Bicrystal Grain Boundary Research Articles

Grain boundary solute segregation across the 5D space of crystallographic character

Solute segregation in materials with grain boundaries (GBs) has emerged as a popular method to thermodynamically stabilize nanocrystalline structures. However, the impact of varied GB crystallographic character on solute segregation has never been thoroughly examined. This work examines Co solute segregation in a dataset of 7272 Al bicrystal GBs that span the 5D space of GB crystallographic character. Considerable attention is paid to verification of the calculations in the diverse and large set of GBs. In addition, the results of this work are favorably validated against similar bicrystal and polycrystal simulations. As with other work, we show that Co atoms exhibit strong segregation to sites in Al GBs and that segregation correlates strongly with GB energy and GB excess volume. Segregation varies smoothly in the 5D crystallographic space but has a complex landscape without an obvious functional form.

First principles study of electronic structure and transport in graphene grain boundaries

Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions. We generated 150 different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB. We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.

Interaction of cracks with grain boundaries in iron bicrystals

Molecular dynamic modelling of seed cracks evolution in iron bicrystals with inclined grain boundaries under uniaxial expansion was carried out. The process of seed crack evolution can be divided into four stages. At the first stage, in the interval of elastic deformations, the seed crack is stationary, and the stresses increase linearly, reaching a maximum value of ~7.0 GPa. At the same time, the atomic volume and stresses at the crack tip before its opening grow significantly faster than the average for the sample. At the second stage, the crack begins to spread into the grain volume. The process of crack propagation leads to an abrupt stress release due to relaxation processes in the areas adjacent to the crack banks and the emission of defects from the crack tip. After reaching the grain boundary, the crack stops and blunts. At the third stage, the crack remains in the grain boundary, and the sample stresses experience significant oscillations, which is caused by the emission of various defects both from the grain boundary and from other interfaces. The emission of defects from the crack tip can cause local migration of the grain boundary, which is formation of a bend on the initially flat surface of the grain boundary. When defects cease to be emitted from the crack tip, the voltage and atomic volume in this region increase rapidly. At the fourth stage, the crack begins to spread into the second grain. It was found that a boundary with a large grain misorientation angle is a more effective barrier restraining crack propagation. Initiation of the seed crack propagation in material is always preceded by an abrupt increase in atomic volume and stresses at the crack tip.

A novel approach for evaluating creep damage and cavitation in copper bicrystals subject to constant load

Creep in metal alloys is an important failure mode for high temperature and stress applications, but despite extensive study it is still not fully understood, particularly the early stage of creep cavity formation. This paper describes a novel constant load cantilever beam test to investigate creep damage and cavitation at grain boundaries in copper bicrystals. Bicrystals of copper have been prepared with the grain boundary oriented normal to the beam long axis, allowing the development of damage at a single boundary to be studied. Tests were conducted at a temperature of 285°C in a vacuum of 10−10 MPa. Creep cavitation is observed in copper bicrystals of {001} and {111} orientation with a 22° rotation at the boundary. We compare these data with observations for a polycrystalline copper specimen.

Tilt grain boundary energy in 〈0001〉/φ bicrystals of pure ice

This work presents measurements of the relative energy ( γgb/ γs) of the tilt grain boundary (GB) in pure ice bicrystals with crystalline misorientation 〈0001〉/ φ. The GBs had different inclinations and were annealed at −5 °C and −18 °C. A GB structure model called the facet model (FM) was applied. The FM assumes that the GB is composed of facets. The facets are oriented with the principal faces of the cell primitive of the coincident site lattice (CSL). The CSL is associated with the bicrystalline misorientation. In general, ( γgb/ γs) varies by more than one order of magnitude between temperatures −5 °C ( γgb/ γs ~ 0.03) and −18 °C ( γgb/ γs ~ 0.35), so GB structural changes are very noticeable with temperature. The GB energy does not depend on the inclination at temperatures near 0 °C, while the dependence on inclination presents a slight significance at −18 °C. Finally, it was observed that the GB relative energy ( γgb/ γs) for bicrystalline 〈0001〉/ φ samples is lower than the relative energy at other crystalline misorientations. These experimental data are unique and could explain the slow GB migration in 〈0001〉/ φ bicrystals and they could help to understand grain growth in polar ice where the most frequent crystalline misorientation is 〈0001〉/ φ.

