Asymmetric Grain Research Articles

The impact of melt flow on the grain orientation in solidifying metal alloys

Directional solidification experiments were performed in PbSn and AlSi alloys under the influence of a rotating magnetic field. The solidification process was started without forced melt convection before applying a sudden onset of the electromagnetic force. Just before and after the initiation of the magnetic field the grain structure was analysed. Electrolytic etching was used to identify grains having different orientations and related EBSD measurements provided quantitative values for the crystallographic orientation of the grains. Whereas a distinct asymmetric grain growth or a deflection of the columnar grains becomes obvious in case of forced melt flow, a perceivable modification of the crystal orientation cannot be detected.

Characterization and atomic modeling of an asymmetric grain boundary

Grain boundaries (GBs) significantly affect the properties of materials. In an effort to examine the phenomena at GBs, many model boundaries, typically symmetric tilt GBs, have been investigated. However, the geometries of symmetric tilt GBs are too restricted to represent commonly occurring interface phenomena properly in polycrystalline materials. Thus, a method of applying density functional theory (DFT) to asymmetric GBs has long been desired. Here, we present a simple geometric method and a new GB model with two surfaces which make it possible to characterize an asymmetric tilt GB and calculate the GB energetics. Our method can be extended to study other geometric asymmetric interfaces in various materials. The proposed technique thus paves the way for DFT-related studies of asymmetric interfaces.

First principles investigation of zinc-induced embrittlement in an aluminum grain boundary

Aluminum–zinc alloys have an extensive range of structural applications, but their susceptibility to intergranular stress corrosion cracking limits their wider use. In the present work the influence of zinc on a ∑5(0 1 2)[1 0 0] aluminum grain boundary was investigated by means of first principles calculations with the full potential linearized augmented plane wave method using two approaches: (i) within the framework of the Rice–Wang thermodynamic model; (ii) by the ab initio tensile test method. We determined the most energetically favorable segregation site of Zn along the Al grain boundary, its segregation energy from the Al grain boundary, the possible fracture paths of the grain boundary with Zn and the corresponding fracture energies. We established that Zn has a large driving force (−0.19 eV atom −1) for segregation from the Al bulk to the asymmetrical grain boundary site, and its segregation reduces the grain boundary strength slightly. Unusually, segregation of larger atomic size Zn leads to grain boundary contraction. Through precise calculations it was confirmed that zinc is a weak embrittler with a potency of +0.05 eV atom −1. Analysis in terms of the relaxed atomic and electronic structures and bonding characters showed that aluminum–zinc bonds have a metallic character in both grain boundary and free surface environments. This work provides a fundamental quantitative understanding of Zn-induced grain boundary embrittlement in Al alloys on the electronic level and establishes that Zn segregation is one of the factors contributing to stress corrosion cracking in Al alloys.

Optical and Transport Properties of Rare-earth Trivalent Ions Located at Different Sites in Sol-gel SnO2

Photoluminescence and photo-excited conductivity data as well as structural analysis are presented for sol-gel SnO2 thin films doped with rare earth ions Eu3+ and Er3+, deposited by sol-gel-dip-coating technique. Photoluminescence spectra are obtained under excitation with various types of monochromatic light sources, such as Kr+, Ar+ and Nd:YAG lasers, besides a Xe lamp plus a selective monochromator with UV grating. The luminescence fine structure is rather different depending on the location of the rare-earth doping, at lattice symmetric sites or segregated at the asymmetric grain boundary layer sites. The decay of photo-excited conductivity also shows different trapping rate depending on the rare-earth concentration. For Er-doped films, above the saturation limit, the evaluated capture energy is higher than for films with concentration below the limit, in good agreement with the different behaviour obtained from luminescence data. For Eu-doped films, the difference in the capture energy is not so evident in these materials with nanoscocopic crystallites, even though the luminescence spectra are rather distinct. It seems that grain boundary scattering plays a major role in Eu-doped SnO2 films. Structural evaluation helps to interpret the electro-optical data.

Shock wave loading and spallation of copper bicrystals with asymmetric Σ3⟨110⟩ tilt grain boundaries

We investigate the effect of asymmetric grain boundaries (GBs) on the shock response of Cu bicrystals with molecular dynamics simulations. We choose a representative Σ3⟨110⟩ tilt GB type, (110)1/(114)2, and a grain size of about 15 nm. The shock loading directions lie on the GB plane and are along [001¯] and [221¯] for the two constituent crystals. The bicrystal is characterized in terms of local structure, shear strain, displacement, stress and temperature during shock compression, and subsequent release and tension. The shock response of the bicrystal manifests pronounced deviation from planar loading as well as strong stress and strain concentrations, due to GBs and the strong anisotropy in elasticity and plasticity. We explore incipient to full spallation. Voids nucleate either at GBs or on GB-initiated shear planes, and the spall damage also depends on grain orientation.

