Chapter 10:Phase Transformations
ISSUES TO ADDRESS ...• Transforming one phase into another takes time.
• How does the rate of transformation depend ontime and T?
Fe
γ(Austenite)
Eutectoid transformation
C FCC
Fe3C (cementite)
α (ferrite)
+(BCC)
• How can we slow the transformation so thatwe can engineer non-equilibrium structures?
• Are the properties of non-equilibriumstructures improved?
Fraction of Transformation
• Fraction transformed depends on time.
Adapted from Fig. 10.1, Callister 6e.
y
log (t)
Fixed T
0
0.5
1
t0.5
fraction transformed time
y = 1− e−ktn
Avrami Eqn.
• Transformation rate depends on T.
1 10 102 1040
50
100 135°
C11
9°C
113°
C10
2°C
88°C
43°Cy (%)
log (t) min
Ex: recrystallization of Cu
r = 1t
0.5
= Ae−Q /RT
activation energy
• r often small: equil not possible!
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.)
Transformations and Undercooling
• Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)
• Eutectoid transf. (Fe-C System):
αferrite
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6
.7
L
γaustenite
γ+L
γ+Fe3C
Fe3C
cementiteα+Fe3C
α+γ
L+Fe3C
(Fe) Co, wt% C
Eutectoid:
0.7
7
727°C
T(°C)
∆T
0.0
22
γ ⇒ α +Fe3C0.77wt%C
0.022wt%C6.7wt%C
Undercooling by ∆T: Ttransf. < 727ºC
Equil. cooling: Ttransf. = 727ºC
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.)
Eutectoid Transformation Rate ~ ∆T
γαααα
α
α
pearlite growth direction
Austenite (γ) grain boundary
cementite (Fe3C)
ferrite (α)
γ
Diffusive flow of C needed
α
αγ γ
α
• Growth of pearlite from austenite:
• Reaction rateincreases with∆T.
Adapted from Fig. 9.13, Callister 6e.
Adapted from Fig. 10.3, Callister 6e.
675°C (∆T smaller)
1 10 102 103time (s)
0
50
100
y (%
pe
arl
ite
) 0
50
100
600°C (∆T larger)
650°C
% a
ust
en
ite
Nucleation and Growth
• Reaction rate is a result of nucleation and growthof crystals.
• Examples:
% Pearlite
0
50
100
Nucleation regime
Growth regime
log (time)t50
Nucleation rate increases w/ ∆T
Growth rate increases w/ T
Nucleation rate high
T just below TE T moderately below TE T way below TENucleation rate low
Growth rate high
γ γ γ
pearlite colony
Nucleation rate med .Growth rate med. Growth rate low
Adapted fromFig. 10.1, Callister 6e.
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.)400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
100
50
01 102 104
T=675°C
y,
% t
ran
sfo
rme
d
time (s)
time (s)
isothermal transformation at 675°C
Ex: Cooling History Fe-C System
• Eutectoid composition, Co = 0.77 wt%C• Begin at T > 727 °C• Rapidly cool to 625 °C 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.)
1 10 102 103 104 105 time (s)
500
600
700
T(°C)
γ γ
γγγγ
γ
Austenite (stable)
Pearlite
0%pearlite
100%
50%
TE (727°C)
Pearlite Morphology
10
µm
• Ttransf just below TE--Larger T: diffusion is faster--Pearlite is coarser.
Two cases:• Ttransf well below TE
--Smaller T: diffusion is slower--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.)
- Larger ∆T: colonies are smaller
- Smaller ∆T: colonies are larger
Non-Equil. Transformation Products: Fe-C
Bainite reaction rate:
rbainite = e−Q /RT
• Bainite:--α lathes (stripes) with long
rods of Fe3C--diffusion controlled.
• Isothermal Transf. Diagram
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.)
(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.)
Fe3C
(cementite)
5 µm
α (ferrite)
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
100% bainite
pearlite/bainite boundary100% pearlite
A
A
Other Products: Fe-C System (1)
60 µm
α(ferrite)
Fe3C
(cementite)
• Spheroidite:--α crystals with spherical Fe3C--diffusion dependent.--heat bainite or pearlite for long times--reduces interfacial area (driving force)
• Isothermal Transf. Diagram
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.)
(Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United States Steel Corporation, 1971.)
10 103 105time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
A
A
Spheroidite100% spheroidite
100% spheroidite
Other Products: Fe-C System (2)
• Martensite:--γ(FCC) to Martensite (BCT)
Adapted from Fig. 10.13, Callister 6e.
(Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United States Steel Corporation.)
• Isothermal Transf. Diagram
xx x
xx
xpotential C atom sites
Fe atom sites
(involves single atom jumps)
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
S
M + AM + A
M + A
0%50%90%
Martentite needlesAustenite
60
µm
• γ to M transformation..-- is rapid!-- % transf. depends on T only.
(Adapted from Fig. 10.11, Callister, 6e.
Time-Temperature-Transformation (TTT) Diagrams
Isothermal TTT Diagram Continuous Cooling TTT Diagram
TTT Diagrams for a Eutectoid Steel
Cooling Ex: Fe-C System (1)
Adapted from Fig. 10.15, Callister 6e.
• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
0%
100%50%
A
S
M + AM + AM + A
0%50%90%
100% Bainite
A
100%A 100%B
Case I
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
Cooling Ex: Fe-C System (2)
• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
0% 100%50%
A
S
M + AM + AM + A
0%50%90%
M + trace of A
A
100%A
Case II
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
Adapted from Fig. 10.15, Callister 6e.
Cooling Ex: Fe-C System (3)
Adapted from Fig. 10.15, Callister 6e.
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800T(°C)
Austenite (stable)
200
P
B
0%
100%50%
A
S
M + AM + AM + A
0%50%90%
50%P, 50%B
A
50%P, 50%A
50%P, 50%A
100%A
50%P, 50%B
Case III
Mechanical Prop: Fe-C System (1)
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.)
Adapted from Fig. 9.27,Callister6e. (Fig. 9.27 courtesy Republic Steel Corporation.)
Adapted from Fig. 9.30,Callister 6e. (Fig. 9.30 copyright 1971 by United States Steel Corporation.)
• Effect of wt%C
• More wt%C: TS and YS increase, %EL decreases.wt%C
0 0.5 10
50
100%EL
Imp
ac
t e
ne
rgy
(Izo
d, f
t-lb
)
0
40
80
300
500
700
900
1100YS(MPa)TS(MPa)
wt%C0 0.5 1
hardness
0.7
7
0.7
7
Co>0.77wt%C
Hypereutectoid
Co<0.77wt%C
Hypoeutectoid
Pearlite (med)ferrite (soft)
Pearlite (med)Cementite
Hypo HyperHypo Hyper
(hard)
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.)
• Fine vs coarse pearlite vs spheroidite
• Hardness: fine > coarse > spheroidite • %AR: fine < coarse < spheroidite
80
160
240
320
wt%C0 0.5 1
Bri
ne
ll h
ard
ne
ss
fine pearlite
coarse pearlitespheroidite
0
30
60
90
wt%C0 0.5 1
Du
cti
lity
(%A
R)
fine pearlite
coarse pearlite
spheroidite
Hypo Hyper Hypo Hyper
Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
• Hardness: fine pearlite << 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.)
0
200
wt%C0 0.5 1
400
600
Bri
ne
ll h
ard
ne
ss martensite
fine pearlite
Hypo Hyper
Tempering Martensite
• reduces brittleness of martensite,• reduces internal stress caused by quenching.
Adapted from Fig. 10.24, Callister 6e. (Fig. 10.24 copyright by United States Steel Corporation, 1971.)
Adapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig. furnished courtesy of Republic Steel Corporation.)
• decreases TS, YS but increases %AR
YS(MPa)TS(MPa)
800
1000
1200
1400
1600
1800
30
4050
60
200 400 600Tempering T (°C)
%AR
TS
YS
%AR
9 µ
m
• produces extremely small Fe3C particles surrounded by α.
Summary: Processing Options
Austenite (γ)
Bainite (α + Fe3C plates/needles)
Pearlite (α + Fe3C layers + a
proeutectoid phase)
Martensite (BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine
Fe3C particles)
slow cool
moderate cool
rapid quench
reheat
Str
en
gth
Du
cti
lity
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
Adapted from Fig. 10.27, Callister 6e.
Chapter 11: Metal AlloysApplications and Processing
ISSUES TO ADDRESS...
• How are metal alloys classified and how are they used?
• What are some of the common fabrication techniques?
• How do properties vary throughout a piece of materialthat has been quenched, for example?
• How can properties be modified by post heat treatment?
Taxonomy of Metals
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.)
Adapted from Fig. 11.1, Callister 6e.
