Introduction to Materials Science

1
Phase Diagrams
Thermodynamics
Solubility of sugar
Binary isomorphous system
Two phases aq.solution solid sugar
Cu/Ni completely soluble
(WLWa)1
FCC structure in solid
(WLWa)C0WLCLWaCa
Level rule
2
Microstructure
Nonequilibrium solidification
Limited solid-state diffusion
Equilibrium solidification
3
Binary Eutectic Systems
Pb/Sn
Cu/Ag
Eutectic point (Invariant)
Eutectic reaction
4
Examples 9.2-3
(a) What phases are present ?
Sb/Sn (60wt Pb) at 150oC
a b
(b) Compositions ?
Ca 10 wt Sn
Cb 98 wt Sn
Density, volume fraction ?
5
Development of Microstructures
Equilibrium solidification
b ??????
6
Eutectic Structure
7
Development of Microstructures
WE?
Wa?
Wb?
8
Zone Refining, Pfann (1952)
Heater
W. G. Pfann. Trans. AIME, 194, 747 (1952)
Crystal
Feed
Melt
Quartz tube
Induction coil
Ge zone refiner
Since then, thousands of materials have been
purified using this technique.
1999????????????, 11/27/1999, ?? C.W. Lan
9
Zone Refining vs Normal Freezing
Zone Refining
Normal freezing
crystal
feed
crystal
melt
Refining zone
Refining zone
C
l
C
Zone leveling
Distance (z)
Distance (z)
1. Complete mixing 2. No solid-state diffusion
3. Planar Interface
-Kz/l
K-1
C C 1-(1-K) e
C C K(1-f )
s
s
0
0
10
Multipass Zone Refining
Lords expression
K 0.1
l/L0.2
Also other numerical calculations for variable
zone size, variable K, etc..
11
Interface Morphology on K
Constitutional supercooling
_at_ Interface
G
T
C
Cellular
Melt
Crystal
V
Distance
Interface breakdown
phase diagram
Dendritic
Crystal
Melt
12
Intermediate Phases
13
Intermetallic Compounds
Mg2Pb
14
Eutectoid Reaction
Peritectic Reaction
15
Congruent Phase Transformation
No composition change during transformation
Incongruent
Congruent transformation
Ni/Ti system
16
Gibbs Phase Rule
Degree of Freedom No. of Unknowns - No. of
Equations
(C - 1) P 2
C (P - 1)
F
T, P
F C - P 2
F 2 - P 1
1 atm
Needs comp. and T to specify the alloy
For a, b phases, P 1. F 2
For a L, P 2 F 1.
T is needed.
Eutectic point, P 3 F 0
Invariant !
17
Iron-Carbon System
d ferrite (BCC)
Austenite (Nonmagnetic)
Ferrite (magnetic)
Austenite (FCC)
Cementite (Fe3C)
Ferrite (BCC)
2.14 wt C in austenite
0.022wt C
Due to the larger interstitial voids
18
Development of Microstructure
Eutectoid composition
Pearlite
19
Hypoeutectoid alloy
Also see p.241 for proeutectoid ferrite and
pearlite ferrite !
20
Hypereutectoid alloys
Proeutectoid cementite cementite formed before
eutectoid reaction
21
The Effects of Other Alloying Elements
The eutectoid temperature can be affected !
The eutectoid composition can be affected !