power piont ch2 phase-transformation-in-metals (1)
TRANSCRIPT
Chapter 1
Phase Transformations in
MetalsFe3C (cementite)- orthorhombic
Martensite - BCT
Austenite - FCC
Ferrite - BCC
Why do we study phase transformations? The 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 HTprocess.
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.
Based on this, we will learn: A. Phase transformations in metals B. Microstructure and property dependence in Fe-C alloy system C. Precipitation Hardening
Topics to be covered:Transformation rateKinetics of Phase Transformation
Nucleation: homogeneous, heterogeneous
Free Energy, GrowthIsothermal Transformations (TTT diagrams)Pearlite, Martensite, Spheroidite, BainiteContinuous CoolingMechanical BehaviorPrecipitation Hardening
Phase TransformationsPhase transformations – change in the number or
character of phases. Simple diffusion-dependent
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 Change in # of phases Change in composition Example: eutectoid reaction
Diffusion-less Example: meta-stable phase : martensite
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.
Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth.
1. Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (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 (0.1-10ºC).
Supercooling During the cooling of a liquid, solidification (nucleation)
will begin only after the temperature has been lowered below the equilibrium solidification (or 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
Kinetics of Solid State ReactionsTransformations involving diffusion depend on time.Time is also necessary for the energy increase associated with
the phase boundaries between parent and product phases.Moreover, nucleation, growth of the nuclei, formation of grains
and grain boundaries and establishment of equilibrium take time.
As a result we can say the transformation rate is a function of time.
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 (e.g., conductivity).
The obtained data is plotted as fraction of the transformation versus logarithm of time.
2
• Fraction transformed depends on time.
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 = 1t0.5
= Ae−Q /RT
activation energy
• r often small: equil not possible
y
log (t)
Fixed T
0
0.5
1
t0.5
FRACTION OF TRANSFORMATION
Percent recrystallization as a function of time and at constant temperature for pure copper.
Transformations & Undercooling
For transformation to occur, must cool to below 727°C
Eutectoid transformation (Fe-Fe3C system):γ ⇒ α + Fe3C
0.76 wt% C0.022 wt% C
6.7 wt% C
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ +Fe3C
α +Fe3C
L+Fe3C
δ
(Fe) C, wt% C
1148°C
T(°C)
αferrite
727°C
Eutectoid:Equil. Cooling: Ttransf. = 727ºC
∆T
Undercooling by Ttransf. < 727°C
0.7
6
0.0
22
Generation of Isothermal Transformation Diagrams• The Fe-Fe3C system, for Co = 0.76 wt% C• A transformation temperature of 675°C.
100
50
01 102 104
T = 675°C%
tra
nsf
orm
ed
time (s)
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Coarse pearlite formed at higher temperatures – relatively soft
Fine pearlite formed at lower temperatures – relatively hard
• Transformation of austenite to pearlite:
γαααα
α
α
pearlite growth direction
Austenite (γ)grain boundary
cementite (Fe3C)
Ferrite (α)
γ
• For this transformation, rate increases with ( ∆T) [Teutectoid – T ].
675°C
(∆T smaller)
0
50
% p
earli
te
600°C
(∆T larger)650°C
100
Diffusion of C during transformation
α
αγ γ
αCarbon diffusion
Eutectoid Transformation Rate ~ ∆T
Eutectoid Transformation RateAt T just below 727°C, very long times (on the order of 105 s) are
required for 50% transformation and therefore transformation rate is slow.
The transformation rate increases as T decreases, for example, at 540°C 3 s is required for 50% completion.
This observation is in clear contradiction with the equation of
This is because in T range of 540°C-727°C, the transformation rate is mainly controlled by the rate of pearlite nucleation and nucleation rate decreases with increasing T. Q in this equation is the activation energy for nucleation and it increases with T increase.
It has been found that 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.
r = 1t0.5
= Ae−Q /RT
activation energy
5
• Reaction rate is a result of nucleation and growth of 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
Nucleation and Growth
Isothermal Transformation Diagrams
solid curves are plotted: one represents the time required at each
temperature for the start of the transformation;
the other is for transformation completion.
The dashed curve corresponds to 50% completion.
The austenite to pearlite transformation will occur only if the alloy is supercooled to below the eutectoid temperature (727 C).˚
Time for process to complete depends on the temperature.
• Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C• Begin at T > 727˚C• Rapidly cool to 625˚C and hold isothermally.
Isothermal Transformation Diagram
Austenite-to-Pearlite
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
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ +L
γ +Fe3C
α +Fe3C
L+Fe3C
δ
(Fe) C, wt%C
T(°C)
727°C∆T
0.7
6
0.0
22
1.13
Str
eng
th
Duc
tility
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
Possible Transformations
Coarse pearlite (high diffusion rate) and (b) fine pearlite
- Smaller ∆T: colonies are larger
- Larger ∆T: colonies are smaller
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
A
A
Bainite: Non-Equilibrium Transformation Products
elongated Fe3C particles in α-ferrite matrix diffusion controlled α lathes (strips) with long rods of Fe3C
100% bainite
100% pearlite
Martensite
Cementite
Ferrite
Bainite Microstructure
Bainite: formed as a result of transformation of austenite.
Bainite consists of ferrite and 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.
The carbon content is typically greater than 0.1%.Bainite transforms to iron and cementite with
sufficient time and temperature.
10
Fe3C particles within an α-ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force – reduction of α-ferrite/Fe3C interfacial area
Spheroidite: Nonequilibrium Transformation
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
Pearlitic Steel partially transformed to Spheroidite
Design a heat treatment to produce the pearlite structure shown in Figure
micrograph of the pearlite lamellae (x 2000)
The effect of the austenitetransformation temperature on the interlamellar spacing in pearlite.
7.14
A
B CD
E
A
B C
The Martensitic Reaction and TemperingMartensiteis : is result of a diffusionless solid-state transfomation .
•The growth rate is so high → nucleation is controlling step.• is an athermal transformation (i.e. the reaction depends only on the temperature, not on the time)
FCC austenite
upon quenching, In steels (< 0.2% C) nonequilibrium supersaturated
BCC martensite structure
In steels (>0.2% C)FCC austenite
upon quenching, BCT (body-centered tetragonal) martensite
BCT caused by the Carbon atoms in the (1/2, 0, 0) site being trapped during the transformation to the BCC
%C(martensite) = %C (the starting austenite)
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
10 103 105 time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
M + AM + A
M + A
0%50%90%
Martensite needlesAustenite
An micrograph of austenite that was polished flat and then allowed to transform into martensite. The different colors indicate the displacements caused when martensite forms.
Iron-carbon alloy with eutectoid composition.
A: Austenite P: Pearlite B: Bainite M: Martensite
Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in the positions and shapes of the TTT curves:
Change transition temperature; Shift the nose of the austenite-to-
pearlite transformation to longer times;
Shift the pearlite and bainite noses to longer times (decrease critical cooling rate);
Form a separate bainite nose;
Effect of Adding Other Elements
4340 Steel
plain carbonsteel
nose
Plain carbon steel: primary alloying element is carbon.
Example 1: Iron-carbon alloy with eutectoid
composition. Specify the nature of the 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 (a) Rapidly cool to 350 ˚C Hold for 104 seconds Quench to room temperature
Bainite, 100%
Martensite, 100%
Example 2: Iron-carbon alloy with
eutectoid composition. Specify the nature of the 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%
Bainite, 50%
Example 3: Iron-carbon alloy with
eutectoid composition. Specify the nature of the 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
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 the continuous cooling of a specimen to room temperature.
TTT diagram (dotted curve) is modified for a CCT diagram (solid curve).
For continuous cooling, the time required for a reaction to begin and end is delayed.
The isothermal curves are shifted to longer times and lower temperatures.
Moderately rapid and slow cooling curves are superimposed on a continuous cooling transformation diagram of a eutectoid iron-carbon alloy.
The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve and ends upon crossing the completion transformation curve.
Normally bainite does not form when an alloy is continuously cooled to room temperature; austenite transforms to pearlite before bainite has become possible.
The austenite-pearlite region (A---B) terminates just below the nose. Continued cooling (below Mstart) of austenite will form martensite.
For continuous cooling of a steel alloy there exists a critical quenching rate that represents the minimum rate of quenching that will produce a totally martensitic structure.
