Nucleation Of Partial Dislocations Research Articles

Amorphization and nanocrystallization of silicon under shock compression

High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory.

Non-twinning deformation mechanism of pure copper under high speed electromagnetic forming

Abstract The conventional high strain rate forming usually induces the occurrence of deformation twins in face-centered-cubic metals such as copper with medium stacking fault energy. In order to investigate the possible mechanical twinning of copper under a new type of high speed processing technique—electromagnetic forming, the pure copper was electromagnetically bulged followed by microstructural characterization through electron backscattered diffraction and transmission electron microscopy. The results revealed a reduction of twin boundaries with increasing plastic strain, indicates a non-twinning deformation mechanism which is different from the deformation twinning observed in other high speed forming process. The physical origin of the present phenomenon is discussed in detail according to the energy barrier and the critical shear stress that are required for the nucleation of perfect and partial dislocations that determine the formation of twins.

Dynamic and atomic-scale understanding of the twin thickness effect on dislocation nucleation and propagation activities by in situ bending of Ni nanowires

Although their mechanical behavior has been extensively studied, the atomic-scale deformation mechanisms of metallic nanowires (NWs) with growth twins are not completely understood. Using our own atomic-scale and dynamic mechanical testing techniques, bending experiments were conducted on single-crystalline and twin-structural Ni NWs (D=∼40nm) using a high-resolution transmission electron microscope (HRTEM). Atomic-scale and time-resolved dislocation nucleation and propagation activities were captured in situ. A large number of in situ HRTEM observations indicated strong effects from the twin thickness (TT) on dislocation type and glide system. In thick twin lamella (TT>∼12nm) and single-crystalline NWs, the plasticity was controlled by full dislocation nucleation. For NWs with twin thicknesses of ∼9nm<TT<∼12nm, full and partial dislocation nucleation occurred from the free surface, and the dislocations glided on multiple systems and interacted with each other during plastic deformation. For NWs with twin thicknesses of ∼6nm<TT<∼9nm, partial dislocation nucleation from the free surface and the gliding of those dislocations on the plane that intersected the twin boundaries (TBs) were the dominant plasticity events. For the NWs with twin thicknesses of ∼1nm<TT<∼6nm, the plasticity was accommodated by a partial dislocation nucleation process and glide parallel to the TBs. When TT<∼1nm, TB migration and detwinning processes resulting from partial dislocation nucleation and glide adjacent to the TBs were frequently observed.

Recoverable plasticity in penta-twinned metallic nanowires governed by dislocation nucleation and retraction.

There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.

ATOMISTIC SIMULATION OF TORSIONAL VIBRATION AND PLASTIC DEFORMATION OF FIVE-FOLD TWINNED COPPER NANOWIRES

In this paper, atomistic simulations have been conducted to investigate the torsional mechanical behaviors of five-fold twinned nanowires (FTNs), including the torsional vibration properties, elasto-plastic deformation behaviors and activation process of the first partial dislocation nucleation. Simulation results show that the fundamental torsional vibration frequency is inversely proportional to the wire length and is independent of the wire radius. Provided that an effective shear modulus of FTNs is used, the classic elastic torsional theory may be applicable to nanoscale. Furthermore, it is found that the plastic deformation of FTNs is dominated by partial dislocation activities. The normalized critical torsional angle corresponding to the onset of plastic deformation increases with the decrease of the wire radius and temperature, while it is almost independent of the wire length and loading rate. In addition, the activation energy of the first partial dislocation nucleation is about several electric voltages and decreases with the increase of the wire radius and applied torsional load.

Atomistic investigations of tensile and shear mechanical properties of nanotwinned copper with embedded defects

The tensile and shear mechanical properties of nanotwinned copper with embedded defects have been investigated by using molecular dynamics simulations. Simulation results show that the stress concentration at the tips of crack-like defects dominates the first partial dislocation nucleation at the onset of plastic deformation, the joint action of twin boundaries and dislocations controls the yield stress at the earlier plastic deformation stage and the dislocation/twin boundary–dislocation interactions determine the plastic flow stage. Furthermore, it is found that the yield stress decreases nonlinearly with the increase of the crack length, while it decreases almost linearly with the increase of the temperature.

