Grain Boundary Systems Research Articles

A new compositional microscopic degree of freedom at grain boundaries in complex compounds: a case study in spinel.

The accurate computational treatment of polycrystalline materials requires the rigorous generation of grain boundary (GB) structures as many quantities of interest depend strongly on the specifics of the macroscopic and microscopic degrees of freedom (DoFs) used in their creation. In complex materials, containing multiple sublattices and where atomic composition can vary spatially through the system, we introduce a new microscopic DoF based on this compositional variation which we find governs observable properties. In spinel - a wide class of complex oxides where this compositional variation manifests as cation inversion - we exploit this DoF to generate and analyze low-energy microstates of two GBs with three spinel chemistries (FeCr2O4, NiCr2O4 and MgAl2O4). This treatment is found to allow for the co-redistribution of cations at the GBs which acts to modify the spatial charge distribution, defect segregation energy and defect transport through these regions. Additionally, we generate low-energy metastable microstates of the GB system with an induced cation disorder, simulating those which may develop as a result of damage events. These are then analyzed to discover their composition and defect transport properties which depend strongly on the amount of induced damage. We conclude that considering this new DoF is important in describing the properties of GBs in complex materials.

Theoretical investigation on yttrium clustering in tungsten grain boundary region and strengthening effect

We have systematically investigated the solution and aggregation behaviors of yttrium (Y) on symmetrically inclined tungsten (W) grain boundary (GB) Σ5(310)/[001] by first-principles simulation. It is found that the most stable site for Y is located on the GB plane, and the solution energy of the Y substitutional site increases with increasing the distance from the GB plane. The charge redistribution of Y atoms is positively correlated with the solution energy, i.e., the deviation of the electrons of Y atoms is beneficial to its solution in the W–GB system. Further, the segregation of multiple Y atoms in the W–GB is clearly observed, where the Y atoms preferentially occupy the GB plane substitutional sites. The alloying Y atoms at the GB plane sites can effectively strengthen the GB when the concentration is less than 3.367%. The stronger binding energy of W–Y than the binding energy of W–W in the GB region is the main factor for the strengthening of the GB.

Grain boundary plane rotation analysis for FCC bicrystal structures using MD simulation

Grain boundary (GB) plane rotation, one of the GB engineering mechanisms, was investigated using the activation–relaxation technique by molecular dynamics simulation. The simulation systems considered in this work are bicrystals with Lennard-Jones-type interatomic potential. The systems of four GB types established are symmetric (SYM), asymmetric (ASYM), symmetric zigzag (SZ), and high-angle zigzag (HZ) models. Of the first two models, [Formula: see text]5 (310) for SYM and [Formula: see text] tilted for ASYM particularly provided reference atomic potential energy distributions and structures at minimum energy state at 0 K. The characteristic of SYM is the discrete atomistic potential distributions which are distinct from ASYM. The other two models based on zigzag-like GBs were created by rotating GB planes about [001] at the center of a [Formula: see text]5 (310)[Formula: see text]GB system for the SZ case and a high-angle GB system for the HZ case. Simulation results show that the initially tilted GBs kinetically transferred to relaxed states for shorter-length GBs through a series of curved GBs. The GBs consist of different combinations of order defect segments, amorphous regions, and defect-free regions. A mechanism proposed is the GB plane rotation, the rate of which is structure-dependent. A low-[Formula: see text] coincidence site lattice boundary section can stabilize the systems at a specified metastable state.

Oxygen vacancy formation in the SrTiO3 Σ5 [001] twist grain boundary from first‐principles

AbstractThe SrTiO3 5 [001] twist grain boundary (GB) is studied using first‐principles density functional theory calculations. Three types of GB structures, SrO/SrO (S/S), SrO/TiO2 (S/T), and TiO2/TiO2 (T/T), are modeled and their relative thermodynamic stabilities are examined. Our calculations show that the S/S and S/T structures can be formed within appropriate synthesis conditions, with the S/S structure thermodynamically favored over the S/T structure within a wide range of chemical potentials, while the T/T structure is unlikely to form. The segregation behavior of oxygen vacancies is also investigated by calculating oxygen vacancy formation energies with respect to the distance from GB plane. In the S/S system, oxygen vacancies tend to segregate to the layer adjacent to the GB layer, while in the S/T system, oxygen vacancies tend to segregate to the GB layer itself. In both S/S and S/T systems, oxygen vacancy formation energy is lower than that in bulk SrTiO3. To clearly show the experimental conditions necessary to promote oxygen vacancy formation in the 2 GB systems, we also generate grain boundary phase diagrams for oxygen vacancy with respect to synthesis temperature and oxygen partial pressure. Our calculations reveal different segregation behaviors and distributions of oxygen vacancies in the S/S and S/T systems, providing a possible avenue for GB engineering.

