Effects Of Free Surfaces Research Articles

Dynamic stability assessment of consequent rock slopes considering multiple transmissions and reflections of stress waves

In this work, Based on the conservation of momentum at the wave fronts and the displacement discontinuity method, a theoretical approach for calculating the dynamic safety factor of consequent rock slopes is proposed. The multiple transmissions and reflections of stress waves on the joints and free surfaces are taken into account. A comparison of UDEC simulation and theoretical calculation was carried out to verify the feasibility of the proposed method. Moreover, parametric studies were conducted to investigate the effects of free surfaces, joint stiffness, amplitude of incident waves, and frequency of incident waves. The results show that the proposed method can predict the time history curve of the safety factor of a slope readily and accurately. Waves reflected from both the slope and top surfaces are found to have a significant effect on the stability of the slope – considering them or not can cause a difference in the safety factors up to 32.5%. The greater the joint stiffness is, the greater the safety factor fluctuates with time. However, when the joint stiffness is sufficiently high, the effect of joint stiffness on the safety factor can be neglected. The approach proposed in this work provides a theoretical tool for the dynamic assessment and design of consequent rock slopes. Highlights An analytical method is proposed for calculating the dynamic safety factor of rock slopes. Multiple transmissions and reflections of stress waves are considered. The proposed method has high calculation accuracy and can be applied in practice.

Surface effects on the crystallization kinetics of amorphous antimony.

Elemental antimony (Sb) is regarded as a promising candidate to improve the programming consistency and cycling endurance of phase-change memory and neuro-inspired computing devices. Although bulk amorphous Sb crystallizes spontaneously, the stability of the amorphous form can be greatly increased by reducing the thickness of thin films down to several nanometers, either with or without capping layers. Computational and experimental studies have explained the depressed crystallization kinetics caused by capping and interfacial confinement; however, it is unclear why amorphous Sb thin films remain stable even in the absence of capping layers. In this work, we carry out thorough ab initio molecular dynamics (AIMD) simulations to investigate the effects of free surfaces on the crystallization kinetics of amorphous Sb. We reveal a stark contrast in the crystallization behavior between bulk and surface models at 450 K, which stems from deviations from the bulk structural features in the regions approaching the surfaces. The presence of free surfaces intrinsically tends to create a sub-nanometer region where crystallization is suppressed, which impedes the incubation process and thus constrains the nucleation in two dimensions, stabilizing the amorphous phase in thin-film Sb-based memory devices.

The effects of free surfaces on deformation twinning in HCP metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and, in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and irregular. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

The Effects of Free Surfaces on Deformation Twinning in HCP Metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and skewed. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging.

In this paper we model the segmental relaxation in poly(2-chlorostyrene) 18 nm freestanding films, using only data on bulk samples to characterize the system, and predict film relaxation times (τ) as a function of temperature that are in semi-quantitative agreement with film data. The ability to translate bulk characterization into film predictions is a direct result of our previous work connecting the effects of free surfaces in films with those of changing pressure in the bulk. Our approach combines the Locally Correlated Lattice (LCL) equation of state for prediction of free volume values (Vfree) at any given density (ρ), which are then used in the Cooperative Free Volume (CFV) rate model to predict τ(T, Vfree). A key feature of this work is that we calculate the locally averaged density profile as a function of distance from the surface, ρav(z), using the CFV-predicted lengthscale, Lcoop(z), over which rearranging molecular segments cooperate. As we have shown in the past, ρav(z) is significantly broader than the localized profile, ρ(z), which translates into a relaxation profile, τ(z), exhibiting a breadth that mirrors experimental and simulated results. In addition, we discuss the importance of averaging the log of position dependent relaxation times across a film sample (〈log τ(z)〉), as opposed to averaging the relaxation times, themselves, in order to best approximate a whole sample-averaged value that can be directly compared to experiment.

Capturing the Effects of Free Surfaces on Threading Dislocation Density Reduction

The big concern with using silicon as a substrate for making Ge and III-V devices is the dislocation density in the epilayers. Dislocations degrade device performance by trapping the photo-generated carriers, dragging down efficiency.In this presentation, we show that dislocation engineering using nanovoids in the heteroepitaxy of Ge/Si can be a plausible path for monolithic integration of high-quality GaAs on Si platform. Possible mechanisms responsible for threading dislocation (TD) density reduction through free surfaces located, either in the Ge layer or in Si substrate that lead to their annihilation or fusion are discussed. By combining various scanning and transmission electron microscopy techniques, it has been possible to link different effects of either of the free wedge surfaces on dislocation, particularly, threading arm pinning by nanovoids. The results suggest that introducing nanovoids favors recombination of TDs by increasing the characteristic interaction distance between neighboring dislocations. In fact, the creation of free surfaces could facilitate interactions between dislocations, enabling the dislocation network to change its connectivity in a way, which facilitates the subsequent annihilation of TD segments. In addition, the voids formed in the silicon substrate could potentially capture and thereby combine many more TDs. The TDs bend towards the voids in Si in order to minimize their length, causing their recombination and elimination at the nearest free surface thus leading to the creation of an almost defect-free Ge layer on Si [1].The use of such a simple, industry friendly, inexpensive and self-organised process through electrochemical etching and thermal annealing could be crucial in cutting manufacturing costs and enabling market penetration of III–V on Si-based devices.[1] Y. A. Bioud et al., “Uprooting defects to enable high-performance III–V optoelectronic devices on silicon,” Nat. Commun., vol. 10, no. 1, 2019.

