Grain Boundary Structure Research Articles

Machine learning interatomic potential with DFT accuracy for general grain boundaries in α-Fe

AbstractTo advance the development of high-strength polycrystalline metallic materials towards achieving carbon neutrality, it is essential to design materials in which the atomic level control of general grain boundaries (GGBs), which govern the material properties, is achieved. However, owing to the complex and diverse structures of GGBs, there have been no reports on interatomic potentials capable of reproducing them. This accuracy is essential for conducting molecular dynamics analyses to derive material design guidelines. In this study, we constructed a machine learning interatomic potential (MLIP) with density functional theory (DFT) accuracy to model the energy, atomic structure, and dynamics of arbitrary grain boundaries (GBs), including GGBs, in α-Fe. Specifically, we employed a training dataset comprising diverse atomic structures generated based on crystal space groups. The GGB accuracy was evaluated by directly comparing with DFT calculations performed on cells cut near GBs from nano-polycrystals, and extrapolation grades of the local atomic environment based on active learning methods for the entire nano-polycrystal. Furthermore, we analyzed the GB energy and atomic structure in α-Fe polycrystals through large-scale molecular dynamics analysis using the constructed MLIP. The average GB energy of α-Fe polycrystals calculated by the constructed MLIP is 1.57 J/m2, exhibiting good agreement with experimental predictions. Our findings demonstrate the methodology for constructing an MLIP capable of representing GGBs with high accuracy, thereby paving the way for materials design based on computational materials science for polycrystalline materials.

A universal descriptor to determine the effect of solutes in segregation at grain boundaries

The control of solute segregation at grain boundaries is of significance in engineering alloy properties. However, there is currently a lack of a physics-informed predictive model for estimating solute segregation energies. Here we propose novel electronic descriptors for grain-boundary segregation based on the valence, electronegativity and size of solutes. By integrating the non-local coordination number of surfaces, we build a predictive analytic framework for evaluating the segregation energies across various solutes, grain-boundary structures, and segregation sites. This framework uncovers not only the coupling rule of solutes and matrices, but also the origin of solute-segregation determinants, which stems from the d- and sp-states hybridization in alloying. Our scheme establishes a novel picture for grain-boundary segregation and provides a useful tool for the design of advanced alloys.

Molecular Dynamics Analysis of Hydrogen Diffusion Behavior in Alpha-Fe Bi-Crystal Under Bending Deformation

The hydrogen embrittlement (HE) phenomenon occurring in drawn pearlitic steel wires sometimes results in dangerous delayed fracture and has been an important issue for a long time. HE is very sensitive to the amount of plastic deformation applied in the drawing process. Hydrogen (H) atom diffusion is affected by ambient thermal and mechanical conditions such as stress, pressure, and temperature. In addition, the influence of stress gradient (SG) on atomic diffusion is supposed to be crucial but is still unclear. Metallic materials undergoing plastic deformation naturally have SG, such as residual stresses, especially in inhomogeneous regions (e.g., surface or grain boundary). In this study, we performed molecular dynamics (MD) simulation using EAM potentials for Fe and H atoms and investigated the behavior of H atoms diffusing in pure iron (α-Fe) with the SG condition. Two types of SG conditions were investigated: an overall gradient established by a bending deformation of the specimen and an atomic-scale local gradient caused by the grain boundary (GB) structure. A bi-crystal model with H atoms and a GB structure was subjected to bending deformation. For a moderate flexure, bending stress is distributed linearly along the thickness of the specimen. The diffusion coefficient of H atoms in the bulk region increased with an increase in the SG value. In addition, it was clearly observed that the direction of diffusion was affected by the existence of the SG. It was found that diffusivity of the H atom is promoted by the reduction in its cohesive energy. From these MD results, we recognize an exponential relationship between the amount of H atom diffusion and the intensity of the SG in nano-sized bending deformation.

