Shuozhi Xu

Size-dependent plastic deformation of twinned nanopillars in body-centered cubic tungsten

Compared with face-centered cubic metals, twinned nanopillars in body-centered cubic (BCC) systems are much less explored partly due to the more complicated plastic deformation behavior and a lack of reliable interatomic potentials for the latter. In this paper, the fault energies predicted by two semi-empirical interatomic potentials in BCC tungsten (W) are first benchmarked against density functional theory calculations. Then, the more accurate potential is employed in large scale molecular dynamics simulations of tensile and compressive loading of twinned nanopillars in BCC W with different cross sectional shapes and sizes. A single crystal, a twinned crystal, and single crystalline nanopillars are also studied as references. Analyses of the stress-strain response and defect nucleation reveal a strong tension-compression asymmetry and a weak pillar size dependence in the yield strength. Under both tensile and compressive loading, plastic deformation in the twinned nanopillars is dominated by dislocatio...

Nanovoid growth in BCC α-Fe: influences of initial void geometry

The growth of voids has a great impact on the mechanical properties of ductile materials by altering their microstructures. Exploring the process of void growth at the nanoscale helps in understanding the dynamic fracture of metals. While some very recent studies looked into the effects of the initial geometry of an elliptic void on the plastic deformation of face-centered cubic metals, a systematic study of the initial void ellipticity and orientation angle in body-centered cubic (BCC) metals is still lacking. In this paper, large scale molecular dynamics simulations with millions of atoms are conducted, investigating the void growth process during tensile loading of metallic thin films in BCC α-Fe. Our simulations elucidate the intertwined influences on void growth of the initial ellipticity and initial orientation angle of the void. It is shown that these two geometric parameters play an important role in the stress–strain response, the nucleation and evolution of defects, as well as the void size/outline evolution in α-Fe thin films. Results suggest that, together with void size, different initial void geometries should be taken into account if a continuum model is to be applied to nanoscale damage progression.

Addressing the discrepancy of finding the equilibrium melting point of silicon using molecular dynamics simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid–liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.

Atomic collision cascades on void evolution in vanadium

Molecular dynamics simulations were performed to study void evolution subject to unidirectional self-bombardment and radiation-induced variation of mechanical properties in single crystalline vanadium. 3D simulation cells of perfect body-centered cubic (BCC) vanadium, as well as those with one, two, four, and six voids, were investigated. For the no void case, the maximum number of defects, maximum volumetric swelling, and the number of defects left in bulk after a sufficiently long recovery period increased with higher primary recoil energy. For the cases containing voids, a primary recoil energy was carefully assigned to an atom so as to initiate a dense collision spike in the voids center, where some self-interstitial atoms gained kinetic energy by secondary replacement collision sequence traveling along the ⟨ 111⟩ direction. It is found that the larger or the greater the number of voids contained initially in the box, the larger the normalized void volume, and the smaller the volumetric swelling after sufficient recovery of systems. In the single void case, the void became elongated along the bombarding direction; in the multiple void cases, the voids coalesced only when the intervoid ligament distance was short. After sufficient relaxation of the irradiated specimen, a hydrostatic tension was exerted on the box, where the voids were treated as dislocation sources. It is shown that with higher primary recoil energy, the yield stress dropped in cases with smaller or fewer voids but rose in those with larger or greater number of voids. This radiation-induced softening to hardening transition with increasing dislocation density can be attributed to the combined effects of the defect-induced dislocation nucleation and the resistance of defects to dislocation motion. Moreover, as the primary recoil energy increased, the ductility of vanadium in the no void case decreased, but was only slightly changed in the cases containing void.

A molecular dynamics study of void interaction in copper

Molecular dynamics simulations in three-dimensional single crystal copper under remote uniaxial tension at high strain rate (109/s) were performed to analyze the microscopic mechanism of dislocation emission and void interaction. Two different cases, characterized by whether the line joining centers of two embedded cylindrical voids was perpendicular to or paralleled the tension direction, were studied using embedded atom method (EAM) potentials. The mean-squared displacements of atoms around the voids marking both yielding and coalescence points were presented. The critical position of the first slide was predicted where shear stress at slip plane was maximum.

A Review on Micro- and Nanoscratching/Tribology at High Temperatures: Instrumentation and Experimentation

High-temperature micro-/nanomechanics has attracted much interest over the last decade, primarily because of the urgent need to understand the mechanical and tribological properties of advanced engineering materials at micro-/nanoscale and the underlying physics controlling such properties at operationally relevant conditions. Recent years have subsequently witnessed the swift growth and development of new high-temperature micro- and nanoscratching/tribology instruments. Here, we present an overview of fundamental principles and developments in these instruments, discuss pertinent findings on the topic in detail, and outline current challenges and promising future directions in the field.

