Phase transformation

77
Chapter 11 Phase Transformations Fe 3 C (cementite)- orthorhombic Martensite - BCT Austenite - FCC Ferrite - BCC

Transcript of Phase transformation

Page 1: Phase transformation

Chapter 11

Phase

TransformationsFe3C (cementite)- orthorhombic

Martensite - BCT

Austenite - FCC

Ferrite - BCC

Page 2: Phase transformation

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

In the study of phase transformations we will

be dealing with the changes that can occur

within a given system e.g.an alloy that can exist

as a mixture of one or more phases

A phase can be defined as a portion of the

system whose properties and composition are

homogeneous and which is physically distinct

from other parts of the system

The components of a system are the different

elements or chemical compound which make up

the system

Phase Transformations

Page 4: Phase transformation

Equilibrium

Any transformation that results in a decrease in Gibbs free energy is

possible

Page 5: Phase transformation

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 6: Phase transformation

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 7: Phase transformation

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 8: Phase transformation

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: 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

g(Austenite)

Eutectoid transformation

C FCC

Fe3C

(cementite)

a(ferrite)

+

(BCC)

Page 9: Phase transformation

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)

g 24 rGS

g = surface tension

Volume (Bulk) Free Energy –

stabilizes the nuclei (releases energy)

GrGV3

3

4

volume unit

energy free volume G

Page 10: Phase transformation

Effect of Temperature

Page 11: Phase transformation

Solidification

TH

Tr

f

m

g

2*

Note: Hf and g 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

g = surface free energy

T = Tm - T = supercooling

r* = critical radius

Page 12: Phase transformation

Effect of Temperature

Page 13: Phase transformation

Growth

• It begins once an embryo has exceeded the

critical size r*

• nucleation will continue to occur simultaneously

with growth

• The growth process will cease in any region

where particles of the new phase meet

• Growth occurs by long-range atomic diffusion

– diffusion through the parent phase, across a

phase boundary, and then into the nucleus.

Page 14: Phase transformation

growth rate

Page 15: Phase transformation
Page 16: Phase transformation

Computation of Critical Nucleus Radius and Activation

Free Energy

(a) For the solidification of pure gold, calculate the critical radius

r*and the activation free energy ΔG* if nucleation is homogeneous.

Values for the latent heat of fusion and surface free energy are -1.16

x109 J/m3 and 0.132 J/m2 , respectively. Use the super-cooling value

found in Table 10.1.

(b) Now calculate the number of atoms found in a nucleus of critical

size. Assume a lattice parameter of 0.413 nm for solid gold at its

melting temperature.

Page 17: Phase transformation

Transformations & Undercooling

• For transformation to occur, must

cool to below 727°C

• Eutectoid transformation (Fe-Fe3C system): g a + Fe3C

0.76 wt% C0.022 wt% C

6.7 wt% C

Fe

3C

(cem

entite

)

1600

1400

1200

1000

800

600

4000 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

ferrite727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºC

T

Undercooling by Ttransf. < 727C

0.7

6

0.0

22

Page 18: Phase transformation

18

Rate of Phase Transformation

Avrami equation => y = 1- exp (-kt n)

transformation complete

log t

Fra

ctio

n tra

nsfo

rme

d, y

Fixed T

fraction

transformed

time

0.5

By convention rate = 1 / t0.5

Fraction

transformed

depends on

time

maximum rate reached – now amount unconverted decreases so rate slows

t0.5

rate increases as surface area increases

& nuclei grow

Avrami relationship - the rate is defined as the inverse of the time to complete half of the

transformation. This describes most solid-state transformations that involve diffusion.

Page 19: Phase transformation

• In general, rate increases as T

r = 1/t0.5 = A e -Q/RT

– R = gas constant

– T = temperature (K)

– A = ‘preexponential’ rate factor

– Q = activation energy

• r is often small so equilibrium is not possible.

Arrhenius expression

Adapted from Fig. 10.11,

Callister 7e. (Fig. 10.11

adapted from B.F. Decker and

D. Harker, "Recrystallization in

Rolled Copper", Trans AIME,

188, 1950, p. 888.)

