Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymme...Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.展开更多
In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondl...In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondly,the existence and uniqueness of local solutions are proved by the Sobolev embedding theorem and the comparison principle.Finally,according to the relevant research data and contents of red fire ants,the diffusion area and nest number of red fire ants were simulated without external disturbance.This paper mainly simulates the early diffusion process of red fire ants.In the early diffusion stage,red fire ants grow slowly and then spread over a large area after reaching a certain number.展开更多
Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformat...Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformation,while the mechanism has not been fully understood at the mesoscale.In this work,a model coupling dendrite growth,thermal-solutal-fluid flow,thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys.Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation.The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds.As the width of the mushy zone gets stable,stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space.There are three characteristic zones of stress distribution along the solidification direction:(i)no stress concentration in the fully solidified regions;(ii)stress developing in the primary dendrite bridging region,and(iii)stress decrease in the inter-dendritic uncontacted zone.The stresses reach maximum near the initial dendrite bridging position.The lower temperature gradients,the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites.Corresponding measures are suggested to reduce LAGBs.展开更多
Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated...Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated the segregation behaviors of an Mg-RE alloy under deformation.We found that the enrichment of solute atoms occurred in{101^(-)1}compressive twin boundaries under compression at 298 K without any annealing in an Mg-RE alloy by scanning transmission electron microscopy and energy-dispersive X-ray analysis.The segregated solutes and precipitates impeded the twin growth,partially contributing to the formation of small-sized{101^(-)1}compressive twins.This research indicates the twin boundaries can be strengthened by segregated solutes and precipitates formed under deformation at room temperature.展开更多
This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo sim...This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo simulations.Two segregation mechanisms,substitutional and interstitial mechanisms,are observed.The intergranular defects,including dislocations,steps and vacancies,and the intervals in structural units are conductive to the prevalence of interstitial mechanism.And substitutional mechanism is favored by the highly ordered twin GBs.Furthermore,the two mechanisms affect the GB structure differently.It is quantified that interstitial mechanism is less destructive to GB structure than substitutional one,and often leads to a segregation level being up to about 6 times higher than the latter.These findings contribute to atomic scale insights into the microscopic mechanisms about how solute atoms are absorbed by GB structures,and clarify the correlation among intergranular structures,segregation mechanisms and kinetics.展开更多
Due to the heterogeneity of rock masses and the variability of in situ stress,the traditional linear inversion method is insufficiently accurate to achieve high accuracy of the in situ stress field.To address this cha...Due to the heterogeneity of rock masses and the variability of in situ stress,the traditional linear inversion method is insufficiently accurate to achieve high accuracy of the in situ stress field.To address this challenge,nonlinear stress boundaries for a numerical model are determined through regression analysis of a series of nonlinear coefficient matrices,which are derived from the bubbling method.Considering the randomness and flexibility of the bubbling method,a parametric study is conducted to determine recommended ranges for these parameters,including the standard deviation(σb)of bubble radii,the non-uniform coefficient matrix number(λ)for nonlinear stress boundaries,and the number(m)and positions of in situ stress measurement points.A model case study provides a reference for the selection of these parameters.Additionally,when the nonlinear in situ stress inversion method is employed,stress distortion inevitably occurs near model boundaries,aligning with the Saint Venant's principle.Two strategies are proposed accordingly:employing a systematic reduction of nonlinear coefficients to achieve high inversion accuracy while minimizing significant stress distortion,and excluding regions with severe stress distortion near the model edges while utilizing the central part of the model for subsequent simulations.These two strategies have been successfully implemented in the nonlinear in situ stress inversion of the Xincheng Gold Mine and have achieved higher inversion accuracy than the linear method.Specifically,the linear and nonlinear inversion methods yield root mean square errors(RMSE)of 4.15 and 3.2,and inversion relative errors(δAve)of 22.08%and 17.55%,respectively.Therefore,the nonlinear inversion method outperforms the traditional multiple linear regression method,even in the presence of a systematic reduction in the nonlinear stress boundaries.展开更多
Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoe...Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoelectric properties has been a hot research topic. This study investigated the effects of phase boundary engineering and domain engineering on (1-x)[0.8Pb(Zr_(0.5)Ti_(0.5))O_(3)-0.2Pb(Zn_(1/3)Nb_(2/3))O_(3)]-xBi(Zn_(0.5)Ti_(0.5))O_(3) ((1-x)(0.8PZT-0.2PZN)- xBZT) ceramic to obtain excellent piezoelectric properties. The crystal phase structure and microstructure of ceramic samples were characterized. The results showed that all samples had a pure perovskite structure, and the addition of BZT gradually increased the grain size. The addition of BZT caused a phase transition in ceramic samples from the morphotropic phase boundary (MPB) towards the tetragonal phase region, which is crucial for optimizing piezoelectric properties. By adjusting content of BZT and precisely controlling position of the phase boundary, the piezoelectric performance can be optimized. Domain structure is one of the key factors affecting piezoelectric performance. By using domain engineering techniques to optimize grain size and domain size, piezoelectric properties of ceramic samples have been significantly improved. Specifically, excellent piezoelectric properties (piezoelectric constant d_(33)=320 pC/N, electromechanical coupling factor kp=0.44) were obtained simultaneously for x=0.08. Based on experimental results and theoretical analysis, influence mechanisms of phase boundary engineering and domain engineering on piezoelectric properties were explored. The study shows that addition of BZT not only promotes grain growth, but also optimizes the domain structure, enabling the polarization reversal process easier, thereby improving piezoelectric properties. These research results not only provide new ideas for the design of high-performance piezoelectric ceramics, but also lay a theoretical foundation for development of related electronic devices.展开更多
The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the...The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.展开更多
The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.T...The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.展开更多
The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mes...The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.展开更多
The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through...The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through post-processing,potentially altering the mechanical properties of the optimized structure.A topology optimization method of Movable Morphable Smooth Boundary(MMSB)is proposed based on the idea of mesh adaptation to solve the problem of jagged boundaries and the influence of post-processing.Based on the ICM method,the rational fraction function is introduced as the filtering function,and a topology optimization model with the minimum weight as the objective and the displacement as the constraint is established.A triangular mesh is utilized as the base mesh in this method.The mesh is re-divided in the optimization process based on the contour line,and a smooth boundary parallel to the contour line is obtained.Numerical examples demonstrate that the MMSB method effectively resolves the jagged boundary issues,leading to enhanced structural performance.展开更多
On February 8,2025,a remote area in the Caribbean Sea was rocked by a large M_(W)7.6(USGS,2025) earthquake,centered 209 km SSW of Georgetown,the capital of the Cayman Islands,and the largest city(population~41 000) of...On February 8,2025,a remote area in the Caribbean Sea was rocked by a large M_(W)7.6(USGS,2025) earthquake,centered 209 km SSW of Georgetown,the capital of the Cayman Islands,and the largest city(population~41 000) of the British Overseas Territories(Figure 1).The earthquake was significant due to its large magnitude,potential regional impact,and the possibility of generating a tsunami.展开更多
Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of...Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of shear and streaming vorticity)under the field description.For an incompressible viscous flow interacting with a stationary wall,the full expressions of the boundary fluxes of the orbital rotation and the spin are derived,for the first time,to elucidate their boundary creation mechanisms.Then,these new findings are successfully extended to the study of the boundary enstrophy dynamics,as well as the Lyman vorticity dynamics as an alternative interpretation to the boundary vorticity dynamics.Interestingly,it is found that the boundary coupling of the longitudinal and transverse processes is only embodied in the boundary spin flux,which is definitely not responsible for the generation of the boundary orbital-rotation flux.In addition,the boundary fluxes of enstrophy are directly associated with the boundary source of the second principal invariant of the velocity gradient tensor(VGT)and the two quadratic forms representing the spin-geometry interaction.The present exposition provides a new perspective and an additional dimension for understanding the vorticity dynamics on boundaries,which could be valuable in clarifying the formation mechanisms of near-wall coherent structures and flow noise at the fundamental level.展开更多
Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary...Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary layer are excited and rapidly increase during the receptivity process,so sufficiently large Reynolds stress causes the basic flow velocity profile to change,and the formation of turbulence is inevitable.展开更多
Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction len...Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction lengths have not been sufficiently investigated. First, this study presents a theoretical scaling analysis and validates it through wind tunnel experiments. It conducts detailed control volume analysis of mass conservation, considering the differences between inviscid and viscous cases. Then, three models for analysing interaction length under gradual expansion waves are derived. Related experiments using schlieren photography are conducted to validate the models in a Mach 2.73 flow. The interaction scales are captured at various relative distances between the shock impingement location and the expansion regions with wedge angles ranging from 12° to 15° and expansion angles of 9°, 12°, and 15°. Three trend lines are plotted based on different expansion angles to depict the relationship between normalised interaction length and normalised interaction strength metric. In addition, the relationship between the coefficients of the trend line and the expansion angles is introduced to predict the interaction length influenced by gradual expansion waves. Finally, the estimation of normalised interaction length is derived for various coefficients within a unified form.展开更多
As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding ...As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding on how Y addition affects the microstructural evolution and mechanical properties,the Y segregation behavior at grain boundaries was investigated in Mg-1wt.%Y and Mg-7wt.%Y alloys at different conditions.The segregation intensity and its dependence on the grain boundary misorientation angle were experimentally characterized and computationally predicted.Strong segregation at grain boundaries was observed in both low and high Y-containing alloys.Y segregation was found to remain in alloy Mg-7Y after high-temperature annealing heat treatment at 540℃.No direct correlation between the Y segregation intensity and the grain boundary misorientation angle could be established based on either the experimental characterization or the atomistic simulation with a spectral model.We thus conclude that grain boundary segregation of Y is independent of grain boundary misorientation angle.展开更多
Metals in advanced nuclear reactors,such as W,often experience microcracks.However,the synergistic effects of high temperature,stress,and specialized structures can improve the self-healing ability of these metals.Mic...Metals in advanced nuclear reactors,such as W,often experience microcracks.However,the synergistic effects of high temperature,stress,and specialized structures can improve the self-healing ability of these metals.Microcrack healing is closely related to crack surface conditions.The order and disorder degree of crack surface atoms may affect crack stability.In this study,first-principles calculations,ab initio molecular dynamics,and surface thermodynamic theory were used to investigate the stability of grain boundary(GB)cracks at 0,293,and 373 K.We compared the energy densities,crack attraction energies,and atomic diffusion behaviors of crack surfaces atΣ3 GBs with those atΣ5 GBs.Adsorption on the nanocrack surface determines the critical nanocrack width.It was found that AlΣ3(111)nanocracks heal at high temperatures,and this healing behavior is closely related to the crack surface energy.Meanwhile,the GB cracks of W heal in an orderly manner at 573 and 1203 K.BY contrast,the GB cracks of Ti remain unhealed.Finally,a high-temperature nanocrack expansion model was developed and used to predict crack behavior under applied stress at different temperatures.展开更多
When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatl...When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatly enhance the operational capabilities of aero-engine.However,due to the“line-of-sight”processing characteristic of PVD process,uneven coating deposition rates occur when preparing coatings on obstructed areas such as blisks.Quantitative research on such phenomena is few,and it is even rarer in the study of aero-engine coatings.Based on the analyses and considerations of the geometric shape of blade surfaces and the influence of both deposition and re-sputtering effect,an ideal model is established to analyze the deposition rate variation along blocked region in complex self-shadowing boundaries.The relative deposition rates at various locations on the blade surface within the inter-blade gaps are quantitatively