Kink boundaries, KBs, in metals are well defined entities that at low disregistry angles are identical to low angle grain boundaries, LAGBs. The latter are comprised of basal dislocations aligned normal to the basal planes in such a way that their strain fields cancel. A ripplocation is best defined as an atomic scale ripple. When ripplocations on adjacent planes nest, they define a ripplocation boundary, RB. Two oppositely signed adjacent RBs define a ripplocation band. Until recently, most RBs have been classified as KBs. In layered crystalline solids, LCS, when deformation is confined in two independent directions, with no possibility of twinning, most KBs should be classified as RBs. Herein, we show that in LCSs, RBs: i) are not atomically sharp; their strain fields are considerably delocalized, especially in comparison to KBs, ii) delaminate at high curvatures; iii) form nanobridges, and iv) are highly strained and, if not trapped, fully reversible. At extreme curvatures, the bonds are sundered and the RBs are no longer reversible and are reminiscent of KBs at the macroscopic scale. The distinction is important because whether delaminations nucleate and/or a material is highly strained has important and crucial ramifications on its deformation and ultimate failure.

Recently, we have established that, when loaded in compression, edge-on, atomic layers in layered solid can fail by buckling. The resulting structure is termed a ripplocation. When more than one layer buckles, they outline standing waves with boundaries that we labeled ripplocation boundaries that are nearly fully recoverable. In this paper, we examine buckling of layers at the centimeter level to explore continuum buckling theory and its applicability to atomic layers. Specifically, we examine buckling by confining and cyclically loading thin steel sheets, edge-on, determining that increasing confining pressure, sheet thickness, and/or decreasing the number of layers increases the buckling load. Concomitantly, the resulting wavelengths and amplitudes are reduced. A nonlinear, folding mechanics model, which accounts for frictional bending and foundation energies, is adapted and verified on our experimental results. We also demonstrate that Coulombic friction between the layers can account for the energy dissipated per cycle. The predicted values of buckling nucleation loads and number of modes from the model are in good agreement---at low levels of confinement---with continuum and atomistic scale results. The wavelength estimates from the model correlate surprisingly well with the continuum buckling results; however, likely due to the complex mechanics at the lower length scales and limiting theoretical assumptions in the derivation, the accuracy decreases at the atomistic scale and at higher confining pressures.

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Kinking is a deformation mechanism ubiquitous to layered systems, ranging from the nanometer scale in layered crystalline solids, to the kilometer scale in geological formations. Herein, we demonstrate its origins in the former through multiscale experiments and atomistic simulations. When compressively loaded parallel to their basal planes, layered crystalline solids first buckle elastically, then nucleate atomic-scale, highly stressed ripplocation boundaries – a process driven by redistributing strain from energetically expensive in-plane bonds to cheaper out-of-plane bonds. The consequences are far reaching as the unique mechanical properties of layered crystalline solids are highly dependent upon their ability to deform by kinking. Moreover, the compressive strength of numerous natural and engineered layered systems depends upon the ease of kinking or lack there of.

Recent work suggests that layered solids deform through buckling of basal planes. When isolated locally, as in graphite, these buckles, termed ripplocations, behave superficially similar to dislocations, but have no Burgers vectors. Through atomistic simulations, we demonstrate the easy transitions of ripplocations in graphite between many closely-spaced energy states, even at low temperatures. Between 60 and 350 K, their migration barrier is estimated at 32 meV, independent of segment length. Ripplocations spontaneously migrate towards vacancies and away from compressive stresses. These results shed more light on this new micromechanism and potentially explain experimental observations that evade sufficient description through dislocation-based models.

M. W. Barsoum,1,* X. Zhao,2 S. Shanazarov,1 A. Romanchuk,1 S. Koumlis,2 S. J. Pagano,2 L. Lamberson,2,† and G. J. Tucker3,‡ 1Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 2Department of Mechanical Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 3Department of Mechanical Engineering, Colorado School of Mines, Golden, Colorado 80401, USA

We have performed direct molecular dynamics (MD) simulations of heteroepitaxial vapor deposition of InxGa1-xN films on nonpolar (11 (2) over bar0) wurtzite-GaN surfaces to investigate strain relaxation by misfit-dislocation formation. The simulated growth is conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN substrate. We apply time-and-position-dependent boundary constraints to affect the appropriate environments for the vapor phase, the near-surface solid phase, and the bulklike regions of the growing layer. The simulations employ a newly optimized Stillinger-Weber In-Ga-N system interatomic potential wherein multiple binary and ternary structures are included in the underlying density-functional theory and experimental training sets to improve the treatment of the In-Ga-N related interactions. To examine the effect of growth conditions, we study amatrix of 63 different MD-growth simulations spanning seven InxGa1-xN-alloy compositions ranging from x = 0.0 to x = 0.8 and nine growth temperatures above half the simulated melt temperature. We found a composition dependent temperature range where all kinetically trapped defects were eliminated, leaving only quasiequilibrium misfit and threading dislocations present in the simulated films. Based on the MD results obtained in this temperature range, we observe the formation of interfacial misfit and threading dislocation arrays with morphologies strikingly close to those seen in experiments. In addition, we compare the MD-observed thickness-dependent onset of misfit-dislocation formation to continuum-elasticity-theory models of the critical thickness and find reasonably good agreement. Finally, we use the three-dimensional atomistic details uniquely available in the MD-growth histories to directly observe the nucleation of dislocations at surface pits in the evolving free surface.

We investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and (11 (2) over bar0) wurtzite-GaN surfaces. The simulated growths are conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN surface; we apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the vapor-phase, the near-surface solid-phase, and the bulk-like regions of the growing layer. The simulations employ newly optimized Stillinger-Weber In-Ga-N-system potentials, wherein multiple binary and ternary structures are included in the underlying density-functional-theory training sets, allowing improved treatment of In-Ga-related atomic interactions. To examine the effect of growth conditions, we study a matrix of >30 different MD-growth simulations for a range of InxGa1-xN-alloy compositions (0 <= x <= 0.4) and homologous growth temperatures [0.50 <= T/T-m*(x) <= 0.90], where T-m*(x) is the simulated melting point. Growths conducted on polar (0001) GaN substrates exhibit the formation of various extended defects including stacking faults/polymorphism, associated domain boundaries, surface roughness, dislocations, and voids. In contrast, selected growths conducted on semi-polar (11 (2) over bar0) GaN, where the wurtzite-phase stacking sequence is revealed at the surface, exhibit the formation of far fewer stacking faults. We discuss variations in the defect formation with the MD growth conditions, and we compare the resulting simulated films to existing experimental observations in InGaN/GaN. While the palette of defects observed by MD closely resembles those observed in the past experiments, further work is needed to achieve truly predictive large-scale simulations of InGaN/GaN crystal growth using MD methodologies. Published by AIP Publishing.

We present a new interatomic potential for solids and liquids called Spectral Neighbor Analysis Potential (SNAP). The SNAP potential has a very general form and uses machine-learning techniques to reproduce the energies, forces, and stress tensors of a large set of small configurations of atoms, which are obtained using high-accuracy quantum electronic structure (QM) calculations. The local environment of each atom is characterized by a set of bispectrum components of the local neighbor density projected onto a basis of hyperspherical harmonics in four dimensions. The bispectrum components are the same bond-orientational order parameters employed by the GAP potential [1]. The SNAP potential, unlike GAP, assumes a linear relationship between atom energy and bispectrum components. The linear SNAP coefficients are determined using weighted least-squares linear regression against the full QM training set. This allows the SNAP potential to be fit in a robust, automated manner to large QM data sets using many bispectrum components. The calculation of the bispectrum components and the SNAP potential are implemented in the LAMMPS parallel molecular dynamics code. We demonstrate that a previously unnoticed symmetry property can be exploited to reduce the computational cost of the force calculations by more than one order of magnitude. We present results for a SNAP potential for tantalum, showing that it accurately reproduces a range of commonly calculated properties of both the crystalline solid and the liquid phases. In addition, unlike simpler existing potentials, SNAP correctly predicts the energy barrier for screw dislocation migration in BCC tantalum.

Deformation processes in nanocrystalline (NC) metals such as dislocation nucleation/absorption and grain boundary shuffling/sliding are mediated at interfaces. Grain boundaries and their junctions fundamentally control NC material inelastic deformation mechanisms. However, material deformation is inherently multiscale and quantifying failure criteria and scaling relations across multiple length/time scales has proven to be difficult. We apply recently developed volume-averaged kinematic variables from continuum mechanics as metrics to assess results of atomistic simulations. These metrics are computed using nearest neighbors and include kinematic quantities such as microrotation and dilatation. Useful insight into the origins of plastic deformation in NC metals is gained by their application, enabling resolution of the contributions of competing mechanisms to inelastic material deformation.

The shear deformation behavior of bicrystalline grain boundaries is analyzed using continuum mechanical metrics extracted from atomistic simulations. Calculating these quantities at this length-scale is premised on determining the atomic deformation gradient tensor using interatomic distances. Employing interatomic distance measurements in this manner permits extension of the deformation gradient formulation to estimate important continuum-scale quantities such as lattice curvature and vorticity. These continuum metrics are calculated from atomic deformation fields produced in 2D and thin 3D equilibrium bicrystalline grain boundary structures under shear at 10 K. Results from these simulations show that interface structure strongly influences the resulting accommodation mechanisms under shear and deformation fields produced in the surrounding lattice. Calculating these continuum quantities at the nanoscale lends insight into localized and collective atomic behavior during shear deformation for various mechanisms, and it is shown that different mechanisms lead to differing behavior. Additionally, the results of these calculations can perhaps serve as an intermediary form to inform continuum models seeking to explore larger-scaled grain boundary deformation behavior in 3D, and to evaluate the veracity of continuum models that overlap the nanoscale.