Capturing the Effects of Free Surfaces on Threading Dislocation Density Reduction

The big concern with using silicon as a substrate for making Ge and III-V devices is the dislocation density in the epilayers. Dislocations degrade device performance by trapping the photo-generated carriers, dragging down efficiency. In this manuscript, a Ge/Si substrate is treated with a post epitaxial process to create a region with a high density of nanovoids within the Ge/Si structure, which acts as a free surface that attracts dislocations, facilitating the subsequent annihilation of their threading arms. Nanovoids are formed in the Ge layer by electrochemical porosification, followed by thermal annealing, creating a new configuration referred to as nanovoid-based Ge/Si virtual substrate (NVS).

Dimensionality effects in crystal plasticity, from 3D silicon to 2D silicene

To characterize the effects of free surfaces on dislocation mobility, the edge dislocation glide process in thin silicon films is modeled using an interatomic potential and first principles calculations. The influence of film thickness is determined, starting with the simulation of a silicene monolayer and then increasing the number of layers. The energy barrier for dislocation glide in silicene was calculated to be 1.5 eV, indicating a relatively high mobility of dislocation defects. In thin Si films a glide mechanism via consecutive bond rotations was identified, with kink nucleation being observed at the free surfaces of the film with subsequent migration. The influence of the free edge in finite size films is shown to be negligible in relation to glide for dislocations at distances from the free edge larger than three Burgers vectors. Molecular dynamics simulations reveal the possibility of more complex but lower energy barrier atomic reconstructions triggered at the free surfaces of the film near the dislocation core that may increase dislocation mobility at higher temperatures.

Atomic Force Microscopy Study of Discrete Dislocation Pile-ups at Grain Boundaries in Bi-Crystalline Micro-Pillars

Compression tests at low strains were performed to theoretically analyze the effects of anisotropic elasticity, misorientation, grain boundary (GB) stiffness, interfacial dislocations, free surfaces, and critical force on dislocation pile-ups in micro-sized Face-Centered Cubic (FCC) Nickel (Ni) and α -Brass bi-crystals. The spatial variations of slip heights due to localized slip bands terminating at GB were measured by Atomic Force Microscopy (AFM) to determine the Burgers vector distributions in the dislocation pile-ups. These distributions were then simulated by discrete pile-up micromechanical calculations in anisotropic bi-crystals consistent with the experimentally measured material parameters. The computations were based on the image decomposition method considering the effects of interphase GB and free surfaces in multilayered materials. For Ni and α -Brass, it was found that the best predicted step height spatial profiles were obtained considering anisotropic elasticity, free surface effects, a homogeneous external stress and a certain critical force in the material to equilibrate the dislocation pile-ups.

The Effect of Internal Free Surfaces on Void Swelling of Irradiated Pure Iron Containing Subsurface Trenches

We studied the effects of internal free surfaces on the evolution of ion-induced void swelling in pure iron. The study was initially driven by the motivation to introduce a planar free-surface defect sink at depths that would remove the injected interstitial effect from ion irradiation, possibly enhancing swelling. Using the focused ion beam technique, deep trenches were created on a cross section of pure iron at various depths, so as to create bridges of thickness ranging from 0.88 μm to 1.70 μm. Samples were then irradiated with 3.5 MeV Fe2+ ions at 475 °C to a fluence corresponding to a peak displacement per atom dose of 150 dpa. The projected range of 3.5 MeV Fe2+ ions is about 1.2 μm so the chosen bridge thicknesses involved fractions of the ion range, thicknesses comparable to the mean ion range (peak of injected interstitial distribution), and thicknesses beyond the full range. It was found that introduction of such surfaces did not enhance swelling but actually decreased it, primarily because there were now two denuded zones with a combined stronger influence than that of the injected interstitial. The study suggests that such strong surface effects must be considered for ion irradiation studies of thin films or bridge-like structures.

Excitonic Aharonov-Bohm Oscillations in Core-Shell Nanowires.

