Class 6
Phase Diagrams
And
Microstructure
Homeworks
• If everyone agrees, homeworks will be in
the lab on a bench in alphabetical groups.
• Placed there this lunchtime so can be
collected after 1.00pm
• Exam
• Multiple Choice – eg how does sodium
and chlorine bond?
Microstructures and Composition
At eutectic composition, and temp, amount of a = 97.5-61.9/97.5-19.2 = 35.6/78.3= 45%
amount of b = 61.9-19.2/97.5-19.2 = 42.7/78.3= 55%
For 70% Pb 30% Sn, a phase if formed in a+liquid range, called pro eutectic a.
Amount of proeutectic a and eutectic a+b is obtained from inverse lever arm rule
Amount of pro eutectic a = 61.9-30.0/61.9-19.2 =31.9/42.7=0.75=75%
Amount of eutectic a+b = 30.0-19.5/61.9-19.2 = 10.5/42.4 = 0.25 = 25%
The composition of the a will be the same independent of whether it is pro eutectic or
eutectic at 80.8%Pb 19.2% Sn.
Lead Tin Microstructures
90 %Pb 10%Sn
70% Pb 30%Sn
38.1%Pb 61.9% Sn
50%Pb 50%Sn
Lead Tin Microstructures
15%Pb 85%Sn
Equiaxed Single Phase Grain Structure
Iron Carbon (Fe3C) Phase Diagram
Primary solid solubility
in a is 0.022wt% C, for
 is 2.14 wt% C.
Eutectoid reaction is
important for steels.
> a + Fe3C at 727C.
At 726C , a 99.978%Fe
0.022%C
Fe3C 93.3%C
6.7%C
Fe3C is a compound and
a Metastable phase, not
an equilibrium phase, but
stable unless thermally
changed.
Eutectoid composition
Is Fe 99.24%, C 0.76%
Eutectoid Composition
At 727C Eutecoid Reaction occurrs.

> a + Fe3C
Above 727C all , equiaxed.
Below 727C, a + Fe3C
Amounts of a and Fe3 C given by Inverse Lever
Arm Rule.
Amount of a = 6.7 – 0.76/ 6.7-0.022 = 5.94/6.678
= 0.89 = 89% a
Amount of Fe3C = 0.76- 0.022/ 6.7-0.022
= 0.738 / 6.678
= 0.11 = 11% Fe3C
a composition is 99.978%Fe, 0.022 C
Fe3C composition if 93.3%Fe, 6.7%C
Eutectoid Formation
a
Carbon moves from a

Fe3C
Iron moves from carbide
Growth Direction
Eutectoid
Alternate ferrite and cementite plate structure in Eutectoid steel.
Hypereutecoid Strucutres
Hypereutecoid – more carbon then eutectoid >0.76%
Start in single phase austenite, , range, grains of .
Cool into two phase region, then some  transforms
to Fe3C on the existing  grain boundaries. Called
“Pro Eutectoid” cementite.  composition follows
phase boundary to the eutectiod composition, so at
727C the remaining  transforms to pearlite, the mixture
of a + Fe3C. Cementite exists as Pro eutectoid and
eutectoid forms.
Hypoeutectoid Steels
Hypoeutecoid steels – less carbon than eutectoid < 0.76%
Start in single phase austenite, , range, grains of .
Cool into two phase region, then some  transforms
to a on the existing  grain boundaries. Called
“Pro Eutectiod” ferrite.  composition follows
phase boundary to the eutectiod composition, so at
727C the remaining  transforms to pearlite, the mixture
of a + Fe3C. Ferrite exists as Pro eutectoid and
eutectoid forms.( Pearlite has narrow plates in this case
of a steel with 0.08%C)