Chemical medium-range order in a medium-entropy alloy

Chemical Short-Range Ordering in Nanoprecipitates Modulates Planar Faults to Enhance Mechanical Properties

Nanoprecipitates strengthen metallic materials by impeding dislocation motion, but they often compromise ductility. This study introduces a novel strategy to address this challenge by incorporating atomic-scale chemical heterogeneity within nanoprecipitates. For the first time, pronounced short-range ordering (SRO) within L12-ordered precipitates of the Co40Ni30Cr20Al5Ti4Ta1 multi-principal element alloy is observed and confirmed, with its formation mechanism elucidated via density functional theory. Experimental and computational results show that the unique atomic configuration reshapes the energy landscape of planar defects, enhancing the strength and work-hardening capacity. The SRO structure elevates the critical shear stress for dislocation-mediated precipitate shearing while reducing the formation energy of superlattice intrinsic stacking faults, thereby promoting nucleation site formation. This work pioneers a method for modulating atomic-scale heterogeneity within ordered structures, advancing high-performance material design.

Nanoscale Indentation-Induced Crystal Plasticity in CrCoNi Medium-Entropy Alloys Containing Short-Range Order

CrCoNi medium-entropy alloys (MEAs), characterised by their high configurational entropies, have become a research hotspot in materials science. Recent studies have indicated that MEAs exhibit short-range order (SRO), which affects their deformation mechanisms. In this study, the micro-mechanisms of SRO within the framework of mesoscale continuum mechanics are mathematically evaluated using an advanced, non-local crystal plasticity constitutive framework. Furthermore, a crystal plasticity model considering the impact of SRO on slip is established. By combining nanoindentation simulations and multi-level grain model tensile simulations, the load-displacement and stress-strain curves demonstrated that the presence of SRO increases the hardness of MEAs. More specifically, considering the distribution of shear strain and geometrically necessary dislocations, the heterogeneity of MEAs increases with an increase in the degree of SRO. This study not only enriches the crystal plasticity theory but also provides references for the microstructure and performance regulation of high-performance multi-level grain structure materials.

Local chemical order enables an ultrastrong and ductile high-entropy alloy in a cryogenic environment

Owing to superior strength-ductility combination and great potential for applications in extreme conditions, high-entropy alloys (HEAs) with the face-centered cubic (FCC) structure have drawn enormous attention. However, the FCC structure limits yield strength and makes the alloys unable to meet ever-increasing demands for exploring the universe. Here, we report a strategy to obtain FCC materials with outstanding mechanical properties in both ambient and cryogenic environments, via exploiting dynamic development of the interstitial-driven local chemical order (LCO). Dense laths composed of the multiscaled LCO domains evolve from planar-slip bands that form in the prior thermomechanical processing, contributing to ultrahigh yield strengths over a wide temperature range. During cryogenic tensile deformation, LCO further develops and promotes remarkable dislocation cross-slip. Together with the deformation-driven transformation and twinning, these factors lead to satisfactory work hardening. The cryogenic loading-promoted LCO, also revealed by ab initio calculations, opens an avenue for designing advanced cryogenic materials.

Remnant Copper Cation-Assisted Atom Mixing in Multicomponent Nanoparticles

Nanostructured high-/medium-entropy compounds have emerged as important catalytic materials for energy conversion technologies, but complex thermodynamic relationships involved with the element mixing enthalpy have been a considerable roadblock to the formation of stable single-phase structures. Cation exchange reactions (CERs), in particular with copper sulfide templates, have been extensively investigated for the synthesis of multicomponent heteronanoparticles with unconventional structural features. Because copper cations within the host copper sulfide templates are stoichiometrically released with incoming foreign cations in CERs to maintain the overall charge balance, the complete absence of Cu cations in the nanocrystals after initial CERs would mean that further compositional variation would not be possible by subsequent CERs. Herin, we successfully retained a portion of Cu cations within the silver sulfide (Ag2S) and gold sulfide (Au2S) phases of Janus Cu2-xS-M2S (M = Ag, Au) nanocrystals after the CERs, by partially suppressing the transformation of the anion sublattice that inevitably occurs during the introduction of external cations. Interestingly, the subsequent CERs on Janus Cu1.81S-M2S (M = Ag, Au), by utilizing the remnant Cu cations, allowed the construction of Janus Cu1.81S-AgxAuyS, which preserved the initial heterointerface. The synthetic strategy described in this work to suppress the complete removal of the Cu cation from the template could fabricate the CER-driven heterostructures with greatly diversified compositions, which exhibit unusual optical and catalytic properties.

