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Abstract
This paper presents an appraisal of investigations which feature statistics of grain boundary plane distributions in polycrystals. The approach taken is to set the review against a background which includes the significance of the grain boundary plane parameter and several methodologies for its investigation in both bicrystals and polycrystals, including high resolution electron microscopy and computer simulation. The digest of data in polycrystals is analysed in the light of tilt and twist characteristics, boundary plane reorientation, the occurrence of low-index boundary planes and correlation with boundary properties. It emerges that the key factors which control the crystallography of boundary planes are the combined effects of material type, twinning (where applicable), microtexture, proximity to a coincidence site lattice and boundary plane inclination with respect to the macroscopic specimen geometry. Grain boundary plane engineering, wherein CSL categorisation is secondary to boundary plane crystallography, is recommended as a more advanced approach and its feasibility is demonstrated.
... High-temperature properties of grain boundaries including grain boundary energy, mobility, diffusion and segregation are reported to depend strongly on the associated crystallographic parameters, as covered in detailed reviews [1][2][3][4]. Five crystallographic macroscopic parameters are: three for misorientation and two for the grain boundary plane normal [1]. ...
... High-temperature properties of grain boundaries including grain boundary energy, mobility, diffusion and segregation are reported to depend strongly on the associated crystallographic parameters, as covered in detailed reviews [1][2][3][4]. Five crystallographic macroscopic parameters are: three for misorientation and two for the grain boundary plane normal [1]. In studies on bicrystals, selected grain boundaries are formed with specific macroscopic parameters, for example a given tilt angle about <1 1 0> axis, with specific grain boundary planes [5,6]. ...
... In metals, binding energy is significant in determining relative energies. For example, a large fraction of coincidence sites on Σ3 or Σ11 on specific planes lead to exceptionally low GB energies [1,5,8,11,21]. In MgO, asymmetric tilts with low-energy surface planes {1 0 0} on one side were found to occur more frequently compared to symmetric tilts with the same misorien- tation [7,17]. ...
Properties of grain boundaries such as grain boundary energy, mobility and diffusion are reported to depend strongly on their crystallography. While studies on ceramic bicrystals with low Σ misorientations have shown highly ordered structures and low energies, studies on dense polycrystalline ceramics often show the significance of grain boundary planes. In the present study, grain boundary plane distributions were studied for yttria-stabilised cubic zirconia with varying grain sizes using Electron Back Scattered Diffraction technique combined with a stereological approach. Despite nearly isotropic grain boundary plane distributions, a highly anisotropic grain boundary character distribution is observed for specific misorientations. Certain low-energy symmetric tilts such as Σ3 and Σ11 are found to occur with high frequencies across the grain size range studied, leading to an inverse correlation between GB energy and frequency of occurrence, consistent with other ceramics studied in literature.
... The importance of the two remaining DOF describing the grain boundary plane orientation has been shown in many papers, e.g. [5][6][7][8][9][10][11][12][13][14][15][16]. The difference between the CSL criterion and the grain boundary classification based on all five DOF in terms of GBE can be illustrated in Fig. 1. ...
... One of the first analyses to emphasize the importance of the grain boundary plane orientation in addition to the CSL relationship was the computer simulation study performed by Wolf and Phillpot [5]. In recent decades, the importance of the plane orientation on the properties of grain boundaries has been demonstrated experimentally by Randle et al. [8,9,14], Rohrer and co-workers [10][11][12]16], and others [13,15]. More experimental as well as theoretical results have been obtained for f.c.c. ...
... The directions parallel to the seed axes (=growth axes) were h311i for grain 1, h210i for grain 2, Fig. 1 Change of the grain boundary character in a polycrystal during GBE procedures. a Currently applied mechanism: change of the misorientation spectrum to increase the proportion of low-R CSL boundaries; b proposed mechanism [8]: grain boundary inclination to establish special orientations by a specific boundary plane. A, B, C, D are individual grains, separated by general, G, and special, S, grain boundaries. ...
Grain Boundary Engineering is a modern concept for the production of polycrystalline materials with optimized properties. It is based on manipulation of the structure to increase the proportion of special grain boundaries. Present technologies based on a simple but insufficient classification of grain boundaries according to the Coincidence Site Lattice model require mutual rotations of all grains in the structure i.e. the recrystallization of the material performed under specific external conditions (annealing in magnetic field etc.). Recently, the effect of grain boundary orientation on properties of polycrystalline materials has been emphasized and a new grain boundary classification has been proposed both suggesting that optimized polycrystalline material can also be produced by reorientation of the grain boundaries between existing grains during suitable annealing. As a contribution to this field, model experiments with growth and annealing of a tricrystal of an Fe-3mass%Si alloy are reported which display inclinations of the grain boundaries to establish low energy orientations supporting the above mentioned ideas.
... Theoretical models, such as the dislocation glide mechanism for low-angle GBs (7,8), the local conservative shuffling of atoms for high-angle GBs (9), and the unified disconnection-mediated mechanism (10,11), have been proposed to describe the GB migration. However, these models do not consider the change of GB structure during and after the migration, which frequently happens in one of the most common GBs in polycrystalline materials, i.e., the asymmetrical tilt GBs (ATGBs) (12), especially in the GB with facets, and could affect their migration behaviors. For instance, Σ11 ATGBs (13) in Cu were found to show unique anisotropic mobility, which is associated with the transformation events at the facet nodes and incommensurate GB facets during the migration (14). ...
... The faceted morphology is a typical feature of ATGBs (12,21), which are formed via GB facet transformation (also termed as GB faceting). Broadly speaking, GB faceting, dissociation, and structural phase transformation all belong to GB complexion transition, as they involve the change of GB structure units (22). ...
Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of ( 1 ¯ 1 ¯ 1 ) / ( 002 ) -type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of ( 111 ) / ( 11 1 ¯ ) -type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.
... In the case of CSL grain boundaries, it corrects the misconception that LAGBs involves in good corrosion resistance for long time. Compared with the LAGBs, there is a high proportion of undistorted coherent atoms in the CSL grains boundaries, resulting in a relatively low interface energy [43,44]. However, the recent work indicated that the CSL model was influenced by boundary orientation and showed that only the coherent twin ∑3 n grain boundaries had high corrosion resistance due to the presence of numerous jointed lattice points with small excess energy [44,45]. ...
... Compared with the LAGBs, there is a high proportion of undistorted coherent atoms in the CSL grains boundaries, resulting in a relatively low interface energy [43,44]. However, the recent work indicated that the CSL model was influenced by boundary orientation and showed that only the coherent twin ∑3 n grain boundaries had high corrosion resistance due to the presence of numerous jointed lattice points with small excess energy [44,45]. Nevertheless, only a small fraction of low-∑3, low-∑9 and low-∑27 grain boundaries are visible, namely coherent twin ∑3 n grain boundaries as represented by cerise, amaranth and jasper lines in Fig. 1g. ...
... This means that it is impossible to correlate the microstructure of the materials with the crystallographic orientations using SR-µCT. EBSD has been widely used to reveal the crystallographic microstructure of the materials with great spatial resolutions, but which only gives 2D surface information of the samples and so that only four of five parameters required for describing a full grain boundary are provided [52], which limits the investigation on the effect of the grain boundaries and crystallographic orientations on the fracture behavior of polycrystalline structural materials. In combination of EBSD and focused ion beam (FIB), EBSD can be extended into 3D-EBSD, while which has to serially section the sample by FIB for 3D reconstruction. ...
... In combination with in-situ SR-µCT characterization in corrosive environment, it is found that a specific range of grain boundaries have special properties and which directly correlate with the crack bridging development. reveal the crystallographic microstructure of the materials with great spatial resolutions, but which only gives 2D surface information of the samples and so that only four of five parameters required for describing a full grain boundary are provided [52], which limits the investigation on the effect of the grain boundaries and crystallographic orientations on the fracture behavior of polycrystalline structural materials. In combination of EBSD and focused ion beam (FIB), EBSD can be extended into 3D-EBSD, while which has to serially section the sample by FIB for 3D reconstruction. ...
Synchrotron radiation computed micro-tomography (SR-µCT) is a non-destructive characterization method in materials science, which provides the quantitative reconstruction of a three-dimension (3D) volume image with spatial resolution of sub-micrometer level. The recent progress in brilliance and flux of synchrotron radiation source has enabled the fast investigation of the inner microstructure of metal matrix composites without complex sample preparation. The 3D reconstruction can quantitatively describe the phase distribution as well as voids/cracks formation and propagation in structural metals, which provides a powerful tool to investigate the deformation and fracture processes. Here, we present an overview of recent work using SR-µCT, on the applications in structural metals.
... It is generally assumed that low ( ≤ 29) CSL boundaries are 'special' even though there is no physical basis for this assumption [1][2][3]5,9,10]. Recent results have indicated that only a subset of low CSLs are special [6,[11][12][13][14][15]. In spite of this assumption, GBE approach has been shown to be effective in enhancing the properties in a variety of low-to-medium stacking fault energy materials [1][2][3]5,10]. ...
... In spite of this assumption, GBE approach has been shown to be effective in enhancing the properties in a variety of low-to-medium stacking fault energy materials [1][2][3]5,10]. This is essentially due to the fact that these materials exhibit prolific multiple twinning during GBE-type thermo-mechanical processing leading to higher fraction of 'special' 3 boundaries terminating on low-index plane [12]. In addition to this, multiple twinning also leads to the generation of 9 and 27 boundaries through the following interactions: ...
... Hence, the fracture of crystalline metals is found to prefer intergranular way [5][6][7]. To improve the reliability of crystalline metals, researchers have tried to add certain GB proportions in GB engineering, especially in those with low Σ (reciprocal density of coincidence sites) [8,9]. However, the mechanisms of intergranular fractures along these special GBs are poorly understood and need to be identified. ...
