Introduction to Materials Science and Engineering

Introduction to Materials Science and Engineering

Eun Soo Park

Office: 33-313

Telephone: 880-7221 Email: espark@snu.ac.kr

Office hours: by appointment

2020 Fall

11. 10. 2020

1

Chapter 10. Phase Transformations

Contents for previous class

2

(Ch1) Thermodynamics and Phase Diagrams (Ch2) Diffusion: Kinetics

(Ch3) Crystal Interface and Microstructure

(Ch4) Solidification: Liquid → Solid

(Ch5) Diffusional Transformations in Solid: Solid → Solid (Ch6) Diffusionless Transformations: Solid → Solid

Background to understand

phase

transformation

Representative Phase

transformation

Contents _Phase transformation course

2 1

0

G G G

∆ = − <

A B

Time

Liquid Undercooled Liquid Solid

<Thermodynamic>

Solidification: Liquid Solid

• Interfacial energy Δ T

Melting: Liquid Solid

• Interfacial energy

SV LV

SL γ γ

γ + <

No superheating required!

No Δ T

T

vapor

Melting and Crystallization are Thermodynamic Transitions

3

4 4

• Nucleation in Pure Metals

• Homogeneous Nucleation

• Heterogeneous Nucleation

• Nucleation of melting

* *

( )

G

S θ G

∆ = ∆

SV LV

SL γ γ

γ + < _(commonly)

V SL

r G

= ∆

∗

2 γ

2 2

2 3 2

3

) (

1 3

16 )

( 3

* 16

T L

T G G

V m SL V

SL

 ∆



 



= 

= ∆

∆ π γ πγ

r* & ΔG* ↓ as ΔT ↑

Solidification : Liquid Solid

2 3 cos cos

4 ( )

A

A B

V S

V V

θ θ θ

− +

= =

+

2 0 2

hom

~ 1 ) } exp{ (

T T

C A f

N

∆

− ∆

≈

• Undercooling ΔT

• Interfacial energy γ

/ S( θ) wetting angle

5

θ

V

V

Barrier of Heterogeneous Nucleation

4

) cos

cos 3

2 ( 3

) 16 3 (

* 16

3 2

3 2

3

θ πγ θ θ

πγ − +

∆ ⋅

=

∆ ⋅

=

∆

V SL V

SL

S G G G

θ θ

 − + 

∆ = ∆  

 

3

*

2 3 cos cos

4

*

sub homo

G G

2 3 cos cos

4 ( )

A

A B

V S

V V

θ θ θ

− +

= =

+

How about the nucleation at the crevice or at the edge?

* *

( ) hom

G het S θ G

∆ = ∆

6

5.4 Overall Transformation Kinetics – TTT Diagram

The fraction of Transformation as a function of Time and Temperature

Plot the fraction of

transformation (1%, 99%) in T-log t coordinate.

→ f (t,T)

Plot f vs log t.

- isothermal transformation

- f ~ volume fraction of β at any time; 0~1

Fig. 5.23 The percentage transformation versus time for different transformation temperatures.

If isothermal transformation,

7

Constant Nucleation Rate Conditions



consider impingement + repeated nucleation effects

( ^f ) ^df _e

df = 1 −

f df _e df

= −

( ^f ) 1 f _e = − ln 1 −

( ) 



 

 −

−

=

−

−

= ^{3 4}

exp 3 1

) ( exp

1 )

( t f t Iv t

f _e π

Johnson-Mehl-Avrami Equation

f

t 1

t 4

∝

before

impingement

1-exp(z)~Z (z ≪1 )

( ) ^kt

f = 1 − exp −

k: _{T sensitive} f(I, v)

n: 1 ~ 4 (depend on nucleation mechanism)

i.e. 50% transform

Exp (-0.7) = 0.5 kt

= 0 . 7

t

= k 0

.

7

⁰

^{. 9}

v t = N

Rapid transformations are associated with (large values of k), or (rapid nucleation and growth rates)

* Short time:

t→∞, f →1

* Long time:

If no change of nucleation mechanism during phase transformation, n is not related to T.

Nucleation and Growth Rates

Nulceation and Growth for Silica

0.E+00 2.E+07 4.E+07 6.E+07 8.E+07 1.E+08 1.E+08 1.E+08

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

Temperature (K)

N u cl eat io n R at e ( sec

)

0 5E-26 1E-25 1.5E-25 2E-25 2.5E-25

G r o w th R a te (m -s e c

Nucleation rate (sec-1) Growth rate (m/sec)

9

Time–Temperature–Transformation Curves (TTT)

 How much time does it take at any one temperature for a given fraction of the liquid to transform

(nucleate and grow) into a crystal?

 f(t,T) ~πI(T)µ(T) ³ t ⁴ /3

where f is the fractional volume of crystals formed, typically taken to be 10 ^-6 , a barely observable crystal volume.

