Chapter 10
Phase
Transformations
in Metals Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 1
Why do we study phase transformations? Tensile strength of an Fe-C alloy of eutectoid
composition can be varied between 700-2000 MPa depending on HT process adopted.
Desirable mechanical properties of a material can be obtained as a result of phase transformations using the right HT process.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 2
Why do we study phase transformations? In order to design a HT for some alloy with
desired RT properties, time & temperature dependencies of some phase transformations can be represented on modified phase diagrams.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 3
Why do we study phase transformations? Based on this, we will learn:
A. Phase transformations in metals
B. Microstructure & property dependence in Fe-C alloy system
C. Precipitation Hardening, Crystallization, Melting, and Glass Transition
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 4
Topics to be covered: Transformation rate
Kinetics of Phase Transformation
Nucleation: homogeneous, heterogeneous
Free Energy, Growth
Isothermal Transformations (TTT diagrams)
Pearlite, Martensite, Spheroidite, Bainite
Continuous Cooling
Mechanical Behavior
Precipitation Hardening Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 5
Phase Transformations (PT) Phase transformations: change in number and / or character of phases
Simple diffusion-dependent PT No change in # of phases
No change in composition
Example: solidification of a pure metal, allotropic transformation, re-crystallization, grain growth
More complicated diffusion-dependent PT Change in # of phases
Change in composition
Example: eutectoid reaction
Diffusion-less PT Example: meta-stable phase : martensite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 6
Phase Transformations -Stages
Most phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 7
Phase Transformations -Stages
Nucleation: process whereby seeds act as templates for crystal growth
1. Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable super-cooling (typically 80-300°C).
2. Heterogeneous nucleation - form at structural in-homogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable “nucleating surface” is already present; requires slight super-cooling (0.1-10°C).
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 8
Supercooling During the cooling of a liquid, solidification
(nucleation) will begin only after temperature has been lowered below the equilibrium solidification (melting) temperature Tm. This phenomenon is termed super-cooling or under-cooling.
The driving force to nucleate increases as T increases
Small super-cooling slow nucleation rate - few nuclei - large crystals
Large super-cooling rapid nucleation rate - many nuclei - small crystals
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 9
Kinetics of Solid State Reactions Transformations involve diffusion which depends on
time.
Time is necessary for the energy increase associated with phase boundaries between parent & product phases.
Nucleation, growth of nuclei, formation of grains & grain boundaries and establishment of equilibrium take time.
Transformation rate is a function of time
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 10
Kinetics of Solid State Reactions The fraction of reaction completed is measured as a
function of time at constant T.
Tranformation progress can be measured by microscopic examination or measuring a physical property (conductivity).
The obtained data is plotted as fraction of transformation versus logarithm of time.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 11
2
• Fraction transformed depends on time.
• Transformation rate depends on T.
• r often small: equil not possible
Fraction of Transformation
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 12
Transformations & Undercooling
For transformation to occur, must
cool to below 727°C
Eutectoid transformation (Fe-Fe3C system):
g a + Fe3C
0.76 wt% C 0.022 wt% C
6.7 wt% C
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
400 0 1 2 3 4 5 6 6.7
L
g
(austenite)
g +L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt% C
1148°C
T(°C)
a
ferrite 727°C
Eutectoid: Equil. Cooling: Ttransf. = 727ºC
T
Undercooling by Ttransf. < 727C
0.7
6
0.0
22
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 13
Generation of Isothermal Transformation Diagrams • The Fe-Fe3C system, for Co = 0.76 wt% C
• A transformation temperature of 675°C.
100
50
0 1 10 2 10 4
T = 675°C %
tra
nsfo
rme
d
time (s)
400
500
600
700
1 10 10 2 10 3 10 4 10 5
Austenite (stable) TE (727C)
Austenite (unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 14
Coarse pearlite formed at higher temperatures – relatively soft
Fine pearlite formed at lower temperatures – relatively hard
• Transformation of austenite to pearlite:
g a a a a
a
a
pearlite growth direction
Austenite (g)
grain boundary
cementite (Fe3C)
Ferrite (a)
g
• For this transformation,
rate increases with ( T)
[Teutectoid – T ]. 675°C
(T smaller)
0
50
% p
earlite 600°C
(T larger) 650°C
100
Diffusion of C during transformation
a
a
g g
a Carbon
diffusion
Eutectoid Transformation Rate ~ T
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 15
Eutectoid Transformation Rate At T just below 727°C, very long times (on order of
105 s) are required for 50% transformation and therefore transformation rate is slow.