A potential mechanism for abnormal grain growth in Ni thin films on c-sapphire

Normal grain growth (NGG) of a (111) textured Ni film on c-sapphire and abnormal grain growth (AGG) of (100) grains at the expense of this (111) texture has been studied as a function of temperature with and without a capping layer. The grain boundaries (GBs) in the Ni film are controlled by the preferred orientation relationships (ORs) adopted by the Ni grains on the sapphire substrate. The 2 variants of a single OR, Ni(111)<11¯0>//Al2O3(0001)<11¯00>, form a (111) mazed bicrystal with Σ3 GBs. The (100) grains have a single OR, Ni(100)<010>//Al2O3(0001)<11¯00> with 3 variants; their GBs within the (111) grains have the (111)<11¯0>//(100)<010> misorientation.(100) AGG within the (111) mazed bicrystal of the 100 nm Ni film takes place above 1023 K. The orientation transition is driven by the biaxial elastic modulus anisotropy which favors the growth of (100) grains over (111) grains, as this reduces the elastic strain energy induced by the thermal mismatch between Ni and sapphire. (100) AGG is suppressed and the NGG of the (111) texture is slowed down when the film is covered by a 10 nm amorphous alumina layer aimed at inhibiting surface diffusion. Thus, it is proposed that as long as the surface can act as a sink for the point defects diffusing along the GBs, the movement of the GBs is correlated to the diffusivity of atoms and vacancies, which is a function of their misorientation and crystallographic GB structure.

Atomic-scale investigation on the structures and mechanical properties of metastable grain boundaries in aluminum

Grain boundary (GB) structural transition in pure metals open up new possibilities for material microstructure design. By managing the distribution of ground-state and metastable GB structures, the beneficial effects of GBs on materials can be maximised. However, the application of GB transitions to develop high-performance aluminum is limited by the inadequate understanding of the metastable GB structure in aluminium and its associated plastic deformation mechanisms. In this work, we firstly used a high-throughput GB generation method to construct multiple GB structure for a given misorientation angle, and screen out the ground-state and metastable GB structures according to the energy principle. Six typical symmetrically tilted GBs were investigated, including Σ5(210), Σ17(410) and Σ17(530) GBs around <001> tilt axis, and Σ9(221), Σ11(332), Σ17(223) GBs around <110> axis. Then, molecular dynamics simulations were carried out to perform uniaxial tensile tests on the six GBs and their corresponding metastable GBs at different temperatures. The results show that the tensile strength of <001> metastable GBs differs slightly from the ground-state GB at various temperatures, but for <110> GBs, the strength of most metastable GBs is significantly stronger or weaker compared to that of ground-state GB at 10 K. An increase in temperature reduced this difference in tensile strength of <110> GBs. According to atomic analysis of the GB structural characteristics, it was found that stress and temperature-induced GB structural transition, GB strain localization, and the prevention of dislocation nucleation or propagation from the dissociated structure are the main causes of the differences in tensile responses between the ground-state and metastable GBs.

Self-diffusion in < 100 > symmetrical tilt grain boundaries in tungsten: Molecular dynamics simulation

The structure and energy of grain boundaries, the energies of point defects formation in them and grain-boundary self-diffusion coefficients in 18 < 100 > symmetrical tilt grain boundaries in tungsten have been determined by atomistic simulation. The dependences of grain boundary energies, defect formation energies in them and activation energy of grain-boundary self-diffusion on the misorientation angle have been calculated. A structural phase transformation has been discovered in Σ5(310) and Σ5(210) boundaries, which affected the diffusion properties of these grain boundaries. The calculations performed are compared with the available experimental data on self-diffusion in grain boundaries of tungsten.

