1. Chapter 5 Phase Equilibria
Materials Science
Chapter 5
Phase Equilibria

2. Phase Diagram A collection of curves showing solubility limits.
In one side of the curve a single phase is present as unsaturated solution (liquid or solid)
Beyond the curve the solubility limit is exceeded, i.e. 2nd phase exists and a mixture of two phases are present.
Useful to know the phases in equilibrium, alloy composition at a desired temperature, Chemical composition of each phase and calculation of the quantity of phase present.

3. Solutions and Mixtures
Different components combine to form solutions or mixtures.
A solution is a phase with more than one component.
A Mixture is a material with more than one component.
It is possible to have a mixture with more than one solution, e.g. lead-tin solder

4. Liquid solubility limits

5. Liquid solubility limits

6. Solid solubility limits

7. Definitions
Phase: part of material that is distinct from others in structure or composition.
Phase boundary: is a region between two phases locates the discontinuinity in structure to from separate phases.
Solution: is a phase with more than one component.
Mixture: a material with more than one phase.
Component: a constituent of a phase with a definite chemical composition.

8. Phase Diagrams (qualitative)
A phase diagram is a collection of solubility limit curves.

9. Cu-Ni Diagram

10. Eutectic Temperature and Composition
Pure lead melts at 327oC; pure tin melts at 232oC.
Alloys of lead and tin melt at lower temperatures.
Example: solder which is widely used for electrical connections. An alloy of this composition melts most readily and thus facilitates production with the a minimum possible damage to adjacent circuit components.
The above alloy is an eutectic alloy.
The eutectic temperature is 183oC; the eutectic composition is 38.1 pb and 61.9 Sn.
The eutectic composition and the eutectic temperature exist at the intersection of the two solubility curves that limit the composition of the liquid.

11. Eutectic Reactions

12. Liquidus and solidus meet at the eutectic temperature and composition.
Freezing Ranges
The range of temperatures over which freezing occurs. It changes with the composition.
Liquidus: the locus of temperatures above which all compositions are liquids.
Solidus: the locus of temperatures below which all compositions are solids.
Liquidus and solidus meet at the eutectic temperature and composition.

13. Freezing Ranges

14. Isotherm Cuts
A traverse across the phase diagram at constant temperature provides a simple sequence of altering one- and two-phase fields.

15. Phase Names and Lables
Brass: is the phase name of an fcc solid solution of zinc in copper.
Bronze: is an fcc alloy of tin in copper.
Sterling Silver: 92.5 Ag and 7.5 Cu.
More often phases are labled with greek letters such as , , , etc..