Atomic-scale observations of distinct segregation behaviors driven by site anisotropy in a near-Σ3 grain boundary

The experimental results on motion of single grain boundaries (GBs) of natural mechanical twin and single fabricated twin GBs as well as on fabricated twin GBs in system with triple junction (TJ) are obtained. The mobility of natural mechanical twin GBs, fabricated single GBs and fabricated GBs with TJ are compared. For the first time the effect of detachment of moving TJ and single natural twin GB from adsorbed atoms is reported. The results on single GB migration are considered in context of triple junction migration as the step to grain growth, i.e. “polycrystal” experiments.

Grain boundaries (GBs) trigger structure-specific chemical segregation of solute atoms. According to the three-dimensional (3D) topology of grains, GBs - although defined as two-dimensional defects - cannot practically be free of curvature. This leads to discrete variations in the GB plane orientations. Topologically required arrays of secondary GB dislocations accommodate these variations as well as deviations from ideal coincidence site lattice GBs. We report here that these pattern-forming secondary GB dislocations can have an additional and, in some cases, even a much stronger effect on GB segregation than defect-free GBs. Using nanoscale correlative tomography combining crystallography and chemical analysis, we quantified the relationship between secondary GB dislocations and their segregation energy spectra for a model Fe-W alloy. This discovery unlocks design opportunities for advanced materials, leveraging the additional degrees of freedom provided by topologically-necessary secondary GB dislocations to modulate segregation.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Understanding the transition mechanism from microscopic to macroscopic yielding is crucial for developing high-strength metallic materials. Basal-to-prismatic slip transfer at grain boundaries (GBs) of Mg alloys is known to trigger macroscopic yielding. Here, the penetration behavior of pileup dislocations at a simple basal–prismatic (BP) boundary was analyzed using molecular dynamics simulations. Edge dislocation penetration creates a step in the BP boundary at the intersection with the slip plane, widened via subsequent penetration of dislocations. Many GBs, including the BP boundary, have a regular GB dislocation network that corresponds to a coincidence site lattice. Dislocation penetrations must gradually destroy the initial GB dislocation network, requiring increasing driving force for subsequent dislocation penetrations; thus, slip transfer at the boundary occurs sporadically and stably via the penetration of individual lattice dislocations. As the width of the step created by dislocation penetration increases, a GB dislocation network can be formed in the step, which can stabilize the energy of the step with specific widths. When the energy of step decreases with increasing step width, a smaller driving force is required for the subsequent penetration of dislocations, and thus slip transfer occurs unstably via the penetration of several sequential lattice dislocations. Once the step has a low energy at a specific width, it strongly obstructs dislocation penetration. This formation and disruption of the GB dislocation network cause intermittent penetration of dislocations. Finally, dislocation penetration at the BP boundary is discussed based on a fracture mechanics approach, which well-reproduces the transition between penetration behaviors and the macroscopic yield stress.

Segregation to defects, in particular to grain boundaries (GBs), is an unavoidable phenomenon leading to changed material behavior over time. With the increase of available computational power, unbiased quantum-mechanical predictions of segregation energies, which feed classical thermodynamics models of segregation (e.g., McLean isotherm), become available. In recent years, huge progress towards predictions closely resembling experimental observations was made by considering the statistical nature of the segregation process due to competing segregation sites at a single GB and/or many different types of co-existing GBs. In the present work, we further expand this field by explicitly showing how compositional disorder, present in real engineering alloys (e.g., steels or Ni-based superalloys), gives rise to a spectrum of segregation energies. With the example of a Σ5 GB in a Ni-based model alloy (Ni-Co-Cr-Ti-Al), we show that the segregation energies of Fe, Mn, W, Nb, and Zr are significantly different from those predicted for pure elemental Ni. We further use the predicted segregation energy spectra in a statistical evaluation of GB enrichment, which allows for extracting segregation enthalpy and segregation entropy terms related to the chemical complexity in multi-component alloys. • Study of solute segregation behavior of Fe, Mn, W, Nb and Zr to a Σ5(210)[001] GB in a Ni-based model alloy. • The use of disordered atomistic models gives rise to the segregation energy spectrum. • Chemical complexity of multi-component alloy leads to stronger segregation behavior than pure Ni.

ZnO is used in a wide variety of applications owing to the electrical properties. Polycrystalline ZnO ceramics have long been used such as varistor, and ZnO films are currently intensively studied for transparent conductor applications. Grain boundary (GB) in ZnO varistor is believed to be the origin of nonlinear current-voltage characteristics, and GB in ZnO films possibly affects the electrical conductivity. It is therefore important to understand the role of ZnO GB on the electrical properties, which should be closely related with the structure in atomic scale. With these viewpoints, we have studied the atomistic structure of ZnO GBs, where the orientation relations of adjacent crystals are well defined. Single GBs studied were obtained by fabricating ZnO bicrystals and the GBs were characterized by scanning transmission electron microscopy (STEM) and theoretical calculations. It is found that coordination number of ions change in ZnO GBs; there are underfold or overfold coordinated ions that are unusual in bulk inside. It is calculated that these atomistic structures alters the electronic structure but would not create deep states in the band gap. On the other hand, when praseodymium (Pr), which is known to be a key dopant element to obtain nonlinear <i>(I-V)</i> characteristics, is added to the GBs, Pr strongly localizes to the GBs and occupies specific atomic sites. Pr facilitates the formation of the acceptorlike defects such as zinc vacancies, which we think that is an important role of Pr on generation of nonlinear <i>(I-V)</i> characteristics. Furthermore, atomic arrangement and localization behavior of Pr are studied for several GBs to obtain fundamental understanding about GB structure formation.

