Dislocation Nucleation Mechanism Research Articles

Accelerated quasicontinuum: a practical perspective on hyper-QC with application to nanoindentation

Hyper-QC is a multiscale method based on the quasicontinuum (QC) method in which time is accelerated using hyperdynamics through the addition of a suitable bias potential. This paper describes the practical details of implementing and carrying out hyper-QC simulations and introduces a novel mechanism-based bias potential for deformation processes in face-centred cubic (fcc) systems. The factors limiting the maximum achievable acceleration are discussed. The method is demonstrated for nanoindentation into a thin film of single crystal fcc nickel at near experimental loading rates. Speed up factors as high as 10,000 are achieved. The simulations reveal a thermally activated dislocation nucleation mechanism with a logarithmic dependence on temperature and indenter velocity in agreement with a theoretical model.

Mechanisms and rate of dislocation nucleation in aluminum-copper alloys near Guinier-Preston zones

This article is devoted to a molecular dynamics simulation study of partial dislocation loop nucleation with respect to its mechanism and rate, and its propagation process under high shear stress in aluminum-copper alloys. The mechanisms of dislocation nucleation near Guinier-Preston (GP) zones of various diameters and concentrations have been analyzed. Dislocation nucleation rates near plain GP Cu-zones with diameters of 3.5, 7.5, and 13.5 nm and at various concentrations have been calculated using the mean lifetime method with temperatures in range of 100 and 700 K. It has been found that depending on the temperature and applied stress, the dislocation can nucleate either from the edge, or from the plain area of a GP zone. The dislocation nucleation is preceded by a generation of defect clusters that are formed due to local opposite atomic shifts in two adjacent (111) planes by the half-length of a Burgers vector of a partial dislocation. The expansion of a partial dislocation loop can be accompanied by the formation of twins via a shift of the atoms in the internal region of the loop. The twin velocity along the direction of the partial dislocation Burgers vector inside the loop can achieve longitudinal sound speed. The speeds of the edge and screw segments of a partial dislocation loop as a function of a shear stress component along the Burgers vector have been estimated. The latter seems to be limited by the shear sound speed.

Strain-rate-induced bcc-to-hcp phase transformation of Fe nanowires**Project supported by the National Natural Science Foundation of China (Grant No. 51571082) and China Postdoctoral Science Foundation (Grant No. 2015M580191).

Using molecular dynamics simulation method, the plastic deformation mechanism of Fe nanowires is studied by applying uniaxial tension along the [110] direction. The simulation result shows that the bcc-to-hcp martensitic phase transformation mechanism controls the plastic deformation of the nanowires at high strain rate or low temperature; however, the plastic deformation mechanism will transform into a dislocation nucleation mechanism at low strain rate and higher temperature. Furthermore, the underlying cause of why the bcc-to-hcp martensitic phase transition mechanism is related to high strain rate and low temperature is also carefully studied. Based on the present study, a strain rate-temperature plastic deformation map for Fe nanowires has been proposed.

Slip dislocation and twin nucleation mechanisms in hcp metals

A new nucleation mechanism is proposed for $$ \{ 10\bar{1}1\} $$ deformation twin in hcp materials. The mechanism is based on the results of atomistic computer simulations. It was found that under high shear stress applied on $$ \{ 10\bar{1}1\} $$ plane (the stress level is about 7 % of shear modulus), the core of a slip dislocation can transform to a twin embryo. The transformation and subsequent twin growth are accompanied by nucleation and migration of interfacial defects including disconnections and stacking faults. The paper provides the analysis of the nature of these defects and describes the reactions between them.

