Alloying Element Segregation and Grain Boundary Reconstruction, Atomistic Modeling

Insights from symmetry: Improving machine-learned models for grain boundary segregation

The effect of grain boundaries (GBs) on deformation mechanisms becomes increasingly important as the volume of deformation reaches the submicrometer and nanoscale. The current work investigates the impact of grain boundaries on the incipient plasticity of small-scale deformations of fcc metals. For this purpose, the behavior of single and bi-crystal Au thin films during nanoindentation are studied, using large-scale atomistic simulations. Various symmetric ⟨110⟩ tilt GBs with a wide range of misorientation angles are included to analyze the effect of GB geometry on the nanoscale plasticity mechanisms. Potentially, GBs can act as a source, sink, or obstacle for lattice dislocation, depending on their geometry, energy level, and distance from the deformation zone. The role of the heterogeneous nucleation and emission of dislocations from GBs on the plasticity and hardness of bicrystals is analyzed. According to our results, the intrinsic free volume involved in the GB region is associated with dislocation nucleation at the GB. The volume of the plastic zone generated beneath the tip and the way it grows is strongly dependent on the GB structure. Dislocation nucleation occurs predominantly in the early stages of indentation at GBs with a dissociated interface structural unit, before the interaction of lattice dislocation and GB. Coherent twin boundaries display the lowest effect on the hardness. Based on our results, there is a strong correlation between the interfacial boundary energy and its effect on the bicrystal hardness. GBs with lower interfacial energy offer a higher barrier against slip transmission and nucleation at the GB.

Grain boundaries (GBs) have a profound impact on mechanical, chemical, and physical properties of polycrystalline materials. Comprehension of atomic and electronic structures of different GBs in materials can help to understand their impact on materials' properties. Here, with aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of a 90° twist GB s in CsPbBr3 is determined, and its impact on electron-hole pair separation is predicted. The 90° twist GB has a coherent interface and the same chemical composition as the bulk except for the lattice twist. Density functional theory (DFT) calculation results indicate that the twist GB has an electronic structure similar to that of the bulk CsPbBr3. An electronic potential at the GBs enhances the separation of photogenerated carriers and promotes the motion of electrons across the GBs. These results extend the understanding of atomic and electronic structure of GBs in halide perovskites and propose a potential strategy to eliminate the influence of GBs by GB engineering.

Zinc oxide (ZnO) is used for a wide range of electrical applications, where its grain boundaries (GBs) possibly influence the physical properties. It is important to determine the GB structure in atomic scale to understand GB effects on the electrical properties. In this study, the atomic structure of a ZnO GB is investigated in detail by scanning transmission electron microscopy and theoretical calculations. It is shown that the atomic structure of ZnO Σ13 [0001]/{130} 32.2° symmetric tilt GB is constructed as an array of structural units (SUs) of six‐ and eight‐membered rings. The GB has two different SU alignments. The dominant structure of the GB is the zig‐zag SU alignment and the secondary one is the straight SU alignment. The relation between the SU alignment and the rotation angle has been determined.

Misorientation-dependent transition between grain boundary migration and sliding in FCC metals

Atomistic investigation on grain boundary effect on helium segregation and clustering in iron

The zero-temperature energies and equilibrium volume expansions of point-defect free asymmetrical grain boundaries (GBs) involving the four densest planes in fcc bicrystals (with some higher-index plane on the other side of the interface) have been determined using an embedded-atom-method potential fitted to Au. It is found that the two asymmetrical tilt GBs at the endpoints of the related GB energy vs twist disorientation curves give rise to pronounced energy cusps. As for symmetrical GBs, a practically linear relationship between the GB energy and equilibrium volume expansion is observed. The volume expansion and the destruction of the perfect-crystal stacking at the GB are shown to cause a pronounced local decrease in the resistance toward shear parallel to the GB plane. 21 refs., 5 figs.

Dependence on grain boundary structure of radiation induced segregation in a 9 wt.% Cr model ferritic/martensitic steel

Segregation of H, C and B to Σ = 5 (0 1 3) α-Fe grain boundary: A theoretical study