 



 

 ∆ −



 





 

 



 





 ∆



 



 ∆

= RT

E T

T RT

n H

I

exp

81 exp 16

π

ν  





 



 



 



  

 



  ∆



 



−  ∆

 

 



= 

m m

T T RT

H T

a N

T fRT 1 exp

) ( ) 3

(

η µ π

Nucleation rates Growth rates

10

Time Transformation Curves for Silica

T-T-T Curve for Silica

200 400 600 800 1000 1200 1400 1600 1800 2000

1 1E+13 1E+26 1E+39 1E+52 1E+65 1E+78

Time (sec)

T em p er at u r e ( K )

11

12

* Time-Temperature-Transformation diagrams

TTT diagrams

12

13

* Continuous Cooling Transformation diagrams

CCT diagrams

13

(b) Eutectoid Transformation (a) Precipitation

Composition of product phases differs from that of a parent phase.

→ long-range diffusion

Which transformation proceeds by short-range diffusion?

Chapter 5. Diffusion Transformations in solid

: diffusional nucleation & growth

β α

α ' → +

β α

γ → +

Metastable supersaturated Solid solution

14

(d) Massive Transformation (e) Polymorphic Transformation (c) Order-Disorder

Transformation

5. Diffusion Transformations in solid

α ' α →

α β →

: The original phase decomposes into one or more new phases which have the same composition as the parent phase, but different crystal structures.

In single component systems, different crystal structures are stable over different temper- ature ranges.

Disorder

(high temp.) Order (low temp.)

Stable metastable

15

16

The Iron-Carbon Phase Diagram Microstructure (0.4 wt%C) evolved by slow cooling (air, furnace) ?

5.6. The Precipitation of Ferrite from Austenite ( γ→α)

(Most important nucleation site: Grain boundary and the surface of inclusions)

Ledeburite

Perlite

* 철-탄소 합금에서 미세조직과 특성의 변화

Transformations & Undercooling

• For transf. to occur, must cool to below 727°C (i.e., must “undercool”)

• ^Eutectoid transf. (Fe-Fe

C system): γ ⇒ α + Fe

C

0.76 wt% C

0.022 wt% C

6.7 wt% C

F e

C ( c em ent it e)

1600 1400 1200 1000 800

600

400 0 1 2 3 4 5 6 6.7

L γ

(austenite) γ+L

γ+Fe

C

α+Fe

C

L+Fe

C δ

(Fe) C, wt%C

1148°C

T(°C)

α ferrite

727°C

Eutectoid:

Equil. Cooling: T _transf. = 727°C ΔT

Undercooling by T _transf. < 727°C

0 .7 6

0 .02 2

Fig. 11.23, Callister &

Rethwisch 9e.

[Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 1, T. B.

Massalski (Editor-in-Chief), 1990.

Reprinted by permission of ASM International, Materials Park, OH.]

* 철-탄소 합금에서 미세조직과 특성의 변화

17

The Fe-Fe ₃ C Eutectoid Transformation

Coarse pearlite  formed at higher temperatures – relatively soft Fine pearlite  formed at lower temperatures – relatively hard

• Transformation of austenite to pearlite:

Adapted from Fig. 11.14, Callister &

Rethwisch 9e.

α γ α α α α

α

pearlite growth direction Austenite ( γ)

grain

boundary

cementite (Fe

C) Ferrite ( α)

γ

• For this transformation, rate increases with

[T

– T ] (i.e., ΔT).

Rethwisch 9e.

675°C (ΔT smaller)

0 50

y (% pear lit e)

(ΔT larger)

650°C

100

Diffusion of C

during transformation

α α

γ γ

α

diffusion

18

Fig. 12.13, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977.

Reproduced by permission of ASM International, Materials Park, OH.]

Generation of Isothermal Transformation Diagrams

• The Fe-Fe

C system, for C

= 0.76 wt% C

• A transformation temperature of 675ºC.

100 50

0 1 10 2 10 4

T = 675°C

y , % t rans for m ed

time (s)

400 500 600 700

1 10 10 ² 10 ³ 10 ⁴ 10 ⁵ Austenite (stable)

T _E (727°C)

Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

Consider:

19

Fig. 12.13, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977.

Reproduced by permission of ASM International, Materials Park, OH.]

Generation of Isothermal Transformation Diagrams

• The Fe-Fe

C system, for C

= 0.76 wt% C

• A transformation temperature of 675ºC.

Consider:

20

• Eutectoid composition, C

= 0.76 wt% C

• Begin at T > 727°C

• Rapidly cool to 625°C

• Hold T (625°C) constant (isothermal treatment)

Austenite-to-Pearlite Isothermal Transformation

Fig. 12.14, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977.

Reproduced by permission of ASM International, Materials Park, OH.]

