse face-centered cubic configurations. The magnitude of the magnetic exchange interactions are found to be comparable to the chemical interactions. Compared to a conventional implicit magnetism CE built from only magnetic ground state configurations, the spin CE shows improved performance on several experimental benchmarks over a broad spectrum of compositions, particularly at higher temperatures due to the explicit treatment of magnetic disorder. We find that SRO is strongly influenced by alloy Cr content, since Cr atoms prefer to align antiferromagnetically with nearest neighbors but become magnetically frustrated with increasing Cr concentration. Using the spin CE, we predict that increasing the Cr concentration in typical austenitic stainless steels promotes the formation of SRO and increases order–disorder transition temperatures. This study underscores the significance of considering magnetic interactions explicitly when exploring the thermodynamic properties of complex transition metal alloys. It also highlights guidelines for customizing SRO through adjustments of alloy composition.

Cu–Mg dilute alloys- Assessment of strengthening, internal friction, and electrical conductivity

Overcoming the strength-conductivity trade-off has long been an issue for commercial copper alloys. However, a synergy between strength and electrical conductivity can be achieved by reasonable adjustment of the microstructure. It is known that the formation of Short-Range Order (SRO) in a solid solution can change the mechanical and electrical behavior of the materials. Currently, the prediction of SRO formation is widely based on computational techniques. In the present study, Miedema's thermodynamical approach is applied for the assessment of alloying elements' potential to form SRO in the copper lattice. According to this methodology, magnesium tends to form SRO with copper. The mechanical, electrical, and structural properties of the oxygen-free copper (Cu-OFC), Cu–Mg0.14 wt%, and Cu–Mg0.50 wt% strips fabricated by continuous extrusion forming process (Conform) have been investigated. According to the optical microscopy investigations, the average grain sizes of the Mg alloyed samples were observed to be lower compared to that of OFC. The Vickers hardness measurements were carried out and the hardness values for Cu-OFC, Cu–Mg0.14, and Cu–Mg0.50 samples were recorded as 56.3 HV, 76.05 HV, and 87.68 HV, and the yield strengths as 80, 150 and 260 MPa, respectively. A decrement in electrical conductivity was observed with the increasing Mg addition. Dynamic elastic modulus test measurements, which refer to internal friction, have been performed by impulse excitation technique to elucidate the presence of SRO in the dilute Cu–Mg systems. The anomalous “bump” formation in the temperature-elastic modulus profiles of the Cu–Mg samples was remarkable which has been attributed to the SRO formation.

Molecular dynamics simulations of the effect of starch on transport of water and ions through graphene nanopores.

We use molecular dynamics simulations to unravel the molecular level mechanisms underlying the structure and dynamics of water and ions flowing through nanoporous starch-graphene membranes. Our findings indicate that there is a significant tendency for the formation of short-range order in close proximity to the graphene membrane surface. This leads to a greater concentration of water and ions, suggesting strong interactions between the membrane and the saltwater solution. Furthermore, we found that the starch-graphene membrane was most efficient in sieving out ions when the starch loading is 15 wt.%, and the pore diameter is 14Å. At these conditions, the starch-graphene membrane showed a high water transport rate and maintained a high level of ion rejection. We investigated the effect of loading of starch and the pore diameter on the pressure-induced transport, structure, and dynamics of Na+, Cl-, and water using the GROMACS 2021.4 package. We further analyze the density profiles of water and ions in the context of ion-polymer and water-polymer interactions and provide mechanistic insights into the piston-induced flow of saltwater through the starch-graphene membranes using Visual Molecular Dynamics (VMD) software.

Modulating hierarchical self-assembly in thermoresponsive intrinsically disordered proteins through high-temperature incubation time

The cornerstone of structural biology is the unique relationship between protein sequence and the 3D structure at equilibrium. Although intrinsically disordered proteins (IDPs) do not fold into a specific 3D structure, breaking this paradigm, some IDPs exhibit large-scale organization, such as liquid–liquid phase separation. In such cases, the structural plasticity has the potential to form numerous self-assembled structures out of thermal equilibrium. Here, we report that high-temperature incubation time is a defining parameter for micro and nanoscale self-assembly of resilin-like IDPs. Interestingly, high-resolution scanning electron microscopy micrographs reveal that an extended incubation time leads to the formation of micron-size rods and ellipsoids that depend on the amino acid sequence. More surprisingly, a prolonged incubation time also induces amino acid composition-dependent formation of short-range nanoscale order, such as periodic lamellar nanostructures. We, therefore, suggest that regulating the period of high-temperature incubation, in the one-phase regime, can serve as a unique method of controlling the hierarchical self-assembly mechanism of structurally disordered proteins.