Phase coherence in nanostructures is at the heart of a wide range of quantum effects such as Josephson oscillations between exciton-polariton condensates in microcavities, conductance quantization in 1D ballistic transport, or the optical (excitonic) Aharonov-Bohm effect in semiconductor quantum rings. These effects only occur in structures of the highest perfection. The 2D semiconductor heterostructures required for the observation of Aharonov-Bohm oscillations have proved to be particularly demanding, since interface roughness or alloy fluctuations cause a loss of the spatial phase coherence of excitons, and ultimately induce exciton localization. Experimental work in this field has so far relied on either self-assembled ring structures with very limited control of shape and dimension or on lithographically defined nanorings that suffer from the detrimental effects of free surfaces. Here, it is demonstrated that nanowires are an ideal platform for studies of the Aharonov-Bohm effect of neutral and charged excitons, as they facilitate the controlled fabrication of nearly ideal quantum rings by combining all-binary radial heterostructures with axial crystal-phase quantum structures. Thanks to the atomically flat interfaces and the absence of alloy disorder, excitonic phase coherence is preserved even in rings with circumferences as large as 200 nm.

Non-local model for diffusion-mediated dislocation climb and cavity growth

To design efficient thermal recovery procedures for structural materials in fusion energy applications it is important to characterise quantitatively the annealing timescales of radiation-induced defect clusters. With this goal in mind, we present an extension of the Green’s function formulation of Gu et al. (2015). for the climb of curved dislocations, to include in the same framework the evaporation and growth of cavities and the effects of free surfaces. This paper focuses on the mathematical foundations of the model, which makes use of boundary integral equations (París and Cañas, 1997) to solve the steady-state vacancy diffusion problem. Numerical results are also presented in the simplified case of a dilute configuration of prismatic dislocation loops and spherical cavities in a finite-size medium, which show good agreement with experimental data on high temperature annealing in ion-irradiated tungsten (Ferroni et al., 2015).

Characterization of silicon surface elastic constants based on different interatomic potentials

Mechanical properties of materials are an important factor in designing nanoscale systems. Several researches and experiments have shown that the mechanical properties of the nano-scale materials are different from those of bulk. One of the major reasons for this difference is that the ratio of surface to volume increases at the nano-scale, and the effects of free surfaces become very important. In this paper, we have measured the surface elastic constants of silicon crystalline structure using different interatomic potentials. The potentials employed here are EDIP (Environment-Dependent Interatomic Potential), Stillinger-Weber and Tersoff, and also different crystalline orientations are examined. In order to obtain surface elastic constants, we have first determined the lattice constant resulting in minimum energy of bulk model for each potential, then we have measured the bulk and film elastic constants using respective models. It is observed that by increasing film thickness, elastic constants approach to those of bulk. Next, using the results of the previous sections, the surface elastic constants are measured and discussed for different surfaces using different potentials.

Anisotropic Elastic Field of 3D Prismatic Dislocation Loops in Bounded and Voided Single Crystal Films

Dislocations in a finite medium bring about image stresses. These image stresses play important roles in the dislocation behavior in finite sized systems such as thin films. Since the ratio of surface to volume is higher for thin films than for bulk materials, dislocation behaviors in thin films are greatly different from those in a corresponding infinite medium, which make it necessary to take into account the effects of free surfaces on the evolution of dislocations in thin films. In the investigations[4, 5], image stresses in an elastic cylinder and thin films are calculated by employing a Fourier transform (FFT) approach and isotropically elastic fields due to dislocations are adopted in their formulation. However, most crystals are anisotropic, and the anisotropic ratio changes with environment physical parameters, such as the temperature, moisture, electron field, magnetic field. A theorem based on anisotropic Stroh’s formula for calculating the image stress of infinite straight dislocations in anisotropic bicrystals has been developed by Barnett and Lothe[6]. Wu et al.[3] recently also make use of the FFT technique to investigate the general dislocation image stresses of cubic thin films, thus extending the formalism by Weinberger et al.[4,5] from isotropic to anisotropic thin films. It is clear that for the assumed in-plane elastic fields to be periodically defined within an unbounded region is an essential and indispensable prerequisite for the above FFT-based approach to be effectively implemented, thus ruling out the possibility of its being employed to analyse image stresses in bounded and/or voided thin cubic films. Our motivation here is then to make an further extension by first calculating the anisotropic elastic fields of dislocation loops in an unbounded thin film with cavities and then invoking FEM and the principle of superposition to seek the image stress solution.

