Douglas E. Spearot

Atomistic simulations of homogeneous dislocation nucleation in single crystal copper

Atomistic simulations are used to investigate how the stress required for homogeneous nucleation of partial dislocations in single crystal copper under uniaxial tension changes as a function of crystallographic orientation. Molecular dynamics is employed based on an embedded-atom method potential for Cu at 10 and 300 K. Results indicate that non-Schmid parameters are required to describe dislocation nucleation for certain single crystal orientations. Specifically, we find that the stereographic triangle can be divided into two regions: a region where dislocation nucleation is dominated by the conventional Schmid factor (the resolved shear stress in the direction of slip) and a region where dislocation nucleation is dominated by the normal factor (the resolved stress normal to the slip plane). A continuum relationship that incorporates Schmid and non-Schmid terms to correlate the stress required for dislocation nucleation over all tensile axis orientations within the stereographic triangle is presented. The significance of this work is that simulation results are cast into an atomistically inspired continuum formulation for partial dislocation loop nucleation in face-centered cubic single crystals.

Atomistic simulations of nanoindentation on the basal plane of crystalline molybdenum disulfide (MoS2)

In the present work, nanoindentation on the basal surface of a crystalline molybdenum disulfide (MoS2) thin film is investigated by molecular statics (MS) calculations. A previously parameterized interatomic potential combining the reactive empirical bond-order and Lennard-Jones potentials is implemented into the LAMMPS molecular simulation package and refined for improved prediction of the mechanical properties of MoS2 at athermal conditions. Nanoindentation simulations are performed using three indenter sizes with specific focus on the incipient plastic deformation event within the MoS2single crystal. MS calculations show that a local phase transformation occurs beneath the indenter at plastic yield without the presence of broken Mo–S bonds. The structural characteristics of the phase transformation are captured using a slip vector analysis. The nanoindentation simulations provide insight into the mechanical response of MoS2 during contact deformation characteristic of both synthesis and application for better design of MoS2 nanoparticle lubricants.

Virtual diffraction analysis of Ni [0 1 0] symmetric tilt grain boundaries

Electron and x-ray diffraction are well-established experimental methods used to explore the atomic scale structure of materials. In this work, a computational method is implemented to produce virtual electron and x-ray diffraction patterns directly from atomistic simulations without a priori knowledge of the unit cell. This method is applied to study the structure of [0 1 0] symmetric tilt low-angle and large-angle grain boundaries in Ni. Virtual electron diffraction patterns and x-ray diffraction 2θ line profiles show that this method can distinguish between low-angle grain boundaries with different misorientations and between low-angle boundaries with the same misorientation but different dislocation configurations. For large-angle Σ5 (2 1 0), Σ29 (5 2 0) and Σ5 (3 1 0) coincident site lattice [0 1 0] symmetric tilt grain boundaries, virtual diffraction methods can identify the misorientation of the grain boundary and show subtle differences between grain boundaries in the x-ray 2θ line profiles. A thorough analysis of the effects of simulation size on the relrod structure in the electron diffraction patterns is presented.

Fracture mechanics of monolayer molybdenum disulfide

Molecular dynamics (MD) modeling is used to study the fracture toughness and crack propagation path of monolayer molybdenum disulfide (MoS(2)) sheets under mixed modes I and II loading. Sheets with both initial armchair and zigzag cracks are studied. The MD simulations predict that crack edge chirality, tip configuration and the loading phase angle influence the fracture toughness and crack propagation path of monolayer MoS(2) sheets. Furthermore, under all loading conditions, both armchair and zigzag cracks prefer to extend along a zigzag path, which is in agreement with the crack propagation path in graphene. A remarkable out-of-plane buckling can occur during mixed mode loading which can lead to the development of buckling cracks in addition to the propagation of the initial cracks.

Effect of point and grain boundary defects on the mechanical behavior of monolayer MoS2 under tension via atomistic simulations

Atomistic simulation is used to study the structure and energy of defects in monolayer MoS2 and the role of defects on the mechanical properties of monolayer MoS2. First, energy minimization is used to study the structure and energy of monosulfur vacancies positioned within the bottom S layer of the MoS2 lattice, and 60° symmetric tilt grain boundaries along the zigzag and armchair directions, with comparison to experimental observations and density functional theory calculations. Second, molecular dynamics simulations are used to subject suspended defect-containing MoS2 membranes to a state of multiaxial tension. A phase transformation is observed in the defect-containing membranes, similar to prior work in the literature. For monolayer MoS2 membranes with point defects, groups of monosulfur vacancies promote stress-concentration points, allowing failure to initiate away from the center of the membrane. For monolayer MoS2 membranes with grain boundaries, failure initiates at the grain boundary and it is found that the breaking force for the membrane is independent of grain boundary energy.