135C 119C 113C 102C 88C 43C

1 10 102 104

Temperature Dependence of

Transformation Rate

Page 20: Phase transformation

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

% t

ran

sfo

rme

d

time (s)

400

500

600

700

1 10 102 103 104 105

Austenite (stable)TE (727C)

Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

Consider:

Page 21: Phase transformation

Coarse pearlite formed at higher temperatures – relatively soft

Fine pearlite formed at lower temperatures – relatively hard

• Transformation of austenite to pearlite:

gaaaa

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

gg

aCarbon

diffusion

Eutectoid Transformation Rate ~ T

Page 22: Phase transformation

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 23: Phase transformation

• 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 24: Phase transformation

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

g (austenite)

g+L

g +Fe3C

a+Fe3C

L+Fe3C

d

(Fe) C, wt%C

T(°C)

727°CT

0.7

6

0.0

22

1.1

3

Page 25: Phase transformation

25

Transformations Involving

Noneutectoid Compositions

Hypereutectoid composition – proeutectoid cementite

Consider C0 = 1.13 wt% C

a

TE (727°C)

T(°C)

time (s)

A

A

A+

C

P

1 10 102 103 104

500

700

900

600

800

A+

P

Adapted from Fig. 11.16,

Callister & Rethwisch 3e. Adapted from Fig. 10.28,

Callister & Rethwisch 3e.

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

4000 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°CT

0.7

6

0.0

22

1.1

3

Page 26: Phase transformation
Page 27: Phase transformation

Str

en

gth

Ductilit

yMartensite

T Martensite bainite

fine pearlite coarse pearlite

spheroidite

General Trends

Possible Transformations

Page 28: Phase transformation

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

- Smaller T:

colonies are

larger

- Larger T:

colonies are

smaller

Page 29: Phase transformation

10 103

105

time (s)10

-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TEA

A

Bainite: Non-Equil 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

Page 30: Phase transformation

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 31: Phase transformation

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

Page 32: Phase transformation

Pearlitic Steel partially transformed to Spheroidite

Page 33: Phase transformation

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

TEA

A

M + AM + A

M + A

0%50%90%

Martensite needlesAustenite

Page 34: Phase transformation

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 35: Phase transformation

Martensite

The martensitic transformation occurs without composition change

The transformation occurs by shear without need for diffusion

The atomic movements required are only a fraction of the

interatomic

spacing

The amount of martensite formed is a function of the temperature to

which the sample is quenched and not of time

Hardness of martensite is a function of the carbon content

→ but high hardness steel is very brittle as martensite is brittle

Steel is reheated to increase its ductility

→ this process is called TEMPERING

Page 36: Phase transformation

Isothermal Transformation Diagram

Iron-carbon alloy

with eutectoid

composition.

A: Austenite

P: Pearlite

B: Bainite

M: Martensite

Page 37: Phase transformation

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 38: Phase transformation

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 39: Phase transformation

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 40: Phase transformation

class quiz (bonus)

1. Describe the microstructure present in a 1045 steel after each

step in the following heat treatments:

a) heat at 820°C, quench to 650°C and hold for 90s, and

quench to 25°C;

b) heat at 820°C, quench to 450°C and hold for 90s, and

quench to 25°C;

c) heat at 820°C, and quench to 25°C;

d) heat at 820°C, quench to 680°C and hold for 100s, and

quench to 25°C;

e) heat at 820°C, quench to 720°C and hold for 100s, quench

to 400°C and hold for 500 s, and quench to 25°C;

