68 research outputs found
Molecular Dynamics Study of Crystal Plasticity during Nanoindentation in Ni Nanowires
Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on-oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of1 ms1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation
Nanoindentation and Plasticity in Nanocrystalline NI Nanowires: A Case Study in Size Effect Mitigation
We examine the processes of spherical indentation and tension in Ni nanowires and thin films containing random distributions of nanoscale grains by molecular dynamics simulations. It is shown that the resistance to nanoindentation of nanocrystalline Ni nanowires with diameters of 12 and 30 nm tends not to depend on the wire diameter and free surfaces, contrary to nanoindentation in single-crystalline nanowires. Accommodation of plastic deformation by grain boundary sliding suggests a mitigation strategy for sample boundary effects in nanoscale plasticity
Grain Growth Behavior at Absolute Zero during Nanocrystalline Metal Indentation
The authors show using atomistic simulations that stress-driven grain growth can be obtained in the athermal limit during nanocrystalline aluminum indentation. They find that the grain growth results from rotation of nanograins and propagation of shear bands. Together, these mechanisms are shown to lead to the unstable migration of grain boundaries via process of coupled motion. An analytical model is used to explain this behavior based on the atomic-level shear stress acting on the interfaces during the shear band propagation. This study sheds light on the atomic mechanism at play during the abnormal grain coarsening observed at low temperature in nanocrystalline metal
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Effect of geometrical constraint condition on the formation of nanoscale twins in the Ni-based metallic glass composite
We investigated the effect of geometrically constrained stress-strain conditions on the formation of nanotwins in -brass phase reinforced Ni59Zr20 Ti16 Si2 Sn3 metallic glass (MG) matrix deformed under macroscopic uniaxial compression. The specific geometrically constrained conditions in the samples lead to a deviation from a simple uniaxial state to a multi-axial stress state, for which nanocrystallization in the MG matrix together with nanoscale twinning of the brass reinforcement is observed in localized regions during plastic flow. The nanocrystals in the MG matrix and the appearance of the twinned structure in the reinforcements indicate that the strain energy is highly confined and the local stress reaches a very high level upon yielding. Both the effective distribution of reinforcements on the strain enhancement of composite and the effects of the complicated stress states on the development of nanotwins in the second-phase brass particles are discussed
Grain boundary energies and cohesive strength as a function of geometry
Cohesive laws are stress-strain curves used in finite element calculations to
describe the debonding of interfaces such as grain boundaries. It would be
convenient to describe grain boundary cohesive laws as a function of the
parameters needed to describe the grain boundary geometry; two parameters in 2D
and 5 parameters in 3D. However, we find that the cohesive law is not a smooth
function of these parameters. In fact, it is discontinuous at geometries for
which the two grains have repeat distances that are rational with respect to
one another. Using atomistic simulations, we extract grain boundary energies
and cohesive laws of grain boundary fracture in 2D with a Lennard-Jones
potential for all possible geometries which can be simulated within periodic
boundary conditions with a maximum box size. We introduce a model where grain
boundaries are represented as high symmetry boundaries decorated by extra
dislocations. Using it, we develop a functional form for the symmetric grain
boundary energies, which have cusps at all high symmetry angles. We also find
the asymptotic form of the fracture toughness near the discontinuities at high
symmetry grain boundaries using our dislocation decoration model.Comment: 12 pages, 19 figures, changed titl
Ideal maximum strengths and defect-induced softening in nanocrystalline-nanotwinned metals
Strengthening of metals through nanoscale grain boundaries and coherent twin boundaries is manifested by a maximum strength—a phenomenon known as Hall–Petch breakdown. Different softening mechanisms are considered to occur for nanocrystalline and nanotwinned materials. Here, we report nanocrystalline-nanotwinned Ag materials that exhibit two strength transitions dissimilar from the above mechanisms. Atomistic simulations show three distinct strength regions as twin spacing decreases, delineated by positive Hall–Petch strengthening to grain-boundary-dictated (near-zero Hall–Petch slope) mechanisms and to softening (negative Hall–Petch slope) induced by twin-boundary defects. An ideal maximum strength is reached for a range of twin spacings below 7 nm. We synthesized nanocrystalline-nanotwinned Ag with hardness 3.05 GPa—42% higher than the current record, by segregating trace concentrations of Cu impurity (\u3c1.0 weight (wt)%). The microalloy retains excellent electrical conductivity and remains stable up to 653 K; 215 K better than for pure nanotwinned Ag. This breaks the existing trade-off between strength and electrical conductivity, and demonstrates the potential for creating interface-dominated materials with unprecedented mechanical and physical properties
Coupled structural and magnetic properties of ferric fluoride nanostructures part I: a Metropolis atomistic study
A modified Metropolis atomistic simulation is proposed to model the structure
of grain boundaries (GBs) and interfaces in ionic nanostructured systems and is
applied to the magnetically interesting case of iron trifluoride (FeF3). We
chose long-range interatomic potentials adjusted on experimental results, and
adapted a previously established Monte Carlo scheme consisting of various
modifications of the simulated annealing/ Metropolis algorithm. Atomic
structures of twisted and tilted GBs as a function of the relative
disorientation of the grains have been achieved yielding close to
experimentally measured properties. This approach takes into account the
structure of the grains far from the interface in order to constrain the
relative orientation of the grains, without any periodic boundary conditions.