Plastic deformation of nanocrystalline copper-antimony alloys

Molecular dynamics simulations are used to evaluate the influence of Sb dopant atoms at the grain boundaries on plastic deformation of nanocrystalline Cu. Deformation is conducted under uniaxial tensile loading, and Sb atoms are incorporated as substitutional defects at the grain boundaries. The presence of randomly dispersed Sb atoms at the grain boundaries does not appreciably influence the mechanisms associated with dislocation nucleation in nanocrystalline Cu; grain boundary ledges and triple junctions still dominate as partial dislocation sources. However, the magnitude of the tensile stress associated with the partial dislocation nucleation event does increase with increasing Sb concentration and also with increasing grain size. The flow stress of nanocrystalline Cu increases with increasing Sb concentration up to 1.0 at.% Sb, with a maximum observed at a grain size of 15 nm for all Sb concentrations (0.0–2.0 at.% Sb).

Atomistic Modeling of Grain Boundaries and Dislocation Processes in Metallic Polycrystalline Materials

The objective of this review article is to provide a concise discussion of atomistic modeling efforts aimed at understanding the nanoscale behavior and the role of grain boundaries in plasticity of metallic polycrystalline materials. Atomistic simulations of grain boundary behavior during plastic deformation have focused mainly on three distinct configurations: (i) bicrystal models, (ii) columnar nanocrystalline models, and (iii) 3D nanocrystalline models. Bicrystal models facilitate the isolation of specific mechanisms that occur at the grain boundary during plastic deformation, whereas columnar and 3D nanocrystalline models allow for an evaluation of triple junctions and complex stress states characteristic of polycrystalline microstructures. Ultimately, both sets of calculations have merits and are necessary to determine the role of grain boundary structure on material properties. Future directions in grain boundary modeling are discussed, including studies focused on the role of grain boundary impurities and issues related to linking grain boundary mechanisms observed via atomistic simulation with continuum models of grain boundary plasticity.

Molecular dynamics simulation of nanoconfinement induced organization of n-decane.

Molecular dynamics (MD) simulations are used to study the behavior of n-decane under sub-10 nm confinement between two gold {111} surfaces. This confinement and dielectric medium are characteristic of those used in nanoscale electromachining (nano-EM) processes; thus, it is important that the behavior of the nanoconfined dielectric medium be investigated for better process understanding. Results obtained via MD simulations indicate that, when confined down to a thickness less than 1 nm, the mechanical boundary conditions trigger organization in the n-decane medium, resulting in two distinct molecular layers. The n-decane chains lie flat on the {111} gold surfaces and show preferred orientation in the close-packed 110 crystallographic directions. A 4-fold increase in the maximum local density as compared with the experimental bulk (liquid) density is observed at the interface between the molecular medium and the gold {111} surfaces, regardless of confinement spacing. Radial distribution function curves are used to quantitatively examine organization of the medium into molecular layers. The deliberate introduction of ledges (atomic steps) on the gold surface triggers a preferred alignment of the n-decane chains toward the boundaries of the ledges.

Heterogeneous dislocation nucleation in single crystal copper–antimony solid-solution alloys

Molecular dynamics (MD) simulations are employed to study the partial dislocation nucleation process in single crystal copper with varying concentrations of antimony (0.0–2.0 at%Sb) under uniaxial tension. A well-established embedded-atom method potential is used to represent the Cu–Cu interactions and a recently developed Lennard-Jones potential is used for the Cu–Sb and Sb–Sb interactions. Antimony atoms are randomly distributed as substitutional defects in the Cu single crystal. MD simulations indicate that the tensile stress required for partial dislocation nucleation in the crystal decreases with increasing concentration of Sb. The strain field around Sb dopant atoms in the Cu lattice reduces the unstable stacking fault energy, which promotes heterogeneous nucleation of partial dislocations and reduces the tensile stresses required for plastic deformation. In addition, the role of Sb on the reduction in the stress required for dislocation nucleation is found to be orientation-dependent. Finally, both temperature and Sb distribution play a role in the statistical variation of the stress required for heterogeneous partial dislocation nucleation; this variation is maximum at moderate levels of Sb concentration (0.20–0.50 at%Sb).

Microstructure stability of nanocrystalline materials using dopants

By definition, nanocrystalline materials have grain sizes, d, less than 100 nm. Due to their reduced grain size, nanocrystalline materials have superior mechanical properties compared to their microcrystalline counterparts. Loss of these unique properties due to grain growth under the effect of high temperature and stress is a limitation to their use in many applications. Recently it has been proposed to use dopants (alloying elements) to reduce the driving force for grain boundary motion, leading to improved microstructural stability and resistance to deformation. Inclusion of dopants has been shown to alter properties of nanocrystalline materials, although their precise effect on mechanical and electrical properties is still unclear. In this brief review article, work done in the domain of stability of polycrystalline materials using dopants and their application in nanocrystalline materials is discussed. The importance of both experiment and molecular dynamics simulations is presented.