calculated and experimentally validated.Furthermore,differences in erosion resistance of the coatings are tested.The conclusions are drawn as follows:the geometric configuration of the obstructed shape and resputtering phenomenon significantly influence the deposition rates within the inner wall of blade gaps.Taking the structural configuration as an example,in a 25 mm×60 mm×15 mm gap,the coating thickness can vary more than 252%from the thickest to the thinnest location.The deposition rates of various locations are proportional to the solid angle of incident ion in more obstructed regions,and the re-sputtering is more prominent in open regions.Obstructive boundaries directly affect the erosion resistance at various locations within the gaps,with erosion failure time decreasing by 40%in heavily blocked region compared to open region.展开更多
This review provides recent advancements in the study of attachment-line boundary layer transition with emphasis on high-speed configurations.As a critical factor influencing aerodynamic performance and thermal manage...This review provides recent advancements in the study of attachment-line boundary layer transition with emphasis on high-speed configurations.As a critical factor influencing aerodynamic performance and thermal management in supersonic and hypersonic systems,the transition mechanisms of three-dimensional attachment-line boundary layers have emerged as a pivotal research frontier in high-speed aerodynamics.This review systematically summarizes the evolution of research on attachment-line boundary layer transition,from early theoretical foundations to modern computational and experimental breakthroughs.A critical examination is presented for two pivotal challenges in high-speed attachment-line boundary layer transition:The resolution of the Gaillard paradox and the leading-edge contamination mechanism.Through systematic synthesis of theoretical developments and empirical evidence,this review identifies critical knowledge gaps while proposing novel methodological approaches for attachment-line boundary layer transition analysis.The review culminates in a strategic framework outlining promising avenues for both fundamental inquiry into attachment-line phenomena and applied engineering solutions in flow control strategies.展开更多
Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 k...Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 kHz are considered to be rep-resentative of the first and second instability modes,respectively.For the first mode,wall pressure fluctuations for both entropy and vorticity wave cases exhibit a strong modulation yet without a growing trend,indicating that the first mode is not generated despite its instability predicted by linear stability theory.The potential reason for this is the absence of postshock slow acoustic waves capable of synchronizing with the first mode.By contrast,for the second mode,a typical three-stage boundary-layer response is observed,consistent with that to slow acoustic waves studied previously.Furthermore,the postshock disturbances outside the boundary layer can be decomposed into the entropy(density/temperature fluctuations)and vorticity components(ve-locity fluctuations),and the latter is shown to play a leading role in generating the second mode,even for the case with entropy waves where the density/temperature fluctuations dominate the postshock regions.展开更多
基金supported by the Scientific and Technological Developing Scheme of Jilin Province under grants no.YDZJ202301ZYTS538the Chinese Academy of Sciences Youth Innovation Promotion Association under grants number 2023234+3 种基金the National Natural Science Foundation of China under grants number U21A20323the Scientific and Technological Developing Scheme of Jilin Province under grants no.SKL202302038the Major Scientific and Technological Projects of Hebei Province under grants No.23291001Zthe Scientific and Technology Project of Hanjiang District.
文摘Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.
基金Supported by National Natural Science Foundation of China(12101482)Postdoctoral Science Foundation of China(2022M722604)+2 种基金General Project of Science and Technology of Shaanxi Province(2023-YBSF-372)The Natural Science Foundation of Shaan Xi Province(2023-JCQN-0016)Shannxi Mathmatical Basic Science Research Project(23JSQ042)。
文摘In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondly,the existence and uniqueness of local solutions are proved by the Sobolev embedding theorem and the comparison principle.Finally,according to the relevant research data and contents of red fire ants,the diffusion area and nest number of red fire ants were simulated without external disturbance.This paper mainly simulates the early diffusion process of red fire ants.In the early diffusion stage,red fire ants grow slowly and then spread over a large area after reaching a certain number.
基金sponsored by the National Natural Science Foundation of China(Grant Nos.52074182,52304406 and U23A20612)the Natural Science Foundation of Shanghai(Grant Nos.22ZR1430700 and 23TS1401900)+1 种基金the National Science and Technology Major Project(No.2017-VII-0008-0102)Neng Ren acknowledges the Startup Fund for Young Faculty at SJTU.