f) heat at 820°C, quench to 720°C and hold for 100s, quench

to 400°C and hold for 10 s, and quench to 25°C; and

g) heat at 820°C, quench to 25°C, heat to 500°C and hold

Page 41: Phase transformation
Page 42: Phase transformation

ALLOY STEELS

Various elements like Cr, Mn, Ni, W, Mo etc are added to plain

carbon

steels to create alloy steels

The alloys elements move the nose of the TTT diagram to the right

→ this implies that a slower cooling rate can be employed to

obtain

martensite → increased HARDENABILITY

The ‘C’ curves for pearlite and bainite transformations overlap in

the case of plain carbon steels

→ in alloy steels pearlite and bainite

transformations can be represented by separate ‘C’ curves

Page 43: Phase transformation

ROLE OF ALLOYING ELEMENTS

• + Simplicity of heat treatment and lower cost

• Low hardenability

• Loss of hardness on tempering

• Low corrosion and oxidation resistance

• Low strength at high temperatures

Plain Carbon Steel

Element Added

Solid solution

• ↑ hardenability

• Provide a fine distribution of alloy carbides during

tempering

• ↑ resistance to softening on tempering

• ↑ corrosion and oxidation resistance

• ↑ strength at high temperatures

• Strengthen steels that cannot be quenched

• Make easier to obtain the properties throughout a larger

section

• ↑ Elastic limit (no increase in toughness)

Alloying elements

• Alter temperature at

which the transformation

occurs

• Alter solubility of C in

a or g Iron

• Alter the rate of various

reactions

Interstitial

Substitutional

Page 44: Phase transformation

Austenite Pearlite

Bainite

Martensite100

200

300

400

600

500

800

Ms

Mf

t →

T

TTT diagram for Ni-Cr-Mo low alloy steel

~1 min

Page 45: Phase transformation

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 Elements4340 Steel

plain

carbon

steel

nose

Plain carbon steel: primary

alloying element is carbon.

Page 46: Phase transformation

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 (dashed 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 47: Phase transformation

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.

Page 48: Phase transformation

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 49: Phase transformation

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 50: Phase transformation

Tempering

Heat below Eutectoid temperature → wait→

slow cooling

The microstructural changes which take place

during tempering

are very complex

Time temperature cycle chosen to optimize

strength and toughness

Cementite

ORF

Ferrite

BCC

Martensite

BCTTemper

)( Ce)( )( ' 3

aa

Page 51: Phase transformation

AustenitePearlite

Pearlite + Bainite

Bainite

Martensite100

200

300

400

600

500

800

723

0.1 1 10 102 103 104 105

Eutectoid temperature

Ms

Mf

t (s) →

T

a + Fe3C

MARTEMPERING

AUSTEMPERING

To avoid residual stresses generated during quenching

Austenized steel is quenched above Ms for homogenization of temperature

across the sample

The steel is then quenched and the entire sample transforms simultaneously

Tempering follows

To avoid residual stresses generated during quenching

Austenized steel is quenched above Ms

Held long enough for transformation to Bainite

Martempering

Austempering

Page 52: Phase transformation

% Carbon →

Har

dn

ess

(R

c) →

20

40

60

0.2 0.4 0.6

Harness of Martensite as a

function of Carbon content

Properties of 0.8% C steel

Constituent Hardness (Rc) Tensile strength (MN / m2)

Coarse pearlite 16 710

Fine pearlite 30 990

Bainite 45 1470

Martensite 65 -

Martensite tempered at 250 oC 55 1990

Page 53: Phase transformation

Examples

• Unusual combinations of properties can be

obtained by producing a steel with a microstructure

containing 50% ferrite and 50% martensite. The

martensite provides strength, and the ferrite

provides ductility and toughness. Design a heat

treatment to produce a dual phase steel in which

the composition of the martensite is 0.60% C.

Page 54: Phase transformation

Mechanical Properties

• Hardness

• Brinell, Rockwell

• Yield Strength

• Tensile Strength

• Ductility

• % Elongation

• Effect of Carbon Content

Page 55: Phase transformation

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)

Page 56: Phase transformation

Mechanical Properties: Fe-C System

Page 57: Phase transformation

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

Page 58: Phase transformation

Hardness as a function of carbon

concentration for steels

Page 59: Phase transformation

Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080) that

has been rapidly quenched to form martensite.

Rockwell C and Brinell Hardness

Page 60: Phase transformation
Page 61: Phase transformation

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.

• 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 62: Phase transformation

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

Page 63: Phase transformation

The supersaturated a solid

solution is usually heated to an

intermediate temperature T2

within the ab region (diffusion

rates increase).

The 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 room temperature.

Strength and hardness of the

alloy depend on the ppt

temperature (T2) and the aging

time at this temperature.