Atomistic simulations are used to investigate how grain boundary structure influences dislocation nucleation under uniaxial tension and compression for a specific class of symmetric tilt grain boundaries that contain the E structural unit. After obtaining the minimum energy grain boundary structure, molecular dynamics was employed based on an embedded-atom method potential for copper at 10 K. Results show several differences in dislocation nucleation with respect to uniaxial tension and compression. First, the average nucleation stress for all 〈1 1 0〉 symmetric tilt grain boundaries is over three times greater in compression than in tension for both the high strain rate and quasistatic simulations. Second, partial dislocations nucleate from the boundary on the {1 1 1} slip plane under uniaxial tension. However, partial and full dislocations nucleate from the boundary on the {1 0 0} and {1 1 1} slip planes under uniaxial compression. The full dislocation nucleation on the {1 0 0} plane for boundaries with misorientations near the coherent twin boundary is explained through the higher resolved shear stress on the {1 0 0} plane compared to the {1 1 1} plane. Last, individual dislocation nucleation mechanisms under uniaxial tension and compression are analyzed. For the vicinal twin boundary under tension, the grain boundary partial dislocation is emitted into the lattice on the same {1 1 1} plane that it dissociated onto. For compression of the vicinal twin, the 1/3〈1 1 1 〉 disconnection is removed through full dislocation emission on the {1 0 0} plane and partial dislocation emission parallel to the coherent twin boundary plane, restoring the boundary to the coherent twin. For the Σ19 boundary, the nearly simultaneous emission of numerous partial dislocations from the boundary result in the formation of the hexagonal close-packed (HCP) phase.

Materials with characteristic structures at nanoscale sizes exhibit significantly different mechani-cal responses from those predicted by conventional, macroscopic continuum theory. For example,nanocrystalline metals display an inverse Hall-Petch effect whereby the strength of the materialdecreases with decreasing grain size. The origin of this effect is believed to be a change in defor-mation mechanisms from dislocation motion across grains and pileup at grain boundaries at mi-croscopic grain sizes to rotation of grains and deformation within grain boundary interface regionsfor nanostructured materials. These rotational defects are represented by the mathematical conceptof disclinations. The ability to capture these effects within continuum theory, thereby connectingnanoscale materials phenomena and macroscale behavior, has eluded the research community.The goal of our project was to develop a consistent theory to model both the evolution ofdisclinations and their kinetics. Additionally, we sought to develop approaches to extract contin-uum mechanical information from nanoscale structure to verify any developed continuum theorythat includes dislocation and disclination behavior. These approaches yield engineering-scale ex-pressions to quantify elastic and inelastic deformation in all varieties of materials, even those thatpossess highly directional bonding within their molecular structures such as liquid crystals, cova-lent ceramics, polymers and biological materials. This level of accuracy is critical for engineeringdesign and thermo-mechanical analysis is performed in micro- and nanosystems. The researchproposed here innovates on how these nanoscale deformation mechanisms should be incorporatedinto a continuum mechanical formulation, and provides the foundation upon which to develop ameans for predicting the performance of advanced engineering materials.4 AcknowledgmentThe authors acknowledge helpful discussions with Farid F. Abraham, Youping Chen, Terry J.Delph, Remi Dingreville, James W. Foulk III, Robert J. Hardy, Richard Lehoucq, Alejandro Mota,Gregory J. Wagner, Edmund B. Webb III and Xiaowang Zhou. Support for this project was pro-vided by the Enabling Predictive Simulation Investment Area of Sandia's Laboratory DirectedResearch and Development (LDRD) program.5

Atomistic simulations are used to investigate the structure and interfacial free volume of 〈110〉 symmetric tilt grain boundaries in copper containing the E structural unit from the Σ9(221)θ=141.1° grain boundary. In this work, a stereologically-based methodology is used to calculate the grain boundary free volume along with the spacing and connectivity of free volume. After generating the minimum energy equilibrium grain boundary, we examine (i) the grain boundary structure, (ii) a measure of free volume associated with the grain boundary, (iii) spatial correlation functions of the distribution of free volume, and (iv) images of grain boundary free volume distribution. Using the results from these calculations, the influence of free volume spatial distribution and grain boundary structure on dislocation dissociation and nucleation is briefly discussed for boundaries with the E structural unit subjected to tensile loading normal to the interface along with the potential implications of free volume connectivity.

Carbamazepine, mainly used as an anticonvulsant but also used for trigeminal neuralgias and other neuralgic pains, is now being used experimentally for affective disorders, nonresponsive psychoses, and dyscontrol. However, carbamazepine dosage must be carefully monitored because low initial doses are equivalent to higher later doses, and the drug's addition to a regimen of other drugs may increase carbamazepine's serum levels. Given alone to a pharmacologically naive inpatient, initial dosage of carbamazepine 200 mg/day can be increased by 100 mg every day or every second day; an outpatient can have the dosage increased by 100 mg every third day. Serum levels and side effects should be monitored. White cell counts usually decrease about 25%, but the decrease is not clinically related to the very rare occurrence of agranulocytosis. A side effect of concern is hepatic toxicity, but few such cases have been reported. The most common side effect is allergic rash, which occurs in about 5% of all patients receiving carbamazepine; antihistamines sometimes bring about a remission of the rash. Generic carbamazepine may cause more problems than Tegretol.