Mechanism-based crystal plasticity modeling of twin boundary migration in nanotwinned face-centered-cubic metals

Nanotwinned (nt) metals are an important subset of nanostructured materials because they exhibit impressive strength and ductility. Several recent investigations on nt face-centered-cubic (FCC) metals indicate that their macroscopic responses emerge from complex microscopic mechanisms that are dominated by dislocation–TB interactions. Under applied stimulus, nt microstructures evolve through migration of twin boundaries (TBs) that may have implications on the material strength and stability. This work focuses on modeling TB migration within finite element framework in an explicit manner and studying its effects on the micromechanics of twinned FCC metals under quasi-static loading conditions. The theoretical setting is developed using three-dimensional single crystal plasticity as a basis wherein the plastic slip on the {111}〈1¯10〉 slip systems in an FCC crystal structure is modeled as visco-plastic behavior. Owing to their governing role, twins are modeled as discrete lamellas with full crystallographic anisotropy. To model TB migration, an additional visco-plastic slip-law for twinning partial systems ({111}〈112¯〉) based on the nucleation and motion of twin partial dislocations is introduced. This size-dependent constitutive law is presumed to prevail in the vicinity of the TB and naturally facilitates TB migration when combined with a twinning condition that is based on the accrual of the necessary shear strain. The constitutive development is implemented within a finite element framework through a User Material (UMAT) facility within ABAQUS/STANDARD®. Detailed micromechanics simulations on model microstructures involving single-grained and polycrystalline topologies are presented.

Atomistic simulation of single crystal copper nanowires under tensile stress: Influence of silver impurities in the emission of dislocations

The transition from elastic to plastic behaviour in single crystal copper nanowires under uniaxial tensile stress at different concentrations of silver (0.0–0.5at.% Ag) and at different temperatures (0.1, 100, and 300K) using the molecular dynamics method is investigated. The tensile stress is applied along 〈100〉 crystallographic orientation and the silver atoms are placed randomly on the surface of the nanowire, as substitutional point defect. The simulations indicate that silver atoms lower slightly the unstable stacking fault energy, making them act as sources of partial dislocation nucleation, due to the local strain field they produce in the lattice structure. The defects generated in the material also act as sources for nucleation, giving rise to a competition of two mechanism. Also, it is observed that the yield point decreases with the temperature and the presence of impurities.

Plasticity‐Controlled Friction and Wear in Nanocrystalline SiC

Wear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size or by reducing the size of the deformation zone. We have combined these two strategies with the goal of understanding the atomistic mechanisms underlying the plasticity‐controlled friction and wear in nanocrystalline (nc) silicon carbide (SiC). We have performed molecular dynamics simulations of nanoscale wear on nc‐SiC with 5 nm grain diameter with a nanoscale cutting tool. We find that grain‐boundary (GB) sliding is the primary deformation mechanism during wear and that it is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull‐out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of GBs. Pile up in nc‐SiC consists of grains that were pulled out during deformation. We compare the wear response of nc‐SiC to single‐crystal (sc) SiC and show that scratch hardness of nc‐SiC is lower than that of sc‐SiC. Our results demonstrate that the higher scratch hardness in sc‐SiC originates from nucleation and motion of dislocations, whereas nc‐SiC is more pliable due to additional mechanism of deformation via GB sliding.

Investigation of crack tip dislocation emission in aluminum using multiscale molecular dynamics simulation and continuum modeling

This work investigates the dislocation nucleation processes that occur at the tip of a crack in aluminum under a broad range of crystallographic orientations and temperatures. A concurrent multiscale molecular dynamics – continuum simulation framework is employed. The results are then interpreted using a Peierls continuum model that uses finite temperature material properties derived from molecular dynamics simulation. Under ramped loading, partial dislocation nucleation at the crack tip is found to lead to both full dislocation emission and twinning, depending upon the orientation, temperature, and magnitude of the applied load in the simulation. The origins of the dependencies are made apparent by the Peierls continuum model. The continuum model suggests that in many instances dislocation nucleation from the crack tip can be considered to be a strain rate independent process, yet still temperature dependent through the temperature dependence of the stacking fault energies and elastic constants.