21

Coarse perlite Fine perlite

22

Fig. 11.23, Callister & Rethwisch 9e.

[Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol.

1, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International, Materials Park, OH.]

Transformations Involving noneutectoid compositions

Hypereutectoid composition – proeutectoid cementite Consider C ₀ = 1.13 wt% C

a T _E (727°C) T(°C)

time (s)

A

A

+ A C

P

1 10 10

10

10

500 700 900

600 800

A + P

Fig. 12.16, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977. Reproduced by permission of ASM International, Materials Park, OH.]

F e

C ( c em ent it e)

1600 1400 1200 1000 800 600

400 0 1 2 3 4 5 6 6.7

L

(austenite) γ

γ+L

γ+Fe

C α+Fe

C

L+Fe

C

δ

(Fe) C, wt%C

T(°C)

727°C

0 .7 6

0 .0 22 1. 13

23

10 10

10

time (s)

10

400 600 800

T(°C)

Austenite (stable)

200

P

B

T

A

A

Bainite: Another Fe-Fe ₃ C Transformation Product

• Bainite:

-- elongated Fe

C particles in α-ferrite matrix

-- diffusion controlled

• Isothermal Transf. Diagram, C

= 0.76 wt% C

Fig. 12.18, Callister & Rethwisch 9e. [Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977. Reproduced by permission of ASM International, Materials Park, OH.]

Fig. 12.17, Callister & Rethwisch 9e.

(From Metals Handbook, Vol. 8, 8th edition, Metallography, Structures and Phase Diagrams, 1973. Reproduced by permission of ASM International, Materials Park, OH.)

Fe

C

(cementite)

5 μm α (ferrite)

100% bainite 100% pearlite

24

Upper Banite in medium-carbon steel Lower Bainite in 0.69wt% C low-alloy steel

At high temp. 350 ~ 550^oC, ferrite laths, K-S

relationship, similar to Widmanstäten plates At sufficiently low temp. laths → plates

Carbide dispersion becomes much finer, rather like in tempered M.

5.8.2 Bainite Transformation

(b) Schematic of growth mechanism. Widmanstatten ferrite laths growth into γ₂. Cementite plates

nucleate in carbon-enriched austenite.

(b) A possible growth mechanism. α/γ interface advances as fast as carbides precipitate at interface thereby removing the excess carbon in front of the α.

Surface tilts by bainite trans. like M trans.

Due to Shear mechanism/ordered military manner

25

• Spheroidite:

-- Fe 3 C particles within an α-ferrite matrix -- formation requires diffusion

-- heat bainite or pearlite at temperature just below eutectoid for long times -- driving force – reduction

of α-ferrite/Fe

C interfacial area

Spheroidite: Another Microstructure for the Fe-Fe ₃ C System

Fig. 12.19, Callister &

Rethwisch 9e.

(Copyright United States Steel Corporation, 1971.)

60 μm α

(ferrite)

(cementite) Fe

C

26

• Martensite:

-- γ(FCC) to Martensite (BCT)

Fig. 12.21, Callister & Rethwisch 9e.

(Courtesy United States Steel Corporation.)

Adapted from Fig. 12.20, Callister & Rethwisch 9e.

Martensite: A Nonequilibrium Transformation Product

Martensite needles Austenite

60 μm

그그그 그그그 그 그그그그.

그그그 그그그 그 그그그그.

x

x x

x x

x

potential C atom sites Fe atom

sites

Adapted from Fig. 12.22, Callister &

Rethwisch 9e.

• Isothermal Transf. Diagram

• γ to martensite (M) transformation.

-- is rapid! (diffusionless)

-- % transformation depends only on T to which rapidly cooled

10 10

10

time (s)

10

400 600 800

T(°C)

Austenite (stable)

200

P

B

T

A

A

M + A M + A M + A

0%

50%

90%

27

γ ^(FCC) α ^{(BCC) + Fe} 3 C

Martensite Formation

slow cooling

tempering quench

M (BCT)

Martensite (M) – single phase

– has body centered tetragonal (BCT) crystal structure

Diffusionless transformation BCT if C ₀ > 0.15 wt% C BCT ^ few slip planes ^ hard, brittle

28

Isothermal Heat Treatment _Example Problems On the isothermal transformation diagram for a 0.45 wt% C, Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:

a) 42% proeutectoid ferrite and 58% coarse pearlite

b) 50% fine pearlite and 50% bainite c) 100% martensite

d) 50% martensite and 50% austenite

29

Solution to Part (a) of Example Problem

a) 42% proeutectoid ferrite and 58% coarse pearlite

Isothermally treat at ~ 680°C -- all austenite transforms

to proeutectoid α and coarse pearlite.

