Application of the embedded atom method to Ni 3 Al
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I. INTRODUCTION There is a great deal of current interest in the Ni 3 Al alloy due to its unusual mechanical properties. Nickel aluminide forms an Ll 2 ordered crystal structure.1 The long-range ordering is responsible for its unusual mechanical property of increasing yield stress with increasing temperature.2 The Ni3Al-based alloys are also resistant to air oxidation due to their ability to maintain an adherent surface oxide film.3 An inherent drawback to using monolithic Ni3Al as a structural material is the tendency of a polycrystalline pure stoichiometric alloy to undergo brittle intergranular fracture. However, recent work has shown that nonideal stoichiometry combined with certain alloy additions, in particular, boron, produces a ductile polycrystalline material.4 Finally, Ni3Al is the most important strengthening constituent of commercial nickel-based alloys and is responsible for these alloys' high-temperature strength and creep resistance. In this paper the first results of the application of the embedded atom method5 (EAM) to the description of Ni-Al alloys are presented. The EAM is a recently developed technique for rapidly computing the total energy of an arbitrary arrangement of atoms in a metal or alloy. The energy is determined from the energy to embed each atom in the electron density due to the surrounding atoms plus a short-range core-core repulsion. The embedding functions and core-core repulsions are determined empirically by fitting to properties of the bulk material. The EAM has been applied successfully to a wide range of problems in pure metals including surfaces,5'6 defect energies,5'7 fracture,5-8 phonons,9 and liquid metals.10 For binary alloys it has been used to determine the segregation at surfaces and internal deJ. Mater. Res. 2(1), Jan/Feb 1987
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fects in binary alloys containing Cu, Ag, Au, Ni, Pd, and Pt. 7 ' 11 The technique has also been used successfully to study the interaction of hydrogen with metal surfaces.5'12 In this work the EAM will be used to compute the energies of point defects, antiphase boundaries, and surfaces of Ni3Al as well as the vibrational properties of the alloy. Section II summarizes the EAM and presents the empirically determined embedding functions and corecore repulsions used to describe the Ni-Al system. In Sec. Ill the predictions of the method for the equilibrium phases of Ni-Al alloys for Al concentrations of less than 50% will be compared with the known phase diagram to provide a test of the validity of the functions. Another test of the functions is provided in Sec. IV, where the elastic constants and vibrational modes of Ni3Al are computed and the elastic constants are compared to experiment. Section V presents the computations of point defect properties including the vacancy formation and migration energies in Ni 3 Al as well as the divacancy binding energy. Next, results of computations of the (100) and (111) antiphase boundary energies will be discussed. The final section describes the results obtained for the ener
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