Asymmetric grain distribution in phthalocyanine thin films

Many electronic and optical properties of organic thin films depend on the precise morphology of grains. Iron phthalocyanine thin films are grown on sapphire substrates at different temperatures to study the effect of grain growth kinematics and to experimentally quantify the grain size distribution in organic thin films. The grain size is measured with an atomic force microscope and the data is processed and analyzed with well-known image segmentation algorithms. For relevant statistics, over 3000 grains are evaluated for each sample. The data show pronounced asymmetric grain growth with increasing deposition temperature from almost spherical grains at room temperature to elongated needlelike shapes at $260\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The average size along the major axis increases from 35 to 200 nm and along the minor axis from 25 to 90 nm. The distribution is almost symmetric at low-deposition temperatures, but becomes lognormal at higher temperatures. Strikingly, the major axis and minor axis of the elliptically shaped grains have different distributions at all temperatures due to the planar asymmetry of the molecule.

Atomistic segregation analysis in an asymmetrical grain boundary for Ni(Ag)

Intergranular segregation is studied for the first time in an asymmetrical grain boundary at the atomistic level. The grain boundary (GB) is the asymmetrical tilt (331)//(11-1) <110> experimentally observed by high resolution transmission electron microscopy (HRTEM) in nickel bicrystal. The metallic alloy chosen for the study is nickel(silver) in the limit of infinity diluted solution. The interactions are modelled with n-body, Finnis-Sinclair like, potentials. The atomic sites are characterized by their exact Voronoï volumes and by the stress density tensors locally exerted. In these respects, the asymmetrical near Σ=11 (331)//(11-1) GB presents a larger diversity of sites than the symmetrical GBs previously studied. The segregation energies are computed and analyzed versus the two ‘driving forces’, the elastic size effect and the excess cohesion energy effect, which play a role in metallic intergranular segregation in the one atom segregation limit. The elastic size effect calculated by the method of virtual impurity represents the main segregation driving force in most cases of the considered bimetallic alloy. This generalizes to an asymmetrical GB results previously obtained on symmetrical GBs.

Charles Crussard's early contributions: Recrystallization in situ and a Grain Boundary study with J. Friedel and B. Cullity

Abstract Charles Crussard's discovery of what he called Recrystallization in situ in 1944 is compared with what Egon Orowan and Robert Cahn named Polygonization. The context of the previous research is recalled in order to show the novelty of these contributions. Progress in accurate deformation and observation techniques such as tensile machines and X-ray methods were important, as were theoretical models about plausible microscopic mechanisms. With Jacques Friedel and Barney Cullity, Ch. Crussard also wrote a pioneering article on grain boundaries, which contains an experimental study of the relative energies of both symmetrical and asymmetrical grain boundaries observed in aluminium together with the first atomistic calculation of energies of symmetrical low and high angle tilt boundaries in several metals. This less well-known contribution is also recalled.

Grain boundary finite length faceting

We study symmetrical and asymmetrical aluminium grain boundary faceting with molecular dynamics simulations employing two embedded atom method potentials. Facet formation, coarsening, and the reversible phase transition of ∑ 3 { 1 1 0 } boundary into ∑ 3 { 1 1 2 } twin, and vice versa, are demonstrated in the simulations and the results are consistent with earlier experimental studies and theoretical models. The ∑ 11 { 0 0 2 } 1 / { 6 6 7 } 2 boundary shows faceting into { 2 2 5 } 1 / { 4 4 1 } 2 and { 6 6 7 } 1 / { 0 0 1 } 2 boundaries and coarsens with a slower rate when compared to ∑ 3 { 1 1 2 } facets. However, facets formed by { 1 1 1 } 1 / { 1 1 2 } 2 and { 0 0 1 } 1 / { 1 1 0 } 2 boundaries from a { 1 1 6 } 1 / { 6 6 2 } 2 boundary are stable against finite temperature annealing. In the above faceted boundary, elastic strain energy induced by atomic mismatch across the boundary creates barriers to facet coarsening. Grain boundary tension is too small to stabilize the finite length faceting in both ∑ 3 { 1 1 2 } twin and asymmetrical { 1 1 1 } 1 / { 1 1 2 } 2 and { 0 0 1 } 1 / { 1 1 0 } 2 facets. The observed finite facet sizes are dictated by facet coarsening kinetics which can be strongly retarded by deep local energy minima associated with atomic matching across the boundary.

