Computer Simulation of Free-Surfaces in Close-Packed Structures
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J.R.Fernndez*, A.M.Monti*" and R.C.Pasianot*" "CONICET, *"Depto. Ciencia de Materiales, Gcia. Desarrollo, CNEA, Avda. Libertador 8250, Zip. 1429, Buenos Aires, Argentina
ABSTRACT The static relaxation method was applied to the study of free-surfaces in fcc (Ni,AI,Ni3 AI) and hcp (Ti,Zr,Mg) lattices bounded by many-body interaction potentials of the EAM-type. The present calculated changes in interlayer spacings were compared with measured and other theoretical results. Vibrational modes of atoms in the surface and in planes below the surface were analyzed in the Einstein and the "cluster" approximations. The latter explicitly includes coupling effects. Frequency variations with the distance to the surface are obtained in the former approximation and vibration modes for a set of atoms in the second one.
INTRODUCTION Starting in the last decade, the structure of free surfaces in bcc, fcc and hcp materials has been extensively studied by computer simulation techniques. Most investigations focus on energy, stress and relaxations on high symmetry surfaces 1-6. Those studies were steered by the development of easy to compute semiempirical many-body potentials (EAM, embedded atom method 7, N-body 8, Glue model
9).
These potentials, at variance with simple pair potentials,
include a local volume term in the energy which renders them more appropriate for surface studies '. This is particularly true for surface relaxations where a better agreement with the rather uncertain experimental results is obtained by employing those many-body models '0.Some of the main features of the theoretical studies are oscillatory surface layer relaxations, as well as surface rippling for the case of alloys. Following standard calculation procedures and adding dynamic analysis, we tackle here the study of two surfaces in close packed structures. The surfaces considered are the ( 11) in Ni, Al (fcc) and Ni3AI (LI 2 alloy), and the (0001) in Mg, ai-Ti and ci-Zr (hcp). Based on symmetry grounds, some analogies are expected in the behavior of those surfaces, though belonging to different crystal structures. Surface energies and layer relaxations are calculated and compared with previous theoretical and experimental results 2. We complement the static results by calculating vibrational properties of the surfaces using two different approaches. Although static studies have been largely considered within the materials science community literature, studies dealing with dynamic aspects are less abundant. In the next section, we give the details of the calculation method used. That section is followed by a report of the present static and dynamic results, which are discussed in the last section. 527
Mat. Res. Soc. Symp. Proc. Vol. 318. 11994 Materials Research Society
CALCULATION METHOD In this work, the atomistic simulations are carried out using EAM-type interatomic potentials. The EAM represents the energy per atom as a sum of a pair term and a local volume or density term 7. The potentials of Voter and Chen " for Ni, Al and the Ni 3A1 alloy,
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