Controllable synthesis of high-entropy alloys

High-entropy alloys (HEAs) involving more than four elements, as emerging alloys, have brought about a paradigm shift in material design. The unprecedented compositional diversities and structural complexities of HEAs endow multidimensional exploration space and great potential for practical benefits, as well as a formidable challenge for synthesis. To further optimize performance and promote advanced applications, it is essential to synthesize HEAs with desired characteristics to satisfy the requirements in the application scenarios. The properties of HEAs are highly related to their chemical compositions, microstructure, and morphology. In this review, a comprehensive overview of the controllable synthesis of HEAs is provided, ranging from composition design to morphology control, structure construction, and surface/interface engineering. The fundamental parameters and advanced characterization related to HEAs are introduced. We also propose several critical directions for future development. This review can provide insight and an in-depth understanding of HEAs, accelerating the synthesis of the desired HEAs.

Harnessing instability for work hardening in multi-principal element alloys

The strength-ductility trade-off has long been a Gordian knot in conventional metallic structural materials and it is no exception in multi-principal element alloys. In particular, at ultrahigh yield strengths, plastic instability, that is, necking, happens prematurely, because of which ductility almost entirely disappears. This is due to the growing difficulty in the production and accumulation of dislocations from the very beginning of tensile deformation that renders the conventional dislocation hardening insufficient. Here we propose that premature necking can be harnessed for work hardening in a VCoNi multi-principal element alloy. Lüders banding as an initial tensile response induces the ongoing localized necking at the band front to produce both triaxial stress and strain gradient, which enables the rapid multiplication of dislocations. This leads to forest dislocation hardening, plus extra work hardening due to the interaction of dislocations with the local-chemical-order regions. The dual work hardening combines to restrain and stabilize the premature necking in reverse as well as to facilitate uniform deformation. Consequently, a superior strength-and-ductility synergy is achieved with a ductility of ~20% and yield strength of 2 GPa during room-temperature and cryogenic deformation. These findings offer an instability-control paradigm for synergistic work hardening to conquer the strength-ductility paradox at ultrahigh yield strengths.

Negative enthalpy alloys and local chemical ordering: a concept and route leading to synergy of strength and ductility

Solid solutions are ubiquitous in metals and alloys. Local chemical ordering (LCO) is a fundamental sub-nano/nanoscale process that occurs in many solid solutions and can be used as a microstructure to optimize strength and ductility. However, the formation of LCO has not been fully elucidated, let alone how to provide efficient routes for designing LCO to achieve synergistic effects on both superb strength and ductility. Herein, we propose the formation and control of LCO in negative enthalpy alloys. With engineering negative enthalpy in solid solutions, genetic LCO components are formed in negative enthalpy refractory high-entropy alloys (RHEAs). In contrast to conventional 'trial-and-error' approaches, the control of LCO by using engineering negative enthalpy in RHEAs is instructive and results in superior strength (1160 MPa) and uniform ductility (24.5%) under tension at ambient temperature, which are among the best reported so far. LCO can promote dislocation cross-slip, enhancing the interaction between dislocations and their accumulation at large tensile strains; sustainable strain hardening can thereby be attained to ensure high ductility of the alloy. This work paves the way for new research fields on negative enthalpy solid solutions and alloys for the synergy of strength and ductility as well as new functions.

