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This paper reviews findings on the anisotropy of the grain boundary energies. After introducing the basic concepts, there is a discussion of fundamental models used to understand and predict grain boundary energy anisotropy. Experimental methods for measuring the grain boundary energy anisotropy, all of which involve application of the Herring equation, are then briefly described. The next section reviews and compares the results of measurements and model calculations with the goal of identifying generally applicable characteristics. This is followed by a brief discussion of the role of grain boundary energies in nucleating discontinuous transitions in grain boundary structure and chemistry, known as complexion transitions. The review ends with some questions to be addressed by future research and a summary of what is known about grain boundary energy anisotropy. [ABSTRACT FROM AUTHOR]

Equal-channel angular pressing (ECAP) is an effective fabrication process for obtaining ultrafine-grained materials. This paper examines the development of homogeneity in materials processed by ECAP with emphasis on samples of pure aluminum and an Al-6061 alloy processed by ECAP for up to 8 passes at room temperature. The Vickers microhardness was recorded on the polished cross-sectional planes of each as-pressed billet and the results are plotted in the form of contour maps to provide a pictorial depiction of the hardness distributions throughout the cross-sections. The factors influencing the homogeneity are examined, including the die corner angle within the ECAP die and the number of imposed passes. It is shown that good homogeneity may be achieved through ECAP processing when the number of passes in ECAP is reasonably high. [ABSTRACT FROM AUTHOR]

In this paper, recent progress in magnesium matrix composite technologies is reviewed. The conventional and new processes for the fabrication of magnesium matrix composites are summarized. The composite microstructure is subsequently discussed with respect to grain refinement, reinforcement distribution, and interfacial characteristics. The mechanical properties of the magnesium matrix composites are also reported. [ABSTRACT FROM AUTHOR]

We provide an explanation, within the framework of a model based on standard mean curvature grain-boundary motion coupled with somewhat unusual boundary conditions, for the paradoxical experimental results reported by Rath and Hu (Trans Am Inst Min Eng, 245:1577, ) regarding grain boundary migration in aluminum bi-crystals. In their experiments, they measured the velocity of the grain boundary in a wedge-shaped bi-crystal of aluminum, and assuming that rt ∝ − r-n, where r is the radius of the grain boundary as measured on the external surface, they found that 3.3 < n < 4, despite the fact that according to much well-established theory and extensive experimental data to be found elsewhere, n should be equal to one. We undertake a theoretical study of the dynamics of an initially cylindrical grain embedded in a 3D-single crystal film. The grain boundary is assumed to maintain axisymmetry and to move by mean curvature motion. Boundary conditions for the grain boundary are taken to be determined by the experimental results of Rath and Hu. We find a solution using a combination of analytical and numerical methods. Our analytical model predicts that rt ∝ fn, where f, the force applied by the grain boundary on the triple junction, is given by f = γgb sinθ, where γgb is the grain boundary energy and θ is the tilt angle of the grain boundary at the triple junction, and n satisfies 3.3 < n < 4 as earlier. This power law prediction is not in accordance with classical Mullins' theory; however, it should be possible to test experimentally, thus allowing our approach to be checked for consistency with experiment. We suggest that the unusual kinetics observed in the experiment are due to grain boundary interaction at the surface triple junction with an external surface covered with aluminum oxide. [ABSTRACT FROM AUTHOR]

The high-temperature resistant Si–Al–C–(O) fibers were prepared through polymer-derived method using continuous polyaluminocarbosilane (PACS) fibers. Evolutions of the crystallization during the pyrolysis of the Si–Al–C–(O) precursor fibers were investigated by a series analysis. The structure of the fibers transforms from organic state to inorganic state and the crystalline phases appear during the pyrolysis. The β-SiC crystallite size increases when the temperature is higher than 1,300 °C. At the same time, the α-SiC appears. At 1,600 and 1,800 °C, the grain size of β-SiC of the fibers is 15.4 and 22.1 nm, respectively. The growth of β-SiC and the appearing of α-SiC have a great influence on the properties of the fibers. The change of the tensile strength of the pyrolysis products is divided into three stages with the growth of the crystal. The tensile strength of the Si-Al-C fibers is higher than 1.9 GPa. [ABSTRACT FROM AUTHOR]

