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At present, gradient‐structure alloys with strong properties can be machined by strong surface plastic deformation. This kind of alloy also has high strength, excellent plastic toughness, and work hardening properties. Herein, nanometer polycrystalline nickel–cobalt alloys with grain gradient are studied. The change of mechanical properties and evolution mechanism of internal defects of graded polycrystalline nickel–cobalt alloys under shear loading are emphatically revealed, and the unusual strengthening mechanism brought about by gradient structure is revealed. The results show that grain gradient microstructure can improve the strength and maintain the toughness of the alloy to a certain extent. The uneven grain boundary distribution due to the gradient structure brings different hindering effects to the dislocation movement. Therefore, the mechanical properties exhibited are different from those of uniform microstructure. [ABSTRACT FROM AUTHOR]

This article provides a comprehensive review of the research progress in the tribology of nickel‐based superalloys (NBS). First, the microstructure, friction, and wear characteristics of NBS are summarized. Second, the tribological features of NBS and the effects of environmental factors on tribological behavior and wear mechanism are elucidated. Third, the microstructure and the corresponding formation mechanism of the NBS tribolayers and the oxidation behavior during friction are analyzed. Fourth, the effect of tribolayer formation on the friction state and wear mechanism is discussed. Moreover, the application of existing finite element simulation technology in NBS friction and wear is reviewed. Finally, the strain‐induced gradient structure (i.e., strengthening layer) is introduced, and the benefits of the gradient structure in the frictional process are analyzed in comparison with the coarse grain. According to these existing reports, future research should focus on elucidating the quantitative relationship between friction state and wear mechanism, investigating the design of wear‐resistant NBS, and expanding its potential applications. These advancements provide a pathway for thoroughly elucidating NBS tribology and promoting the application of strengthening technologies. [ABSTRACT FROM AUTHOR]

Functionally graded composites with fibre gradients were prepared with three different ceramic matrices: ZrB 2 -SiC-Y 2 O 3 , ZrB 2 -MoSi 2 and SiC-Y 2 O 3 , with short carbon fibres in amounts ranging from 0–50%. Short term oxidation tests at 1650 °C in air were carried out to study the influence of the graded architecture and fibre content on the structural stability and oxidation resistance. The composite layers were perfectly joined at the interface, with no visible defects. Even after oxidation testing, no spallation phenomena were observed. ZrB 2 -MoSi 2 matrix proved to be the most effective in protecting the composite from oxidation thanks to the formation of a compact ZrO 2 -SiO 2 scale. • Fibre gradient, ultra-refractory ceramic composites were produced by sintering. • Bulk ceramic layers and fibre reinforced layers were joined without cracks or spallation. • Oxidation tests at 1650 °C did not affect the structural integrity of the graded composites. • Out of the three ceramic matrices tested, ZrB 2 /MoSi 2 proved to be the most performing. [ABSTRACT FROM AUTHOR]

PbS quantum dot (QD) infrared (IR) solar cells that can absorb low‐energy photons are promising photovoltaic devices to improve utilization of sunlight energy by broadening absorption range of the sunlight spectrum to short‐wave infrared region. For PbS QD photovoltaics, depleted heterojunction established between photoactive layer and ZnO electron transport layer (ETL) is critical to determine the performance of devices. However, undesired defects in ZnO films and mismatched energy levels have limited the improvement of device properties. Herein, a novel and simple gradient ZnO ETL is developed for achieving high‐performance QD IR solar cells by depositing a Cs‐doped ZnO thin layer on the pristine ZnO film. The resulting gradient ETL exhibits significantly suppressive trap states and a much smoother surface, efficiently reducing the trap‐assisted nonradiative recombination at the interfaces. Meanwhile, realigned energy levels of the gradient ZnO ETL facilitate the transport and extraction of photogenerated carriers. As a result, the champion device shows a high IR open circuit voltage (VOC) of 0.446 V and IR power conversion efficiency (PCE) of 1.27% under 1100 nm filtered illumination. The VOC and PCE are 0.507 V and 11.04% under AM1.5 illumination, respectively. These results demonstrate a promising strategy for exploiting high‐performance infrared optoelectronic devices. [ABSTRACT FROM AUTHOR]

Precipitation strengthening plays a vital role in enhancing the mechanical properties of metallic materials, typically achieved through heat treatment. However, deformation-induced precipitation (DIP) in multicomponent alloys (MAs) without the assistance of annealing is rarely reported, owing to their inherent high entropy and sluggish diffusion effects. This work reveals the significant formation of nanoscale and microscale precipitates in the dual-phase Fe49.3Co23Ni23C0.85Mn1Si2.85 MA triggered by surface severe plastic deformation (SSPD) through industrial shot blasting. The SSPD process leads to the formation of ultrafine gradient structures comprising nanocrystalline and submicron-crystalline zones. Submicron SiO2 precipitates emerge at the interfaces of both these zones, while various Mn5Si3 and Si nanoparticles precipitate within the nanocrystalline zone. This phenomenon can be ascribed to the high density of generated substructures, the accelerated diffusion of atoms, and the reduction in solid solubility limits in the SPD-driven non-equilibrium state.

The methods for fabrication of the functionally-gradient materials, which have a high complex of unique mechanical, technological and special properties, when they working on impact, wear, fatigue, experiencing increased cyclic and alternating loads, are reviewed. Considered materials are used in aerospace engineering, energy, and other industries characterized by extremely unfavourable, extreme operating conditions of critical parts, structural elements, and assemblies. Different methods for fabrication of the gradient structures are considered, in particular, the method of severe plastic deformation of metals and the process of active bending of copper. The effect of grain size, texture, and grain-growth gradients on the deformation mechanisms and mechanical properties of gradient- nanostructured nickel, as well as functionally-gradient ceramic materials are studied. The review may be of interest to researchers and scientists in the field of materials science, metallurgy, and nanotechnology.

