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Chapter 11 Phase Transformatio ns Fe 3 C (cementite)- orthorhombic Martensite - BCT Austenite - FCC Ferrite - BCC

Phase transformation physical metallurgy

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Page 1: Phase transformation physical metallurgy

Chapter 11

Phase Transformations

Fe3C (cementite)- orthorhombic

Martensite - BCT

Austenite - FCC

Ferrite - BCC

Page 2: Phase transformation physical metallurgy

Phase Transformations• 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

Page 3: Phase transformation physical metallurgy

Phase Transformations

Phase 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,

recrystallization, grain growth

More complicated diffusion-dependent Change in # of phases Change in composition Example: eutectoid reaction

Diffusionless Example: metastable phase - martensite

Page 4: Phase transformation physical metallurgy

Phase Transformations 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.

• Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (typically 80-300°C).

• Heterogeneous nucleation - form at structural inhomogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable “nucleating surface” is already present; requires slight supercooling (0.1-10ºC).

Page 5: Phase transformation physical metallurgy

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 supercooling (or undercooling.

The driving force to nucleate increases as T increases

Small supercooling slow nucleation rate - few nuclei - large crystals

Large supercooling rapid nucleation rate - many nuclei - small crystals

Page 6: Phase transformation physical metallurgy

Nucleation of a spherical solid particle in a liquid

Liquid

The change in free energy G (a function of the internal energy and enthalpy of the system) must be negative for a transformation to occur.

Assume that nuclei of the solid phase form in the interior of the liquid as atoms cluster together-similar to the packing in the solid phase.

Also, each nucleus is spherical and has a radius r.

Free energy changes as a result of a transformation: 1) the difference between the solid and liquid phases (volume free energy, GV); and 2) the solid-liquid phase boundary (surface free energy, GS).

Transforming one phase into another takes time.

G = GS + GV

Fe

(Austenite)

Eutectoid transformation

C FCC

Fe3C(cementite)

(ferrite)

+(BCC)

Page 7: Phase transformation physical metallurgy

r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy)

Homogeneous Nucleation & Energy Effects

GT = Total Free Energy = GS + GV

Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)

24 rGS

= surface tension

Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)

GrGV3

3

4

volume unit

energy free volume G

Page 8: Phase transformation physical metallurgy

Solidification

TH

Tr

f

m

2*

Note: Hf and are weakly dependent on T

r* decreases as T increases

For typical T r* ~ 10 nm

Hf = latent heat of solidification (fusion)

Tm = melting temperature

= surface free energy

T = Tm - T = supercooling

r* = critical radius

Page 9: Phase transformation physical metallurgy

Transformations & Undercooling

• For transformation to occur, must cool to below 727°C

• Eutectoid transformation (Fe-Fe3C system): + 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

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) C, wt% C

1148°C

T(°C)

ferrite

727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºC

T

Undercooling by Ttransf. < 727C

0.7

6

0.0

22

Page 10: Phase transformation physical metallurgy

2

• Fraction transformed depends on time.

fraction transformed time

y1 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

Page 11: Phase transformation physical metallurgy

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

% tr

ansf

orm

ed

time (s)

400

500

600

700

1 10 102 103 104 105

0%pearlite

100%

50%

Austenite (stable) TE (727C)Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

Consider:

Page 12: Phase transformation physical metallurgy

Coarse pearlite formed at higher temperatures – relatively soft

Fine pearlite formed at lower temperatures – relatively hard

• Transformation of austenite to pearlite:

pearlite growth direction

Austenite ()grain boundary

cementite (Fe3C)

Ferrite ()

• For this transformation, rate increases with ( T) [Teutectoid – T ].

675°C (T smaller)

0

50

% p

earli

te600°C

(T larger)650°C

100

Diffusion of C during transformation

Carbon diffusion

Eutectoid Transformation Rate ~ T

Page 13: Phase transformation physical metallurgy

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

pearlite colony

Nucleation rate med Growth rate med. Growth rate low

Nucleation and Growth

Page 14: Phase transformation physical metallurgy

Isothermal Transformation Diagrams2 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.

