1. Materials Engineering
Lecture 6:
Phase diagrams 2.

2. Phase composition change on cooling

3. Eutectic Binary Diagram
While “moving” downwards along composition M, the solid

4. Binary Eutectic Systems (Cu-Ag)
a-phase: FCC Ag + Cu solute;
b-phase: FCC Cu + Ag solute;
Solvus line: solid solubility limit;
Eutectic (Eu): the composition having the lowest melting point.
Eutectic reaction:

5. Invariant points in Phase Diagrams
Points, for which even a slight change of composition of temperature in any direction causes a phase change, are referred to as invariant points.
Invariant points are:
Eutectic: on solidus, having the lowest melting temperature;
Eutectoid: between two solid states;
Peritectic: binary liquid “splits” into two different solid phases.

6. Phase Diagram with Intermediates: Pb-Mg
To simplify the diagram, it can be cut into two separate diagrams along the vertical line of Mg2Pb. Then two eutectic systems are shown. The analysis is then much simpler.

7. Iron-Carbon Phase diagram
6.67 wt% of C = Fe3C (cementite).
All practical iron alloys have < 6.67 wt% of C.
Compositions richer in carbon are just solutions of cementite (mp = 1840ºC) in solid graphite (mp = 3650ºC)

8. Ternary phase diagram: Chromium-Nickel steel

9. Equilibrium cooling: Ni-Cu

10. Non-equilibrium cooling: Ni-Cu

11. Microstructure development: Pb-Sn alloy, equilibrium cooling

12. Microstructures of Pb-Sn alloys
20 wt% Sn wt% Sn
62 wt% Sn wt% Sn

13. Musts of this lecture
Invariant points: eutectic, eutectoid, peritectic;
Ternary diagrams;
Equilibrium and non-equilibrium cooling.
Development of Microstructure