High-angle Tilt Grain Boundaries Research Articles

Phase-field crystal study on the influence of nanoscale intergranular structures on grain boundary segregation

Grain boundary (GB) segregation can substantially affect the performance of materials by greatly changing the chemical compositions of GB. It is well known that GB segregation is essentially attributed to the structural differences between the bulk of grain and GB. Nevertheless, we still lack a clear understanding about the correlation between nanoscale intergranular structures and solute segregation. In this work, by using the phase-field crystal model, we performed atomic scale simulations to investigate the segregation of Li atoms to symmetric ⟨110⟩ tilt GBs in binary Al–Li alloys. It was found that the amount of segregated solute increases proportionally to GB misorientation angle in the case of low-angle tilt GBs, and converges at high-angle tilt GBs, except some special GBs with coincidence-site lattice. This is analogous to the dependences of GB energy and density on the misorientation angle. The correlations among GB structure, misfit strain around GBs and solute segregation are quantified at atomic scale. In low-angle tilt GBs, Li atoms are segregated to the compress zone around the core of intergranular dislocations to release the misfit strain energy. In the general high-angle tilt GBs, since the GB structure and misfit strain energy is uniform, the segregated atoms distribute homogeneously along GB. Particularly, the regular arrangement of structural units in some low Σ GBs lead to very low misfit strain energy, and accordingly to a periodically distributed and very low amount of solute segregation.

Understanding Grain Boundary Electrical Resistivity in Cu: The Effect of Boundary Structure.

Grain boundaries (GBs) in metals usually increase electrical resistivity due to their distinct atomic arrangement compared to the grain interior. While the GB structure has a crucial influence on the electrical properties, its relationship with resistivity is poorly understood. Here, we perform a systematic study on the resistivity–structure relationship in Cu tilt GBs, employing high-resolution in situ electrical measurements coupled with atomic structure analysis of the GBs. Excess volume and energies of selected GBs are calculated using molecular dynamics simulations. We find a consistent relation between the coincidence site lattice (CSL) type of the GB and its resistivity. The most resistive GBs are in the high range of low-angle GBs (14°–18°) with twice the resistivity of high angle tilt GBs, due to the high dislocation density and corresponding strain fields. Regarding the atomistic structure, GB resistivity approximately correlates with the GB excess volume. Moreover, we show that GB curvature increases resistivity by ∼80%, while phase variations and defects within the same CSL type do not considerably change it.

Atomic-scale study of compositional and structural evolution of early-stage grain boundary precipitation in Al–Cu alloys through phase-field crystal simulation

Interfacial solute clustering is an essential step preceding grain boundary (GB) precipitation. Both states, i.e., clusters and precipitates, alter the mechanical, chemical, and corrosion properties of materials. Continuum models cannot capture the atomic details of these phenomena, specifically of the transition from clustering to precipitation. We thus use the structural phase-field crystal (XPFC) model to study the compositional and structural evolution during GB clustering in Al–Cu alloys. The results show that the compositional evolution is dominated by solute segregation to lattice defects at the very beginning and then by confined spinodal decomposition along the GBs. The latter leads to a steep increase in the concentration and then the formation of disordered clusters. This structure acts as a precursor for phase nucleation, just like the decomposed solid solution, and Guinier–Preston zones are the precursors of the thermodynamically stable Al2Cu phase in the interior of grains. Two modes of spinodal decomposition are found. (a) On low-angle tilt GBs, spinodal decomposition occurs at the dislocations that constitute the GB. (b) On high-angle tilt GBs, spinodal decomposition takes place inside the entire GB plane. In either case, the structural transition from the disordered low-dimensional precursor states to an ordered phase state takes place following the compositional enrichment. These results shed light on atomic-scale early-stage GB decomposition and precipitation processes in Al–Cu alloys and enrich our knowledge about the coupling effects between compositional and structural evolution during GB phase transformation phenomena.

Structural Stabilities and Electronic Properties of High-Angle Grain Boundaries in Perovskite Cesium Lead Halides

Organometal trihalide perovskites are emerging as very promising photovoltaic materials, which is rivaling that of single crystal silicon solar cells despite their polycrystalline nature with relatively high density of grain boundaries (GBs). There is a lack of understanding of the effects of GBs on halide perovskites as their presence in silicon and other photovoltaic materials is generally detrimental to their photovoltaic properties. Using first-principles calculations, we systematically investigate the geometric structures of high-angle tilt GBs in halide perovskites CsPbX3 (X = Cl, Br, and I) starting from the coincidence site lattice model and refining using crystal shifts and lattice expansion. Electronic density of states calculations reveal that GBs in halides perovskites do not generate midgap states because of the large distance between the unsaturated atoms and the atomic reconstructions in the GB region. However, we show that the GBs can induce different very shallow states near the valence b...

Effect of applied stress on vacancy segregation near the grain boundary in nickel

We report a first-principles density functional theory investigation on the vacancy segregation near the nickel Σ5(210) grain boundary (GB) under applied stress. Our total energy calculations within generalized-gradient approximation reveal that the applied stress can hardly alter the energy preference of a vacancy at different atomic layers away from the GB plane, especially in the range of practical stresses. The segregation enthalpy, nevertheless, gets more (less) significant with the increasing compressive (tensile) stress as a result of the vacancy being smaller in volume at the boundary. Our first-principles result that high-angle tilt GBs in nickel are likely to act as a source (sink) in emitting (absorb) vacancies under a tensile (compressive) stress is not in accordance with the prediction based on a thermodynamic model. The numerical results are discussed with electronic structure analysis.

High-Resolution Electron Microscopy of Grain Boundary Migration

ABSTRACTAtomic-scale grain boundary (GB) migration has been directly observed by high-resolution transmission electron microscopy (HREM). Atomic-scale motion of high-angle tilt GBs as well as twist and general GBs at gold island grains with a number of planar facets has been studied at ambient and elevated temperatures. GB migration mechanisms depend on GB structure and geometry. Strong indications for cooperative effects has been found. In this case, as has been proposed before, atoms may undergo small shifts in their lattice positions to be incorporated into the growing grain in a collective mode. At high temperature and in the absence of a strong driving force such small lattice regions are observed to fluctuate back and forth between the two grains. Faceted GBs typically move in spurts. This appears to be inherent to GB migration, whenever the motion is controlled by different structural entities. For some GB geometries the motion was found to proceed by the lateral propagation of atomic-scale steps.

Defects in the Structure of Σ=27 <110> Tilt Grain-Boundaries in Ge.

ABSTRACTGeneral high-angle tilt grain-boundaries may be described by an arrangement of repeating structural units. A particular defect in the normal arrangement of structural units in a Σ=27 <110> tilt grain-boundary in Ge is reported.The defect is characterized by a short segment of (111) facet at the interface, and is associated with a screw dislocation in the boundary plane. This defect accommodates a slight misorientation of the boundary away from the perfect tilt configuration, andmay represent a type of structural unit present in other tilt boundaries containing a small twist component.