Multiscale modeling of nanoindentation in copper thin films via the concurrent coupling of the meshless Hermite-Cloud method with molecular dynamics

This paper investigates the 2D nanoindentation of a copper thin film using a concurrent multiscale method. The method uses molecular dynamics (MD) simulation in the atomistic region, the strong-form meshless Hermite-Cloud method in the continuum region and a handshaking algorithm to concurrently couple them. A fully atomistic simulation is also carried out to validate the multiscale method. The results, namely the load versus indentation depth graph obtained from the multiscale method shows only slight quantitative variation from that of the full atomistic model. More importantly, the graphs from both simulations show a similar trend thus validating the 2D multiscale method. The displacement profile without discontinuities further supports the efficiency of the multiscale method in ensuring smooth exchange of information between the atomistic and continuum domains. The material properties extracted from the simulation include the force/unit length values obtained by dividing the maximum load on the indenter by its contact perimeter, instead of the hardness value obtained in 3D simulations. By restricting the atomic scale detail to the critical regions beneath the indenter, the multiscale method effectively saves computational resources to more than one order (close to 13 times less for this problem), thus making it feasible to simulate problems of larger dimensions that are not amenable to complete atomistic simulations.     

Bridging Atomistic/Continuum Scales in Solids with Moving Dislocations

We propose a multiscale method for simulating solids with moving dislocations. Away from atomistic subdomains where the atomistic dynamics are fully resolved, a dislocation is represented by a localized jump profile, superposed on a defect-free field. We assign a thin relay zone around an atomistic subdomain to detect the dislocation profile and its propagation speed at a selected relay time. The detection technique utilizes a lattice time history integral treatment. After the relay, an atomistic computation is performed only for the defect-free field. The method allows one to effectively absorb the fine scale fluctuations and the dynamic dislocations at the interface between the atomistic and continuum domains. In the surrounding region, a coarse grid computation is adequate.     

Quasicontinuum analysis of the effect of tool geometry on nanometric cutting of single crystal copper

A dynamic multiscale simulation based on quasicontinuum method (QC) has been conducted to study the effect of tool geometry in nanometric cutting process of single crystal copper. In the simulation, the many-body EAM potential is used for the interactions between copper atoms in of the workpiece. The simulation captures the atomistic behaviors of material removal mechanisms from the free surface and the mobility of dislocations and their interactions with the computational cost of local atomistic simulation method. Simulations are performed on single crystal copper to study the atomistic details of material removal, chip formation, sub-surface deformation, and machining mechanism. The simulation results demonstrate that tool edge radius has significant effect on chip formation and subsurface deformation, because the effective rake angle varies with the tool edge radius. In addition, different effective rake angles result in different stress states and smoother surface can be obtained under bigger clearance angle. The variations of tangential force, normal force as well as the ratio of normal force to tangential force are obtained to analyze the effects of tool edge radius, rake angle and clearance angle in quantitative way.     

基于桥域理论的Cu单晶纳米切削跨尺度仿真研究

桥域方法是一种典型的跨尺度仿真研究方法.基于桥域理论,本文分析了原子和连续介质耦合区域的处理问题,即在耦合区采用不同的权重计算系统的能量,通过Lagrange乘子法对原子和连续介质位移进行约束.采用桥域方法,建立了单晶Cu米纳切削的跨尺度仿真模型,获得了单晶Cu纳米切削的材料变形机理.同时,研究了不同切削速度对纳米切削过程和原子受力分布的影响,仿真结果表明:随着切削速度的提高,切削区原子所受的力值增大,切屑变形系数减小,已加工表面变质层厚度增加.本文基于桥域理论,实现了Cu单晶纳米切削跨尺度的建模和仿真, 关键词： 桥域法 纳米切削 单晶Cu 切削速度     

A multiscale coupling method for the modeling of dynamics of solids with application to brittle cracks

We present a multiscale model for numerical simulations of dynamics of crystalline solids. The method combines the continuum nonlinear elasto-dynamics model, which models the stress waves and physical loading conditions, and molecular dynamics model, which provides the nonlinear constitutive relation and resolves the atomic structures near local defects. The coupling of the two models is achieved based on a general framework for multiscale modeling – the heterogeneous multiscale method (HMM). We derive an explicit coupling condition at the atomistic/continuum interface. Application to the dynamics of brittle cracks under various loading conditions is presented as test examples.     

Multiscale theory of fluctuating interfaces: renormalization of atomistic models

We describe a framework for the multiscale analysis of atomistic surface processes which we apply to a model of homoepitaxial growth with deposition according to the Wolf-Villain model and concurrent surface diffusion. Coarse graining is accomplished by calculating renormalization-group (RG) trajectories from initial conditions determined by the regularized atomistic theory. All of the crossover and asymptotic scaling regimes known from computer simulations are obtained, but we also find that two-dimensional substrates show an intriguing transition from smooth to mounded morphologies along the RG trajectory.     

Atomistic simulation of InGaN/GaN quantum disk LEDs

In this work electronic and optoelectronic properties of InGaN/GaN nanocolumn quantum disk LEDs have been studied with the multiscale simulation tool tiberCAD. Calculations have been performed with an atomistic tight-binding model. Results shows that emission energies have a minor dependence on the nanocolumn dimension while In concentration in the active region is a critical parameter.     