文摘Low-angle grain boundaries(LAGBs)are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties.The formation of LAGBs is related to dendrite deformation,while the mechanism has not been fully understood at the mesoscale.In this work,a model coupling dendrite growth,thermal-solutal-fluid flow,thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys.Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation.The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds.As the width of the mushy zone gets stable,stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space.There are three characteristic zones of stress distribution along the solidification direction:(i)no stress concentration in the fully solidified regions;(ii)stress developing in the primary dendrite bridging region,and(iii)stress decrease in the inter-dendritic uncontacted zone.The stresses reach maximum near the initial dendrite bridging position.The lower temperature gradients,the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites.Corresponding measures are suggested to reduce LAGBs.
基金support from Interdisciplinary Research Project for Young Teachers of USTB Fundamental Research Funds for the Central Universities(Grant no.FRF-IDRY-23-030).
文摘Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys.Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing.In this study,we investigated the segregation behaviors of an Mg-RE alloy under deformation.We found that the enrichment of solute atoms occurred in{101^(-)1}compressive twin boundaries under compression at 298 K without any annealing in an Mg-RE alloy by scanning transmission electron microscopy and energy-dispersive X-ray analysis.The segregated solutes and precipitates impeded the twin growth,partially contributing to the formation of small-sized{101^(-)1}compressive twins.This research indicates the twin boundaries can be strengthened by segregated solutes and precipitates formed under deformation at room temperature.
基金supported by grants from the National Natural Science Foundation of China(Nos.52031017,51801237)the National Key Laboratory of Science and Technology on High-strength Structural Materials in Central South University,China(No.6142912200106).
文摘This study aims to clarify the mechanisms for the grain boundary(GB)segregation through investigating the absorption of excess solute atoms at GBs in Al−Cu alloys by using the hybrid molecular dynamics/Monte Carlo simulations.Two segregation mechanisms,substitutional and interstitial mechanisms,are observed.The intergranular defects,including dislocations,steps and vacancies,and the intervals in structural units are conductive to the prevalence of interstitial mechanism.And substitutional mechanism is favored by the highly ordered twin GBs.Furthermore,the two mechanisms affect the GB structure differently.It is quantified that interstitial mechanism is less destructive to GB structure than substitutional one,and often leads to a segregation level being up to about 6 times higher than the latter.These findings contribute to atomic scale insights into the microscopic mechanisms about how solute atoms are absorbed by GB structures,and clarify the correlation among intergranular structures,segregation mechanisms and kinetics.
基金funded by the National Key R&D Program of China(Grant No.2022YFC2903904)the National Natural Science Foundation of China(Grant Nos.51904057 and U1906208).
文摘Due to the heterogeneity of rock masses and the variability of in situ stress,the traditional linear inversion method is insufficiently accurate to achieve high accuracy of the in situ stress field.To address this challenge,nonlinear stress boundaries for a numerical model are determined through regression analysis of a series of nonlinear coefficient matrices,which are derived from the bubbling method.Considering the randomness and flexibility of the bubbling method,a parametric study is conducted to determine recommended ranges for these parameters,including the standard deviation(σb)of bubble radii,the non-uniform coefficient matrix number(λ)for nonlinear stress boundaries,and the number(m)and positions of in situ stress measurement points.A model case study provides a reference for the selection of these parameters.Additionally,when the nonlinear in situ stress inversion method is employed,stress distortion inevitably occurs near model boundaries,aligning with the Saint Venant's principle.Two strategies are proposed accordingly:employing a systematic reduction of nonlinear coefficients to achieve high inversion accuracy while minimizing significant stress distortion,and excluding regions with severe stress distortion near the model edges while utilizing the central part of the model for subsequent simulations.These two strategies have been successfully implemented in the nonlinear in situ stress inversion of the Xincheng Gold Mine and have achieved higher inversion accuracy than the linear method.Specifically,the linear and nonlinear inversion methods yield root mean square errors(RMSE)of 4.15 and 3.2,and inversion relative errors(δAve)of 22.08%and 17.55%,respectively.Therefore,the nonlinear inversion method outperforms the traditional multiple linear regression method,even in the presence of a systematic reduction in the nonlinear stress boundaries.
