Surface Dislocation Research Articles

Reaching near-theoretical strength by achieving quasi-homogenous surface dislocation nucleation in MgO particles

The structural stability of the ceramic nanoparticle (NP) is essential for its versatile applications in broad fields. Due to inevitable imperfections on the surfaces, the NP usually yields at a stress that is far below its theoretical strength. Here we propose an effective way to approach the theoretical strength of the NPs by simply homogenizing the primary imperfections. Electron beam irradiation is utilized to modify the original uneven surface and achieve surface nano-crystallization in MgO NPs, which leads to more homogeneous stress distribution intrinsically as proved by numerical simulations. As a result, quasi-homogeneous surface nucleation of dislocations occurs and raises the compressive strength to be three times higher than the as-received particles with the original surface. The near-theoretical strength gained here indicates that the imperfection in materials can be homogenized to optimize the properties to approach that of perfect crystals.

Deformation mechanism of WC single crystals under nanoindentation: Effects of surface defects and orientation on pop-in and hysteresis

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they were absent when the loading was purely elastic.

Shear displacement gradient in X-ray Bragg coherent diffractive imaging.

Bragg coherent X-ray diffractive imaging is a cutting-edge method for recovering three-dimensional crystal structure with nanoscale resolution. Phase retrieval provides an atomic displacement parallel to the Bragg peak reciprocal lattice vector. The derivative of the displacement along the same vector provides the normal strain field, which typically serves as a proxy for any structural changes. In this communication it is found that the other component of the displacement gradient, perpendicular to the reciprocal lattice vector, provides additional information from the experimental data collected from nanocrystals with mobile dislocations. Demonstration on published experimental data show how the perpendicular component of the displacement gradient adds to existing analysis, enabling an estimate for the external stresses, pinpointing the location of surface dislocations, and predicting the dislocation motion in insitu experiments.

Synthesis, Characterization, and Photocatalytic Property of Ba-Doped ZnO Nanoparticles Synthesized Using Facile Precipitation

This work represents a facile synthesis of ZnO and Ba-doped ZnO (BaZnO) for using as photocatalysts. ZnO and BaZnO are sub-micron particles and nanoparticles, respectively, with similar hexagonal wurtzite polycrystalline structures. Even the size of the lower particles and crystals of BaZnO, compared with ZnO, correspond to the improved specific surface area and dislocation density. The existence of surface chemical defects was detected for both ZnO and BaZnO. For use as photocatalysts, both ZnO and BaZnO were dispersed into a carbaryl solution activated with ultra-violet irradiation. The BaZnO had a higher degradation rate constant of 17.34 × 10−2 min−1 than ZnO (12.40 × 10−2 min−1), an improvement of 39.8%. The improvement is facilitated by the synergistic effects of high specific surface areas and defects caused by small particle sizes and high dislocation density, respectively. Therefore, it can be demonstrated that Ba is an effective dopant for modifying suitable BaZnO nanoparticles to enhance photocatalytic degradation of the toxic carbaryl insecticide.

Performance of the Yangbi MS 6.4 earthquake disaster in different geomorphic units in Yunnan

An MS 6.4 earthquake occurred in Yangbi, Yunnan province, on May 21, 2021. According to related investigations, the macro-epicenter of the earthquake is 6 km northwest of Yangbi County, and the seismogenic structure is the NW-trending Weixi-Qiaohou fault. The earthquake area is located in the hinterland of the Hengduan Mountains in the northwest of Yunnan province, a region dominated by high and medium-high mountains, with deep canyons and tectonic basins in between. Various geomorphic features are derived from drastic topographic changes and huge geological differences in the earthquake area. There are a variety of buildings in the earthquake-affected zone, including civil and brick-wood structures ones with weak seismic performance, as well as brick-concrete and frame ones with better seismic performance. This paper summarizes and analyzes different characteristics of the earthquake in different geomorphic units through field investigations of different buildings and geological disasters in the affected area. The results show that under the same earthquake intensity, the damage to most buildings (located in slope areas or rooted in weak strata) is amplified by the earthquake. The earthquake has exerted an obvious propagation effect along the direction of the seismogenic structure. Moreover, local ground fissures will aggravate the damage to the buildings even without surface dislocation. Thus, we suggest that attention should be paid to the ground fissures caused by the slope effect. The fissure areas may also be the disaster spot of collapses and landslides in case of a high-magnitude earthquake.

