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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]

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]

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]

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]

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]

[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]

[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]

• A gradient layer about 300 μm is formed on the surface after USRP. • Tensile properties and fatigue resistance are improved after USRP. • USRP reduces the ratcheting effect, prolongs fatigue life. • Transition of crack initiation mode is a reason for fatigue strengthening. Surface gradient structures with small to large grain sizes were obtained on 18CrNiMo7-6 steel by ultrasonic surface rolling processing (USRP). Both tensile properties and fatigue resistance have been enhanced. Based on the gradient layer generated after USRP, the fatigue strengthening mechanism was discussed. After USRP, the reduction of the ratcheting strain reduced the fatigue damage of the material under the same stress level and prolonged the fatigue life. Meanwhile, USRP changed the grain orientation of the surface layer, the initiation mode of the fatigue crack was transformed from the surface to the interior at a low stress level. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor nanoparticles possess unique size, shape and composition dependent optical properties, which make them appealing candidates for light-emitting devices (LEDs). However, due to enhanced reabsorption and energy transfer associated losses at high nanoparticle concentrations, which are required for LEDs, photoluminescence quantum yield of the nanoparticles significantly decreases, limiting external quantum efficiency of the LEDs. Therefore, Stokes shift engineered nanoparticles that allow for the suppression of the reabsorption and energy transfer losses are of great practical interest. Here, we report on gradient alloy C d S e 1 − x S x / C d S core-shell nanoplatelets, which provide tunable and large Stokes-shifted narrow emission. We demonstrate that the spectrally separation of absorption and emission peaks of C d S e 1 − x S x / C d S core-shell nanoplatelets is due to their gradient composition. Gradient alloy C d S e 1 − x S x / C d S core-shell nanoplatelets reveal an order of magnitude larger Stokes shift (310 – 422 meV) compared to that of conventional C d S e / C d S core-shell nanoplatelets. Moreover, unlike C d S e / C d S core-shell nanoplatelets, emission wavelength of gradient alloy C d S e 1 − x S x / C d S core-shell nanoplatelets can be finely controlled within the range of 500 – 600 nm via changing the composition of the core and thickness of the shell. We believe that the developed here gradient alloy C d S e 1 − x S x / C d S core-shell nanoplatelets, which provide tunable and large Stokes-shifted emission will be of interest for the development of high-efficiency LEDs with controllable optical properties. [Display omitted] • Colloidal gradient alloy C d S e 1 − x S x / C d S core-shell NPLs are developed. • Gradient alloy C d S e 1 − x S x / C d S core-shell NPLs provide a tunable emission wavelength. • C d S e 1 − x S x / C d S core-shell NPLs exhibit large Stokes-shifted narrow emission. • Optical properties of C d S e 1 − x S x / C d S NPLs can be tuned by varying their composition. [ABSTRACT FROM AUTHOR]

Hierarchical structures and gradient structures have become research hotspots due to their excellent crashworthiness. However, hierarchical gradient structures that combine hierarchical structures and gradient structures are rarely reported. Inspired by the gradient distribution of biological structure vascular bundles and the excellent energy absorption capacity of hierarchical structures, this paper innovatively combines hierarchical structures and gradient structures to propose a new type of bioinspired hierarchical gradient multicell tube (BHGMT). Crashworthiness studies have shown that the high-order BHGMT has a much better crashworthiness than the low-order BHGMT. The specific energy absorption of the 4th order BHGMT under the same mass is 216.26% that of the 0th order BHGMT. A comparison with other existing hierarchical structures under the same mass conditions shows that the proposed hierarchical gradient structure also has better crashworthiness. Subsequently, the systematic development of the influence of structural parameters such as hierarchy, section thickness, and crisscross rib thickness on the parameterization of BHGMT was carried out. Finally, theoretical research on the mean crushing force of the 3rd order BHGMT was carried out, and the theoretical solution was shown to be in agreement with the numerical simulation results. • Bioinspired hierarchical gradient multicell tubes are proposed to enhance crashworthiness. • Introduction of hierarchical gradients can effectively improve various crashworthiness indicators. • The influence of different crisscross ribs thickness on crashworthiness is studied. • A theoretical model is proposed to predict the mean crushing force of bioinspired hierarchical gradient multicell tubes. [ABSTRACT FROM AUTHOR]

• Laser shock peening is an effective way to introduce gradient structures that improve the mechanical properties of high entropy alloy (HEA). • A hardened layer of several hundred micrometers is produced on the surface of HEA via laser shock peening. • Twin boundaries, dense dislocations, and grain refinement in the laser affected layer result in enhanced yield strength of HEA. • Excellent plasticity during loading of the laser-treated HEA is mainly accommodated by dislocations in the laser unaffected core region. For most metallic materials, surface hardening via modifying the near-surface microstructure is an effective method for improving mechanical properties. Among these processes, laser shock peening (LSP), which is versatile and nondestructive to the fabricated product shape, has received much attention. The present work has shown that the mechanical properties of CrFeCoNiMn 0.75 Cu 0.25 high entropy alloy (HEA) have a significant improvement after LSP treatment. Yield strength more than two times of as-cast alloys is achieved in HEA samples treated by LSP for 4 cycles, due to the introduction of a gradient microstructure comprising subgrains, dense dislocations and nano-twins near the treated surface. The combination of dislocation hardening and mechanical twinning improves the strain hardening ability for the LSP-treated HEA, leading to excellent plasticity during tensile loading. Grain refinement also takes place via a faulting process of dislocation dissociation on nearly every {111} plane. Then, TEM observations and finite simulation of the LSP processed HEA samples after different levels of accumulated strain demonstrate that further tensile deformation is mainly accommodated by dislocations in the core region of the sample unaffected by the LSP, while the top LSP-affected layer accommodates little further plasticity as it is sufficiently hardened by the LSP. The present results indicate the possibility to develop gradient-structured HEAs with excellent mechanical properties using laser shock peening. [ABSTRACT FROM AUTHOR]

