Phase Diagrams

Question 1

What is a Phase Diagram?

Answer 1

Describe the conditions (temperature, pressure, composition) under which a phase is stable. Provide info about systems at equilibrium.

Question 2

What is a phase?

Answer 2

A region of a substance that is:

1. Uniform in chemical composition
2. Physically distinct
3. Mechanically separable

Question 3

What is a solid solution?

Answer 3

A solid in which the two or more components are compatible and form a single phase.

Question 4

What is the Solubility Limit?

Answer 4

The maximum concentration of solute atoms that may dissolve in the solvent at a specific temperature to form a solid solution. Adding solute past the solubility limit yields another solid solution or compound that has a different composition.

Question 5

Single Component Phase diagrams

Answer 5

Unary phased diagrams display temperature vs. pressure. eg. H2O phase diagram.

Question 6

Two Component Phase diagrams

Answer 6

Binary phase diagrams, must now consider temperature and composition. Two types of binary phase diagrams:
Type #1 : complete solubility of two components (isomorphous)
Type #2 : Limited solubility of two components (eutectic)

Question 7

Microstructures in isomorphous alloys

Answer 7

When cooled from the melt, the microstructure of a binary isomorphous alloy transforms from liquid to a solid solution.

Question 8

Binary Eutectic Systems

Answer 8

Occur when 2 elements completely dissolve into each other in the liquid state, but have only limited solubility in the solid state.

Question 9

Phase Transformations

Answer 9

Most reactions include nucleation and growth.

Question 10

Isothermal Transformation Diagrams

Answer 10

Plot of temperature vs, logarithm of time for a steel alloy of definite composition. Determines when transformations begin and end for an isothermal heat treatment of a previously austenized alloy.

Question 11

Pearlite

Answer 11

• Transformation rate increases with decreasing temperature
• Coarse pearlite: alternating ferrite and cementite layers are relatively thick
• Fine pearlite: alternating ferrite and cementite layers are relatively thin

Question 12

Bainite

Answer 12

Forms between the temperature at which martensite and pearlite form.
-microstructure consists of alpha-ferrite and fine dispersion of cementite

Question 13

Spheroidite

Answer 13

-Steel alloy composed of either pearlite or bainite microstructure is heated and held at a temperature below the eutectoid point for a long time.

Question 14

Martensite

Answer 14

• A metastable iron phase supersaturated carbon that is the product of a diffusionless transformation from austenite.
• Quenched to a relatively low temperature.