Aluminum is one of the world’s most widely used metals, in large part due to its superior strength-to-weight ratio, while alumina (Al2O3) is one of the most strongly bonded compounds in existence (with enthalpy of formation of 1674.4 kJ/mol)1,2 and is widely used as a catalyst support, for structural ceramics, and as a substrate for film growth3,4,5,6 Among the various polymorphs of alumina, corundum (α-Al2O3) is the most stable phase at the ambient conditions and has been extensively studied7 The Al/α-Al2O3 interface is of enormous scientific and technological significance due to the crucial role it plays in a range of important applications such as metal-ceramic composites, protective coating for Al, casting and smelting processes, microelectronics, corrosion/wear protection and catalysis8,9,10,11 As a consequence, large amount of experimental8,12,13,14,15,16,17,18,19,20,21,22 and theoretical23,24,25,26,27,28,29,30,31,32,33 research work has been devoted to this interface.
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Based on high-resolution transmission electron microscopy, the primary orientational relationship at the Al/α-Al2O3 interface has been observed to be one that matches the close-packed planes and directions in the two phases17 In this orientation the Al(111) plane is parallel to the Al2O3(0001) basal plane and the following directions are parallel to each other:
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As is the case for other similar metal/Al2O3 interfaces2,34,35, the stoichiometry of clean Al/α-Al2O3 interface is a function of the oxygen partial pressure. Previous first principles based analysis25 has suggested that the clean Al/Al2O3 interface can be stoichiometric, oxygen terminated, or aluminum terminated, depending on the ambient oxygen partial pressure. For the relatively low oxygen partial pressures36, usually associated with sessile drop experiments, the interface is predicted to be stoichiometric. However, for relatively high oxygen partial pressures, prevalent during fracture experiments, the interface has been suggested to be oxygen terminated25. Recent experimental37 and theoretical31 studies reporting atomically abrupt Al(liquid)/α-Al2O3(solid) interface stabilized by self-regulated interfacial Al vacancies have suggested an aluminum terminated interface.
Electronic structure and work of adhesion have been obtained from first principles based quantum mechanical computations by Batyrev and Kleinman26 and Siegel et al.28,29 assuming a coherent interface. Streitz and Mintmire32,33,38 employed a force field accounting for the electrostatic interaction associated with charge transfer in their MD simulations to study the solid Al/Al2O3 interface. However, unlike the experimentally-observed abrupt interface17, the force field predicted that O atoms should rapidly diffuse into the Al lattice, resulting in a highly disordered region at the interface33. Zhang et al.30 have studied the adhesion and nonwetting to wetting transition in the Al/α-Al2O3 interface using a reactive force field (ReaxFF) approach39. They find that the evaporation of Al atoms and diffusion of O atoms in α-Al2O3 primarily lead to the wetting of liquid Al on the oxide surface.
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Nanolayered laminated composites of aluminum and aluminum oxide with interlayer spacing ranging from 50 to 500 nm have been prepared in the past40 and found to exhibit interesting deformation mechanism with extensive ductility. However, an atomistic understanding of the observed mechanical behavior is presently lacking. Although available dislocation dynamics models are capable of exploring evolution of interface dislocation networks and can predict the microscopic properties of the nanolayered composites during mechanical deformations at large length scales, such models depend upon atomistic input to define interfacial properties and dislocation reaction rules. A deeper understanding of the interface structure can be crucial for understanding the macroscopic mechanical behavior of the nanolayered composite heterostructures.
However, despite the significance of interfacial structure, studies addressing them have remained quite scarce in the literature. The reason for this scarcity is two fold. On the one hand, accurate ab initio simulation techniques are not efficient enough to practically deal with such large systems with several thousand systems41. On the other hand, sophisticated atomistic potentials required for molecular dynamics (MD) simulations, that can treat both metal and ceramic systems on equal footing and adequately describe charge transfer effects at the interface, were unavailable until recently. As a first step towards this direction, the goal of the present study is to elucidate the coherent and semi-coherent interface structures at the Al(111)/α-Al2O3(0001) interface. This work is expected to provide novel information on dislocation patterns that can form at the interface to accommodate misfit strain as well as on the factors that govern the formation of these patterns. The resulting knowledge from the present study is expected to serve as an input for the subsequent dislocation dynamics models42,43 to understand and predict the macroscopic mechanical behavior of Al/α-Al2O3 composite heterostructures.
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