Effect of Alloying Elements on the Short-Range Orders and Atomic Diffusion Behavior of Liquid Al-9Si Cast Alloys.

To investigate the influence of alloying elements (Zn, Mg, and Cu) on the structural and dynamical properties of liquid Al-9Si alloy, we conducted ab initio molecular dynamics (AIMD) simulations. Our results indicate that the structure of Al-Si-M ternary alloys is determined with a combination of atomic radii and mixing enthalpy, while the dynamic properties are primarily influenced by electronic structure of the alloying elements. Specifically, the addition of Cu promotes the formation of Al-Cu short-range order (SRO), while Zn has a higher propensity for Zn-Zn SRO. The Al-Cu SRO in liquid alloy may serve as the precursor for the Al2Cu reinforcing phase in Al-Si-Cu alloys. Upon the addition of Mg, a greater number of relatively stable perfect and distorted icosahedral structures, as well as hcp and bcc ordered structures with lower energies, are observed. Additionally, the presence of Mg leads to a reduction in the atomic diffusion rates of Al and Si, while Cu and Zn exhibit complex diffusion behavior influenced by the presence of Si atoms.

Formation of Chemical Short-Range Order in Equiatomic CoNiCrFeMn High-Entropy Alloy. Atomistic MD/MC Simulation

Abstract—The formation of short-range order in the equiatomic high-entropy alloy (HEA) CoNiCrFeMn during annealing at moderate temperatures was studied using atomistic MD/MC simulation, including the exchange of atoms in the Monte Carlo (MC) scheme and the relaxation of its positions by the molecular dynamics (MD) method. It has been found that two types of chemical short range order (CSRO) regions are formed during annealing. One of them consist mainly of Fe and Co atoms, while others are enriched by Cr with Ni and Mn atoms at their boundaries. It is shown that the formation of short-range order includes several stages, the sequence of which is determined by the value of Cr–Cr, Fe–Co and Ni–Mn interatomic interactions.

Orientation dependence of the effect of short-range ordering on the plastic deformation of a medium entropy alloy

Multi-principal-element alloys (also known as medium and high entropy alloys) offer a much larger and richer design space than conventional alloys, providing opportunities for discovering new functionalities and their governing physics. Some of these alloys exhibit an outstanding combination of high strength and ductility, linked to the activation of various deformation modes triggered by low-energy stacking faults. However, a pressing question remains: Is the plasticity of medium- and high-entropy alloys governed only by stacking fault energy, or does atomic short-range order (SRO) play a role? To answer this, we investigated how SRO affects the deformation in single-crystalline NiCoCr, with previous contradictory findings. First, we established unique experimental evidence for SRO formation in bulk single crystals using high-energy synchrotron transmission X-Ray Diffraction. By tuning the degree of SRO by aging at high temperatures, twinning density and strain-induced martensitic phase transformation can be significantly increased in the [110] and [111] orientations under tension, increasing the tensile ductility; yet, no increase was observed along the [001] orientation due to lack of TWinning-Induced Plasticity (TWIP) or TRansformation-Induced Plasticity (TRIP), indicating a strong crystallographic orientation dependence. Our first-principles thermodynamic calculations unequivocally show SRO exists and governs the observed microstructural evolution and deformation hardening behavior. Here we find direct proof that SRO triggers a simultaneous TWIP and TRIP in NiCoCr, a rare microstructural evolution path. Our findings establish that the interplay of SRO and plasticity could be exploited to alter deformation modes and yield unprecedented mechanical response in medium- and high-entropy alloys.

Influence of short-range order on diffusion in multiprincipal element alloys from long-time atomistic simulations

Short-range order (SRO) is an important structural characteristic in multiprincipal element alloys (MPEAs) that plays a crucial role in their superior mechanical and physical properties. The development of SRO is usually tangled with structural evolution associated with diffusion-induced atomic transport, making it challenging to accurately determine the diffusion coefficients in MPEAs based on conventional modeling techniques. In this study, we combine machine learning with kinetic Monte Carlo to compute the self-diffusion coefficients in NiCoCr and NiFeCr MPEAs with distinct SRO tendencies under long-time structural evolution. Our results show that the long-term diffusion process governed by thermodynamic and kinetic factors can promote SRO formation due to chemically biased exchange between vacancy and specific components in the alloy. We further demonstrate that accurate diffusion coefficients in MPEAs can only be obtained through long-term simulations allowing for sufficient structural evolution. This study underlines the shortcomings of the presently widely employed techniques to study diffusion in MPEAs and highlights the intricate coupling between SRO and diffusion.

