Showing papers by "David J. Srolovitz published in 2013"

Large-scale molecular dynamics simulations of wear in diamond-like carbon at the nanoscale

Abstract: We perform large-scale molecular dynamics simulations on diamond-like carbon to study wear mechanism and law at the nanoscale. Our simulations show that material loss during sliding varies linearly with normal load and sliding distance, consistent with Archard's law. Our simulations also show that the number of chemical bonds across the contact interface during sliding correlates well with friction force, but not with material loss, indicating that friction and wear follow different mechanisms. Our analysis reveals the following wear mechanism: the shear traction causes mass accumulation at the trailing end of contact, which is then lost by a cluster detachment process.

Effect of strain on the stacking fault energy of copper: A first-principles study

Abstract: The intrinsic stacking fault energy (SFE) of copper under volumetric, longitudinal, and shear strains is investigated using density functional theory (GGA-PBE). Calculations are performed using a copper slab model aligned perpendicular to the (111) intrinsic stacking fault plane. The calculated SFE for unstrained copper is \ensuremath{\gamma} $=$ 41 mJ/m${}^{2}$. Results show a strong dependence of \ensuremath{\gamma} on strain and distinct behavior for different types of strain: (a) volumetric and longitudinal in the direction perpendicular to the stacking fault, (b) longitudinal parallel to the stacking fault, and (c) shear parallel to the stacking fault. In the first case (a), the SFE decreases monotonically with strain with a slope $d$\ensuremath{\gamma}/$d$\ensuremath{\epsilon}${|}_{\ensuremath{\varepsilon}=0}$ $=$ \ensuremath{-}0.44 J/m${}^{2}$ and \ensuremath{-}0.87 J/m${}^{2}$ for volumetric and longitudinal, respectively, and with ${d}^{2}$\ensuremath{\gamma}/$d$\ensuremath{\epsilon}${}^{2}$ g 0. In contrast, for longitudinal strain parallel to the stacking fault (b), the SFE dependence exhibits ${d}^{2}$\ensuremath{\gamma}/$d$\ensuremath{\epsilon}${}^{2}$ 0 with a maximum at \ensuremath{\epsilon} \ensuremath{\approx} \ensuremath{-}0.015. For the case of shear parallel to the stacking fault (c), the SFE is nearly constant at small and moderately large strain, but drops rapidly at very large strain (by a factor of 1/3 for $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}{111}$ shear at \ensuremath{\epsilon} $=$ \ifmmode\pm\else\textpm\fi{}0.1). For large $\ensuremath{\langle}11\overline{2}\ensuremath{\rangle}{111}$ shear strains, the SFE can either increase or decrease at large strain depending on the sign of the strain. In volumetric or longitudinal (perpendicular to the stacking fault) tension and longitudinal strain in the boundary plane (and for some shear directions), the SFE can become negative, implying a limit on the stability of the fcc crystal structure. The strong dependence of the SFE on strain suggests deep implications for the mechanical properties, microstructural evolution, and dynamic plasticity of metals at high pressure, during severe plastic deformation, and in shock-loading conditions.

Microstructure versus Flaw: Mechanisms of Failure and Strength in Nanostructures

Abstract: Understanding failure in nanomaterials is critical for the design of reliable structural materials and small-scale devices with nanoscale components. No consensus exists on the effect of flaws on fracture at the nanoscale, but proposed theories include nanoscale flaw tolerance and maintaining macroscopic fracture relationships at the nanoscale with scarce experimental support. We explore fracture in nanomaterials using nanocrystalline Pt nanocylinders with prefabricated surface notches created using a "paused" electroplating method. In situ scanning electron microscopy (SEM) tension tests demonstrate that the majority of these samples failed at the notches, but that tensile failure strength is independent of whether failure occurred at or away from the flaw. Molecular dynamics simulations verify these findings and show that local plasticity is able to reduce stress concentration ahead of the notch to levels comparable with the strengths of microstructural features (e.g., grain boundaries). Thus, failure occurs at the stress concentration with the highest local stress whether this is at the notch or a microstructural feature.