The fracture of crystalline metals preferentially occurs in intergranular way. This work investigates orientation effect on intergranular fracture behaviors along symmetrical tilt coherent and incoherent Σ3 grain boundaries (GBs) in bcc iron. The analytical results obtained based on Rice concept and numerical results obtained based on molecular dynamics theory are presented. The two inconsistencies observed between the analytical and numerical results achieved for stacking fault formation on the plane coincident with crack plane and twinning formation on the plane not coincident with crack plane are fully discussed. The results in this work show that intergranular cracks on coherent Σ3 GB prefer to propagate in a ductile way, while those on incoherent Σ3 GB prefer to propagate in a brittle way. Along both symmetrical tilt coherent and incoherent Σ3 GBs, intergranular crack propagation depends on advance direction and usually presents directional anisotropy due to non-mirror symmetrical atomic distribution along the plane vertical to crack advance direction. Intergranular crack propagation also depends on front direction. On account of the differences in the type and nucleation ability of plastic behavior, intergranular cracks with different front directions have different ductile-brittle levels and corresponding models have different maximum tensile stresses. Investigation on orientation effect can provide a good reference to improve material reliability.
... Deformation and annealing treatments cause microstructural alteration through recovery, recrystallisation and grain growth [61]. The interfacial energy of high energy boundaries can be reduced through different microstructural evolutionary mechanisms such as: (i) reorientation of grain boundary plane into lower energy status [62]; (ii) formation of low energy boundaries by multiple twinning and grain boundary dissociation [63]; and (iii) reduction in grain boundary area by grain growth [64]. During thermo-mechanical treatments, a combination of all these mechanisms may take place concurrently. ...
Microstructures of type 304 austenitic stainless steel, produced through thermo-mechanical processing, were analysed with large area EBSD and optical image analysis assessments of the attacked grain boundary cluster after DL-EPR testing. The thermo-mechanically processed microstructures were exposed to acidified potassium tetrathionate (K2S4O6) solution under tensile stress and the lengths and distributions of the initiated intergranular crack nuclei were assessed. The crack populations were quantified by fitting a Gumbel extreme value statistics distribution to evaluate their characteristic crack length. A factor (susceptibility parameter) is introduced to rank the degree of susceptibility to intergranular stress corrosion cracking of thermo-mechanically processed microstructures. This accounts for the network connectivity of the sensitised grain boundaries, the grain size and the degree of sensitisation. Similar rankings are obtained for this susceptibility parameter and characteristic crack lengths of the assessed microstructures, in which the thermo-mechanical treatments increased the population of grain boundaries with resistance to stress corrosion cracking.
... The Olmsted's data-base consists of fcc GBs (388 for each material type) with diverse crystallographic character. While it is well-known that the boundary-plane orientation influences structure and properties of GBs [77][78][79][80][81], a quantitative understanding of this role was lacking. To address this challenge, Homer et al. [71] classified the GBs in Olmsted's data-base according to their misorientations and plotted GB energies and mobilities in their corresponding BPl fundamental zones. ...
Grain boundaries (GBs) influence a wide array of physical properties in polycrystalline materials and play an important role in governing microstructural evolution under extreme environments. While the importance of interfaces is well documented, their properties are among the least understood of all the defect types present in engineering material systems. This is due to the vast configurational space of interfaces, resulting in a diverse range of structures and properties. The complexity associated with GB structures is related to the different crystallographic degrees of freedom – the misorientation, the boundary-plane orientation and the relative translations between the adjoining crystals. These unique challenges can be addressed by leveraging high-throughput simulations of GB properties and developing machine learning algorithms grounded in the concepts of bicrystallography of interfaces. To demystify the relationships between crystallography and properties, the symmetry aspects of GBs are reviewed with an emphasis on boundary-plane orientations and disconnection line defects. To quantify structure-property relationships, recent advances in describing GB structures using the polyhedral unit model and the gaussian-based approximation of local atomic environments are discussed. Finally, examples of predicting GB structure-property relationships using machine learning techniques are summarized. As part of a special issue, the goal of this review article is to motivate machine learning strategies that are informed by bicrystallography and novel structural descriptors for developing reliable crystallography-structure-property relationships for grain boundaries.
... The longtime hypothesis that low-Σ GBs should have relatively low energies (see e.g., Refs. [40,41]) is clearly not supported by the present data. Consider the Σ3 GBs, for example, the energies spread out over a range of about 300 mJ/m 2 for the TIGBs (Fig. 9a) and 400 mJ/m 2 for the TWGBs (Fig. 9b). ...
... To this end, MD simulations are performed on four different copper symmetric tilt GBs, i.e., R9-(114), R9-(221), R11-(113) and R11-(332), containing an SFT of three different sizes. The main reason why we select these four GBs in this study is that GBs with misorientations of R9 and R11 are usually observed in the polycrystalline materials and GBs tend to have GB plane with smaller indexes in realistic materials [32]. The influence of SFT on the shear properties of GBs are studied. ...
... The variation in the energies of Σ3 boundaries highlight the crucial role of the boundary plane on grain boundary properties. Similar conclusions were drawn from the experimental and computational results for copper [61,147]. Disorientation angle alone is not sufficient to determine the grain boundary energy. ...
Plastic deformation of classical crystalline materials is mostly dominated by dislocations and their mutual interactions. In nanocrystalline (nc) metals, different grain boundary mechanisms may exist in addition to the dislocation-based mechanisms. The dependency on, among other, the grain shape, grain orientation, initial dislocation density, grain boundary structure and external conditions will promote one or two deformation mechanisms over others. These dominant mechanisms dictate the overall response of nc metal. The influence of the microstructural features needs to be better understood individually and collectively. In the scope of the thesis, 3D discrete dislocation dynamics (DD) simulations were performed on three micron-sized single grains of same volume but differing in aspect ratios. Localization of plastic deformation was observed to decrease with increasing grain aspect ratio. Due to the enhanced cross-slip mechanism, grains with higher aspectratio exhibit a softer behavior. The anisotropic plastic response of elongated grains was quantified interms of the magnitude of back-stress on each slip system. Further, a polycrystalline version of dislocation dynamics code coupled with a finite elements was used, to study the mechanical behavior of free-standing palladium thin films with columnar grains. The initial dislocation density considered in the simulations is close to the one measured experimentally. DD simulations of a polycrystal with 12 equally sized hexagonal grains properly reproduce the strain hardening behavior. The increase in strength observed with decreasing film thickness was captured using a heterogenous grain size distribution of the polycrystal. The key element is that the probability of smaller grains with no inital dislocations is increasingwith decreasing thickness of the film. Difference in the back-stress contributions arising from the grain size distribution in the film was also quantified. Finally, by adapting Read’s model, the influence of a static, electrically-charged dislocation on electrical properties in semiconductors was studied.
... Grain boundaries are classically defined using five geometric degrees of freedom 24,25 ; three degrees of freedom describe the misorientation between grains and two degrees of freedom describe the orientation of the boundary plane. However, the number of parameters required to describe the complex coupling between interfacial structure and mechanical behavior during intergranular fracture encompasses not only the five degrees of freedom describing the geometry of the grain boundary, but also the stress incompatibility controlling the deformation at the grain boundary 26 , the nature of slip activity (single slip versus multiple slip, mechanisms for slip transfer) in each neighboring crystal 27,28 , and likely other parameters. ...
All grain boundaries are not equal in their predisposition for fracture due to the complex coupling between lattice geometry, interfacial structure, and mechanical properties. The ability to understand these relationships is crucial to engineer materials resilient to grain boundary fracture. Here, a methodology is presented to isolate the role of grain boundary structure on interfacial fracture properties, such as the tensile strength and work of separation, using atomistic simulations. Instead of constructing sets of grain boundary models within the misorientation/structure space by simply varying the misorientation angle around a fixed misorientation axis, the proposed method creates sets of grain boundary models by means of isocurves associated with important fracture-related properties of the adjoining lattices. Such properties may include anisotropic elastic moduli, the Schmid factor for primary slip, and the propensity for simultaneous slip on multiple slip systems. This approach eliminates the effect of lattice properties from the comparative analysis of interfacial fracture properties and thus enables the identification of structure-property relationships for grain boundaries. As an example, this methodology is implemented to study crack propagation along Ni grain boundaries. Segregated H is used as a means to emphasize differences in the selected grain boundary structures while keeping lattice properties fixed.
... The former is quantified by the Σ number. The recombination activity of GBs is affected either by the GB structure (intrinsic effect) or decoration of impurities (extrinsic effect) [1][2][3][4][5]. Therefore, the ways in which the performance of solar cells can be improved by the control of the crystal defect character have been the subject of investigation. ...
We investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc-Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc-Si ingots with different artificial GB characters. PDG combined with an originally developed multiple-cycle gettering technique at low temperature was introduced on intentionally Fe-contaminated mc-Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple-cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud-like photoluminescence signal around contaminated GBs was observed for artificial Σ5-GBs and tilt-GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc-Si ingots, which could strongly affect the PDG efficiency.
... AFM is a very sensitive instrument which visualizes the moving crystal plate on the distance of 0.1 nm [46]. The crystals with cubic structure like CrN are disposed to twinning [47]. The reason for the formation of twins may be connected with mechanical stress or the atom impurity during growth of the crystal, causing the crystal shear stresses. ...
Cr–O–N coatings were formed by cathodic arc evaporation at different O2 / (N2 + O2) relative oxygen concentrations onto HS6-5-2 (DIN standard) steel substrates. The chemical and phase composition, surface morphology on the as-deposited coatings were investigated by Wavelength Dispersive Spectrometry, X-ray Diffraction, Atomic Force Microscopy, Scanning Electron Microscopy and Raman Spectroscopy. The coatings deposited in pure nitrogen atmosphere had a cubic CrN structure. Structural properties of the coatings synthesized in mixed oxygen and nitrogen atmosphere depended strongly on the relative oxygen concentration. High relative oxygen concentration caused amorphization of the coating. XRD diffraction lines were shifted and broadened, indicating increasing stress and decreasing mean crystallite size, from 109 nm for coatings deposited at relative oxygen concentration equal 0% to 23 nm at 20%. The coatings formed at 50% relative oxygen concentration had a rhombohedral Cr2O3 structure with a grain size of about 11 nm and lattice distortion of about 2%. Increasing the relative oxygen concentration also increased the surface roughness and the fraction of the surface rate covered by macroparticles.
... In the present work, we focus on just the five crystallographic parameters: [13] three rotation angles to describe the misorientation between the two crystals that form the boundary and two spherical angles that specify the orientation of the boundary plane. [14] Both misorientations [15] and boundary plane orientations [16] affect GB properties. While the misorientations of grains on the surface of a polycrystal may be measured using electron backscatter diffraction (EBSD), [17,18] finding the boundary plane orientation requires three-dimensional (3D) characterization of GB shapes and locations. ...