A multiphase‐gradient‐structured medium‐Mn steel with dislocation/stacking fault (SF) density gradient, austenite fraction gradient, and grain size gradient is designed and fabricated by cyclic torsion. It exhibits higher yield strength (YS) (942 MPa), ultimate tensile strength (1295 MPa), and tensile toughness (290 mJ mm−3) than its homogeneous counterpart (672, 1273 MPa, and 278 mJ mm−3, respectively). High‐density‐interlaced SFs are generated at the surface region because of the change in strain path and stress state during cyclic torsion. These massive interactive SFs promote the persistent martensite transformation of the retained austenite in the surface region within the entire uniform strain range. Therefore, in addition to the central layer, the surface region in the gradient structures provides strong strain hardening. Moreover, the continuous martensite transformation in the surface region allows the gradient structure to be retained even after tensile failure, which is beneficial for improving the heterogeneous‐deformation‐induced (HDI) hardening of the gradient‐structured sample during tensile deformation. Thus, the combination of active and persistent transformation‐induced plasticity effect and HDI hardening contributes to its excellent tensile toughness and ductility. Quantitative analysis indicates that HDI strengthening and dislocation strengthening play a dominant role in the sample's ultrahigh YS. [ABSTRACT FROM AUTHOR]

The established approach to materials design for additive manufacturing (AM) consists of attempting to reproduce the uniform structures and properties of conventionally processed materials. While this certainly helped facilitate material certification and the rapid introduction of AM technologies in several industries, the opportunity to exploit unique features of specific AM processes to generate spatially varying microstructures–and hence novel materials, remains largely untapped. This work presents a method for manufacturing materials through laser powder bed fusion (LPBF), in which control over the spatial variation in phase composition and mechanical properties is achieved. This technique is demonstrated using 17‐4 precipitation‐hardened stainless steel (17‐4PH), by controlling spatial modulation of energy densities during printing. This results in local control of ferrite/martensite volume fractions, allowing the fabrication of metal/metal architected composites with hard/brittle regions interspersed with soft/tough regions. Local variations of ~20% in tensile strength and ~150% in elongation are achieved, with a spatial resolution of ~100 microns. The approach is general and robust, fully compatible with commercially available LPBF equipment, and applicable to virtually any multi‐phase alloy system. This work shifts the paradigm from attempting to print components with uniform properties to manufacturing alloys with controlled spatial property gradients. [ABSTRACT FROM AUTHOR]

This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small‐scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high‐strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively. [ABSTRACT FROM AUTHOR]

Transparent bamboo (TB) is a new type of biomass and light transmitting material with potential to serve as substitute for transparent wood. However, the potential of the bamboo structure in preparing transparent composites is underexplored. Inspired by the functional gradient structure of bamboo, a simple and efficient two‐step strategy to prepare TB with different transmittance levels, fiber textures, and colors is used. The performance of single‐ and multilayer TB with various fiber volume contents prepared using different lamination methods is evaluated. It is found that the optical and mechanical properties of TB differ with the fiber volume content and the lamination method used. TB composed of veneers with low fiber volume shows the highest light transmittance (44.6%), whereas TB composed of veneers with high fiber volume has the highest tensile strength (45.7 MPa). In terms of water resistance, cross lamination inhibits expansion in width (water absorption rate, 16.37–21.26%) and thickness (thickness swelling rate, 10.22–21.74%). The lamination structure and fiber volume content in the preparation of TB should be selected according to the specific application scenarios. The graded utilization of bamboo veneer with a fiber gradient can effectively realize the rational allocation of bamboo resources and promote sustainable development. [ABSTRACT FROM AUTHOR]

The methods for fabrication of the functionally-gradient materials, which have a high complex of unique mechanical, technological and special properties, when they working on impact, wear, fatigue, experiencing increased cyclic and alternating loads, are reviewed. Considered materials are used in aerospace engineering, energy, and other industries characterized by extremely unfavourable, extreme operating conditions of critical parts, structural elements, and assemblies. Different methods for fabrication of the gradient structures are considered, in particular, the method of severe plastic deformation of metals and the process of active bending of copper. The effect of grain size, texture, and grain-growth gradients on the deformation mechanisms and mechanical properties of gradient-nanostructured nickel, as well as functionally-gradient ceramic materials are studied. The review may be of interest to researchers and scientists in the field of materials science, metallurgy, and nanotechnology. [ABSTRACT FROM AUTHOR]

The results of electron beam processing of Mn-Zn ferrite samples using pulsed and continuous electron beams in the fore-vacuum pressure range (10 Pa) are presented. We find that continuous electron beam processing leads to surface structuring of the ferrite, changes in elemental composition on the surface, and electrical property modification. The degree of ferrite parameter changes exhibits a threshold behavior. For surface processing temperatures below 900 °C, changes are barely noticeable, while for temperatures over 1100 °C the surface resistance decreases by more than an order of magnitude to values of less than 3 kOhm. Electron beam processing with millisecond pulse duration and pulse energy density exceeding 15 J/cm2 results in the formation of low zinc content melt islands, while the remaining surface area (outside the islands) elemental content and ferrite properties remain largely unchanged. The thickness of the modified layer depends on the processing mode and can be controlled over the range of 0.1–0.5 mm. Due to its low resistance, the modified layer can be utilized to enhance the RF-absorbing properties of the ferrite, which is important in the design of modern magnetic elements of electronic equipment. [ABSTRACT FROM AUTHOR]

Lightweight Al/B 4 C composites with a gradient lamellar structure were fabricated through centrifugal freezing and pressure infiltration. Their ceramic volume fraction showed a continuous increase from the central region to the surface region, the increase being from approximately 23 vol% to 44 vol%, and the spatial variation of their composition and microstructure led to a gradient change in their mechanical properties. The bending strength and hardness increased with the B 4 C content, and they attained the highest values of 424 ± 16 MPa and 130.6 ± 4.2 HV 0.3 at the surface region of the composite, respectively. By contrast, the work of fracture reached the maximum value (3763 ± 268 J/m2) at the central region of the composite because of the high metal content and the existence of multiple extrinsic toughening mechanisms. The proposed method is versatile and economical and is suitable for the preparation of composites with heterogeneous structures and gradient properties. [ABSTRACT FROM AUTHOR]