Dislocation plasticity in equiatomic NiCoCr alloys: Effect of short-range order

Equiatomic NiCoCr solid solutions have been shown by recent experiments and atomistic simulations to display exceptional mechanical properties that have been suggested to be linked to nanostructural short-range order (SRO) features that may arise from thermal treatments, such as annealing or/and aging. Here we use hybrid Monte Carlo--molecular dynamics simulations to gain further insights of thermal effects on the SRO formation as well as the edge dislocation plasticity mechanisms of equiatomic NiCoCr face-centered cubic solid solution. For that purpose, we utilize two well-known NiCoCr interatomic potentials, one of which displays well-documented SRO, believed to be linked to experimental evidence and labeled as the Li-Sheng-Ma potential, while the other (Farkas-Caro) does not. We use these two potentials to discern short-range ordering (from inherent randomness in random solid solutions) and understand how SROs influence dislocation depinning dynamics in various thermal annealing scenarios. In this context, we used robust, scale-dependent metrics to infer a characteristic SRO size in the Li-Sheng-Ma case by probing local concentration fluctuations which otherwise indicate uncorrelated patterns in the Farkas-Caro case in a close agreement with random alloys. Our Voronoi-based analysis shows meaningful variations of local misfit properties owing to the presence of SROs. Using relevant order parameters, we also report on the drastic increase of chemical ordering within the stacking fault region. More importantly, we find that the Li-Sheng-Ma potential leads to excellent edge dislocation depinning strength with low stacking fault width. Our findings indicate an enhanced roughening mechanism due to the SROs-misfit synergy that leads to significant improvements in dislocation glide resistance. We argue that the improvements in alloy strength have their atomistic origins in the interplay between nanoscopic SROs and atomic-level misfit properties.

Improved mechanical properties of Co-free high-entropy Cantor alloy: A first-principles study

High-entropy alloys (HEAs) have received attention because of their excellent mechanical and thermodynamic properties. A recent study revealed that Co-free face-centered cubic HEAs could improve strength and ductility, which is crucial for nuclear materials. Here we implemented first-principles calculations to explore the fundamental mechanism for enhancing the mechanical properties in Co-free Cr25Fe25Ni25Mn25 alloy. We found that the local lattice distortion of Co-free HEA was more significant than that of the well-known Cantor alloy. Furthermore, the short-range order formation in Co-free HEA caused the highly fluctuated stacking fault energy. Thus, the significant local lattice distortion and the non-uniform solid solution states comprising low- and high-stacking fault regions improve strength and ductility.

Effect of copper addition on tensile behavior in Fe-Cr-Ni stable austenitic stainless steel

Effects of Cu on tensile properties and deformation mechanisms were investigated in the stable Fe-Cr-Ni austenitic stainless steel with different wt% of Cu. Addition of Cu slightly decreased the yield strength and negligibly affected the ultimate tensile strength, while it induced a considerably large reduction of fracture elongation. Deformation twins were generated during the tensile deformation, and fraction of deformation twins decreased with increasing Cu content. However, degree of twinning was similar, regardless of Cu content, at the same tensile strain, which indicated that development of twins was interfered by earlier fracture triggered by Cu addition and hardly contributed to different hardening behavior at the early stage of deformation. Cu significantly altered the motion of dislocations. The low-Cu-containing alloy mainly developed tangled dislocations and cellular structures, whereas the high-Cu-containing alloy did planar slip and high density of dislocation walls. Although the addition of Cu decreased the shear modulus and increased the stacking fault energy, dislocation substructure changed from tangled to planar slip with increasing Cu content. This indicated that slip planarity of high-Cu-containing alloys might be originated from the formation of short-range ordering or clustering, thereby leading to the local stress concentration and reduction of the elongation.

