Electronic Structure of Planar Faults and Point Defects in High Temperature Intermetallics
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ELECTRONIC STRUCTURE OF PLANAR FAULTS AND POINT DEFECTS IN HIGH TEMPERATURE INTERMETALLICS J.M. MACLAREN t and C. WOODWARD t
t Department of Physics, Tulane University, New Orleans, LA 70118. S U.E.S Inc., Dayton, OH 45432.
ABSTRACT First principles electronic structure calculations, using the layer Korringa-KohnRostoker method, are reported for isolated planar faults in TiAl. The calculated fault energies are discussed in the context of suggested superdislocation separation reactions. The influence of dilute impurities on fault energies are treated using the coherent potential approximation. Using this approach, the variation of fault energies in TiAl resulting from stoichiometry changes and from the addition of Mn axe calculated, and compared to recent experimental data.
INTRODUCTION The macroscopic properties of materials are often controlled by the occurence, distribution and interactions of the defects present in the crystal. Crystals containing defects, such as planar faults, are more difficult to treat with a first principles electronic structure approach owing to the reduced symmetry. Multiple Scattering Theory (MST) provides, in principle, a formalism in which to address low symmetry systems. Practicle applications require the use of symmetry, though this can be translational symmetry, semi-infinite periodicity, or a combination of both. A further benefit of MST is that effects of disorder on electronic structure can be addressed simply within the coherent potential approximation
(CPA) [1].
In this paper we will first discuss the theory behind the layer Korringa-Kohn-Rostoker (LKKR) electronic structure approach method applied to ordered materials [2] and the extension, using the CPA, to disordered materials. Secondly, we will report calculations of the electronic properties of planar defects in stoichiometric TiAl, off-stoichiometric TiA1, and TiAl with dilute additions of Mn. High temperature intermetallics, such TiAl, have many attractive properties for aerospace applications including low density, high melting temperatures and high temperature strength retention [4]. TiAl has a brittle-ductile transition at 7000C. The mechanism of the transition, however, is not well understood. Improving the ductility of TiAl has been the focus of experimental work, and additions of Mn appear to have benefical effects [5]. This work is part of an ongoing theoretical effort to understand the properties of high temperature intermetallics and in particular how features in the crystal bonding influence dislocation core structure and mobility [6).
THEORETICAL APPROACH Ordered Interfaces The electronic structure of an isolated interface in an ordered system is calculated using the layer Korringa-Kohn-Rostoker (LKKR) method. In this section we will summarize the LKKR technique, a more detailed discussion can be found in ref 2. In the LKKR method the solid is partitioned into planes of atoms within which two dimensional symmetry is imposed. The one electron potentials are assumed to be the spherically symmetric muffin-t
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