Highlights Microwave absorber with nanoscale gradient structure was proposed for enhancing the electromagnetic absorption performance. Outstanding reflection loss value (−62.7 dB), broadband wave absorption (6.4 dB with only 2.1 mm thickness) in combination with flexible adjustment abilities were acquired, which is superior to other relative graded distribution structures. This strategy initiates a new method for designing and controlling wave absorber with excellent impedance matching property in practical applications. Abstract In the present paper, a microwave absorber with nanoscale gradient structure was proposed for enhancing the electromagnetic absorption performance. The inorganic–organic competitive coating strategy was employed, which can effectively adjust the thermodynamic and kinetic reactions of iron ions during the solvothermal process. As a result, Fe nanoparticles can be gradually decreased from the inner side to the surface across the hollow carbon shell. The results reveal that it offers an outstanding reflection loss value in combination with broadband wave absorption and flexible adjustment ability, which is superior to other relative graded distribution structures and satisfied with the requirements of lightweight equipment. In addition, this work elucidates the intrinsic microwave regulation mechanism of the multiscale hybrid electromagnetic wave absorber. The excellent impedance matching and moderate dielectric parameters are exhibited to be the dominative factors for the promotion of microwave absorption performance of the optimized materials. This strategy to prepare gradient-distributed microwave absorbing materials initiates a new way for designing and fabricating wave absorber with excellent impedance matching property in practical applications.

The reaction-diffusion master equation (RDME) is a lattice-based stochastic model for spatially resolved cellular processes. It is often interpreted as an approximation to spatially continuous reaction-diffusion models, which, in the limit of an infinitely large population, may be described by means of reaction-diffusion partial differential equations. Analyzing and understanding the relation between different mathematical models for reaction-diffusion dynamics is a research topic of steady interest. In this work, we explore a route to the hydrodynamic limit of the RDME which uses gradient structures. Specifically, we elaborate on a method introduced in [J. Maas and A. Mielke, J. Stat. Phys., 181 (2020), pp. 2257--2303] in the context of well-mixed reaction networks by showing that, once it is complemented with an appropriate limit procedure, it can be applied to spatially extended systems with diffusion. Under the assumption of detailed balance, we write down a gradient structure for the RDME and use the method in order to produce a gradient structure for its hydrodynamic limit, namely, for the corresponding RDPDE. [ABSTRACT FROM AUTHOR]

Conventional methods for creating gradient tissue structures using extrusion-based 3D printing (E3DP) systems involve using mixers to create homogeneous materials. However, commercially available mixers can generate high fluid pressure and shear force that harm cells. Hence, a new mixer-integrated printhead was developed, inspired by stirring-rod-assisted mixing processes. The as-designed mixing system was validated through computational fluid dynamics (CFD) simulations using input parameters including ink viscosity, density, and velocity vector and output parameters including material volume fraction, average fluid pressure, and shear force. Six-stir element mixer units were used to comprehensively analyze material mixing while considering the fluid volume fraction after flowing through the mixers. Printheads integrated with multiple discrete mixers with consistent spiral directions, performing well when introducing gaps, were used to reduce the cell pressure and shear force. The optimal mixer design, with two 1.62-mm gaps and six-stir element mixer units sharing the same spiral structure, reduced the average fluid pressure and shear force by 3.196e+4 Pa and 0.368e+2 Pa, respectively, compared with a commercial static mixer. Finally, a mixer-integrated printhead was manufactured, matching well with the original design. This research could improve E3DP systems and serve as a design reference for 3D printing structures of varying stiffness.

Advancing hole transport layers (HTL) to realize large‐area, flexible, and high‐performance perovskite solar cells (PSCs) is one of the most challenging issues for its commercialization. Here, a self‐assembled gradient Ti3C2Tx MXene incorporated PEDOT:PSS HTL is demonstrated to achieve high‐performance large‐area PSCs by establishing half‐caramelization‐based glucose‐induced MXene redistribution. Through this process, the Ti3C2Tx MXene nanosheets are spontaneously dispersed and redistributed at the top region of HTL to form the unique gradient distribution structure composed of MXene:Glucose:PEDOT:PSS (MG‐PEDOT). These results show that the MG‐PEDOT HTL not only offers favorable energy level alignment and efficient charge extraction, but also improves the film quality of perovskite layer featuring enlarged grain size, lower trap density, and longer carrier lifetime. Consequently, the power conversion efficiency (PCE) of the flexible device based on MG‐PEDOT HTL is increased by 36% compared to that of pristine PEDOT:PSS HTL. Meanwhile, the flexible perovskite solar minimodule (15 cm2 area) using MG‐PEDOT HTL achieve a PCE of 17.06%. The encapsulated modules show remarkable long‐term storage stability at 85 °C in ambient air (≈90% efficiency retention after 1200 h) and enhanced operational lifetime (≈90% efficiency retention after 200 h). This new approach shows a promising future of the self‐assembled HTLs for developing optoelectronic devices. [ABSTRACT FROM AUTHOR]

Gradient structures have been created in single crystal nickel-based superalloys (SX alloys) via surface mechanical creep-feed grinding treatment (SMCGT). It has been found that these gradient structures are mainly composed of nano-sized grains, sub-micron-sized grains, dislocation structures, and the matrix material of single crystals along the depth from the treated surface. In addition, the evolution of such structures is found to be dominated by the dislocation movements which run through both γ channels and γ' precipitates, subdividing the two types of microstructures into various dislocation structures, and eventually introducing the refined grains into the surface layer. Furthermore, the evolution process of gradient structures primarily originates from the mechanical effect between abrasive grits and workpiece material, owing to the large grinding force (up to 529 N) and low grinding temperature (less than 150 °C) during the unique creep-feed grinding treatment in the present investigation. Due to the typical grain refinement, the hardness of the nanostructures exhibits the largest value of around 10 GPa in the surface layer, approximately 26% higher than that of the matrix material. This study further enhances the understanding of the microstructure–property relationship of SX alloys subjected to creep-feed grinding treatment and contributes to achievement of high-performance components. [ABSTRACT FROM AUTHOR]