Tension-compression asymmetry in plasticity of nanotwinned 3C-SiC nanocrystals

Encompassing nanoscale thin twins in metals induces diverse influences, either strengthening triggered by the lattice dislocation blockage effects or softening prompted by dislocation emission from coherent twin boundary (CTB)/grain boundary (GB) intersections as well as CTB migration; yet the deformation mechanism remains poorly understood in ceramic nanostructures possessing covalent bonds. Here, we report the results of uniaxial compressive and tensile stress loading of twin-free and nanotwinned nanocrystalline cubic silicon carbide (3C-SiC) ceramic attained by large-scale molecular dynamics simulations. We find a strong and unique tension-compression asymmetry in strength of nanocrystalline ceramics, much higher than that of metals. We demystify that strength and ductility behaviour do not correlate simply with the amount of dislocation density, voids, intergranular disordered phase, and total strain energy; instead, it arises from a complex interplay of the aforementioned features and structural characteristics, e.g., GB and triple junction area distribution along/normal to the direction of straining as well as the capability of strain accommodation by the GBs and CTBs, with the dominant role of the structural characteristics in nanotwinned samples. Our results also reveal that primarily intergranular crack propagation and fracture along the GBs transpires, and not along the CTBs, resulting from the high energy of the GBs. However, a high density of nanoscale twins in the 3C-SiC nanocrystals could give rise to the alternation of the crack path from intergranular to intragranular type induced by shear, which brings about the glide of Shockley partials along the CTBs and subsequent formation of CTB steps and twin plane migration.Encompassing nanoscale thin twins in metals induces diverse influences, either strengthening triggered by the lattice dislocation blockage effects or softening prompted by dislocation emission from coherent twin boundary (CTB)/grain boundary (GB) intersections as well as CTB migration; yet the deformation mechanism remains poorly understood in ceramic nanostructures possessing covalent bonds. Here, we report the results of uniaxial compressive and tensile stress loading of twin-free and nanotwinned nanocrystalline cubic silicon carbide (3C-SiC) ceramic attained by large-scale molecular dynamics simulations. We find a strong and unique tension-compression asymmetry in strength of nanocrystalline ceramics, much higher than that of metals. We demystify that strength and ductility behaviour do not correlate simply with the amount of dislocation density, voids, intergranular disordered phase, and total strain energy; instead, it arises from a complex interplay of the aforementioned features and structural char...

Modeling dislocations and heat conduction in crystalline materials: atomistic/continuum coupling approaches

ABSTRACT Dislocations and heat conduction are essential components that influence properties and performance of crystalline materials, yet the modelling of which remains challenging partly due to their multiscale nature that necessitates simultaneously resolving the short-range dislocation core, the long-range dislocation elastic field, and the transport of heat carriers such as phonons with a wide range of characteristic length scale. In this context, multiscale materials modelling based on atomistic/continuum coupling has attracted increased attention within the materials science community. In this paper, we review key characteristics of five representative atomistic/continuum coupling approaches, including the atomistic-to-continuum method, the bridging domain method, the concurrent atomistic–continuum method, the coupled atomistic/discrete-dislocation method, and the quasicontinuum method, as well as their applications to dislocations, heat conduction, and dislocation/phonon interactions in crystalline materials. Through problem-centric comparisons, we shed light on the advantages and limitations of each method, as well as the path towards enabling them to effectively model various material problems in engineering from nano- to mesoscale. Abbreviations: AtC: atomistic-to-continuum; BCC: body-centred cubic; BDM: bridging domain method; CAC: concurrent atomistic–continuum; CADD: coupled atomistic/discrete-dislocation; DDD: discrete dislocation dynamics; DDf-MD: discrete diffusion-molecular dynamics; DOF: degree of freedom; ESCM: embedded statistical coupling method; FCC: face-centred cubic; GB: grain boundary; XFEM: extended finite element method; MD: molecular dynamics; MS: molecular statics; PK: Peach-Koehler; QC: quasicontinuum

Concurrent atomistic-continuum simulations of uniaxial compression of gold nano/submicropillars

ABSTRACT In this work, uniaxial compression of nano/submicropillars in Au with the initial diameter D between 26.05 and 158.53 nm was modelled by concurrent atomistic-continuum simulations. Two models with distinct surface facets were employed to explore the surface facets-dependent extrinsic size effects on the plastic deformation of pillars. It is found that (i) the yielding in pillars with smooth surfaces was controlled by dislocation nucleation from the two ends of the pillars, and (ii) in pillars with faceted surfaces, dislocations were initiated from the sharp edges on the surface. As a result of the differences in the plastic deformation mechanism between the two models, the yield stress decreased exponentially and increased nearly linearly with respect to an increasing D in pillars with smooth and faceted surfaces, respectively.

Generalized Continua Concepts in Coarse-Graining Atomistic Simulations

Generalized continuum mechanics (GCM) has attracted increased attention in the context of multiscale materials modeling, an example of which is a bottom-up GCM model, called the atomistic field theory (AFT). Unlike most other GCM models, AFT views a crystalline material as a continuous collection of lattice points; embedded within each point is a unit cell with a group of discrete atoms. As such, AFT concurrently bridges the discrete and continuous descriptions of materials, two fundamentally different viewpoints. In this chapter, we first review the basics of AFT and illustrate how it is realized through coarse-graining atomistic simulations via a concurrent atomistic-continuum (CAC) method. Important aspects of CAC, including its advantages relative to other multiscale methods, code development, and numerical implementations, are discussed. Then, we present recent applications of CAC to a number of metal plasticity problems, including static dislocation properties, fast moving dislocations and phonons, as well as dislocation/grain boundary interactions. We show that, adequately replicating essential aspects of dislocation fields at a fraction of the computational cost of full atomistics, CAC is established as an effective tool for coarse-grained modeling of various nano/micro-scale thermal and mechanical problems in a wide range of monatomic and polyatomic crystalline materials.