We use high-energy X-ray diffraction microscopy (HEDM) to characterize the microstructure of Ni-base alloy 725. HEDM is a non-destructive technique capable of providing three-dimensional reconstructions of grain shapes and orientations in polycrystals. The present analysis yields the grain size distribution in alloy 725 as well as the grain boundary character distribution (GBCD) as a function of lattice misorientation and boundary plane normal orientation. We find that the GBCD of Ni-base alloy 725 is similar to that previously determined in pure Ni and other fcc-base metals. We find an elevated density of Σ9 and Σ3 grain boundaries. We also observe a preponderance of grain boundaries along low-index planes, with those along (1 1 1) planes being the most common, even after Σ3 twins have been excluded from the analysis. © 2016 The Minerals, Metals & Materials Society and ASM International
... The 〈110〉 tilt GBs are a preferred interface configuration and of importance for face-centered cubic (fcc) materials [22]. Therefore, in the present work, we carried out MD simulations on some 〈11 0〉 textured GBs with misorientation angle θ=53.1°, ...
Molecular dynamics simulations were carried out to investigate the mechanical property and the deformation mechanisms of Cu bicrystal with non-planar structured grain boundaries (GBs) under uniaxial tension and compression. The simulation results showed that the non-planar GBs could change their equilibrium configurations under the applied stress, and the deformation mechanisms varied when altering the misorientation angle. The stacking fault energy curve was affected by the stress perpendicular to the slip plane and therefore has an influence on the dislocation nucleation mechanisms. Previous studies have revealed a ubiquitous tension/compression (T/C) strength asymmetry of many ultra-fine or nanocrystalline materials, and a higher compressive strength was usually reported. However, in the present study, the bicrystal samples with non-planar structured GBs show a higher tensile strength than the compressive one. The unusual T/Casymmetry property has an implication that the GBs with non-planar structure can play a significant role in affecting the mechanical properties of nanostructured materials.
... The distribution of grain boundary planes for the S3 boundary is strongly peaked at the (111) positions, i.e., the grain boundary planes are perpendicular to the [111] disorientation axis and they are twist boundaries. This corresponds to the common twin-boundary configuration found in many cubic materials [33]. Boundaries with this character make up about one quarter of all of the boundaries in the CuInSe 2 thin film. ...
Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 absorbers exhibit record conversion efficiencies of up to 22.6%. There is still a lack of a quantitative connection between the grain-boundary character distribution (GBCD) and the corresponding electrical and optoelectronic properties. The present work uses microstructural data from a CuInSe2 thin film acquired by electron backscatter diffraction (EBSD) to evaluate the GBCD. The most prominent features of the GBCD of CuInSe2 are Σ3 twin boundaries and the Σ9 and Σ27a symmetric tilt grain boundaries. Moreover, combining EBSD with electron-beam-induced current and cathodoluminescence (measurements on the same identical area) on a CuInSe2/Mo/glass stack provide the means to relate the grain-boundary character with the corresponding electrical and optoelectronic signals across the grain boundary. In part, determining this relationship is accomplished by means of correlation analysis using measurement data from more than 100 grain boundaries. However, the crystallographic, electrical and optoelectronic data showed no strong correlations, which is attributed to atomic reconstruction found in atomic planes adjacent to planar defects in polycrystalline CuInSe2 thin films and corresponding reductions of excess charge densities at these defects.
... These four GBs are selected in this study due to two main reasons. One is that GB misorientations such as P 9 and P 11are usually observed in the polycrystalline materials and GBs tend to have GB plane with smaller indexes (Randle, 1998). On the other hand, it will be shown that four GBs under shear on the GB plane exhibit three representative predominant plastic deformation behaviors connecting to the GBs in the polycrystalline materials during the plastic deformations (Barai and Weng, 2009;Berbenni et al., 2013;Cahn et al., 2006;Peron-Luhrs et al., 2014;Prieto-Depedro et al., 2015;Molinari, 2004, 2005;Taupin et al., 2014;Wang, 2009, 2010). ...
Grain boundaries (GBs) in the polycrystalline and nanocrystalline materials are usually at their non-equilibrated states due to the plastic deformations. Thus, the point defect sink efficiencies of non-equilibrated GBs may be different from those of equilibrated counterparts, which may influence the irradiation tolerance of materials. In this paper, we firstly performed the shear responses of four copper symmetric tilt grain boundaries (GBs). The plastic deformation modes of four GBs include GB sliding, shear-coupling and complex mechanism due to atom-shuffling, partial dislocation nucleation and local GB dissociations. We then study the energetics of point defects interacting with a series of GB configurations undergone plastic deformations. It is found that the plastic deformation dominated by the sliding and shear-coupling has no effect on the point defect sink efficiency of GB in comparison with initial GB states. However, sink efficiencies of GB configurations produced from the complex deformation mode are generally intensified, for both vacancy and self-interstitial atom. In addition, the residual stress in the crystals due to the dislocation nucleating from GB affects the point defect concentration in the crystals. On the other hand, complex deformation mechanism drives GBs to higher energy states with highly disordered structures. As a result, the distribution of lower point defect formation energies extends a larger distance from GB, which may therefore favor GB absorbing the point defects nearby.
... In order to understand materials' behaviour and develop new materials tailored to ever-higher industrial needs, it is of the utmost importance to be able to probe these crystallite properties (e.g. [160,161,162,163]). Conversely, these properties can also yield information on ancient metal working methods long forgotten [164]. ...
Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.
... The former is quantified by Ʃ number. The recombination activity of GBs is affected either by GB structure (intrinsic effect) or decoration of impurity (extrinsic effect) [1][2][3][4][5]. Therefore, the ways in which the performance of solar cells can be improved by the control of crystal defect character have been the subject of investigation. ...
We attempted to clarify the impact of grain boundaries (GBs) types in multicrystalline silicon (mc-Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow the mc-Si ingots with different artificial GB characters. PDG was combined with originally developed multiple-cycle gettering technique at a low-temperature to enhance external and internal gettering. As a result, a significant positive PDG effect was found when PDG with multiple-cycle technique was used, as can be seen by an increase in lifetimes after PDG. A bright cloud-like photoluminescence (PL) signal around contaminated GBs was observed for artificial 5 and tilt GBs after PDG, showing that internal gettering efficiency was enhanced, leaving a cleaner area around them. This result suggests the importance of control of crystal defect character as well as impurities in mc-Si ingot which could strongly affect the PDG efficiency.
Grain boundary structures with high resistance to intergranular fracture are the target of grain boundary design. In this work, grain boundary elimination in front of crack tip is observed in two special bcc iron bicrystals through molecular dynamics simulations under mode I loading. Grain boundary elimination depends on crack advance direction and enhances resistance to intergranular fracture. Direction-dependent elimination leads to directional anisotropy of intergranular crack propagation. By analytical analysis and molecular dynamics simulation, grain boundary elimination is found to be attributed to the activities of twinning and dislocation. All twinning bands are formed by atomic slip along ordinary twinning direction, but all dislocations are nucleated by atomic slip along anti-twinning direction. Mechanisms of twinning formation and dislocation nucleation are revealed by calculating energy barriers of atomic slip and shear stress field. According to the mechanism of grain boundary elimination, conditions for grain boundary elimination are proposed to find all special grain boundaries. Results show that twist grain boundaries cannot be eliminated under mode I loading, while tilt grain boundaries with axes of 〈110〉 can. This work provides a good reference for grain boundary design.
Fatigue lives of grain boundaries (GBs) and component crystals were measured by using [123]/[335] and [5 9 13]/ [579] Cu bicrystals with a perpendicular GB at room temperature in air. The results show that the GBs were always the preferential sites for fatigue crack initiation and propagation in the two groups of bicrystals under cyclic tensiontension loading as well as push-pull straining control. When an [123]/[335] bicrystal with reduced section area was tested by cyclic tension-tension loading, the [335] component crystal always had a relatively higher fatigue life than the [123] component crystal. As a result, the fatigue life increased in the order of GB, [123] and [335] component crystals. When the two groups of bicrystals were subjected to cyclic push-pull straining, the GB of the [5 9 13]/[579] bicrystal showed a higher fatigue life than that of the [123]/[335] bicrystal, which was suggested to be partially attributed to the difference in the component crystal orientations in the two bicrystals. By using scanning electron microscopy and electron channeling contrast techniques, the fatigue damage features on the surfaces and fatigue fractography were examined. Based on the present results, the fatigue cracking mechanisms along GBs and persistent slip bands in Cu bicrystals are discussed.
Nanocrystalline nickel produced by pulsed electrolysis was heat-treated to produce grain sizes from nanoscale to microscale. A special polish allowed to image the specimen with an atomic force microscope (AFM) down to a grain size of 30 nm. Micro- and instrumented nanohardness of these specimens were examined. A NI-AFM (Nanoindenting AFM) was used to measure the interaction between grain boundaries and dislocations. Nanoindentation was performed always in the center of the grains. When the size of the indent was kept constant (constant strain) it could be shown that the hardness scales with the dislocation density within the grains. However, when the size of the indent approached the grain size, the plastic zone spread over several grains and a decrease in hardness was observed. In addition, with deceasing grain size, grain boundary sliding was observed even at room temperature.
It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress.
During grain growth in 2D systems triple junction kinetics may significantly influence not only the rate of grain growth but also the geometric evolution of the grain boundary network. In this contribution we analyse results from in-situ heating experiments coupled with electron backscatter diffraction analysis using a columnar Al foil. It is shown that (a) the von Neumann– Mullins relation is not always satisfied, (b) there is a marked discontinuity and heterogeneity in TJ motion in space and time and (c) grain boundaries evolve from predominately curved to straight boundaries. To interpret these results we introduce 3 main types of triple junctions Tc, Tp, and Ts, which are made up by at least 2 concave, 2 convex, and 3 straight boundaries, respectively. Analyses show that triple junction drag plays a significant role during grain growth. Data suggest that the drag effect of Tp is higher than that of Tc. In addition, triple junctions with 3 low-angle boundaries are exceptionally stable, while those with at least 2 high-angle boundaries are unstable and tend to disappear during grain growth. Only ~ 20% of grain boundaries exhibit steady state triple junction motion.
