Diffusion induced grain boundary migration in the Ag–Zn system

Diffusion induced grain boundary migration (DIGM) has been studied in the Ag–Zn system by exposing polycrystalline Ag to Zn vapor with a Ag-25 wt.% Zn alloy as the source of Zn. The time and temperature dependence of the migration distance has been studied in the temperature range 660 to 810 K. The composition profile was obtained on the sheet cross-section along a line perpendicular to the edge to determine D_bδ at each temperature. Similarly, the Zn concentration profile was obtained from the region swept by the migrating grain boundary. The coherency strain energy, the total chemical free energy change and the effective free energy change were calculated. The regular solution model was used for calculating the free energy change. It has been observed that a fraction of the total free energy has been used for volume diffusion in front of the migrating grain boundary. The instantaneous rate of migration has been observed to be directly proportional to the chemical free energy change and the coherency strain energy. The instantaneous rate of migration versus the composition graph has indicated that the driving force for DIGM in the Ag–Zn system is the coherency strain energy.The fine-grained layer formed at the surface follows a parabolic growth behavior. The diffusion coefficients calculated from the composition profile as well as from the rate of growth of the fine-grained layer are of the same order of magnitude. The diffusivity values are four to six orders of magnitude higher than the volume diffusion coefficients. From the activation energy and the diffusivities it is clear that DIGM in the Ag–Zn system occurs by the diffusion of Zn along the grain boundaries of polycrystalline Ag.     

Grain boundary sliding and migration in copper: the effect of vacancies

The atomic structure and mechanism of the interface sliding of the Σ = 5(2 1 0)[0 0 1] symmetric tilt grain boundary (GB) in copper and its interaction with vacancies at an elevated temperature has been studied using a computationally efficient potential based on the Embedding Atom Method in connection with the finite temperature Monte Carlo technique. Grain boundary sliding is performed for pure copper as well as copper containing a vacancy at a selected position. The discontinuous changes of the GB energy at certain sliding distances are associated with GB migrations. Elevated temperature reduces the grain boundary sliding/migration energy by a factor of about 2 but does not increase the rate of migration. Migration of the GB is mediated by the flow of atoms along the interface in coordination with the atoms in bulk. The sliding and migration properties partially depend on the position of the vacancy in the GB core. We found that the grain boundary sliding energy profile in the presence of a vacancy placed at the interface increased the GB energy, but reduces the sliding energy. The sliding process invokes the interface migration in such a way that the vacancy effectively migrates to a more convenient position and reduces the GB energy.     

The role of grain boundary energy in grain boundary complexion transitions

Recent findings about the role of the grain boundary energy in complexion transitions are reviewed. Grain boundary energy distributions are most commonly evaluated using measurements of grain boundary thermal grooves. The measurements demonstrate that when a stable high temperature complexion co-exists with a metastable low temperature complexion, the stable complexion has a lower energy. It has also been found that the changes in the grain boundary energy lead to changes in the grain boundary character distribution. Finally, recent experimental observations are consistent with the theoretical prediction that higher energy grain boundaries transform at lower temperatures than relatively lower energy grain boundaries. To better control microstructures developed through grain growth, it is necessary to learn more about the mechanism and kinetics of complexion transitions.     

Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

Grain growth and shrinkage are essential to the thermal and mechanical stability of nanocrystalline metals,which are assumed to be governed by the coordinated deformation between neighboring grain boundaries(GBs)in the nanosized grains.However,the dynamics of such coordination has rarely been reported,especially in experiments.In this work,we systematically investigate the atomistic mechanism of coordinated GB deformation during grain shrinkage in an Au nanocrystal film through combined state-of-the-art in situ shear testing and atomistic simulations.We demonstrate that an embedded nanograin experiences shrinkage and eventually annihilation during a typical shear loading cycle.The continu-ous grain shrinkage is accommodated by the coordinated evolution of the surrounding GB network via dislocation-mediated migration,while the final grain annihilation proceeds through the sequen-tial dislocation-annihilation-induced grain rotation and merging of opposite GBs.Both experiments and simulations show that stress distribution and GB structure play important roles in the coordinated defor-mation of different GBs and control the grain shrinkage/annihilation under shear loading.Our findings establish a mechanistic relation between coordinated GB deformation and grain shrinkage,which reveals a general deformation phenomenon in nanocrystalline metals and enriches our understanding on the atomistic origin of structural stability in nanocrystalline metals under mechanical loading.     

