Thermodynamics of the substantially ordered Cu3Au-type alloys

Ipser

1999

phys. stat. sol. (b)

A statistical‐thermodynamic model for binary nonstoichiometric L12‐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 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., 1996) gives clearly the best agreement, resulting in very low vacancy concentrations (of the order of 10—7 to 10—9). Thus the thermal disorder and the 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 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.

Section: Discussionsupporting

confidence: 41%

Section: Discussionmentioning

confidence: 93%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

Krachler

Ipser

1999

phys. stat. sol. (b)

“…Various authors present statistical-thermodynamic models and derive expressions for the composition dependence of the thermodynamic properties in ordered intermetallic phases. [18,[22][23][24][25][26][27][28][29][30][31] Most of these models are based on evaluations of disorder parameters which were usually defined as the concentrations of point defects at the stoichiometric composition. Such models were also developed for B2 and L1 2 phases, [3][4][5][6][7][8]18,20,22,[25][26][27][28][29]31,32] assuming purely substitutional disorder caused by anti-structure atoms.…”

Section: Application Of the Wagner-schottky Model And Ab-initio Calcumentioning

confidence: 99%

Binary L1₂ Intermetallics: A Statistical‐Thermodynamic Modeling of Ordering Phenomena, Behavior and Properties

Zeitschrift anorg allge chemie

2012

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 L1 2 intermetallic 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 noninteracting point defects for description of the arrangement of anti-* Dr. O. Semenova

A Statistical‐Thermodynamic Modeling of Ordering Phenomena in Binary Intermetallic B2‐ and L1₂‐Structures

Krachler

2012

Adv Eng Mater

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.