CHAPTER 10:

PHASE TRANSFORMATIONS

ISSUES TO ADDRESS...
• Transforming one phase into another takes time.

Fe Fe C
3
Eutectoid
g transformation (cementite)
(Austenite) +
C
a
FCC (BCC)
(ferrite)

• How does the rate of transformation depend on

time and T?
• How can we slow down the transformation so that
we can engineering non-equilibrium structures?
• Are the mechanical properties of non-equilibrium
structures better?
Chapter 10- 1
 FRACTION OF TRANSFORMATION
• Fraction transformed depends on time.

Adapted from
Fig. 10.1,
Callister 6e.

• Transformation rate depends on T.

Adapted from Fig.
10.2, Callister 6e.
(Fig. 10.2 adapted
from B.F. Decker
and D. Harker,
"Recrystallization in
Rolled Copper",
Trans AIME, 188,
1950, p. 888.)

• r often small: equil not possible!

Chapter 10- 2
TRANSFORMATIONS & UNDERCOOLING

Adapted from Fig.

9.21,Callister 6e. (Fig.
9.21 adapted from Binary
Alloy Phase Diagrams,
2nd ed., Vol. 1, T.B.
Massalski (Ed.-in-Chief),
ASM International,
Materials Park, OH, 1990.)

Chapter 10- 3
EUTECTOID TRANSFORMATION RATE ~ DT
• Growth of pearlite from austenite:

Adapted from
Fig. 9.13,
Callister 6e.

• Reaction rate
increases with
DT.
Adapted from
Fig. 10.3,
Callister 6e.

Chapter 10- 4
 NUCLEATION AND GROWTH
• Reaction rate is a result of nucleation and growth
of crystals.

Adapted from
Fig. 10.1, Callister 6e.

• Examples:

Chapter 10- 5
 ISOTHERMAL
TRANSFORMATION DIAGRAMS
• Fe-C system, Co = 0.77wt%C
• Transformation at T = 675C.

Adapted from Fig. 10.4,Callister 6e.

(Fig. 10.4 adapted from H. Boyer (Ed.)
Atlas of Isothermal Transformation
and Cooling Transformation Diagrams,
American Society for Metals, 1977, p.
369.)

Chapter 10- 6
EX: COOLING HISTORY Fe-C SYSTEM
• Eutectoid composition, Co = 0.77wt%C
• Begin at T > 727C
• Rapidly cool to 625C and hold isothermally.

Adapted from Fig.

10.5,Callister 6e.
(Fig. 10.5 adapted from
H. Boyer (Ed.) Atlas of
Isothermal
Transformation and
Cooling Transformation
Diagrams, American
Society for Metals,
1997, p. 28.)

Chapter 10- 7
 PEARLITE MORPHOLOGY
Two cases:
• Ttransf just below TE • Ttransf well below TE
--Larger T: diffusion is faster --Smaller T: diffusion is slower
--Pearlite is coarser. --Pearlite is finer.

Adapted from Fig. 10.6 (a) and (b),Callister 6e. (Fig. 10.6 from R.M. Ralls et al., An Introduction to
Materials Science and Engineering, p. 361, John Wiley and Sons, Inc., New York, 1976.)

- Smaller DT: - Larger DT:

colonies are colonies are
larger smaller

Chapter 10- 8
 NON-EQUIL TRANSFORMATION
PRODUCTS: Fe-C
• Bainite:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled. Fe3C
• Isothermal Transf. Diagram (cementite)
a (ferrite)

5 mm
(Adapted from Fig. 10.8, Callister, 6e.
(Fig. 10.8 from Metals Handbook, 8th ed.,
Vol. 8, Metallography, Structures, and
Phase Diagrams, American Society for
Metals, Materials Park, OH, 1973.)

Adapted from Fig. 10.9,Callister 6e.