Highlights: Microwave absorber with nanoscale gradient structure was proposed for enhancing the electromagnetic absorption performance. Outstanding reflection loss value (−62.7 dB), broadband wave absorption (6.4 dB with only 2.1 mm thickness) in combination with flexible adjustment abilities were acquired, which is superior to other relative graded distribution structures. This strategy initiates a new method for designing and controlling wave absorber with excellent impedance matching property in practical applications. In the present paper, a microwave absorber with nanoscale gradient structure was proposed for enhancing the electromagnetic absorption performance. The inorganic–organic competitive coating strategy was employed, which can effectively adjust the thermodynamic and kinetic reactions of iron ions during the solvothermal process. As a result, Fe nanoparticles can be gradually decreased from the inner side to the surface across the hollow carbon shell. The results reveal that it offers an outstanding reflection loss value in combination with broadband wave absorption and flexible adjustment ability, which is superior to other relative graded distribution structures and satisfied with the requirements of lightweight equipment. In addition, this work elucidates the intrinsic microwave regulation mechanism of the multiscale hybrid electromagnetic wave absorber. The excellent impedance matching and moderate dielectric parameters are exhibited to be the dominative factors for the promotion of microwave absorption performance of the optimized materials. This strategy to prepare gradient-distributed microwave absorbing materials initiates a new way for designing and fabricating wave absorber with excellent impedance matching property in practical applications. [ABSTRACT FROM AUTHOR]

Cellular structures are broadly used because of their exclusive properties in tissue engineering. This research proposes a new method, both in design and manufacturing, to engineer their mechanical properties considering gradient material and geometrical features and evaluate the possibility of using created structures as bone implants. Schwarz-primitive surface has been utilized to design cellular structures with different porosities and unit cell sizes. A total of 18 cellular structures were designed and fabricated using the SLS 3D printer with a new unconventional approach in adjusting the settings of the machine, and their mechanical properties were extracted. The structures' internal properties were evaluated using the FESEM. Comparing the mechanical compressive test results showed that adjustments in material and geometry improved mechanical properties (such as the compressive moduli, compressive strength, and yield strength). For instance, in 3 mm samples, the elastic modulus in material gradient and geometrical gradient structures is 20% and 73 % higher than the minimum values of the uniform structure. FESEM imaging revealed that adjusting the absorbed energy by powders (controlled by laser characteristics) leads to the formation of natural voids with diameters in the range of 6 to 144 μm for the gradient structures. Evaluation of the designed structures showed that 6 of them (4 uniform porosity and 2 geometrically gradient) have mechanical behavior of the desired tissue. The research outcomes can assist in optimizing manufactured parts by SLS 3D printers with internal and external controlled properties to obtain more desirable mechanical characteristics, especially for tissue engineering applications.

Permanent-magnet materials are critical components for green energy technologies, but the intrinsic trade-off between saturation magnetization and coercivity limits their achievable energy density. Here, a new concept of ordered structures is proposed to develop high-performance permanent magnets. This strategy enables a rare combination of high magnetization and high coercivity by introducing new magnetization reversal mechanisms associated with ordered structures. Implications and challenges of building ordered structures for strong magnets are discussed. This Perspective aims to draw the attention of materials research community to the emerging ordered magnets and to motivate active investigations on this topic for creating transformative magnets.

High‐performance electromagnetic (EM) absorbing materials are in urgent need due to severe EM pollution caused by fast development of communication technology and electronic devices. Gradient structure benefits EM absorbing performance. However, the relevant study is very limited, and the relationship between component layers and overall performance is unclear. Herein, a gradient structure containing impedance matching layer, lossy layer, and reflective layer is built via repeated casting‐drying method using Ti3C2Tx MXene and polyvinyl alcohol. With optimized gradient structure, the minimum reflection loss (RL) reaches −74.8 dB with effective absorbing bandwidth (EAB) covering the whole X‐band. The good absorbing performance is ascribed to the good impedance matching, which is derived from the stepwisely increased permittivity of each layer. Moreover, it is found that RL reaches the lowest value when lossy layer permittivity is close to the average permittivity of impedance matching layer and reflective layer. In addition, the decrease of lossy layer permittivity causes the decrease of total thickness of the gradient structure. This work demonstrates the effectiveness of gradient structure toward strong and broadband microwave absorption and provides rules for designing proper gradient structures to satisfy different requirements. [ABSTRACT FROM AUTHOR]

High‐performance dielectric capacitors are critical for advanced electronics and electrical power systems. Polymeric dielectrics have been widely investigated for using in dielectric capacitors because of their high dielectric constants, flexibility, low density, and easy processability. However, it is still challenging to achieve simultaneous improvements in both energy density and efficiency in these dielectric polymers. Here, we report ferroelectric polyvinylidene‐fluoride‐based all‐organic dielectric polymer films with continuous compositional gradient structure are reported by modulating the spatial distribution of polymethyl‐methacrylate components, prepared via a facile and scalable additive manufacturing method. The continuous out‐of‐plane composition gradient in the all‐organic dielectric polymer films allows to tune electrical and mechanical behaviors, and thus induces a notably enhanced breakdown strength by modulating the electromechanical breakdown process related to the coupling of local electric field and stress. An ultrahigh discharge energy density of 38.8 J cm−3 along with a high discharge efficiency of >80% is achieved at the electric field of 800 kV mm−1 in the gradient polymer films, which is the highest energy density reported thus far in polymer‐based dielectrics including their nanocomposites and the highest energy efficiency achieved along with an energy density of >30 J cm−3. [ABSTRACT FROM AUTHOR]

