Volume 95, Issue 4

Recent advances both in experimental instrumentation and computing power have made it possible to interrogate the distribution of internal interfaces in polycrystals and the three dimensional structure of the grain boundary network with an unprecedented level of detail. The purpose of this paper is to review techniques that can be used to study the mesoscopic crystallographic structure of grain boundary networks and to summarize current findings. Recent studies have shown that grain surfaces within dense polycrystals favor the same low energy planes that are found on equilibrium crystal shapes and growth forms of crystals in contact with another phase. In the materials for which comprehensive data exists, the distribution of grain boundaries is inversely correlated to the sum of the energies of the surfaces of the grains on either side of the boundary.

A statistical method for the experimental study of the grain boundary network topology is proposed. The experimental binary classification of grain boundaries is based on their wetting behavior (wetted and non-wetted grain boundaries). The grain boundary energy distribution is used to describe the intergranular liquid network topology by means of the percolation theory. The model is applied to the experimental study of grain boundary wetting of polycrystalline zinc and aluminum by liquid gallium. Elemental mapping in a scanning electron microscope is used for cluster determination. The experimental observations are in accordance with the theory.

The motion of connected boundary systems was investigated in Al and Zn tricrystals. It was found that triple junctions can exert a drag force on the adjoining boundaries owing to a lower mobility. This drag manifests itself by a deviation of the dihedral angles at the triple junction from the equilibrium angles due to grain boundary surface tensions. The strength of triple junction drag can be expressed in terms of a criterion Λ = ( m tj ∙ a/m b ) , where m tj is the triple junction mobility, m b the boundary mobility, and a is the grain size. Therefore, the effect should be particularly pronounced in fine-grained and nanocrystalline materials. It is shown that the von Neumann–Mullins relation of 2D grain growth is modified by triple junction drag such that there is no unique relationship between growth rate and the number of sides anymore. For very small Λ , the triple junctions control grain structure evolution during grain growth, which leads to polyhedral grain shapes with flat boundaries. The theoretical concept is supported by computer simulations of grain growth and in-situ annealing experiments in the SEM.

The influence of the stacking-fault energy on the interaction between a lattice dislocation and a Σ3 grain boundary has been investigated in copper. The entrance processes of the dissociated dislocations as observed by transmission electron microscopy (conventional, weak-beam and image contrast simulation) are supported by atomistic simulations. They may be explained, on the basis of theoretical models, by the occurrence of a constriction phenomenon. The observed processes constitute the first steps of the intergranular stress accommodation which plays an important role in the plastic deformation of polycrystals.

This brief review describes the major features of grain boundary mobility in metals and current limitations in our theoretical understanding. Although thermally activated exchange of atoms across boundaries provides a basic picture of grain boundary correct to within an order of magnitude, no refinement of this theory is available that accounts for the experimental characteristics of grain mobility as it depends on crystallographic type. An example is given of a simple extension involving density of kinks on the two surfaces that comprise a boundary to show that the resulting estimate of mobility is essentially different from the experimentally observed dependence.

Recent achievements in in situ transmission electron microscopy (TEM) observation of the discontinuous precipitation reaction have been reviewed. The steady-state period of growth of this reaction can be characterized by two different morphologies. The regular morphology is usually disturbed by instabilities, resulting in an irregular morphology which leads to serious changes in the shape of the reaction front, the changes in the growth direction, branching and re-nucleation of the β lamellae. The termination of the discontinuous precipitation reaction was attributed to the phenomena of colonies impingement or formation and growth of a precipitate-free zone ahead of the reaction front. The aspects of further in situ studies are also indicated taking into account unsolved problems or untouched areas.

The reaction of planar <100>-tilt and twist grain boundaries to mechanical stresses is reviewed. It was found that the motion of low-angle grain boundaries as well as of high-angle grain boundaries can be induced by an imposed external mechanical stress field. The migration of grain boundaries was measured in-situ using X-ray diffraction to localize the grain boundary position continuously. From the observed activation enthalpies, conclusions can be reached regarding the migration mechanisms of the stress-induced grain boundary motion. The motion of planar low- and high-angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low- and high-angle grain boundaries was observed for tilt as well as for twist grain boundaries.

An attempt is made to assess the efficiency of drag effects by different structural elements of a polycrystal on grain growth. The rate of grain area change is chosen as a measure of stability of a grain structure, and the inhibition of grain growth is pairwise evaluated among all drag effects considered. In aluminium, at temperatures of about 200 °C, triple junction drag was found to be most effective. The derived hierarchy of drag efficiency can be used as an effective engineering tool to assess and compare the role of chemistry and crystal defects on the microstructural stability of nanocrystalline and fine-grained materials.

In previous computer simulation studies of the atomic structure of twin boundaries and defects such as twinning dislocations in the HCP metals, conventional periodic boundary conditions along the dislocation line and fixed conditions in the other two directions were used. Thus, extensive defect motion could not be considered. A method has now been developed to simulate a twin boundary containing a step with dislocation character, with full periodicity in the boundary plane. This model may be used for investigating either the static or dynamic properties of such interfacial defects.

Thin films of perovskite-based material, exhibiting colossal magnetoresistance properties have been structurally characterized. The lattice mismatch with the substrate and the associated stress can induce different types of distortions in the film, resulting in different crystal structures. This opens the possibility to design new structures with predictable physical properties. The structure of epitaxial La 0.1 Sr 0.9 MnO 3 on a (100) SrTiO 3 substrate and of the epitaxial composite (La 0.67 Ca 0.33 MnO 3 ) 1− x (MgO) x films on a (100)MgO substrate have been investigated by electron microscopy. Novel mechanisms for misfit accommodation in epitaxial films have been detected. For the La 0.1 Sr 0.9 MnO 3 /(100)SrTiO 3 film, where there is only a small mismatch ( a F ≈ a S ), the formation of periodic microtwins is observed. For the composite (La 0.67 Ca 0.33 MnO 3 ) 1– x (MgO) x /(100)MgO films under a 3D stress (mismatch ≈ 8 %), a phase transition from low to high symmetry is detected.