The misorientation of 515 grain boundaries has been determined using electron backscatter diffraction data from an 18 μm thick copper foil with columnar grain structure and a preferential {110} surface orientation. The energy of the grain boundaries was determined from the dihedral angles in the vicinity of grain boundary thermal grooves. The experimental grain boundary energy vs. misorientation angle shows deep minima for the low-angle grain boundaries and small minima corresponding to the Σ3 and Σ9 grain boundaries. Only a small fraction of the coincidence site lattice grain boundaries demonstrate an increased occurrence frequency (compared to a random orientation distribution) and low energy. In parallel, the grain boundary energy for a subset of 400 symmetrical tilt grain boundaries was calculated using molecular statics simulations. There is a good agreement between the experiment and molecular statics modeling.
In this work, we used the selective laser melting (SLM) fabricated Co-Cr alloy with prominent residual strain, extremely non-equilibrium microstructures, and low stacking fault energy as a precursor to fabricate materials with the optimal grain boundary character distribution. The grain boundary engineering (GBE) of the Co-Cr alloy was achieved by a simple heat treatment of the SLM-fabricated Co-Cr alloy. The obtained GBE Co-Cr alloy exhibited 81.47% of special grain boundaries (Σ3ⁿ n=1, 2, 3)), while it substantially disrupted the connectivity of the random high-angle boundaries, successfully reducing the propensity of intergranular degradation. Slow strain rate tests (SSRTs) showed that the GBE Co-Cr alloy possessed lower stress corrosion cracking (SCC) susceptibility and higher ductility in the corrosive environment (0.9% NaCl solution) than in the air. The high fraction of special boundaries, coupled with the stress-induced martensitic transformation (SIMT) in the GBE Co-Cr alloy yielded these results, which unique and rarely simultaneously satisfied for common structural materials. The current “SLM induced GBE strategy” offers a novel approach towards customized GBE materials with high SCC resistance and ductility in the corrosive environment, shedding new light on developing high-performance structural materials.
The surface properties of weathering steel (WS) is very important for its service performance and safety, and the localized corrosion induced by inclusions is closely related to the surface properties of WS and its application. In the current work, a common spherical (Al, Mg, Ca, Mn)-oxy-sulfide inclusion was selected to investigate the corrosion evolution of complex inclusion and its effect on localized corrosion on WS surface. The results indicate the inclusion in WS consists of (Ca, Mn) sulfides part and (Ca, Al, Mg) oxides part with complex core-shell structure. Locally preferential dissolution occurs in (Ca, Mn) sulfides part as well as metal matrix around the inclusions. Furthermore, both parts of the inclusions with poor conductivity and high-density dislocation at metal matrix around the inclusions was found, which suggests that traditional micro-galvanic corrosion cell may not be the cause of inclusion-induced localized corrosion on WS surface at initial stage of corrosion. The variation in maximum and average depth around the inclusion or selected region with immersion time indicates that localized corrosion induced by inclusions is overwhelmed by uniform corrosion of WS in the latter stage of immersion, then the rust formed on WS surface consists of two layers.
Changes in the chemistry of internal interfaces, particularly grain boundaries, are known to affect the macroscopic properties of a wide range of material systems. Solute segregation to grain boundaries is dependent on, amongst other factors, the physical structure of the grain boundary.
We demonstrate how complementary use of transmission Kikuchi diffraction (TKD) and atom probe tomography (APT) can provide a more holistic characterisation of grain boundaries in a variety of materials. Structural information is reported from TKD data for a model steel, a titanium alloy, and a multicrystalline silicon sample. Complementary APT analyses are used to determine the segregation behaviour to these interfaces.
A novel specimen preparation protocol allows for the grain boundary to be positioned more reliably within the apex of an APT specimen. Meanwhile, a method that allows a grain boundary’s five macroscopic degrees of freedom to be determined from TKD data alone is also proposed.
In this study, the grain boundary energy of symmetric and asymmetric iron bicrystals is calculated for Σ3, Σ9 boundaries with 110 tilt axis and Σ5 with <100> tilt axis. The calculations are carried out using molecular dynamics simulations with the embedded-atom method potential. A modified method for creating grain boundary atomic structure is proposed that has sufficient accuracy and its computation cost will be considerably lower than the previously used methods. The effect of three parameters namely rigid body movement, overlapping distance, and reduction side is investigated and compared to previous studies and the optimal parameters are introduced which leads to a better performance in bicrystal modeling.
It is well known that the fatigue of metallic materials is governed by the accumulation of plastic slip, which ultimately leads to the initiation and propagation of cracks. Whether a material exhibits an infinite fatigue life or not is therefore determined by the complex interplay between plastic deformation processes, the formation of small crack nuclei and the direct and indirect interactions of cracks with the surrounding microstructure. It is this complex interplay with the microstructure that makes predictive modeling of fatigue in metals such an arduous task, even though the first empirical expressions for the fatigue life have been developed more than hundred years ago. For this reason, most macroscale material models for large-scale computer simulations are still directly parameterized through costly fatigue experiments instead of considering microstructural information and lower-scale deformation mechanisms. The focus of this work is therefore to investigate the fundamental mechanisms that are of relevance to metal fatigue: crack nucleation by slip accumulation at grain boundaries (GBs), crack propagation along GBs, dislocation-crack interactions, and the influence of crack front curvature, which is especially important as long as the cracks are very small. Studying the direct defect-defect interactions characteristic for these processes require atomic-scale resolution. Atomistic modeling methods, such as molecular dynamics (MD) simulations, are therefore ideally suited for their close and detailed investigation. Since atomistic simulations come with their own challenges in terms of limited time and length scale, it is important to note that the present work is intended to lay the foundations for the future developments of predictive, mechanism-based and microstructure-sensitive material models for large-scale fatigue simulations, rather than being by itself quantitatively predictive. Instead, we present several qualitative and semi-quantitative observations, which should also hold true in real materials. I.e., that dislocation pile ups are more critical for crack nucleation at GBs than homogeneously distributed dislocations and vacancies; that furthermore, the fracture behavior and toughness is markedly influenced by (i) crystal orientations, (ii) GB structures, (iii) pre-existing dislocations, and (iv) crack front curvature. Finally, future directions for atomistic modeling of fatigue damage are presented.
To obtain a fundamental understanding of the effect of structure and geometry of grain boundary on the diffusion kinetics in nanocrystalline materials, the influence of grain boundary misorientation on the effective diffusion coefficient (apparent diffusivity) in nanocrystalline aluminum was investigated using molecular dynamics simulations. Nine series of [001] symmetric tilt grain boundaries, including high and low symmetric boundary planes, were studied. The apparent diffusivity in the samples was calculated in the temperature range from 423 K to 823 K by monitoring the mean square displacement of atoms as a function of simulation time. A temperature dependence of the effective diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that the apparent diffusivity is anisotropic and it is a strong function of grain boundary misorientation at low and high temperatures. At all temperatures, Σ29 [001]/(520) symmetric tilt grain boundary with misorientation angle of 43.68° exhibits the highest effective diffusion coefficient among the investigated grain boundaries. The simulation results show that the activation energy and pre-exponential factor are affected significantly by the grain boundary misorientation angle. Moreover, the results indicated that the misorientation dependence of activation energy for diffusion exhibits two local maxima, which correspond to two symmetric tilt grain boundaries. Additional calculation of misorientation dependence of the pre-exponential factor shows two local minima at the same symmetric tilt grain boundaries. The misorientation dependence of the effective diffusion coefficient was explained on the basis of grain boundary energy and the crystallographic structure of grain boundary.
This paper presents the results of a study on grain boundary characteristics in cold deformed and annealed 304HCu grade austenitic stainless steel (SS 304HCu) using electron backscatter diffraction. The microstructure exhibited an increasing fraction of Σ1 to 29 coincidence site lattice boundaries with annealing temperature, resulting up to ~60 pct at 1573 K with 92 pct contribution from Σ3-type twin boundary. However, the twin boundary interaction at the triple points with a network of Σ3–Σ3–Σ9 was found to decrease from 4 to 0.5 pct with annealing temperature. To understand the resultant boundary advancement of the Σ3n (n = 1, 2, 3) boundaries, their migration was traced in the annealed specimen. However, in the specimen with extended annealing Σ3 boundary fraction was found to be higher with a concomitant decrease in the boundary fraction generated by the Σ3 interactions. In this study, a procedure to analyze the coherency of Σ3 boundaries and its interfaces that form due to Σ3 interactions has been evolved based on single-section analysis using the pole concentration across the grains. Further, a crystallographic description of the two planes meeting at the interface of Σ3-type boundary has been provided by adopting serial sectioning methods, which help to understand the morphological changes. The quantitative deviation from exact coherent Σ3 has been estimated to be within ~6 deg in this study.
Distributions of silicon carbide grain boundary types (random high angle, low angle, and coincident site lattice-related boundaries), were compared in irradiated tristructural isotropic-coated fuel particles from the Advanced Gas Reactor-1 experiment exhibiting high (>80%) and low (<19%) Ag-110m retention. Grain orientation from transmission electron microscope-based precession electron diffraction data, and, ultimately, grain boundary distributions, indicate irradiated particles with high Ag-110m retention correlate with lower relative fractions of random, high-angle grain boundaries. An inverse relationship between the random, high-angle grain boundary fraction and Ag-110m retention was found and is consistent with grain boundary percolation theory. Also, the SiC grain boundary distribution in an irradiated, low Ag-110m retention, Variant 1 particle was virtually identical to that of a previously reported as-fabricated (unirradiated) Variant 1 TRISO particle. Thus, SiC layers with grain boundary distributions associated with low Ag-110m retention may have developed during fabrication and were present prior to irradiation, assuming significant microstructural evolution did not occur during irradiation. Finally, irradiation levels up to 3.6 × 10²⁵ n/m² and 16.7% fissions per initial metal atom were found to have little effect on association of fission product precipitates with specific grain boundary types in particles exhibiting between 19% and 80% Ag-110m retention.
