Recent molecular dynamics simulations of the growth of ͓Ni 0.8 Fe 0.2 /Au͔ multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited ͑111͒ ͓NiFe/CoFe/Cu͔ multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the ͑111͒ surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

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Periodic cellular metals with honeycomb and corrugated topologies are widely used for the cores of light weight sandwich panel structures. Honeycombs have closed cell pores and are well suited for thermal protection while also providing efficient load support. Corrugated core structures provide less efficient and highly anisotropic load support, but enable cross flow heat exchange opportunities because their pores are continuous in one direction. Recent advances in topology design and fabrication have led to the emergence of lattice truss structures with open cell structures. These three classes of periodic cellular metals can now be fabricated from a wide variety of structural alloys. Many topologies are found to provide adequate stiffness and strength for structural load support when configured as the cores of sandwich panels. Sandwich panels with core relative densities of 2-10% and cell sizes in the millimetre range are being assessed for use as multifunctional structures. The open, three-dimensional interconnected pore networks of lattice truss topologies provide opportunities for simultaneously supporting high stresses while also enabling cross flow heat exchange. These highly compressible structures also provide opportunities for the mitigation of high intensity dynamic loads created by impacts and shock waves in air or water. By filling the voids with polymers and hard ceramics, these structures have also been found to offer significant resistance to penetration by projectiles.

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A preliminary study of a promising bi-layer environmental barrier coating (EBC) designed to reduce the susceptibility of SiC composites to hot water vapor erosion is reported. The EBC system consisted of a silicon bond coat and a pore-free ytterbium disilicate (YbDS; Yb 2 Si 2 O 7 ) topcoat. Both layers were deposited on -SiC substrates using a recently optimized air plasma spray method. The two layers of the coating system had coefficients of thermal expansion (CTE) that were well matched to that of the substrate, while the YbDS has been reported to have a moderate resistance to silicon hydroxide vapor forming reactions in water vapor rich environments. Thermal cycling experiments were conducted between 110 °C and 1316 °C in a flowing 90 % H 2 O/10 % O 2 atmospheric pressure environment, and resulted in the formation of a thermally grown (silica) oxide (TGO) at the silicon-ytterbium disilicate interface. The TGO layer exhibited linear oxidation kinetics consistent with oxidizer diffusion through the ytterbium silicate layer controlling its thickening rate. The effective diffusion coefficient of the oxidizing species in the YbDS layer was estimated to be 2x10 -12 m 2 s -1 at 1316 o C. Slow steam volatilization of the YbDS topcoat resulted in the formation of a thin, partially protective, high CTE ytterbium monosilicate layer on the outside of the YbDS coating. Progressive edge delamination of the coating system was observed with steam exposure time, consistent with water vapor volatilization of the TGO edges that were directly exposed to the environment. This was aided by outward bending of the delaminated region to relax TGO and YbMS surface layer stresses developed during the cooling phase of each thermal cycle.

Atomistic simulations using interatomic potentials are widely used for analyzing phenomena as diverse as crystal growth and plastic deformation in all classes of materials. The potentials for some material classes, particularly those for metal oxides, are less satisfactory for certain simulations. Many of the potentials currently utilized for metal oxides incorporate a fixed charge ionic component to the interatomic binding. However, these fixed charge potentials incorrectly predict the cohesive energy of ionic materials, and they cannot be used to simulate oxidation at metal surfaces or analyze metal/oxide interfaces where the local ion charge can be significantly different from that in the bulk oxide. A recent charge transfer model proposed by Streitz and Mintmire has in part successfully addressed these issues. However, we find that this charge transfer model becomes unstable at small atomic spacings. As a result, it cannot be used for the studies of energetic processes such as ion bombardment ͑e.g., plasma-assisted vapor deposition͒ where some ions closely approach the others. Additionally, the Streitz-Mintmire charge transfer model cannot be applied to systems involving more than one metal element, precluding study of the oxidation of metal alloys and dissimilar metal oxide/metal oxide interfaces. We have analyzed the origin of these limitations and propose a modified charge transfer model to overcome them. We then unify metal alloy embedded atom method potentials and the modified form of the charge transfer potential to create a general potential that can be used to explore the oxidation of the metallic alloy and the energetic vapor deposition of oxides, and to probe the structure of dissimilar metal oxide/metal oxide or metal alloy/oxide multilayers. Numerical procedures have been developed to efficiently incorporate the potential in molecular dynamics simulations. Several case studies are presented to enable the potential fidelity to be assessed, and an example simulation of the vapor deposition of aluminum oxide is shown to illustrate the potential utility.

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