Fe3C
cementite
Metal Alloys
Steels
Ferrous Nonferrous
Cast Irons Cu Al Mg Ti<1.4wt%C 3-4.5wt%C
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γaustenite
γ+L
γ+Fe3Cα
ferriteα+Fe3C
α+γ
L+Fe3C
δ
(Fe) Co, wt% C
Eutectic:
Eutectoid:0.77
4.30
727°C
1148°C
T(°C)
Steels<1.4wt%C
Cast Irons3-4.5wt%C
microstructure: ferrite, graphite cementite
Steels
Low Alloy High Alloy
low carbon <0.25wt%C
med carbon 0.25-0.6wt%C
high carbon 0.6-1.4wt%C
Uses auto struc. sheet
bridges towers press. vessels
crank shafts bolts hammers blades
pistons gears wear applic.
wear applic.
drills saws dies
high T applic. turbines furnaces V. corros. resistant
Example 1010 4310 1040 4340 1095 4190 304
Additions noneCr,V Ni, Mo
noneCr, Ni Mo
noneCr, V, Mo, W
Cr, Ni, Mo
plain HSLA plainheat
treatableplain tool
austentitic stainless
Name
Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++
increasing strength, cost, decreasing ductilityBased on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e.
Basic Ideas in Alloying Steels:1. Phase Partitioning at Austenite/Pearlite Interface
Carbide FormersV, Ti, Nb
Ferrite Formers (solid solution)Ni, Si, Mn
Paritioning at the Austenite/Pearlite InterfaceSlows transformationAllows Bainite or Martensite to form on cooling
Basic Ideas in Alloying Steels:2. Alloying to Control the Eutectoid Transformation
AlloyingControl ‘Nose’ in TTT DiagramControl Eutectoid Temperature and C Composition
Cr: added (~8 – 12wt %) to make steel ‘stainless’Ni: High concentrations to stabilize austenite – austenitic steels
Hardenability of Steels
• Ability to form martensite• Jominy end quench test to measure hardenability.
• Hardness versus distance from the quenched end.
24°C water
specimen (heated to γ phase field)
flat ground
4”
1”
Ha
rdn
ess
, HR
C
Distance from quenched end
Adapted from Fig. 11.10, Callister 6e. (Fig. 11.10 adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.)
Adapted from Fig. 11.11, Callister 6e.
Why Hardness Changes with Position
• The cooling rate varies with position.
Adapted from Fig. 11.12, Callister 6e. (Fig. 11.12 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.)
distance from quenched end (in)Ha
rdn
ess
, HR
C
20
40
60
0 1 2 3
600
400
200A → M
A → P
Martensite
Martensite + Pearlite
Fine Pearlite
Pearlite
0.1 1 10 100 1000
T(°C)
M(start)
Time (s)
0
0%100%
M(finish)
Hardenability vs. Alloy Content
• Jominy end quenchresults, C = 0.4wt%C
• "Alloy Steels"(4140, 4340, 5140, 8640)--contain Ni, Cr, Mo
(0.2 to 2wt%)--these elements shift
the "nose".--martensite is easier
to form.
Adapted from Fig. 11.13, Callister 6e. (Fig. 11.13 adapted from figure furnished courtesy Republic Steel Corporation.)
T(°C)
10-1 10 103 1050
200
400
600
800
Time (s)
M(start)M(90%)
TE
A Bshift from A to B due to alloying
Cooling rate (°C/s)
Ha
rdn
ess
, HR
C
20
40
60
100 20 30 40 50Distance from quenched end (mm)
210100 3
4140
8640
5140
1040
50
80
100
%M4340
Quenching Medium and Geometry
• Effect of quenching medium:
Mediumairoil
water
Hardnesssmall
moderatelarge
Severity of Quenchsmall
moderatelarge
• Effect of geometry:When surface-to-volume ratio increases:
--cooling rate increases--hardness increases
Positioncenter
surface
Cooling ratesmalllarge
Hardnesssmalllarge
Nonferrous Alloys
NonFerrous Alloys
• Cu AlloysBrass: 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 ρ: 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.
aircraft parts & packaging)
• Mg Alloys-very low ρ: 1.7g/cm3 -ignites easily -aircraft, missles
• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals
-Ag, Au, Pt -oxid./corr. resistant
• Ti Alloys-lower ρ: 4.5g/cm3
vs 7.9 for steel -reactive at high T -space applic.
Based on discussion and data provided in Section 11.3, Callister 6e.