The fatigue crack growth prediction near the threshold regime remains one of the most challenging fields in fatigue research. In previous studies, the threshold effects have been incorporated into fatigue crack growth models upon modification of the Paris Law in an empirical fashion. In this work, without a-priori assumptions and empirical constants, we derived the threshold levels with a combination of molecular dynamics and continuum calculations. We illustrate that the threshold value is non-unique and depends on the state of the microstructure surrounding the crack. Also, in reality, we envisage very low fatigue crack growth rates near the ‘threshold’ and not a cut-off value, which is consistent with experimental trends. In the model, the microstructure is characterized by grain boundary types, grain size, and initial dislocation density. We derive friction stresses for forward and reverse motion of crack tip dislocations interacting with grain boundaries which allow determination of irreversible crack tip displacements. We illustrate the benefits of sigma-3 grain boundaries, finer grain sizes and shielding dislocations at the crack tip on improving the near-threshold fatigue crack growth behavior.
The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.
We investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc-Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc-Si ingots with different artificial GB characters. PDG combined with an originally developed multiple-cycle gettering technique at low temperature was introduced on intentionally Fe-contaminated mc-Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple-cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud-like photoluminescence signal around contaminated GBs was observed for artificial sigma 5-GBs and tilt-GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc-Si ingots, which could strongly affect the PDG efficiency. Copyright (c) 2016 John Wiley & Sons, Ltd.
The SEM/electron back scattered diffraction and serial sectioning procedure have been used to study the five parameter description of grain boundaries in a polygonal ferrite microstructure of 9Cr-1Mo steel. A routine has been evolved to correlate the successive images of a selected region and determine the grain boundary plane morphology. The relative misorientation of the crystallites, in terms of misorientation angle-axis (ω,ȓ), the grain boundary inclination angles [azimuth (γ) and polar (β) angles], and the crystallographic description of the two meeting planes have been studied and compared with random distribution. The low-angle boundaries are found to be persistently present in higher amounts after the grain growth induced by extended annealing.
A facet is defined as ‘one side of a many-sided body.’ With regard to grains in a polycrystal, a facet can be an intergranular or transgranular fracture surface (Field 1997; Randle and Hoile, 1998; Randle, 1999), or the surface of a grain or phase boundary (Randle, 1997). Figure 23.1 shows the appearance of intergranular fracture surfaces in an aluminium alloy revealed as a result of liquid metal embrittlement by gallium. This is, of course, an unusual view of grain boundaries; usually they are revealed by etching a polished surface to show the trace of the grain boundary network. Frequently, fracture surfaces and grain boundaries have very specific crystallographic identities. This is also true of other planar features in materials, such as intergranular or transgranular microcracks, (Liu et al., 1992) or slip traces (Blochwitz et al., 1996; Lin and Pope, 1996; Raabe et al., 1997). It is possible to extend the capabilities of EBSD to obtain the crystallographic indices of these flat surfaces (i.e., planes). Such a strategy is a precursor to a three-dimensional view of microtexture. This chapter gives an overview of the state-of-the-art of EBSD analysis of facets and surfaces.
In this study, the fission product precipitates at silicon carbide grain boundaries from an irradiated TRISO particle were identified and correlated with the associated grain boundary characteristics. Precession electron diffraction in the transmission electron microscope provided the crystallographic information needed to identify grain boundary misorientation and boundary type (i.e., low angle, random high angle or coincident site lattice (CSL)-related). The silicon carbide layer was found to be composed mainly of twin boundaries and small fractions of random high angle and low angle grain boundaries. Most fission products were found at random, high-angle grain boundaries, with small fractions at low-angle and CSL-related grain boundaries. Palladium (Pd) was found at all types of grain boundaries while Pd-uranium and Pd-silver precipitates were only associated with CSL-related and random, high-angle grain boundaries. Precipitates containing only Ag were found only at random, high-angle grain boundaries, but not at low angle or CSL-related grain boundaries.
Magnetically anisotropic PrCo5 permanent bulk magnet prepared by the hot deformation method has been reported for its crystallographic orientation-dependant magnetic properties. Although the PrCo5 crystals strongly favor the {0001} orientation texture, the crystallographic orientation textures of PrCo5 crystallites differ at various radial locations. From the bulk center to the edge, the c-axis orientation texture decreases from 5.380 MRD to 4.090 MRD, whereas the remanence decreases from 8.93 kGs to 7.58 kGs, and the coercivity increases from 4.34 kOe to 6.56 kOe. This paper also reports the alteration of boundary plane orientation textures resulted from the corresponding energy anisotropy at different locations, and suggests development of grain boundaries with lower boundary energies to have significant impact on the magnetic properties with regard to remanence and energy density. The current work therefore proposes an elegant approach of attaining desired magnetic properties in PrCo5 magnets by fine tuning the crystallographic orientation.
Dislocation-density based multiple-slip constitutive formulations and specialized computational schemes are introduced to account for grain-boundary (GB) effects in polycrystalline aggregates. New kinematically based interfacial grain-boundary regions and formulations are introduced to account for dislocation-density transmission, absorption, and pile-ups that may occur due to CSL grain-boundary misorientations.
Crystallographic characteristics of a large number of ∑3 boundaries have been investigated in a nickel superalloy. The results indicated that {111}, vicinal-to-{111} (within 10° of {111} planes) and not-{111} ∑3s were all present in the sample population. At ∑3/∑3/∑9 junctions a combination of one {111} and one vicinal-to-{111} ∑3 was more likely to occur than two {111} or two not-{111} ∑3s, and an explanation is proposed. An analysis of the interface planes for the not- {111} ∑3s indicated that more than half the boundaries in this category could not have {211} {211}, {774}{855}, {111}{511}, {001}{221} or {110}{411} planes.
Since 1976, grain boundary engineering (GBE) has been used to improve the properties of a material (e.g., strength and corrosion resistance in polycrystalline materials). The concept of GBE is to produce a high population of special grain boundaries (coincidence-site lattice grain boundaries, CSL GBs) with certain misorientations to replace general GBs with a random misorientation. Previous studies have demonstrated that special GBs exhibit superior properties of low defect, and this promotes the strength and corrosion and oxidation resistance of materials. Compared with metals, only few studies have reported on the application of GBE to ceramics. The main problem is that ceramics are too brittle to undergo a thermomechanical process (conventional GBE process of metals) to orientate grains for special GBs. To the best of our knowledge, only 3% (near the random distribution of 1.5%) of Σ3 GBs, one of the common CSL GBs, has been reported for polycrystalline strontium titanate (SrTiO3). We suggested that a high population of approximately 9% Σ3 GBs can be achieved in polycrystalline SrTiO3 by controlling the shapes and surface planes of nanocyrstals in SrTiO3 (initial powder). In addition, the formation mechanism of CSL GBs is discussed.
Directional solidification of Al-4.5Cu alloy refined by adding Al-5Ti-1B has been carried out to investigate the texture formation and grain boundary characteristic of the paramagnetic crystal under a high magnetic field. OM and EBSD were applied to analyze the microstructures solidified at different temperature gradients (G) and magnetic field intensities (B). The results show that at the temperature gradient of 27 K/cm, the orientations of fcc α-Al grains without magnetic field are random. However, as a high magnetic field is imposed, the easy magnetization axes<310> of the α-Al grains are aligned parallel to the direction of the magnetic field leading to <310> texture. Meanwhile, the ratio of coincidence site lattice (CSL) grain boundaries increases with the increment of magnetic field intensity and reaches its maximum value at 4 T, but decreases as the magnetic field enhances further. On the other hand, when the temperature gradient is elevated, columnar dendrite morphology is exhibited without magnetic field; while a 6 T high magnetic field is introduced, the columnar dendrites are broken and equiaxed grains of random orientations are obtained. The alignment behavior of the free crystals in melt could be attributed to the magnetic crystalline anisotropy of α-Al. Moreover, the influence of fluid flow on the texture formation and CSL grain boundary development under magnetic field is discussed. The absence of convection is benefit for grain reorientation and CSL boundary formation. The application of high static magnetic field will inhibit the macro-scale convection. However, the interaction between thermoelectric current and magnetic field will cause micro-scale fluid flow, i.e., thermoelectric magnetic convection (TEMC). The TEMC will give rise to perturbation near the solid-liquid interface leading to the appearance of freckles as well as the decreasing of the ratio of CSL boundary. Moreover, it is proposed that the formation of CSL boundary is associated with the rotation of the free grains in melt along specific crystallographic axes by magnetic torque.
Large scatters in segregation enthalpies of both the high-coincidence Σ = 5, 36.9°[100], and the non-coincidence 45°[100] symmetrical and asymmetrical tilt grain boundaries clearly display the important role of grain boundary plane orientation in interfacial properties and confirm existence of special asymmetrical tilt grain boundaries. These interfaces consist of combinations of pairs of crystallographic planes which belong to low levels of Paidar's classification of grain boundary planes and which create special symmetrical tilt grain boundaries.
Grain boundary specialness is most often discussed in terms of the coincident site lattice (CSL) theory. Recent contributions by Adams and co-workers claim that a certain class of misorientations, defined in group theory as having a multiplicity greater than one, respond differently than random boundaries. The present work critically examines this claim and offers a physical description of the mathematical boundary on which these misorientations lie. Careful review of the available literature offers little support for CSL theory as a means of defining grain boundary specialness. Three-dimensional descriptions of the crystallographic structure of grain boundaries are essential in defining specialness. Data from damaged materials are presented which offer qualified support of Adams' claims. Detailed investigations are required to further restrict the specialness criteria and reduce the domain of boundaries classified as special.
A methodology is presented to characterize the crystallography of individual fracture surface facets. Electron backscatter patterns (EBSP's) from a metallographic section through a facet identify grain orientation, and quantitative tilt fractography identifies facet orientation; these results are combined to establish fracture facet crystallography. For this technique, facet electropolishing is not required, the facet alignment procedure is accurate and quick, and the method can be generalized to different microstructures, test environments, or facet orientations. Method accuracy is illustrated for 25 to 50 μm fatigue crack facets in an unrecrystallized Al–Li–Cu alloy (AA2090) that has 5 μm thick subgrains in elongated grains that are 10 to 200 μm thick. The fine subgrain structure and tortuous fatigue crack profile precludes the use of other diffraction techniques for determining AA2090 facet crystallography. EBSP and tilt fractography results demonstrate that vacuum fatigue cracks in AA2090 are nearly parallel to local {111} planes.