A distinct grain boundary (GB) is formed when two crystallization fronts collide in metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si) thin films. This has been used to study carrier transport across a single dominant GB. The average number of traps per unit area is found to be about 1.6/spl times/ 10/sup 12//cm/sup 2/ in this GB, significantly higher than that associated with the regular GBs in the bulk of MILC poly-Si. Though this single GB occupies a negligible fraction of the total device volume, it has been found to significantly affect both the resistance of MILC resistors and the leakage current of MILC thin-film transistors.

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

Screening and manipulation by segregation of dopants in grain boundary of Silicon carbide: First-principles calculations

We study the atomic structure of [001] symmetrical tilt grain boundaries (GBs) with stoichiometric composition in the B2 compound NiAl using molecular statics with an embedded-atom potential. The simulations are performed in the misorientation range 0° ⩽ θ 90° in every 3° in average. The multiplicity of stable GB structures is addressed by using the γ-surface technique combined with full atomic relaxation. The GBs typically have only a few nonidentical stable structures. All other structures either are symmetrically related to those few structures or are their strained variants and reduce to one of them if relative translations of the grains are allowed. All ground-state structures are composed of only four types of structural unit, or their variants obtained by site substitution. Many structural features of the GBs can be understood in terms of the significant atomic size effect peculiar to NiAl. The ground-state Σ = 5, (310), θ = 36.87° and Σ = 5,(210),θ = 53.13° GBs, as well as all GBs with intermediate orientations, show a relative shift of the grains by 1/2[001]. All such GBs follow the structural unit model with the two Σ = 5 GBs being the delimiting boundaries. For all other orientations, the ground-state GB structures have no shift along the tilt axis and are consistent with the structural unit model only at θ< 30° and θ> 65°. In some GBs, the primary or secondary GB dislocations are observed to dissociate in partials associated with GB steps. Many GBs contain regions of antiphase boundaries on (100) and (110) planes. The behaviour of GBs under applied shear stress parallel to the GB plane is discussed.

Dependency of grain boundary dislocation network configuration on generalized stacking fault energy surface in FCC metals

Effect of low-angle grain boundaries on morphology and variant selection of grain boundary allotriomorphs and Widmanstätten side-plates

Grain boundaries (GBs) are material imperfections that significantly impact material properties. Understanding how their atomic structure deviates from ideal symmetric orientations is crucial for establishing fundamental structure–property relationships. In this study, we utilized aberration-corrected scanning transmission electron microscopy, geometric phase analysis and nanobeam electron diffraction (NBED) to examine the structure of a series of near- Σ 5 ( 310 ) [ 001 ] tilt grain boundaries in copper and to explore the formation of GB defects and their associated strain field evolution on different length scales. Globally, the GB appears flat with no noticeable defects, as confirmed by NBED strain mapping. On the atomic-scale, however, various types of GB defects are observed. When a slight deviation in the misorientation is introduced, a patterning emerges featuring characteristic structural units from the Σ 5 ( 310 ) [ 001 ] and Σ 5 ( 210 ) [ 001 ] tilt boundaries. This pattern can be interpreted as secondary GB dislocations, a conclusion that is supported by GB structure prediction. Since these defects are confined to within the GB core, their associated strain field does not extend into the adjacent bulk grains. The structural landscape of the GB becomes more complex when GB plane inclination is also present, such as a wavy morphology or staircase-like architecture. The wavy morphology shows an unusual V-shape of the expansion and compression zones of the GB facet junctions that continue to extend into the bulk crystals for several nanometers. Our investigation into GB structure, particularly its inherent defects, is a prerequisite towards gaining atomic-scale insights into their potential impact on material properties.

The diffusion–microstructure correlations for grain boundaries (GBs) in the technologically-relevant Ni-based 602CA alloy are investigated. Prolonged annealing treatments up to 744 h create distinct GB complexions with specific segregation–precipitation–structure states. Globular M23C6-type carbides at straight GBs and plate-like carbides together with NiAl-enriched (γ′-type) particles at hackly GBs are found to co-exist. Moreover, an atomic-scale GB spinodal-like decomposition, especially at straight GBs, is observed. The co-existence of the two distinct states of general high-angle GBs, indicated by tracer diffusion experiments and verified by a detailed structure examination, is explained via state-of-the-art measurements of local elastic strains. In a course of annealing at 873 K, the relatively “fast” diffusivities are found to increase by a factor of 10 or more as a result of a coupled evolution of the GB plate-like precipitates and the irregular GB structures, whereas the relatively ”slow” diffusivites remained practically unchanged representing the contributions of straight interfaces with spherical precipitates. Thus, the diffusion properties of high-angle GBs evolve together with characteristic changes of GB complexions distinguished by a growth of carbide- and γ′-type precipitates and a concomitant generation of GB dislocation networks. The obtained results provide novel insights into grain boundary tailoring by utilizing structure – kinetics correlations involving segregation, precipitation and the evolution of interface defects.

Physical properties of ceramics are often changed by presence of grain boundaries (GBs). One possible cause of the change is the grain boundary (GB) itself, because the GB atomic structure is different from that of the bulk. Another possible cause would be dopant segregation at the GBs. Thus, it is important to understand GB atomic structure and dopant segregation at the GBs at the atomic level. Here, we are going to present our studies on atomic structure and dopant (Pr) segregation at ZnO [0001] tilt GBs. We fabricated undoped and Pr-doped [0001] tilt GBs within ZnO bicrystals [1]. Atomic structure of the undoped ZnO GB was observed by high-resolution transmission electron microscopy (HRTEM), and that of the Pr-doped ZnO GB was observed using high-angle annular dark-field scanning TEM (HAADF-STEM). Atomistic calculations were performed for both of the undoped and the Pr-doped ZnO GBs to obtain further information in detail. The calculation results were compared with the HRTEM and the HAADF-STEM images.