High‐performance electromagnetic (EM) absorbing materials are in urgent need due to severe EM pollution caused by fast development of communication technology and electronic devices. Gradient structure benefits EM absorbing performance. However, the relevant study is very limited, and the relationship between component layers and overall performance is unclear. Herein, a gradient structure containing impedance matching layer, lossy layer, and reflective layer is built via repeated casting‐drying method using Ti3C2Tx MXene and polyvinyl alcohol. With optimized gradient structure, the minimum reflection loss (RL) reaches −74.8 dB with effective absorbing bandwidth (EAB) covering the whole X‐band. The good absorbing performance is ascribed to the good impedance matching, which is derived from the stepwisely increased permittivity of each layer. Moreover, it is found that RL reaches the lowest value when lossy layer permittivity is close to the average permittivity of impedance matching layer and reflective layer. In addition, the decrease of lossy layer permittivity causes the decrease of total thickness of the gradient structure. This work demonstrates the effectiveness of gradient structure toward strong and broadband microwave absorption and provides rules for designing proper gradient structures to satisfy different requirements.

Abstract Polymeric coatings with randomly distributed dielectric nanoparticles have attracted intensive attention in the passive daytime radiative cooling application. Here, we propose a modified Monte–Carlo method for investigating the spectral response and cooling performance of polymer coating with gradient‐dispersed nanoparticles. Using this method, we carry out a quantitative analysis on the solar reflectance, infrared emittance and cooling power of four categories of gradient structures. It is shown that the gradient profile of particle distribution at the near‐surface region has a significant influence on the overall performance of the coatings. Compared to a randomly distributed structure, the downward size‐gradient structure exhibits superiority in both solar reflectance and cooling power. The presented gradient design, also applicable to porous structures, provides an effective and universal strategy for significantly improving the cooling performance of radiative cooling coatings.

Permanent-magnet materials are critical components for green energy technologies, but the intrinsic trade-off between saturation magnetization and coercivity limits their achievable energy density. Here, a new concept of ordered structures is proposed to develop high-performance permanent magnets. This strategy enables a rare combination of high magnetization and high coercivity by introducing new magnetization reversal mechanisms associated with ordered structures. Implications and challenges of building ordered structures for strong magnets are discussed. This Perspective aims to draw the attention of materials research community to the emerging ordered magnets and to motivate active investigations on this topic for creating transformative magnets. Engineering ordered structures opens up new opportunities to create strong magnets, in which unprecedentedly high properties could be achieved with less alloying content and critical elements. [ABSTRACT FROM AUTHOR]

Gradient structures have been created in single crystal nickel-based superalloys (SX alloys) via surface mechanical creep-feed grinding treatment (SMCGT). It has been found that these gradient structures are mainly composed of nano-sized grains, sub-micron-sized grains, dislocation structures, and the matrix material of single crystals along the depth from the treated surface. In addition, the evolution of such structures is found to be dominated by the dislocation movements which run through both γ channels and γ’ precipitates, subdividing the two types of microstructures into various dislocation structures, and eventually introducing the refined grains into the surface layer. Furthermore, the evolution process of gradient structures primarily originates from the mechanical effect between abrasive grits and workpiece material, owing to the large grinding force (up to 529 N) and low grinding temperature (less than 150 °C) during the unique creep-feed grinding treatment in the present investigation. Due to the typical grain refinement, the hardness of the nanostructures exhibits the largest value of around 10 GPa in the surface layer, approximately 26% higher than that of the matrix material. This study further enhances the understanding of the microstructure–property relationship of SX alloys subjected to creep-feed grinding treatment and contributes to achievement of high-performance components.

The surface integrity of structural materials after surface mechanical strengthening has a significant effect on service performance. Herein, the 50CrMnMoVNb spring steels with different microstructures and strengths are surface strengthened by shot peening (SP) with the same parameter, and the effect of matrix strength on surface integrity, including the roughness, residual stress, and gradient structure, is systematically studied. The experimental results show that both the line roughness and surface roughness decrease with the increase in matrix strength; the microhardness enhancement at the topmost surface layer and thickness of gradient layer decrease with the increase in matrix strength, whereas the surface residual stress and the maximum compressive residual stress display an increasing trend with the increase in matrix strength, and both the depth at the maximum compressive residual stress and the thickness of compressive residual stress zone are stable with a value of ≈75 and ≈300 μm, respectively. Inspired by these results and discussions, the surface integrity of the shot peened (SPed) metallic materials would be improved greatly by composition adjustment and microstructure optimization to regulate the matrix strength, as well as the SP process optimization. [ABSTRACT FROM AUTHOR]

Molecular dynamic simulations are used to correlate gradient structure and mechanical properties of a CoNiCrFeMn high‐entropy alloy (HEA) by characterizing the structural evolution and dislocation substructures during tensile loading. The gradient distributions of deformation faults, dislocations, and grain size from the surface to the center of samples are explored in detail. Quantitative analysis indicates that improvement of strength is attributed to the high densities of dislocations and deformation faults in the grain interior. Moreover, the results reveal that the energy barrier for nucleation of deformation faults in the deformed layer of gradient HEA is higher than that of the ungradient sample. The high strength and work hardening of gradient HEA areattributed to the bundles of concentrated dislocations, which are distributed in grain interiors. This study helps to fundamental understanding of the deformation mechanisms of HEAs with gradient structure, which can be used to strength‐ductility trade‐off. [ABSTRACT FROM AUTHOR]