The texture in thin films develops during processing steps such as deposition and annealing. Recent studies show that texture plays an important role in stress voiding, thermal hillock formation, grain collapse and electromigration failure. Specifically, electromigration failure depends on the grain misorientation distribution, which describes the probability of different grain boundaries and, therefore, links the grain boundary structure to the mass transport that takes place primarily along the grain boundaries. To understand the relationship between the grain misorientation and electromigration lifetime in aluminum thin films, the texture was measured on three sets of films from different manufacturing conditions. The frequency of occurrence of coincidence site lattice (CSL) grain boundaries, which represent special misorientations between grains, was obtained, and electromigration tests were done for all three conditions. Experimental results show that the lifetime of patterned films increases as the amount of 111 texture and the frequency of CSL boundaries increased. © 1995, Canadian Institute of Mining and Metallurgy. All rights reserved.
Grain boundary faceting in a well annealed polycrystalline niobium of high purity p300K =3000 p4.2 K has been studied in the paper. A small tendency of the given material to faceting has been revealed (I°o faceted boundaries). A steplike structure was observed in high-angle grain boundaries near to coincidence orientations with 57 = 11, 13, 17, 19, 29 and low-angle grain boundaries with an axis (I10) and 0 ~ 10 °. Analysis of high-angle boundaries faceting has been carried out according to a model of coincidence site lattice (CSL). Two types of facets have been observed-crystallographic facets with orientation corresponding to close-packed planes of CSL and accommodation facets; they were located at the triple grain boundaries joints and at the surface. Orientation of the later is determined by boundary equilibrium conditions at triple joints and on the surface. It has been shown that the faceting of boundaries sensitive to inclination was due to the existence of pinning points giving the boundary orientation.
A classification of grain boundaries based on the separation of the atomic planes parallel to the boundary plane is presented. The results are compatible with computer simulations of the grain boundary atomistic structure interpreted in the framework of the structural unit model. The classification scheme can be used as a guideline for the theoretical and experimental investigation of the grain boundaries with special properties.
Intergranular cracking of aluminum alloys occurs by a variety of processes in a host of structural components. Cracks emanate from holes and corners of support structures and heterogeneously spread along grain boundaries. In the present study, cracked specimens were investigated using local orientation measurement techniques. True representation of grain boundaries in spaces of five or eight dimensions was emphasized in characterizing the microstructure. Orientation of the grain boundary plane with respect to the stress axis and with respect to the crystal lattice was found to play an important role in determining GB “specialness” in addition to misorientation of the crystallite lattices.
Conspicuous microstructural changes were observed for a variety of metallic materials during elevated temperature cyclic deformation, owing to grain growth, grain boundary motion and reorientation of the grain boundaries under 45° with respect to the stress axis. These phenomena are associated with an obvious change of microtexture and orientation correlation between neighbouring grains. The current investigation focused on the evolution of microstructure and microtexture during high temperature low cycle fatigue in pure nickel. The orientations of the individual grains were determined by means of the electron backscattering diffraction technique in a scanning electron microscope. The spatial arrangement of the grain boundaries was measured using optical microscopy. The results are discussed with regard to the effect of grain boundary character on the propensity for grain boundary damage by motion and alignment.
The mode of fracture and the dictile-to-brittle transition temperature (DBTT) of high purity iron (\gtrsim99.999%) have been investigated with two sets of specimens of different grain boundary character. Wire specimens of 0.4 mm diameter were tested in tension at a strain rate of 8.33×10−5/s.The grain structure of one set of specimens is of bamboo-type with high angle boundaries, which are nearly parallel to the {100} or {110} planes of the grains separated by the boundary and nearly normal to the specimen axis. As expected, these specimens fracture in the intergranular mode and DBTT is between 110 and 125 K. Specimens of the other set have coarse grain structure. Most of the boundaries are of small angle or near-twin type with random boundary plane orientation. These specimens fracture in the transgranular mode at and below 50 K; DBTT for the intergranular fracture, if any, is below 4.2 K. This is in contrast to the occurrence of intergranular fracture even at 77 K for less pure iron specimens (99.99% or below) with the grain boundary character of the second type.Thus, DBTT for the intergranular fracture, which is the common fracture mode in pure iron, depends strongly on the purity and on the grain boundary character of iron specimens. If the grain boundary character is unfavorable to the intergranular fracture, DBTT decreases from above 77 K to below 4.2 K by increasing the purity from 99.99% to 99.999%. In specimens favorable to the intergranular fracture, DBTT is about 120 K even in 99.999% pure iron.
Transmission electron microscopy and electron diffraction were used to study grain-boundary precipitation in an Al-4.0Cu-0.5Mg-0.5Ag (wt%) alloy. Low-angle grain-boundaries were found to nucleate Ω precipitates on the {111}α planes even when the {100}α habit planes of the competinng θ′ metastable phase were closer to the grain-boundary plane. High-angle grain-boundaries, which were random in nature and had relatively large energy, nucleated Ω precipitates predominantly. A few S precipitates and a θ precipitate (G IV/V orientation) were found to co-exist with Ω in these boundaries. The proximity of the grain-boundary plane to the {111}α plane on which grain-boundary Ω nucleated was found to be particularly important in both low and high-angle grain-boundaries, similar to results for the θ′ phase in AlCu alloys.
The structure and crystallography of two special high-angle grain boundaries which have been observed in spinel are discussed in terms of coincident-site lattice concepts. The two grain boundaries correspond to relatively high-Σ values; the value of Σ = 99 is higher than that which is usually considered to be significant for grain boundaries. It is proposed that it is not the actual value of Σ which is important for these interfaces but rather the fact that the misorientations which are responsible for producing the grain boundaries cause several pairs of low-index crystal lattice planes to be nearly parallel to one another at the interface. The relationship between this interpretation and the frequent observation of both low-Σ interfaces and asymmetric grain boundaries in this and other materials is emphasized.
The zero-temperature energies and equilibrium volume expansions of point-defect free grain boundaries (GBs) on the two densest planes of fcc copper, bcc molybdenum and cubic-diamond silicon have been determined, using an Embedded-Atom-Method potential for Cu, a Finnis-Sinclair potential for Mo, and the Stillinger-Weber potential for Si. It is found that in all three structures the energies of the GBs on the second-densest planes are about two to three times higher than of those on the densest planes. For the metals a strong correlation between GB energy and volume expansion at the GB is observed. Owing to its covalent nature of binding such a correlation is not found for Si. It is illustrated that atoms in very close contact (up to about 10 percent closer than the perfect-crystal nearest-neighbor distance) are mainly responsible for the very large anisotropy in the GB energy. Since the strong repulsive forces between such atoms should be active even in complex interface systems (such as meal-ceramic interfaces or strained-layer superlattices), we suggest that the energetics of even more general interfaces is strongly influenced by Pauli's principle.
Electron backscatter diffraction has been used in conjunction with a scanning electron microscope to obtain orientation measurements of individual grains in an interstitial-free steel. The microstructure of the material reveals the presence of “raised” grains with respect to the surface. Electron backscatter diffraction reveals that the majority of these grains are {100} 〈011〉 oriented in the normal direction to the surface. Furthermore, the electron backscatter diffraction patterns associated with this grain type are of poor quality, indicating that they retain a high degree of deformation that may ultimately detract from the properties of the materials. Grain-boundary analysis revealed that low-angle boundaries are unlikely to exist between “raised” grains and other grain types, which may play a role in adhesion characteristics of zinc coatings. The electron backscatter diffraction technique was additionally used to demonstrate that a texture gradient exists between the surface and the midplane of the material.
Theoretical and experimental results are presented, with the primary objective of improving the resistance of convectional polycrystalline alloys to intergranular degradation phenomena, through the application of grain boundary design and control. Geometric considerations are discussed, which show that, as a consequence of both energetic and crystallographic constraints associated with twinning, a grain boundary character distribution (GBCD), consisting entire of low σ grain boundaries, is attainable. A geometric model of crack propagation through active intergranular paths is used to evaluate the potential effects of σ grain boundary fraction and grain size on intergranular cracking. The effect of the GBCD on intergranular stress corrosion cracking and intergranular corrosion in a nickel-based alloy 600 (Ni16Cr9Fe) is determined. Important factors in achieving microstructural optimization of alloy 600 are presented. These results provide direct experimental support for the model of intergranular crack propagation, and demonstrate the importance of grain boundary structure control for enhancing the resistance of a material to intergranular degradation.
Grain boundaries play an important role in most properties of materials. This role is strongly dependent on the crystallography and on the purity of the grain boundary, these two factors being correlated. It is thus necessary to establish the grain-boundary character distribution in a polycrystal to understand the grain-boundary contribution to an overall property. The main purpose of this study is to investigate the influence of phosphorus on the distributions of the grain-boundary misorientations and inclinations in iron, which we refer to as grain-boundary texture.
Analytical electron microscopy (AEM) has been used to examine the relationship between grain boundary structure and the segregation of chromium in a sensitised AISI 316 stainless steel. Fifty grain boundaries have been analysed. Among the boundaries there was a threefold difference in full width half maximum (FWHM) of the chromium concentration profiles. This variation is interpreted in terms of the different long-range stress fields created by different combinations of structural units in each boundary. From the narrowness of their profiles, it is deduced that the SIGMA = 3, 11, 13a, 13b and 29a boundaries are favoured while the SIGMA = 9 boundary is non-favoured. There was no correlation between FWHM and grain boundary chromium concentration. Additionally, we observe that it is not pre-requisite for the boundary plane to be close to {111} for a chromium carbide to nucleae. There is also no correlation between the boundary normal and either the FWHM of the chromium concentration profile or the grain boundary chromium concentration.
The distribution of grain boundary types along intergranular cracks in Ni3Al was measured, by Σ value, and compared to the distribution in the bulk, using statistically significant sample sizes. It was found that low angle (Σ1) and symmetrical Σ3 boundaries (twins) are particularly strong, and all high angle boundaries, independent of their Σ values are weak. In particular, low Σ, high angle boundaries, as a group, are also weak. These results are in qualitative agreement with predictions based on the structural unit model and imply that the fracture strength of an intergranularly brittle polycrystalline aggregate can be increased only by increasing the fraction of low angle and symmetrical Σ3 boundaries.
