Effect of interface dislocation Burgers vectors on elastic fields in anisotropic bicrystals

A recent anisotropic elasticity formalism for quantifying interface dislocation arrays is used to compute the variations in long-range elastic fields and short-range strain energies of misfit dislocations due to changes in the Burgers vectors of the interface dislocations. The importance of selecting proper reference states for tilt and twist grain boundaries as well as a heterophase interface formed by tetragonal crystals is discussed in terms of partitioning of strain and rotation fields. For constrained interfaces consistent with the Frank–Bilby equation, the present work shows that examining the strain energies of different admissible dislocation configurations may be used as a criterion for predicting the most favorable structures.National Science Foundation (U.S.) (NSF Grant No. 1150862

Structure, shear resistance and interaction with point defects of interfaces in Cu–Nb nanocomposites synthesized by severe plastic deformation

Atomistic modeling is used to investigate the shear resistance and interaction with point defects of a Cu–Nb interface found in nanocomposites synthesized by severe plastic deformation. The shear resistance of this interface is highly anisotropic: in one direction shearing occurs at stresses <1200 MPa, while in the other it does not occur at all. The binding energy of vacancies, interstitials and He impurities to this interface depends sensitively on the binding location, but there is no point defect delocalization, nor does this interface contain any constitutional defects. These behaviors are markedly dissimilar from a different Cu–Nb interface found in magnetron sputtered composites. The dissimilarities may, however, be explained by quantitative differences in the detailed structure of these two interfaces.MISTI-France Seed Fun

Non-coherent Cu grain boundaries driven by continuous vacancy loading

We use atomistic modeling to study the response of three non-coherent grain boundaries (GBs) in Cu to continuous loading with vacancies. Our simulations yield insights into the structure and properties of these boundaries both near and far from thermal equilibrium. We find that GB energies vary periodically as a function of the number of vacancies introduced. Each GB has a characteristic minimum energy state that recurs during continuous vacancy loading, but in general cannot be reached without removing atoms from the boundary. There is no clear correlation of GB energies with GB specific excess volumes or stresses during vacancy loading. However, GB stresses increase monotonically with specific excess volumes. Continuous vacancy loading gives rise to GB migration and shearing, despite the absence of applied loads. Successive vacancies introduced into some of the boundaries accumulate at the cores of what appear to be generalized vacancy dislocation loops. We discuss the implications of these findings for our understanding of grain boundary sink efficiencies under light ion irradiation.United States. Dept. of Energy. Office of Basic Energy Sciences. Center for Materials in Irradiation and Mechanical Extremes (Award 2008LANL1026

Coarsening by network restructuring in model nanoporous gold

Using atomistic modeling, we show that restructuring of the network of interconnected ligaments causes coarsening in a model of nanoporous gold. The restructuring arises from the collapse of some ligaments onto neighboring ones and is enabled by localized plasticity at ligaments and nodes. This mechanism may explain the occurrence of enclosed voids and reduction in volume in nanoporous metals during their synthesis. An expression is developed for the critical ligament radius below which coarsening by network restructuring may occur spontaneously, setting a lower limit to the ligament dimensions of nanofoams

Determining the Burgers vectors and elastic strain energies of interface dislocation arrays using anisotropic elasticity theory

A formalism for describing interface dislocation arrays linking the Frank–Bilby equation and anisotropic elasticity theory under the condition of vanishing far-field stresses is developed. The present approach enables the determination of a unique reference state for interface misfit dislocations, within which the Burgers vectors of individual dislocations are defined and allows for the unequal partitioning of elastic fields between neighboring crystals. The elastic strain energies of interface dislocation arrays are computed using solutions for short-range elastic fields. Examples of applications to simple interfaces are given, namely symmetric tilt and twist grain boundaries, as well as a pure misfit heterophase interface.United States. Dept. of Energy. Office of Basic Energy Sciences (Award 2008LANL1026)National Science Foundation (U.S.) (Grant 1150862