基金National Natural Science Foundation of China (52202139, 52072178)。
文摘Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoelectric properties has been a hot research topic. This study investigated the effects of phase boundary engineering and domain engineering on (1-x)[0.8Pb(Zr_(0.5)Ti_(0.5))O_(3)-0.2Pb(Zn_(1/3)Nb_(2/3))O_(3)]-xBi(Zn_(0.5)Ti_(0.5))O_(3) ((1-x)(0.8PZT-0.2PZN)- xBZT) ceramic to obtain excellent piezoelectric properties. The crystal phase structure and microstructure of ceramic samples were characterized. The results showed that all samples had a pure perovskite structure, and the addition of BZT gradually increased the grain size. The addition of BZT caused a phase transition in ceramic samples from the morphotropic phase boundary (MPB) towards the tetragonal phase region, which is crucial for optimizing piezoelectric properties. By adjusting content of BZT and precisely controlling position of the phase boundary, the piezoelectric performance can be optimized. Domain structure is one of the key factors affecting piezoelectric performance. By using domain engineering techniques to optimize grain size and domain size, piezoelectric properties of ceramic samples have been significantly improved. Specifically, excellent piezoelectric properties (piezoelectric constant d_(33)=320 pC/N, electromechanical coupling factor kp=0.44) were obtained simultaneously for x=0.08. Based on experimental results and theoretical analysis, influence mechanisms of phase boundary engineering and domain engineering on piezoelectric properties were explored. The study shows that addition of BZT not only promotes grain growth, but also optimizes the domain structure, enabling the polarization reversal process easier, thereby improving piezoelectric properties. These research results not only provide new ideas for the design of high-performance piezoelectric ceramics, but also lay a theoretical foundation for development of related electronic devices.
基金supported by the National Key Research and Development Program of China(2022YFB3505503)the National Natural Science Foundation of China(52201230)+2 种基金the Key R&D Program of Shandong Province(2022CXGC020307)the China Postdoctoral Science Foundation(2022M71204)the Beijing NOVA Program(Z211100002121092).
文摘The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.
基金supported by the National Key Research and Development Program of China(Nos.2021YFB3503003,2021YFB3503100,and 2022YFB3505401).
文摘The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.
基金partially supported by the National Natural Science Foundation of China(Nos.92271103,12202191)。
文摘The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.
基金supported by the National Natural Science Foundation of China(Grant 12472113).
文摘The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through post-processing,potentially altering the mechanical properties of the optimized structure.A topology optimization method of Movable Morphable Smooth Boundary(MMSB)is proposed based on the idea of mesh adaptation to solve the problem of jagged boundaries and the influence of post-processing.Based on the ICM method,the rational fraction function is introduced as the filtering function,and a topology optimization model with the minimum weight as the objective and the displacement as the constraint is established.A triangular mesh is utilized as the base mesh in this method.The mesh is re-divided in the optimization process based on the contour line,and a smooth boundary parallel to the contour line is obtained.Numerical examples demonstrate that the MMSB method effectively resolves the jagged boundary issues,leading to enhanced structural performance.
文摘On February 8,2025,a remote area in the Caribbean Sea was rocked by a large M_(W)7.6(USGS,2025) earthquake,centered 209 km SSW of Georgetown,the capital of the Cayman Islands,and the largest city(population~41 000) of the British Overseas Territories(Figure 1).The earthquake was significant due to its large magnitude,potential regional impact,and the possibility of generating a tsunami.