Surface deformation, phase transition and dislocation mechanisms of single crystalline 6H-SiC in oblique nano-cutting

Compared with the conventional orthogonal cutting, the oblique nano-cutting of 6H-SiC by tilting the tool at different angles, i.e., the tool inclination angle, is systematically investigated via molecular dynamics (MD) simulations. Focusing on the deformation mechanism, surface deformation including elastic recovery and deformation layer thickness is studied considering the edge ovalization effect in the cutting plane of oblique cutting. Moreover, the variation of cutting force is also analyzed, which relates to the improvement of processing accuracy and the structural phase transition. In particular, the transition path is clarified by coordination number analysis and virtual X-ray diffraction, which shows that the amorphization process is achieved via an initial transformation from the wurtzite structure to a fivefold coordinated orthorhombic intermediate structure, and then to an amorphous phase. Finally, dislocation analysis reveals that the basal dislocation systems dominate the dislocation behaviors, while in oblique nano-cutting especially at the tool inclination angle of 30°, the dislocation system of {0001}<11¯00> (i.e., the basal 1/3<11¯00> Shockley partial dislocation of 6H-SiC) is more activated due to the inclined-cutting-edge-derived shear stress. Furthermore, stacking faults are also formed beneath the bottom of cutting grooves due to the slip upon fivefold coordinated atoms plane.

Reaction–drift–diffusion models from master equations: application to material defects

We present a general method to produce well-conditioned continuum reaction–drift–diffusion equations directly from master equations on a discrete, periodic state space. We assume the underlying data to be kinetic Monte Carlo models (i.e. continuous-time Markov chains) produced from atomic sampling of point defects in locally periodic environments, such as perfect lattices, ordered surface structures or dislocation cores, possibly under the influence of a slowly varying external field. Our approach also applies to any discrete, periodic Markov chain. The analysis identifies a previously omitted non-equilibrium drift term, present even in the absence of external forces, which can compete in magnitude with the reaction rates, thus being essential to correctly capture the kinetics. To remove fast modes which hinder time integration, we use a generalized Bloch relation to efficiently calculate the eigenspectrum of the master equation. A well conditioned continuum equation then emerges by searching for spectral gaps in the long wavelength limit, using an established kinetic clustering algorithm to define a proper reduced, Markovian state space.

Study on the relationship between surface and dislocation of nanoporous copper under cyclic shear loading

In this paper, molecular dynamics simulation is conducted to study the relationship between the surface and dislocation of nanoporous copper under cyclic shear loading. The results show that the dislocation and the surface have both mutual promotion and competition relationship. On one hand, the surface becomes rough owing to reconstruction caused by cyclic shear, which promotes the dislocation activities, while the dislocation activities also accelerate the process of surface reconstruction. On the other hand, there is a competition between surface reconstruction and dislocation activities: in the early cycles, the dislocation density is low, and surface reconstruction dominates stress release; in the late cycles, the surface tends to be stable, and then, the dislocation activities dominate.

Defect-Rich, Highly Porous PtAg Nanoflowers with Superior Anti-Poisoning Ability for Efficient Methanol Oxidation Reaction.

The design of efficient and sustainable Pt-based catalysts is the key to the development of direct methanol fuel cells. However, most Pt-based catalysts still exhibit disadvantages including unsatisfied catalytic activity and serious CO poisoning in the methanol oxidation reaction (MOR). Herein, highly porous PtAg nanoflowers (NFs) with rich defects are synthesized by using liquid reduction combining chemical etching. It is demonstrated that the proportion of precursors determines the inhomogeneity of alloy elements, and the strong corrosiveness of nitric acid to silver leads to the eventual porous flower-like structure. Impressively, the optimal etched Pt1 Ag2 NFs have the mixed defects of surface steps, dislocations, and bulk holes, and their mass activity (1136mA mgPt-1 ) is 2.6 times higher than that of commercial Pt/C catalysts, while the ratio of forward and backward peak current density (If /Ib ) can reach 3.2, exhibiting an excellent anti-poisoning ability. Density functional theory calculations further verify their high anti-poison properties from both an adsorption and an oxidation perspective of CO intermediate. The introduction of Ag makes it easier for CO to be oxidized and removed. This study provides a facile approach to prepare rich defects and porous alloy with excellent MOR performance and superior anti-poisoning ability.