A finite difference approach with velocity interfacial conditions for multiscale computations of crystalline solids

We propose a class of velocity interfacial conditions and formulate a finite difference approach for multiscale computations of crystalline solids with relatively strong nonlinearity and large deformation. Full atomistic computations are performed in a selected small subdomain only. With a coarse grid cast over the whole domain and the coarse scale dynamics computed by finite difference schemes, we perform a fast average of the fine scale solution in the atomistic subdomain to force agreement between scales. During each coarse scale time step, we adopt a linear wave approximation around the interface, with the wave speed updated using the coarse grid information. We then develop a class of velocity interfacial conditions with different order of accuracy. The interfacial conditions are straightforward to formulate, easy to implement, and effective for reflection reduction in crystalline solids with strong nonlinearity. The nice features are demonstrated through numerical tests.     

A damping boundary condition for atomistic-continuum coupling

The minimization of spurious wave reflection is a challenge in multiscale coupling due to the difference of spatial resolution between atomistic and continuum regions. In this study, a new damping condition is presented for eliminating spurious wave reflection at the interface between atomistic and continuum regions. This damping method starts by a coarse–fine decomposition of the atomic velocity based on the bridging scale method. The fine scale velocity of the atoms in the damping region is reduced by applying nonlinear damping coefficients. The effectiveness of this damping method is verified by one-and two-dimensional simulations.     

Time-domain observation of the spinmotive force in permalloy nanowires

The spinmotive force associated with a moving domain wall is observed directly in Permalloy nanowires using real time voltage measurements with proper subtraction of the electromotive force. Whereas the wall velocity exhibits nonlinear dependence on magnetic field, the generated voltage increases linearly with the field. We show that the sign of the voltage reverses when the wall propagation direction is altered. Numerical simulations explain quantitatively these features of spinmotive force and indicate that it scales with the field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization.     

Multiscale models of hard-soft composite media

The status of models of composite media for ultra-high density recording is reviewed. It is shown that the usual micromagnetic formalism may break down when dealing with interfaces where rapid spatial changes in magnetisation may occur. A multiscale approach is outlined, in which atomistic models are coupled with micromagnetic concepts to provide an improved physical model. The theory is applied to FePt/FeRh bilayers, which have potential as media for heat-assisted magnetic recording.     

Matching conditions in atomistic-continuum modeling of materials

A new class of matching conditions between the atomistic and continuum regions is presented for the multiscale modeling of crystals. They ensure the accurate passage of large scale information between the atomistic and continuum regions and at the same time minimize the reflection of phonons at the interface. These matching conditions can be made adaptive if we choose appropriate weight functions. Applications to dislocation dynamics and friction between two-dimensional atomically flat crystal surfaces are described.     

Solitary deflection waves on the supersonic domain wall in yttrium orthoferrite

The moving antiferromagnetic vortices are accompanied by solitary deflection waves. These waves allow to investigate generation and nonlinear dynamics of the antiferromagnetic vortices on the moving domain wall with the help of the two- and three-fold digital high speed photography. On the quasi-relativistic domain wall the vortex dynamics is quasi-relativistic with the limiting velocity c=20 km/s, which is equal to the spin-wave velocity. The solitary deflection waves dynamics can be explained assuming existence of the gyroscopic force. A theory for the gyroscopic force in the orthoferrite domain wall is elaborating by A.K. Zvezdin et al. currently. We present a comparison of the theoretical and experimental results on the dynamics of the solitary deflection waves, which accompany the antiferromagnetic vortices in the domain wall of orthoferrites.     

Steady motion of the magnetic vortex under the magnus force in weak ferromagnets

The problem of steady motion of the magnetic vortex in a moving domain wall under the action of the Magnus force in weak ferromagnets was studied. Dynamic bending of the domain wall containing a moving vortex was analyzed. The formulas describing the dependences of the vortex velocity on the velocity of the domain wall in which it moves were derived.     

Mechanism of void nucleation and growth in bcc Fe: atomistic simulations at experimental time scales

Evolution of small-vacancy clusters in bcc Fe is simulated using a multiscale approach coupling an atomistic activation-relaxation method for sampling transition-state pathways with environment-dependent reaction coordinate calculations and a kinetic Monte Carlo simulation to reach time scales on the order of ~10? s. Under vacancy-supersaturated condition, di- and trivacancy clusters form and grow by coalescence (Ostwald ripening). For cluster size greater than four we find a transition temperature of 150 °C for accelerated cluster growth, as observed in positron annihilation spectroscopy experiments. Implications for the mechanism of stage-IV radiation-damage-recovery kinetics are discussed.     

Magnus force and magnetic vortex dynamics in weak ferromagnets

The problem of steady motion of the magnetic vortex in a moving domain wall under the action of the Magnus force in weak ferromagnets was studied. Dynamic bending of the domain wall containing a moving vortex was analyzed. The formulas describing the dependences of the vortex velocity on the velocity of the domain wall in which it moves were derived.     

Coupling atomistic and finite element approaches for the simulation of optoelectronic devices

Multiscale methods coupling quantum mechanical/atomistic models such as envelope function and tight binding approaches with continuous media models e.g. for strain or electronic transport are very useful for an accurate simulation of modern and emerging electronic and optoelectronic devices based on nanostructured active regions. We present simulations using TiberCAD whose main focus is on providing an integrated multiscale/multiphysics simulation environment.     

Cu刃型扩展位错附近局部应变场的原子模拟研究

通过结合virial应变分析技术的准连续介质多尺度模拟方法研究了金属Cu刃型扩展位错的局部应变场.结果表明在距离位错核心几十纳米的区域内晶体处于小变形状态,virial应变计算结果与弹性理论预测结果符合得相当好,当距离位错核心仅几纳米时,晶格畸变加剧,virial应变极大值出现在扩展位错两端的Shockley分位错芯部.进一步分析表明Shockley分位错芯部严重畸变区大致呈长轴7b1、短轴3b1的椭圆形,其中b1为分位错柏氏矢量的长度.