Page 15: Phase transformation physical metallurgy

• 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

Page 16: Phase transformation physical metallurgy

Transformations Involving Noneutectoid Compositions

Hypereutectoid composition – proeutectoid cementite

Consider C0 = 1.13 wt% C

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) C, wt%C

T(°C)

727°CT

0.7

6

0.0

22

1.13

Page 17: Phase transformation physical metallurgy

Str

engt

h

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Possible Transformations

Page 18: Phase transformation physical metallurgy

Coarse pearlite (high diffusion rate) and (b) fine pearlite

- Smaller T: colonies are larger

- Larger T: colonies are smaller

Page 19: Phase transformation physical metallurgy

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-Equil Transformation Products elongated Fe3C particles in -ferrite matrix diffusion controlled lathes (strips) with long rods of Fe3C

100% bainite

100% pearlite

Martensite

Cementite

Ferrite

Page 20: Phase transformation physical metallurgy

Bainite Microstructure

• 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 (considered semi-stable below 150°C).

Page 21: Phase transformation physical metallurgy

10

Fe3C particles within an -ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force – reduction of -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

Page 22: Phase transformation physical metallurgy

Pearlitic Steel partially transformed to Spheroidite

Page 23: Phase transformation physical metallurgy

single phase body centered tetragonal (BCT) crystal structure BCT if C0 > 0.15 wt% C

Diffusionless transformation BCT few slip planes hard, brittle % transformation depends only on T of rapid cooling

Martensite Formation

• Isothermal Transformation Diagram

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

Page 24: Phase transformation physical metallurgy

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.

Page 25: Phase transformation physical metallurgy

Isothermal Transformation Diagram

Iron-carbon alloy with eutectoid composition.

A: AusteniteP: PearliteB: BainiteM: Martensite

Page 26: Phase transformation physical metallurgy

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.

Page 27: Phase transformation physical metallurgy

Example 11.2: 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 (a) Rapidly cool to 350 ˚C Hold for 104 seconds Quench to room temperature

Bainite, 100%

Page 28: Phase transformation physical metallurgy

Martensite, 100%

Example 11.2: 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%

Page 29: Phase transformation physical metallurgy

Bainite, 50%

Example 11.2: 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 (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

Page 30: Phase transformation physical metallurgy

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.

Page 31: Phase transformation physical metallurgy

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.

Page 32: Phase transformation physical metallurgy

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

Page 33: Phase transformation physical metallurgy

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.

Page 34: Phase transformation physical metallurgy

Mechanical Properties

• Hardness• Brinell, Rockwell• Yield Strength• Tensile Strength• Ductility• % Elongation• Effect of Carbon Content

Page 35: Phase transformation physical metallurgy

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)

Page 36: Phase transformation physical metallurgy

Mechanical Properties: Fe-C System

Page 37: Phase transformation physical metallurgy

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 -ferrite. US Steel Corp.

Tempered Martensite

4340 steel

Page 38: Phase transformation physical metallurgy

Hardness as a function of carbon concentration for steels

Page 39: Phase transformation physical metallurgy

Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080); room temperature.

Rockwell C and Brinell Hardness

Page 40: Phase transformation physical metallurgy
Page 41: Phase transformation physical metallurgy

Precipitation Hardening

• The 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

Page 42: Phase transformation physical metallurgy

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 ( phase solid solution supersaturated with B atoms; alloy is soft, weak-no ppts).

Phase Diagram for Precipitation Hardened Alloy

Page 43: Phase transformation physical metallurgy

The supersaturated solid solution is usually heated to an intermediate temperature T2 within the region (diffusion rates increase).

The 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

Page 44: Phase transformation physical metallurgy

0 10 20 30 40 50wt% Cu

L+L

+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 solid solution)-- Pt C: reheat to nucleate small particles within phase.

Temp.

Time

-- Pt A: solution heat treat (get solid solution)

Pt A (solution heat treat)

B

Pt B

C

Pt C (precipitate

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).

Page 45: Phase transformation physical metallurgy

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.

Page 46: Phase transformation physical metallurgy

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: y ~

1S

PRECIPITATION STRENGTHENING

Page 47: Phase transformation physical metallurgy

Several stages in the formation of the equilibrium PPT () phase. (a)supersaturated solid solution; (b)transition (”) PPT phase; (c)equilibrium phase within the matrix phase.

Page 48: Phase transformation physical metallurgy

• 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.

%E

L (2

in s

ampl

e)10

20

30

0 1min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time

Page 49: Phase transformation physical metallurgy

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.

Page 50: Phase transformation physical metallurgy

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 room temperature.