Low-angle Tilt Grain Boundaries Research Articles

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.

Atomistic evaluation of the dislocation transmission across tilt and twist low-angle grain boundaries in body-centered cubic iron

This study systematically and qualitatively evaluates the resistance for dislocation transmission across tilt/twist low-angle grain boundaries (LAGBs), which mainly consist of edge/screw dislocations (“GB dislocations”). The motion of the incident lattice edge dislocation is disturbed by the LAGBs, while the edge dislocation is transmitted to the adjacent grain by applying large shear stress if the tilt and twist angles are sufficiently low. It was observed that the twist LAGB is a stronger obstacle compared to the tilt LAGB for the incident lattice edge segment even though the misorientation angle of both tilt and twist LAGBs is the same because of the reconstruction of the GB-dislocation structures at the twist LAGB owing to the incident lattice dislocation. When we compare the incident lattice edge and screw components, the barrier effect of the GB on the transmission is stronger for the screw component than for the edge component. The stronger barrier effect can be explained mainly by the higher line tension of the screw component based on the dislocation bow-out model. Accordingly, the dominant factors of the GB-strengthening effect in LAGB are the incident lattice dislocation type, the structure of GB dislocations, and dislocation–dislocation reactions. These results indicate that the GB-strengthening factors are dominated by the type of incident lattice dislocation and the GB-dislocation character in addition to the macroscopic misorientation of the GB.

High-temperature fatigue strength of grain boundaries with different misorientations in nickel-based superalloy bicrystals

Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.

The atomic scale mechanisms of the interaction between pore and grain boundary during sintering

The grain boundary (GB)-pore interaction is crucial to grain evolution and densification in materials sintering. In this work, by using the phase-field crystal model, we performed atomic scale simulations to investigate the microscopic mechanisms for the GB-pore interaction. The influence of pore on grain growth is clarified. The results indicate that a singular pore retards grain growth mainly through inhibiting grain rotation. Two modes, the pinning and bypassing mode, of GB-pore interaction are observed. In the pinning mode, the moving circular GB of a shrinking grain is pinned by the two junctions connecting the GB and the pore surface in the case of high GB misorientation, or by absorbing intergranular dislocations in the case of small GB misorientation. High misorientation GBs interact with a pore through the two junctions directly, while small misorientation GBs through the long range elastic field of the intergranular dislocations. The bypassing mode particularly occurs in the case of a very low angle tilt GB and a nano-pore. The GB quickly bypasses the pore locating between two neighboring intergranular dislocations through local lattice distortion.

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.

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.

Slip of low-angle tilt grain boundary (110) in FCC metals at perpendicular shear

Grain boundaries (GBs) make a significant contribution into the kinetics of plastic deformation of both polycrystalline and nanocrystalline metals. Among others, motion of GBs is an important phenomenon, which has to be taken into account in the consideration of the thermo-mechanical response of poly- and nanocrystals. In this work, we study motion of low-angle tilt GBs and the accompanying relaxation of shear stresses in the case, when the shear is applied perpendicular to GB. Using the molecular dynamics (MD) simulations, we consider bicrystals made of aluminum, copper and nickel as FCC metals with different stacking fault energies. MD data are used for construction and verification of a GB motion model, which is based on the previously proposed equation of motion of a solitary dislocation in a single crystal. Within this model, a low-angle tilt GB is presented as a wall of periodically located perfect edge dislocations. The main driving factors are the external shear stress and the local stress distribution due to the plastic relaxation in the trace of each dislocation creating an area of partially relaxed stresses behind the shifted GB. These factors define the average GB velocity during the main stage of motion, while dislocation-dislocation interactions between the walls and inside each wall define details of the motion, such as the curvature of GBs and their mutual acceleration at the late stage of interaction of two approaching opposite GBs.

Atomic resolution analyses on defects in nanocrystalline Cu-based alloys generated by severe plastic deformation

Bulk nanocrystalline (NC) materials possess high volume fraction (up to 50%) of grain boundaries (GBs), which contributes significantly to their mechanical and physical properties. Nevertheless, high density of dislocations at GBs and the associated strain fields complicate the analyses on the fine GBs structures of NC materials. Here, taking NC Cu-based alloys as model systems, the atomic-scale configurations of dislocations and typical GBs triggered by severe plastic deformation are well investigated using aberration-corrected high-resolution transmission electron microscopy and geometric phase analysis. A periodic array of extended dislocations with a spacing of 2.2 nm is found to form low-angle GBs (LAGBs) in NC Cu-based alloys. The interaction between dislocations and twins play a crucial role in forming the GBs of NC. Twin boundaries (TBs) in NC Cu-based alloys can serve as barriers for trapping dislocations, which in turn tune the TBs directly evolving into LAGBs or high-angle GBs. In addition, a symmetric 6° [001] tilt LAGB is also analyzed to be composed of a/2〈100〉 edge dislocations, which are activated by the a/2〈110〉 full dislocation dissociation or simple shear deformation of {100} planes. Our study offers insights into the atomic resolution structures of dislocations and GBs in NC materials.

