A Statistical-Thermodynamic Model for Intermetallic Phases with L12-Structure and Its Application to the Compound Ni3Al

Abstract

A statistical-thermodynamic model for binary nonstoichiometric L1 2 -phases has been developed based on a mean-field approximation. Vacancies and anti-structure atoms are allowed on both sublattices as possible point defects, and the expressions for the defect concentrations as functions of composition and temperature have been derived. From these the compositional variation of the thermodynamic activities of the two components can be calculated using energies of formation of the four types of point defects as parameters. The model equations are applied to the intermetallic compound Ni 3 Al using defect formation energies from the literature, and the corresponding curves are compared with experimental aluminum activities at 1400 and 1600 K. As it turns out, one particular set of energy parameters (Debiaggi et al., 1996) gives clearly the best agreement, resulting in very low vacancy concentrations (of the order of 10- to 10 -9 ). Thus the thermal disorder and the deviation from stoichiometry in Ni 3 Al is in principle caused entirely by anti-structure atoms. Their concentrations (referred to the total number of lattice sites) at the exactly stoichiometric composition are found to be 0.0075 at 1400 K, i.e. 3% of the At-sites are occupied by Ni-atoms and 1% of the Ni-sites by At-atoms. It is argued that this combination of statistical thermodynamics and experimental activity data permits an assessment of the reliability of the theoretically derived values of the defect formation energies in non-stoichiometric phases.