Tension/compression asymmetry of grain boundaries with non-planar structure

Molecular dynamics simulations were carried out to investigate the mechanical property and the deformation mechanisms of Cu bicrystal with non-planar structured grain boundaries (GBs) under uniaxial tension and compression. The simulation results showed that the non-planar GBs could change their equilibrium configurations under the applied stress, and the deformation mechanisms varied when altering the misorientation angle. The stacking fault energy curve was affected by the stress perpendicular to the slip plane and therefore has an influence on the dislocation nucleation mechanisms. Previous studies have revealed a ubiquitous tension/compression (T/C) strength asymmetry of many ultra-fine or nanocrystalline materials, and a higher compressive strength was usually reported. However, in the present study, the bicrystal samples with non-planar structured GBs show a higher tensile strength than the compressive one. The unusual T/C asymmetry property has an implication that the GBs with non-planar structure can play a significant role in affecting the mechanical properties of nanostructured materials.

Manipulating dislocation nucleation and shear resistance of bimetal interfaces by atomic steps

By means of atomistic simulations and interface dislocation theory, the mechanism of dislocation nucleation and shear resistance of various stepped fcc/bcc interfaces are comparatively studied using the Kurdjumov-Sachs (KS) Cu/Nb interface as a prototype. It is found that the introduction of atomic steps at the flat Cu{111}/{110}Nb KS interface does not change the most preferred slip systems, but influences the nucleation sites at the interface during tension loading, indicating that the flat and stepped interfaces possesses comparable energetic barriers for dislocation nucleation. During shear loading, the steps may significantly enhance the resistance to interface sliding by propagating partial dislocations that facilitate the emission and growth of parallel twins via cross slip. When the parallel twins are not favored or are hindered, the interface sliding will dominate in a “climbing peak-to-valley” manner. These results provide an effective pathway to solve the trade-off dilemma between dislocation nucleation and interface sliding by appropriately manipulating atomic steps at the flat interface in the design of high-strength metallic materials.

Universal scaling laws for homogeneous dislocation nucleation during nano-indentation

We perform atomistic simulations to study the mechanism of homogeneous dislocation nucleation in two dimensional (2D) hexagonal crystals during nanoindentation with a circular indenter of radius R. We study both a realistic embedded atom method (EAM) potential for Al in addition to simple pair-wise potentials: Lennard-Jones, Morse, and Hookean springs. The nucleation process is governed by the vanishing of the energy associated with a single energy eigenmode. The critical eigenmode, or dislocation embryo, is found to be localized along a line (or plane in 3D) of atoms with a lateral extent, ξ, at some depth, Y⁎, below the surface. For all interatomic potentials, the scaled critical load, Fc/R, and scaled critical contact length, Cc/R, decrease to R-independent values in the limit of large R. However, ξ/R and Y⁎/R display non-trivial scaling with R despite the R independence of Fc/R and Cc/R. We show that although both the interaction potential and the orientation of the lattice affect the prefactors in the scaling relations, all the scaling laws are robust. Furthermore, we show that a stability criterion proposed by Van Vliet et al. based on the minimum eigenvalue, Λ, of the local acoustic tensor predicts the location, orientation, and polarization of the dislocation embryo with a high degree of accuracy for all potentials and crystallographic orientations. However, we also show that, for all crystallographic orientations and interaction potentials, Λ erroneously indicates instability before the true instability occurs.

Effects of layer thickness and strain rate on mechanical properties of copper-gold multilayer nanowires

Effects of individual layer thickness and strain rate on the mechanical behavior of copper-gold multilayer nanowires as well as the dislocation nucleation mechanism under a uniform tensile loading are investigated using molecular dynamics method. Simulations indicate that the highest yield strength increases with the increase of the individual layer thickness. Furthermore, the result also shows that the mechanical properties in the tensile process at different strain rates are dramatically different from each other, where the dislocation motion and twinning deformation are at a lower strain rate, while the individual atoms are at a higher strain rate for leading to amorphization. The general conclusions derived from this work can provide a guideline for the design of high performance multilayer composite materials.

The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation.

Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures.