Molecular dynamics study of surface effects on atomic migration near aluminum grain boundary

In this paper, atomic migration near grain boundary of aluminum wiring line in micro-electronic device is analyzed by molecular dynamics (MD) simulation. Interatomic potential used is based on effective-medium theory (EMT). It is shown that junction point composed of grain boundary and surface is a region where active movement of atoms (atomic diffusion) appears. Under tensile loading, not only atomic diffusion but also slip between atomic layers (dislocation movement) is activated in the junction region. If there exists a certain constraint on the surface due to, for example, a passivation film attached on the aluminum line, atomic rearrangement near the junction changes remarkably.     

Orientation dependence of dislocation structure evolution during cold rolling of aluminum

A well-organized dislocation structure forms in many polycrystalline metals during plastic deformation. This structure is described qualitatively with no explanation of the quantitative characterization. In this work, the evolution of dislocation structure in commercial purity aluminum is described by means of the excess dislocation density and by quantitative characterization of the cell structure as seen on a plane surface. The measurements were performed on a pseudo-internal surface of a split specimen deformed by channel die deformation. The results show a clear dependence of cell structure formation on orientation of the crystallite with respect to the imposed deformation gradient with the largest excess dislocation density occurring in grains of {0 1 1}[1 2 2] orientation for plane strain deformation. Neighboring grain and non-local effects are shown to be of importance in the type of dislocation structures that evolve.     

Phase field simulations of elastic deformation-driven grain growth in 2D copper polycrystals

In this work, a phase field grain growth model coupled with a spectral stress calculation method is used to investigate the effect of applied elastic deformation on grain growth in 2D copper polycrystals with isotropic grain boundary properties. The applied deformation accelerates the grain growth compared to a relaxed polycrystal, though the effect of the deformation decreases rapidly with time. The softest grain orientations with respect to the applied deformation grow at the expense of other orientations, though they have higher elastic energy density. Due to a rapid decrease in the elastic energy stored in the system, the GB energy eventually dominates the growth leading to a linear change in the average grain area with time. Increasing the magnitude of the applied deformation accelerates the growth, while increasing the temperature accelerates the growth but decreases the effect of the applied deformation.     

Impact of grain boundary junctions on grain growth in polycrystals with different grain sizes

Effect of finite mobility of triple and quadruple grain boundary junctions on grain growth is studied by numerical simulation. They retard the growth process and transform its kinetics from parabolic one into a sequence consisting of exponential, linear and parabolic steps. This relates even to polycrystals with large initial grain sizes. Such junctions can contribute to microstructure stabilization in nanomaterials.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

Abstract

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with ∑3(011), ∑5(001) and ∑9(011) coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}(001) (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.

The ∑5 001l and ∑9 011l twist boundaries showed higher migration speed than ∑3(011) twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (?θ) from the ideal orientation relationship for a coincidence boundary. The increase of ?θ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the ∑9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed. © 2000 Elsevier Science Ltd. All rights reserved.     

An intrinsic correlation between driving force and energy barrier upon grain boundary migration

The migration of grain boundary (GB), which plays a key role in the microstructural evolution of polycrystalline materials, remains mysterious due to the unknown relationship between GB mobility associated with specific geometry and external conditions (e.g. temperature, stress, etc., hence the thermodynamic driving force). Combining the rate equation of GB migration with molecular dynamics simulations, an intrinsic correlation between driving force and energy barrier for the migration of various types of GBs (i.e. twist, symmetric tilt, asymmetric tilt, and mixed twist-tilt) is herein explored, showing the decrease of energy barrier with increasing thermodynamic driving force.     