Precipitation Heat Treatment

Page 64: Phase transformation

Solution Heat Treatment

• Heat treatable aluminum alloys gain strength from subjecting the material to a sequence of processing stepscalled solution heat treatment, quenching, and aging.

• The primary goal is to create sub-micron sized particles in the aluminum matrix, called precipitates that in turn influence the material properties.

• While simple in concept, the process variations required (depending on alloy, product form, desired final property combinations, etc.) make it sufficiently complex that heat treating has become a professional specialty.

• The first step in the heat treatment process is solution heat treatment. The objective of this process step is to place the elements into solution that will eventually be called upon for precipitation hardening.

• Developing solution heat treatment times and temperatureshas typically involved extensive trial and error, partially due to the lack of accurate process models.

Page 65: Phase transformation

Aging-microstructure

• The supersaturated solid solution is unstable and if, left alone, the excess qwill precipitate out of the a phase. This process is called aging.

• Types of aging:

– Natural aging process occurs at room temperature

– Artificial aging If solution heat treated, requires heating to speed up the precipitation

Page 66: Phase transformation

Overaging

• After solution heat treatment the material is ductile,

since no precipitation has occurred. Therefore, it may

be worked easily.

• After a time the solute material precipitates and

hardening develops.

• As the composition reaches its saturated normal state,

the material reaches its maximum hardness.

• The precipitates, however, continue to grow. The fine

precipitates disappear. They have grown larger, and as

a result the tensile strength of the material decreases.

This is called overaging.

Page 67: Phase transformation

Precipitation Heat Treatment

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.

Guinier-Preston (GP) zones - Tiny clusters

of atoms that precipitate from the matrix in

the early stages of the age-hardening

process.

Page 68: Phase transformation

Hardness vs. Time

The hardness and tensile strength vary

during aging and overaging.

Page 69: Phase transformation

• 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

ten

sile

str

en

gth

(M

Pa

)

200

300

400

1001min 1h 1day 1mo 1yr

204°C149°C

• %EL reaches minimum

with precipitation time.

%E

L(2

in s

am

ple

)10

20

30

01min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time

Page 70: Phase transformation

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 71: Phase transformation

Aluminum rivets

Alloys that experience significant

precipitation hardening at room

temp, 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.

Page 72: Phase transformation

Several stages in the formation of the equilibrium

PPT (q) phase.

(a) supersaturated a solid solution;

(b) transition (q”) PPT phase;

(c) equilibrium q phase within the a matrix phase.

Page 73: Phase transformation

0 10 20 30 40 50wt% Cu

La+La

aqq

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 B: quench to room temp.

(retain a solid solution)-- Pt C: reheat to nucleate

small q particles within

a phase.

Temp.

Time

-- Pt A: solution heat treat

(get a solid solution)

Pt A (solution heat treat)

B

Pt B

C

Pt C (precipitate q)

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 74: Phase transformation

24

• Hard precipitates are difficult to shear.

Ex: Ceramics in metals (SiC in Iron or Aluminum).

• Result: y ~

1

S

PRECIPITATION STRENGTHENING

Page 75: Phase transformation

Aging

• Aging either at room or moderately elevated temperature after the quenching process is used to produce the desired final product property combinations.

• The underlying metallurgical phenomenon in the aging process is precipitation hardening. Due to the small size of the precipitate particles, early understanding was hampered by the lack of sufficiently powerful microscopes to actually see them.

• With the availability of the transmission electron microscope (TEM) with nanometer-scale resolution, researchers were able to actually image many precipitate phases and build on this knowledge to develop improved aluminum alloy products.

Page 76: Phase transformation

Aluminum

• Aluminum is light weight, but engineers want to improve the strength for high performance applications in automobiles and aerospace.

• To improve strength, they use precipitation hardening.

Age-hardening heat treatment phase diagram

Page 77: Phase transformation

Quenching

• Quenching is the second step in the process.

• Its purpose is to retain the dissolved alloying elements in solution for subsequent precipitation hardening.

• Generally the more rapid the quench the better, from a properties standpoint, but this must be balanced against the concerns of part distortion and residual stressif the quench is non-uniform.

Changes in Microstructure due to quenching