Structure-property relationships in actinide containing molten salts – A review: Understanding and modelling the chemistry of nuclear fuel salts

Molten salts have recently received increased attention worldwide as key materials for sustainable and low-carbon energy supply technologies (e.g. thermal energy storage, concentrated solar power technologies, fission and fusion nuclear reactors) thanks to their appealing thermo-physical properties. In particular, they are used as fuel and coolant in the Molten Salt Reactor (MSR), considered a breakthrough technology for the next generation of nuclear fission reactors. This review focuses on the thermochemical and thermo-physical properties of actinide bearing fluoride and chloride salts, used as nuclear fuel in MSRs, and more specifically on the structure–property relationships. In the last 15 years, the knowledge on the structural properties of actinide containing salts has grown quite significantly thanks to the development of dedicated high temperature in situ X-ray Absorption Spectroscopy measurements, and to the advancement of atomistic simulations. These have evidenced the formation of short-range order in the liquid, which contributes to storing energy in the salt and directly influences transport and thermodynamic excess properties. This interrelationship is illustrated in the present article, which covers both experimental and computational information, as well as most recent developments in the modelling methods at the mesoscale of the structural, thermodynamic, density and viscosity properties using the CALPHAD methodology.

Short-range ordering and its impact on thermodynamic property of high-entropy alloys

Short-range ordering and atomic segregation have been observed experimentally in high-entropy alloys (HEAs). Understanding their impact on the melting temperature is crucial to the rational design of HEAs. Here we perform molecular dynamics simulations combined with Monte Carlo calculations to study the effects of elemental concentration, short-range ordering and atomic segregation on the melting temperature of AlCoFeNiCux HEAs (x = 0, 0.5 and 1). The simulation results demonstrate that the melting temperature of HEAs increases with decreasing Cu concentration and Cu atoms tend to break bonds more readily than other four types of atoms (Al, Co, Fe and Ni) during heating, even though Cu is not the one with the lowest melting temperature among the constituent component in pure form (melting temperature: Al < Cu < Ni < Co < Fe). We find that the reduced melting temperature is due to the formation of more short-range ordering and atomic segregation with phase boundary when the Cu concentration increases. The formation of short-range ordering and atomic segregation within Cu atoms are consistent with previous experimental observation of Al0.5CoCrFeNiCu HEAs and simulation results of CoCrFeNiCux HEAs (x = 0.4, 0.7 and 1) using another interatomic potential. Our findings highlight the importance of short-range ordering and atomic segregation in influencing the melting temperature of HEAs and provide guide for designing HEAs with desired thermodynamic properties.

Low-Temperature Synthesis of a Porous High-Entropy Transition-Metal Oxide as an Anode for High-Performance Lithium-Ion Batteries.

Transition-metal oxides (TMOs) are promising anode materials for high-performance lithium-ion batteries (LIBs) because of their abundant reserves and high theoretical capacity. However, the poor conductivity, unstable solid electrolyte interface (SEI) film, and poor cycling stability still limit their practical applications. As a novel kind of anode material, a high-entropy oxide (HEO) is a single-phase crystal structure composed of multiple metal elements, demonstrating a huge potential for energy storage applications due to the synergistic effect of various metal species. Herein, we have designed the porous spinel-phase HEO (Cr0.2Fe0.2Co0.2Ni0.2Zn0.2)3O4 synthesized at low temperature by a sol-gel method. On the one hand, the unique porous nanostructure not only promotes transport of the electrolyte but also alleviates the volume change of active materials upon cycling. On the other hand, the stabilization effect of entropy can suppress the formation of cation short-range order within the crystalline structure of HEO by a lattice distortion effect, thus guaranteeing a fast lithium-ion transport and achieving an excellent electrochemical performance. As a result, the as-prepared HEO-450 electrode delivers 1022 mAh/g after 1000 cycles at 1 A/g and 220 mAh/g at an ultrahigh current density of 30 A/g, respectively.