Crystallographic parameters of low energy grain boundaries in silver and copper in the temperature range 0.8-0.996 Tm have been studied. The investigations have been carried out by sintering single crystalline spheres to single crystalline plates having surfaces parallel to low index planes. In the above temperature range the most important energy cusps were found to be associated with [110] tilt boundaries. This result indicates that the parallelity of close packed rows of atoms from both grains surfaces leads to low energy grain boundary structures. The distribution of misorientation angles for low energy [1 1 0] tilt boundaries is almost continuous at 0.8 Tm. At higher temperatures the spheres select some orientations as more preferred than others. This set of orientations is smaller than predicted according to recent geometrical criteria for low energy boundaries. It can be interpreted in terms of grain boundary faceting in such a way that it contains compact structural units of low energy. There seems to be no connection between the type of structural units in the grain boundary and the present crystallographic criteria. This is the reason why these criteria fail to select between nonsymmetrical boundaries of higher and lower energy.
Relation between the amount of P-segregation and grain boundary crystallography in an FeNiCr alloy was studied. A grain boundary etching method was utilized for analyzing P-segregation, and the crystallography of grain boundaries was characterized by making channelling pattern and trace analysis. Macroscopic geometric characteristics of grain boundaries which were found to be effective in suppressing the amount of P-segregation are (1) low-angle disorientation (2) σ3 orientation relation, (3) \ ̃gs3 (quasi-twin) orientation relation (4) boundary orientation close to the orientation of the coherent twin boundary,(5) boundary plane having low indices, and (6) boundary straightness. It was suggested that an essential geometric factor on the atomic scale which controls the amount of P-segregation in high-angle boundaries is their coherency or free volume.
Using the rotating-sphere-on-a-plate method, the relatively low energy orientations of grain boundaries in Cu, Ni and two Cu-Ni alloys lying, on average, parallel to the (100) surface of the plate were measured. Pole figures were plotted by using an X-ray texture goniometer. The data were analysed to determine whether special Σ-value boundaries existed (up to Σ35b), and crystallographic parameters such as Σ-value, boundary plane, rotation axis and angle, twist and/or tilt components were tabulated. The measured low energy orientations were compared with previous measurements from other studies and with predictions from geometric criteria.
The validity of proposed grain boundary models is tested by comparing the predicted and the experimentally observed boundaries of low energy. The method used to identify the boundaries of low energy was the coherent rotation during annealing of single crystal balls of copper sintered onto a copper single crystal plate. The experimentally observed boundaries of low energy are at variance with the predictions of grain boundary models based on geometry. The observations support a grain boundary model based on a periodic arrangement of structural units, the energy of which is controlled by the interaction energy between the atoms. Furthermore, the experimental results suggest that two types of boundaries may be distinguished in metals: "electron insensitive" and "electron sensitive" boundaries. To a first approximation the energy of electron insensitive boundaries is controlled by the geometry of the atomic arrangement in the boundary, whereas the energy of the second group depends on the electron structure. In all metals with the same lattice structure, the electron insensitive boundaries are obtained for identical orientation relationships. The orientation relationships corresponding to low energy boundaries of the "electron sensitive" type vary as a function of the electron structure of the material.
The grain boundary premelting temperature (GBPMT) Tm in thin foils of 99.99 mass% copper bicrystals deformed in tension to a strain of 0.4 increased from approximately 0.5TM up to near the bulk melting point TM with increasing oxidation time. The GBPMT Tm in thin foils of the bicrystal whose grain boundary plane was parallel to active screw dislocations was lower than that to active edge dislocations. The difference between the GBPMT in 99.99 and 99.9999 mass% copper polycrystals were not so significant. The difference between the GBPMT of aluminum bicrystals and the of copper bicrystals depends upon the difference between the properties of the corresponding oxide films and would probably be related to the difference in the elastic anisotropy between them. Plastic strain would decrease the GBPMT at least in copper.
Atomic-scale detail in 〈100〉 tilt grain boundaries (GB) of high-purity bicrystals of NiO has been studied with a 400 kV high-resolution electron microscope. Crystallinity is always maintained right up to the grain boundary and there is a strong tendency for coherent matching of atomic planes across the GB in symmetric and asymmetric GBs, and for all misorientations. Low-angle boundaries are characterized by their primary dislocation structure which is, depending on the GB plane, composed of a〈100〉 and a〈110〉 type dislocations. High-angle boundaries, even those close to σ = 5, often take on asymmetric configurations with the boundary close to a (100) plane in one of the crystals. The observation of symmetric as well as highly asymmetric facets at high angles suggests that both configurations correspond to local minima in the free energy. These results are discussed in terms of current models of tilt GBs of NiO.
A large sample population of grain-boundary geometries in annealed polycrystalline nickel has been collected and analysed. The data include all five degrees of freedom, that is, the grain misorientation plus the crystallographic orientation of the boundary plane. The most significant category of boundaries on the basis of those geometries that could give rise to 'special' properties were symmetric and asymmetric tilt boundaries in the Sigma = 3 system. Together, these accounted for nearly half the sampled boundaries. Certain plane combinations occurred with an above-average frequency, whereas others, e.g. the (211) symmetric tilt boundary, were not observed at all. The data were compatible with recent calculations on the energies of asymmetric tilt boundaries. Furthermore, there was some correlation between Sigma = 3 boundaries and the inclination of the boundary relative to the specimen surface, which could be explained by grain-boundary area and connectivity arguments.
The crystallographic parameters and faceting during annealing of Σ = 9; 27; 81; 243 special grain boundaries in stainless steel and copper are investigated by TEM technique. The preferential facet orientation parallel to closely packed {Okl} planes of CSL is supposed to be due to a special feature of the Σ = 3n boundary formation, that is realized by multiple interaction of Σ = 3 twin boundaries. A strong agreement is observed between the experimental rotation angles and theoretical ones. It is concluded that Σ = 3n boundaries may serve as standards for further investigations of grain boundaries in polycrystalline materials.[Russian Text Ignored].
The type and frequency of grain boundaries, the so-called grain boundary character distribution (GBCD), has been determined in rapidly solidified and subsequently annealed Fe-6·5 mass% Si alloy ribbon by the scanning electron microscopy-electron channelling pattern (SEM-ECP) technique. High frequencies of low-angle boundaries and coincidence boundaries with Σ3, Σ9, Σ11, Σ17 and Σ19 were observed in a fully annealed ribbon with well defined {110} texture. The total frequency of low-angle boundaries and coincidence boundaries is almost one-half of all grain boundaries. The coincidence boundaries which occurred more frequently are exactly those predicted theoretically from the coincidence orientations for ⟨110⟩ rotation in cubic crystals, similar to those observed previously in {100} textured ribbons of the same alloy produced by the same processing method. The presence of a close relationship between the type of texture and GBCD has been confirmed by experiment on differently textured ribbons of the same material.
High-resolution electron-microscopy experiments are combined with computer simulations of tilt grain boundaries (GBs) in Au to investigate the preferred GB planes in ∑ = 9 and ∑ = 11 bicrystals. The energies calculated for a variety of symmetric and asymmetric GBs suggest that asymmetric GB-plane orientations are often preferred over symmetric ones. Experimentally it is found that symmetric and asymmetric GBs coexist for each misorientation, and that the observed atomic-scale facets are consistent with the computed energies. In accordance with earlier observations, it is suggested that a significant fraction of the GBs in polycrystalline materials may indeed be asymmetric.Work supported by the U.S. Department of Energy, BES-Materials Sciences, under contract W-31-109-Eng-38. The U.S. Government retains a non-exclusive, royalty free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.
We report reflectance (R) measurements of polypyrrole (PPy) films with d.c. conductivity of 350 S/cm at 300 K. The reflectance is metal-like in the IR at all temperatures, with R increasing in the far-IR to about 100% at temperatures below 50 K. The optical conductivity, σ(ω), and the real part of the dielectric function, ϵ1(ω), are not typical of a Drude metal. At 10 K, ε1(ω) = − A/ω2 for h̵ω < 200 cm−1 while σ(ω) decreases in the same spectral range, implying a narrow peak in σ(ω) at ħω well below 20 cm−1 with oscillator strength corresponding to about 2% of the total free-carrier oscillator strength. ‘Metallic’ PPy is, therefore, a conductor with a gap in the spectrum of charged excitations; the d.c. conductivity comes from a narrow peak in σ(ω) at or near ω = 0.
Analyse de la correlation par un potentiel a n corps et par un simple potentiel de paire. Etude du role des effets a n corps. Identification des phenomenes entierement independants du potentiel et representatifs des proprietes generiques des metaux
The low energy interphase boundaries between noble metals (Au, Cu) and various ionic crystals (LiF, KCl, NaCl, MgO, A12O3, mica) were determined at 550°C by means of the boundary energy induced rotation of small (~ 1 μm) spheres. The systems were chosen so that the effect of lattice mismatch (varied between 1.3 and 35.1%), the effect of lattice structure (cubic/cubic and hexagonal/cubic) and chemical effects could be studied. The results obtained suggest that boundary models based on the coincidence concept are not applicable to interphase boundaries between noble metals and ionic crystals because the low energy boundaries observed were not of the coincidence type and existing coincidence orientation relationships did not result in low energy boundaries. However, the atomic structure of the low energy interphase boundaries observed may be understood in terms of the following "lock-in model". A low energy interphase boundary results if the close packed rows of atoms at the "surface" of the metal crystal fit into the "valleys" between close packed rows of atoms at the "surface" of the ionic crystal. This model seems to predict correctly the experimentally observed correlations between the interfacial energy and the boundary inclination, the lattice mismatch and the lattice structure of the two phases involved.
Although the interpretation of experiments in such fields as the shapes of small particles and the thermal etching of surfaces usually involves problems of kinetics rather than mere equilibrium considerations, it is suggested that a knowledge of the relative free energies of different shapes or surface configurations may provide a useful perspective. This paper presents some theorems on these relative free energies which follow from the Wulff construction for the equilibrium shape of a small particle, and some relations between atomic models of crystal surfaces and the surface free energy function used in this construction. Equilibrium shapes of crystals and of noncrystalline anisotropic media are classified, and it is pointed out that the possibilities for crystals include smoothly rounded as well as sharp-cornered forms. The condition is formulated for thermodynamic stability of a flat crystal face with respect to formation of a hill-and-valley structure. A discussion is presented of the limitations on the applicability of the results imposed by the dependence of surface free energy on curvature; and it is concluded that these limitations are not likely to be serious for most real substances, though they are serious for certain idealized theoretical models.