Transformation rate increases as T decreases
Example: at 540°C, 3 s is required for 50% completion.
This observation is in clear contradiction with the equation of
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 16
Eutectoid Transformation Rate This is because in T range of 540°C-727°C,
transformation rate is mainly controlled by the rate of pearlite nucleation and nucleation rate decreases with T increase.
Q in this equation is the activation energy for nucleation and it increases with T increase.
At lower T, the austenite decomposition is diffusion controlled and the rate behavior can be calculated using Q for diffusion which is independent of T.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 17
• Reaction rate is a result of nucleation and growth of crystals
• Examples:
Nucleation and Growth
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 18
Isothermal Transformation Diagrams
Solid curves are plotted:
one represents time
required at each
temperature for start of
transformation
the other is for
transformation
completion
Dashed curve
corresponds to 50%
completion
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 19
Isothermal Transformation Diagrams
Austenite to pearlite
transformation will
occur only if alloy is
supercooled to below
eutectoid temperature
(727˚C).
Time for process to
complete depends on
temperature.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 20
• Eutectoid iron-carbon alloy; Co = 0.76 wt% C • Begin at T > 727˚C • Rapidly cool to 625˚C and hold isothermally.
Isothermal Transformation Diagram
Austenite-to-Pearlite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 21
Transformations Involving Noneutectoid Compositions
Hypereutectoid composition – proeutectoid cementite
Consider C0 = 1.13 wt% C
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
400 0 1 2 3 4 5 6 6.7
L
g (austenite)
g +L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt%C
T(°C)
727°C T
0.7
6
0.0
22
1.1
3
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 22
Str
en
gth
Ductilit
y
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
Possible Transformations
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 23
Coarse pearlite (high diffusion rate) and (b) fine pearlite
- Smaller T:
colonies are larger
- Larger T:
colonies are smaller
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 24
10 10 3
10 5
time (s) 10
-1
400
600
800
T(°C) Austenite (stable)
200
P
B
TE A
A
Bainite: Non-Equilibrium Transformation Products
elongated Fe3C particles in a-ferrite matrix
diffusion controlled
a lathes (strips) with long rods of Fe3C
100% bainite
100% pearlite
Martensite
Cementite
Ferrite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE
25
Bainite Microstructure
Bainite: formed as a result of transformation of austenite
Bainite consists of ferrite & cementite and diffusion processes take place as a result.
This structure looks like needles or plates.
There is no proeutectoid phase in bainite.
Bainite consists of acicular (needle-like) ferrite with very small cementite particles dispersed throughout.
Carbon content is typically greater than 0.1%.
Bainite transforms to iron & cementite with sufficient time and temperature.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 26
10
Fe3C particles within an a-ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times
driving force – reduction of a-ferrite/Fe3C interfacial area
Spheroidite: Nonequilibrium Transformation
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 27
Pearlitic Steel partially transformed to Spheroidite
Friday, January 02, 2015
Dr. Mohammad Suliman Abuhaiba, PE
28
single phase
body centered tetragonal (BCT) crystal structure
BCT if C0 > 0.15 wt% C
Diffusion-less transformation
BCT few slip planes hard, brittle
% transformation depends only on T of rapid cooling
Martensite Formation
10 10 3
10 5 time (s) 10
-1
400
600
800
T(°C) Austenite (stable)
200
P
B
TE A
A
M + A M + A
M + A
0% 50% 90%
Martensite needles Austenite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 29
A micrograph of austenite that was polished flat and then allowed to transform into martensite. The different colors indicate the displacements caused when martensite forms.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 30
Isothermal Transformation Diagram
Iron-carbon alloy with eutectoid composition.