Gradient-structured (GS) materials are capable of displaying high strength without compromising ductility, which can result in damage-tolerant structures. However, due to the difficulties in fabricating bulk GS materials, there has been only limited studies on the fracture behavior in GS metals. In the present work, the impact toughness of the macroscale GS pure Ni plates was investigated using instrumented Charpy impact testing. The gradient orientation was found to have a significant influence on the impact toughness of GS Ni. For gradient structures that transition from coarse grains (CG) to nano-grains (NG), termed CG→NG gradients (in the present study from ∼8 μm to ∼30 nm), the absorbed energy and the tensile strength were increased, respectively, by 1.6 and 2.3 times from those exhibited by uniform coarse-grained structures, demonstrating a simultaneous enhancement in strength and impact toughness. Analysis of load-displacement curves revealed that the resistance to both crack initiation and propagation were significantly enhanced as the crack penetrated through the CG→NG gradient structure, leading to markedly rising dynamic R-curve behavior estimated from nonlinear-elastic fracture mechanics J- based measurements. The superior fracture resistance in the CG→NG gradient structure was found to originate from sustained ductile fracture by microvoid coalescence, taking place not only in the initial CG zone, but also within the latter NG regions where adiabatic shear bands form during impact; in these latter regions, plasticity becomes enhanced due to grain coarsening induced by recrystallization under the dynamic loading. The present work not only reveals how the dynamic fracture resistance can be significantly enhanced in GS metals, but also provides structure-design strategies for developing superior metallic materials for impact engineering applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A concept is proposed for increasing the efficiency of a rock-crushing tool. The concept uses basics of the thermodynamics of open systems. Based on the proposed concept, the following promising approaches to increasing the efficiency of a rock-crushing tool are identified: Co-phase doping, creating gradient constraints in the volume of the rock-breaking component, and formation of mesostructures in sintered hard alloys. [ABSTRACT FROM AUTHOR]

Ideal biomedically engineered hydrogels should imitate the physiological environment of natural bone tissue healing. The healing ability of soft tissue is stronger than that of bone tissue, thus leading to premature growth of soft tissue into the bone tissue area of the defect and resulting in poor bone tissue healing at the defect site. In addition, good cellular affinity is necessary to allow for postimplantation fusion with surrounding tissues. Inspired by the adhesion mechanism of natural mussels, a polydopamine‐polyacrylamide (PDA‐PAM) single‐network hydrogel and a PDA‐PAM/ACP hydrogel composed of amorphous hydroxyapatite (ACP) is designed to maintain sufficient free catechol groups in the hydrogel by preventing excessive oxidation of dopamine. The hydrogels are found to confer a strong healing ability without stimulation and to have cellular affinity. More notably, separation of the ACP layer and PDA‐PAM hydrogel is not easy to achieve in a synthetic hydrogel. It is found that the hydrogels (PDA‐PAM/ACP) promote bone formation. The gel up‐regulates the expression of osteogenic genes including alkaline phosphatase, runt‐related transcription factor 2, and osteocalcin. In vivo, 8‰ PDA‐PAM/ACP gel is found to effectively repair skull defects. [ABSTRACT FROM AUTHOR]

The paper describes the magnetoelectric (ME) effect in a gradient laminate structure consisting of a LiNbO3/Ni/metglas multilayer based on a bidomain lithium niobate single crystal with a Y + 128° cut. The main purpose of the work is to obtain a self‐biased structure. A theoretical and experimental study is conducted to determine the optimal thickness of the Ni film for creating such a structure. The application of self‐biased structures based on gradient bidomain crystals in ME devices can significantly reduce their weight and dimensions as well as improve their electrical characteristics. [ABSTRACT FROM AUTHOR]

Magnesium alloys are essential for lightweight manufacturing. Nevertheless, defects such as low hardness and poor wear resistance severely restrict their further applications. Ultrasonic impact treatment (UIT) was used to simultaneously fabricate surface Ti-MoS 2 self-lubricating coatings and interior gradient structures on AZ91D magnesium alloy for the first time. The surface morphology and microstructure were investigated, the tribological properties were evaluated at various normal loads, and the formation principles and anti-wear mechanisms of coatings and gradient structures were revealed. The findings indicate that the coatings present a particle-stacking microstructure. The gradient structures were characterized by grain, dislocation, and hardness distribution from greater to lesser along the depth direction. At all loads, the UIT samples exhibited superior anti-wear properties. Compared to untreated samples, the UIT sample's wear volume at 5 N, 15 N, and 25 N loads was reduced by 89.78 %, 84.23 %, and 43.88 %, respectively. The wear surface analysis indicates that the Ti-MoS 2 coatings effectively enhance the lubricity of friction surfaces. Additionally, gradient structure works when the coatings fail and reduces the shear deformation, further reducing wear. This work presents a new method for fabricating simultaneously surface coatings and interior gradient structures using UIT, contributing to improving the anti-wear properties of magnesium alloys and enhancing their utilization in lightweight manufacturing fields. [Display omitted] • A new method for simultaneous fabrication of coatings and gradient structures by ultrasonic impact treatment was proposed • Surface Ti-MoS 2 self-lubricating coatings and interior gradient structures were prepared on AZ91D alloys • The proposed method effectively improves the wear resistance of AZ91D alloys due to the dual anti-wear mechanism [ABSTRACT FROM AUTHOR]

• A gradient structural high entropy alloy induced by laser shock peening exhibits excellent strength-ductility synergy. • Dense shear bands appear on the surface of high entropy alloy after multiple laser shock peening to accommodate the plastic deformation. • Plastic deformation incompatibility stimulates the interaction and accumulation of defects in gradient structural alloys. • The back stress contributes to the high strain hardening for the studied HEA during uniaxial tensile loading. Suppressing strain localization in hard layers was crucial for making gradient alloys ductile. Here, a typical structure in CrFeCoNiMn 0.75 Cu 0.25 high entropy alloy (HEA), which combines gradient distributions along the depth for grain width and twin density, was fabricated using the laser shock peening (LSP) treatment. In situ digital image correlation methods elucidated the mechanisms of the excellent mechanical properties in HEAs treated by LSP for 4 cycles from the perspective of local strain. Dense shear bands were nucleated at the low-strain stage, and remained stable evolution during the entire plastic deformation, thus, leading to an excellent tensile ductility (∼40 %). The plastic deformation incompatibility results in two-dimensional stress states and lateral strain gradient near the plastic-elastic interfaces, which stimulates the interaction and accumulation of micro-defects, thus, improving the strain hardening capacity of alloys. These observations will reveal the mechanistic origin of the gradient structured alloys with superior strength-ductility from a new perspective, which also provide a guidance for optimizing mechanical performances of HEAs. [ABSTRACT FROM AUTHOR]