One concludes that a long-range coulombic falloff of the interatomic potentials
is necessary to obtain GB structures presenting a correct local topology but
with a smooth transition from crystalline to amorphous states. The structural
features are finally discussed in terms of topological aspects and local
magnetic structure.Comment: submitted to JMM
Simulation of dimensionality effects in thermal transport
The discovery of nanostructures and the development of growth and fabrication
techniques of one- and two-dimensional materials provide the possibility to
probe experimentally heat transport in low-dimensional systems. Nevertheless
measuring the thermal conductivity of these systems is extremely challenging
and subject to large uncertainties, thus hindering the chance for a direct
comparison between experiments and statistical physics models. Atomistic
simulations of realistic nanostructures provide the ideal bridge between
abstract models and experiments. After briefly introducing the state of the art
of heat transport measurement in nanostructures, and numerical techniques to
simulate realistic systems at atomistic level, we review the contribution of
lattice dynamics and molecular dynamics simulation to understanding nanoscale
thermal transport in systems with reduced dimensionality. We focus on the
effect of dimensionality in determining the phononic properties of carbon and
semiconducting nanostructures, specifically considering the cases of carbon
nanotubes, graphene and of silicon nanowires and ultra-thin membranes,
underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
Anomalous Heat Conduction and Anomalous Diffusion in Low Dimensional Nanoscale Systems
Thermal transport is an important energy transfer process in nature. Phonon
is the major energy carrier for heat in semiconductor and dielectric materials.
In analogy to Ohm's law for electrical conductivity, Fourier's law is a
fundamental rule of heat transfer in solids. It states that the thermal
conductivity is independent of sample scale and geometry. Although Fourier's
law has received great success in describing macroscopic thermal transport in
the past two hundreds years, its validity in low dimensional systems is still
an open question. Here we give a brief review of the recent developments in
experimental, theoretical and numerical studies of heat transport in low
dimensional systems, include lattice models, nanowires, nanotubes and
graphenes. We will demonstrate that the phonon transports in low dimensional
systems super-diffusively, which leads to a size dependent thermal
conductivity. In other words, Fourier's law is breakdown in low dimensional
structures
Quasicontinuum study of the shear behavior of defective tilt grain boundaries in Cu
Atomistic simulations using the quasicontinuum method are used to study the role of vacancy defects and angström-scale voids on the mechanical behavior of five tilt bicrystals containing grain boundaries (GBs) that have been predicted to exhibit characteristic deformation processes of nanocrystalline and nanotwinned metals: GB-mediated dislocation emission, interface sliding, and shear-coupled GB migration. We demonstrate that such nanoscale defects have a profound impact on interfacial shear strength and underlying deformation mechanisms in copper GBs due to void-induced local stresses. In asymmetric high and low angle GBs, we find that voids become preferential sites for dislocation nucleation when the void size exceeds 4 Å. In symmetric 9(221) GBs prone to sliding, voids are shown to shield the local shear stress, which considerably reduces the extent of atom shuffling at the interface. In symmetric Sigma5(210) and Sigma27(115) GBs, we find that the effect of voids on shear-coupled GB migration depends on the GB tilt direction considered, as well as on the size and number of voids. Remarkably, large voids can completely abate the GB migration process in Sigma 27(115) GBs. For all GB types, the interfacial shear strength is shown to decrease linearly as the volume fraction of voids at the interface increases; however, this study also suggests that this decrease is much more pronounced in GBs deforming by sliding than by dislocation nucleation or migration, owing to larger void-induced stresses
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