Chapter 10 Phase in Metals - الصفحات الشخصية | الجامعة...

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.


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


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


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.


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).


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


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


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.


2

• Fraction transformed depends on time.

• Transformation rate depends on T.

• r often small: equil not possible

Fraction of Transformation


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


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


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


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


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.


• Reaction rate is a result of nucleation and growth of crystals

• Examples:

Nucleation and Growth


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


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.


• 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


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


Str

en

gth

Ductilit

y

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 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.


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


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


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.


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.


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%


Martensite, 100%

Example 2: Iron-carbon alloy with

eutectoid composition. Specify nature of final


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 nature of final


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 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.


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.


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.


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


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


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)


Mechanical Properties: Fe-C System


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


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 a-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.

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


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


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 & Brinell Hardness


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


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


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).


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.


24

• Hard precipitates are difficult to shear.

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

• Result: y ~

1

S

Precipitation Strengthening


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.


• 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


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.


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.


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Chapter 10 Phase in Metals - الصفحات الشخصية | الجامعة...

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Transcript of Chapter 10 Phase in Metals - الصفحات الشخصية | الجامعة...