Fast and scalable prediction of local energy at grain boundaries: machine-learning based modeling of first-principles calculations

We propose a new scheme based on machine learning for the efficient screening in grain-boundary (GB) engineering. A set of results obtained from first-principles calculations based on density functional theory (DFT) for a small number of GB systems is used as a training data set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis scheme, we can increase the training data set significantly. We use atomic radial distribution functions and additional structural features as atom descriptors to predict atomic energies and GB energies simultaneously using the least absolute shrinkage and selection operator, which is a recent standard regression technique in statistical machine learning. In the test study with fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energy changes at and near GBs at a glance, even if we collected training data from only 10 GB systems. The present scheme can emulate time-consuming DFT calculations for large GB systems with negligible computational costs, and thus enable the fast screening of possible alternative GB systems.

A multiscale model of grain boundary structure and energy: From atomistics to a continuum description

Grain boundaries play an important role in the mechanical and physical properties of polycrystalline metals. While continuum macroscale simulations are appropriate for modeling grain boundaries in coarse-grained materials, only atomistic simulations provide access to the details of the grain boundary (GB) structure and energy. Hence, a multiscale description is required to capture these GB details. The objective of this paper is to consolidate various approaches for characterizing grain boundaries in an effort to develop a multiscale model of the initial GB structure and energy. The technical approach is detailed using various 〈100〉,〈110〉 and 〈111〉 symmetric tilt grain boundaries in copper and aluminum. Characteristic features are: (i) GB energies obtained from atomistic simulations and boundary period vectors from crystallography, (ii) structural unit and dislocation descriptions of the GB structure and (iii) the Frank–Bilby equation to determine the dislocation content. The proposed approach defines an intrinsic net defect density scalar that is used to accurately compute the GB energy for these GB systems. The significance of the present work is that the developed atomistic-to-continuum approach is suitable for realistically inserting the initial GB structure and energy into continuum level frameworks.

Adapted solute drag model for impurity-controlled grain boundary motion

Abstract

Atomistic Investigation of the Role of Grain Boundary Structure on Hydrogen Segregation and Embrittlement in α-Fe

Material strengthening and embrittlement are controlled by complex intrinsic interactions between dislocations and hydrogen-induced defect structures that strongly alter the observed deformation mechanisms in materials. In this study, we reported molecular statics simulations at zero temperature for pure α-Fe with a single H atom at an interstitial and vacancy site, and two H atoms at an interstitial and vacancy site for each of the 〈100〉, 〈110〉, and 〈111〉 symmetric tilt grain boundary (STGB) systems. Simulation results show that the grain boundary (GB) system has a smaller effect than the type of H defect configuration (interstitial H, H-vacancy, interstitial 2H, and 2H-vacancy). For example, the segregation energy of hydrogen configurations as a function of distance is comparable between symmetric tilt GB systems. However, the segregation energy of the 〈100〉 STGB system with H at an interstitial site is 23 pct of the segregation energy of 2H at a similar interstitial site. This implies that there is a large binding energy associated with two interstitial H atoms in the GB. Thus, the energy gained by this H-H reaction is ~54 pct of the segregation energy of 2H in an interstitial site, creating a large driving force for H atoms to bind to each other within the GB. Moreover, the cohesive energy values of 125 STGBs were calculated for various local H concentrations. We found that as the GB energy approaches zero, the energy gained by trapping more hydrogen atoms is negligible and the GB can fail via cleavage. These results also show that there is a strong correlation between the GB character and the trapping limit (saturation limit) for hydrogen. Finally, we developed an atomistic modeling framework to address the probabilistic nature of H segregation and the consequent embrittlement of the GB. These insights are useful for improving ductility by reengineering the GB character of polycrystalline materials to alter the segregation and embrittlement behavior in α-Fe.

Motion of the flat grain boundary in zinc

The motion of the individual flat random grain boundary (GB) in zinc is studied. The results are compared with those obtained earlier by the author while studying the motion of individual flat boundaries of special type. It was shown that the flat segments of the random GBs are the most mobile. The mobility of a GB system with a \([10\bar 10]\) tilt GB having a shape of a half-loop with a flat moving GB segment (facet) is an order of magnitude lower in the same temperature range.