Multiscale simulation of nanometric cutting of single crystal copper——effect of different cutting speeds

A multiscale simulation has been performed to determine the effect of the cutting speed on the deformation mechanism and cutting forces in nanometric cutting of single crystal copper. The multiscale simulation model, which links the finite element method and the molecular dynamics method, captures the atomistic mechanisms during nanometric cutting from the free surface without the computational cost of full atomistic simulations. Simulation results show the material deformation mechanism of single crystal copper greatly changes when the cutting speed exceeds the material static propagation speed of plastic wave. At such a high cutting speed, the average magnitudes of tangential and normal forces increase rapidly. In addition, the variation of strain energy of work material atoms in different cutting speeds is investigated.

Multiscale simulation and nanoindentation experimental study of initial plasticity of Fe single crystal

It is very important to understand the initial plastic behavior of metals at microscale.In order to research the initial plasticity of body centered cubic metals in micro-/nano-scale,the multiscale simulation method and experimental study were used to study the nanoindentation process of Fe single crystal.The results show that the first abruption of load-displacement curve in nanoindentation of Fe single crystal can be attributed to the first transition from elastic to plastic deformation characterized by the dislocation emission.

A machine-learning-enabled approach for bridging multiscale simulations of CNTs/PDMS composites

Benefitting from the interlaced networking structure of carbon nanotubes(CNTs),the composites of CNTs/polydimethylsiloxane(PDMS)have found extensive applications in wearable electronics.While hierarchical multiscale simulation frameworks exist to optimize the structure parameters,their wide applications were hindered by the high computational cost.In this study,a machine learning model based on the artificial neural networks(ANN)embedded graph attention network,termed as AGAT,was proposed.The datasets collected from the micro-scale and the macro-scale simulations are utilized to train the model.The ANN layer within the model framework is trained to pass the information from micro-scale to macro-scale,while the whole model is aimed to predict the electro-mechanical behavior of the CNTs/PDMS composites.By comparing the AGAT model with the original multiscale simulation results,the data-driven strategy is shown to be promising with high accuracy,demonstrating the potential of the machine-learning-enabled approach for the structure optimization of CNT-based composites.

MULTISCALE SIMULATION OF INCIPIENT PLASTICITY AND DISLOCATION NUCLEATION ON NICKEL FILM DURING TILTED FLAT-ENDED NANOINDENTATION

Multiscale simulations of the tilted flat-ended nanoindentation with different tilted angles （from 5° ～ 30°） on the （-1 1 0） surface of nickel crystal were studied using the QC method. The model of the indentation is a flat-end indenter inclined by an angle ε driven into a half- plane vertically. Load-displacement responses, initiM stages of the plasticity deformations and dislocation emissions for nickel film at different inclined angles were obtained and analyzed as well. An energy criterion was successfully proposed to analyze the critical load for the first dislocation emission beneath the edge of the indenter. Simulation results agree well with analytical ones. An elastic model based on the contact theory and the Peierls-Nabarro dislocation model were combined to analyze when and where the dislocation will be emitted beneath the lower surface of an inclined indenter. Results indicate that the key parameter is the ratio of the contact half- width to the position of the slip plane. This parameter shows the range in which a dislocation will probably be emitted. This mechanism explains the simulation results well. This work is of value for understanding the mechanism of dislocation emissions of FCC crystals under tilted flat- ended nanoindentation while providing approaches to predicting when the first dislocation will be emitted and where subsequent dislocations will probably be emitted.

A multiscale material point method for impact simulation

To better simulate multi-phase interactions involving failure evolution, the material point method （MPM） has evolved for almost twenty years. Recently, a particle-based multiscale simulation procedure is being developed, within the framework of the MPM, to describe the detonation process of energetic nano-composites from molecular to continuum level so that a multiscale equation of state could be formulated. In this letter, a multiscale MPM is proposed via both hierarchical and concurrent schemes to simulate the impact response between two microrods with different nanostructures. Preliminary results are presented to illustrate that a transition region is not required between different spatial scales with the proposed approach.

