Atomistic model of amorphous materials

An atomistic model, the tight-bond cluster model, was constructed for amorphous materials. This model contains essentially three major parts: (I) clusters with tight bonding; (II) redefined loosely bonded free-volume regions between clusters; and (III) interconnecting zones, which interconnect the clusters. The reversible transformation between the free-volume regions and interconnecting zones around glass-transition temperature determines the amorphous solid and supercooled liquid status. The atomistic model presented in this short article also contributes to a fundamental understanding of the structure of amorphous materials at the atomic level as well as the relationship between the structure and mechanical and physical properties of amorphous metallic glasses in solid and supercooled liquid states.     

Modeling homogeneous precipitation with an event-based Monte Carlo method: Application to the case of Fe–Cu

A precipitation model for an event-based kinetic Monte Carlo (EKMC) method is presented. It is based on atomic-scale computations of the emission and absorption rates of monomers by clusters. Clusters are considered as single objects that emit monomers close to them, at higher rates than predicted by the mean field cluster dynamics method. We show that a law based on continuous diffusion equation can be used to account for absorption, provided the reaction distances between clusters are accurately computed. The model is shown to reproduce quantitatively results obtained by atomistic kinetic Monte Carlo methods when only monomers are mobile. The kinetics obtained by EKMC is faster than the one given by cluster dynamics, which highlights the limits of such a mean-field method, especially at high solute concentrations. When applied to the precipitation of Cu in Fe, which involves the mobility of clusters, the EKMC model shows good agreement with experimental results.     

Influence of cluster mobility on Cu precipitation in α-Fe: A cluster dynamics modeling

A cluster dynamics model has been parametrized to quantitatively reproduce results obtained by atomistic kinetic Monte Carlo (AKMC) modeling on the precipitation of Cu in α-Fe under thermal aging. The cluster mobility, highlighted by AKMC, is shown to have a significant effect on the precipitation kinetics and can reconcile the experimentally observed fast kinetics with the relatively low diffusivity of Cu monomers.     

Concurrent atomistic and continuum simulation of strontium titanate

This paper presents a concurrent atomistic–continuum methodology (CAC) to simulate the dynamic processes of dislocation nucleation and migration as well as crack initiation and propagation in complex crystals. The accuracy and efficiency of the method is tested with respect to the molecular dynamics (MD) method through simulations of the dynamic fracture processes in strontium titanate under a combination of tension and shear loading and the dislocation behavior under nanoindentation. CAC simulation results demonstrated a smooth passage of cracks and dislocations through the atomistic–continuum interface without the need for additional constitutive rules or special numerical treatment. Although some accuracy is lost in CAC simulations as a consequence of a 98.4% reduction in the degrees of freedom, all the CAC results are qualitatively and quantitatively comparable with MD results. The stacking fault width and nanoindentation hardness measured in the CAC simulations agrees well with existing experimental data. Criteria for cleavage and slip in ionic materials are verified. The need to include the internal degrees of freedom of atoms in concurrent atomistic–continuum methods for polyatomic crystalline materials is confirmed.     

Irreversible atomic rearrangements in elastically deformed metallic glasses

The atomistic behaviour of elastically deformed Ni₅₀Zr₅₀ metallic glasses obtained at different quenching rates was studied by molecular dynamics. Deformation induces the irreversible rearrangement of atomic clusters of various chemical composition. The relative amount of Ni and Zr atoms participating in irreversible rearrangements depends on the quenching rate. The rearrangements are related to the potential energy difference between initial and final cluster configurations, connected in turn with volume effects. The role of local structures was investigated by focusing the attention on icosahedral coordination. The numerical findings indicate that icosahedral clusters become involved in rearrangements only after the average strain has overcome a certain value. Therefore, at small elastic strain only small atomic clusters with spatial organization different from the icosahedral one rearrange. The different involvement in rearrangements of different local structures is tentatively related to apparent local strain.     