Molecular dynamics simulations on the intergranular crack propagation of magnesium bicrystals

As an important micro-structure, grain boundary plays a significant role in the micro-structure evolution and mechanical property of metallic materials. In this study, comprehensive molecular dynamics simulations were performed to investigate the propagation mechanisms of intergranular cracks along [11-00] symmetric tilt grain boundaries in magnesium bicrystals under tensile loading conditions. The effects of grain boundary misorientation angle, temperature and solid solution were considered. The results show: At low temperature, most of the cracks with different misorientation angles propagate by the brittle cleavage mechanism, which generally agrees with the theoretical predictions based on the Rice’s model. With increasing the temperature, the crack propagation mechanism changes, on the one hand, from brittle cleavage to void nucleation, growth and coalescence; on the other hand, the local plastic deformation becomes favorable near the crack tip, due to the dislocation emission and nanograin nucleation via the split of original grain boundary, which relaxes the stress concentration and shields the crack tip. Also, the brittle-to-ductile transition is observed due to the effect of temperature. By adding the alloying elements of Al, Ca, Zn, it is seen that the strength of the grain boundary is enhanced. This work provides insights into the understanding of the ductility of magnesium and its alloys.

Dislocation interactions at the grain boundary in FCC bicrystals: An atomistically-informed dislocation dynamics study

To understand the underlying mechanisms that control the mechanical properties of nanostructured metals, an insight into the role of the grain boundary in dislocation-driven plastic deformation is vital. The grain boundary has been observed as a dislocation source, sink, or having no effect, which in turn, gives rise to different macroscopic mechanical responses. With this motivation, atomistic simulations and three-dimensional dislocation dynamics simulations were performed to investigate dislocation interactions at various grain boundaries and their role in the plastic deformation of face-centered cubic (FCC) bicrystalline micropillars. The atomistically-informed dislocation dynamics simulations show that bicrystalline samples containing a high angle grain boundary (HAGB) display hardening and higher flow stresses compared to single crystals, while micropillars with a coherent twin boundary (CTB) show similar flow stresses to the reference single crystalline samples. This is due to the transparency of the grain boundary to slip transmission, which is observed in the atomistic simulations. Interestingly, allowing dislocation glide on the grain boundary exhibits a decrease in flow stress as slip transmission becomes easier.

On incipient plasticity in the vicinity of grain boundaries in aluminum bicrystals: Experimental and simulation nanoindentation study

The local mechanical behavior near <100> symmetric tilt grain boundaries in aluminum bicrystals were studied by the nanoindentation technique as well as by computer simulations. Experimentally, grain boundaries with misorientation angles 8.7°, 13.8° and 18.8° were examined. Two well pronounced pop-in events were observed on the load – penetration depth curves measured during indentation in the close vicinity of the studied boundaries. The load of the pop-ins, observed close to the boundaries, practically did not differ from that obtained in the grain interior. This was interpreted as evidence that the boundaries with misorientations in the examined angular range do not represent specific sites for sources of lattice dislocations. The load, at which the second pop-ins took place, substantially increased with increasing misorientation angle of the examined boundaries. Quasistatic molecular dynamics simulations were performed to identify the details of the interaction between grain boundary and dislocations generated during indentation. For this purpose, the bicrystals with similar geometry and misorientation angles, as investigated experimentally, were computed. The simulation results showed that the direct transmission of incoming dislocations across the grain boundary was the primary mechanism for the plastic flow transfer past the boundary. The analysis of the results of both experiments and simulations provided evidence that the capability of grain boundaries to act as a barrier for the motion of incoming dislocations depends crucially on grain boundary structure.

