1. Chapter 10: Phase Transformations
ISSUES TO ADDRESS...
• Transforming one phase into another takes time. Fe g
(Austenite)
Eutectoid
transformation C
FCC
Fe3C
(cementite) a
(ferrite) +
(BCC)
• How does the rate of transformation depend on
time and T ?
The tensile strength of just the eutectoid composition of steel can be varied between MPa depending on the heat treatment / rate of cooling that is chosen.
• How can we slow down the transformation so that
we can engineer non-equilibrium structures?
• Are the mechanical properties of non-equilibrium
structures better?

2. The tensile strength of an iron–carbon alloy of eutectoid composition (0.76 wt% C) can be varied between approximately 700 MPa (100,000 psi) Mpa (300,000 psi) depending on the heat treatment employed!

3. Phase Transformations
Nucleation
nuclei (seeds) act as template to grow crystals
for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss
once nucleated, grow until reach equilibrium
Driving force to nucleate increases as we increase T
supercooling (eutectic, eutectoid)
superheating (peritectic)
In Chapter 9 we looked at the equilibrium phase diagram . This indicated phase structure if we wait long enough. But due to slow diffusion may not reach equilibrium We need to consider time- kinetics - energy of phase boundaries may be high. – also nucleation
Transformation rate
How fast do the phase transformations occur?
First need nuclei (seeds) to form for the rest of the material to crystallize
Small supercooling  few nuclei - large crystals
Large supercooling  rapid nucleation - many nuclei,
small crystals

4. Solidification: Nucleation Processes
Homogeneous nucleation
nuclei form in the bulk of liquid metal
requires supercooling (typically °C max)
Heterogeneous nucleation
much easier since stable “nucleus” is already present
Could be wall of mold or impurities in the liquid phase
allows solidification with only ºC supercooling

5. Homogeneous Nucleation & Energy Effects
Surface Free Energy- destabilizes
the nuclei (it takes energy to make
an interface)
g = surface tension
DGT = Total Free Energy
= DGS + DGV
Volume (Bulk) Free Energy –
stabilizes the nuclei (releases energy)
r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy)
Adapted from Fig.10.2(b), Callister 7e.

6. Solidification Note: HS = strong function of T
HS = latent heat of solidification
Tm = melting temperature
g = surface free energy
DT = Tm - T = supercooling
r* = critical radius
Note: HS = strong function of T
 = weak function of T
 r* decreases as T increases
For typical T r* ca. 100Å

7. t10_01_pg318.jpg
t10_01_pg318

8. Information Available from Transformation Curves
The size of the product phase particles will depend on transformation temperature.
Transformations that occur at temperatures near to Tm correspond to low nucleation and high growth rates, few nuclei form that grow rapidly. (coarse grain structure)
Transformations at lower temperatures, nucleation rates are high and growth rates low, which results in many small particles (e.g., fine grains).
It is possible to produce non-equilibrium phase structures, when materials are cooled very rapidly, to a relatively low temperature where the transformation rate is extremely low.

9. Rate of Phase Transformations
Kinetics - measure approach to equilibrium vs. time
Hold temperature constant & measure conversion vs. time
How is conversion measured?
X-ray diffraction – have to do many samples
electrical conductivity – follow one sample
sound waves – one sample

10. Rate of Phase Transformation
All out of material - done
Fixed T
Fraction transformed, y
0.5
maximum rate reached – now amount
unconverted decreases so rate slows
rate increases as surface area increases & nuclei grow
t0.5
log t
Adapted from Fig ,
Callister 7e.
S.A. = surface area
By convention r = 1 / t0.5

11. Rate of Phase Transformations
135C
119C
113C
102C
88C
43C 1 10
102
104
Adapted from Fig , Callister 7e. (Fig adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888.)
In general, rate increases as T 
r = 1/t0.5 = A e -Q/RT
R = gas constant
T = temperature (K)
A = preexponential factor
Q = activation energy
Arrhenius expression
r often small: equilibrium not possible!

