Chemical Short-range Order Research Articles

An AIMD investigation on the structural properties of Fe-SiO2 melt for converting SiO2 inclusions into the reinforcing particles in Fe matrix

It is essential to understand the structural properties of Fe-SiO2 melt for converting SiO2 inclusions into the reinforcing particles in Fe matrix by liquid metallurgical synthesis. In this work, the structural behaviors of Si and O in Fe27SiO2, Fe24(SiO2)2, Fe21(SiO2)3, and Fe18(SiO2)4 melts at 2800 K and the formation of SiOn clusters in Fe24(SiO2)2 melt (20 at.% SiO2) from 2800 K to 2000 K are investigated by ab-initio molecular dynamics simulations. According to the pseudo-binary mixing enthalpies, partial pair correlation functions, and chemical short-range order, the critical composition range for the formation of SiOn clusters in Fe-SiO2 melt at 2800 K is from 20 at.% SiO2 to 30 at.% SiO2, and the critical temperature range in the quenching process of Fe24(SiO2)2 melt is from 2600 K to 2400 K. By further analyzing the distribution of partial coordination numbers and bond angles, the quenching process of Fe24(SiO2)2 melt from 2800 K to 2000 K can be divided into three stages: the homogenous Fe-SiO2 melt from 2800 K to 2600 K, the dispersive formation of SiOn clusters containing a considerable number of SiO4 tetrahedrons from 2600 K to 2200 K, the gathering of SiOn clusters contributed by the connection of SiO4 tetrahedrons in the pattern of OSi2 from 2200 K to 2000 K.

Nanostructure and dislocation interactions in refractory complex concentrated alloy: From chemical short-range order to nanoscale B2 precipitates

Refractory complex concentrated alloys (RCCAs) have emerged as a promising class of structural materials, demonstrating exceptional mechanical performance in aggressive environments. However, the complex atomic environments, significant lattice distortion, and vast compositional space of RCCAs present challenges to understanding the mechanisms that govern structure-property relationships. In this study, we explore the dislocation mechanisms in three model quaternary RCCAs, namely Mo25Nb10Ta25W40 (at. %), Mo25Nb25Ta25W25, and Mo25Nb40Ta25W10 using large-scale atomistic simulations and machine learning based Spectral Neighbor Analysis Potential. Our atomistic simulations examine how the chemical composition and local ordering influence the mobility of both edge and screw dislocations, and how lattice distortion and diffuse anti-phase boundary energy (DAPBE) affect dislocation behaviors during nanostructural evolution. Notably, with the increase in Nb concentration in the model RCCAs, both DAPBE and lattice distortion are simultaneously enhanced as the chemical short-range order (CSRO) evolves into nanoscale B2 precipitates. This evolution results in high lattice distortion due to the lattice mismatch between B2 precipitates and the random matrix. Consequently, B2 nanoprecipitates provide a stronger pinning effect, hindering edge dislocation motion while promoting cross-slip of screw dislocations, leading to a reduced screw-to-edge ratio in slip resistance and mobility discrepancy. These findings offer valuable insights into dislocation behaviors and interactions with ordered precipitates, highlighting the importance of exploring non-equiatomic compositions and advancing beyond CSRO in RCCAs. This study has implications for optimizing alloy compositions and processing methods for superior performance in aggressive environments.