This curve will just miss the nose where pearlite transformation begins
Continuous cooling diagram for a 4340 steel alloy and several cooling curves superimposed.
This demonstrates the dependence of the final microstructure on the transformations that occur during cooling.
Alloying elements used to modify the critical cooling rate for martensite are chromium, nickel, molybdenum, manganese, silicon and tungsten.
Mechanical PropertiesHardnessBrinell, RockwellYield StrengthTensile StrengthDuctility% ElongationEffect of Carbon Content
Mechanical Properties: Influence of Carbon Content
C0 > 0.76 wt% C
Hypereutectoid
Pearlite (med)
Cementite(hard)
C0 < 0.76 wt% CHypoeutectoid
Pearlite (med)
ferrite (soft)
Mechanical Properties: Fe-C System
Example: P10.37For a eutectoid steel, describe isothermal heat treatments that would be required to yield specimens having the following Brinell Hardnesses:
1. 180 HB2. 220 HB3. 500 HB
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.
The ductility and toughness of the material can be enhanced by heat treatment called tempering. This also helps to release any internal stress.
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
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 α-ferrite. US Steel Corp.
Tempered Martensite
4340 steel
Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness.
The hardness and strength may be due to large area of phase boundary per unit volume of the material.
The phase boundary acts like a barrier for dislocations. The continuous ferrite phase in tempered martensite adds ductility and toughness to the material.
The size of the cementite particles is important factor determining the mechanical behavior.
As the cementite particle size increases, material becomes softer and weaker. The temperature of tempering determines the cementite particle size. Since martensite-tempered martensite transformation involves diffusion, Increasing T will accelerate the diffusion and rate of cementite particle growth and rate of softening as a result.
Tempered Martensite
Hardness as a function of carbon concentration for steels
Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080); room temperature.
Rockwell C and Brinell Hardness
Example: P10.25Briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 steel from microstructure to another.
1. (Martensite + Ferrite + Bainite) to (Martensite + Ferrite + pearlite + Bainite)
2. (Martensite + Ferrite + Bainite) to (spheroidite)3. (Martensite + Ferrite + Bainite) to (tempered
Martensite)
Precipitation HardeningThe strength and 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
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 (α phase solid solution supersaturated with B atoms; alloy is soft, weak-no ppts).
Phase Diagram for Precipitation Hardened Alloy
The supersaturated α solid solution is usually heated to an intermediate temperature T2 within the α+β region (diffusion rates increase).
The β 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 room temperature.
Strength and hardness of the alloy depend on the ppt temperature (T2) and the aging time at this temperature.
Precipitation Heat Treatment – the 2nd stage
0 10 20 30 40 50wt% Cu
Lα+Lα
α+θθ
θ+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 B: quench to room temp. (retain α solid solution)-- Pt C: reheat to nucleate small θ particles within α phase.
Temp.
Time
-- Pt A: solution heat treat (get α solid solution)
Pt A (solution heat treat)
B
Pt B
C
Pt C (precipitate θ)
At room temperature the stable state of an aluminum-copper alloy is an aluminum-rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having nominal composition CuAl2 (θ).
Precipitation Heat Treatment – the 2nd stage PPT behavior is represented in
the diagram: With increasing time, the
hardness increases, reaching a maximum (peak), then decreasing in strength.
The reduction in strength and 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.
24
• Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum).
Large shear stress needed to move dislocation toward precipitate and shear it.
Side View
Top View
Slipped part of slip plane
Unslipped part of slip plane
S
Dislocation “advances” but precipitates act as “pinning” sites with spacing S.
precipitate
• Result: σy ~
1
S
PRECIPITATION STRENGTHENING
Several stages in the formation of the equilibrium PPT (θ) phase. (a)supersaturated α solid solution; (b)transition (θ”) PPT phase; (c)equilibrium θ phase within the α matrix phase.
• 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
tens
ile s
tren
gth
(MP
a)
200
300
400
1001min 1h 1day 1mo 1yr
204°C
non-
equi
l. so
lid s
olut
ion
man
y sm
all
prec
ipita
tes
“age
d”
few
er la
rge
prec
ipita
tes
“ove
rage
d”149°C
• %EL reaches minimum with precipitation time.
%EL
(2 in
sam
ple)
10
20
30
0 1min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time
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.
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 room temperature.