Kink Nucleation Research Articles

Quantum mechanical tunnel nucleation of kink-solitons in a random potential

In connection with the development of nanotechnology, the sizes of the objects under study have approached the values at which the fundamental quantum mechanical laws of dynamics come into play. An important component of such dynamics in extended systems is the process of domain formation and propagation of their boundaries. In quasi-one-dimensional systems, these boundaries are peculiar quasiparticles—topological kink-solitons or simply kinks. Various processes such as switching states or phase transformations of a system can be described in terms of nucleation and propagation of kinks. With decreasing temperature, the thermal activation mechanism of nucleation of kinks is replaced by quantum mechanical subbarrier tunneling. The insufficient perfection of materials imposes often uncontrolled perturbations that modify the regularities of the tunnel formation of kinks. The influence of perturbations of the type of spatial ‘Gaussian noise’ is modeled in the work. The distribution of the rate of quantum mechanical tunnel nucleation of kinks over realizations of random fields is studied. It is shown that even relatively weak perturbations, as compared to the threshold of metastability, lead to a wide spectrum of kink nucleation times and a significant modification of the dependence of their typical characteristics on the driving force.

Tunneling Nucleation of Kink Pairs on Dislocations in the Peierls Potential Relief with Random Distortions

In view of the recent increased interest in the mechanical properties of quantum crystals, the study of the low-temperature dynamics of dislocations is necessary. Tunneling through the barriers created by periodic relief of the crystal lattice, which leads to the formation of pairs of kinks, makes an important contribution to this dynamics. Incomplete perfection of crystals often imposes uncontrollable perturbations that modify the features of the process of the tunneling nucleation of kinks. This article presents the modelling of perturbation effect by the random fields of internal stresses such as spatial “Gaussian noise.” The average value of the realizations of the rate of quantum-mechanical tunnel formation of kink pairs in the random fields is calculated. It is shown that even perturbations that are relatively weak in comparison with the crystalline relief lead to a significant modification of the kink formation rate dependence on a driving force.

Conditions controlling kink crack nucleation out of, and delamination along, a mixed-mode interface crack

This paper analyzes the competition between kink nucleation and interface fracture for an interface crack (without a putative kink flaw) subject to mixed-mode loading. The simulations utilize a distributed cohesive zone approach that embeds cohesive elements throughout the entire mesh; dynamic crack path evolution occurs through a loss of cohesive traction associated with a critical separation between elements. The simulations identify mesh densities that lead to mesh-independent results for randomly oriented triangular meshes, and provide clear guidelines regarding parameters that recover toughness-controlled cracking (i.e. linear elastic fracture mechanics). The results demonstrate that, when the when the normalized bulk toughness is far from the transition between kinking and delamination, crack direction and critical loads are identical to those predicted by He and Hutchinson, who analyzed cracking from a putative flaw associated with the maximum energy release rate. Near the transition between fracture modes, kink nucleation depends on the relative size of the interface and bulk process zones, such that additional criteria are needed (beyond those postulated by He and Hutchinson). Regime maps are presented which indicate regions of kink nucleation versus delamination as a function of controlling cohesive parameters.

Understanding thermally-activated glide of [formula omitted] screw dislocations in UO2 – A molecular dynamics analysis

As shown experimentally, the interplay between screw and edge dislocations in uranium dioxide (UO2) determines the low-temperature plasticity in this material. For the latter, neither the mobility of screw dislocations nor the mechanisms of their glide had been assessed – up until now. It is particularly interesting to evaluate the mobility of 1/2〈110〉{110} screw dislocations, supposedly the least mobile in UO2 due to the allegedly extremely high Peierls barrier of their motion. To address this issue, molecular dynamics simulations of dislocation glide are conducted on Lomonosov/MVS-10P supercomputers with LAMMPS software, and post-processing is done using DXA/OVITO. Under changing temperature and stress, the following variations of thermally-activated glide are found: nucleation and expansion of double kinks, formation and recombination of 1/6〈112〉 Shockley partials, self-pinning and production of debris, formation of sessile 1/3〈111〉 Frank loops. Velocity function of 1/2〈110〉{110} dislocations calculated at temperatures T=500–2000 K and shear stresses σ=10–1000 MPa shows a weak temperature dependence and becomes higher than the velocity of 1/2〈110〉{001} edge dislocations at temperatures T<1250 K.

