Olga Semenova

The main objective of this paper is aimed towards a correct statistical‐thermodynamic modeling and description of the effects and changes associated with the creation of different defects in the crystal structure and their connection and influence on the properties of the binary L1₂intermetallic systems. This was done by the using of a General Defect Correlation Model (GDCM), employing applications of the Bragg‐Williams‐Gorsky mean‐field approximation. The obtained results are compared to a model based on the Wagner‐Schottky approach with application of the grand canonical ensemble for non‐interacting point defects for description of the arrangement of anti‐structure and vacancy defects. The basic concept includes the evaluation of such characteristics of the intermetallic materials with L1₂structures as the virtual critical temperatures of order‐disorder transformations, the pair‐wise interaction and defect formation energies, thermodynamic activities and defect concentrations, and also descriptions of long‐ranged (LRO) and short‐ranged (SRO) atomic correlations in the crystal lattice (including the possible correlation of nearest neighbor point defect combinations), which is difficult to obtain by other methods. The model calculations also are tested using experimental data on thermodynamic and structural properties.

Statistical‐thermodynamic models for the description of ordering in non‐stoichiometric intermetallic phases can be very helpful for the design of new functional materials for various purposes. Here, we present a review of methods for describing B2 (BCC) and L1₂ (FCC) – type binary alloys. The free energy of binary alloys where ordered structures are formed, is evaluated by statistical approaches employing applications of the Bragg–Williams–Gorsky mean‐field approximation. In the original mean field method, the degree of order in the neighborhood of a given site in the crystal is determined by the average state of order throughout the macroscopic crystal. Actually, the force tending to produce long‐range order depends on the fluctuations in the configuration, an effect which is neglected in the mean field approximation but is taken under consideration, e.g., in the Quasi‐Chemical Approach (QCA) by Guggenheim and its advanced applications, in various sophisticated treatments using the Cluster Variation Method (CVM) originally developed by Kikuchi, as well as in the Defect Correlation Model (DCM). The latter model is a less sophisticated method based on the simultaneous description of short‐range order and long‐range order by use of properly defined non‐overlapping clusters. All these methods allow the “pure” mean‐field results to be substantially refined, particularly at high temperatures, and for alloys with large deviations from the stoichiometric composition. Defect concentration‐alloy composition diagrams and activity‐alloy composition diagrams are shown for several methods with focus on the QCA and DCM approaches. The calculated results are compared with each other and with experimental data on thermodynamic and structural properties from the literature.