The influence of annealing twin density on CSL distributions in f.c.c. materials is discussed. Geometric considerations are presented which show that as a consequence of both energetic and crystallographic constraints associated with twinning, a CSL distribution consisting entirely of low-Σ boundaries is attainable. Considered the limit of grain boundary character control for high angle grain boundaries, such a ‘twin-limited’ CSL distribution is expected to arise when the annealing twin frequency approaches 2/3.Der Einfluß der Anlaßzwillingsdichte auf die CSL-Verteilung in kubisch flächenzentrierten Materialien wird diskutiert. Geometrische Betrachtungen zeigen, daß als Folge von energetischen und kristallographischen Randbedingungen bei der Zwillingsbildung eine CSL-Verteilung erreichbar ist, die vollständig aus Grenzflächen mit niedrigen Σ-Werten besteht. Eine solche durch „Zwillingsbildung begrenzte” CSL-Verteilung, die als Grenzfall der Korngrenzencharakterkontrolle für Großwinkelkorngrenzen angesehen werden kann, wird erwartet, wenn der Anteil der Anlaßzwillinge 2/3 erreicht.
Polycrystalline thin films have attracted increased research attention in recent times, due to their importance in semiconductor, magnetic, magneto-optic and other applications. These films are often found to exhibit a strong fiber texture with one of the low index directions, , , perpendicular to the film plane. The grain boundaries in such polycrystalline films are usually perpendicular to the film surface implying a tilt character, i.e., the rotation axis is parallel to the grain boundary plane, and also parallel to the common film normal. According to Neumann`s principle, a symmetric boundary should represent an extremum -- either a minimum or a maximum -- of the interfacial energy with respect to changes of boundary plane, and Wolf has shown by computer simulation that symmetric tilt boundaries often correspond to energy minima. If symmetric tilt boundaries generally have lower energy than other boundaries, one might expect a larger than random proportion of all the boundaries to be of this type. In this paper, the authors report a preliminary determination of the proportion of grain boundaries (in the immediate vicinity of triple junctions) that have a symmetric tilt character in polycrystalline gold thin films, prepared by thermal evaporation and subsequent annealing.
Recently, a method has been devised for measuring the boundary orientations using backscattered Kikuchi diffraction (BKD, otherwise known as electron backscattering, EBS). The work reported demonstrates that BKD can be efficiently used to measure both the misorientation across grain boundaries and also the orientation of boundary planes. In nickel it has been shown that the boundaries of grains which are situated along the corner of a rectangular specimen rotate so as to minimize their interfacial energy. For non-coincidence site lattices related grains, boundaries tend to align normal to the edge of the specimen, while {Sigma} = 3 and {Sigma} = 9 CSLs tend to rotate to tilt configuration, particularly asymmetric tilts such as {l brace}111{r brace}/ {l brace}115{r brace} or {l brace}110{r brace}/{l brace}114{r brace}.
The presence of grain boundaries in polycrystalline materials affects the materials properties and performance. Recently it has been realized that boundaries can be manipulated to give better properties, and the design and control of grain boundaries is now an area of strong research interest in the search for high performance engineering materials. Grain boundaries can be classified using the Coincident Site Lattice Model (CSL), which defines the periodicity, i.e., the degree of fit between the two lattices which constitute the boundary. Using this model it is possible to divide boundaries into categories: low angle (up to 15{degree} misorientation), CSL and random i.e., high angle non-CSL. Some CSL boundaries have been shown to have special properties: an example from recent research in the same program as that currently reported has shown that twin boundaries ({Sigma} = 3 in CSL notation) in High Nitrogen Austenitic Stainless Steels do not favor the formation of Cr{sub 2}N precipitates. The research presented here examines grain boundary inclinations of surface grains in austenitic steel specimens which have been isothermally aged at higher 700 C or 800 C. Grain boundary plane crystallography has also been obtained for the 800 C aged sample.
The authors conclude that essentially all the boundaries in the rotating copper and silver sphere-on-a-plate experiments at 0.96 T/sub M/ and 0.99 T/sub M/ could not have been completely melted. Instead, it appears that they remained crystalline and that the rotations occurred because of the dependence of their energy on crystal misorientation. Possible situations include a variety of cases where the boundary is considerably disordered but where the atomic positions are still highly correlated with the atomic positions in the two grains adjoining the boundary slab. This conclusion is consistent with numerous previous interpretations of these experiments. The observation that all boundaries rotated into a relatively small number of discrete misorientations is also consistent with the conclusions that the boundary structures are related to the structures of the relatively low energy boundaries into which they rotated. Finally, the authors' conclusion is also consistent with the experimental results obtained by transmission electron microscopy where no evidence for boundary melting was found in different experiments with aluminum performed at 0.96 T/sub M/ and 0.999 T/sub M/.
It is shown that it is possible to combine rotations that lead to coincidence-site lattice relationships and derive new coincidence- site lattices. This has applications to studies of twinning; in specific orientations the plane of the boundary can have more atoms in coincidence-site lattice positions than is given by the lattice relationship; the continuity of planes across a grain boundary separating two crystals with a coincidence-site lattice relationship exhibits an m to n correspondence, where m and n are integers. This has applications in the study of contrast from such boundaries in field-ion micrographs.
The fine structure of (001) twist boundaries in gold was systematically studied by transmission electron microscopy. About 200 such boundaries were prepared under controlled conditions by welding single crystal films together face-to-face at various twist angles Θ. An almost continuous study of the structure was then made over the entire range 0 ⋜ Θ⋜45°.Orthogonal grids of grain boundary misfit screw dislocations were found in the vicinities of critical angles Θc (Θc=0, 22–6, 28.1 and 36.9°). producing various high density coincidence site boundaries. Measured geometrical properties of the grids near each Θc (i.e. grid spacings, orientation and dislocation diffraction contrast) were completely consistent with those predicted for a boundary structure consisting of a suitable misfit dislocation grid embedded in the high density coincidence site interface corresponding to Θc.Fine structure was not detected in appreciable angular ranges lying between the Θc. This may have been due in part to the relatively small Burgers vectors and small spacings of many of the grids expected in these regions. On the basis of expected energy versus Θ behaviour of such boundaries, it was concluded that misfit dislocation structures may actually exist over a substantial portion of the total Θ range.
A simple form of multi-ion interaction has been constructed for the purpose of atomistic simulation of transition metals. The model energy consists of a bonding term, which is the square-root of a site density ρi, summed over atoms i, and a repulsive pairwise term of the form The site density ρi is defined as sum over neighbouring sites j of a cohesive potential (R ij). Both V and are assumed to be short-ranged and are parameterized to fit the lattice constant, cohesive energy and elastic moduli of the seven body-centred-cubic (b.c.c.) transition metals. The result is a simple model which, unlike a pair-potential model, can account for experimental vacancy-formation energies and does not require an externally applied pressure to balance the “Cauchy pressure”.
Discontinuous precipitation in an austenitic Fe-Mn-V-C alloy has been studied using transmission electron microscopy of extraction replicas and thin foils. The vanadium carbide precipitates formed during the discontinuous reaction are present either as particles or as long fibres. The coincidence site lattice model for high-angle grain boundaries has been used to relate each precipitate morphology to the nature of the grain boundary at which it forms. Fibrous precipitation occurs behind those boundaries in which there is a comparatively high density of coincidence sites, whereas particulate precipitation occurs at low coincidence boundaries.
Conspicuous microstructural changes were observed for a variety of metallic materials during elevated temperature cyclic deformation, owing to grain growth, grain boundary motion and reorientation of the grain boundaries under 45° with respect to the stress axis. These phenomena are associated with an obvious change of microtexture and orientation correlation between neighbouring grains. The current investigation focused on the evolution of microstructure and microtexture during high temperature low cycle fatigue in pure nickel. The orientations of the individual grains were determined by means of the electron backscattering diffraction technique in a scanning electron microscope. The spatial arrangement of the grain boundaries was measured using optical microscopy. The results are discussed with regard to the effect of grain boundary character on the propensity for grain boundary damage by motion and alignment.
Résumé
Nous avons observé des changements microstructurels visibles pour une variété de matériaux métalliques au cours d'une déformation cyclique à température élevée, dû à la croissance des grains, au mouvement des joints de grains et à la réorientation des joints de grains à moins de 45° par rapport à l'axe de la contrainte. Ces phénomènes sont associés à un changement évident de la microtexture et de la corrélation d'orientation entre les grains adjacents. La présente étude est centrée sur l'évolution de la microstructure et de la microtexture d'une fatigue cyclique basse à haute température dans du nickel pur. Nous avons déterminé l'orientation individuelle des grains au moyen de la technique de diffraction rétrograde des électrons dans un microscope electronique à balayage. Nous avons mesuré l'organisation spatiale des joints de grains en utilisant la microscopie optique. Nous discutons les résultats en considerant les effets du caractère des joints de grains sur la tendance d'endommagement des joints de grains cause par le mouvement et l'alignement.
This paper reports measurements of large sample populations of grain boundary geometries – both grain misorientations and boundary plane orientations – in annealed nickel. Two heat treatment schedules are represented: one in which a specimen is heated at 1000°C for 1 h (the ‘fast’ specimen) and another which combines this treatment with a slow heating and cooling cycle (the ‘slow’ specimen). There was a marked difference between the proportions of ‘geometrically special’ boundaries in the two data sets, with the slow set containing almost double the proportion of coincident site lattice boundaries (CSLs) observed for the fast set. Furthermore, of the CSLs in the slow set, half were tilts or twists (mostly asymmetrical tilts), compared with less than a quarter of the CSLs in the fast set. The interpretation of these data is that kinetic factors have a strong influence on the evolution of populations of ‘special’ boundaries.MST/1431
The texture in thin films develops during processing steps such as deposition and annealing. Recent studies show that texture plays an important role in stress voiding, thermal hillock formation, grain collapse and electromigration failure. Specifically, electromigration failure depends on the grain misorientation distribution, which describes the probability of different grain boundaries and, therefore, links the grain boundary structure to the mass transport that takes place primarily along the grain boundaries. To understand the relationship between the grain misorientation and electromigration lifetime in aluminum thin films, the texture was measured on three sets of films from different manufacturing conditions. The frequency of occurrence of coincidence site lattice (CSL) grain boundaries, which represent special misorientations between grains, was obtained, and electromigration tests were done for all three conditions. Experimental results show that the lifetime of patterned films increases as the amount of 111 texture and the frequency of CSL boundaries increased.