Shear banding instability in multicomponent metallic glasses: Interplay of composition and short-range order

The shear-banding instability in quasistatically driven bulk metallic glasses emerges from collective dynamics, mediated by shear transformation zones and associated nonlocal elastic interactions. It is also phenomenologically known that sharp structural features of shear bands are typically correlated to the sharpness of the plastic yielding transition, being predominant in commonly studied alloys composed of multiple different elements, that have very different atomic radii. However, in the opposite limit where elements' radii are relatively similar, plastic yielding of bulk metallic glasses is highly dependent on compositional and ordering features. In particular, a known mechanism at play involves the formation of short-range order dominated by icosahedra-based clusters. Here, we report on atomistic simulations of multicomponent metallic glasses with different chemical compositions showing that the degree of strain localization is largely controlled by the interplay between composition-driven icosahedra-ordering and collectively-driven shear transformation zones. By altering compositions, strain localization ranges from diffuse homogenized patterns to singular crack-like features. We quantify the dynamical yielding transition by measuring the atoms' susceptibility to plastic rearrangements, strongly correlated to the local atomic structure. We find that the abundance of short-range ordering of icosahedra within rearranging zones increases glassy materials' capacity to delocalize strain. This could be understood on the basis of structural heterogeneities that are enhanced by the presence of local order. The kind of plastic yielding can be often qualitatively inferred by the commonly used compositional descriptor that characterizes element associations, the misfit parameter ${\ensuremath{\delta}}_{a}$, and also by uncommon ones, such as shear-band width and shear-band dynamics' correlation parameters.

Atomic Short-Range Order in a Cation-Deficient Perovskite Anode for Fast-Charging and Long-Life Lithium-Ion Batteries.

Perovskite-type oxides are widely used for energy conversion and storage, but their rate-inhibiting phase transition and large volume change hinder the applications of most perovskite-type oxides for high-rate electrochemical energy storage. Here, it is shown that a cation-deficient perovskite CeNb3 O9 (CNO) can store a sufficient amount of lithium at a high charge/discharge rate, even when the sizes of the synthesized particles are on the order of micrometers. At 60 C (15 A g-1 ), corresponding to a 1 min charge, the CNO anode delivers over 52.8% of its capacity. In addition, the CNO anode material exhibits 96.6% capacity retention after 2000 charge-discharge cycles at 50 C (12.5 A g-1 ), indicating exceptional long-term cycling stability at high rates. The excellent cycling performance is attributed to the formation of atomic short-range order, which significantly prevents local and long-range structural rearrangements, stabilizing the host structure and being responsible for the small volume evolution. Moreover, the extremely high rate capacity can be explained by the intrinsically large interstitial sites in multiple directions, intercalation pseudocapacitance, atomic short-range order, and cation-vacancy-enhanced 3D-conduction networks for lithium ions. These structural characteristics and mechanisms can be used to design advanced perovskite electrode materials for fast-charging and long-life lithium-ion batteries.

Radiation-assisted chemical short-range order formation in high-entropy alloys

The chemical short-range order (CSRO) in a face-centered cubic equiatomic NiCoFeCrMn high-entropy alloy was revealed directly from advanced transmission electron microscopy in this work. Further, instead of controlling CSRO by conventional heat treatment, we show a new possibility of tailoring CSRO by radiation. The multiple types of atoms in such high-entropy system are more prone to form CSRO under radiation compared to that only under thermodynamic equilibrium conditions, due to the rearrangement of the chemical ordering during the non-equilibrium thermal spike period and the radiation-enhanced diffusion mechanism. Our results show an increase in the fraction of CSRO area by ∼45% after high-temperature irradiation with a dose of 50 DPA. Accordingly, we demonstrated that enhanced CSRO can be achieved by controlling the irradiation dose and temperature, which provides a new route to explore the kinetic evolution and effect of CSRO in complex alloy systems.

Synergetic effect of Si addition on mechanical properties in face-centered-cubic high entropy alloys: a first-principles study

High-entropy alloys (HEA) have been receiving increased attention for their excellent mechanical properties. Our recent study revealed that Si-doped face-centered cubic (FCC) HEAs have great potential to improve both strength and ductility. Here, we carried out first-principles calculations in cooperation with Monte Carlo simulation and structural factor analysis to explore the effect of Si addition on the macroscopic mechanical properties. As a result, Si addition increased the local lattice distortion and the stacking fault energy (SFE). Furthermore, the short-range order formation in Si-doped alloy caused highly fluctuated SFE. Thus, the heterogeneous solid solution states in which low and high stacking fault regions are distributed into the matrix were nucleated. This unique feature in Si-doped FCC-HEA induces ultrafine twin formation in Si-doped alloys, which can be a dominant factor in improving both strength and ductility.

Formation of Short-Range Atomic Order in Cu–Pd Alloys with a Low Palladium Content: Resistometric Study

Formation of Short-Range Atomic Order in Cu–Pd Alloys with a Low Palladium Content: Resistometric Study