Combining gradient structure and supersaturated solid solution to achieve superior mechanical properties in WE43 magnesium alloy

• Surface mechanical attrition treatment was employed to successfuly produce a gradient nanostructured layer on WE43 magnesium alloy. • High-resolution TEM was mainly performed to successfully dissect the refinement mechanisms of the gradient nanostructure. • A supersaturated solid solution is formed by re-dissolving the Mg 3 RE second phase into nanostructured α-Mg matrix phase in the gradient layer. • Gradient structure and supersaturated solid solution cooperatively contribute to the extraordinary mechanical properties. In this study, surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy. X-ray diffraction, energy dispersive X-ray spectrometer, and high-resolution transmission electron microscope observations were mainly performed to uncover the microstructure evolution responsible for the refinement mechanisms. It reveals that the grain refinement process consists of three transition stages along the depth direction from the core matrix to the topmost surface layer, i.e., dislocation cells and pile-ups, ultrafine subgrains, and randomly orientated nanograins with the grain size of ~40 nm. Noticeably, the original Mg 3 RE second phase is also experienced refinement and then re-dissolved into the α-Mg matrix phase, forming a supersaturated solid solution nanostructured α-Mg phase in the gradient refined layer. Due to the cooperative effects of grain refinement hardening, dislocation hardening, and supersaturated solid-solution hardening, the gradient nanostructured WE43 alloy contributes to the ultimate tensile strength of ~435 MPa and ductility of ~11.0%, showing an extraordinary strain hardening and mechanical properties among the reported severe plastic deformation-processed Mg alloys. This work provides a new strategy for the optimization of mechanical properties of Mg alloys via combining the gradient structure and supersaturated solid solution.

Effect of laser shock peening on fatigue properties of U75VG rail flash-butt welding joints

Rail-welded joints have an important impact on the safety of high-speed railways. Laser shock peening (LSP) is performed on these joints to improve their bending fatigue performance. This study examines the effect of LSP treatment on residual stress, surface microstructure, and surface morphology of the rail base of U75VG flash-butt welding joints. First, the characteristics of the LSP-treated and untreated specimens were evaluated, then the fatigue test was carried out. The results indicated that the LSP treatment generated compressive residual stress with an average value of −436.2 MPa on the surface, and the depth of the corresponding stress layer exceeded 1.5 mm. Both the surface dislocation density and hardness were increased. The fatigue S-N curves indicated that LSP treatment increased the fatigue life of the specimens, particularly under low bending stress conditions. The fatigue limit increased by 6.7% from 264.0 MPa. The fatigue fracture revealed that high-density dislocations enhanced the resistance to the fatigue crack growth, resulting in the crack propagation zone near the LSP-treated surface having a rough morphology with steps. The mass-produced pits on the LSP-treated surface and the induced compressive residual stress could be significant at the crack initiation phase.

Deformation Mechanism of WC Single Crystals Under Nanoindentation: Effects of Surface Defects and Orientation on Pop-In and Hysteresis

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they when the loading was purely elastic.

In Situ TEM tensile testing of bicrystals with tailored misorientation angles

Grain boundaries have complex structural features that influence strength, ductility and fracture in metals and alloys. Grain boundary misorientation angle has been identified as a key parameter that controls their mechanical behavior, but the effect of misorientation angle has been challenging to isolate in polycrystalline materials. Here, we describe the use of bicrystal Au thin films made using a rapid melt growth process to study deformation at a single grain boundary. Tensile testing is performed on bicrystals with different misorientation angles using in situ TEM, as well as on a single crystalline sample. Plastic deformation is initiated through dislocation nucleation from free surfaces. Grain boundary sliding is not observed, and failure occurs away from the grain boundary through plastic collapse in all cases. The failure behavior in these nanoscale bicrystals does not appear to depend on the misorientation angle or grain boundary energy but instead has a more complex dependence on sample surface structure and dislocation activity.