Atomistic simulation of chemical short-range order on the irradiation resistance of HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) have been considered as one of the potential structural material candidates for fourth-generation nuclear reactors and fusion reactors due to their excellent irradiation resistance. Current studies have shown that the chemical short-range order (CSRO) usually exists in HEAs, which has a significant effect on the mechanical properties and irradiation resistance of HEAs. Refractory high entropy alloys (RHEAs), as a new class of HEAs have better mechanical properties at high temperatures than face-centered cubic (FCC) HEAs, and therefore have better prospects of application in the nuclear field. In this study, CSRO and its effect on the irradiation resistance of HfNbTaTiZr are analyzed via molecular dynamics (MD) and Monte Carlo (MC). The primary cascade simulations, multi-cascade simulations and surface bombardment simulations are carried out to simulate the generation and accumulation of irradiation damage. The results of the primary cascade simulations and surface bombardment simulations of CSRO models show that the presence of CSRO induces cascade splitting into subcascades. The presence of subcascades reduces the thermal peak enhancement effect and thus lowers the recombination rate of Frenkel pairs (FPs) in the damage zone when FPs concentrations are low. However, the creation of subcascades increases the size of the damage zone caused by the cascade. Thus, when the concentrations of FPs are high, the larger area of damage zone allows more of the already existing FPs to be included, thus promoting their recombination, i.e., impedes their accumulation when concentrations are high. These subcascades lower the recombination of FPs at low FPs concentrations but inhibit their accumulation at high FPs concentrations. The presence of CSRO is also beneficial in inhibiting the growth of point defect clusters, which further improves the resistance of HfNbTaTiZr to dislocation generation. Furthermore, the presence of CSRO facilitates the irradiation-induced phase transition. But it is found that HfNbTaTiZr shows suppression of HCP cluster growth. And the tendency to break down large HCP clusters into smaller ones is demonstrated in the CSRO model. From our calculations we also find that the irradiation-induced HCP atoms have a higher potential energy relative to the matrix. The potential energy difference between those energetic HCP atoms and the matrix can lead to generating a great number of insurmountable barriers pervading the matrix and largely suppressing the long-term mobility of FPs, thus limiting their aggregation and growth into clusters.

Ordering-facilitated lower hydrogen embrittlement sensitivity in a prototype high-entropy alloy

The ageing treatment (450 °C/60 h) led to the spinodal decomposition at grain boundaries (GBs) and the development of chemical short-range orders (CSROs) in the alloy matrix, increasing the strength and effectively reducing the HE sensitivity of the equiatomic FeMnNiCoCr alloy. Firstly, the emergence of NiMn- and Cr-rich orders at GB regions reduces hydrogen diffusion rates along GBs and limits hydrogen enrichment at these sites. Secondly, CSROs promote hydrogen accumulation within the grain interior and impede hydrogen penetration, resulting in a shallower hydrogen-affected depth in the aged sample. Moreover, the modulation of hydrogen distribution by microstructure initiates transgranular cracking in the aged sample.

Composition-dependent diffusion and viscosity behavior in liquid Ti–Al–Ni ternary alloys

We conducted ab initio molecular dynamics simulations to investigate the dynamic properties of the liquid Ti–Al–Ni ternary alloy system at 2033 K. Through simulations across the entire composition range, we revealed the complex compositional dependence of self-diffusion coefficients and viscosity. The self-diffusion coefficients of Ti, Al, and Ni exhibit nonlinear distribution characteristics, while viscosity shows non-monotonic compositional dependence, with lower viscosity in Al-rich regions and higher viscosity in Ni-rich regions. To understand the structural origins of these behaviors, we analyzed chemical short-range order and local fivefold symmetry structures using Warren-Cowley parameters and fivefold symmetry parameters, uncovering a close correlation between atomic-scale structural features and dynamic properties. Evaluation of the Stokes-Einstein relation revealed varying degrees of deviation from its predicted viscosity values across composition regions. We observed a negative correlation between free volume fraction and viscosity, with varying strengths in different regions. These findings provide new perspectives for understanding the complex dynamic properties of Ti–Al–Ni liquid alloys.

Effect of static magnetic field on the chemical short-range order of CoCrFeMnNi high-entropy alloy prepared by laser powder bed fusion

Recently, laser powder bed fusion (LPBF) has emerged as a potential technique for manipulating chemical short-range order (CSRO) in high-entropy alloys (HEAs), eliminating the need for homogenization treatment. The application of the magnetic field in LPBF has shown promise in refining the microstructure and optimizing the mechanical properties. However, the precise influence of magnetic field on the CSRO of LPBF-processed HEAs remains inadequately understood. Therefore, in this study, we investigate the impact of the static magnetic field on the CSRO of LPBF-processed HEAs. The findings reveal that the presence of the static magnetic field results in a reduction in CSRO, including the size and area fraction. This behavior can be attributed to the influence of the static magnetic field on the melt flow, which is perturbed by the electromagnetic damping effect and thermos-electromagnetic forces, thereby impeding the free diffusion of atoms and ultimately leading to a decrease in CSRO.