Lithium (Li) metal batteries are highly sought after for their exceptional energy density. However, their practical implementation is impeded by the formation of dendrites and significant volume fluctuations in Li, which stem from the uneven distribution of Li-ions and uncontrolled deposition of Li on the current collector. Here, an amino-functionalized reduced graphene oxide covered with polyacrylonitrile (PrGN) film with an electric field gradient structure is prepared to deal with such difficulties. This novel current collector serves to stabilize Li-metal anodes by regulating Li-ion flux through vertically aligned channels formed by porous polyacrylonitrile (PAN). Moreover, the amino-functionalized reduced graphene oxide (rGN) acts as a three-dimensional (3D) host, reducing nucleation overpotential and accommodating volume expansion during cycling. The combination of the insulating PAN and conducting rGN creates an electric field gradient that promotes a bottom-up mode of Li electrodeposition and safeguards the anode from interfacial parasitic reactions. Consequently, the electrodes exhibit exceptional cycle life with stable voltage profiles and minimal hysteresis under high current densities and large areal capacities., (© 2023 Wiley-VCH GmbH.)

Programmed deformations are widespread in nature, providing elegant paradigms to design self‐morphing materials with promising applications in biomedical devices, flexible electronics, soft robotics, etc. In this emerging field, hydrogels are an ideal material to investigate the deformation principle and the structure‐deformation relationship. One crucial step is to construct heterogeneous structures in a facile yet effective way. Herein, we provide a focus review on different deformation modes and corresponding structural features of hydrogels. Photolithography is a versatile approach to control the outer shape of the hydrogel and spatial distribution of the component in the hydrogel, endowing the patterned hydrogels with programmed internal stress and thus controllable deformations. Specifically, cooperative deformations take place in periodically patterned hydrogels with in‐plane gradients, and multiple morphing structures are formed in one patterned hydrogel using selective preswelling to direct the buckling of each unit. The structural control strategy and deformation principles should be applicable to other materials with broad applications in diverse areas. Patterning‐assisted deformations: This review summarizes recent progress on the programmed deformations of hydrogels, in which the heterogeneous structures are constructed with the assistance of photolithography. The control of outer shape or spatial distribution of the component affords the patterned hydrogels programmable deformations and promising applications in soft actuators, biomedical devices, etc. [ABSTRACT FROM AUTHOR]

Evident tension-compression yield asymmetry limited the potential use of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy as a structural material. Pre-torsion deformation (free-end torsion) was conducted on the extruded alloy and its effects on microstructure and tensile-compressive mechanical properties were investigated in this study. It was found that the as-extruded alloy exhibited homogenous microstructure and <100>−<110 > double-peak fiber texture. Gradient structures along the radial direction were formed after pre-torsion, which contained gradient distribution of dislocations, grain size, {102} extension twin lamellas and texture components. <100>−<110 > double-peak fiber texture was gradually weakened with the torsional angle increasing from 90° to 135° and 180°, and it was related with the re-orientation of extension twinning and dislocation movement. Moreover, extension twinning was more easily to be activated in grains with <110> fiber texture component, and this phenomenon was revealed by calculating the global Schmid factor of extension twinning under pure shear stress state. Tensile and compressive yield strength increased monotonically with the increasing torsional angle. Meanwhile, ductility almost remained unchanged. Tension-compression yield asymmetry was gradually reduced, and it was eliminated after 180° pre-torsion, which was attributed to the texture weakening. The studying provided a promising and cost-effective method to enhance yield strength and reduce tension-compression yield asymmetry while also keep ductility for extruded Mg alloys. [ABSTRACT FROM AUTHOR]

We consider a system of independent particles and a system of reacting particles on a discrete state space. For the independent case, we rigorously prove a dynamic large-deviation principle for the empirical measure-integrated flux pair, taking the one-way fluxes rather than net fluxes into account. We prove the large deviations under deterministic initial conditions, and under random initial conditions satisfying a large-deviation principle. We formally calculate similar one-way flux large deviations for the more general reacting particles case. Next, we use these results to derive a number of principles from Macroscopic Fluctuation Theory like time-reversal principles and generalised gradient structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • A direct 4D printing technology for achieving and precisely controlling complex ceramic architectures. • This study can help establish a novel paradigm for designing ceramics with complex structures. • Precursors transfer to complex structure ceramics without intervention of a manual physical force and stimuli. • Superior compressive strength. Shape-morphing ceramics with complex geometries can be applied in several scenarios; however, the fabrication of such structures remains challenging owing to the brittleness and stiffness of ceramics. This paper proposes a direct four-dimensional (4D) printing technology for achieving and precisely controlling complex ceramic architectures that may be transformed, in a free-standing manner, from a pre-programmed gradient structure after sintering without intervention of a manual physical force and stimuli. The proposed method involves three steps: printing, curing, and sintering. The designability and flexibility of the proposed approach were demonstrated by preparing different topologies, such as fingers in a palm (multi-curvature), leaves (anisotropic morphing), a dragonfly (high-precision localised deformation), and an intricate structure (self-locking). The obtained ceramics exhibited excellent mechanical properties. This study can help establish a novel paradigm for designing ceramics with complex structures, with potential for application in various fields such as aerospace and biomedical engineering. [ABSTRACT FROM AUTHOR]