基金Project supported by the National Natural Science Foundation of China(No.12402262)。
文摘Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of shear and streaming vorticity)under the field description.For an incompressible viscous flow interacting with a stationary wall,the full expressions of the boundary fluxes of the orbital rotation and the spin are derived,for the first time,to elucidate their boundary creation mechanisms.Then,these new findings are successfully extended to the study of the boundary enstrophy dynamics,as well as the Lyman vorticity dynamics as an alternative interpretation to the boundary vorticity dynamics.Interestingly,it is found that the boundary coupling of the longitudinal and transverse processes is only embodied in the boundary spin flux,which is definitely not responsible for the generation of the boundary orbital-rotation flux.In addition,the boundary fluxes of enstrophy are directly associated with the boundary source of the second principal invariant of the velocity gradient tensor(VGT)and the two quadratic forms representing the spin-geometry interaction.The present exposition provides a new perspective and an additional dimension for understanding the vorticity dynamics on boundaries,which could be valuable in clarifying the formation mechanisms of near-wall coherent structures and flow noise at the fundamental level.
文摘Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary layer are excited and rapidly increase during the receptivity process,so sufficiently large Reynolds stress causes the basic flow velocity profile to change,and the formation of turbulence is inevitable.
基金co-supported by the National Natural Science Foundation of China (No. 12172175)the National Science and Technology Major Project, China (No. J2019-II0014-0035)the Science Center for Gas Turbine Project, China (Nos. P2022-C-II-002-001, P2022-A-II-002-001)
文摘Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction lengths have not been sufficiently investigated. First, this study presents a theoretical scaling analysis and validates it through wind tunnel experiments. It conducts detailed control volume analysis of mass conservation, considering the differences between inviscid and viscous cases. Then, three models for analysing interaction length under gradual expansion waves are derived. Related experiments using schlieren photography are conducted to validate the models in a Mach 2.73 flow. The interaction scales are captured at various relative distances between the shock impingement location and the expansion regions with wedge angles ranging from 12° to 15° and expansion angles of 9°, 12°, and 15°. Three trend lines are plotted based on different expansion angles to depict the relationship between normalised interaction length and normalised interaction strength metric. In addition, the relationship between the coefficients of the trend line and the expansion angles is introduced to predict the interaction length influenced by gradual expansion waves. Finally, the estimation of normalised interaction length is derived for various coefficients within a unified form.
基金supported by PRISMS(PRedictive Integrated Structural Materials Science)center which is located at University of Michigan and funded by U.S.Department of Energy,Office of Basic Energy Science,Division of Materials Science and Engineering(Grant award number DE-SC0008637)support from Michigan Center for Materials Characterization(MC2)at University of Michigan and Canmet MATERIALS,Natural Resources Canada+1 种基金the Extreme Science and Engineering Discovery Environment(XSEDE)Stampede2 at the TACC through allocation TG-MSS160003the National Energy Research Scientific Computing Center(NERSC),a U.S.Department of Energy Office of Science User Facility operated under Contract No.DE-AC02-05CH11231。
文摘As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding on how Y addition affects the microstructural evolution and mechanical properties,the Y segregation behavior at grain boundaries was investigated in Mg-1wt.%Y and Mg-7wt.%Y alloys at different conditions.The segregation intensity and its dependence on the grain boundary misorientation angle were experimentally characterized and computationally predicted.Strong segregation at grain boundaries was observed in both low and high Y-containing alloys.Y segregation was found to remain in alloy Mg-7Y after high-temperature annealing heat treatment at 540℃.No direct correlation between the Y segregation intensity and the grain boundary misorientation angle could be established based on either the experimental characterization or the atomistic simulation with a spectral model.We thus conclude that grain boundary segregation of Y is independent of grain boundary misorientation angle.
基金supported by the National Natural Science Foundation of China(Nos.12175323,11832019,and 11705264)project supported by the State Key Laboratory of Powder Metallurgy,Central South University,Guangdong Basic and Applied Basic Research Foundation(2023A1515012692)the National Natural Science Foundation of China Original Exploration Project(12150001).