Atomistic Model Analysis of Local and Global Instabilities in Crystals at Finite Temperature

There have been a lot of studies dedicated to structural instability in solids. For local instability, theoretical (ideal) strength of crystals has been extensively studied with ab initio calculations. Global instability taking into account the collective motion of atoms involved in deformation has also been investigated. However, these studies have usually been done at 0 K and little has been understood about the effect of temperature. In this study, we demonstrate computational approaches to the effect of temperature on local and global instabilities. Ideal shear strength (ISS) of silicon at finite temperatures is calculated by molecular dynamics (MD) simulations with an empirical potential. ISS is obtained as a function of temperature. Our results imply that, unlike metals, the reduction in ISS by temperature cannot be estimated simply by taking into account thermal expansion of volume. In addition, global instability for dislocation nucleation in a Cu thin film model under tension is investigated. We first evaluated instability modes at 0 K with increasing strain, and then performed MD simulations at 50 K. After the nucleation of a partial dislocation, the second dislocation can be one to create a twin or one to create another partial dislocation. These different deformations can be understood as the competition of latent instability modes that have relatively small eigenvalues.

Atomistic Deformation Mechanisms in Copper Nanoparticles

The compression of the face-centered-cubic copper nanosphere is investigated by using molecular dynamics simulations. In the elastic regime, the response of nanosphere can be described by the classic Hertzian contact theory. The plastic deformation begins with the nucleation of partial dislocations from the contact edges. These partials dislocations glide and intersect to form pyramid sessile dislocation locks, which play the key role in the hardening of incipient plasticity. As the compression increases, partial dislocations nucleate from the free surface and their expansion leads to deformation twinning, which not only enhances the strength but also ductility.

Bending properties of Ag nanowires with pre-existing surface defects

Materials used in the engineering always contain imperfections or defects which significantly affect their performances. Based on the large-scale molecular dynamics simulation and the Euler–Bernoulli beam theory, the influence from different pre-existing surface defects on the bending properties of Ag nanowires (NWs) is studied in this paper. It is found that the nonlinear-elastic deformation, as well as the flexural rigidity of the NW is insensitive to different surface defects for the studied defects in this paper. On the contrary, an evident decrease of the yield strength is observed due to the existence of defects. In-depth inspection of the deformation process reveals that, at the onset of plastic deformation, dislocation embryos initiate from the locations of surface defects, and the plastic deformation is dominated by the nucleation and propagation of partial dislocations under the considered temperature. Particularly, the generation of stair-rod partial dislocations and Lomer–Cottrell lock are normally observed for both perfect and defected NWs. The generation of these structures has thwarted attempts of the NW to an early yielding, which leads to the phenomenon that more defects does not necessarily mean a lower critical force.

Plasticity of Cu nanoparticles: Dislocation-dendrite-induced strain hardening and a limit for displacive plasticity

The plastic behaviour of individual Cu crystallites under nanoextrusion is studied by molecular dynamics simulations. Single-crystal Cu fcc nanoparticles are embedded in a spherical force field mimicking the effect of a contracting carbon shell, inducing pressure on the system in the range of gigapascals. The material is extruded from a hole of 1.1–1.6 nm radius under athermal conditions. Simultaneous nucleation of partial dislocations at the extrusion orifice leads to the formation of dislocation dendrites in the particle causing strain hardening and high flow stress of the material. As the extrusion orifice radius is reduced below 1.3 Å we observe a transition from displacive plasticity to solid-state amorphisation.