The roles of grain orientation and grain boundary characteristics in the enhanced superelasticity of Cu71.8Al17.8Mn10.4 shape memory alloys

Through texture and grain boundary control by continuous unidirectional solidification, the continuous columnar-grained polycrystalline Cu_71.8Al_17.8Mn_10.4 shape memory alloys were prepared and possess a strong 〈0 0 1〉 texture along the solidification direction and straight low-energy grain boundary. The alloys show excellent superelasticity of 10.1% improved from 3% for ordinary polycrystalline counterpart and with a tiny residual strain of less than 0.3% after unloading. There are some reasons for the enhanced superelasticity: (1) The martensitic transformation of all grains with strong 〈0 0 1〉-oriented texture occur at the same time under the tensile loading, which can avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain can be obtained along 〈0 0 1〉 grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions of continuous columnar-grained polycrystals can significantly reduce the blockage of martensitic transformation at the grain boundaries. These results provide a reference to structure design of high-performance polycrystalline Cu-based shape memory alloys.     

Simultaneous grain boundary motion,grain rotation,and sliding in a tricrystal

Grain rotation and grain boundary (GB) sliding are two important mechanisms for grain coarsening and plastic deformation in nanocrystalline materials. They are in general coupled with GB migration and the resulting dynamics, driven by capillary and external stress, is significantly affected by the presence of junctions. Our aim is to develop and apply a novel continuum theory of incoherent interfaces with junctions to derive the kinetic relations for the coupled motion in a tricrystalline arrangement. The considered tricrystal consists of a columnar grain embedded at the center of a non-planar GB of a much larger bicrystal made of two rectangular grains. We examine the shape evolution of the embedded grain numerically using a finite difference scheme while emphasizing the role of coupled motion as well as junction mobility and external stress. The shape accommodation at the GB, necessary to maintain coherency, is achieved by allowing for GB diffusion along the boundary.     

The importance of grain boundary complexions in affecting physical properties of polycrystals

In recent decades researchers have revealed a rich variety of grain boundary segregation phenomena, including interfacial phase, or complexion, transitions. Grain boundary complexion transitions have been shown to induce discontinuous changes in materials properties as a function of temperature and chemical potential, and have been used to explain phenomena that had previously evaded satisfactory explanation. This review article discusses how grain boundary complexions relate to mass transport and mechanical properties, by highlighting both what is understood and emphasizing topics requiring additional study.     

Modelling of the grain size probability distribution in polycrystalline nanomaterials

Theoretical analyses have always resulted in nanomaterials’ grain size probability distribution being of varied form: approximately either lognormal, Rayleigh, normal, Weibull, etc. The isotropic Hillert’s model of grain growth which is more suitable for soap froth has been frequently used to establish these distributions with the hope of approximating experimental observations. Observed grain growth in nanomaterials shows departures from the Hillert’s model.In the present paper, the probability distribution of grain size in nanomaterials is dealt with. Use is made of a modified model of grain growth in polycrystalline nanomaterials developed recently by the authors. The modified model accounts for grain growth caused by curvature driven grain boundary migration and grain rotation-coalescence mechanisms. Since the grain size in the aggregate is random, the stochastic counterpart of the expression governing the incremental change in individual grain size is obtained by the addition of two fluctuation terms.The integro-differential equation governing the development of the probability density function of the grain size is obtained which is the generalised Fokker–Planck–Kolmogorov equation. Numerical solution to the integro-differential equation is obtained.Results from analytical modelling of grain size probability distribution in polycrystalline nanomaterials are different if the effect of grain rotation-coalescence mechanism on grain growth process is taken into account and, further, due to the addition of the fluctuation terms. Results also depend on the nature of the fluctuation term, which is a material property as the fluctuation in grain sizes varies from one material to another. It is shown that many of the major attributes of grain growth, such as self similarity (probability density approaching a stationary one), can be predicted by the solution of the Fokker–Planck–Kolmogorov equation.     