The multiscale simulation of metal organic chemical vapor deposition growth dynamics of GaInP thin film

As a Group III–V compound, GaInP is a high-efficiency luminous material. Metal organic chemical vapor deposition (MOCVD) technology is a very efficient way to uniformly grow multi-chip, multilayer and large-area thin film. By combining the computational fluid dynamics (CFD) and the kinetic Monte Carlo (KMC) methods with virtual reality (VR) technology, this paper presents a multiscale simulation of fluid dynamics, thermodynamics, and molecular dynamics to study the growth process of GaInP thin film in a vertical MOCVD reactor. The results of visualization truly and intuitively not only display the distributional properties of the gas’ thermal and flow fields in a MOCVD reactor but also display the process of GaInP thin film growth in a MOCVD reactor. The simulation thus provides us with a fundamental guideline for optimizing GaInP MOCVD growth.

Numerical modeling and simulation of PEM fuel cells: Progress and perspective

This paper provides a comprehensive review on the research and development in multi-scale numerical modeling and simulation of PEM fuel cells. An overview of recent progress in PEM fuel cell modeling has been provided. Fundamental transport phenomena in PEM fuel cells and the corresponding mathematical formulation of macroscale models are analyzed. Various important issues in PEM fuel cell modeling and simulation are examined in detail, including fluid flow and species transport, electron and proton transport, heat transfer and thermal management, liquid water transport and water management, transient response behaviors, and cold-start processes. Key areas for further improvements have also been discussed.

Investigation of grain boundary activity in nanocrystalline Al under an indenter by using a multiscale method

Grain boundary activity in nanocrystalline Al under an indenter is studied by using a multiscale method. It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations. The transition processes might result in grain coarsening and refinement events. Dislocation reflection generated by a piece of stable grain boundary is also observed, because of the complex local atomic structure within the nanocrystalline Al. This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.

A Simulation Approach Including Underresolved Scales for Two-Component Fluid Flows in Multiscale Porous Structures

.In this study,we develop computational models and a methodology for accurate multicomponent flow simulation in underresolved multiscale porous structures[1].It is generally impractical to fully resolve the flow in porous structures with large length-scale differences due to the tremendously high computational expense.The flow contributions from underresolved scales should be taken into account with proper physics modeling and simulation processes.Using precomputed physical properties such as the absolute permeability,K_(0),the capillary pressure-saturation curve,and the relative permeability,K_(r),in typical resolved porous structures,the local fluid force is determined and applied to simulations in the underresolved regions,which are represented by porous media.In this way,accurate flow simulations in multiscale porous structures become feasible.To evaluate the accuracy and robustness of this method,a set of benchmark test cases are simulated for both single-component and two-component flows in artificially constructed multiscale porous structures.Using comparisons with analytic solutions and results with much finer resolution resolving the porous structures,the simulated results are examined.Indeed,in all cases,the results successfully show high accuracy with proper input of K_(0),capillary pressure,and K_(r).Specifically,imbibition patterns,entry pressure,residual component patterns,and absolute/relative permeability are accurately captured with this approach.

Geometrically-Compatible Dislocation Pattern and Modeling of Crystal Plasticity in Body-Centered Cubic(BCC)Crystal at Micron Scale Dedicated to Professor Karl Stark Pister for his 95th birthday

The microstructure of crystal defects,e.g.,dislocation patterns,are not arbitrary,and it is possible that some of them may be related to the microstructure of crystals itself,i.e.,the lattice structure.We call those dislocation patterns or substructures that are related to the corresponding crystal microstructure as the Geometrically Compatible Dislocation Patterns(GCDP).Based on this notion,we have developed a Multiscale Crystal Defect Dynamics(MCDD)to model crystal plasticity without or with minimum empiricism.In this work,we employ the multiscale dislocation pattern dynamics,i.e.,MCDD,to simulate crystal plasticity in body-centered cubic(BCC)single crystals,mainlyα-phase Tantalum(α-Ta)single crystals.The main novelties of the work are:(1)We have successfully simulated crystal plasticity at micron scale without any empirical parameter inputs;(2)We have successfully employed MCDD to perform direct numerical simulation of inelastic hysteresis of the BCC crystal;(3)We have used MCDD crystal plasticity model to demonstrate the size-effect of crystal plasticity and(4)We have captured cross-slip which may lead to size-effect.

