Ab initio Simulation of Vacancy Processes in Ni 3 Al
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ABSTRACT Properties of point defects such as antisites and vacancies in Ni 3A1 are studied by means of ab initiocalculations for supercells. Temperature dependent quantitities such as defect formation energies are derived by means of a grandcanonical ensemble. Stimulated by experiments of residual resistivities suggesting an outstandingly large activation energy of 4.6 eV due to Al vacancies, several models for point like defects are treated in combination with calculated migration barriers for nearest neighbor jumps and also the six-jump model.
INTRODUCTION Atomic jumps via next neighbor sites can be studied-aside of usual tracer diffusion experiments-by investigation of long-range ordering processes. Changes of temperature in long- range ordered intermetallic compounds are accompanied by a change in the degree of order. Such a change must occur via atomic jumps from one sublattice to the other. Whereas in the B2 structure each jump leads to formation of anti-site atoms (i.e. atoms sitting on the wrong sublattice) this is not necessarily the case for the majority atom type such as Ni in the L12-ordered compound Ni 3A1. By measurements of residual resistivities, Pfeiler and Kozubski[ 1] have investigated extensively order-order transitions in Ni3AA.They concluded, that two simultaneously acting processes with drastically different reaction rates are involved in the change of the degree of order both having an equal activation enthalpy of 4.6 eV. Since Ni* tracer diffusion experiments yielded activation energies of about 3 eV only, the high activation enthalpy of 4.6 eV is thought to be connected to the formation of vacancies on the Al-sublattice.
Long-range diffusion and ordering are governed by the same parameters. The activation enthalpy is considered to consist of two contributions, namely of the enthalpy for vacancy formation and vacancy migration. Since from experiment the data cannot be further analyzed and the high activation energy is not understood, ab initio calculations are necessary. In order to treat the formation of defects properly, a thermodynamical model based on a grandcanonical ensemble has to be used which is based on ab initio results. This is the aim of the present study. Computationaland thermodynamical aspects Two ab initio methods were applied: i) the Vienna ab initio simulation package (VASP) [2] based on ultrasoft pseudopotentials; and ii) the all-electron full-potential linearized augmented planewave (FLAPW) method [3]. For the majority of calculations VASP was used. All data were calculated within the generalized gradient approximation (GGA) for exchange and correlation, since ground state properties of Ni 3 AA are then in excellent agreement to experimental data (e.g. aexp=3.572 A, acaic=3.576 A; Bex =1.75 Mbar and=1.77 Mbar). Both rather different ab initio approaches agree better than 10% for alf calculated quantities. KK5.15.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
Point defects and their migration are modeled with supercells of 32 atoms for which
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