Unsupervised Calculation of Free Energy Barriers in Large Crystalline Systems.

The calculation of free energy differences for thermally activated mechanisms in the solid state are routinely hindered by the inability to define a set of collective variable functions that accurately describe the mechanism under study. Even when possible, the requirement of descriptors for each mechanism under study prevents implementation of free energy calculations in the growing range of automated material simulation schemes. We provide a solution, deriving a path-based, exact expression for free energy differences in the solid state which does not require a converged reaction pathway, collective variable functions, Gram matrix evaluations, or probability flux-based estimators. The generality and efficiency of our method is demonstrated on a complex transformation of C15 interstitial defects in iron and double kink nucleation on a screw dislocation in tungsten, the latter system consisting of more than 120 000 atoms. Both cases exhibit significant anharmonicity under experimentally relevant temperatures.

Phase Stability and Anisotropic Sublimation of Cubic Ge–Sb–Te Alloy Observed by In Situ Transmission Electron Microscopy

Phase stability and anisotropic sublimation dynamics of the cubic Ge–Sb–Te alloy have been investigated by in situ transmission electron microscopy (TEM). The starting point of the phase-transition study is an epitaxially aligned Ge1Sb2Te4 grain on a Si(111) substrate. Upon in situ heating, the cubic phase remains stable up to the sublimation point without a transition to the thermodynamically stable trigonal crystal structure which is attributed to Si diffusion into the Ge1Sb2Te4 grain. The sublimation process is made visible with atomic resolution. The anisotropic process leads to the formation of stable {111} facets via kink nucleation on stable steps and subsequent sublimation from those kink sites as predicated by the terrace-step-kink model. Kink nucleation sites are identified and are in accordance with the broken-bond model approach.

Atomistic simulations on ductile-brittle transition in ⟨111⟩ BCC Fe nanowires

Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of ⟨111⟩ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10–1000 K employing a constant strain rate of 1 × 108 s−1. The results indicate that at low temperatures (10–375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2⟨111⟩ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450–1000 K. At 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2⟨111⟩ and immobile ⟨100⟩ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in ⟨111⟩ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transition increases with increasing nanowire size. The change in fracture behavior has been discussed in terms of the relative variations in yield and fracture stresses and change in slip behavior with respect to temperature. Further, the dislocation multiplication mechanism assisted by the kink nucleation from the nanowire surface observed at high temperatures has been presented.

Uncovering the Atomistic Mechanism for Calcite Step Growth.

Determining a complete atomic-level picture of how minerals grow from aqueous solution remains a challenge as macroscopic rates can be a convolution of many reactions. For the case of calcite (CaCO3 ) in water, computer simulations have been used to map the complex thermodynamic landscape leading to growth of the two distinct steps, acute and obtuse, on the basal surface. The carbonate ion is found to preferentially adsorb at the upper edge of acute steps while Ca2+ only adsorbs after CO32- . In contrast to the conventional picture, ion pairs prefer to bind at the upper edge of the step with only one ion, at most, coordinated to the step and lower terrace. Migration of the first carbonate ion to a growth site is found to be rate-limiting for kink nucleation, with this process having a lower activation energy on the obtuse step.