Temperature dependence of migration features of self-interstitials in zirconium

Molecular dynamics simulations are conducted to study self-interstitial migration in zirconium. By defining crystal lattice points where more than one atom is present in corresponding Wigner-Seitz cells, as the locations of self-interstitial atoms （LSIAs）, three types of events are identified as LSIA migrations：the jump remaining in one 〈1120〉 direction （ILJ）, the jump from one 〈1120〉 to another 〈1120〉 direction in the same basal plane （OLJ）, and the jump from one basal plane to an adjacent basal plane （OPJ）. The occurrence frequencies of the three types are calculated. ILJ is found to be a dominant event in a temperature range from 300 K to 1200 K, but the occurrence frequencies of OLJ and OPJ increase with temperature increasing. The total occurrence frequency of all jump types has a good linear dependence on temperature. Moreover, the migration trajectories of LSIAs in the hcp basal-plane is not what is observed if only conventional one-or two-dimensional migrations exists; rather, they exhibit the feature that we call fraction-dimensional. Using Monte Carlo simulations, the potential kinetic effects of fraction-dimensional migration, which is measured by the average number of lattice sites visited per jump event （denoted by nSPE）, are analysed. The significant differences between the nSPE value of the fraction-dimensional migration and those of conventional one-and two-dimensional migrations suggest that the conventional diffusion coefficient cannot give an accurate description of the underlying kinetics of SIAs in Zr. This conclusion could be generally meaningful for the cases where the low-dimensional migration of defects are observed.

Numerical Modelling of the Powder Metallurgical Manufacturing Chain of High Strength Sintered Gears

This paper presents a digital model for the powder metallurgical(PM)production chain of high-performance sintered gears based on an integrated computational materials engineering(ICME)platform.Discrete and finite element methods(DEM and FEM)were combined to describe the macroscopic material response to the thermomechanical loads and process conditions during the entire production process.The microstructural evolution during the sintering process was predicted on the meso-scale using a Monte-Carlo Model.The effective elastic properties were determined by a homogenization method based on modelling a representative volume element(RVE).The results were subsequently used for the FE modelling of the heat treatment process.Through the development of multi-scale models,it was possible obtain characteristics of the microstructural features.The predicted hardness and residual stress distributions allowed the calculation of the tooth root load bearing capacity of the heat-treated sintered gears.

Particle-based hybrid and multiscale methods for nonequilibrium gas flows

Over the past half century,a variety of computational fluid dynamics(CFD)methods and the direct simulation Monte Carlo(DSMC)method have been widely and successfully applied to the simulation of gas flows for the continuum and rarefied regime,respectively.However,they both encounter difficulties when dealing with multiscale gas flows in modern engineering problems,where the whole system is on the macroscopic scale but the nonequilibrium effects play an important role.In this paper,we review two particle-based strategies developed for the simulation of multiscale nonequilibrium gas flows,i.e.,DSMC-CFD hybrid methods and multiscale particle methods.The principles,advantages,disadvantages,and applications for each method are described.The latest progress in the modelling of multiscale gas flows including the unified multiscale particle method proposed by the authors is presented.

