Bulk and Defect Properties of Ordered Intermetallics: A First-Principles Total-Energy Investigation
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BULK AND DEFECT PROPERTIES OF ORDERED INTERMETALLICS: A FIRST-PRINCIPLES TOTAL-ENERGY INVESTIGATION C. L. FU, Y.-Y. YE,* AND M. H. YOO Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831 ABSTRACT First-principles quantum mechanical calculations based on local-density-functional theory have been used to investigate the fundamental factors that govern the deformation and fracture behavior of ordered intermetallic alloys. Unlike in Ni 3 AI, the calculated elastic constants and shear fault energies indicate that anomalous yield strength behavior is not likely to occur in Ni 3 Si. From the calculated Griffith strength and a phenomenological theory relating fracture toughness to ideal cleavage strength, Ni 3 Si is predicted to be ductile with respect to cleavage fracture. For TiAl, we find the absence of structural vacancies due to the strong Ti-Al bonding and similar atomic radii for Ti and Al. For NiAl, the defect structure is found to be dominated by two types of defects - monovacancies on the Ni sites and substitutional antisite defects on the Al sites. For FeAl, on the other hand, we find a more complex defect structure, which is closely related to the importance of electronic structure effect in FeAl. More importantly, we predict the strong tendency for vacancy clustering in FeAl due to the large binding energy found for divacancies. Effects of thermomechanical history on microhardness are discussed in terms of the calculated results. 1. INTRODUCTION During the past decade the theoretical tools for studying the properties of materials have been developed to the point that it is possible to understand many of their properties from first principles. This understanding has been used both in interpretive and predictive modes. It is now well established that the bonding behavior in intermetallics has various characteristics, which include metallic bonding, directional bonding, strongly hybridized states from p- and delectron, and charge transfer effect. These electronic structure factors can manifest themselves in the atomic-level bonding interactions, which, in turn, determine the physical and mechanical properties of alloys as well as their response to intrinsic and extrinsic defects. Thus, there is a growing recognized need, in the field of intermetallics, for the type of atomistic details that first principles calculations can provide, and which have proved so useful in aiding the understanding of the interactions that govern alloy behavior. The framework of modern first-principle calculations of the ground state properties of solids is based on the local density functional (LDF) theory [1]. With this theory the complicated many-body problem of interacting nuclei and electrons is mapped onto a set of decoupled single particle-like differential equations, i.e. effective one-particle Schr&dinger equations that describe the interaction of a single electron with the assembly of nuclei and electrons through an 'effective' one-particle potential. This effective potential is give
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