Glass transition in single poly(ethylene oxide) chain: A molecular dynamics simulation study

ABSTRACTLocal dynamics of single poly(ethylene oxide) chain in various environments (bulk, film, and isolated systems) has been characterized by the reorientation functions of various backbone bond vectors. Within any observation time, the variations of these reorientation functions with the temperature can be well described by the Kohlrausch−Williams−Watts (KWW) like equation, in which the fitted temperature parameter is identified as the glass transition temperature (Tg). The so‐obtained Tg for that polymer faithfully reveals the effects of the observation time, chain flexibility and vector range on the local dynamics. Furthermore, it is found that the KWW like relation is also applicable to the temperature‐dependence of the fraction of frozen atoms or torsions defined by the trajectory radii of gyration or the conformational transitions. Consequently, different motions lead to different values of Tg for the same system. Despite all, the consistent trend can be yielded, namely, Tg (bulk) > Tg (film) > Tg (isolated), which captures the effects of free surfaces on enhanced dynamics. In addition, dynamics heterogeneity in the systems can be quantitatively revealed. The newly proposed method holds a bright promise to predict the Tg values of complex polymers especially for comparisons. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 178–188

Capturing the effects of free surfaces on void strengthening with dislocation dynamics

Void strengthening in crystalline materials refers to the increase in yield stress due to the impediment of dislocation motion by voids. Dislocation dynamics (DD) is a modeling method well suited to capture the physics, length scales, and time scales associated with void strengthening. However, previous DD simulation of dislocation–void interactions have been unable to accurately account for the strong image forces acting on the dislocation due to the void’s free surface. In this article, we employ a finite-element-based DD method to determine the obstacle strength of voids, defined as the critical resolved shear stress for a dislocation to glide past an array of voids. Our results demonstrate that the attractive image forces between the dislocation and free surface significantly reduce the obstacle strength of voids. Effects of surface mobility and stress concentrations around the void are also explored and are shown to have minimal effect on the critical stress. Finally, a new model relating void size and spacing to obstacle strength is proposed.

3D simulations of grain growth in polycrystalline sheets and rods: Effects of free surface and shape of specimen

A simple and efficient method for grain growth simulation is presented to account for the existence of free surfaces in polycrystalline solids. This method extends a phase-field model of grain growth, controlling the enthalpy and mean temperature of the system to maintain macroscopic shape of specimens. Effects of free surfaces and specimen shapes on the evolution of grain structure and the shape of individual grains in polycrystalline sheets and rods are examined.

Response of a shallow asymmetric V-shaped canyon to antiplane elastic waves.

This study focuses on the theoretical aspect of topographic scattering induced by a shallow asymmetric V-shaped canyon under plane shear horizontal-wave incidence. An analytical approach, based on the region-matching technique, is applied to derive a rigorous series solution, which is more general than that in a previous study. For the wave functions constrained in two angular directions, a novel form of Graf's addition formula is derived to arbitrarily shift the local coordinate system. Barrier geometry, angle of incidence and wave frequency are taken as the most significant parameters in exploring the topographic effects of localized concave free surfaces on ground motions. Both surface and subsurface motions are presented. Comparisons with previously published results and boundary-element solutions show good agreement. Frequency-domain results indicate that, for the high-frequency case at a low grazing angle (corresponding to the potential case in teleseismic propagation), the high levels of amplified motions occur mostly on the illuminated side of the canyon. When the windward slope is steeper, the peak amplitude values, at least 2.4 times larger than those of free-field responses, tend to increase. Time-domain simulations display how a sequence of scattered waves travel and attenuate at regional distances.

Superelasticity and fatigue in oligocrystalline shape memory alloy microwires

In oligocrystalline shape memory alloys, the total grain boundary area is smaller than the surface area of the specimen, leading to significant effects of free surfaces on the martensitic transformation and related shape memory and superelastic properties. Here we study sample size effects upon the superelastic characteristics of oligocrystalline microwires after one loading cycle and after many. Cu–Zn–Al wires with diameters ranging from ∼100 down to ∼20 μm are fabricated by the Taylor liquid processing technique and characterized through both uniaxial cyclic tensile testing and mechanically constrained thermal cycling. The energy dissipated per superelastic cycle increases with decreasing wire diameter, and this size effect is preserved after extensive cycling despite a significant transient evolution of the superelastic response for early cycles. We also present fatigue and fracture data, indicating that oligocrystalline wires of this normally brittle alloy can exhibit fatigue lifetimes two orders of magnitude improved over conventional polycrystalline Cu–Zn–Al.

Size-dependent yield stress in twinned gold nanowires mediated by site-specific surface dislocation emission

Large-scale molecular dynamics simulations were performed to demonstrate the synergistic effects of twin boundaries and free surfaces on dislocation emission in gold nanowires under tensile loading. It is revealed that the addition of nanoscale twins to crystalline nanowires can act to either increase or decrease their resistance to slip in tension, depending on both sample diameter and number of twins per unit length. Site-specific surface dislocation emission and image forces due to twin boundaries are used to explain the size-dependence of yield stress in twinned gold nanowires.