Intrinsic tensile brittleness of tilted grain boundaries and its shear toughening

In the endeavors of working with microstructures in polycrystalline metals for better strength and ductility, grain boundaries (GBs) are placed at the front burner for their pivotal roles in plastic deformation. Often the mechanical properties of polycrystalline metals are governed by mutual interactions among GBs and dislocations. A thorough comprehension of GB deformation is therefore critical for the design of metals of superb performance. In this research, we investigated the mechanical behavior of symmetric tilt grain boundaries in face-centered cubic (F.C.C.) nickel, which may be subject to tension, shearing, and mixing-mode load using molecular dynamics simulations. We observed that (1) there exist four types of micro deformation mechanisms in GBs, and illustrate at the atomistic scale their distinctions and their dependence on the activation of lattice slip in the crystal; (2) GBs are intrinsically brittle under tension but exhibit ductile behavior during shearing. Shifting from pure tension with increasing shear component during mixing-mode load leads to GB toughening; and (3) there lacks conceivable dependence of GB tensile strength on tilted GBs, in contrast to a relatively rough trend of greater shear strength in GBs of large misorientation. GB energy shows no direct connection with GB strength, as broadly reported in existing literature. This research enhances our mechanistic understanding of GB plasticity in crystalline metals, and points to a potential way of making strong-yet-tough polycrystalline metals through GB engineering: in addition to GB structure manipulation, tuning the loading mode of GBs may open another avenue for their better performance.

Influences of symmetric tilt grain boundary on the stretching deformation behaviors of Cu bicrystals

The microstructure of grain boundary (GB) becomes an important factor affecting the strength of nanomaterials during plastic deformation. In this work, Cu bicrystals with different coincidence site lattice (CSL) interfaces are constructed, and the influences of the symmetric tilt interface on the mechanical properties and plastic deformation behaviors during uniaxial tension are studied by molecular dynamics simulation. The results indicate that the peak stress of tensile stress-strain curves for most bicrystals is lower than its monocrystalline samples, except CSL3, CSL11 and CSL57 bicrystals with a continuous and single structural unit. The CSL3 bicrystal has the highest strength, while CSL33 bicrystal has the lowest strength among all the tested samples. The CSL11 and CSL57 bicrystals with a lower GB energy have a higher stability of interface, which delays the yielding. Moreover, the generation of twin and stair-rod dislocation at the interfaces strengthens the CSL11 and CSL57 bicrystals. The CSL3 almost has the same parameters as its monocrystalline sample S3, massive research has confirmed that the sample with a higher density twin (CSL3 interface) exhibits higher combined properties of strength and plasticity. This work may provide a guideline for GB engineering to improve material properties by controlling the GB structure.

Intergranular fracture, grain-boundary structure, and dislocation-density interactions in FCC bicrystals

A dislocation-density based crystalline plasticity (DCP) and nonlinear finite element (FE) analysis were used to predict, and fundamentally understand how and why fracture nucleation and propagation are related to the interrelated microstructural mechanisms of dislocation-density pileups, GB structure, orientation, and total and partial dislocation density interactions within and adjacent for a random low angle grain boundary (LAGB) and a random high angle GB (HAGB). The GB orientations and structures were obtained from micropillar experiments, such that LAGBs and the HAGBs can be accurately represented and used for the modeling predictions. The normal stress, density of pileups, and dislocation-density accumulation along and within the GB were higher for the low angle GB bicrystal. These interrelated phenomena delineate how fracture for high angle GBs nucleate and propagate at lower nominal strains than the lower angle GB bicrystal case. These predictions underscore how fundamental mechanisms can be identified and used to understand how failure nucleates and propagates for different GB structures and orientations.

Method of Determining the Relationship between Grain Structure and Relative Energies of Grain Boundaries

Method of Determining the Relationship between Grain Structure and Relative Energies of Grain Boundaries

Mechanical response and grain boundary behavior of (HfNbTaTiZr)C high-entropy carbide ceramics and its constituent binary carbides

The structure and mechanical response of grain boundaries (GBs) are essential for predicting the mechanical properties of polycrystalline materials. Understanding the properties of GBs in carbide ceramics is essential for the development of applications of high entropy carbide ceramics (HECCs) in extreme environments. In this work, we investigated the intrinsic properties and mechanical responses of {210}/[001] GBs in (HfNbTaTiZr)C HECC subjected to shear deformation and its constituent binary carbides using first-principles calculations. The results showed that the GBs in Group VB carbides undergo migration, whereas the formation of CC bonds within the GBs in Group IVB carbides can inhibit GB migration and result in the failure of supercells due to the breakage of metal-C bonds. Therefore, tailoring the GBs in (HfNbTaTiZr)C based on metal atoms from different groups can induce a "pinning" effect, potentially enhancing plasticity without sacrificing strength. Furthermore, HECC exhibits higher GB migration stress compared to conventional binary carbides. These results can provide in-depth insights into the mechanical behavior of GB structure of high-entropy carbide ceramics.

