1. Phase Diagrams And Microstructure
Class 6
Phase Diagrams
And
Microstructure

2. Homeworks
If everyone agrees, homeworks will be in the lab on a bench in alphabetical groups.
Placed there Wednesday so can be collected after 1.00pm
Lab schedule starts Wed 2.00pm room 121 Crawford Hall.
NOTE First Exam move to Oct 10th
Multiple Choice – eg how does sodium and chlorine bond?

3. Microstructures and Composition
At eutectic composition, and temp, amount of a = / = 35.6/78.3= 45%
amount of b = / = 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 = / =31.9/42.7=0.75=75%
Amount of eutectic a+b = / = 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.

4. Lead Tin Microstructures
38.1%Pb 61.9% Sn
90 %Pb 10%Sn
70% Pb 30%Sn
50%Pb 50%Sn

5. Lead Tin Microstructures
15%Pb 85%Sn
Equiaxed Single Phase Grain Structure

6. 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 %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%

7. Eutectoid Composition
At 727C Eutecoid Reaction occurrs.
> a + Fe3C
Above 727C all g, equiaxed.
Below 727C, a + Fe3C
Amounts of a and Fe3 C given by Inverse Lever
Arm Rule.
Amount of a = 6.7 – 0.76/ = 5.94/6.678
= 0.89 = 89% a
Amount of Fe3C = /
= / 6.678
= 0.11 = 11% Fe3C
a composition is %Fe, C
Fe3C composition if 93.3%Fe, 6.7%C

8. Eutectoid Formation Growth Direction g a Carbon moves from a Fe3C
Iron moves from carbide
Growth Direction

9. Alternate ferrite and cementite plate structure in Eutectoid steel.

10. Hypereutecoid Strucutres
Hypereutecoid – more carbon then eutectoid >0.76%
Start in single phase austenite, g, range, grains of g.
Cool into two phase region, then some g transforms
to Fe3C on the existing g grain boundaries. Called
“Pro Eutectoid” cementite. g composition follows
phase boundary to the eutectiod composition, so at
727C the remaining g transforms to pearlite, the mixture
of a + Fe3C. Cementite exists as Pro eutectoid and
eutectoid forms.

11. Hypoeutectoid Steels
Hypoeutecoid steels – less carbon than eutectoid < 0.76%
Start in single phase austenite, g, range, grains of g.
Cool into two phase region, then some g transforms
to a on the existing g grain boundaries. Called
“Pro Eutectiod” ferrite. g composition follows
phase boundary to the eutectiod composition, so at
727C the remaining g 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)

12. Homeworks.
From the Pb­Sn phase diagram, for a 70% Pb, 30%Sn alloy, at what temperature does solid first form.  At the eutectic temperature, how much pro­eutectic phase is present and how much eutectic phase?  What are the compositions at 100oC?
For a steel containing 0.4% C:-
a) At what temperature does austenite start to transform on cooling from 950oC? 
b) How much proeutectoid phase is present at the eutectoid temperature 
c) What is its composition?