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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]

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]

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]

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]

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]

• 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]

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]

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]