Grand canonically optimized grain boundary phases in hexagonal close-packed titanium

Grain boundaries (GBs) profoundly influence the properties and performance of materials, emphasizing the importance of understanding the GB structure and phase behavior. As recent computational studies have demonstrated the existence of multiple GB phases associated with varying the atomic density at the interface, we introduce a validated, open-source GRand canonical Interface Predictor (GRIP) tool that automates high-throughput, grand canonical optimization of GB structures. While previous studies of GB phases have almost exclusively focused on cubic systems, we demonstrate the utility of GRIP in an application to hexagonal close-packed titanium. We perform a systematic high-throughput exploration of tilt GBs in titanium and discover previously unreported structures and phase transitions. In low-angle boundaries, we demonstrate a coupling between point defect absorption and the change in the GB dislocation network topology due to GB phase transformations, which has important implications for the accommodation of radiation-induced defects.

Can we predict mixed grain boundaries from their tilt and twist components?

One of the major challenges towards understanding and further utilizing the properties and functional behaviors of grain boundaries (GB) is the complexity of general GBs with mixed tilt and twist character. Here, we report the correlations between mixed GBs and their tilt and twist components in terms of structure, energy and stress field by computationally examining 7440 silicon GBs. Such correlations indicate that low angle mixed GBs are formed through the reconstruction mechanisms between their superposed tilt and twist components, which are revealed as the energetically favorable dissociation, motion and reaction of dislocations and stacking faults. In addition, various complex disconnection network structures are discovered near the conventional twin and structural unit GBs, implying the role of disconnection superposition in forming high angle mixed GBs. By unveiling the energetic correlation, an extended Read-Shockley model that predicts the general trends of GB energy is proposed and confirmed in various GB structures across different lattices. Finally, this work is validated in comparison with experimental observations and first-principles calculations.

Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270 μs, corresponding to Q = 5 million, and a highest observed value of 501 μs. Through materials analysis techniques and numerical simulations we investigate the dominant source of energy loss, and devise and demonstrate a strategy toward its mitigation. Growing aluminum films thicker than 300 nm reduces the presence of oxide, a known host of defects, near the substrate-metal interface, as confirmed by time-of-flight secondary ion mass spectrometry. A loss analysis of coplanar waveguide resonators shows that this results in a reduction of dielectric loss due to two-level system defects. The correlation between the enhanced performance of our devices and the film thickness is due to the aluminum growth in columnar structures of parallel grain boundaries: transmission electron microscopy shows larger grains in the thicker film, and consequently fewer grain boundaries containing oxide near the substrate-metal interface.

Structural evolution and mechanical response of multiple Al Σ5(210) grain boundaries with segregation of solute atoms: First-principles study

Grain boundary (GB) segregation of solute atoms is a key factor affecting the microstructure and macroscopic properties of materials. In this study, first-principles calculations were carried out to investigate the effect of solute atoms (Mg and Cu) segregation on the structural evolution and mechanical response of Al ground-state Σ5(210) GB (GB-Ⅰ) and metastable Σ5(210) GBs (GB-Ⅱ and GB-Ⅲ). The GB energy, segregation energy, and the theoretical tensile strength of the multiple Σ5(210) GBs were calculated. The results show that both Mg and Cu atoms tend to segregate in the boundary plane, thus reducing GB energy and improving GB stability. The segregation of Cu atoms reduced the GB energy more significantly than that of Mg atoms. The segregation of solute atoms can change the symmetric GB structure into an asymmetric one or induce GB phase transformation. The theoretical strength of GB-I is weaker than that of the metastable GB-II with higher GB energy, suggesting that grain boundaries with higher energy are not necessarily unstable. In addition, the effect of solute atom segregation on the GB strength depends not only on the type of element but also on the specific GB structure. The weakening effect of Mg segregation in GB-I and GB-II is due to the weak MgAl bond which enlarges the low charge density region of GB and increases the free volume of GB. The strengthening of GB-I and GB-II by Cu segregation mainly depends on the stronger AlCu bond than AlAl bond along the GB fracture path. The enhanced strength of GB-III with Mg and Cu segregation is attributed to the GB structural phase transformation caused by the segregation of solute atoms. The calculation results further elucidate the relationship between element segregation and grain boundary characteristics.