Organic-Inorganic Hybrid Glasses of Atomically Precise Nanoclusters

Organic-inorganic atomically precise nanoclusters provide indispensable building blocks for establishing structure-property links in hybrid condensed matter. However, robust glasses of ligand-protected nanocluster solids have yet to be demonstrated. Herein, we show [Cu4I4(PR3)4] cubane nanoclusters coordinated by phosphine ligands (PR3) form robust melt-quenched glasses in air with reversible crystal-liquid-glass transitions. Protective phosphine ligands critically influence the glass formation mechanism, modulating the glasses' physical properties. A hybrid glass utilizing ethyldiphenylphosphine-based nanoclusters, [Cu4I4(PPh2Et)4], exhibits superb optical properties, including >90% transmission in both visible and near-infrared wavelengths, negligible self-absorption, near-unity quantum yield, and high light yield. Experimental and theoretical analyses demonstrate the structural integrity of the [Cu4I4(PPh2Et)4] nanocluster, i.e., iodine-bridged tetranuclear cubane, has been fully preserved in the glass state. The strong internanocluster CH-π interactions found in the [Cu4I4(PPh2Et)4] glass and subsequently reduced structural vibration account for its enhanced luminescence properties. Moreover, this highly transparent glass enables performant X-ray imaging and low-loss waveguiding in fibers drawn above the glass transition. The discovery of "nanocluster glass" opens avenues for unraveling glass formation mechanisms and designing novel luminescent glasses of well-defined building blocks for advanced photonics.

Sub-Ångstrom-scale structural variations in high-entropy oxides

High-entropy oxides (HEOs) are a special class of materials that utilize the concept of high-entropy alloys (HEAs) with five or more elements randomly distributing at a single sublattice in near-equiatomic proportions. HEOs have been attracting increasing attention owing to their many outstanding physical and chemical properties. However, unlike HEAs, for which local chemical compositions, order/disorder behaviors, and property-structure relationships have been comprehensively investigated, detailed information on the atomic-scale chemical and structural features and their correlations with functionalities in HEOs so far is still not sufficient. Herein, we select four typical HEOs with pyrochlore, spinel, perovskite and rock-salt type structures, and directly observe and quantify sub-Ångstrom-scale structure variations in different manners by means of advanced aberration-corrected scanning transmission electron microscopy techniques. Visualization and quantification of local structural variations and lattice distortions in the current work may show a valuable example for future investigations on local fluctuating structures and their relationships with properties in more systems of HEOs.

On the origin of diffuse intensities in fcc electron diffraction patterns

Interpreting diffuse intensities in electron diffraction patterns can be challenging in samples with high atomic-level complexity, as often is the case with multi-principal element alloys. For example, diffuse intensities in electron diffraction patterns from simple face-centred cubic (fcc) and related alloys have been attributed to short-range order1, medium-range order2 or a variety of different {111} planar defects, including thin twins3, thin hexagonal close-packed layers4, relrod spiking5 and incomplete ABC stacking6. Here we demonstrate that many of these diffuse intensities, including [Formula: see text]{422} and [Formula: see text]{311} in ⟨111⟩ and ⟨112⟩ selected area diffraction patterns, respectively, are due to reflections from higher-order Laue zones. We show similar features along many different zone axes in a wide range of simple fcc materials, including CdTe, pure Ni and pure Al. Using electron diffraction theory, we explain these intensities and show that our calculated intensities of projected higher-order Laue zone reflections as a function of deviation from their Bragg conditions match well with the observed intensities, proving that these intensities are universal in these fcc materials. Finally, we provide a framework for determining the nature and location of diffuse intensities that could indicate the presence of short-range order or medium-range order.

Exceptionally High Saturation Magnetic Flux Density and Ultralow Coercivity via an Amorphous-Nanocrystalline Transitional Microstructure in an FeCo-Based Alloy

High saturation magnetic flux density (B_s ) of soft magnetic materials is essential for increasing the power density of modern magnetic devices and motor machines. Yet, increasing B_s is always at the expense of high coercivity (H_c ), presenting a general trade-off in the soft magnetic material family. Here, superior comprehensive soft magnetic properties, i.e., an exceptionally high B_s of up to 1.94 T and H_c as low as 4.3 A m^-1 are unprecedentedly combined in an FeCo-based alloy. This alloy is obtained through a composition design strategy to construct a transitional microstructure between amorphous and traditional nanocrystalline alloys, with nanocrystals (with < 5 nm-sized crystal-like regions around) sparsely dispersed in an amorphous matrix. Such transitional microstructure possesses extremely low magnetic anisotropy caused by the annihilation of quasi-dislocation dipoles, and a strong magnetic exchange interaction, which leads to excellent comprehensive magnetic properties. The results provide useful guidelines for the development of the next generation of soft magnetic materials, which are promising for applications of high-frequency, high-efficiency, and energy-saving devices.