High-field vortices in dense chains of O- and π-shifted Josephson junctions

Sequences of spatially interchanging 0- and π-shifted Josephson junctions are encountered in superconductor-ferromagnet-superconductor heterostructures, asymmetric grain boundaries in thin films of YBCO, and YBCO/Nb zigzag junctions. In this paper we demonstrate existence of Josephson vortices in applied fields much higher than the Josephson penetration field. The high-field vortices exist in narrow field intervals near equidistant fields Hn. When the applied field is in any of these intervals, the flux per junction is nø 0, where n in an integer. We show that high-field vortices are much longer than the vortices carrying flux in each of the 0- or π-shifted junctions. As a result the field within the vortices is much lower than the background field. High-field vortices carry one flux quanta or half-flux quanta and are free to move, unlike the semi-fluxons in low fields that are pinned by the contacts between 0- and π-junctions. In the presence of a transport current the high-field vortices are subjected to the Lorentz force and move to one side of the chain, reflect, move to the other side, etc. This periodic motion generates constant voltage across the chain with resonances similar to the zero-field-steps. If the vortices carry half-flux quanta, the resonances will appear at half the voltage one would expect for zero-field-steps.

Grain-boundary plane orientation dependence of electrical barriers at Σ5 boundaries in SrTiO 3

Dependence of the electrical properties on grain-boundary plane orientation is examined by a combination of high-resolution transmission electron microscopy, impedance spectroscopy, and electron energy-loss spectrometry using two kinds of SrTiO 3 Σ5 ([1 0 0]/36.8°) bicrystalline grain boundaries: symmetric (3 1 0) (18.4°/18.4°) and asymmetric (8.4°/28.4°). While the symmetric grain boundary is observed to be straight with the symmetric (3 1 0)//(3 1 0) plane orientation, the asymmetric grain boundary is faceted into symmetric (3 1 0)//(3 1 0) and (2 1 0)//(2 1 0), and asymmetric (1 0 0)//(4 3 0). Grain-boundary impedance is observed only in the asymmetric grain boundary, and the electron energy-loss spectrometry quantification indicates that the asymmetric (1 0 0)//(4 3 0) facets are more oxygen-deficient than the symmetric ones. The results suggest that the asymmetric (1 0 0)//(4 3 0) facets are the most resistive among the three different facets.

Effect of impurities on grain boundary cohesion in bcc iron

The effect of B, N and O impurities placed in interstitial and substitutional positions at symmetric Σ5(210) Fe grain boundary is studied by means of first principles calculations. Full relaxation of supercell shape and volume is applied which results in stable asymmetric grain boundaries, depending on the impurity and its position. In all cases the big shifts (0.47–2.33Å) of grains with respect to each other are observed. The equilibrium distance between the grains is decreased for impurities in substitutional positions, and increased for atoms in interstitial sites, compared to the relaxed clean GB. We found that nitrogen both in interstitial and substitutional positions, and boron in substitutional position enhance cohesion while oxygen in both positions, and interstitial boron, are embrittlers. The magnetic moments of Fe atoms at clean grain boundary are remarkably increased compared to the bulk. They tend to the bulk value in the middle of the grain in an oscillatory way.

Shapiro steps in Josephson junctions with alternating critical current density

We treat theoretically Shapiro steps in tunnel Josephson junctions with spatially alternating critical current density. Explicit analytical formulas for the width of the first integer (normal) and half-integer (anomalous) Shapiro steps are derived for short junctions. We develop a coarse-graining approach, which describes Shapiro steps in the voltage-current curves of the asymmetric grain boundaries in yttrium barium copper oxide thin films and different superconductor-ferromagnet-superconductor Josephson-type heterostructures.

Structures and energies of Σ 3 asymmetric tilt grain boundaries in copper and aluminium

The objective of this research is to use atomistic simulations to investigate the energy and structure of symmetric and asymmetric Σ3 ⟨110⟩ tilt grain boundaries. A nonlinear conjugate gradient algorithm was employed along with an embedded atom method potential for Cu and Al to generate the equilibrium 0 K grain boundary structures. A total of 25 ⟨110⟩ grain boundary structures were explored to identify the various equilibrium and metastable structures. Simulation results show that the Σ3 asymmetric tilt grain boundaries in the ⟨110⟩ system are composed of only structural units of the two Σ3 symmetric tilt grain boundaries. The energies for the Σ3 grain boundaries are similar to previous experimental and calculated grain boundary energies. A structural unit and faceting model for Σ3 asymmetric tilt grain boundaries fits all of the calculated asymmetric grain boundary structures. The significance of these results is that the structural unit and facet description of all Σ3 asymmetric tilt grain boundaries may be predicted from the structural units of the Σ3 coherent twin and incoherent twin boundaries for both Cu and Al.

Atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on p-Si substrates due to thermal treatment

The plane-view high-resolution transmission electron microscopy (HRTEM) images in ZnO thin films grown on p-Si substrates showed that (101¯0) asymmetric grain boundaries with a periodic array of strain contrast features existed in a sparse columnar structure for as-grown ZnO thin films and that (112¯0) asymmetric grain boundaries and (8513¯0) symmetric grain boundaries existed in a dense columnar structure for annealed ZnO thin films. The atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on Si substrates due to thermal treatment are described on the basis of the HRTEM results.

Interaction mechanism between edge dislocations and asymmetrical tilt grain boundaries investigated via quasicontinuum simulations

The interactions between edge dislocations and grain boundaries---dislocation pileup, dislocation absorption, and dislocation transmission---are studied by performing quasicontinuum simulations. The $⟨112⟩$ asymmetrical tilt grain boundaries with different misorientation angles are used. The atomic configurations and stress fields of equilibrium and nonequilibrium asymmetrical grain boundaries are investigated in detail by comparison with analytical models. The influence of the grain boundary structure on the stress concentration due to dislocation pileup and the accommodation of extrinsic dislocations in the grain boundaries are also examined by using low- and high-angle grain boundaries. The critical forces on the dislocation in small-angle tilt grain boundaries for it to eject from the boundaries are evaluated by atomic simulations, and the results are compared with dislocation theory. It is also found that the rearrangement of the grain boundary dislocations with local grain boundary sliding in the local region, where the extrinsic dislocation is absorbed, is the characteristic accommodation mechanism of low-angle asymmetrical grain boundaries. The effects of the interaction between dislocations and grain boundaries on the mechanical properties of coarse-grained metals with dislocation sources in their grain and on those of nanocrystalline metals without sources in their grain are also discussed.

Correlation between the atomic structures and the misorientation angles of [0001]-tilt grain boundaries at triple junctions in ZnO thin films grown on Si substrates

The correlation between the atomic structures and the misorientation angles of [0001]-tilt grain boundaries at triple junctions in ZnO thin films grown on Si substrates was investigated by using high-resolution transmission electron microscopy (HRTEM) measurements. The HRTEM images showed three symmetric grain boundaries and one asymmetric grain boundary around the triple junction in the ZnO film. The correlation between the atomic structures and the misorientation angles of the grain boundaries at triple junctions in ZnO films is described on the basis of the HRTEM results.

Comparative studies of microstructure and impedance of small-angle symmetrical and asymmetrical grain boundaries in SrTiO 3

Several low-angle symmetrical [0 0 1]-tilt grain boundaries and an asymmetrical grain boundary with both tilt and twist components were examined by high-resolution transmission electron microscopy, conventional transmission electron microscopy, and impedance spectroscopy. The symmetrical tilt grain boundaries consist of isolated dislocation cores and the Burgers vectors are translation vectors of the crystal. Strain analysis shows that the strain fields around the dislocation cores overlap with increasing misorientation angle. The a.c. impedance response of the low-angle grain boundaries reveals a marked blocking effect for charge transport across the low-angle symmetrical grain boundaries, which increases with increasing misorientation angle. In the case of the mixed asymmetrical grain boundary, however, the blocking effect is low despite the large misorientation. The resistance and electrical potential of this grain boundary were calculated and compared with those obtained from the symmetrical grain boundaries. A relationship between microstructure and macroscopic electrical property can approximately be arrived by considering anion vacancies to be preferentially situated in highly distorted regions such as dislocation cores.

Relation between grain boundary segregation and grain boundary character in FCC alloys

An analytical model of segregation at grain boundaries, which takes into account all five macroscopic parameters of grain boundary character, has been developed. The model is based on a combination of previous bond energy treatments of grain boundary energy and of segregation to free surfaces. It is tested by comparing its predictions against previous computations of segregation to symmetrical twist grain boundaries in simple fcc alloys obtained by Monte Carlo simulations in conjunction with embedded atom method potentials. The comparison shows good agreement with the previous computer simulations. Examples of model predictions in the case of asymmetric grain boundaries are also provided.

Challenges of interfacial classification for grain boundary engineering

The most widely used characteristic of grain boundary type in grain boundary engineering (GBE) is the reciprocal density of coincidence sites Σ. Low values of Σ are supposed to indicate special boundaries. A systematic study of segregation at well characterised grain boundaries in Fe–Si alloys resulted in detailed classification of [100] tilt grain boundaries refining the coincidence site lattice (CSL) concept. This classification agrees well with the CSL concept in characterising the low Σ {015}, {013} and {012} symmetrical grain boundaries as special, however, the CSL concept fails in classifying the asymmetrical boundaries. It was clearly proved that general boundaries also exist for the Σ=5 CSL relationship while a special boundary appears in the non-coincidence 45°[100] misorientation. The important result of this classification – the finding that each asymmetrical grain boundary formed by the (011) plane is special – evokes new possibilities for GBE technologies.