Intermetallic Compounds
Antifluorite Structure:
• FCC Unit cell with Anions
occupying FCC sites
• Cations occupying 8
octahedral interstitial sites
Mg2Pb
Intermetallic compounds form lines - not areas - because
stoichiometry (i.e. composition) is exact.
Intermetallic Compounds
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Monotectic
Monotectic Reaction: L
cool
heat
L1 + Solid
Pb and Zn do not mix in solid state:
• RT: Cu in Pb < 0.007%
• RT: Pb in Cu ~ 0.002 – 0.005%
Cu + L1
Crystal
Structure
electroneg
r (nm)
Pb
FCC
1.8
0.175
Cu
FCC
1.9
0.128
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
26.8%
Eutectoid & Peritectic
Eutectoid Reaction: 1 solid phase  2 solid phases

cool
heat
 + Fe3C
(727ºC)
intermetallic compound
- cementite
Peritectic Reaction: liquid + solid 1  solid 2
+L
cool
heat

(1493ºC)
Eutectoid and Peritectic
Copper-Zinc Binary Equilibrium Phase Diagram:
Eutectoid & Peritectic
Peritectic transition  + L
+L
Cu-Zn Phase diagram
Eutectoid transition 
+


Congruent vs Incongruent
Congruent phase transformation: no compositional change associated with
transformation
Examples:
• Allotropic phase transformations
• Melting points of pure metals
• Congruent Melting Point
Incongruent phase transformation: at
least one phase will experience change in
composition
Examples:
• Melting in isomorphous alloys
• Eutectic reactions
• Pertectic Reactions
• Eutectoid reactions
Ni
Ti
Iron-Carbon System
Diagram is not ever plotted past 12 wt%
Cementite
Hägg carbide
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Iron Carbon Phase Diagram
 ferrite,
BCC
Formation of Ledeburite
FCC
A3
ACM
A1 (Eutectoid Temperature)
Formation of Pearlite
 ferrite
BCC
Steel
Cast Irons
Source: Reed-Hill, Abbaschian, Physical
Metallurgy Principles, 3rd Edition,
PWS Publishing Company, 1994.
Cementite – What is it?
Iron Carbide – Ceramic Compound
Purple: Carbon atoms
Orange: Iron atoms
• Cementite has an orthorhombic lattice with approximate
parameters 0.45165, 0.50837 and 0.67297 nm.
• There are twelve iron atoms and four carbon atoms per
unit cell, corresponding to the formula Fe3C.
Source: http://www.msm.cam.ac.uk/phase-trans/2003/Lattices/cementite.html
H. K. D. H. Bhadeshia
Pearlite: What is it?
• The eutectoid transformation:
 (0.77% C)
 (0.02%C) + Fe3C (6.67%C)
• Alternate lamellae of ferrite and cementite w/ ferrite
as the continuous phase
• Diffusional Transformation
• “Pearlite” name is related to the regular array of the
lamellae in colonies. Etching attacks the ferrite
phase more than the cementite. The raised and
regularly spaced cementite lamellae act as
diffraction gratings and a pearl-like luster is
produced by the diffraction of light of various
wavelengths from different colonies [1]
Pearlite
• Two phases appear in definite
ratio by the lever rule:

6.67  0.77
 88%
6.67
cementite 
Reed-Hill, Abbaschian, 1994, [5]
0.77  0
 12%
6.67
• Since the densities are same
(7.86 and 7.4) lamellae widths
are 7:1
• Heterogeneous nucleation and
growth of pearlite colonies – but
typically grows into only 1 grain
Lamellae Nucleation
Reed-Hill, Abbaschian, 1994
Reed-Hill, Abbaschian, 1994
Interlamellar Spacing
• Interlamellar spacing l is almost constant in pearlite
formed from  at a fixed T
• Temperature has a strong effect on spacing – lower T
promotes smaller l
– Pearlite formed at 700oC has l ~ 1 mm and Rockwell C - 15
– Pearlite formed at 600oC has l ~ 0.1 mm and Rockwell C - 40
• Zener and Hillert Eq. for spacing [1]:
l
4  / Fe3C TE
H V T
/Fe3C = Interfacial energy per unit area of /Fe3C boundary
TE = The equilibrium temperature (Ae1)
HV = The change in enthalpy per unit volume b/t  and /Fe3C
T = The undercooling below Ae1
Effect of Undercooling on l
Krauss, Steels, 1995
Effect of Interlamellar Spacing
Stone et al, 1975
Iron-Carbon (Fe-C) Phase Diagram
T(°C)
1600

L+
1200
-Eutectoid (B) [0.77 %C]:
   + Fe3C
-Eutectic (A) [4.32 %C]:
L   + Fe3C
L
 +L

(austenite)
 
 
1000

800
600
S
 +Fe3C
727°C = Teutectoid
R
S
1
0.76
L+Fe3C
R
B
400
0
(Fe)
A
1148°C
2
3
+Fe3C
4
5
6
Fe3C (cementite)
-Peritectic (C) [0.17%C]:1400
C
C eutectoid
3 invariant points:
6.7
4.30
Co, wt% C
Fe3C (cementite-hard)
 (ferrite-soft)
Hypoeutectoid Steel
T(°C)
1600

 
 
 +L

1200
(austenite)
 
 
1000




 
800
1148°C
 + Fe3C
727°C
r s
RS
w  =s/(r +s) 600
w  =(1- w )
400
0
(Fe)
pearlite
 + Fe3C
1
C0
w pearlite = w 
0.76

w  =S/(R+S)
w Fe3 =(1-w  )
C
L+Fe3C
pearlite
2
3
4
5
6
Fe3C (cementite)
L
1400
6.7
Co , wt% C
100 mm
proeutectoid ferrite
Proeuctectoid Ferrite – Pearlite
0.38 wt% C: Plain Carbon – Medium Carbon Steel