A review on recent progress of refractory high entropy alloys: From fundamental research to engineering applications

Refractory high-entropy alloys (RHEAs) have emerged as frontier materials for high-temperature structural applications due to their exceptional mechanical properties and thermal stability. This review aims to provide a comprehensive and in-depth summary of the latest research progress in the field of RHEAs. By systematically analyzing 253 publications since 2022, this review presents a panoramic overview of the current research status in the field of RHEAs, covering aspects from alloy design, microstructure, processing techniques, mechanical behavior, to physicochemical properties. Key strengthening and toughening mechanisms, such as solid solution strengthening, precipitation strengthening, and grain boundary strengthening, are extensively analyzed. The high-temperature mechanical performance, oxidation resistance, and adaptability of RHEAs in extreme environments including corrosion and irradiation, are critically reviewed, and the potential application value of these research findings in aerospace, nuclear energy, biomedical, and other fields is discussed. Simultaneously, the multidisciplinary characteristics of RHEAs research has revealed the trend of the field evolving from fundamental research to practical applications. Furthermore, with the aid of advanced characterization techniques and computational methods, the review elucidates the controlling effects of chemical short-range order, defect structures, and other factors on the performance evolution of RHEAs, highlighting the importance of multi-principal element synergistic effects. Based on summarizing the key scientific issues and technological bottlenecks faced by RHEAs, the article provides a forward-looking perspective on future research directions, emphasizing development strategies that integrate computation and experiments, and accelerate the transformation of fundamental research into engineering applications, thus providing insights and guidance for developing a new generation of high-performance RHEAs.

Local atomic configuration and dynamic properties of liquid Ag-Si eutectic alloy by ab initio molecular dynamics

The structural evolution and dynamic properties of liquid Ag89Si11 eutectic alloy during rapid cooling were explored by ab initio molecular dynamics. The calculated pair distribution functions and structure factors at 1173 K agree well with the experimental reference data. The comprehensive analyses including Honeycutt-Andersen index, Voronoi tessellation, and atomistic cluster alignment reveal a gradual enhancement of chemical short-range order with the decreasing temperature and a dominant structural shifting towards perfect and distorted icosahedra. Additionally, the dynamic properties of liquid Ag89Si11 eutectic alloy were obtained and the diffusion coefficients follow Arrhenius relationship.

Vacancy formation free energy in concentrated alloys: Equilibrium vs. random sampling

Special Quasi-random Structures (SQSs) are often used to model disordered alloys in small simulation cells. Yet, SQS-based sampling yields defect formation energies that do not match the equilibrium values, for instance measured in atomic Monte Carlo simulations, due to the lack of chemical short-range order in random samples. In this paper, our approach to computing chemical potentials in alloys through random sampling techniques is extended to the computation of vacancy formation free energies. By rigorous thermodynamic derivation, we show that vacancy formation free energies computed from randomly sampled local chemical environments must be corrected to ensure its consistency between various available calculations. Indeed, irrespective of the choice of the species being replaced by a vacancy, we should arrive at the same value of the vacancy formation free energy. We propose a simple way to compute this correction term, and compare the outcome with equilibrium Monte Carlo results and with approximations found in the literature.

The effect of chemical short-range order on incipient plastic behavior in FCC structured high entropy alloys

Chemical short-range order (CSRO) was assumed as one of the most important structure feature of high entropy alloys and the influence of CSRO on mechanical properties is a fundamental issue yet to be fully understood. In this work, we performed extensively nanoindentation experiments on CoCrFeNiAlx alloys to study the effect of CSRO on the incipient nanomechanical properties. The statistical nature of strengths at the first pop-in event was analyzed to gain insight of deformation mechanisms. All samples examined here exhibit bimodal distribution which indicate non-unique dislocation nucleation mechanisms. The bimodal distributions can be decomposed into two Gaussian distributions and the activation volumes can be obtained in the range of 0.73–1.38b3. The peaks shift to higher stress level after the development of CSRO. The heterogeneous dislocation nucleation plays a dominant role at low indentation stress with the aid of pre-existing crystalline defects. The homogeneous dislocation nucleation mechanism prevails when indentation stress close to theoretical values. The transmission electron microscopy characterization indicates the presence of chemical ordering in the aged samples. Both the degree of chemical ordering and lattice distortion are much higher in the Al containing HEAs due to the distinctive difference of properties in Al and other transition element atoms.