Comparison of molecular dynamics simulation methods for the study of grain boundary migration

Abstract: In the present study, grain boundary (GB) mobility was determined by molecular dynamics (MD) simulations using two different techniques: the applied strain method and the adapted interface random walk method. The first method involves a driving force while the second method does not. Nevertheless, both methods led to essentially the same values of the GB mobility. This shows that the GB mobility is independent of the nature of the driving force, provided that it is low enough that the linear velocity?driving force relationship is properly sampled. The case studied here can be viewed as a validated reference case that can be used in future studies to test new techniques to determine the GB mobility. For this purpose we provide the full information about the interatomic potential we employed and the initial atomic configurations. Finally, we use the obtained results to discuss whether any existing MD simulation data agree with experimental data on pure metals.

Statistical topology of three-dimensional Poisson-Voronoi cells and cell boundary networks.

Abstract: Voronoi tessellations of Poisson point processes are widely used for modeling many types of physical and biological systems. In this paper, we analyze simulated Poisson-Voronoi structures containing a total of 250000000 cells to provide topological and geometrical statistics of this important class of networks. We also report correlations between some of these topological and geometrical measures. Using these results, we are able to corroborate several conjectures regarding the properties of three-dimensional Poisson-Voronoi networks and refute others. In many cases, we provide accurate fits to these data to aid further analysis. We also demonstrate that topological measures represent powerful tools for describing cellular networks and for distinguishing among different types of networks.

Manipulating Ferroelectric Domains in Nanostructures Under Electron Beams

Abstract: Freestanding BaTiO3 nanodots exhibit domain structures characterized by distinct quadrants of ferroelastic 90° domains in transmission electron microscopy (TEM) observations. These differ significantly from flux-closure domain patterns in the same systems imaged by piezoresponse force microscopy. Based upon a series of phase field simulations of BaTiO3 nanodots, we suggest that the TEM patterns result from a radial electric field arising from electron beam charging of the nanodot. For sufficiently large charging, this converts flux-closure domain patterns to quadrant patterns with radial net polarizations. Not only does this explain the puzzling patterns that have been observed in TEM studies of ferroelectric nanodots, but also suggests how to manipulate ferroelectric domain patterns via electron beams.

Atomic-scale analysis of liquid-gallium embrittlement of aluminum grain boundaries

Abstract: In this work, we explore the role of atomistic-scale energetics on liquid-metal embrittlement of Al due to Ga. Ab initio and molecular mechanics were employed to probe the binding energies of vacancies and segregation energies of Ga for , and STGBs in Al. We found that the GB local arrangements and resulting structural units have a significant influence on the magnitude of vacancy binding energies. For example, the mean vacancy binding energy for , , and STGBs at 1st layer was found to be -0.63 eV, -0.26 eV, and -0.60 eV. However, some GBs exhibited vacancy binding energies closer to bulk values, indicating interfaces with zero sink strength, i.e., these GBs may not provide effective pathways for vacancy diffusion. The results from the present work showed that the GB structure and the associated free volume also play significant roles in Ga segregation and the subsequent embrittlement of Al. The Ga mean segregation energy for , and STGBs at 1st layer was found to be -0.23 eV, -0.12 eV and -0.24 eV, respectively, suggesting a stronger correlation between the GB structural unit, its free volume, and segregation behavior. Furthermore, as the GB free volume increased, the difference in segregation energies between the 1st layer and the 0th layer increased. Thus, the GB character and free volume provide an important key to understanding the degree of anisotropy in various systems. The overall characteristic Ga absorption length scale was found to be about ~10, 8, and 12 layers for , , and STGBs, respectively. Also, a few GBs of different tilt axes with relatively high segregation energies (between 0 and -0.1 eV) at the boundary were also found. This finding provides a new atomistic perspective to the GB engineering of materials with smart GB networks to mitigate or control LME and more general embrittlement phenomena in alloys.

