Garritt J. Tucker

Spectral neighbor analysis method for automated generation of quantum-accurate interatomic potentials

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.

Shear deformation kinematics of bicrystalline grain boundaries in atomistic simulations

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.

The mechanical behavior and deformation of bicrystalline nanowires

The competition between free surfaces and internal grain boundaries as preferential sites for dislocation nucleation during plastic deformation in aluminum bicrystalline nanowires is investigated using molecular dynamics simulations at room temperature. A number of nanowires containing various minimum energy interfaces are studied under uniaxial compression at a constant applied strain rate to provide a broad, inclusive look at the competition between the two types of sources. In addition, we conduct a detailed study on the role of the grain boundaries to act as a source, sink, or obstacle for lattice dislocations, as a function of grain boundary structure. This work compares the behavior of bicrystalline nanowires containing both random high-angle boundaries and a series of symmetric tilt grain boundaries to further elucidate the effect of interface structure on its behavior. The results show that grain boundaries in nanowires can be preferred nucleation sites for dislocations and twin boundaries, in addition to efficient sinks and pinning points for migrating dislocations. Plastic deformation behavior at high imposed strains is linked to the underlying deformation processes, such as twinning, dislocation pinning, or dislocation exhaustion/starvation. We also detail some important reactions between lattice dislocations and grain boundaries observed in the simulations, along with the activation of a single-arm source. This work suggests that the cooperation of numerous mechanisms and the structure of internal grain boundaries are crucial in understanding the deformation of bicrystalline nanowires.

Evidence for Bulk Ripplocations in Layered Solids.

Plastically anisotropic/layered solids are ubiquitous in nature and understanding how they deform is crucial in geology, nuclear engineering, microelectronics, among other fields. Recently, a new defect termed a ripplocation–best described as an atomic scale ripple–was proposed to explain deformation in two-dimensional solids. Herein, we leverage atomistic simulations of graphite to extend the ripplocation idea to bulk layered solids, and confirm that it is essentially a buckling phenomenon. In contrast to dislocations, bulk ripplocations have no Burgers vector and no polarity. In graphite, ripplocations are attracted to other ripplocations, both within the same, and on adjacent layers, the latter resulting in kink boundaries. Furthermore, we present transmission electron microscopy evidence consistent with the existence of bulk ripplocations in Ti3SiC2. Ripplocations are a topological imperative, as they allow atomic layers to glide relative to each other without breaking the in-plane bonds. A more complete understanding of their mechanics and behavior is critically important, and could profoundly influence our current understanding of how graphite, layered silicates, the MAX phases, and many other plastically anisotropic/layered solids, deform and accommodate strain.

Atomistic simulations of dislocation pinning points in pure face-centered-cubic nanopillars

Single arm sources have been utilized to explain source-dependent strength in a variety of small-scale structures and have been observed to control plasticity in experiments. In this work, we investigate the stability of single arm sources using molecular dynamics focusing on Lomer?Cottrell dislocations as pinning points. We show that these segments are not stable enough to create static pinning points. We also show that some artificially created pinning points can act as stable sources, which allows us to investigate the strength of single arm sources and their dynamics in nanopillars. Finally, we show using constant strain rate molecular dynamics that single arm sources can be created and destroyed by interacting dislocations nucleated from free surfaces and grain boundaries.

Molecular dynamics studies of defect formation during heteroepitaxial growth of InGaN alloys on (0001) GaN surfaces

We investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and ([Formula: see text]) 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 In x Ga 1-x N-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 ([Formula: see text]) 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.

Achieving Radiation Tolerance through Non-Equilibrium Grain Boundary Structures

Many methods used to produce nanocrystalline (NC) materials leave behind non-equilibrium grain boundaries (GBs) containing excess free volume and higher energy than their equilibrium counterparts with identical 5 degrees of freedom. Since non-equilibrium GBs have increased amounts of both strain and free volume, these boundaries may act as more efficient sinks for the excess interstitials and vacancies produced in a material under irradiation as compared to equilibrium GBs. The relative sink strengths of equilibrium and non-equilibrium GBs were explored by comparing the behavior of annealed (equilibrium) and as-deposited (non-equilibrium) NC iron films on irradiation. These results were coupled with atomistic simulations to better reveal the underlying processes occurring on timescales too short to capture using in situ TEM. After irradiation, NC iron with non-equilibrium GBs contains both a smaller number density of defect clusters and a smaller average defect cluster size. Simulations showed that excess free volume contribute to a decreased survival rate of point defects in cascades occurring adjacent to the GB and that these boundaries undergo less dramatic changes in structure upon irradiation. These results suggest that non-equilibrium GBs act as more efficient sinks for defects and could be utilized to create more radiation tolerant materials in future.

Simulated defect growth avalanches during deformation of nanocrystalline copper

In this work we introduce a method to capture the proliferation of material defects that carry inelastic deformation, in microstructures simulated through isobaric–isothermal molecular dynamics. Based on the premise that inelastic dissipation is accompanied by a local temperature rise, our method involves analyzing the response of a chain of Nosé–Hoover thermostats that are coupled to the atomic velocities, while the microstructure deforms under the influence of a ramped external stress. We report results obtained from the uniaxial deformation of two nanocrystalline copper microstructures and show that our analysis allows the dissipative signal of a variety of inelastic events to be effectively unified via an ‘avalanche’ of dissipation. Based on this avalanche, we quantitatively compare dissipation for inelastic deformation under tension vs. compression, observing a significant tension–compression asymmetry in this regard. It is concluded that the present method is useful for discerning critical points that correspond to collective yield and inelastic flow.

Introduction to Atomistic Simulation Methods

In this chapter we give a synopsis of classical simulation methods for atomic and molecular systems. We discuss the fundamental principles and empirical potentials underlying molecular statics and dynamics. We also introduce the connection to statistical mechanics and the estimation of macroscale material properties. In addition to theoretical aspects of atomistic simulation methods, we provide an overview of practical aspects, and the tools and simulation packages that are currently available.

Continuum Metrics for Atomistic Simulation Analysis

Atomistic modeling and simulations have become an invaluable tool for both the Materials Science and Mechanics communities to study and understand the nanoscale structure–property relationships in materials. Many of these studies focus specifically on the nanomechanics and fundamental deformation mechanisms that influence materials properties. However, the usefulness and impact of these simulations to influence the scientific community and materials design are ultimately hinged on the information extracted from the model or simulation. As such, a new and emerging focus in the atomistic modeling community is to develop more innovative avenues to produce more meaningful data. It is this goal that motivates the current chapter. This chapter presents an overview of a novel toolset for atomistic modeling and simulations to more effectively quantify the fundamental kinematics of atomistic structures. These new metrics are based on well-established theory from continuum mechanics, but provide unique insight into atomic-level processes that underly the nanomechanics of materials.