First Principles Calculation of Cooperative Atom Migration in L1 2 Ni 3 Al
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First Principles Calculation of Cooperative Atom Migration in L12 Ni3Al H. Schweiger1,2, R. Podloucky1, W. Wolf1, W. Püschl2 and W. Pfeiler2 1 Institut für Physikalische Chemie, University of Vienna, Liechtensteinstrasse 22a/I/3, A-1090 Vienna, Austria 2 Institut für Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria ABSTRACT Recent Monte-Carlo simulations of order relaxations in L12-ordered Ni3Al reproduced the simultaneous action of two processes as experimentally observed by residual resistometry. It was shown that the fast process is related to the fast annihilation/creation of nearest neighbour antisite pairs. These findings are now strongly corroborated by a new supercell approach of ab initio quantum mechanical calculations describing the simultaneous displacement of Ni and Al atoms on their way to their respective antisite positions. Studies of single jumps suggest that such a cooperative migration of Ni and Al is necessary in order to prevent Al antisites from jumping back into their regular position. Relaxation of neighbouring atoms was taken into account. Thus, a minimum migration barrier of about 3 eV was derived which together with the calculated formation enthalpy of a Ni vacancy of 1.5 eV amounts to 4.5 eV, in remarkable agreement with the high activation enthalpy of 4.6 eV as observed experimentally. INTRODUCTION The extraordinary high-temperature mechanical and corrosion properties of Ni3Al make this intermetallic compound a leading candidate from a technological and a scientific standpoint [1]. These properties mainly are a consequence of chemical long-range ordering of the alloy atoms in the L12 superstructure. For thermodynamic reasons the state of order depends on temperature. It is changed by jumps of atoms between the two different sublattices, this way creating or annihilating antisite defects. Therefore the investigation of so-called ‘order-order’ relaxations yield results complementary to usual tracer diffusion experiments using a Ni* tracer. Ni* can easily diffuse via its own sublattice; the same holds true for Al antisite atoms. Presuming a sufficient number of Al-antisites to be present self diffusion in Ni3Al was recently explained in this way [2]. Changes in the degree of order, however, need atom jumps between different sublattices that change the concentration of antisite atoms correspondingly. In order-order relaxation experiments the system is kicked out of its current equilibrium state of order by a small and sudden temperature change resulting in a subsequent relaxation to a new equilibrium state of order. For such experiments in a so-called directly ordering alloy like Ni3Al, where the order/disorder temperature equals the melting temperature or even virtually lies above it [3], extremely small changes in the degree of order with temperature are to be expected. It turned out in recent years that measuring residual electrical resistivity is a very sensitive indicator for these fine variations of order, indeed at present the only suitable experimental m
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