Comprehensive analysis of ordering in CoCrNi and CrNi2 alloys

Chemical Short-Range Order (CSRO) has attracted recent attention from many researchers, creating intense debates about its impact on material properties. The challenges lie in confirming and quantifying CSRO, as its detection proves exceptionally demanding, contributing to conflicting data in the literature regarding its true effects on mechanical properties. Our work uses high-precision calorimetric data to unambiguously prove the existence and, coupled with atomistic simulations, quantify the type of CSRO. This methodology allows us to propose a mechanism for its formation and destruction based on the heat evolution during thermal analysis and facilitates a precise identification of local ordering in CoCrNi alloys. Samples of CoCrNi (Co33Cr33Ni33) and CrNi2 (Cr33Ni66) alloys are fabricated in varying ordered states, extensively characterized via synchrotron X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. Samples with considerably different ordered states are submitted to tensile tests with in-situ synchrotron X-ray diffraction. We demonstrate, despite inducing varied CSRO levels in CoCrNi, no significant alterations in overall mechanical behavior emerge. However, the CrNi2 alloy, which undergoes long-range ordering, experiences significant shifts in yield strength, ultimate tensile stress and ductility.

Chemical short-range order increases the phonon heat conductivity in a refractory high-entropy alloy

We study the effects of the chemical short-range order (SRO) on the thermal conductivity of the refractory high-entropy alloy HfNbTaTiZr using atomistic simulation. Samples with different degrees of chemical SRO are prepared by a Monte Carlo scheme. With increasing SRO, a tendency of forming HfTi and TiZr clusters is found. The phonon density of states is determined from the velocity auto-correlation function and chemical SRO modifies the high-frequency part of the phonon density of states. Lattice heat conductivity is calculated by non-equilibrium molecular dynamics simulations. The heat conductivity of the random alloy is lower than that of the segregated binary alloys. Phonon scattering by SRO precipitates might be expected to reduce scattering times and, therefore, decrease thermal conductivity. We find that, in contrast, due to the increase of the conductivity alongside SRO cluster percolation pathways, SRO increases the lattice heat conductivity by around 12 %. This is expected to be a general result, extending to other HEAs.

Unveiling the thermodynamic landscape of liquid Ti–Al–Ni alloys through first-principles simulations

We conducted ab initio molecular dynamics simulations to systematically examine the composition-dependent thermodynamic properties and atomic-scale interactions in liquid Ti–Al–Ni alloys throughout the entire ternary phase space at 2033 K. The calculated enthalpies of mixing demonstrated exothermic tendencies, with a distinct minimum in the composition of Ti0.0Al0.50Ni0.50, indicating significant Al–Ni attractive interactions. Incorporating ternary interaction parameters into the Redlich-Kister-Muggianu equation enabled accurate modeling of the complex variations in mixing enthalpy. Analysis of partial pair correlation functions and structure factors revealed chemical and topological short-range ordering (SRO), as well as medium-range ordering, within the liquid alloy. Quantifying deviations from ideal configurational entropy clarifies the coupling between chemical SRO and topological SRO, significantly impacting the overall Gibbs energy of mixing. This comprehensive atomistic study provides insights into the thermodynamics of Ti–Al–Ni alloys, paving the way for tailoring their properties for high-performance applications.

Strong and ductile Resinvar alloys with temperature- and time-independent resistivity

Materials with well-defined electrical resistivity that does not change with temperature or time are important in robotics, communication and automation. However, the challenge of designing such materials has remained elusive due to the temperature-dependent electron-phonon scattering. Moreover, resistive electrical conductors used in flexible and mobile systems under high mechanical loads must possess both high strength and ductility. Achieving such multi-properties presents a fundamental challenge. Here, we solve this problem by combining multicomponent alloy design with atomic-scale chemistry tuning. We term the resultant material ‘Resinvar’ alloy, due to its invariable resistivity (148 μΩ·cm) over wide temperature ranges from room temperature to 723 K. The alloy also has high tensile strength (948 MPa) at large tensile elongation (53%). The distorted lattice, chemical short-range order and ordered coherent nanoprecipitates in the material enable the invariant resistivity via promoting temperature-independent inelastic electron scattering, and contribute to the excellent strength-ductility synergy by manipulating dislocation slip.

Machine Learning-Enabled Tomographic Imaging of Chemical Short-Range Atomic Ordering.

