Minor impurities can cause catastrophic fracture of normally ductile metals. Here, a classic example is represented by the sulfur embrittlement of nickel, whose atomic-level mechanism has puzzled researchers for nearly a century. In this study, coupled aberration-corrected electron microscopy and semi-grand-canonical-ensemble atomistic simulation reveal, unexpectedly, the universal formation of amorphous-like and bilayer-like facets at the same general grain boundaries. Challenging the traditional view, the orientation of the lower-Miller-index grain surface, instead of the misorientation, dictates the interfacial structure. We also find partial bipolar structural orders in both amorphous-like and bilayer-like complexions (a.k.a. thermodynamically two-dimensional interfacial phases), which cause brittle intergranular fracture. Such bipolar, yet largely disordered, complexions can exist in and affect the properties of various other materials. Beyond the embrittlement mechanism, this study provides deeper insight to better understand abnormal grain growth in sulfur-doped Ni, and generally enriches our fundamental understanding of performance-limiting and more disordered interfaces.

First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric ∑5 (210) tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid Monte Carlo and molecular dynamics simulations. Density functional theory calculations confirm this prediction. This first-order coupled structural and adsorption transformation, which produces two variants of asymmetric bilayers, vanishes at an interfacial critical point. A GB complexion (phase) diagram is constructed via semigrand canonical ensemble atomistic simulations for the first time.

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Nacre has attracted widespread interest because its unique hierarchical structure, which is assembled by 95 wt% brittle aragonite and 5 wt% soft organic materials, leads to several orders of improvement in fracture toughness. Apart from the well proposed toughening mechanisms such as mineral bridges and tablets interlocks, the organic materials including biopolymers between tablets and proteins exist within a tablet can also potentially improve the toughness. In this work, we employ a novel approach combining steered molecular dynamics (SMD) and classical molecular dynamics (MD) to build a model of mineral-protein composite to mimic nacre tablet. The critical role of protein in improving the fracture toughness of nacre is investigated for the first time. MD simulations of single crystalline aragonite, polycrystalline aragonite and mineral-protein composite under uniaxial tensile loading are performed, and the obtained constitutive responses are compared with experimental measurements of nacre under tension. It is shown that the fracture toughness of mineral-protein composite is significantly larger than that of single crystalline or polycrystalline aragonite. Detailed atomic configuration analyses reveal that the fracture of individual computer model is governed by its unique failure mechanisms. Dislocation motion and phase transformation are observed during the failure of single crystalline aragonite. Polycrystalline aragonite fails by the inter-granular cleavage, as well as phase transformation within grain. It is surprisingly noted that other than the stretching of protein chains on grain boundaries, intra-granular fracture is triggered in mineral-protein composites. Proteins serve as strong glue between the inorganic nanograins. It is believed that the strong electrostatic interaction between protein and aragonite nanograins, combined with the remarkable plastic ductility of protein lead to the intra-granular failure, which consequently enhance the fracture toughness of the whole specimen.

2017-08-25T16:30:33

This paper presents the new development of a concurrent atomistic-continuum (CAC) method in simulation of the dynamic evolution of defects in polycrystalline polyatomic materials. The CAC method is based on a theoretical formulation that extends Kirkwood's statistical mechanical theory of transport processes to a multiscale description of crystalline materials. It solves for both the deformation of lattice cells and the internal deformation within each lattice cell, making it a suitable method for simulations of polyatomic materials. The simulation results of this work demonstrate that CAC can simulate the nucleation of dislocations and cracks from atomistically resolved grain boundary (GB) regions and the subsequent propagation into coarsely meshed grain interiors in polycrystalline strontium titanate without the need of supplemental constitutive equations or additional numerical treatments. With a significantly reduced computational cost, CAC predicts not only the GB structures, but also the dynamic behaviour of dislocations, cracks and GBs, all of which are comparable with those obtained from atomic-level molecular dynamics simulations. Simulation results also show that dislocations tend to initiate from GBs and triple junctions. The angle between the slip planes and the GB planes plays a key role in determining the GB-dislocation reactions.

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO3. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO3 contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

Atomistic simulations have been used to study the deformation mechanisms of nanocrystalline pure Al and Al-Mg binary alloys. Voronoi tessellation was used to fully create a three dimensional polycrystalline model with a grain size of 10 nm, while hybrid Monte Carlo and molecular dynamic simulations were used to achieve both mechanical and chemical equilibrium in nanocrystalline Al-5 at. % Mg. The results of tensile tests show an improved strength, including the yield strength and ultimate strength, through doping 5 at.% Mg into nanocrystalline aluminum. The results of atomic structures clearly reveal the multiple strengthening mechanisms related to doping in Al-Mg alloys. At the early deformation stage, up to an applied strain of 0.2, the strengthening mechanism of dopants exhibits as dopant pinning grain boundary (GB) migration. However, at the late deformation stage, which is close to failure of nanocrystalline materials, dopants can prohibit the initiation of intergranular cracks and also impede propagation of existing cracks along the GBs, thus improving the flow stress of Al-Mg alloy.