Suppressing the early strain localization at the nanostructured topmost layer is crucial for achieving better tensile ductility in the gradient structure. Thus, structures with combined gradient distributions along the depth for both grain size and volume fraction of precipitates were designed and introduced in a high entropy alloy by surface mechanical attrition treatment and aging. Yield strength and uniform elongation were observed to be simultaneously improved in the structures with combined gradients as compared to the corresponding structures with only grain size gradient. More severe strain gradients and higher density of geometrically necessary dislocations were observed to be produced at various domain boundaries in the structures with combined gradients, resulting in stronger hetero-deformation-induced (HDI) extra hardening for better tensile properties. Shearing and bowing hardening mechanisms were observed for L1 2 and B2 precipitates, respectively. Higher volume fractions of B2 and L1 2 phases at the topmost layer induce stronger precipitation hardening, which compensates the diminished strain hardening due to the reduced grain size at the topmost layer for better tensile ductility in the structures with combined gradients. The observed higher yield strength in the structures with combined gradients have been discussed based on mechanisms of dislocation strengthening, precipitation strengthening and HDI strengthening. [Display omitted]. [ABSTRACT FROM AUTHOR]

In this study, the microstructure evolution and mechanical properties of Mg-15Gd-1Zn-0.4Zr (GZ151K, wt%) alloys treated by ultrasonic surface rolling process (USRP) were investigated. Microscopic analysis methods, including atomic force microscopy, electron backscatter diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction, as well as mechanical performance tests, including microhardness and uniaxial tensile tests, were employed to analyse the strengthening mechanisms of the GZ151K alloy deformed by the USRP. Results showed that the surface morphology of the alloy was significantly improved compared to that of the untreated sample. In addition, the deformation process of GZ151K alloy introduced a large number of twins and dislocations inner the grains, and formed twin and dislocation gradient structures due to the stress/strain gradient along the normal direction. Furthermore, twinning deformation and dislocation slip during USRP alternately occurred and competed with each other to form a higher volume fraction of sub-grains and twin boundaries. Therefore, the microhardness and tensile strength after USRP were significantly enhanced by twin boundaries and dislocation strengthening, whereas the ductility decreased owing to the initiation and extension of microcracks. However, when the pressure during the USRP was 0.3 MPa, minimal surface roughness and superior mechanical properties could be achieved under the twin and dislocation strengthening. • The twin and dislocation gradient of GZ151K magnesium was found by electron backscatter diffraction. • The mechanical performance and strengthening mechanisms of gradient nano-twin GZ151K alloy were obtained. • The microstructural and textural evolution of gradient nano-twin GZ151K alloy was displayed. [ABSTRACT FROM AUTHOR]

This work describes the in-plane uniaxial tensile mechanical properties of two-dimensional graded rectangular perforations metamaterials using numerical homogenization finite element approaches benchmarked by experimental results. The metamaterial configuration is based on graded patterns of centre-symmetric perforated cells that can exhibit an auxetic (negative Poisson's ratio) behavior. Global and local equivalent mechanical properties of the metamaterial are measured using digital image correlation techniques mapped over Finite Element models to identify strain patterns and related stress distributions at different scales. The samples and their numerical counterpart are parametrized against the spacing and aspect ratios of the cells. The overall stiffness behavior of the graded perforated metamaterial plates features a higher degree of compliance that depends both on the geometries of the cells of the graded areas, but also on the graded pattern used. Local Poisson's ratio effects show a general constraint of the auxetic behavior compared to the case of uniform plates, but also interesting and controllable shape changes due to the uniaxial tensile loading applied. [ABSTRACT FROM AUTHOR]

Gradient materials have gained tremendous attention due to their unprecedented mechanical performance. Here, a scanning electron microscopy (SEM)-based approach is reported to characterize the microstructural evolution in a gradient high-entropy alloy (HEA) produced by the shot peening treatment, which involves electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The gradient distributions of nanograins, nanotwins, and dislocations from the surface to the center of the HEA were revealed in detail. The present approach shows comparable resolution to traditional bright field transmission electron microscopy (TEM) imaging, yet with far higher efficiency and affordability. More intriguingly, we uncovered the dislocation substructures and the corresponding geometrically necessary dislocation (GND) density distribution simultaneously below the micrometer scale. The strengthening effect resulted from the change of GND density accounts for the variation of the hardness at the depth larger than 75 μm. The present study provides a route to better understanding the mechanical properties and deformation mechanisms of gradient materials. Unlabelled Image • An SEM-based approach is reported to characterize the gradient high-entropy alloy. • The present approach shows comparable resolution to normal TEM imaging. • GND density distribution is quantitative measured by HR-EBSD. [ABSTRACT FROM AUTHOR]