12. Nucleation and Growth
• Reaction rate is a result of nucleation and growth
of crystals.
% Pearlite 50
100
Nucleation
regime
Growth
log (time) t
0.5
Nucleation rate increases with T
Growth rate increases with T
Adapted from
Fig , Callister 7e.
• Examples:
T just below TE
Nucleation rate low
Growth rate high g
pearlite
colony
T moderately below TE g
Nucleation rate med .
Growth rate med.
Nucleation rate high
T way below TE g
Growth rate low

13. Eutectoid Transformation Rate
• Growth of pearlite from austenite:
Adapted from Fig. 9.15, Callister 7e. g a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
Diffusive flow
of C needed a g
• Recrystallization
rate increases
with DT.
Adapted from Fig , Callister 7e.
675°C
(DT smaller) 50
y (% pearlite)
600°C
(DT larger)
650°C
100
Coarse pearlite  formed at higher T - softer
Fine pearlite  formed at low T - harder

14. Transformations & Undercooling•
Eutectoid
transf. (Fe-C System): g Þ a +
Fe3C •
Can make it occur at:
0.76 wt% C
6.7 wt% C
...727ºC (cool it slowly)
0.022 wt% C
...below 727ºC (“undercool” it!)
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
Co , wt%C
1148°C
T(°C)
ferrite
727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC DT
Undercooling by DTtransf. < 727C
0.76
0.022
Adapted from Fig. 9.24,Callister 7e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

15. Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% C
• Transformation at T = 675°C.
100
T = 675°C y,
% transformed 50 2 4 1 10 10
time (s)
400
500
600
700 1 10 2 3 4 5
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Adapted from Fig ,Callister 7e.
(Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)

16. Effect of Cooling History in Fe-C System
• Eutectoid composition, Co = 0.76 wt% C
• Begin at T > 727°C
• Rapidly cool to 625°C and hold isothermally.
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C) 1 10 2 3 4 5
time (s)
Adapted from Fig ,Callister 7e.
(Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) g g

17. Non-Equilibrium Transformation Products: Fe-C
• Bainite:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram
Fe3C
(cementite) a
(ferrite) 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50%
pearlite/bainite boundary A
100% pearlite
100% bainite
5 mm
(Adapted from Fig , Callister, 7e. (Fig from Metals Handbook, 8th ed.,
Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)
Adapted from Fig , Callister 7e.
(Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

18. Spheroidite: Fe-C System
(Adapted from Fig , Callister, 7e. (Fig copyright United States Steel Corporation, 1971.)
60 m a
(ferrite)
(cementite)
Fe3C
• Spheroidite:
--a grains with spherical Fe3C
--diffusion dependent.
--heat bainite or pearlite for long times
--reduces interfacial area (driving force)

19. Martensite Formation  (FCC)  (BCC) + Fe3C slow cooling quench
M (BCT)
tempering
M = martensite is body centered tetragonal (BCT)
Diffusionless transformation BCT if C > 0.15 wt%
BCT  fewer slip planes  hard, brittle

20. Martensite: Fe-C System
--g(FCC) to Martensite (BCT)
Martensite needles
Austenite
60 m x
potential
C atom sites
Fe atom
sites
(involves single atom jumps)
(Adapted from Fig , Callister, 7e.
• Isothermal Transf. Diagram 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50% A
M + A
90%
(Adapted from Fig , Callister, 7e. (Fig courtesy United States Steel Corporation.)
Adapted from Fig , Callister 7e.
• g to M transformation..
-- is rapid!
-- % transf. depends on T only.

21. Materials of Importance
A relatively new group of metals that exhibit
an interesting (and practical) phenomenon
are the shape-memory alloys (or SMAs).
One of these materials, after having been deformed, has the ability to return to its pre-deformed size and shape upon being subjected to an appropriate heat treatment—that is, the material “remembers” its previous size/shape!
Time-lapse photograph that demonstrates the shape memory
effect. A wire of a shape-memory alloy
(Nitinol) has been bent and treated such that its
memory shape spells the word “Nitinol”. The wire is
then deformed and, upon heating (by passage of an
electric current), springs back to its pre-deformed
shape; this shape recovery process is recorded on the
photograph. [Photograph courtesy the Naval Surface
Warfare Center (previously the Naval Ordnance
Laboratory)].
Deformation normally is carried out at a relatively low temperature, whereas, shape memory occurs upon heating. Materials are capable of recovering after significant amounts of deformation.
A SMA may have two crystal structures (or phases), and the shape-memory effect involves diffusion-less phase transformations between them.

22. Shape Memory Effect

23. Applications of SMAs
There is a host of applications for SMA alloys for example:
eyeglass frames
tooth-straightening braces
collapsible antennas
greenhouse window openers
anti scald control valves on showers
fire sprinkler valves
self-extending coronary stents, and bone anchors.
Shape-memory alloys fall into the classification of “smart materials” (Section 1.5) since they sense and respond to environmental changes (i.e., temperature).

