A statistical-thermodynamic model for intermetallic phases with L1(2)-structure and its application to the compound Ni3Al

A statistical-thermodynamic model for binary nonstoichiometric Lit-phases h as been developed based on a mean-field approximation. Vacancies and anti-s tructure atoms are allowed on both sublattices as possible point defects, a nd the expressions for the defect concentrations as functions of compositio n and temperature have been derived. From these the compositional variation of the thermodynamic activities of the two components can be calculated us ing energies of formation of the four types of point defects as parameters. The model equations are applied to the intermetallic compound Ni3Al 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., 199 6) gives clearly the best agreement, resulting in very low vacancy concentr ations (of the order of 10(-7) to 10(-9)). Thus the thermal disorder and th e deviation from stoichiometry in Ni3Al 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 Al-sites are occupied by Ni-atoms and 1% of the Ni-sites by Al-atoms. It is argued that this combination of statistica l thermodynamics and experimental activity data permits an assessment of th e reliability of the theoretically derived values of the defect formation e nergies in non-stoichiometric phases.