A new strain-rate-induced deformation mechanism of Cu nanowire: Transition from dislocation nucleation to phase transformation

The plastic deformation mechanism of Cu nanowires is studied using the molecular dynamics simulation method by applying uniaxial tension along the [100] direction at a constant strain rate and constant temperature. At high strain rate and low temperature, we find a new face-centered cubic–body-centered cubic–hexagonal close-packed (fcc–bcc–hcp) phase transformation mechanism, which controls the plastic deformation of the Cu nanowire. If we raise the nanowire’s temperature at a high strain rate, the plastic deformation mechanism will transform from the fcc–bcc–hcp phase transformation mechanism to the well-known momentum-induced-melting mechanism. On the other hand, if we reduce the applied strain rate to a certain level, the plastic deformation mechanism will transform into a dislocation nucleation mechanism. Based on the present study we have proposed a strain rate–temperature plastic deformation map for Cu nanowires. This map tells a vivid story about the transition among the three different plastic deformation mechanisms, and will help us develop a deep understanding of the plastic deformation of Cu nanowires.

Search for Dislocation Free Helium 4 Crystals.

The giant plasticity of ^4He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996–1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm^2, sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10^4–10^6 per cm^2, that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.

Atomistic modeling of defect-induced plasticity in CuNb nanocomposites

CuNb nanocomposites have proven stable under light-ion irradiation, although their response to heavy-ion bombardment and subsequent loading is still uncertain. In this paper we analyze the change in mechanical properties of model CuNb nanocomposites in vacancy supersaturated environments, mimicking certain effects introduced by heavy-ion irradiation. We have performed compression tests using molecular dynamics for different defect contents. The presence of defects substantially modifies the system response upon external loading. The dislocation nucleation mechanism changes with the interface atomic density. Stacking fault tetrahedra in the Cu layer may act as dislocation sources to lower significantly the yield stress and the same applies for voids present in the Nb layer. We have analyzed in detail the dislocation-interface interaction mechanisms under different conditions, showing how dislocations react with the misfit dislocations present at the interface to modify the atomic structure of the boundary, which suggests that the interface could be designed to optimize its defect healing properties. We conclude that heavy-ion irradiation softens the multilayer nanocomposites and that interfaces help in recovering these materials as they absorb the dislocations created from irradiation-induced defects.

Nanoindentation Size Effect Interpreted by the Dislocation Nucleation Mechanism

The instability criterion of atomic structure is used to investigate the nanoindentation size effect based on the dislocation nucleation mechanism. Both the dislocation nucleation location and the emission direction under indentation can be predicted. The nanoindentation size effect for hardness can be interpreted with a critical condition of the dislocation nucleation through adopting an Atomic Finite Element Method which is associated with the instability criterion. The present results agree well with the analytical solution based on the Rice model. The present results show that the nanoindentation size effect with the flat indenter can be expressed by using the dislocation nucleation concept. From dislocation nucleation point of view, the applied average force (hardness) required for dislocation emission is proportional to the inverse of square root of the width of the nano-indenter (contact zone size).

Atomistic simulations and continuum modeling of dislocation nucleation and strength in gold nanowires

The strength of true metallic nanowires and nanopillars (diameters below 100 nm) is known to be higher than the strength of bulk metals and is most likely controlled by dislocation nucleation from free surfaces. Dislocation nucleation is a thermally activated process that is sensitive to both temperature and strain rate. However, most simulations rely on high strain rate molecular dynamics to investigate strength and nucleation, which is limited by short molecular dynamics time scales. In this work, the energetics of dislocation nucleation in gold nanowires are computed using atomistic simulations, and transition state theory is used to estimate the strength at experimental strain rates revealing detailed information outside the realm accessible to molecular dynamics simulations. This allows investigation into the competition between thermally activated dislocation nucleation and other failure mechanisms such as elastic and structural instabilities. Additionally, the mechanisms of dislocation nucleation are compared against analytical continuum models which allow a better understanding of the nucleation process including the effects of the wire surfaces. This study helps clarify and consolidate our understanding of the nature of dislocation nucleation in small structures.