Material simulations with tight-binding molecular dynamics

Tight-binding molecular-dynamics has recently emerged as a useful method for atomistic simulation study of realistic materials. The method incorporates electronic structure calculation into molecular dynamics through an empirical tight-binding Hamiltonian and bridges the gap between ab initio molecular dynamics and simulations using empirical classical potentials. This article reviews some achievements and discusses some recent developments in materials simulations with tight-binding molecular dynamics.     

Aluminum Pitting Corrosion in Halide Media: A Quantum Model and Empirical Evidence

The phenomenon of localized damage of aluminum oxide surface in the presence of halide anions was scrutinized at an atomistic level, through the cluster approach and density functional theory. The phenomenon was also investigated empirically through Tafel polarization plots and scanning electron microscopy. A distinct behavior witnessed in the fluoride medium was justified through the hard-soft acid-base principle. The atomistic investigations revealed the greatest potency for chloride entrance into the metal oxide lattice and rationalized to the severity of damage. The interaction of halide anions with the oxide surface causing some displacements on the position of Al atoms provides a mechanistic insight of the phenomenon.     

Effect of the substrate temperatures on the epitaxial rearrangement of the deposited Au nanoclusters

As the existence of charged clusters in the gas phase, which were predicted by the charged cluster model, has been confirmed for many thin film processes, it becomes important to understand the deposition dynamics of the clusters. In this study, deposition behaviors of the cluster of 1985 Au atoms on the (100) gold surface were studied at the deposition temperatures of 300, 700 and 1000 K by molecular dynamics (MD) simulation using the embedded atom method (EAM) potential. After 320 picoseconds of cluster landing on the surface at 300 and 700 K, only a part of the cluster rearranged into the epitaxial orientation with the substrate with the rest of the cluster making a grain boundary at 300 K and a twin at 700 K. At 1000 K, however, the cluster fully underwent epitaxial recrystallization. These results imply that the high rate of epitaxial deposition by clusters of a few nanometers is possible as long as the substrate temperature is sufficiently high as was previously suggested in the charged cluster model.     

Understanding pop-in phenomena in FeNi3 nanoindentation

In this work, the spherical nanoindentation of FeNi₃ crystal was simulated by molecular dynamics (MD) for (0 1 0), (1 1 0) and (1 1 1) surfaces to study the origin of pop-in phenomena at the atomistic level. Simulated results demonstrate that the critical load, indentation modulus as well as dislocation nucleation processes are strongly dependent upon crystallographic orientations. The pop-in behaviors observed in nanoindentation are associated with dislocation reactions. The first pop-in event indicates the homogeneous nucleation of the 1/6<1 1 2>-type partial dislocations. Activated partial dislocations can transform into dislocation locks through complex dislocation reactions.     

Molecular Dynamics Study of Tension Process of Ni-Based Superalloy

To understand the atomistic mechanisms of tension failure of Ni-based superalloy,in this study,the classical molecular dynamics(MD) simulations were used to study the uniaxial tension processes of both the Ni/Ni_3 Al interface systems and the pure Ni and Ni_3 Al systems.To examine the effects of interatomic potentials,we adopted embedded atom method(EAM)and reactive force field(ReaxFF) in the MD simulations.The results of EAM simulations showed that the amorphous structures and voids formed near the interface,facilitating further crack propagation within Ni matrix.The EAM potentials also predicted that dislocations were generated and annihilated alternatively,leading to the oscillation of yielding stress during the tension process.The ReaxFF simulations predicted more amorphous formation and larger tensile strength.The atomistic understanding of the defect initiation and propagation during tension process may help to develop the strengthening strategy for controlling the defect evolution under loading.     

Atomistic Modeling of Solidification Phenomena Using the Phase-Field-Crystal Model

In recent years, a fundamental understanding of solidification and its behavior has been gained through molecular dynamics simulations and the phase-field method, the first of which is limited to short time scales and the latter of which does not represent interface and elastoplastic properties accurately. Recently, the phase-field-crystal (PFC) method, a continuum method operating on atomistic length scales and diffusive time scales, has helped bridge the multiple scale gap between molecular dynamics and phase field. This review surveys the advances of PFC models in the context of various solidification phenomena.     

