Phase Diagrams

Materials Selection and Testing

Phase diagrams

Importance

In design and control of heat treating procedures (why?) Some properties of materials are functions of their microstructures, and, consequently, of their thermal histories Most phase diagrams represent stable (equilibrium) states and microstructures, They are useful in understanding the development and preservation of nonequilibrium structures and their attendant properties (often the case that these properties are more desirable than those associated with the equilibrium state) Remember: strong correlation between microstructure and mechanical properties!!

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Definitions

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Component: pure metals and/or compounds of which an alloy is composed.

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For example, in a copper–zinc (brass), the components are Cu and Zn. Solute and solvent are also common terms

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System: series of possible alloys consisting of the same components, but without regard to alloy composition (e.g., the iron–carbon system) Solid solution: consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained

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Dr Abdul Alami

Solubility limit

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At some specific temperature, there is a maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution The addition of solute in excess of this solubility limit results in the formation of another solid solution or compound that has a distinctly different composition. Sugar-water system: This also depends on temperature.

Dr Abdul Alami

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Sugar-water system

Dr Abdul Alami

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Phases

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Phase: homogeneous portion of a system that has uniform physical and chemical characteristics (pure materials are considered phases)

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Example: sugar-water syrup is a phase, solid sugar is another

If more than one phase is present in a system, each will have distinct properties,...