文摘Metals in advanced nuclear reactors,such as W,often experience microcracks.However,the synergistic effects of high temperature,stress,and specialized structures can improve the self-healing ability of these metals.Microcrack healing is closely related to crack surface conditions.The order and disorder degree of crack surface atoms may affect crack stability.In this study,first-principles calculations,ab initio molecular dynamics,and surface thermodynamic theory were used to investigate the stability of grain boundary(GB)cracks at 0,293,and 373 K.We compared the energy densities,crack attraction energies,and atomic diffusion behaviors of crack surfaces atΣ3 GBs with those atΣ5 GBs.Adsorption on the nanocrack surface determines the critical nanocrack width.It was found that AlΣ3(111)nanocracks heal at high temperatures,and this healing behavior is closely related to the crack surface energy.Meanwhile,the GB cracks of W heal in an orderly manner at 573 and 1203 K.BY contrast,the GB cracks of Ti remain unhealed.Finally,a high-temperature nanocrack expansion model was developed and used to predict crack behavior under applied stress at different temperatures.
基金financially supported by the Shaanxi Provincial Science and Technology Innovation Team,China(No.2024RS-CXTD-26)。
文摘When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatly enhance the operational capabilities of aero-engine.However,due to the“line-of-sight”processing characteristic of PVD process,uneven coating deposition rates occur when preparing coatings on obstructed areas such as blisks.Quantitative research on such phenomena is few,and it is even rarer in the study of aero-engine coatings.Based on the analyses and considerations of the geometric shape of blade surfaces and the influence of both deposition and re-sputtering effect,an ideal model is established to analyze the deposition rate variation along blocked region in complex self-shadowing boundaries.The relative deposition rates at various locations on the blade surface within the inter-blade gaps are quantitatively calculated and experimentally validated.Furthermore,differences in erosion resistance of the coatings are tested.The conclusions are drawn as follows:the geometric configuration of the obstructed shape and resputtering phenomenon significantly influence the deposition rates within the inner wall of blade gaps.Taking the structural configuration as an example,in a 25 mm×60 mm×15 mm gap,the coating thickness can vary more than 252%from the thickest to the thinnest location.The deposition rates of various locations are proportional to the solid angle of incident ion in more obstructed regions,and the re-sputtering is more prominent in open regions.Obstructive boundaries directly affect the erosion resistance at various locations within the gaps,with erosion failure time decreasing by 40%in heavily blocked region compared to open region.
基金supported by the National Natural Science Foundation of China(NSFC)(Grant Nos.12388101,12202242 and 12172195)the computational resources pro〓〓vided by the Yancheng Supercomputing Center(Grant No.20231001)。
文摘This review provides recent advancements in the study of attachment-line boundary layer transition with emphasis on high-speed configurations.As a critical factor influencing aerodynamic performance and thermal management in supersonic and hypersonic systems,the transition mechanisms of three-dimensional attachment-line boundary layers have emerged as a pivotal research frontier in high-speed aerodynamics.This review systematically summarizes the evolution of research on attachment-line boundary layer transition,from early theoretical foundations to modern computational and experimental breakthroughs.A critical examination is presented for two pivotal challenges in high-speed attachment-line boundary layer transition:The resolution of the Gaillard paradox and the leading-edge contamination mechanism.Through systematic synthesis of theoretical developments and empirical evidence,this review identifies critical knowledge gaps while proposing novel methodological approaches for attachment-line boundary layer transition analysis.The review culminates in a strategic framework outlining promising avenues for both fundamental inquiry into attachment-line phenomena and applied engineering solutions in flow control strategies.
基金supported by the National Natural Science Foundation of China(GrantNo.12072231).
文摘Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 kHz are considered to be rep-resentative of the first and second instability modes,respectively.For the first mode,wall pressure fluctuations for both entropy and vorticity wave cases exhibit a strong modulation yet without a growing trend,indicating that the first mode is not generated despite its instability predicted by linear stability theory.The potential reason for this is the absence of postshock slow acoustic waves capable of synchronizing with the first mode.By contrast,for the second mode,a typical three-stage boundary-layer response is observed,consistent with that to slow acoustic waves studied previously.Furthermore,the postshock disturbances outside the boundary layer can be decomposed into the entropy(density/temperature fluctuations)and vorticity components(ve-locity fluctuations),and the latter is shown to play a leading role in generating the second mode,even for the case with entropy waves where the density/temperature fluctuations dominate the postshock regions.