In solids, chemical short-range order (CSRO) refers to the self-organization of atoms of certain species occupying specific crystal sites. CSRO is increasingly being envisaged as a lever to tailor the mechanical and functional properties of materials. Yet quantitative relationships between properties and the morphology, number density, and atomic configurations of CSRO domains remain elusive. Herein, it is showcased how machine learning-enhanced atom probe tomography (APT) can mine the near-atomically resolved APT data and jointly exploit the technique's high elemental sensitivity to provide a 3D quantitative analysis of CSRO in a CoCrNi medium-entropy alloy. Multiple CSRO configurations are revealed, with their formation supported by state-of-the-art Monte-Carlo simulations. Quantitative analysis of these CSROs allows establishing relationships between processing parameters and physical properties. The unambiguous characterization of CSRO will help refine strategies for designing advanced materials by manipulating atomic-scale architectures.

Multiple origins of extra electron diffractions in fcc metals.

Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article explores the many nonordering origins of commonly reported features using a combination of experimental electron microscopy and multislice diffraction simulations, which suggest that diffuse intensities largely represent thermal and static displacement scattering. A number of observations may reflect additional contributions from planar defects, surface terminations incommensurate with bulk periodicity, or weaker dynamical effects.

Effect of local chemical order on monovacancy diffusion in CoNiCrFe high-entropy alloy

High-entropy alloys (HEAs) are promising materials for nuclear applications. Understanding the influence of local chemical order (LCO) on defect diffusion and evolution in these alloys is crucial for enhancing their resistance to radiation damage. In this study, we used molecular dynamics simulation to investigate the effect of LCO on monovacancy diffusion in CoNiCrFe HEA. Alongside the Warren-Cowley parameter, which quantifies the degree of local elemental ordering, we propose a new parameter to characterize the spatial scale of LCOs. Based on their degree, LCO structures have been classified as either chemical medium-range order (CMRO) or chemical short-range order (CSRO). Our study reveals a non-monotonic variation in vacancy diffusion coefficients, transitioning from random solid solution to CSRO and then to CMRO structures. Moreover, we observe a preference for vacancy diffusion in low-energy Fe, Co-rich regions, with their spatial distribution and spatial connectivity significantly influencing the vacancy diffusion. Due to the spatial scale of the inhomogeneity introduced by LCO, both global average diffusion parameters and single migration barriers cannot fully reflect the actual diffusion dynamics. Therefore, our study emphasizes the importance of understanding the preferred diffusion pathways and the associated energy landscapes to fully assess the defect diffusion dynamics in HEAs. This deeper investigation into localized diffusion behaviors influenced by LCO is crucial for evaluating and enhancing the radiation damage tolerance of HEAs.

Impact of atomic packing and electronic structure on icosahedral short-range order and glass-forming ability of Zr–Cu metallic glasses

ABSTRACT Chemical and topological short-range ordering are important for glass formation in metallic alloys. Yet, it remains little understood from the viewpoint of the electronic bonding among the constituent elements of the alloys. In this work, we study CuxZr100−x, (46 ≤ x ≤ 70) metallic glasses, where the atomic packing efficiency and the electronic structure of the full icosahedra, the key short-range order structures, have been investigated in detail. To capture a realistic picture of the electronic bonding in the icosahedra in the as-quenched metastable state, DFT study of the electronic structure of the icosahedra extracted from classical molecular dynamics simulation is carried out. Results for the CunZr13−n , (4 ≤ n ≤ 9) clusters reveal that Cu6Zr7, Cu7Zr6 and Cu8Zr5 are the most abundant, electronically stable in Cu50Zr50 , Cu58Zr42 and Cu64Zr36, respectively. These three clusters also exhibit pseudo-gap in the density of states at the Fermi level – a feature linked with glass-forming ability according to the nearly-free electron approach. The partial density of states demonstrates the effect of small changes in the stoichiometry of the clusters on the s, p and d states of Cu and Zr in Cu–Cu and Cu–Zr bonding and hence, the stability of the clusters.

Local Lattice Distortion and Chemical Short-Range Order in High-Entropy Alloys

Local Lattice Distortion and Chemical Short-Range Order in High-Entropy Alloys

Machine Learning Enhanced Tomographic Imaging of Chemical Short-range Order in Fe-based Solid Solutions

Machine Learning Enhanced Tomographic Imaging of Chemical Short-range Order in Fe-based Solid Solutions