Surface sensibility and stability of AlGaN/GaN ion-sensitive field-effect transistors with high Al-content AlGaN barrier layer

As pH sensors, AlGaN/GaN ion-sensitive field-effect transistors (ISFETs) with stacked and high Al-content AlGaN barrier have been fabricated to evaluate the sensing ability and surface reliability. It demonstrates that the stacked barrier shows mild influence on the two-dimensional electron gas channel. In addition, the AlGaN/GaN ISFETs present good pinch-off characteristics in three kinds of solutions with different pH values, showing a sensitivity of approximately 55.5 mV/pH. However, threshold voltage shifting as well as the output current decreasing were observed after worked in alkaline solution. The surface analysis results confirm that the surface was electrochemical etched into many voids, resulting in a positive shift on the threshold voltage. This etching process is related to the surface oxide and dislocation on the high Al-content AlGaN barrier layer. Therefore, the degradation in electrical performances can be attributed to the transformation of AlGaN into surface oxide as well as the followed dissolution in alkaline solution.

Highly Conductive n-Al0.65Ga0.35N Grown by MOCVD Using Low V/III Ratio

Highly conductive silicon-doped AlGaN and ohmic contacts are needed for deep-UV LEDs and ultrawide bandgap electronics. We demonstrate improved n-Al0.65Ga0.35N films grown by metal–organic chemical vapor deposition (MOCVD) on sapphire substrates using a low V/III ratio (V/III = 10). A reduced V/III ratio improves repeatability and uniformity by allowing a wider range of silicon precursor flow conditions. AlxGa1−xN:Si with x &gt; 0.5 typically has an electron concentration vs. silicon concentration trend that peaks at a particular “knee” value before dropping sharply as [Si] continues to increase (self-compensation). The Al0.65Ga0.35N:Si grown under the lowest V/III conditions in this study does not show the typical knee behavior, and instead, it has a flat electron concentration trend for [Si] &gt; 3 × 1019 cm−3. Resistivities as low as 4 mΩ-cm were achieved, with corresponding electron mobility of 40 cm2/Vs. AFM and TEM confirm that surface morphology and dislocation density are not degraded by these growth conditions. Furthermore, we report vanadium-based ohmic contacts with a resistivity of 7 × 10−5 Ω-cm2 to AlGaN films grown using a low V/III ratio. Lastly, we use these highly conductive silicon-doped layers to demonstrate a 284 nm UV LED with an operating voltage of 7.99 V at 20 A/cm2, with peak EQE and WPE of 3.5% and 2.7%, respectively.

Temperature effect on nanoporous gold under uniaxial tension and compression

Nanoporous gold (NP-Au) is of great interest to researchers due to its high surface area; and accordingly, the wide range of applications that the material can be utilized for especially those where high temperature is involved. Therefore, the effect of temperature on NP-Au is studied by performing Molecular Dynamics (MD) simulations at temperatures between 300 K and 700 K. Moreover, an Arrhenius type formulation is proposed to modify existing scaling laws to capture the temperature effect. Also, a series of temperature dependent modifications to an existing dislocation based constitutive model are proposed. The simulation results show that while the elastic modulus and yield stress are temperature dependent, their tension–compression asymmetries are not. Under both compression and tension, material strength is controlled by surface stresses and dislocation mobility. However, the dislocation density required to plastically deform the material is found to be completely temperature independent under tension, and becomes temperature dependent under compression once there is sufficient amount of ligaments merging and collapse.