(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Chapter 10- 9
Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)
OTHER PRODUCTS: Fe-C SYSTEM (1)
• Spheroidite:
--a crystals with spherical Fe3C a
--diffusion dependent. (ferrite)
--heat bainite or pearlite for long times
--reduces interfacial area (driving force) Fe3C
• Isothermal Transf. Diagram (cementite)

60 mm
(Adapted from Fig. 10.10, Callister,
6e. (Fig. 10.10 copyright United
States Steel Corporation, 1971.)

Adapted from Fig. 10.9,Callister 6e.

(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of
Isothermal Transformation and Cooling
Transformation Diagrams, American Society for
Metals, 1997, p. 28.)

Chapter 10- 10
 OTHER PRODUCTS: Fe-C SYSTEM (2)
• Martensite:
--g(FCC) to Martensite (BCT)
(involves single atom jumps)

60 mm
x
Fe atom potential
x x
sites x x C atom sites
x (Adapted from Fig.
10.11, Callister, 6e.

• Isothermal Transf. Diagram

Martentite needles
Austenite
(Adapted from Fig. 10.12, Callister,
6e. (Fig. 10.12 courtesy United
States Steel Corporation.)
Adapted
from Fig.
10.13, • g to M transformation..
Callister 6e.
-- is rapid!
-- % transf. depends on T only.

Chapter 10- 11
COOLING EX: Fe-C SYSTEM (1)

Adapted
from Fig.
10.15,
Callister 6e.

Chapter 10- 12
COOLING EX: Fe-C SYSTEM (2)

Adapted
from Fig.
10.15,
Callister 6e.

Chapter 10- 13
COOLING EX: Fe-C SYSTEM (3)

Adapted
from Fig.
10.15,
Callister 6e.

Chapter 10- 14
 MECHANICAL PROP: Fe-C SYSTEM (1)

Adapted from Fig. 9.27,Callister Adapted from Fig. 9.30,Callister

6e. (Fig. 9.27 courtesy Republic 6e. (Fig. 9.30 copyright 1971 by
Steel Corporation.) United States Steel Corporation.)

Adapted from Fig.

10.20, Callister 6e.
(Fig. 10.20 based on
data from Metals
Handbook: Heat
Treating, Vol. 4, 9th
ed., V. Masseria
(Managing Ed.),
American Society for
Metals, 1981, p. 9.)

Chapter 10- 15
MECHANICAL PROP: Fe-C SYSTEM (2)

Adapted from Fig. 10.21, Callister

6e. (Fig. 10.21 based on data from
Metals Handbook: Heat Treating,
Vol. 4, 9th ed., V. Masseria
(Managing Ed.), American Society
for Metals, 1981, pp. 9 and 17.)

Chapter 10- 16
MECHANICAL PROP: Fe-C SYSTEM (3)
• Fine Pearlite vs Martensite:

Adapted from Fig. 10.23,

Callister 6e. (Fig. 10.23
adapted from Edgar C. Bain,
Functions of the Alloying
Elements in Steel, American
Society for Metals, 1939, p.
36; and R.A. Grange, C.R.
Hribal, and L.F. Porter, Metall.
Trans. A, Vol. 8A, p. 1776.)

• Hardness: fine pearlite << martensite.

Chapter 10- 17
 TEMPERING MARTENSITE
• reduces brittleness of martensite,
• reduces internal stress caused by quenching.

Adapted from Adapted from

Fig. 10.25, Fig. 10.24,
Callister 6e. Callister 6e.
(Fig. 10.25 (Fig. 10.24
adapted from copyright by
Fig. furnished United States
courtesy of Steel
Republic Steel Corporation,
Corporation.) 1971.)

Chapter 10- 18
SUMMARY: PROCESSING OPTIONS
Adapted from
Fig. 10.27,
Callister 6e.

Chapter 10- 19
 ANNOUNCEMENTS
Reading:

Core Problems:

Self-help Problems:

Chapter 10- 0