24. Phase Transformations of Alloys
Effect of adding other elements
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard    + Fe3C
transformation
Adapted from Fig , Callister 7e.

25. Dynamic Phase Transformations
On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:
50% fine pearlite and 50% bainite
100% martensite
50% martensite and 50% austenite

26. Example Problem for Co = 0.45 wt%
50% fine pearlite and 50% bainite
first make pearlite
then bainite
fine pearlite
 lower T
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Adapted from Fig ,
Callister 5e.

27. Example Problem for Co = 0.45 wt%
100 % martensite – quench = rapid cool
50 % martensite
and 50 % austenite
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%) c)
T (°C) d)
Adapted from Fig ,
Callister 5e.

28. Mechanical Prop: Fe-C System (1)
• Effect of wt% C
Pearlite (med)
Pearlite (med) C
ementite
ferrite (soft)
(hard)
Co < 0.76 wt% C
Co > 0.76 wt% C
Adapted from Fig. 9.30,Callister
7e. (Fig courtesy Republic Steel Corporation.)
Adapted from Fig. 9.33,Callister 7e. (Fig copyright 1971 by United States Steel Corporation.)
Hypoeutectoid
Hypereutectoid
300
500
700
900
1100
YS(MPa)
TS(MPa)
wt% C
0.5 1
hardness
0.76
Hypo
Hyper
wt% C
0.5 1 50
100
%EL
Impact energy (Izod, ft-lb) 40 80
0.76
Hypo
Hyper
Adapted from Fig , Callister 7e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)
• More wt% C: TS and YS increase
, %EL decreases.

29. Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite 80
160
240
320
wt%C
0.5 1
Brinell hardness
fine
pearlite
coarse
spheroidite
Hypo
Hyper 30 60 90
wt%C
Ductility (%AR)
fine
pearlite
coarse
spheroidite
Hypo
Hyper
0.5 1
Adapted from Fig , Callister 7e. (Fig 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.)
• Hardness:
fine > coarse > spheroidite
• %RA:
fine < coarse < spheroidite

30. Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
200
wt% C
0.5 1
400
600
Brinell hardness
martensite
fine pearlite
Hypo
Hyper
Adapted from Fig , Callister 7e. (Fig 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 )
• Hardness: fine pearlite << martensite.

31. Tempering Martensite • • • reduces brittleness of martensite,
• reduces internal stress caused by quenching.
YS(MPa)
TS(MPa)
800
1000
1200
1400
1600
1800 30 40 50 60
200
400
600
Tempering T (°C)
%RA TS YS
Adapted from Fig , Callister 7e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.)
Adapted from Fig , Callister 7e. (Fig copyright by United States Steel Corporation, 1971.)
9 mm
produces extremely small Fe3C particles surrounded by a. • •
decreases TS, YS but increases %RA

32. Summary: Processing Options
Austenite (g)
Bainite
(a + Fe3C plates/needles)
Pearlite
(a + Fe3C layers + a
proeutectoid phase)
Martensite
(BCT phase
diffusionless
transformation)
Tempered
(a + very fine
Fe3C particles)
slow
cool
moderate
rapid
quench
reheat
Strength
Ductility
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Adapted from Fig , Callister 7e.

33. Taxonomy of Metals

34. Taxonomy of Metals Steels Cast Irons <1.4 wt% C 3-4.5 wt% C
Alloys
Steels
Ferrous
Nonferrous
Cast Irons Cu Al Mg Ti
<1.4wt%C
3-4.5
wt%C
Adapted from Fig. 11.1, Callister 7e.
Steels
<1.4 wt% C
Cast Irons
3-4.5 wt% C
microstructure:
ferrite, graphite
cementite Fe 3 C
cementite
1600
1400
1200
1000
800
600
400 1 2 4 5 6
6.7 L g
austenite +L
+Fe3C a
ferrite +
L+Fe3C d
(Fe)
Co , wt% C
Eutectic:
Eutectoid:
0.76
4.30
727°C
1148°C
T(°C)
Adapted from Fig. 9.24,Callister 7e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed.,
Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