The stability and morphology of a moving interface between cation conductors AX and BX during an electric field-driven solid state reaction has been studied theoretically and experimentally. Linear stability analysis and kinetic Monte Carlo simulations with nearest-neighbour interactions show that the morphological stability of the interface is essentially determined by the difference in the ionic conductivities of the two crystals. Simulated morphologies in the unstable regime range from fingers to fractals. Experiments have been performed with diffusion couples AgCl/KCl (immiscible, large difference in conductivities) and AgCl/CuCl (partially miscible, nearly identical conductivities). As predicted by the theoretical calculations, we observe morphological instabilities of the initially planar interface if the “faster phase” grows at the expense of the “slower phase”. The observed morphologies are discussed and compared with the morphologies obtained by kinetic Monte Carlo simulations.

If bulk or thin-film ceramics consisting of more than one component are subjected to a high temperature during processing or use, interfacial solid–solid reactions occur between the components. To understand this type of reactions, the atomic structure of various reactive oxide/pyrochlore and oxide/spinel model interfaces is studied by high-resolution transmission electron microscopy. Kinetics of the pyrochlore- and spinel-forming reactions at these interfaces are shown to depend on the atomic structure of the interface, in particular on that of the interfacial dislocations. During the reaction, these dislocations have to move together with the advancing interface. Accordingly, reaction kinetics are determined by the mode of dislocation movement and, thus, by the Burgers vector geometry of the interfacial dislocations. The type of interfacial dislocations is, in turn, influenced by the stress conditions during the initial stage of the solid state reaction.

It is demonstrated that the formation of reaction zone morphologies periodic in time and space can be considered as a manifestation of the Kirkendall effect accompanying interdiffusion in the solid state. In a diffusion-controlled interaction, the Kirkendall marker plane can bifurcate, which is attributed to diverging vacancy fluxes in the reaction zone. When the Kirkendall plane is present in a phase layer, it attracts in situ -produced inclusions of “secondary-formed phase”, which, in turn, can result in a highly patterned microstructure.

The wetting of tin-silver-based alloys on Al 2 O 3 has been studied using the sessile-drop configuration. Small additions of Ti decrease the contact angle of Sn-3 wt.% Ag alloys on alumina from 150° to 25°. However, a wide variability in contact angles and spreading rates is observed. The variability is related to the kinetics of Ti dissolution in the alloy and the formation of triple-line ridges. Enhanced spreading is not accompanied by the formation of a continuous reaction layer at the metal/ceramic interface. Furthermore, no reaction product could be detected after tests performed at temperatures below 800 °C. The average mechanical strength of metal-ceramic bonds fabricated under parallel conditions ranged between 40–110 MPa. In all cases, fracture of the joints occurred at the metal-ceramic interface. The formation of a discontinuous reaction product decreases the strength of the bonds fabricated at higher temperatures.

Sintering metal particle (Cu or Ni) reinforced alumina composites, doped with CaO and SiO 2 additives, resulted in metal particles found as occlusions within alumina grains, and located at alumina grain boundaries and triple junctions. In composites sintered without additives, only isolated cases of metal particle occlusion were found. In the doped samples, intergranular films were observed at alumina grain boundaries and metal-alumina interfaces, for both types of particles. The occluded particles were found to have a limited size-range of 250 ± 20 nm, while particles at alumina grain boundaries had an average size of ~1.4 ± 0.3 μm. It is suggested that the higher driving force for grain growth during the initial stages of sintering, together with an increase in grain boundary mobility due to the intergranular films, results in increased particle–grain boundary detachment during the initial stages of sintering.

Intergranular boundaries, structures and phases are key microstructural components in advanced ceramic materials where dopants often play a leading role. This extended abstract summarizes the observations and analyses of the chemistry and structural evolutions that take place at intergranular boundaries and pockets during high-temperature deformation processes in Si-based ceramics. Quantitative analysis of these intergranular structures by analytical transmission electron microscopy is proved very important here to understand the dynamic development of the microstructure, which can shed light into the mechanism for plasticity and creep behaviors in high temperature structural ceramics such as SiC and Si 3 N 4 .

Determining the atomic structure, mechanisms and dynamics of interface motion between two phases with high-index orientation relationships is of central importance for understanding the mechanisms of the massive transformation in solids, as well as for obtaining a fundamental understanding of the motion of grain and interphase boundaries with limited or no coherency. In this investigation, both static and in-situ high-resolution transmission electron microscopy and three-dimensional near-coincident-site atom modeling were used to determine the atomic structure, growth mechanisms and dynamics of massive transformation interfaces in a TiAl alloy. These experiments show that massive γ M grains can have high-index orientation relationships and structurally incoherent interfaces with respect to the parent (retained) α 2 phase, although the degree of commensurability in the interface can vary, depending on the interface orientation. It is possible to form one-dimensionally commensurate interfaces, where planes of atoms in the two phases meet edge-to-edge across the interface. In-situ high-resolution transmission electron microscopy observations of several α 2 /γ M interfaces during growth revealed evidence of both continuous and stepwise mechanisms of interface motion, depending on the orientation of the interface plane for a given orientation relationship between the two phases.