A: Austenite
P: Pearlite
B: Bainite
M: Martensite Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE
31
Other elements (Cr, Ni, Mo, Si and
W) may cause significant changes in
positions and shapes of TTT curves:
Change transition temperature
Shift nose of austenite-to-pearlite
transformation to longer times
Shift pearlite & bainite noses to
longer times (decrease critical
cooling rate)
Form a separate bainite nose
Effect of Adding Other Elements
4340 Steel
plain carbon steel
nose
Plain carbon steel: primary
alloying element is carbon.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 32
Example 1: Iron-carbon alloy with eutectoid
composition. Specify nature of final
microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete & homogeneous austenitic structure.
Treatment (a) Rapidly cool to 350 ˚C Hold for 104 seconds Quench to room temperature
Bainite, 100%
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 33
Martensite, 100%
Example 2: Iron-carbon alloy with
eutectoid composition. Specify nature of final
microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (b) Rapidly cool to 250 ˚C Hold for 100 seconds Quench to room temperature
Austenite, 100%
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 34
Bainite, 50%
Example 3: Iron-carbon alloy with
eutectoid composition. Specify nature of final
microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (c) Rapidly cool to 650˚C Hold for 20 seconds Rapidly cool to 400˚C Hold for 103 seconds Quench to room temperature
Austenite, 100%
Almost 50% Pearlite, 50% Austenite
Final: 50% Bainite, 50% Pearlite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 35
Continuous Cooling Transformation Diagrams
Isothermal heat treatments are not
the most practical due to rapidly
cooling and constant maintenance
at an elevated temperature.
Most heat treatments for steels
involve continuous cooling of a
specimen to RT.
TTT diagram (dotted curve) is
modified for a CCT diagram (solid
curve).
For continuous cooling, time
required for a reaction to begin &
end is delayed.
Isothermal curves are shifted to
longer times & lower temperatures.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 36
Moderately rapid & slow cooling curves are superimposed on a CCT diagram of a eutectoid iron-carbon alloy.
Transformation starts after a time period corresponding to intersection of cooling curve with the beginning reaction curve and ends upon crossing completion transformation curve.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE
37
CCT Diagrams
Normally bainite does not form when an alloy is continuously cooled to RT
Austenite transforms to pearlite before bainite has become possible.
Austenite-pearlite region (A---B) terminates just below the nose.
Continued cooling (below Mstart) of austenite will form martensite.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE
38
CCT Diagrams
For continuous cooling of a steel alloy there exists a critical quenching rate that represents minimum rate of quenching that will produce a totally martensitic structure.
This curve will just miss the nose where pearlite transformation begins
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 39
CCT Diagrams
CCT diagram for a 4340
steel alloy & several
cooling curves
superimposed.
This demonstrates the
dependence of final
microstructure on
transformations that
occur during cooling.
Alloying elements used to
modify critical cooling
rate for martensite are Cr,
Ni, Mo, Mn, Si and W
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 40
CCT Diagrams
Mechanical Properties: Influence of Carbon Content
C0 > 0.76 wt% C
Hypereutectoid
Pearlite (med)
Cementite (hard)
C0 < 0.76 wt% C
Hypoeutectoid
Pearlite (med)
ferrite (soft)
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 41
Mechanical Properties: Fe-C System
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 42
Martensite is hard but also very brittle so that it can not be used in most of the applications.
Any internal stress that has been introduced during quenching has a weakening effect.
Ductility and toughness of the material can be enhanced by heat treatment called tempering. This also helps to release any internal stress.
Tempered Martensite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 43
Tempering is performed by heating martensite to a T below eutectoid temperature (250°C-650°C) and keeping at that T for specified period of time.
The formation of tempered martensite is by diffusion.
Tempered Martensite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 44
Tempered martensite is less brittle than martensite; tempered at 594 °C.
Tempering reduces internal stresses caused by quenching.
The small particles are cementite; the matrix is a-ferrite. US Steel Corp.
Tempered Martensite
4340 steel
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 45
Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness.
Hardness & strength may be due to large area of phase boundary per unit volume of the material.