Changing the Atomic and Electronic Structures of Oxide Grain Boundaries with Non-Contacting Electric Fields

Changing the Atomic and Electronic Structures of Oxide Grain Boundaries with Non-Contacting Electric Fields

Atomic Structure and Chemistry of High-Entropy Oxide Grain Boundaries revealed by STEM Imaging, Strain Mapping, and Spectroscopy

Atomic Structure and Chemistry of High-Entropy Oxide Grain Boundaries revealed by STEM Imaging, Strain Mapping, and Spectroscopy

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

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

Effects of mechanical stress, chemical potential, and coverage on hydrogen solubility during hydrogen-enhanced decohesion of ferritic steel grain boundaries: A first-principles study

Hydrogen-enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen embrittlement, a phenomenon that severely impacts structural materials such as iron and iron alloys. Grain boundaries (GBs) play a critical role in this mechanism, where they can provide trapping sites or act as hydrogen diffusion pathways. The interaction of H with GBs and other crystallographic defects, and thus the solubility and distribution of H in the microstructure, depends on the concentration, chemical potential, and local stress. Therefore, for a quantitative assessment of HEDE, a generalized solution energy in conjunction with the cohesive strength as a function of hydrogen coverage is needed. In this paper, we carry out density functional theory calculations to investigate the influence of H on the decohesion of the Σ5(310)[001] and Σ3(112)[11¯0] symmetrical tilt GBs in bcc Fe, as examples for open and close-packed GB structures. A method to identify the segregation sites at the GB plane is proposed. The results indicate that at higher local concentrations, H leads to a significant reduction of the cohesive strength of the GB planes, significantly more pronounced at the Σ5 than at the Σ3 GB. Interestingly, at finite stress, the Σ3 GB becomes more favorable for H solution, as opposed to the case of zero stress, where the Σ5 GB is more attractive. This suggests that, under certain conditions, stresses in the microstructure can lead to a redistribution of H to the stronger grain boundary, which opens a path to designing H-resistant microstructures. To round up our study, we investigate the effects of typical alloying elements in ferritic steel, C, V, Cr, and Mn, on the solubility of H and the strength of the GBs. Published by the American Physical Society 2024

Approaches and challenges in whole-nanoparticle refinements from neutron total-scattering data

This study considers critical data reduction steps and data analysis approaches required to determine explicitly the atomic arrangements in nanoparticles from time-of-flight neutron total scattering. A practical procedure is described for removing parasitic backgrounds caused by the incoherent scattering of hydrogen inevitably present in most nanoparticle samples and normalizing the recovered coherent scattering intensities onto an absolute scale. A model-free analysis is presented of a pair-distribution function derived from total scattering that can be used to determine thermal expansion coefficients and particle sizes directly from experimental data without knowledge of the material's structure. Finally, atomistic whole-nanoparticle refinements of yttrium-doped ceria nanoparticles from neutron total-scattering data are demonstrated using the reverse Monte Carlo method implemented in the RMCProfile software. These results reveal a strong dependence of the cation–oxygen and oxygen–oxygen distances on the coordination numbers, which leads to gradients of these distances near the particle surface. The details are dependent on the surface coverage by ligands and adsorbates and on the structure of grain boundaries in nanocrystalline agglomerates. The refined models confirm the expectations of more substantial disorder at particle surfaces, with a distorted surface layer extending over several coordination shells. The results highlight the feasibility of whole-nanoparticle refinements from neutron data, calling for further development of data reduction and analysis procedures.