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Detail Summery about Points Covered, Equilibrium Diagrams , Eutectoid and Peritectic Reactions, Ceramic Phase Diagrams, The Gibbs Phase Rule, Hypoeutectoid Alloys.
Typology: Study notes
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1
2
2
Note: intermetallic compounds exist as a line on the diagram
distinct chemical formula.
19 wt% Mg-81 wt% Pb
4
Eutectoid transformation
Peritectic transformation + L
5
phases
Solid
1
↔ Solid
2
3
3
C (For Fe-C, 727C, 0.76 wt% C)
cool
heat
Peritectic - liquid and one solid phase transform to a 2nd
solid phase
Solid
1
2
cool
heat
cool
heat
7
2
3
8
2
3
(made up of SiO 2
) are candidate refractories.
Composition (wt% alumina)
T(°C)
1400
1600
1800
2000
2200
0 20 40 60 80 100
alumina
mullite
mullite
mullite
Liquid
(L)
mullite
+crystobalite
crystobalite
alumina + L
3Al
2
O
3
-2SiO
2
10
Gibbs Phase Rule
Phase diagrams and phase equilibria are subject to the laws of thermodynamics.
Gibbs phase rule is a criterion that determines how many phases can coexist within a
system at equilibrium.
P: # of phases present
F: degrees of freedom (temperature, pressure, composition)
C: components or compounds
N: noncompositional variables
For the Cu-Ag system @ 1 atm for a single phase P:
N=1 (temperature), C = 2 (Cu-Ag), P= 1 (, L)
This means that to characterize the alloy within a single phase
field, 2 parameters must be given: temperature and composition.
If 2 phases coexist, for example, L L, then according to GPR, we have 1
degree of freedom: F = 2 + 1 – 2= 1. So, if we have Temp or composition, then we can
completely define the system.
If 3 phases exist (for a binary system), there are 0 degrees of freedom. This means the
composition and Temp are fixed. This condition is met for a eutectic system by the
eutectic isotherm.
11
changes in crystal structure before it
melts.
ferrite ( iron) has a BCC crystal
structure.
transformation to FCC austenite (
iron) at 912 ˚C (1674 ˚F).
back to BCC phase ferrite and melts
at 1538 ˚C (2800 ˚F).
3
C) an
intermediate compound is formed at
6.7 wt% C.
have carbon contents less than 6.7 wt
% C.
iron and forms a solid solution with
the phases
16
Iron-Carbon (Fe-C) Phase Diagram
points
- Eutectoid (B):
+Fe
3
↔
Fe
3
Fe
3
C (cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.
(austenite)
+Fe
3
C
+Fe
3
(Fe)
C, wt% C
1148°C
727°C = T
eutectoid
Result: Pearlite =
alternating layers of
and Fe
3
C phases,
not a separate phase.
120 m
B
A
L+Fe
3
C
Fe
3
C (cementite-hard)
(ferrite-soft)
↔
Eutectoid reaction:
+Fe
3
Austenite – 0.76 wt% C
Ferrite - 0.022 wt% C
Cementite - 6.70 wt% C
Redistribution of carbon by
diffusion
19
Fe
3
C (cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.
(austenite)
3
C
3
C
L+Fe
3
C
(Fe)
C, wt% C
1148°C
727°C
C
0
pearlite
20
eutectoid
(727°C) is proeutectoid.
