We have recently developed a novel molecular-dynamics simulation method to grow polycrystals from a melt containing randomly oriented crystalline seeds. The resulting microstructures contain only randomly oriented (i.e., high-energy) grain boundaries. We find that these grain boundaries, which are highly constrained by their close proximity to grain junctions, are highly disordered in fcc metals and amorphous in silicon. From simulations of infinitely extended high-energy grain boundaries in bicrystals, we find that such highly disordered and amorphous grain boundaries are actually the thermodynamic ground state; by contrast, low-energy grain boundaries are crystalline. High-energy grain boundaries in diamond, however, are structurally ordered at the expense of a significant amount of graphite-like bonding. We show that these complex grain boundary structures have important effects on properties including grain boundary diffusion (fcc metals and silicon), grain boundary diffusion creep (silicon) and grain boundary electrical activity and strength (diamond). The implications for engineering materials with prescribed properties are discussed.

First principles density functional calculations are used to study the structure and properties of ultrathin films of alumina on Al(111) and Ru(0001) substrates. We focus on ∼5 Å two O-layer films, naturally produced by oxidizing NiAl, Ni3Al, and Al deposited on Ru(0001). The interface consists of chemisorbed l×1 oxygen on the underlying metal [1], with a nearly coplanar layer of Al2O3 above. The lowest energy structure of the Al-sublattice is found to consist of zig-zag rows of octahedral and tetrahedral Al ions, resembling the bulk κ-phase. Eleven different adsorbed metals, spanning the periodic table, have been studied [2]. At low coverage the bond is purely ionic; at high coverage most metals bind principally by polarization. Contrary to speculations, isolated vacancies are found on MgO(100) not to directly promote metal nucleation [3], and we suggest this behavior is general. However, ad-OH is found not only to promote island nucleation, but can also, in high concentrations, produce wetting of alumina by Cu [4].

Interest in ceramic composites arises from their potential for dramatically improved damage tolerance, as compared to monolithic ceramics, while delivering similar high-temperature properties. The remarkable toughening effect resulting from the proper combination of brittle phases is dependent upon the failure process developing in a particular way; matrix cracks must deflect into fiber debonding cracks that run in or near the fiber/matrix interface and permit some relative displacement. In early composites the requirements for crack deflection and good composite behavior seem to have been almost fortuitously satisfied by native C layers; however, the most productive uses of ceramic composites are dependent upon the development of fiber coatings that not only promote the desired interfacial failure, but also satisfy the additional requirement of being oxidation resistant. Though conceptually simple, the management of a failure process—specifically, predetermining the progress of cracks on a very local level—places a very demanding and unprecedented requirement on the materials designer. Historically, the details of fracture events were of interest principally for insight into means of avoiding them. The composite problem imposes the new requirement that we design to promote a special kind of fracture, and control post-fracture fiction within a certain range. This truly is a higher level of sophistication than heretofore attained, and its complexities have deterred many investigators from seriously addressing the problem. Nevertheless, there has been progress both in depth of understanding and in actual achievement of viable systems. Moreover, very interesting properties of several families of materials have been uncovered. These issues will be outlined and discussed in more detail with the objective of developing a broad view of the lessons learned and opportunities revealed.

Fiber-matrix adhesion is a variable to be optimized so that optimum composite mechanical properties can be achieved in polymer matrix composites. The contemporary view of adhesion rests on an “interphase” model in which not only the actual chemical and physical interactions between fiber and matrix are considered but also the structure and properties of both the fiber and the matrix in the region near the interface. The optimum design methodology starts with the specification of the fiber and matrix from a structural consideration. Once the constituents are selected, the focus is on the creation of a beneficial fiber-matrix “interphase”. This region where the fiber and matrix interact has to be designed for both “processing” and “performance”. Although no quantitative algorithm is available for interphase optimization, various thermodynamic principles coupled with experimental data can be used to qualitatively design the optimum interphase. Examples will be presented to illustrate how this interface can be engineered with surface treatments and sizings or coatings to insure thorough wetting, protection of the fiber, chemical bonding between fiber and matrix, toughness and desirable failure modes.