The microstructures of 20CrNi2Mo steel underneath the contact surface were examined after dry sliding. Scanning Electronic Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Backscattered Diffraction (EBSD) and an ultra-micro-hardness tester were used to characterize the worn surface and dry sliding wear-induced layer. Martensite laths were ultra-refined due to cumulative strains and a large strain gradient that occurred during cyclic loading in wear near the surface. The microstructure evolution in dominant abrasive wear differs from that in adhesive wear. In dominant abrasive wear, only bent martensite laths with high-density deformation dislocations were observed. In contrast, in dominant adhesive wear, gradient structures were formed along the depth from the wear surface. Cross-sectional TEM foils were prepared in a focused ion beam (FIB) to observe the gradient structures in a dry sliding wear-induced layer at depths of approximately 1–5 μm and 5–20 μm. The gradient structures contained nano-laminated structures with an average thickness of 30–50 nm and bent martensite laths. We found that the original martensite laths coordinated with the strain energy and provided origin boundaries for the formation of gradient structures. Geometrically necessary boundaries (GNBs) and isolated dislocation boundaries (IDBs) play important roles in forming the nano-laminated structures. [ABSTRACT FROM AUTHOR]

A new severe plastic deformation process, the authors refer to as High Pressure Tube Shearing (HPTS), is proposed. This type of deformation processing enhances the strength of the walls of metallic tubes by producing gradient microstructures with ultrafine grained layers in near-surface regions. The thickness of the layers associated with a gradient in microstructure can be controlled by tuning the rotational and translational speeds of the process. The paper describes several examples of steel and titanium tubes processed by different variants of HPTS. The possibility of producing gradient microstructures with ultrafine grained layers at inner or outer surface of a tube, or at both surfaces is demonstrated by in-depth theoretical analysis, finite element simulations, and experimental investigation of the microstructure and texture of tube walls. [ABSTRACT FROM AUTHOR]

[Display omitted] • To develop a universal mixer with novel mixing principle for direct ink material mixing; • To detail the methodology to design and integrate the mixer and printhead to achieve direct material mixing; • To complement the mode of constructing gradient tissue structures by extrusion-based 3D printing; • To enable the possibility of adjustable matrix stiffness for cells if cell-loaded inks are involved or cells are seeded after printing. Conventional methods for creating gradient tissue structures using extrusion-based 3D printing (E3DP) systems involve using mixers to create homogeneous materials. However, commercially available mixers can generate high fluid pressure and shear force that harm cells. Hence, a new mixer-integrated printhead was developed, inspired by stirring-rod-assisted mixing processes. The as-designed mixing system was validated through computational fluid dynamics (CFD) simulations using input parameters including ink viscosity, density, and velocity vector and output parameters including material volume fraction, average fluid pressure, and shear force. Six-stir element mixer units were used to comprehensively analyze material mixing while considering the fluid volume fraction after flowing through the mixers. Printheads integrated with multiple discrete mixers with consistent spiral directions, performing well when introducing gaps, were used to reduce the cell pressure and shear force. The optimal mixer design, with two 1.62-mm gaps and six-stir element mixer units sharing the same spiral structure, reduced the average fluid pressure and shear force by 3.196e+4 Pa and 0.368e+2 Pa, respectively, compared with a commercial static mixer. Finally, a mixer-integrated printhead was manufactured, matching well with the original design. This research could improve E3DP systems and serve as a design reference for 3D printing structures of varying stiffness. [ABSTRACT FROM AUTHOR]

In a previous work we devised a framework to derive generalised gradient systems for an evolution equation from the large deviations of an underlying microscopic system, in the spirit of the Onsager–Machlup relations. Of particular interest is the case where the microscopic system consists of random particles, and the macroscopic quantity is the empirical measure or concentration. In this work we take the particle flux as the macroscopic quantity, which is related to the concentration via a continuity equation. By a similar argument the large deviations can induce a generalised gradient or GENERIC system in the space of fluxes. In a general setting we study how flux gradient or GENERIC systems are related to gradient systems of concentrations. This shows that many gradient or GENERIC systems arise from an underlying gradient or GENERIC system where fluxes rather than densities are being driven by (free) energies. The arguments are explained by the example of reacting particle systems, which is later expanded to include spatial diffusion as well.

Treating critical-size bone defects with autografts, allografts, or standardized implants is challenging since the healing of the defect area necessitates patient-specific grafts with mechanically and physiologically relevant structures. Three-dimensional (3D) printing using computer-aided design (CAD) is a promising approach for bone tissue engineering applications by producing constructs with customized designs and biomechanical compositions. In this study, we propose 3D printing of personalized and implantable hybrid active scaffolds with a unique architecture and biomaterial composition for critical-size bone defects. The proposed 3D hybrid construct was designed to have a gradient cell-laden poly(ethylene glycol) (PEG) hydrogel, which was surrounded by a porous polycaprolactone (PCL) cage structure to recapitulate the anatomical structure of the defective area. The optimized PCL cage design not only provides improved mechanical properties but also allows the diffusion of nutrients and medium through the scaffold. Three different designs including zigzag, zigzag/spiral, and zigzag/spiral with shifting the zigzag layers were evaluated to find an optimal architecture from a mechanical point of view and permeability that can provide the necessary mechanical strength and oxygen/nutrient diffusion, respectively. Mechanical properties were investigated experimentally and analytically using finite element analysis (FEA), and computational fluid dynamics (CFD) simulation was used to determine the permeability of the structures. A hybrid scaffold was fabricated via 3D printing of the PCL cage structure and a PEG-based bioink comprising a varying number of human bone marrow mesenchymal stem cells (hBMSCs). The gradient bioink was deposited inside the PCL cage through a microcapillary extrusion to generate a mineralized gradient structure. The zigzag/spiral design for the PCL cage was found to be mechanically strong with sufficient and optimum nutrient/gas axial and radial diffusion while the PEG-based hydrogel provided a biocompatible environment for hBMSC viability, differentiation, and mineralization. This study promises the production of personalized constructs for critical-size bone defects by printing different biomaterials and gradient cells with a hybrid design depending on the need for a donor site for implantation.