Direct Approach to Atomistic Ab Initio Studies of Precipitate Growth in Alloys
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1229-LL02-09
Direct Approach to Atomistic Ab Initio Studies of Precipitate Growth in Alloys Flemming J. H. Ehlers and Randi Holmestad Norwegian University of Science and Technology NTNU, Department of Physics, Høgskoleringen 5, N-7491 Trondheim, Norway ABSTRACT Theoretical atomistic ab initio investigations of a coherent, compositionally abrupt interface between two assumed defect free metallic systems with appreciable lattice mismatch are discussed. The supercell based modelling scheme employed (i) excludes a presumed weak long range tail of the strain field into the two systems and (ii) breaks down the interface into irreducible segments, distorted by the 'assumed bulk' subsystem boundary conditions. This allows for any segment at the interior of the interface to be modelled at the atomic level. INTRODUCTION The atomic structure and formation energy of an interface between two subsystems is of crucial importance to clarifying the influence and evolution of this interface. For precipitates in metal alloys, the issue is central to structural materials optimization, being intimately connected with the alloy hardening precipitate-dislocation interaction and limits to precipitate growth [1]. The knowledge on precipitates in alloys has increased dramatically in recent years with improvement of transmission electron microscopy techniques for direct structural imaging [2]. With a clarified precipitate structure, atomistic ab initio interface modelling is straightforward in principle, but still hampered in practice for the case of a realistically sized precipitate fully surrounded by host material. Metals are particularly challenging [3]. This paper presents a scheme for modelling a coherent and compositionally abrupt precipitate/host lattice interface without using systems containing thousands of atoms. The supercell approximation is adopted, with the model system describing a segment of a chosen interface. The spatially constant strain field is confined to the range around the interface where the actual field is appreciable. This allows for a well defined description of an arbitrary segment at the interior of the interface, through a subsystem boundary condition. Tests are provided for the [-310]Al interface between Al and β'', the main hardening Al-Mg-Si alloy precipitate [4]. THEORY The typical metal alloy main hardening precipitate is coherent with the host lattice, but with a significant lattice mismatch (several %) with the host along one or two directions. The growth of nanostructures along low mismatch directions being naturally favoured, these precipitates are small, numerous, and highly elongated (plates or needles), with well-defined structural interface orientations where coherency is retained. Both their high number density and the appreciable precipitate/host lattice strain field play fundamental roles in alloy hardening [1]. Interface region modelling for a plane segment central on the interface
Ab initio studies of the precipitate/host lattice system conventionally focus on only an irreducible segment o
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