Atomic structure of grain boundaries in UO2 Bicrystals: A coupled high resolution transmission electron Microscopy/Atomistic simulation approach

In this work, high resolution transmission electron microscopy and numerical simulation at the atomic scale were coupled, for the first time to our knowledge, to study the atomic structure of two Σ5 symmetric tilt grain boundaries in uranium dioxide (UO2) bicrystals. We show that whereas the Σ5 (021)/[100] is fully symmetric, with the grain boundary plane forming a mirror symmetry plane, a rigid body translation exists between the crystals in the Σ5 (031)/[100]. We particularly believe that the excellent agreement between experimentally observed and simulated structures highlights the relevance of the coupled approach. Such result emphasize that the use of CRG and Yakub empirical potentials including a Morse contribution can be relied upon to model the behavior of UO2.

Atomistically informed crystal plasticity analysis of deformation behavior of alloy 690 including grain boundary effects

In this paper, we predict the deformation behavior of alloy 690 by explicitly modeling finite thickness grain boundaries within the crystal plasticity framework. This is realized by using a blend of quasi-static tensile experiments, microstructural characterization using scanning electron microscopy, atomistic simulations and crystal plasticity finite element analysis. We begin with material characterization to probe the type of grain boundaries present in the Ni-based alloy 690. Inspired by physically justified multiscale modeling schemes, we then use atomistic simulations within the framework of embedded-atom method to quantify activation parameters for dislocation nucleation from those grain boundaries in Ni bicrystals. The kinetic activation parameters are then passed on to a transition state theory based crystal plasticity model. At this scale, the grain boundaries are explicitly modeled using finite elements and prescribed distinct constitutive parameters obtained from lower scale atomistic simulations. The predictive capabilities of the adopted methodology are demonstrated by capturing the uniaxial tensile behavior of alloy 690 at different temperatures (250C–6000C) (stress-strain curves as well as texture evolution). Our implementation of grain boundaries in crystal plasticity is built upon a standard representative volume element used in most polycrystalline simulations. It is envisioned that this comprehensible approach will provide a compelling platform for unifying many other interface related deformation processes at the mesoscale. Eventually, the demonstrated methodology can expedite in apprehending the structure-property relationships through apt use of physically justified multiscale modeling schemes.

Influence of crystallographic orientation on the void growth at the grain boundaries in bi-crystals

Void growth and morphology evolution in fcc bi-crystals are investigated using crystal plasticity finite element method. For that purpose, representative volume element of bi-crystals with a void at the grain boundary are considered in the analysis. Grain boundary is assumed initially perpendicular/coaxial with the straight sides of the cell. Fully periodic boundary conditions are prescribed in the representative volume element and macroscopic stress triaxiality and Lode parameter are kept constant during the whole deformation process. Three different pairs of crystal orientations characterized as hard-hard, soft-soft and soft-hard have been employed for modelling the mechanical response of the bi-crystal. Simulations are performed to study the implications of triaxiality, Lode parameter and crystallographic orientation on slip mechanism, hardening and hence void evolution. The impact of void presence and its growth on the heterogeneity of lattice rotation and resulting grain fragmentation in neighbouring areas is also analysed and discussed.

Thermodynamic stability and kinematic accessibility of curved grain boundaries in directionally solidified bicrystals

Grain boundaries always have a positive excess free energy that drives grain growth and structural evolution during high-temperature heat treatments. Grain boundaries can, however, become trapped in a metastable configuration because of the geometry or composition of the specimens in which they are contained. For example, second phase dispersoids can pin grain boundaries in a polycrystal, and thermal grooves can pin grain boundaries in thin films and wires. In these examples of frustrated grain boundary systems, further reduction in the grain boundary area is energetically preferred, but grain boundary migration is arrested because energy is required to create additional grain boundary surface area and de-pin from the obstacle. In this work, we consider an idealized class of frustrated grain boundaries, namely curved grain boundaries in geometrically complex bicrystals. By calculating the energy landscape of the grain boundary configuration space, we show that the local stability of these curved grain boundaries is a sensitive function of the bicrystal shape. We also assess the trajectory of curved grain boundaries in bicrystals grown via directional solidification to determine which grain boundary configurations are physically realizable. By comparing these two analyses, we find that a curved grain boundary may be thermodynamically metastable but kinematically inaccessible, highlighting the general importance of thermodynamics and kinematics in descriptions of frustrated grain boundary systems encountered in a wide range of different materials.