The misorientation behavior was investigated in the region of a symmetrical tilt $$ {\hbox{ $ < $112$ > $ }} $$ boundary with a misorientation of 15.7°. The strain and orientation distributions were obtained by DIC (digital-image-correlation) and EBSD (electron backscattering diffraction) techniques to characterize the kinematical behavior of the grain boundary. In order to obtain a misorientation gradient, the misorientation was used in representation of the axis–angle description. This formation of the orientation gradient is found to have a common rotation axis in the [11−2] direction. [ABSTRACT FROM AUTHOR]

Atomistic simulations are employed to investigate the structure of Σ3 asymmetric tilt grain boundaries (ATGBs) with boundary planes rotated about the $$\langle 110 \rangle$$ misorientation axis in Cu and Al. Results show that the structural units (SUs) and faceting of all 25 Σ3 ATGBs in Cu and Al intermediate to the coherent twin boundary and the symmetric incoherent twin boundary can be completely defined in terms of SUs for these two symmetric boundaries. A structural unit and faceting description for Σ3 asymmetric tilt grain boundaries is presented. Interestingly, this description is identical for both low stacking fault energy Cu and high stacking fault energy Al; only the dissociation width of the D structural unit on the incoherent twin facet differs in Cu and Al. A model based upon the coincidence plot and the structural units of the Σ3 coherent and incoherent twin boundaries is shown to accurately describe the structural units and faceting for all calculated Σ3 asymmetric tilt grain boundaries in this study. This model can also be extended to continuum descriptions of these boundaries to facilitate higher scale computational models. [ABSTRACT FROM AUTHOR]

Ba0.9Sr0.1TiO3 powder was processed at 80°C by reacting Ti sol in aqueous solutions that contained BaCl2, SrCl2 and NaOH at atmospheric pressure. Well-crystallized, spherical, nanosizes powders were formed by this method. The powders were found to have a cubic structure, which was retained even after heating at 900°C. Sintering at 1400°C, led to the formation of a tetragonal structure with a secondary phase of Ba6Ti17O40. Abrupt grain growth was observed at 1400°C. The electrical response of the sample sintered at 1400°C has three electrically different regions. Each region of the sample is represented by different RC element. Element 1 ( R 1 C 1) is the most resistive and its capacitance ishigh (0.5 nFcm−1) indicating a thin region, probably the grain boundary. Element 2 ( R 2 C 2) shows a smaller resistance value compared to element 1. The capacitance value of element 2 is temperature-dependent and displays a Curie–Weiss behaviour, indicative of a ferroelectric material above T c. The lower capacitance of C 2 (15 pFcm−1) indicates that it is a much thicker region than element 1 and can be assigned as a ferroelectric bulk region. Element 3 is probably an electrode effect. [ABSTRACT FROM AUTHOR]

Experiments were conducted on electrodeposited (ED) nanocrystalline (nc) Ni with an average initial grain size of about 20 nm at 393 K to study the shape of the creep curves. In addition, microstructure was examined by means of transmission electron microscopy (TEM). The results show that the creep curves are characterized by the presence of a well-defined steady-state stage. An examination of the microstructure indicates that while grain growth occurs during deformation, the grain size attains a constant value once steady state creep is approached. A comparison between grain size measurements obtained by the TEM technique and those obtained via the X-ray diffraction method shows that the use of the latter method may lead to an underestimation of the value of the average grain size. [ABSTRACT FROM AUTHOR]

This review surveys the characteristics of face-centered cubic (fcc) metals and alloys processed by equal-channel angular pressing (ECAP). The significance of the Hall–Petch relationship for ultra-fine grained structures is examined and the dependence of the saturated stress obtained in ECAP on the absolute melting temperature is described and discussed. In addition, the flow processes at low temperatures in ultrafine-grained materials and the microstructural evolution of the dislocation densities and precipitates in some alloys of practical importance are also considered briefly. [ABSTRACT FROM AUTHOR]