Atomistic modeling of interfaces and their impact on microstructure and properties

Atomic-level modeling of materials provides fundamental insights into phase stability, structure and properties of crystalline defects, and to physical mechanisms of many processes ranging from atomic diffusion to interface migration. This knowledge often serves as a guide for the development of mesoscopic and macroscopic continuum models, with input parameters provided by atomistic models. This paper gives an overview of the most recent developments in the area of atomistic modeling with emphasis on interfaces and their impact on microstructure and properties of materials. Modern computer simulation methodologies are discussed and illustrated by several applications related to thermodynamic, kinetic and mechanical properties of materials. Existing challenges and future research directions in this field are outlined.     

Modelling precipitation in binary alloys by cluster dynamics

Cluster dynamics is an original way to bridge the gap between atomistic simulations and macroscopic approaches of precipitation, but its application to alloys of high solubility limit and solute concentration raise a number of difficulties. The underlying thermodynamic model has been recently extended to treat this type of situation. New tools are presented to explore some of the consequences of this extension, validated by comparing with kinetic Monte-Carlo simulations.     

晶界元素偏聚及其机理的研究进展

晶界成分偏聚对多晶材料的力学性能有很大影响,探明影响晶界成分偏聚的因素和偏聚机理,对改善材料性能和优化设计合金系统具有重要意义。本文通过归纳近几年来国内外原子尺度研究晶界成分偏聚及其机理的研究进展,分析了间隙原子,置换型原子和空位在晶界的偏聚及其对材料性能的影响,讨论了成分偏聚与合金元素择优占位行为的关系,总结了晶界成分偏聚对材料力学性能影响的根本原因。     

Effects of quenching rate on amorphous structures of Cu46Zr54 metallic glass

Metallic glass shows some superior properties different from crystalline, but the nature of amorphous structure and structural change during glass transition have not been completely understood yet. Molecular dynamics simulation provides intuitive insight into the microstructure and properties at atomistic level. Before probing into the microstructures of metallic glass with molecular dynamics (MD) simulation, it is important to obtain amorphous state first. In the current work, we reproduce the process of manufacturing metallic glass in laboratory including the melting, equilibrating and quenching procedure with molecular dynamics simulations. The structure changing at melting point and glass transition temperature are investigated with the different cooling processing. The partial radial distribution function (PRDF) is applied as a criterion to judge the final amorphous state obtained considering the quenching at different cooling rates and the effects of cooling rate on the formation of amorphous structures are further discussed.     

Behavior of Vacancies and Interstitials at Semicoherent Interfaces

Using atomistic simulations on a model semicoherent interface, we show that the formation, migration, and clustering of vacancies and interstitials at semicoherent interfaces depend on the structure of the misfit dislocation network of the interface. Interfacial point defects trap at misfit dislocation intersections and migrate from one intersection to another along misfit dislocations by a multistage process. Interfacial point defect clusters are thermodynamically less stable than individual defects and the largest cluster size depends on the spacing between misfit dislocation intersections. Our results show that the behavior of interfacial point defects is intimately connected to interface structure.     

金属玻璃纳米晶化机制研究进展

金属玻璃是一类具有结构和功能应用前景的新型金属材料,是目前物理和材料学科最为活跃的研究领域之一。由于处于热力学亚稳态,金属玻璃在合适的外界条件下会自发地向相应的晶态相发生转变,导致晶化事件的发生。研究金属玻璃的纳米晶化不仅有重要的科学意义,同时也可对金属玻璃的应用提供理论指导。简要介绍了目前几种代表性的金属玻璃纳米晶化微观机制：经典形核理论、基于耦合通量模型的形核机制、基于相分离的纳米晶形核长大机制、有序原子集团沉积机制、非经典形核理论、大过冷度条件下纳米晶化的微观机制等,同时结合作者课题组近年来在这方面的研究进展,对各种机制进行了评述,最后对未来金属玻璃纳米晶化机制研究中需要重视的几个问题进行了简单展望。