The ultimate goal of modeling—Simulation of system and plant performance

Modern production processes in chemical, pharmaceutical and biological industries are characterized by complex process structures, which consist of different apparatuses and process steps. Modeling the entire process requires simulating all units altogether, while taking into account interconnections between them, Nevertheless, in the area of solids processing, there is nowadays an unfilled gap from the side of computer support of process modeling in allowing effective optimization and prediction of the behavior of the whole plant, This paper presents a tool for flowsheet simulation which allows the simulation of the stationary behavior of complex processes dealing with solids and its extension towards dynamic modeling, Also, a new simulation concept is proposed on the basis of the multiscale approach. On the macroscale, fiowsheet simulation is performed with the help of the SolidSim system. Parameters for the macromodels in Solid-Sim are predicted by microscale simulation. The models for the two scales are then coupled by inter-scale communication laws. Application of the proposed modeling concept is shown by an example of fluidized bed granulation.

A Hot Cracking Initiation Criterion Based on Solidification Liquid Film Characteristic and Microstructure

Herein,a hot cracking initiation criterion based on the characteristics of solidification liquid film and the microstructure was proposed,which integrated both the mechanical and non-mechanical factors during solidification.The criterion also took the effect of the shrinkage volume of the solid-liquid two-phase in the mushy zone,the flow behavior of the liquid film and the microstructure on the feeding behavior into account.Meanwhile,the effect factors of hot cracking initiation such as alloy composition,microstructure,mold design and process condition were included in this criterion,and it could quantitatively calculate whether hot cracks occurred under a certain state or not during solidification.The criterion was utilized to predict whether hot cracks occurred in Al-4.0 wt%Cu alloy in different initial solidification states or not,which was consistent with the experimental results and verified its reliability.According to the criterion expression,Vfeeding*was related with five effect factors includingη,ΔP*,l*,r*and n,in which r*and n were in positive correlation with Vfeeding*whileη,ΔP*and l*were in negative correlation with that,which provided a good instructive significance for mold design,process optimization and composition and microstructure regulation of alloys and simultaneously further enriched the mechanism and influencing factors of hot cracking initiation.Furthermore,a multiscale simulation method for calculating the characteristic parameters of hot tearing behavior during solidification was also provided in this study.

A Macro Projective IntegrationMethod in 2D Microscopic System Applied to Nonlinear Ion Acoustic Waves in a Plasma

In the Equation-free framework,amacro-coarse projective integrationmethod consists of two parts:the time stepper and time projection on macro scale.The first one consists of lifting,micro simulation and restriction.For extracting directly from microscopic simulations the information which would be obtained from the macroscopic model of two-dimensional microscopic systems,the time stepper based on the one-dimensional cumulative distribution functions,the marginal cumulative and appropriate number of the conditional cumulative distributions,is introduced.Here this procedure is tested on the nonlinear ion acoustic wave in a plasma.The numerical micro-solver is the one dimensional electrostatic particle-in-cell code.It is shown that particle correlations related to wave structures are better preserved by the new model.The lifting step is critically related to the noise in system.The enlarged noise,rise of correlations,trapping of particles during the wave steepening can seriously violate the basic assumptions of the equation-free approach.

Electric field induced magnetization reversal in magnet/insulator nanoheterostructure

Electric-field control of magnetization reversal is promising for lowpower spintronics.Here in a magnet/insulator nanoheterostructure which is the fundamental unit of magnetic tunneling junction in spintronics,we demonstrate the electric field induced 180°magne-tization switching through a multiscale study combining firstprinciples calculations and finite-temperature magnetization dynamics.In the model nanoheterostructure MgO/Fe/Cu with insu-lator MgO,soft nanomagnet Fe and capping layer Cu,through firstprinciples calculations we find its magnetocrystalline anisotropy linearly varying with the electric field.Using finite-temperature magnetization dynamics which is informed by the first-principles results,we disclose that a room-temperature 180°magnetization switching with switching probability higher than 90%is achievable by controlling the electric-field pulse and the nanoheterostructure size.The 180°switching could be fast realized within 5 ns.This study is useful for the design of low-power,fast,and miniaturized nanoscale electric-field-controlled spintronics.