Uncovering the Atomistic Mechanism for Calcite Step Growth

AbstractDetermining a complete atomic‐level picture of how minerals grow from aqueous solution remains a challenge as macroscopic rates can be a convolution of many reactions. For the case of calcite (CaCO3) in water, computer simulations have been used to map the complex thermodynamic landscape leading to growth of the two distinct steps, acute and obtuse, on the basal surface. The carbonate ion is found to preferentially adsorb at the upper edge of acute steps while Ca2+ only adsorbs after CO32−. In contrast to the conventional picture, ion pairs prefer to bind at the upper edge of the step with only one ion, at most, coordinated to the step and lower terrace. Migration of the first carbonate ion to a growth site is found to be rate‐limiting for kink nucleation, with this process having a lower activation energy on the obtuse step.

Mechanical properties of Fe-rich Si alloy from Hamiltonian

The physical origins of the mechanical properties of Fe-rich Si alloys are investigated by combining electronic structure calculations with statistical mechanics means such as the cluster variation method, molecular dynamics simulation, etc, applied to homogeneous and heterogeneous systems. Firstly, we examined the elastic properties based on electronic structure calculations in a homogeneous system and attributed the physical origin of the loss of ductility with increasing Si content to the combined effects of magneto-volume and D03 ordering. As a typical example of a heterogeneity forming a microstructure, we focus on grain boundaries, and segregation behavior of Si atoms is studied through high-precision electronic structure calculations. Two kinds of segregation sites are identified: looser and tighter sites. Depending on the site, different segregation mechanisms are revealed. Finally, the dislocation behavior in the Fe–Si alloy is investigated mainly by molecular dynamics simulations combined with electronic structure calculations. The solid-solution hardening and softening are interpreted in terms of two kinds of energy barriers for kink nucleation and migration on a screw dislocation line. Furthermore, the clue to the peculiar work hardening behavior is discussed based on kinetic Monte Carlo simulations by focusing on the preferential selection of slip planes triggered by kink nucleation.

Reaction pathway analysis for shuffle-set 60° perfect dislocation in Si

In this work, the EDIP potential is employed for representing silicon and the shuffle-set 60° perfect dislocation motion is investigated by reaction pathway analysis. There are three possible shuffle-set 60° perfect dislocation core structures named as S1, S2 and S3. The activation energy barriers of the kink migration and nucleation in S1and S2 types are calculated by CI-NEB method. The simulation results show that the critical resolved shear strain of the shuffle-set dislocation in S1 type is around 5%, and the S1 type is the dominate one in the shear strain region of 0 to 5%. During the shear strain from 5to 11.81%, the dislocation moves as the S1 core kink nucleation and migration, meanwhile the S1 dislocation core is in process of transforming into S2. More interestingly, both S1 and S2 dislocation core structures is observed along the dislocation line in this shear strain regime, which could response to the missing observation of long segment dislocation line in the experiment.

Peierls potential and kink-pair mechanism in high-pressureMgSiO3perovskite: An atomic scale study

The motion of [100](010) screw dislocations via a kink-pair mechanism is investigated in high-pressure ${\mathrm{MgSiO}}_{3}$ perovskite by means of atomistic calculations and an elastic interaction model for kink nucleation. Atomistic calculations based on the nudged elastic band method provide the Peierls potential, which is shown to be dynamically asymmetric and stress dependent. The elastic interaction model adjusted to match kink width computed atomistically, is used to evaluate the critical nucleation enthalpy. We demonstrate that the kink-pair mechanism in ${\mathrm{MgSiO}}_{3}$ perovskite is controlled by the nucleation of kinks along the [100] screw dislocation.