35. Taxonomy of Steels
Low Carbon Steels: Contain < 0.25% Carbon; produced in greatest quantities, least expensive, non heat treatable (unable to form Martensite). Soft, ductile and tough; typical uses: Automobile body parts,sheets and cans.
High Strength Low Alloy Steels (HSLA): Low Carbon steels, containing alloying elements, e.g. Cu, V, Ni and Mo (combined conc. < 10%). Higher strength & corrosion resistance cf. low carbon steels. Used in bridges, towers, pressure vessels &
support columns in high rise buildings.
Medium Carbon Steels: Contain % wt% Carbon, Can be heat treated by austenitizing, quenching, tempering etc. Higher strength cf. low carbon steels, however less tough and ductile. Used in railway wheels and tracks, gears and other structural components requiring high strength, wear resistance and toughness.
High Carbon Steels: Contain wt% Carbon; are the hardest, strongest and least ductile of all steels. Almost always used in hardened and tempered condition. High wear resistance and are capable of holding a cutting edge. Used in cutting tools, dies (for shaping materials), knives, razors, blades, springs and high strength wires.
Stainless Steels: Contain atleast 11 wt% Cr; Highly resistant to corrosion in a variety of environments. Typical uses: Steam boilers, heat treating furnaces, aircraft, missiles & nuclear power generating units.

36. Ferrous Alloys Iron containing – Steels - cast irons
Nomenclature AISI & SAE
10xx Plain Carbon Steels
11xx Plain Carbon Steels (resulfurized for machinability)
15xx Mn (10 ~ 20%)
40xx Mo (0.20 ~ 0.30%)
43xx Ni ( %), Cr ( %), Mo ( %)
44xx Mo (0.5%)
where xx is wt% C x 100
example: steel – plain carbon steel with 0.60 wt% C
Stainless Steel -- >11% Cr

37. Cast Iron Ferrous alloys with > 2.1 wt% C
more commonly wt%C
low melting ( C cf > 15000C for steels) (also brittle) so easiest to cast
Cementite decomposes to ferrite + graphite
Fe3C  3 Fe () + C (graphite)
generally a slow process
So phase diagram for this system is different (Fig 12.4)

38. Fe-C True Equilibrium Diagram
1600
1400
1200
1000
800
600
400 1 2 3 4 90 L g +L
 + Graphite
Liquid +
Graphite
(Fe)
Co , wt% C
0.65
740°C
T(°C)
 + Graphite
100
1153°C
Austenite
4.2 wt% C
a + g
Graphite formation promoted by
Si > 1 wt%
slow cooling
Cast irons have graphite
Adapted from Fig. 11.2,Callister 7e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed.,
Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

39. Limitations of Ferrous Alloys
Relatively high density
Relatively low conductivity
Poor corrosion resistance

40. Nonferrous Alloys NonFerrous Alloys • Cu Alloys • Al Alloys
Brass: Zn is subst. impurity
(costume jewelry, coins,
corrosion resistant)
Bronze
: Sn, Al, Si, Ni are
subst. impurity
(bushings, landing
gear)
Cu-Be :
precip. hardened
for strength
• Al Alloys
-lower r
: 2.7g/cm3
-Cu, Mg, Si, Mn, Zn additions
-solid sol. or precip.
strengthened (struct.
aircraft parts
& packaging)
NonFerrous
Alloys
• Mg Alloys
-very low r
: 1.7g/cm3
-ignites easily -
aircraft, missiles
• Ti Alloys
-lower r
: 4.5g/cm3
vs 7.9 for steel
-reactive at high T -
space applic.
• Refractory metals
-high melting T
-Nb, Mo, W, Ta
• Noble metals
-Ag, Au, Pt -
oxid./corr. resistant
Based on discussion and data provided in Section 11.3, Callister 7e.

41. Heat Treatments Annealing Quenching Tempered Martensite a) b) c) TE
800
Austenite (stable) a) b)
Annealing TE
T(°C) A
Quenching P
600
Tempered Martensite B
400 A
100%
Adapted from Fig , Callister 7e.
50% 0% c) 0%
200
M + A
50%
M + A
90% -1 3 5 10 10 10 10
time (s)

42. Summary • Time Temperature Transformation diagrams
• Diffusive & Diffusion-less transformations
• How to design different microstructures without changing
composition of steel.
• Steels: increase TS, Hardness (and cost) by adding
--C (low alloy steels)
--Cr, V, Ni, Mo, W (high alloy steels)
--ductility usually decreases w/additions.
• Non-ferrous:
--Cu, Al, Ti, Mg, Refractory, and noble metals. .