Metal Fabrication Methods (1)
CASTING JOININGFORMING
Ao Ad
force
dieblank
force
• Forging(wrenches, crankshafts)
• Rolling(I-beams, rails)
• Extrusion(rods, tubing)
Adapted from Fig. 11.7, Callister 6e.
ram billet
container
containerforce
die holder
die
Ao
Adextrusion
roll
AoAd
roll
• Drawing(rods, wire, tubing)
often atelev. T
tensile force
AoAddie
die
Forming Temperature
• Hot working--recrystallization--less energy to deform--oxidation: poor finish--lower strength
• Cold working--recrystallization--less energy to deform--oxidation: poor finish--lower strength
• Cold worked microstructures--generally are very anisotropic!--Forged --Fracture resistant!
Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.), John Wiley and Sons, Inc., 1996. (a) Fig. 10.5, p. 410 (micrograph courtesy of G. Vander Voort, Car Tech Corp.); (b) Fig. 10.6(b), p. 411 (Orig. source: J.F. Peck and D.A. Thomas, Trans. Metall. Soc. AIME, 1961, p. 1240); (c) Fig. 10.10, p. 415 (Orig. source: A.J. McEvily, Jr.and R.H. Bush, Trans. ASM 55, 1962, p. 654.)
(a) (b) (c)
--Swaged
Metal Fabrication Methods (2)
plasterdie formedaround waxprototype
FORMING JOININGCASTING• Sand Casting
(large parts, e.g.,auto engine blocks)
Sand Sand
molten metal
• Investment Casting(low volume, complex shapese.g., jewelry, turbine blades)
wax
• Die Casting(high volume, low T alloys)
• Continuous Casting(simple slab shapes)
molten
solidified
Metal Fabrication Methods (3)
CASTINGFORMING JOINING• Powder Processing
(materials w/low ductility)
pressure
heat
point contact at low T
densification by diffusion at higher T
area contact
densify
• Welding(when one large part isimpractical)
• Heat affected zone:(region in which themicrostructure has beenchanged).
Adapted from Fig. 11.8, Callister 6e.(Fig. 11.8 from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.)
piece 1 piece 2
fused base metal
filler metal (melted)base metal (melted)
unaffectedunaffectedheat affected zone
Thermal Processing of Metals
Annealing: Heat to Tanneal, then cool slowly.
Types of Annealing
• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)
• Stress Relief: Reduce stress caused by:
-plastic deformation -nonuniform cooling -phase transform.
• Normalize (steels): Deform steel with large grains, then normalize to make grains small.
• Full Anneal (steels): Make soft steels for good forming by heating to get γ, then cool in furnace to get coarse P.
• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for
15-25h.
Based on discussion in Section 11.7, Callister 6e.
Precipitation Hardening
• Particles impede dislocations.• Ex: Al-Cu system• Procedure:
--Pt A: solution heat treat(get α solid solution)
--Pt B: quench to room temp.--Pt C: reheat to nucleate
small θ crystals withinα crystals.
• Other precipitationsystems:• Cu-Be• Cu-Sn• Mg-Al
Pt A (sol’n heat treat)
Pt B
Pt C (precipitate θ)
Temp.
Time
Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)
Adapted from Fig. 11.20, Callister 6e.
300
400
500
600
700
0 10 20 30 40 50wt%Cu(Al)
Lα+Lα
α+θθ
θ+L
T(°C)
A
B
C
composition range needed for precipitation hardening
CuAl2
Precipitate Effect on TS, %El
• 2014 Al Alloy:
• TS peaks withprecipitation time.
• Increasing T acceleratesprocess.
• %EL reaches minimumwith precipitation time.
precipitation heat treat time (h)
ten
sile
str
en
gth
(M
Pa
)
300
400
500
2001min 1h 1day 1mo 1yr
204°C
149°C
non-
equi
l. so
lid s
olut
ion
man
y sm
all
prec
ipita
tes
“ag
ed”
fe
wer
larg
e
pre
cipi
tate
s
“ove
rage
d”%
EL
(2
in s
am
ple
)10
20
30
0 1min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time (h)
Adapted from Fig. 11.25 (a) and (b), Callister 6e. (Fig. 11.25 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.)
SUMMARY
• 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.
• Fabrication techniques:--forming, casting, joining.
• Hardenability--increases with alloy content.
• Precipitation hardening--effective means to increase strength in
Al, Cu, and Mg alloys.