Metallographic investigation of the discs used in tests to simulate gear tooth contact has revealed the occurrence of several microstructural features that are present only in fatigued specimens: e.g. dark etching regions (DER) and white etching bands (WEB). In addition, a new type of microstructural constituent has been observed close to the surface, below asperities. This feature is referred to as the plastic deformation region (PDR). For the first time the hardness of these fatigue-induced phases was accurately determined by the nanoindentation technique. It has been found that DER initiates at the prior austenite grain boundaries. Fatigue cracks initiate and propagate preferentially at the boundaries of PDR and ultimately lead to the formation of micropitting. [ABSTRACT FROM AUTHOR]

The atomic and electronic structures of [0001]/( $$\bar{1}\bar{2}30$$) Σ 7 symmetric tilt grain boundary in an undoped ZnO bicrystal were investigated by high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. HRTEM imaging and atomistic calculations revealed that the grain boundary was composed of at least two types of structural units. It was also found that one of the structural units has two threefold-coordinated atoms per a unit and the other has two fivefold-coordinated atoms. First-principles calculations indicated that these atoms with various coordination numbers do not form deep unoccupied electronic states in the band gap of ZnO, which is in consistency with a linear current-voltage characteristic observed for the bicrystal with the Σ 7 boundary. [ABSTRACT FROM AUTHOR]

We analyze the results of molecular-dynamics simulations of the interfacial (Kapitza) resistance of representative grain boundaries in Si. Simulations of the interactions of phonon wave packets with the Si grain boundaries show that the scattering process depends strongly on both the branch and wavelength of the incident phonons. This approach has the potential for providing detailed spectral information to mesoscale simulations of thermal transport in interfacial systems. [ABSTRACT FROM AUTHOR]

Using the contactless microwave phase-shift technique (μ W-PS) and High Resolution Transmission Electron Microscopy (HRTEM), we show that the twist and mixed parts of a < 110 > Σ = 51(θ = 16.10∘) grain boundary in germanium (Ge) are electrically active. We also show that we can passivate the electrically active grain boundaries by sulfur segregation which has been studied by energy filtering HRTEM. Atomistic simulations show that the most favorable places for this segregation are the high energy sites of grain boundary. [ABSTRACT FROM AUTHOR]

We have studied the effect of plastic deformation by compression on the occurrence of discontinuous precipitation in Al-30% Zn alloy after ageing at two different temperatures (348 and 423 K) has been studied. Optical microscope, scanning electron microscope, X-ray diffraction and differential scanning calorimetry were used for characterization. During ageing of undeformed alloy, the grain boundary of supersaturated solid solution represents the favorite site of precipitation by discontinuous mechanism. We found that the occurrence of discontinuous precipitation depends mainly on the degree of plastic deformation before ageing. The grain boundary act as reaction front and migrates in case of low degree of prior deformation. [ABSTRACT FROM AUTHOR]

Decomposition of solid solution taking into account the grain boundaries segregations is considered in the framework of generalized Cahn-Hilliard model for finite systems. Two types—“stripe” and “drop-wise”—of the surface-directed decomposition found below spinodal depend on concentration of a wetting component. We demonstrate that grain boundaries segregations are able to drastically change the phase equilibrium inside the grain when its size becomes less than the critical value. As a result, decomposition of the solid solutions easily appears for nanograined materials. [ABSTRACT FROM AUTHOR]

Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (α), carbides at ferrite GBs, some pearl ite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, V)(C, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to α : α GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at α : α GBs, in particular no 5, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V. [ABSTRACT FROM AUTHOR]

Experiments on silicate liquid phase sintering of YSZ ceramics with addition of 0.5 mol % of rare earth ions have been done in order to study the effect of these ions on the kinetics of grain growth and the expulsion of glass through the grain boundaries. Kinetics follow a third power law in the following order YPr > YPrEr > YY > YEr. The expelled glass does not spread over the ceramic grains and its mass is inversely related to grain size. Glass phase separation inside the grain boundaries is found to be a necessary condition for glass expulsion. [ABSTRACT FROM AUTHOR]

An introduction is presented to articles on intergranular and interphase boundaries in materials science published in this issue.