Nonstoichiometry and Defect Mechanism in Intermetallics with L1 2 -Structure
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Nonstoichiometry and Defect Mechanism in Intermetallics with L12-Structure Herbert Ipser, Olga P. Semenova, Regina Krachler, Agnes Schweitzer, Wenxia Yuan1, Ming Peng1, and Zhiyu Qiao1 Inst. f. Anorganische Chemie, Universität Wien, Währingerstraße 42, A-1090 Wien, Austria 1 Dept. of Physical Chemistry, Univ. of Science and Technology Beijing, P.R. China 100083 ABSTRACT A statistical-thermodynamic model was derived which allows to describe thermodynamic activities in intermetallic compounds with L12-structure as a function of composition and temperature. The energies of formation of the four types of point defects (anti-structure atoms and vacancies on both sublattices) were used as adjustable parameters. The model was applied to the three compounds Ni3Al, Ni3Ga, and Pt3Ga, and it permitted to estimate for the first time the defect formation energies for Ni3Ga and to provide initial estimates for Pt3Ga.
INTRODUCTION Intermetallic compounds with the cubic L12-structure have attracted considerable scientific interest in recent years. One prominent example is the nonstoichiometric compound Ni3Al which has gained technological importance in the development of so-called superalloys due to some of its unique properties as, for example, high-temperature strength and excellent corrosion resistance [1-3]. Obviously, many of the outstanding properties of Ni3Al can be related to the type and amount of defects present in thermodynamic equilibrium and to the variation of different defect concentrations with temperature and composition. One possible way to obtain information about types of point defects which are present in the crystal lattice as well as their concentrations is the combination of statistical thermodynamics and accurate experimental thermodynamic data, as shown previously by two of the authors for B2- and B8-phases [4-6]. Thus it has been the aim of the present research to derive a statistical model for L12-phases which would be able to describe the thermodynamic activities as a function of temperature and composition and to apply the corresponding model equations to experimental activity data for the intermetallic compounds Ni3Al, Ni3Ga, and Pt3Ga which crystallize in this structure type. THEORETICAL MODEL The cubic L12 crystal structure can be divided into two sublattices, the α-sublattice (face centered positions) and the β-sublattice (corner positions); in the ideally ordered crystal, all α-sites are occupied by A-atoms and all β-sites by B-atoms, thus yielding the A3B stoichiometry. Four types of point defects are allowed in the lattice, both as thermal defects at temperatures T > 0 K and as constitutional defects which are responsible for the deviation from stoichiometry: anti-structure atoms and vacancies on both sublattices. The possibility of interstitial defects is neglected. For the derivation of the statistical model the A3B crystal is taken as an open system which is allowed to exchange both energy and matter with its surrounding. Consequently the N6.11.1
grand partition function Ξ and the correspo
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