原子論に基づく鉄合金のマクロ降伏強度予測のための理論モデルの構築

It is well known that the mechanical strength of iron is significantly changed by alloying. However, atomistic origin and underlying mechanism are still unclear. Since the strength change with respect to solute concentration is very sensitive and highly non-linear, the way of empirical prediction may contribute little to designing the mechanical strength by alloying. In this study, we theoretically construct a model which predicts temperature, strain rate, and solute concentration dependencies on critical resolved shear stress (CRSS) and yield stress of BCC iron alloys with dilute substitutional solutes based on atomistic analysis of the dislocation-solute atom interaction. In the coarse-grained BCC polycrystalline metals, the mechanical strength and deformation are dominated by screw dislocation motion consisting of kink nucleation and migration processes. Thus, our model is based on atomistically computed activation free energies for kink nucleation and migration of screw dislocation. We eventually apply our model to Fe-Si dilute alloy system as a representative example of BCC dilute alloys, and the theoretically predicted CRSS by our model is compared with an experimental one.

Magnesite Step Growth Rates as a Function of the Aqueous Magnesium:Carbonate Ratio

Step velocities of monolayer-height steps on the (1014) magnesite surface have been measured as functions of the aqueous magnesium:carbonate ratio and saturation index (SI) using a hydrothermal atomic force microscope. At SI ≤ 1.9 and 80–90 °C, step velocities were found to be invariant with changes in the magnesium:carbonate ratio, an observation in contrast with standard models for growth and dissolution of ionically bonded, multicomponent crystals. However, at high saturation indices (SI = 2.15), step velocities displayed a ratio dependence, maximized at magnesium:carbonate ratios slightly greater than 1:1. Traditional affinity-based models could not describe growth rates at the higher saturation index. Step velocities also could not be modeled solely through nucleation of kink sites, in contrast to other minerals whose bonding between constituent ions is also dominantly ionic in nature, such as calcite and barite. Instead, they could be described only by a model that incorporates both kink nucleation...

Simulating acoustic emission: The noise of collapsing domains

Microstructural changes during mechanical shear of a ferroelastic or martensitic material and their signature in acoustic emission (AE) spectroscopy during strain-induced yield and detwinning are investigated by computer simulation. Complex domain patterns are generated during the main yield event, which leads to large displacements of surface atoms and emission of acoustic waves. Loading beyond the yield point leads, eventually, to a simplification of the domain patterns by local movements of needle domains, the nucleation and movement of kinks in domain walls, and the collapse of domains spanning the entire sample (from surface to surface). These microstructural changes lead to much weaker acoustic emissions than those near the yield point. Nucleation/collapse during a yield event involves an energy drop of some 3.7 meV/atom; the collapse of spanning domains releases 0.56 meV/atom, a kink crashing into the surface changes the energy by 0.017 meV/atom, and the collapsing vertical needle changes the energy by 0.017 meV/atom. All these energy bursts can, in principle, be seen by AE. The large energy spread means that AE spectroscopy measures a mixture of events whereby weak and strong signals may signify smaller and bigger events of the same kind or different microstructural changes with intrinsically different signal strengths. In order to disentangle the various contributions, other observables are needed, such as the time-dependent strain matrix of the deformed sample.

Thermal and athermal crackling noise in ferroelastic nanostructures

The evolution of ferroelastic microstructures under external shear is determined by large-scale molecular dynamics simulations in two and three dimensions. Ferroelastic pattern formation was found to be almost identical in two and three dimensions, with only the ferroelastic transition temperature changing. The twin patterns generated by shear deformation depend strongly on temperature, with high wall densities nucleating under optimized temperature conditions. The dynamical tweed and mobile kink movement inside the twin walls is continuous and thermally activated at high temperatures, and becomes jerky and athermal at low temperatures. With decreasing temperature, the statistical distributions of dynamical tweed and kinks vary from a Vogel–Fulcher law to an athermal power-law distribution . During the yield event, the nucleation of needles and kinks is always jerky, and the energy of the jerks is power-law distributed. Low-temperature yield proceeds via one large avalanche. With increasing temperature, the large avalanche is thermally broken up into a multitude of small segments. The power-law exponents reflect the changes in temperature, even in the athermal regime.