Insight into the Melting Behavior of Au–Pt Core–Shell Nanoparticles from Atomistic Simulations

Comprehensive understanding of thermodynamic properties of metallic nanoparticles is of significance for their utility in catalysis. In this article, we have employed molecular dynamics simulations with quantum Sutton–Chen many-body potentials to examine the thermal stability of Au–Pt core–shell nanoparticles with different sizes during continuous heating. Our study shows that, for fixed particle size, the melting temperature is independent of core size for a small core while it is linearly decreased with a rising core radius for a large core. Diverse melting mechanisms have been discovered for different-core-sized nanoparticles. For a small core, the melting is progressively developed from the surface into the core, similar to that of monometallic nanoparticles. For a moderate or large core, an inhomogeneous melting has been found in these nanoparticles. The nucleation and activity of Shockley partial dislocations have initialized the local structural instability of the core–shell interface, leading to the inhomogeneous premelting of the Au core and the Pt shell for the moderate core. Nevertheless, when the core is large enough (resulting superthin shell), the diffusion of Au atoms from the core into the shell plays a dominant role in the destruction of the core–shell interface. This study provides a fundamental perspective on the melting behaviors of bimetallic (even multimetallic) nanoparticles at the atomistic level.

Activation energy for nucleation of partial dislocation from grain boundaries

The nudged elastic band method has been used to calculate the activation energies for the nucleation of the leading partial dislocation from grain boundaries in pure f.c.c. copper under tensile loading at 0K. Two planar grain boundaries, Σ5{210} and Σ7{530}, and a Σ5{210} grain boundary containing a ledge (stepped grain boundary) as a grain boundary defect have been studied. The activation energy for the nucleation of the partial dislocation is found to be significantly lower from the grain boundary containing the defect and that there is no real correlation between the activation energy and the change in interfacial energy of the interface during the nucleation process. This investigation shows that classic molecular dynamics simulations cannot be used to determine activation energies and additionally, and more importantly, that dislocation nucleation is much more likely to occur at defect sites. However, it is found that the activation energy from planar grain boundaries increases sharply as the external load is decreased away from the critical stress, and that the critical stress for partial dislocation nucleation can be found simply with molecular dynamics simulations.

Mechanical Characterization of Co/Cu Multilayered Nanowires

The mechanical deformation properties of (110) Co/Cu multilayered nanowires were studied by Molecular Dynamics under uniaxial tensile and compressive stresses. The potential of the immiscible CoCu system was modeled by a second-moment tight-binding approximation. Stress-strain curves at different conditions were obtained and the elastic modulus and yield stress were analyzed. Both magnitudes are approximately independent of the strain rate, except at high values. They decrease linearly with increasing temperature. Below a volume-to-surface-area ratio, their values drastically increase and diverge from the bulk values. If the thickness of the Cu sublayers increases, the Young's modulus and yield stress decrease, although in a different way. The elastic modulus decreases linearly and the yield stress falls steeply whenever Cu is present in the nanowire, since the lattice distortion takes place firstly and fundamentally in Cu sublayers. The change in the axial stress at the interface is little significant on average and rather localized. Unlike, the transverse stress has a non-uniform distribution along the Cu sublayer, especially at the yield point. The Young's modulus and yield stress are larger in tension than in compression. Under tensile stress, nanowires slip via partial dislocation nucleation and propagation. Unlike, compressive deformation of nanowires takes place via both partial and full dislocations.

Cyclic Cantilever Bending of Copper Nanowhiskers

AbstractA cyclic bending test with cantilever geometry is developed for probing local mechanical properties of single crystalline Cu nanowhiskers (NWs). A lower limit for the yield stress for such whiskers is derived at ≈5 GPa. Since all NWs exhibit the axis parallel to the 〈1 1 0〉 crystallographic direction, the resolved shear stress can be calculated. The values of ≈2 GPa lie close to the theoretical strength limit and predict partial dislocation nucleation as the governing mechanism at the onset of plastic deformation. The yield stress is practically independent of the whisker diameter.

Effect of size on mechanical behavior of Au pillars by molecular dynamics study

The influence of specimen size on the mechanical behavior of Au pillars is studied by means of molecular dynamics (MD) simulations with the EAM potential. Under compression at 300 K, as the deformation of pillars is in the plastic stage, nucleation of partial dislocations is observed. The coupling effect of surface stress and thermal activation is considered when analyzing the size effect on the yield property of the Au pillars. It appears that both the tensile stress component and the temperature in the surface layer impart significant effect on the mechanical behaviors of the nano-sized Au pillars.

Heterogeneous dislocation nucleation from surfaces and interfaces as governing plasticity mechanism in nanoscale metals

Abstract