Phase boundary acts like a barrier for dislocaitons. The continuous ferrite phase in tempered martensite adds ductility and toughness to the material.
Tempered Martensite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 46
The size of cementite particles is important factor determining the mechanical behavior.
As the cementite particle size increases, material becomes softer and weaker.
Temperature of tempering determines the cementite particle size.
Since martensite-tempered martensite transformation involves diffusion, Increasing T will accelerate diffusion and rate of cementite particle growth and rate of softening as a result.
Tempered Martensite
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 47
Hardness as a function of carbon
concentration for steels
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 48
Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080); room temperature.
Rockwell C & Brinell Hardness
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 49
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 50
Precipitation Hardening
Strength & hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix.
Other alloys that can be precipitation hardened or age hardened:
Copper-Beryllium (Cu-Be)
Copper-Tin (Cu-Sn)
Magnesium-Aluminum (Mg-Al)
Aluminum-Copper (Al-Cu)
High-strength Aluminum alloys Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 51
Criteria:
Maximum solubility of 1 component in the other (M);
Solubility limit that rapidly decreases with decrease in temperature (M→N).
Process:
Solution Heat Treatment – first heat treatment where all solute atoms are dissolved to form a single-phase solid solution.
Heat to T0 and dissolve B phase.
Rapidly quench to T1
Nonequilibrium state (a phase solid solution supersaturated with B atoms; alloy is soft, weak-no ppts).
Phase Diagram for Precipitation Hardened Alloy
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 52
Supersaturated a solid solution is usually heated to an intermediate temperature T2 within a+b region (diffusion rates increase).
b precipitates (PPT) begin to form as finely dispersed particles. This process is referred to as aging.
After aging at T2, the alloy is cooled to RT
Strength & hardness of alloy depend on ppt temperature (T2) and aging time at this temperature.
Precipitation Heat Treatment –2nd stage
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 53
0 10 20 30 40 50 wt% Cu
L a+L a
a+q q
q+L
300
400
500
600
700
(Al)
T(°C)
composition range available for precipitation hardening
CuAl2
A
Precipitation Hardening Particles impede dislocation
motion
Ex: Al-Cu system
Procedure: Pt A: solution heat treat (get a
solid solution)
Pt B: quench to RT (retain a
solid solution)
Pt C: reheat to nucleate small
q particles within a phase.
Temp.
Time
Pt A (solution heat treat)
B
Pt B
C
Pt C (precipitate q)
At RT the stable state of an Al-
Cu alloy is AL-rich solid
solution (α) and an intermetallic
phase with a tetragonal crystal
structure having nominal
composition CuAl2 (θ).
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 54
Precipitation Heat Treatment – the 2nd stage
PPT behavior is represented in the diagram:
With increasing time, hardness increases, reaching a maximum, then decreasing in strength.
Reduction in strength & hardness after long periods is overaging (continued particle growth)
Small solute-enriched regions in a solid solution where the lattice is identical or somewhat perturbed from that of the solid solution are called Guinier-Preston zones.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 55
24
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
• Result: y ~
1
S
Precipitation Strengthening
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 56
Several stages in the formation of equilibrium PPT (q) phase. (a) supersaturated a solid solution; (b) transition (q”) PPT phase; (c) equilibrium q phase within the a matrix phase.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 57
• 2014 Al Alloy: • TS peak with precipitation time.
• Increasing T accelerates
process.
Influence of Precipitation Heat Treatment on Tensile Strength (TS), %EL
precipitation heat treat time
tensile
str
ength
(M
Pa)
200
300
400
100 1min 1h 1day 1mo 1yr
204°C 149°C
• %EL reaches minimum with precipitation time.
%E
L (
2 in s
am
ple
) 10
20
30
0 1min 1h 1day 1mo 1yr
204°C 149 °C
precipitation heat treat time
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 58
Effects of Temperature
Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 59
Aluminum rivets Alloys that experience significant
precipitation hardening at room temp and after short periods must be quenched to and stored under refrigerated conditions.
Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal RT.
Friday, January 02, 2015 Dr. Mohammad Suliman Abuhaiba, PE 60