Phase Diagrams

Question 1

What are isomorphous alloys?

Answer 1

Characterised by complete solubility of the two components

Question 2

What can be determined from a phase diagram?

Answer 2

The fraction of phase present at equilibrium

Question 3

What is the composition and phase fraction of a single phase?

Answer 3

Composition is C_0 and the phase fraction is 1

Question 4

How is the phase fraction for a 2-phase region determined?

Answer 4

Using the tie-line and Lever rule

Question 5

What parameter is determined by the lever rule?

Answer 5

Mass fraction

Question 6

What is a eutectic reaction?

Answer 6

A reaction in which a single liquid changes phase into a 2-phase solid

Question 7

Describe ferrite.

Answer 7

-At room temperature the structure is BCC
-Low solubility of C atoms in interstitial sites
-Can dissolve Cr, Si, W and Mo by substitution

Question 8

Describe Austenite.

Answer 8

-At elevated temperature the structure is FCC
-Can dissolve more carbon interstitially than Ferrite (Alpha-iron)
-Can dissolve by substitution large amounts of Ni and Mn

Question 9

Describe Carbide.

Answer 9

-Iron and carbon form the compound cementite
-Hard and brittle phase
-There can be other alloying elements in the cementite such as Cr, Mn and Mo (carbides)

Question 10

What is a eutectoid reaction?

Answer 10

Involves the transformation of solid solution to two phases

Question 11

Describe hypoeutectoid steels.

Answer 11

-At 1000 degrees there is a single austenite phase
-At 800 degrees there are two phases, austenite and proeutectoid ferrite
-At 730 degrees the grain boundaires are plated by proeutectoid ferrite
-Cooling below 730 degrees, forms pearlite (a mixture of ferrite and carbide)