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FIRST PRINCIPLES CALCULATIONS OF THE EQUILIBRIUM MECHANICAL PROPERTIES OF SIMPLE METALS AND ORDERED INTERMETALLIC ALLOYS MICHAEL J. MEHL, * JEAN E. OSBURN, * DIMITRI A PAPACONSTANTOPOULOS, * AND BARRY M. KLEIN* *Complex Systems Theory Branch, Naval Research Laboratory, Washington DC 20375-5000

ABSTRACT High-strength, light-weight intermetallic compounds which are ductile at high temperatures are of great technological interest. Purely experimental searches for improved intermetallic materials are time consuming and expensive. Theoretical studies can shorten the experimental work by focusing on candidate Although current ab compounds which have several of the desired properties. initio density functional calculations are not adequate to calculate material properties at high temperatures, it is possible to compute the equation of state and elastic moduli of ordered compounds with several atoms in a unit Known correlations between equilibrium and high temperature properties cell. can then be used to point the way for experiments. We have demonstrated the power of this approach by applying the linearized augmented plane wave (LAPW) method to calculate the equation of state and elastic moduli for several simple metals (Al, Ca, and Ir), binary and binary L1o and RuZr), AlRu, AlCo, AlNi, cubic intermetallics (SbY, Most of the calculated equilibrium lattice intermetallics (AlTi and IrNb). constants are within 3% of the experimentally observed lattice constants. Although the available experimental information about the elastic moduli is limited by the lack of single crystal data for most of these materials, we are in excellent agreement (within 10%) with the available experimental data, We also use known correlations between except for the shear modulus of IrNb. the elastic moduli and the melting temperature to predict melting temperatures In general the agreement there is good agreement of the intermetallics. between theory and experiment, indicating that we can qualitatively predict melting temperatures.

Introduction The rapid development of materials technology demands the increased use of metals which retain their strength and yet are ductile at high temperatures. The materials which The metal should also be as light-weight as possible. approach these ideals are not the simple single crystals which have been Indeed, intermetallic alloys are complex traditionally treated by theorists. structures, with defects, impurities, and grain boundaries present even at low In fact, these imperfections help to stabilize the metal and temperatures. determine its physical properties. Currently computational physics is unable to exactly model the defect is However, it principles. systems described above starting from first possible to extract information from more limited calculations which may point For example, systems the way for systems worthy of experimental attention. which have a high melting point but low strength might be hardened by the addition of impurities. In this paper we use the full-potential linearized augmented-pla