Fusion of Stacked Nanowires: From Atomistic to Analytical Models

AbstractFusion of metallic nanowires (NWs) is of increasing interest for fabricating printed devices. Atomistic simulations of inter‐NW neck growth during thermal fusion of vertically stacked silver nanowires (NWs) with nonorthogonal axes are performed, a geometric configuration that is commonly seen in applications. High NW rotation during fusion is uncovered surprisingly and found that it accelerates inter‐NW neck growth beyond that explainable by conventional geometric arguments. Rotation‐regulated surface diffusion and dislocation generation are found to be the culpable mechanisms and are shown to be dominant in distinct regimes of initial NW orientation. Motivated by these atomistic observations, an original analytical model of inter‐NW neck growth is formulated and validated. The model accurately predicts the unusual trends in neck growth with six orders of magnitude lesser computational effort than atomistic simulations. Further, it can handle nonisothermal temperature histories over millisecond time scales for NWs up to 100 nm in diameter, a capability that is beyond the reach of typical atomistic simulations. The impact of the revealed spatial disparity of nanoscale neck growth on the properties of random‐packed NW assemblies, and the foundational role of the model in rational design and processing of printed multi‐NW assemblies for a range of applications are discussed.

Surfactant effects on structural, optical and morphological characteristics of microwave irradiated CdO nanostructures

Surfactant based microwave irradiated and alkaline medium involving wet chemical method was adopted to prepare CdO nanostructures. The phase evaluation of prepared and annealed CdO nanostrucutures has been analyzed by XRD and FTIR. The average size of the crystallite was estimated to be 54 nm for the NH4OH reagent and 60 nm for the NaOH reagent in the assisted synthesis. Specific surface area, lattice parameter, X-ray density, dislocation density, microstrain, texture coefficient and Williamson Hall have been calculated from the XRD profile. UV transmittance and the K-M plot of DRS and PL spectra were used to examine the optical properties of CdO nanostructures. The spherical and rod-like structures of CdO prepared with strong and weak alkaline medium have been recorded through SEM and TEM micrographs. The four-step process of hydration, intermediate formation, metal hydroxide formation, and metal oxide formation confirms that the experiment in the presence of 0.1 M sodium hydroxide results in rod-shaped nanostructures.

Site dependence of surface dislocation nucleation in ceramic nanoparticles

The extremely elevated strength of nanoceramics under compression arises from the necessity to nucleate highly energetic dislocations from the surface, in samples that are too small to contain pre-existing defects. Here, we investigate the site dependence of surface dislocation nucleation in MgO nanocubes using a combination of molecular dynamics simulations, nudged-elastic-band method calculations and rate theory predictions. Using an original simulation setup, we obtain a complete mapping of the potential dislocation nucleation sites on the surface of the nanoparticle and find that, already at intermediate temperature, not only nanoparticle corners are favorable nucleation sites, but also the edges and even regions on the side surfaces, while other locations are intrinsically unfavorable. Results are discussed in the context of recent in situ TEM experiments, sheding new lights on the deformation mechanisms happening during ceramic nanopowder compaction and sintering processes.

Depth-profiling of residual stress and microstructure for austenitic stainless steel surface treated by cavitation, shot and laser peening

While the general characteristics of various peening techniques have been established, there have been few comparative studies. Here we compare the variation of the residual stresses and microstructural characteristics with depth for 316L austenitic stainless steel treated by cavitation peening (CP), shot peening (SP) and laser peening (LP) all peened to similar intensity levels. While the plastically affected depths were similar in all cases (~400 μm), the SP specimen showed the most extensive near surface plastic deformation, deformation twinning, dislocation density and compressive residual stress. To counterbalance this, the compressive residual stresses extended deeper for the LP and CP. Across the three treatments, a similar dependency was found between diffraction peak broadening and hardness. The dislocation density at the surface determined by the diffraction line profile analysis (LPA) for the SP specimen (4.9 × 10 15 m −2 ) was approximately 2.5 times that for the CP and LP specimens (2.0 × 10 15 and 2.1 × 10 15 m −2 ). Electron backscatter diffraction (EBSD) shows that the extensive work introduced by the SP had generated planar defects near to the surface. The increase in yield stress estimated from the hardness corresponded with the increase in dislocation density obtained by the LPA. • Shot peening (SP) introduces larger near-surface stresses while cavitation peening and laser peening penetrate deeper. • SP gives the highest nearsurface hardness of the three treatments. • SP also introduces a larger number of dislocations and planar defects than the others. • The hardness at a given FWHM is broadly the same regardless of the kind of the peening treatment. • The increase in yield stress estimated from the hardness correlates with the increase in dislocation density.