Crystal plasticity model for BCC iron atomistically informed by kinetics of correlated kinkpair nucleation on screw dislocation

The mobility of dislocation in body-centered cubic (BCC) metals is controlled by the thermally activated nucleation of kinks along the dislocation core. By employing a recent interatomic potential and the Nudged Elastic Band method, we predict the atomistic saddle-point state of 1/2〈111〉 screw dislocation motion in BCC iron that involves the nucleation of correlated kinkpairs and the resulting double superkinks. This unique process leads to a single-humped minimum energy path that governs the one-step activation of a screw dislocation to move into the adjacent {110} Peierls valley, which contrasts with the double-humped energy path and the two-step transition predicted by other interatomic potentials. Based on transition state theory, we use the atomistically computed, stress-dependent kinkpair activation parameters to inform a coarse-grained crystal plasticity flow rule. Our atomistically-informed crystal plasticity model quantitatively predicts the orientation dependent stress–strain behavior of BCC iron single crystals in a manner that is consistent with experimental results. The predicted temperature and strain-rate dependencies of the yield stress agree with experimental results in the 200–350K temperature regime, and are rationalized by the small activation volumes associated with the kinkpair-mediated motion of screw dislocations.

The effect of hydrogen atoms on the screw dislocation mobility in bcc iron: A first-principles study

We investigate the effect of hydrogen on the mobility of a screw dislocation in body-centered cubic (bcc) iron using first-principles calculations, and show that an increase of screw dislocation velocity is expected for a limited temperature range. The interaction energy between a screw dislocation and hydrogen atoms is calculated for various hydrogen positions and dislocation configurations with careful estimations of the finite-size effects, and the strongest binding energy of a hydrogen atom to the stable screw dislocation configuration is estimated to be 256±32meV. These results are incorporated into a line tension model of a curved dislocation line to elucidate the effect of hydrogen on the dislocation migration process. Both the softening and hardening effect of hydrogen, caused by the reduction of kink nucleation enthalpy and kink trapping, respectively, are evaluated. A clear transition between softening and hardening behavior at the lower critical temperature is predicted, which is in qualitative agreement with experimental observation.

Cálculo eficiente de las energías de formación de escalones dobles en materiales BCC

Motion of screw dislocations in BCC materials at low temperature is believed to be related to the formation of mobile kinks on the dislocation line. Therefore, the accurate prediction of kink nucleation energies is required to fully describe mobility of screw dislocations in these materials. Studies of fundamental dislocation processes at atomic length scale are numerically and computationally intensive problems. This work studies the calculation of zero-stress formation energies of kink-pair configurations for BCC crystals. Our model for stored energy associated to a dislocation line configuration is based on the theory of discrete dislocations of Ariza and Ortiz. Its value is computed efficiently using an algorithm developed on the NVIDIA Compute Unified Device Architecture (CUDA). Results confirm those obtained using atomistic potentials and first principles calculations, and those based on the continuum theory of dislocations.

The effect of light doping on the creep of β-Sn single crystals induced by superconducting transition

Yield strength and characteristics of the transient creep in β-Sn single crystals doped with substitutional impurities In, Cd, Zn at the concentration of 0.01 at. % were measured at T = 1.6 K. The transient creep in samples with (100) 〈010〉 plastic slip orientation was observed after their normal–superconducting transition. As was earlier shown by the authors, plastic flow in such a slip system in pure β-Sn at low temperatures is determined by the dislocations overcoming Peierls barriers through the mechanism of nucleation and expansion of paired kinks (fluctuation creep stage) or over-barrier dislocation motion (dynamic creep stage). Comparison of the creep behavior in the pure and doped specimens allowed obtaining the information on the influence of impurity atoms on kinetics and dynamics of the motion of dislocation string in Peierls energy landscape. It was found that In and Cd impurities form weak obstacles for dislocations and cause a minor increase in yield strength while stimulating the dynamic creep stage. On the contrary, Zn atoms produce high barriers for dislocation motion, significantly increase the yield point and strongly reduce the contribution of dynamic effects.