It has been shown that the concepts used to determine the equilibrium shape of crystals can be extended to determine the conditions under which grain boundaries will be fully wetted, partially wetted, or not wetted by a second phase. Recent experimental observations on the equilibrated morphologies of solid or fluid wetting phases along grain boundaries, reveal features that are predicted, and in some cases required, by this construction. Theory distinguishes between cases where surfaces are smoothly curved or where there are facets, edges and corners. In the latter case the conventional comparison of the energy of the original grain boundary with the sum of the surface energy of the two surfaces of the wetting layer leads to erroneous predictions. The correct predictions are obtained by comparing the Wulff shape of the grain boundary (the interfacial energy minimizing shape for a fixed volume of material) with a carefully defined sum of Wulff shapes of the surfaces of the wetting layer. Where orientations that are wetted join with those that are not, there is almost always an abrupt change of orientation. Faceting on two hierarchical levels can occur. Microscopic morphology changes along macroscopically curved surfaces follow well defined rules predicted by theory. The analogy between the thermodynamics of interface faceting and phase transformations allows the well known concepts of phase equilibria to be used to understand the predicted structures. The predictions of the model will be used to identify the nature of the faceting observed in alumina in the presence of a second phase.

The use of the electron back scattering pattern (EBSP) technique of electron diffraction (also referred to as EBSD and BKD) to study microstructure continues to gain popularity in the materials characterization community. The technique's ability to rapidly measure lattice orientation with good relative accuracy provides a statistically powerful tool for investigating the misorientation character of interfaces. It is certainly possible to gather basic misorientation data on thousands of interfaces in only a couple of hours. This invited talk will discuss the detection and characterization of interfaces in polycrystalline materials using the automated-EBSP technique with specific discussion of sub-grain and twin boundaries in deformed and/or annealed materials. This paper will also attempt to demonstrate with data from γγ' Ni-base superalloys that the process of acquiring, with high accuracy, all the spatial, orientation, and chemical information associated with boundaries during an extended data acquisition is a substantial experimental challenge. Combining quantitative image analysis of traditional high-resolution scanning electron micrographs with the EBSP data can help correct some of the distortions commonly encountered in EBSP data.

Thermally-grown oxide scales on high temperature alloys can provide oxidation protection if they are slow-growing, dense and adherent to the substrate. The factors affecting the integrity of the metal/oxide interface during dynamic oxidation processes are complex and include interfacial segregation, interface morphology, near-interface substrate properties, as well as the development of stresses in the growing oxide scale. Minor additions of reactive elements (RE) to the base metal have been shown to improve the oxidation performance of many high temperature materials. Studies on a variety of alloys have shown that the presence of reactive elements in the alloy affects segregation processes at both the metal/oxide and oxide/oxide interfaces. Whereas segregation to the metal/oxide interface can affect the scale adherence, segregation to the oxide/oxide interfaces in RE-containing systems has been proposed to result in changes in transport mechanisms as well as changes in the scale microstructure itself. High spatial resolution analytical electron microscopy techniques have been used to provide information on the microstructure and microchemistry of the scale and the metal/oxide and oxide/oxide interfaces. In general, in systems which exhibit improved oxidation performance, a consistent set of interfacial segregation phenomena and microstructural features were observed. Examples will be shown from a variety of nickel- and iron-based alumina formers. These kinds of studies, combined with traditional scanning electron microscopy studies of oxide scales can lead to the development of a more complete link between RE doping, interfacial segregation, interfacial/scale microstructure, and oxidation performance.

In a surprising variety of cases, interfaces in normally ductile materials can undergo timedependent brittle cracking under the influence of a tensile stress, either applied externally or existing as an internal residual stress. The connecting feature in all these cases is the presence of a surface-adsorbed element that is highly mobile in comparison to the constituent elements of the material. As in the phenomena of diffusion creep and diffusive growth of cavities at high temperatures, the driving force for this cracking is the work done by the tensile stress when a surface atom enters the solid. At temperatures below about 0.5 Tm of the solid, this occurs mainly along grain boundaries. Examples of systems that have been studied in some detail include cracking of alloy steels by sulfur, Cu-Sn alloys by tin, and nickel-based alloys by oxygen. Because the cracking involves diffusive penetration along grain boundaries, the rate of cracking is highly sensitive to grain-boundary structure and composition, and these variables offer opportunities to control the problem. We are aiming at a quantitative understanding of the effects of grain-boundary structure, stress, and temperature on this phenomenon by crack-growth experiments on bicrystals, by atomistic modeling of the stress-driven diffusion, and by micromechanical modeling of the events occurring at the tip of a growing crack.