Alloys Gibbs energy

The alloy Gibbs energy is the difference between the Gibbs energy of formation and the chemical potentials of the ions in the electrolyte. 

The tables in this section contain values of the enthalpy and Gibbs energy of formation, entropy, and heat capacity at 298.15 K (25°C). No values are given in these tables for metal alloys or other solid solutions, for fused salts, or for substances of undefined chemical composition. 

The oxidation of nickel-copper alloys provides an example of die dependence of the composition of the oxide layer on the composition of the alloy. Nickel-copper alloys depart from Raoult s law, but as a first approximation can be taken as ideal. The Gibbs energy change for the reaction... 

In order to examine the possible relationship between the bulk thermodynamics of binary transition metal-aluminum alloys and their tendency to form at underpotentials, the room-temperature free energies of several such alloys were calculated as a function of composition using the CALPHAD (CALculation of PHAse Diagrams) method [85]. The Gibbs energy of a particular phase, G, was calculated by using Eq. (14),... 

It should be remembered that the CALPHAD approach is based on the hypothesis that, for all the phases and structures existing across the complete alloy system, entire Gibbs energy vs. composition curves may be constructed even by extrapolation into regions where they are unstable or metastable. A particular case concerns the pure component elements for which the relative Gibbs energy for the different crystal structures (the so-called lattice stabilities) must also be established and defined as a function of temperature (and pressure). 

The consequence of the positive interactions is that alloys which lie between xi and X2 can lower their Gibbs energy by forming two-phase structures. One phase which is y4-rich with composition Xi and the other which is B-rich with composition... 

X2- The lowering of Gibbs energy, by forming multi-phase structures rather than a series of continuous solutions, is the reason for some of the fundamental features of alloy phase diagrams and will be discussed later in section 3.7. 

Figure 3.11 shows the G/x diagram at 600 K. If an alloy of composition xq were single phase it would have a Gibbs energy Go- However, if it could form a mixture of two phases, one with composition x and the other with composition x, it could lower its total Gibbs energy to G, where G is defined by the equation... 

It is commonplace to assume a form of the Gibbs energy function which excludes the pressure variable for solid-state phase transformations, as the magnitude of the PAV term is small at atmospheric pressures. This is of course not the case in geological systems, or if laboratory experiments are deliberately geared to high-pressure environments. Klement and Jayaraman (1966) provide a good review of the data available at the time when some of the earliest CALPHAD-type calculations were made (Kaufman and Bernstein 1970, Kaufman 1974). Much work was also carried out on specific alloy systems such as Fe-C (Hilliard 1963) and the Tl-In system (Meyerhoff and Smith 1963). 

Figure 9.7. Schematic diagram showing first iteration stage in the Gibbs energy minimisation process of an alloy with composition zo in Ni-Cu at 1523 K.

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