Interaction between lattice dislocations and low-angle grain boundaries in Ni via molecular dynamics simulations

Low-angle grain boundaries (LAGBs) may show up frequently as distinct dislocation products such as in the processes of work hardening, recovery and recrystallisation of metals and alloys. To reveal their mechanical behaviours, interactions between lattice dislocation and symmetric tilt and twist LAGBs are studied with molecular dynamics simulations. It is shown that dislocation reaction and slip transmission depend on the structure of LAGB, the character of incident dislocation and the particular glide planes inhabiting the incoming slip. For tilt LAGBs, a free slip-transmission process is identified where dislocations can be forced to penetrate through the boundary without inducing dislocation reaction. Otherwise, the incident slip tends to be trapped or absorbed by those intrinsic grain boundary dislocations. With increasing the applied strain, a number of dislocation reactions can be triggered, which may lead to indirect slip transmission across the boundary.

Point defect sink efficiency of low-angle tilt grain boundaries

We examine the common assumption that grain boundaries (GBs) are ideal (or perfect) sinks for point defects by comparing and contrasting its implications with an explicit model of a low-angle tilt GB described by an array of edge dislocations which annihilate point defects by climbing. We solve the resultant diffusion equation in the absence and presence of irradiation-induced point defects. The GB sink efficiency depends on the physical parameters describing the boundary geometry (i.e., misorientation), material properties, and/or irradiation conditions (point defect generation and annihilation within the interior of grains). When the constituent dislocation spacing is small (large misorientation), the GB sink efficiency approaches that of the ideal sink. However, for small misorientations, the GB sink efficiency drops rapidly to zero and the ideal sink assumption for the GB fails dramatically. We derive a reduced dimension description of GBs where the influence of GB structure is captured in a single parameter in a Robin boundary condition for the diffusion equation. For the case of a low-angle tilt GB, we explicitly relate this parameter to the GB structure. We discuss the generality of this approach for cases where the low-angle GB model applies and parameterize the model so that it accurately reproduces the results of the two-dimensional dislocation model. The applicability of the approach to more general GBs is discussed as well as the implication of these results for predicting grain size effects under irradiation conditions.

Application of the Peierls–Nabarro Model to Symmetric Tilt Low-Angle Grain Boundary with Full <a> Dislocation in Pure Magnesium

Three types of symmetric (11\(\bar{2}\)0) tilt low-angle grain boundaries (LAGBs) with array of basal, prismatic, and pyramidal edge full <a> dislocations in pure Mg have been studied by using the improved Peierls–Nabarro model in combination with the generalized stacking fault energy curve. The results show that with decreasing distance between the dislocations in all the three types of tilt LAGBs, the stress and strain fields are gradually suppressed. The reduction extent of the stress and strain fields decreases from the prismatic to basal to pyramidal dislocations. The variation of dislocation line energy (DLE) for all tilt LAGBs is divided into three stages: DLE changes slightly and linearly when the distance is larger than 300 Å, ~10%; DLE declines exponentially and quickly when the distance goes from 300 to 100 Å, ~70%; and finally, the descent speed lowers when the distance is smaller than 100 Å and the dislocation core energy is nearly half of the DLE. The grain boundary energy (GBE) decreases when the tilt angle of LAGB increases from 1° to 2° for all cases. The tilt LAGB consists of pyramidal dislocations always has the largest GBE, while that with array of prismatic dislocations has the smallest one in the whole range. The Peierls stress of dislocation in tilt LAGB is nearly unchanged, the same as that of single dislocation. This work is useful for further study of dissociated dislocation, solute segregation, precipitate nucleation in tilt LAGB and its interaction with single dislocations.

