BinarySolutions

BinarySolutions - Binary Solutions Composition as a...

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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Binary Solutions ¾ Composition as a thermodynamic variable ¾ Gibbs free energy of binary solutions ¾ Entropy of formation and Gibbs free energy of an ideal solution ¾ Chemical potential of an ideal solution ¾ Regular solutions: Heat of formation of a solution ¾ Activity of a component, Henry±s and Raoult±s laws ¾ Real solutions: interstitial solid solutions, ordered phases, intermediate phases, compounds ¾ Equilibrium in heterogeneous systems Reading: Chapter 1.3 of Porter and Easterling, Chapters 9.5, 9.6, 9.9, 9.10 of Gaskell
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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Solid Solutions (terminology) Solid solutions are made of a host (the solvent or matrix) which dissolves the minor component ( solute ). The ability to dissolve is called solubility . ¾ Solvent : in an alloy, the element or compound present in greater amount ¾ Solute : in an alloy, the element or compound present in lesser amount ¾ Solid Solution : 9 homogeneous 9 maintains crystal structure 9 contains randomly dispersed impurities (substitutional or interstitial) ¾ Second Phase : as solute atoms are added, new compounds / structures are formed, or solute forms local precipitates ¾ Solubility Limit of a component in a phase is the maximum amount of the component that can be dissolved in it (e.g. alcohol has unlimited solubility in water, sugar has a limited solubility, oil is virtually insoluble). The same concepts apply to solid phases: Cu and Ni are mutually soluble in any amount (unlimited solid solubility), while C has a limited solubility in Fe . Whether the addition of impurities results in formation of solid solution or second phase depends the nature of the impurities, their concentration and temperature, pressure±
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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Composition as a thermodynamic variable Real materials are almost always mixtures of different elements rather than pure substances: in addition to T and P, composition is also a variable. To understand conditions for equilibrium and phase diagrams (like the one below) we have to understand how the Gibbs free energy of a given phase depends on composition, temperature and pressure. Since many real experiments are performed at fixed pressure of 1 atm, we will focus on G(T, composition).
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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Gibbs free energy of a binary solution (I) Let±s consider a binary solution of A and B atoms that have the same crystal structures in their pure states and can be mixed in any proportions - form a solid solution with the same crystal structure ( unlimited solid solubility ). Example: Cu and Ni. 1 mol of homogeneous solid solution contains X A mol of A and X B mol of B. X A and X B are the mole fractions of A and B in the alloy.
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This note was uploaded on 03/05/2012 for the course MSE 305 taught by Professor Zhigilei,l during the Spring '08 term at UVA.

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BinarySolutions - Binary Solutions Composition as a...

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