The Effect of Magnetic Flux Focusing on the Current–Voltage Characteristics of YBa2Cu3O7- δ Grain Boundary Josephson Junctions

In order to obtain experimental evidence for the influence of flux focusing effect on the characteristics of planar grain boundary (GB) Josephson junctions, we measured the current–voltage characteristics and magnetic-field-dependent current–voltage steps of integrated YBa2Cu3O7- δ bicrystal GB Junctions with different junction widths. Under the self-magnetic field, the critical current of the GB junction is proportional to the square root of the junction width, which is different from the traditional linear relationship between the critical current and the junction width. When a vertical magnetic field is applied, the flux flow current–voltage step could be observed, and the step voltage increases monotonously with an increase in the junction width. When a magnetic field rotating in the bc plane is applied, the voltage value of the flux flow step decreases slowly as the magnetic field inclined angle increases when the field inclined angle is relatively large. Based on these results, we demonstrate the presence of the magnetic flux focusing effect on YBa2Cu3O7- δ GB Josephson junctions. The size of flux focusing in the GB is closely associated with the width of the junctions and the orientation of the magnetic field.

Shock-induced migration of asymmetry tilt grain boundary in iron bicrystal: A case study of Σ3 [110]**Project supported by the Fundamental Research for the Central Universities of China, the National Key Laboratory Project of Shock Wave and Detonation Physics of China, the Science and Technology Foundation of National Key Laboratory of Shock Wave and Detonation Physics of China, the National Key

Many of our previous studies have discussed the shock response of symmetrical grain boundaries in iron bicrystals. In this paper, the molecular dynamics simulation of an iron bicrystal containing Σ3 [110] asymmetry tilt grain boundary (ATGB) under shock-loading is performed. We find that the shock response of asymmetric grain boundaries is quite different from that of symmetric grain boundaries. Especially, our simulation proves that shock can induce migration of asymmetric grain boundary in iron. We also find that the shape and local structure of grain boundary (GB) would not be changed during shock-induced migration of Σ3 [110] ATGB, while the phase transformation near the GB could affect migration of GB. The most important discovery is that the shock-induced shear stress difference between two sides of GB is the key factor leading to GB migration. Our simulation involves a variety of piston velocities, and the migration of GB seems to be less sensitive to the piston velocity. Finally, the kinetics of GB migration at lattice level is discussed. Our work firstly reports the simulation of shock-induced grain boundary migration in iron. It is of great significance to the theory of GB migration and material engineering.

Dislocation nucleation from damaged grain boundaries in face centered cubic metals – An atomistic study

Towards unraveling the underlying behavior of grain boundaries containing a pre-existing void, we here present a computational approach to quantify the resistance of voided grain boundaries to plasticity evolution in face centered cubic metals. This is realized by conducting an atomistic modeling survey on 104 distinct grain boundaries in both Ni and Cu bicrystals subjected to uniaxial tension perpendicular to the interface plane. Within the framework of embedded-atom method, molecular dynamics simulations are performed to evaluate the critical stress and energy barrier required for dislocation nucleation from the voided interfaces. The resulting set of data is then analyzed to gage correlations between the strength of voided boundaries and common grain boundary descriptors in an effort to inform plasticity models at higher length scales. It is found that macroscopic grain boundary descriptors fail to uniquely predict the strength of damaged grain boundaries, thus suggesting some other mechanistic rationale for the role of such boundaries in governing dislocation nucleation under uniaxial tension in metals. Simulations indicate a direct correlation between the strength and ability of a grain boundary to store energy during tensile deformation. Additionally, it is also observed that a simple statistical distribution can represent dislocation nucleation stresses in face centered cubic metals. These correlations can definitely benefit the materials community in formulating rules for multi-scale materials modeling.