Point defect stability in a semicoherent metallic interface

We present a comprehensive density functional theory (DFT) -based study of different aspects of one vacancy and He impurity atom behavior at semicoherent interfaces between the low-solubility transition metals Cu and Nb. Such interfaces have not been previously modeled using DFT. A thorough analysis of the stability and mobility of the two types of defects at the interfaces and neighboring internal layers has been performed and the results have been compared to the equivalent cases in the pure metallic matrices. The different behavior of fcc and bcc metals on both sides of the interface has been specifically assessed. The modeling effort undertaken is the first attempt to study the stability and defect energetics of noncoherent Cu/Nb interfaces from first principles, in order to assess their potential use in radiation-resistant materials.Seventh Framework Programme (European Commission) (Project RAD-INTERFACES)Spain. Ministerio de Economia y Competividad (Project NANO-EXTREM, Ref. MAT2012-38541)United States. Dept. of Energy. Office of Basic Energy Sciences (Award 2008LANL1026

Bayesian inference of substrate properties from film behavior

We demonstrate that by observing the behavior of a film deposited on a substrate, certain features of the substrate may be inferred with quantified uncertainty using Bayesian methods. We carry out this demonstration on an illustrative film/substrate model where the substrate is a Gaussian random field and the film is a two-component mixture that obeys the Cahn–Hilliard equation. We construct a stochastic reduced order model to describe the film/substrate interaction and use it to infer substrate properties from film behavior. This quantitative inference strategy may be adapted to other film/substrate systems.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0008926

Mechanisms of plastic deformation in amorphous silicon by atomistic simulation using the Stillinger-Weber potential

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 205-212).Molecular dynamics simulation of amorphous silicon (a-Si) using the Stillinger- Weber potential reveals the existence of two distinct atomic environments: one solidlike and the other liquidlike. The mechanical behavior of a-Si when plastically deformed to large strain can be completely described by the mass fraction [phi] of liquidlike material in it. Specifically, samples with higher [phi] are more amenable to plastic flow, indicating that liquidlike atomic environments act as plasticity "carriers" in a-Si. When deformed under constant pressure, all a-Si samples converge to a unique value of [phi] characteristic of steady state flow. Discrete stress relaxations were found to be the source of low-temperature plastic flow in a-Si in deformation simulations by potential energy minimization. These relaxations are triggered when a local yielding criterion is satisfied in a small cluster of atoms. The atomic rearrangements accompanying discrete stress relaxations are describable as autocatalytic avalanches of unit shearing events. Every such unit event centers on a clearly identifiable change in bond length between the two split peaks of the second nearest neighbor shell in the radial distribution function (RDF) of bulk a-Si in steady-state low.by Michael J. Demkowicz.Ph.D

Computational design of patterned interfaces using reduced order models

Patterning is a familiar approach for imparting novel functionalities to free surfaces. We extend the patterning paradigm to interfaces between crystalline solids. Many interfaces have non-uniform internal structures comprised of misfit dislocations, which in turn govern interface properties. We develop and validate a computational strategy for designing interfaces with controlled misfit dislocation patterns by tailoring interface crystallography and composition. Our approach relies on a novel method for predicting the internal structure of interfaces: rather than obtaining it from resource-intensive atomistic simulations, we compute it using an efficient reduced order model based on anisotropic elasticity theory. Moreover, our strategy incorporates interface synthesis as a constraint on the design process. As an illustration, we apply our approach to the design of interfaces with rapid, 1-D point defect diffusion. Patterned interfaces may be integrated into the microstructure of composite materials, markedly improving performance.United States. Dept. of Energy. Office of Basic Energy Sciences (Award 2008LANL1026)National Science Foundation (U.S.) (Grant 1150862

Phenomenology and kinematics of discrete plastic deformation events in amorphous silicon : atomistic simulation using the Stillinger-Weber potential

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 62-64).The need to understand plastic deformation in amorphous covalently bonded materials arose from the unique mechanical properties of disordered intergranular layers in nc-TiN/a-Si₃N₄ ceramic composites. Silicon was chosen as a model disordered network solid for the purpose of conducting feasible atomistic computer simulations of plastic deformation. Amorphous silicon structures were created by melting and quenching using a molecular dynamics algorithm. These structured were plastically deformed by conjugate gradient static energy minimization. Atomic level analysis was carried out using appropriately generalized notions of stress and strain. Plastic deformation was found to occur in a series of discrete stress relaxations, each one of which was accompanied by a well localized atomic level rearrangement. The transforming regions were roughly ellipsoidal in shape and involved the cooperative motion 100-500 atoms spanning a length scale of 0.7-2.5nm. This length scale is large in comparison to the typical thickness of disordered intergranular layers in nanocrystalline ceramic composites, indicating that the plastic relaxation process in such intergranular layers cannot be the same as the one found in bulk amorphous covalent solids.by Michael J. Demkowicz.S.M