Phase diagrams

Question 1

What is a phase?

Answer 1

A region in a system that has uniform chemical and physical properties

Question 2

Characteristics of a phase

Answer 2

same structure or atomic arrangement throughout roughly the same composition and properties throughout a definite interface between the phase and any surrounding phases

Question 3

What is an equilibrium phase?

Answer 3

the most stable state of matter at given conditions has the lowest Gibbs energy no net flow of matter in or out of the phases at a phase boundary, the phases on either side of the boundary are stable

Question 4

Solid solution

Answer 4

It is formed when a solute is dispersed throughout the crystal structure of the solvent

some will form a solid solution at a wide range of concentrations and some over a narrow range some will not form solid solutions

Question 5

Binary isomorphous phase diagram

Answer 5

• 2 components
• isomorphous as there is only one solid phase
• the elements show complete solid solubility at all concentrations
• 2 separate phases
• 3 separate phase fields

Question 6

Liquidus temperature

Answer 6

the temperature above which the whole system is a liquid

the liquidus line is the upper line in the graph showing the liquidus temperature for different alloy compositions

Question 7

Solidus temperature

Answer 7

the temperature below which the whole system is solid

the solidus line is the lower line in the graph showing the solidus temperature for different alloy compositions

Question 8

Freezing range

Answer 8

the temperature difference between the liquidus and the solidus at a specific composition two phases coexist in this range pure metals do not have a freezing range and solidify at a single temperature

Question 9

General Gibbs phase rule

Answer 9

P + F = C + 2

Question 10

Degrees of freedom

Answer 10

variables that can be varied independently and define the system e.g. temperature, composition, pressure

Question 11

Condensed phase rule

Answer 11

P + F = C + 1 pressure can be assumed to be constant for solids thus removing one degree of freedom

Question 12

Which variables can be changed when F = 2?

Answer 12

in the same region, the temperature and composition can be varied independently and maintain the state

Question 13

Which variables can be changed when F = 1?

Answer 13

the temperature can be changed other variables need to change to maintain the state

Question 14

Which variables can be changed when F = 0?

Answer 14

neither the temperature or composition can be changed

Question 15

What is required to define a system?

Answer 15

number of phases present composition of those phases fraction of the system which each phase makes up

Question 16

The Lever rule

Answer 16

equation

Question 17

Eutectic phase diagrams

Answer 17

2 species which have no solid solubility at all the melting point of the mixture typically lower than that of either of the individual components

Question 18

Define the eutectic point

Answer 18

It is a mixture of the elements which melts at a single temperature.

Below the eutectic temperature, the whole system is solid and a mixture

The eutectic horizontal is the temperature below which the whole system is solid

Question 19

Hypoeutectic and hypereutectic systems

Answer 19

A system with more A than at the eutectic point is called a hypoeutectic system.

A system with more B than at the eutectic point is called a hypereutectuc system.

Question 20

Define a primary phase

Answer 20

It is a phase which forms at temperatures above the eutectic horizontal

Question 21

Partial solid solubility

Answer 21

the solvus line/curve gives the limit of solubility of the whole system in a solvent phase with respect to the phase in equilibrium with it