Crystal Plasticity Modeling of Void Growth on Grain Boundaries in Ni-Based Superalloys

In this work, we explore the effect of misorientation angles of crystal orientations between two grains along the grain boundary (GB) on void growth behavior in polycrystalline Ni-based superalloys by using a crystal plasticity finite element method. Quantitative analysis is conducted to study the coupled roles of the crystal orientation and stress triaxiality in void growth in bicrystals. Based on our simulation results, we find that, as the main loading axis perpendicular to the GB, voids grow more slowly on tilt GBs in bicrystals than those in single and bicrystal samples with twist GBs, while the void growth in single- and bicrystal samples with twist GBs exhibited almost the same rate and increased with the stress triaxiality levels. The interaction between two crystals bonded with the GB activates the effective Schmid factors in each crystal, which results in asymmetric distribution of the equivalent plastic strain around the void and induces distinct irregularly shaped voids during deformation.

Effect of Electrical Pulses on the Mechanical Behavior of Single Crystals of Nickel-Based CMSX-4 Superalloy and the Mobility of Low-Angle Grain Boundary in Aluminum Bicrystals

The electroplastic effect is studied on mechanically loaded single crystals of nickel-based CMSX-4 superalloy and pure aluminum bicrystals. Both objects of study exhibit elevated plasticity under electrical pulses. Traces of the electroplastic deformation that occurs in CMSX-4 single crystals at current densities J higher than 3 kA mm−2 and aluminum bicrystals at J exceeding 1.6 kA mm−2 are analyzed using a scanning electron microscope.

Interaction of Hydrogen Atoms with Grain Boundaries in Palladium Bicrystals

Hydrogen diffusion in palladium bicrystals containing a small-angle twist or tilt boundary or a large-angle boundary similar to a special boundary is investigated using molecular dynamics simulation. We assess the effect of grain boundaries on the hydrogen diffusion process. The types of grain boundaries considered here are shown to differ in their absorption activity for hydrogen. The temporal grain-boundary segregation of hydrogen atoms can be accounted for in terms of their coordination, which differs significantly from that in the grain bulk.

Coupled motion of grain boundaries and the influence of microcracks: A phase-field-crystal study

With phase-field-crystal, the two-dimensional coupled motion of grain boundary (GB) in triangular lattice crystal is studied. The migration mechanism of symmetric and asymmetric GBs in bicrystals is explored. Considering microcracks can hardly be avoided in materials, it is necessary to study the influence of microcracks on coupled GB motion. As shown in our simulation results, the influence of microcracks can be classified into three categories: (I) GB passes microcracks and is not impeded, cracks maintain unchanged; (II) Microcracks hinder GB motion, cracks migrate along GB and shrink; (III) GB motion is blocked by microcracks, cracks extend along GB and enlarge. The three kinds of results have great dependence on the misorientation and inclination angles of GB. Moreover, coupled GB motion can also lead to grain growth. The shear-induced grain growth process is simulated. As cracks can impede coupled GB motion, the shear-induced grain growth can also be hindered by cracks.

Interactions between displacement cascades and Σ3〈110〉 tilt grain boundaries in Cu

With large-scale molecular dynamics simulations, we investigate systematically the interaction of displacement cascades with a set of Σ3〈110〉 tilt grain boundaries (GBs) in Cu bicrystals at low ambient temperatures, as regards irradiation-induced defect production/absorption and GB migration/faceting. Except for coherent twin boundary, GBs exhibit pronounced preferential absorption of interstitials, which depends on initial primary knock-on atom distance from GB plane and inclination angle. GB migration occurs when displacement cascades overlap with a GB plane, as induced by recrystallization of thermal spike, and concurrent asymmetric grain growth. Faceting occurs via expanding coherent twin boundaries for asymmetric GBs.