Garnet-type Li7La3Zr2O12 (LLZO) and its derivatives are considered promising candidate materials for all-solid-state Li-ion batteries as solid electrolytes due to their high Li+ conductivities of around 10-4 S cm-1 at room temperature and wide electrochemical window. Despite the larger number of detailed studies utilizing various experimental and theoretical approaches, the experimentally evaluated Li+ conductivities did not precisely match the computationally predicted values. The discrepancy between the ion conductivities can be attributed to the ambivalent properties of grain boundaries (GBs). GB structure is a critical parameter that controls the macroscopic properties of materials. However, various technological limitations of both the experimental and theoretical approaches utilized for elucidating the Li+ conduction behavior of GBs have restricted our ability of reaching a deeper understanding of the processes occurring at the atomic level. In this study, we investigated the Li+conducting behaviors of various tilted GBs of cubic LLZO with a garnet framework by using molecular dynamics approaches. Nine stoichiometric equilibrium GB models were used to determine Li+conductivity. It was found that the GB conductivity was smaller than the bulk one regardless of the orientation. In particular, the conductivities perpendicular to the GBs were characterized by the lowest values. Some factors affecting Li conductivity near the tilted GBs may be related to their stability. The relationship between the structural distortions at the GBs and the Li+ conducting characteristics were evaluated in terms of the GB energies and radial distribution function for the La-La, Zr-Zr, and O-O interactions in the LLZO bulk and GBs. We found that the Li+ conductivities are highly correlate with the structural distortions at the GBs. In order to elucidate the Li+ diffusion characteristics at the GBs in more detail, the variations of the atomic concentration near the GBs were calculated along the axis perpendicular to the GB surface. The atomic population at bulk region was constantly modulated, while that at GB region was anomalously dispersed. The average atomic concentration at the GBs was lower than that at the bulk. In particular, the observed decrease in the Li+ concentration suggests the formation of Li-deficient sites across the GB center, which represents the primary reason for the degraded ionic conductivity in the GB region. In fact, the Li ionic conductivity perpendicular to the GBs decreased with decreasing Li+ concentration at the GBs, which was caused by the slower diffusion of Li+. Thus, preventing the distortion of the GB crystallographic structure and the related drop in the Li+concentration should be considered one of the top priorities when designing GBs with desirable properties. Acknowledgement This work was partially supported by CREST, JST Grant Number JPMJCR1322, Japan.

On the structure, stress fields and energy of nonequilibrium grain boundaries

Ni/Al nanolaminates are reactive materials with customizable combustion characteristics. A common approach to synthesize the repeating Ni and Al nanolayers is physical vapor deposition, which often results in columnar grains with ⟨111⟩ texture and grain diameters on the order of a single layer thickness. Changes in grain size have been reported to affect combustion rates, yet the role of individual grain boundaries (GBs) on this process is unclear. Thus, this work investigates the role of the GB structure on atomic diffusion/dissolution and the resulting combustion reaction via molecular dynamics simulations. Nanolaminate combustion is simulated in bicrystal models containing columnar symmetric tilt GBs with ⟨111⟩ misorientation axis perpendicular to the Ni/Al interface. A range of GB misorientation angles is studied, and combustion in a Ni/Al nanolaminate without GBs is simulated for comparison. Combustion in bicrystal models reveals a rise in temperature with an exponential form prior to complete Al melting, while the model without GBs shows a linear temperature increase. Diffusion coefficients are measured for each bicrystal model, and separate Arrhenius fits are used to identify the first three combustion stages. Models containing higher energy GBs generally have higher diffusion coefficients and lower activation energies prior to complete melting of Al, while the GB structure shows little effect on dissolution after the Al layer melts. Thus, the GB structure plays a key role in Ni/Al nanolaminate ignition sensitivity but does not impact runaway combustion.

Grain boundaries (GBs) in two-dimensional (2D) materialsare knownto dramatically impact material properties ranging from the physical,chemical, mechanical, electronic, and optical, to name a few. Predictinga range of physically realistic GB structures for 2D materials iscritical to exercising control over their properties. This, however,is nontrivial given the vast structural and configurational (defect)search space between lateral 2D sheets with varying misfits. Here,in a departure from traditional evolutionary search methods, we introducea workflow that combines the Graph Neural Network (GNN) and an evolutionaryalgorithm for the discovery and design of novel 2D lateral interfaces.We use a representative 2D material, blue phosphorene (BP), and identify2D GB structures to test the efficacy of our GNN model. The GNN wastrained with a computationally inexpensive machine learning bond orderpotential (Tersoff formalism) and density functional theory (DFT).Systematic downsampling of the training data sets indicates that ourmodel can predict structural energy under 0.5% mean absolute errorwith sparse (<2000) DFT generated energy labels for training. Wefurther couple the GNN model with a multiobjective genetic algorithm(MOGA) and demonstrate strong accuracy in the ability of the GNN topredict GBs. Our method is generalizable, is material agnostic, andis anticipated to accelerate the discovery of 2D GB structures.

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

We analyze recent results of atomistic computer simulations of grain boundary (GB) diffusion in metals. At temperatures well below the bulk melting point T m GB diffusion occurs by random walk of individual vacancies and self-interstitials. Both defects are equal participants in the diffusion process and can move by a large variety of diffusion mechanisms, many of which are collective transitions. GB diffusion coefficients can be computed by kinetic Monte Carlo simulations. At high temperatures, the presence of large concentrations of point defects is likely to alter the diffusion mechanisms. Molecular dynamics simulations of GB structure and diffusion in copper reveal a continuous GB premelting in close vicinity of T m . However, diffusion in high-energy GBs becomes almost independent of the GB structure (“universal”) at temperatures well below T m . This behavior can be tentatively explained in terms of heterophase fluctuations from the solid to the liquid phase. The exact diffusion mechanisms in the presence of heterophase fluctuations are yet to be established.