The influence of grain boundary segregation on the rate of contact melting in metals

The contact melting of polycrystalline solid solutions with metals is studied. The linear correlations between the average rate of contact melting and (i) the energy of interaction between the impurity atoms and grain boundaries and (ii) differences in properties of constituents of solid solutions (namely differences in surface energies, electron work functions and generalized statistical V.K. Semenchenko moments) are revealed. Found relations demonstrate the importance of grain boundary segregation in the contact melting phenomenon.     

The interplay between osteoblast functions and the degree of nanoscale roughness induced by grain boundary grooving of nanograined materials

From the perspective of osseointegration, nanograined/ultrafine-grained (NG/UFG) metals provide surfaces that are different from conventional coarse-grained (CG) polycrystalline metals because of the high fraction of grain boundaries. We describe here the interplay between the cellular response and grain boundary grooving as a potential approach to enhance osteoblast functions and facilitate the biomechanical interlocking and anchorage. This is accomplished by making a relative comparison of osteoblast response of NG/UFG grains electrochemically grooved to different depths to induce different degree of nanoscale roughness with planar NG/UFG surfaces, under identical biological environment. Electrochemically grooved NG/UFG structures indicated significant attachment and proliferation, and consequently enhanced modulation of cellular response that was significantly different from planar (non-grooved) NG/UFG substrate. Consistent with cell attachment and proliferation, immunofluorescence microscopy and computational analysis indicated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on electrochemically-grooved NG/UFG substrates. These observations are indicative of accelerated response of cell-substrate interaction and activity. The behavior is attributed to average nanoscale roughness and high surface hydrophilicity of the nanoengineered surface.     

Interaction of lattice dislocations with a grain boundary during nanoindentation simulation

Interaction of dislocations with a Σ = 5 (210) [001] grain boundary was investigated using molecular dynamics simulation with EAM potentials. The results showed that the dislocation transmitted across the grain boundary during nanoindentation and left a step in the boundary plane. Burgers vector analysis suggested that a partial dislocation in grain I merged into the grain boundary and it was dissociated into another partial dislocation in grain II and a grain boundary dislocation, introducing a step in the grain boundary. Simulation also indicated that, after the transmission, the leading partial dislocation in the grain across the boundary was not followed by the trailing partials, expanding the width of the stacking fault. The results suggested that the creation of the step that accompanied grain boundary motion and expansion of the stacking fault caused resistance to nanoindentation.     

Resistance to cleavage cracking and subsequent shearing of high-angle grain boundary

In a previous experimental study, it was observed that the break-through process of a cleavage front across a high-angle grain boundary can be highly nonuniform. While the central part of the boundary can be cleaved quite smoothly, the rest parts must be sheared apart. In this paper, the trapping effect of grain boundary shearing is analyzed in considerable detail. Before the shearing is completed, the crack flanks are locally pinned together and a bridging stress must be provided. The bridging stress has a negative contribution to the local stress intensity at the cleavage front segment that penetrates across the grain boundary, and thus the crack growth driving force must be increased. A closed-form equation is derived to relate the overall fracture resistance to the fracture mode through an energy analysis.     

Misorientation dependence of grain boundary migration in advanced ultra-supercritical Ni-based superalloy

ABSTRACT

The microstructures of a Ni-based superalloy used in the advanced ultra-supercritical condition were investigated after creep deformation. The grain boundary migrated during tertiary creep. Accompanied by the migration of grain boundaries, the coarsened γ′ phase with rodlike shape was formed and the precipitate-free zones emerged around this coarsened γ′. The distributional misorientation angle of these grain boundaries was from 45° to 65°. After the examination of the distribution characteristics of the cracks, it was found that the intergranular cracks did not propagate through the precipitate-free zones with the coarsened γ′.