Miedema solid solutions

Some aspects of the mentioned relationships have been presented in previous chapters while discussing special characteristics of the alloying behaviour. The reader is especially directed to Chapter 2 for the role played by some factors in the definition of phase equilibria aspects, such as compound formation capability, solid solution formation and their relationships with the Mendeleev Number and Pettifor and Villars maps. Stability and enthalpy of formation of alloys and Miedema s model and parameters have also been briefly commented on. In Chapter 3, mainly dedicated to the structural characteristics of the intermetallic phases, a number of comments have been reported about the effects of different factors, such as geometrical factor, atomic dimension factor, etc. on these characteristics. 

The early work of Schwarz and Johnson (1983) used a prediction of the underlying thermodynamics of the Au-La system to explain the relative stability of the liquid/amorphous phase in their elemental layered composites (Fig. 11.7). However, they utilised the method proposed by Miedema (1976) for thermodynamic stability of the liquid/amorphous phase. There are clear limitations to the Miedema approach firstly it is not guaranteed to produce the correct phase diagram and therefore phase competition is at best only approximated, and secondly, the thermodynamics of the terminal solid solutions are chosen quite arbitrarily. 

Seah [33] and Miedema [34] have both reported means to estimate AGs. A major factor determining AGs hi the case of a dilute solid solution is the difference in the heats of sublimation... 

The values given by Miedema are the formation enthalpies of solid solutions of A in B or vice versa. The solved metal is embedded into the matrix of the solvent. Using a Haber-Born cycle as shown in Figure 2.39 the sublimation enthalpy of the solved metal embedded in the matrix of the solvent can be calculated. Results of sublimation enthalpies of the solved metal embedded in the matrix of the solvent are shown in Table 2.4b. 

Phase stabiUties of metastable solid solutions (amorphous, bcc, hep) and stable Zr(Fe,Cr)2 phase have been calculated by [2002Rod] using Miedema s model in order to explain alloy amorphization and Fe depletion of the Zr(Fe,Cr)2 precipitates in Zircaloy irradiated at intermediate temperatures. 

Use the Miedema parameters to derive the amount of surface enrichment in solid solutions of metals. 

Here AH , d/ffj represent the monovacancy energies of R and M atoms in the pure metals R and M, and Fr and are the atomic volumes of these metals. The quantities/m and/r are the atomic fractions of M and R atoms surrounding a given M site in RM . All these quantities are listed by Miedema for practically all metals, so that A Hu can be calculated for given metal combinations RM as a function of concentration n. In fig. 5 the vacancy energies at Ni sites in various LaNi intermetallics have been calculated and plotted as a function of Ni concentration. Note that d/ffj in La-rich compounds is considerably less than in pure Ni metal. The initial concentration independence of results from the fact that in intermetallic compounds the Ni atoms try to surround themselves with an optimal number of the larger La atoms. The broken line represents the results for a solid solution, realized more or less in amorphous La-Ni alloys. 

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