Homogeneous nucleation of dislocations

The mechanism and stochastic properties of the homogeneous nucleation of dislocations have been studied. An approach has been proposed for determining the nucleation rate based on consideration of the lifetimes of a metastable state. Primary data have been obtained using the molecular dynamics method. The dependences of the nucleation rate on the shear stresses have been determined for several temperatures. An approximation of the obtained results in terms of the Arrhenius equation has been proposed. The regions of shear stresses and temperatures in which the mechanism of homogeneous dislocation nucleation can be realized have been estimated.

Entropic effect on the rate of dislocation nucleation

Dislocation nucleation is essential to our understanding of plastic deformation, ductility, and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significantly larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviors are expected to occur in other nucleation processes in solids.

Dislocation Behavior in ZrC Particles during Elevated Temperature Compressive Deformation of a 30 vol.% ZrC p/W Composite

A 30 vol.% ZrCp/W composite has been deformed in compression in the temperature range of 1200–1600°C. Dislocation nucleation mechanism in ZrC particles is discussed by analyzing the harmonious deformation between tungsten-matrix and ZrC particles. Thermal activation apparently increases the mobility of screw segments, resulting in the formation of many kinetics jogs and thermodynamics jogs above 1300°C. The formation mechanisms of the dislocation configurations are studied.

Effect of pre-straining on the size effect in molybdenum pillars

The effect of prior deformation on mechanical behavior as a function of size is investigated for body-centered cubic (bcc) molybdenum (Mo) pillars. Experiments were performed using focused ion beam (FIB) manufactured [0 0 1] and [2 3 5] Mo micro/nanopillars, which were compressed, re-FIB machined, and compressed again. Unlike in bulk materials, pre-straining has a negligible effect on stress–strain behavior of the pillars, suggesting that dislocation storage does not occur in small-scale bcc specimens. The prevailing mechanism behind the size effect is attributed to dislocation nucleation mechanisms.

Nanoindentation response of nickel surface using molecular dynamics simulation

The mechanisms of dislocation nucleation on a nickel (Ni) (001) surface under nanoindentation behaviours are investigated using molecular dynamics simulation. The characteristic mechanisms include the molecular models of a thermal layer (TL) and thermal with a free layer (TFL), multi-step load/unload cycles, tilt angles and shapes of the indenter, and slip vectors. The model of a TL has higher reaction force than a TFL. The maximum forces of nanoindentation decrease with increasing time of the multi-step load/unload cycle. The indenter with the tilt angle has larger force to act on the molecular model than the indenter along the normal direction. The effect of the indentation shape is presented such that the conical tip has larger load force to act on the molecular model. The defects along Shockley partials on the (111) plane are produced during nanoindentation involving nucleation, glide and slip.

Incipient plasticity of twin and stable/unstable grain boundaries during nanoindentation in copper

An incipient plastic deformation of several types of grain boundaries subjected to nanoindentation was investigated by atomistic simulations. Our previous study showed that the dislocation nucleation in the inner region of the defect-free metals occurs when the resolved shear stress exceeds a particular stress level slightly higher than the ideal shear strength. However, crystal defects such as grain boundaries undermine the nucleation resistance. In this paper, we examined the dislocation nucleation mechanism at the twin and several coincidence site lattice grain boundaries and the resulting weakening of the dislocation nucleation resistance. We found that for the twin and the relatively stable $\ensuremath{\Sigma}11(\overline{1}13)[110]$ grain boundary, the primary slip deformation is activated on the grain-boundary plane prior to the defect-free region because of the low fault energy of the grain boundaries during slip deformation. Subsequently, the secondary slip is activated from the grain boundary. On the other hand, the dislocation is initially generated from the heterogeneous grain-boundary plane for the unstable high-energy grain boundaries.