PHASE FIELD CRYSTAL SIMULATION OF STRAIN EFFECTS ON DISLOCATION MOVEMENT OF PREMELTING GRAIN BOUNDRIES AT HIGH TEMPERATURE

The properties of modern materials, especially superplastic, nanocrystalline or composite materials, depend critically on the properties of internal interfaces such as grain boundaries(GBs) and interphase boundaries(IBs). All processes which can change the properties of GBs and IBs affect drastically the behaviour of polycrystalline metals and ceramics. In this work, the annihilation processes of low-angle symmetric tilt GBs and dislocations during plastic deformation in the representative system of these materials near but below the melting point and the temperature at liquid-solid coexistence line were simulated using the phase-field crystal model, respectively. The results show that local premelting occurs around lattice dislocations near the melting point but the dislocation structure in the premelting region does not change, while the region become significantly larger when the system reaches the melting temperature. After premelting, deformation to the system causes dislocations in the premelting GB to begin to glide then annihilate with opposite Burgers vectors via the movement, finally the GB and the premelting region disappear. The annihilation mechanisms of dislocations are similar to those for premelting conditions. The more the temperature is closer to the melting point, the more obvious the atomic lattice around the premelting region is softened leading to the atomic binding strength around the dislocations being lowered. Only at this moment, the lattice atoms enable to reduce the resistance of the dislocation motion and accelerate its velocity during deformation. At the temperature reaching to the liquid-solid coexisting region in the simulation, the original premelting regions are induced to develop into bigger ones by the external strain acting. During this process, it can be seen some interactions including the multiplication dislocation pairs, the rotation of dislocation pairs and their annihilation. Furthermore, the shape of the premelting region changes with the variation of the interaction of dislocations inside the region, it is observed that the premelting regions approach each other and consolidate together,then decompose and segregate from each other. Although the shape of the premelting region changes with the applied strain, these regions do not disappear at the end of the simulation, totally different those in lower premelting temperature.

Dislocation interactions and low-angle grain boundary strengthening

The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {110}〈111〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈100〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈100〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈100〉 dislocations. 〈100〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈100〉 dislocations. As an example, we report how the mobility of 〈100〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction.

Tilt angle dependences of vortex structure and critical current density at low-angle grain boundaries inYBa2Cu3O7−xfilms

A tilt angle dependence of vortex structure was studied at low-angle grain boundaries (LAGBs) in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ films. The 2\ifmmode^\circ\else\textdegree\fi{} tilt LAGB did not degrade ${J}_{c}$ of the film in a self-field, demonstrating the existence of Abrikosov vortices at the 2\ifmmode^\circ\else\textdegree\fi{} LAGB. The magnetic field dependence of the flux flow resistance indicated the existence of Abrikosov Josephson vortices at the 5\ifmmode^\circ\else\textdegree\fi{} tilt LAGB. At the 10\ifmmode^\circ\else\textdegree\fi{} tilt LAGB, a Josephson junction behavior was observed in a self-field, indicating the existence of Josephson vortices. The influence of the LAGBs on critical current density was understood based on these vortices.

High-resolution Z-contrast imaging and EELS study of functional oxide materials

Functional complex-oxide materials show a wide variety of properties and behaviors that cannot be found in any other class of materials, including high-temperature superconductivity and colossal magneto resistance. Consequently, this group of oxide materials has become the focus of many experimental as well as theoretical studies, aiming at understanding the fundamental mechanisms and properties that govern these complex structures. Here, we will review our high-resolution Z-contrast imaging and electron energy-loss studies of two complex-oxide materials systems, more specifically low-angle tilt grain-boundaries in YBa 2Cu 3O 7 (YBCO), and the spin-state transition in LaCoO 3. It will be shown that the O K-edge pre-peak can be used to quantify the hole-concentration in the vicinity of the dislocation core in YBCO, as well as to determine the Co 3+ spin-state in LaCoO 3.

Duality of dislocation content of grain boundaries

The Frank–Bilby equation (FBE) can give many solutions for the dislocation content of a grain boundary (GB); most of them are considered to have little physical reality except in limited ranges of the angles which characterize low-angle GBs. We explore two such solutions, each of which is an accurate description for a different low-angle tilt GB with the same tilt axis. We develop a model that uses the two solutions to predict the coupling factor between normal motion and the shear strain produced by any (low- or high-angle) GB. Within this model, the two FBE solutions give rise to two possible modes of coupled GB motion which for the same GB under the same stress differ in direction of motion. Using molecular dynamics simulations we confirm our model. We find positive and/or negative coupled GB motion for all misorientation angles, and the model gives accurate predictions for the shear produced even by high-angle GBs where individual dislocations cannot be resolved. At low temperatures dual behaviour is observed for the same high-angle GB and applied stress: both coupling modes are found with a switch between them after some time. This switch signifies a change in the effective dislocation content. Dual behaviour indicates that both solutions for the dislocation content can be meaningful for the same GB. At higher temperatures only one mode is seen for each GB under the same shear stress, and the switch between the two modes seems to occur discontinuously at some high misorientation angle.