Iron unit cell

Question 1

Ferrite

Answer 1

> interstitial solid soln of C in BCC α-Fe

> a = 0.286 nm

Question 2

What is γ-Fe

Answer 2

FCC, a = 0.365nm

Question 3

What is δ-Fe

Answer 3

BCC, a = 0.293nm

Question 4

What sites do carbon atoms occupy in ferrite? Why?

Answer 4

octahedral, there is less net average displacement

Question 5

What is elastic strain energy equal to?

Answer 5

The square of the displacement of the Fe atoms

Question 6

Why is solubility of C in ferrite very small?

Answer 6

High elastic strain energy associated with placement of carbon in octahedral sites

Question 7

What is austenite?

Answer 7

intersitial solid solution of C in FCC γ-Fe

Question 8

Why do carbon atoms occupy octahedral sites in FCC austenite?

Answer 8

The octahedral site voids are larger than the tetrahedral ones

Question 9

The solubility of C in FCC austenite is ______ than that of C in BCC ferrite. Why?

Answer 9

much higher; the strain energy is substantially lower

Question 10

Cementite

Answer 10

> intermetallic line compund
Fe3C
density is similar to ferrite
it is a metastable phase, but won’t decompose into equilibrium phases ferrite and graphite because kinetics are slow