Deconstructing the high-temperature deformation of phase-separating alloys

Abstract: At high temperatures, a microstructure evolves in order to lower the energy (including interfacial and elastic) of the system. Microstructure evolution can be influenced by applied loads if the elastic constants are anisotropic and/or inhomogeneous. When plastic deformation occurs during microstructure coarsening (e.g., under creep conditions), dislocations modify microstructure evolution (e.g., through relaxing misfit and conversion of interfaces from coherent to semicoherent) and microstructure evolution leads to changes in the plastic deformation behavior. Here, we employ phase field simulations to examine the interplay between plasticity, phase separation and microstructural coarsening. In particular, we separately control microstructure evolution, stress effects and plastic deformation in order to deconstruct the observed deformation behavior. We show that in the absence of an applied stress, the alloy with dislocation sources coarsens more quickly than that without and that the presence of dislocations reorients two-phase interfaces. A comparison of the stress‐strain curves for alloys with microstructure that evolves during deformation with those for which the microstructure is static shows that simultaneous microstructure evolution leads to (1) lower effective elastic moduli, (2) a peak in the stress‐strain curve (it is monotonic in the absence of microstructural evolution) and (3) lower large-strain flow stresses. The decreaseinelasticmodulusistheresultofthereorientationofthemicrostructure with time (the two phases have different stiffnesses). We elucidate the microstructural sources of these changes. (Some figures may appear in colour only in the online journal)

Flaw-driven Failure in Nanostructures

Abstract: Understanding failure in nanomaterials is critical for the design of reliable structural materials and small-scale devices that have components or microstructural elements at the nanometer length scale. No consensus exists on the effect of flaws on fracture in bulk nanostructured materials or in nanostructures. Proposed theories include nanoscale flaw tolerance and maintaining macroscopic fracture relationships at the nanoscale with virtually no experimental support. We explore fracture mechanisms in nanomaterials via nanomechanical experiments on nanostructures with pre-fabricated surface flaws in combination with molecular dynamics simulations. Nanocrystalline Pt cylinders with diameters of ~120 nm with intentionally introduced surface notches were created using a template-assisted electroplating method and tested in uniaxial tension in in-situ SEM. Experiments demonstrate that 8 out of 12 samples failed at the notches and that tensile failure strengths were ~1.8 GPa regardless of whether failure occurred at or away from the flaw. These findings suggest that failure location was sensitive to the presence of flaws, while strength was flaw-insensitive. Molecular dynamics simulations support these observations and show that incipient plastic deformation commences via nucleation and motion of dislocations in concert with grain boundary sliding. We postulate that such local plasticity reduces stress concentration ahead of the flaw to levels comparable with the strengths of intrinsic microstructural features like grain boundary triple junctions, a phenomenon unique to nano-scale solids that contain an internal microstructural energy landscape. This mechanism causes failure to occur at the weakest link, be it an internal inhomogeneity or a surface feature with a high local stress.

Role of atomistic-scale energetics on liquid-metal embrittlement of aluminum due to gallium

Abstract: In this work, the role of atomistic-scale energetics on liquid-metal embrittlement of Al due to Ga was explored. Ab initio and molecular mechanics were employed to probe the formation energies of vacancies and segregation energies of Ga for , and STGBs in Al. We found that the GB local structural units (SUs) have a significant influence on the magnitude of vacancy formation energies. For example, the mean vacancy formation energy for , , and STGBs at 1st layer was found to be 0.25, 0.62, and 0.28 eV. However, some GBs exhibited vacancy formation energies closer to bulk values, indicating interfaces with zero sink strength, i.e., these GBs may not provide effective pathways for vacancy diffusion. The results from the present work showed that the GB structure and the associated free volume also play significant roles in Ga segregation and the subsequent embrittlement of Al. The Ga mean segregation energy for , and STGBs at 1st layer was found to be -0.23, -0.12 and -0.24 eV, respectively, suggesting a stronger correlation between the GB SU, its free volume, and segregation behavior. Furthermore, as the GB free volume increased, the difference in segregation energies between the 1st layer and the 0th layer increased. Thus, the GB character and free volume provide an important key to understanding the degree of anisotropy in various systems. The overall characteristic Ga absorption length scale was found to be about ~5, 6, and 9 layers for , , and STGBs, respectively, at which the mean segregation energies approach the bulk values. Also, a few GBs of different tilt axes with relatively high segregation energies at the boundary were also found. This finding provides a new atomistic perspective to the GB engineering of materials with smart GB networks to mitigate or control LME and more general embrittlement phenomena in alloys.