A + B A + P A + α

A

B P

A

50%

0 200 400 600 800

0.1 10 10

10

time (s)

M (start) M (50%) M (90%)

Fe-Fe

C phase diagram, for C

= 0.45 wt% C

T (°C)

Figure 12.39, Callister & Rethwisch 9e.

(Adapted from Atlas of Time-Temperature Diagrams for Irons and Steels, G. F. Vander Voort, Editor, 1991. Reprinted by permission

of ASM International, Materials Park, OH.) 30

b) 50% fine pearlite and 50% bainite

T (ºC)

A + B A + P A + α

A

B P

A

50%

0 200 400 600 800

0.1 10 10

10

time (s)

M (start) M (50%) M (90%)

Fe-Fe

C phase diagram, for C

= 0.45 wt% C

Then isothermally treat at ~ 470°C

– all remaining austenite transforms to bainite.

Isothermally treat at ~ 590°C – 50% of austenite transforms

to fine pearlite.

Figure 12.39, Callister & Rethwisch 9e.

(Adapted from Atlas of Time-Temperature Diagrams for Irons and Steels, G. F. Vander Voort, Editor, 1991. Reprinted by permission of ASM International, Materials Park, OH.)

Solution to Part (b) of Example Problem

31

c) 100% martensite – rapidly quench to room temperature

d) 50% martensite

& 50% austenite

-- rapidly quench to

~ 290°C, hold at this temperature

T (°C)

A + B A + P A + α

A

B P

A

50%

0 200 400 600 800

0.1 10 10

10

time (s)

M (start) M (50%) M (90%)

Fe-Fe

C phase diagram, for C

= 0.45 wt% C

d)

c)

Figure 12.39, Callister & Rethwisch 9e.

(Adapted from Atlas of Time-Temperature Diagrams for Irons and Steels, G. F. Vander Voort, Editor, 1991. Reprinted by permission of ASM International, Materials Park, OH.)

Solution to Part (c) & (d) of Example Problem

32

33

Phase Transformations of Alloys

Effect of adding other elements Change transition temp.

Fig. 12.23 alloy steel (type 4340)

Cr, Ni, Mo, Si, Mn

retard γ  α + ^Fe

C reaction (and formation of pearlite, bainite)

Fig. 12.22 Iron (pure Fe)-carbon alloy

33

Continuous Cooling Transformation Diagrams

Conversion of isothermal

transformation diagram to continuous cooling

transformation diagram (TTT vs CCT diagram)

Cooling curve

Fig. 12.25, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977.

Reproduced by permission of ASM International, Materials Park, OH.]

34

그림 12.26 공석 조성을 갖는 철-탄소 합금의 연속 냉각 변태도 위에 그려진 적당한 급랭과 서냉온도 곡선

그림 12.27 공석 조성의 철-탄소 합금의 연속 냉각 변태도와 냉각 곡선. 냉각 중에 일어나는 변태에 따른 최종 미세조직 변화를 볼 수 있다.

35

36

그림 12.27

4030 합금강의 연속 냉각 변태도와 여러 조건의 냉각 곡선. 냉각 중에

일어나는 변태에 따른 최종 미세조직의 변화를 볼 수 있다.

Mechanical Props: a. Influence of C Content

Fig. 11.29, Callister & Rethwisch 9e.

(Courtesy of Republic Steel Corporation.)

• Increase C content: TS and YS increase, %EL decreases C

< 0.76 wt% C

Hypoeutectoid Pearlite (med)

ferrite (soft)

Fig. 11.32, Callister & Rethwisch 9e.

(Copyright 1971 by United States Steel Corporation.)

C

> 0.76 wt% C Hypereutectoid

Pearlite (med) Cementite

(hard)

Fig. 12.29, Callister &

Rethwisch 9e.

[Data taken from Metals Handbook: Heat Treating, Vol. 4, 9th edition, V.

Masseria (Managing Editor), 1981. Reproduced by permission of ASM International, Materials Park, OH.]

300 500 700 900 YS(MPa) 1100 TS(MPa)

wt% C

0 0.5 1

hardness

0 .7 6

Hypo Hyper

wt% C

0 0.5 1

0 50 100

%EL

Im p ac t e ne rg y ( Iz od , ft -l b)

0 40 80

0 .7 6

Hypo Hyper

37

Mechanical Props:

b. Fine Pearlite vs. Coarse Pearlite vs. Spheroidite

Fig. 12.30, Callister & Rethwisch 9e.

[Data taken from Metals Handbook: Heat Treating, Vol. 4, 9th edition, V. Masseria (Managing Editor), 1981. Reproduced by

permission of ASM International, Materials Park, OH.]

• Hardness:

• %RA:

fine > coarse > spheroidite fine < coarse < spheroidite

80 160 240 320

0 0.5 wt%C 1

B ri n e ll h ar d n es s

fine pearlite

coarse pearlite spheroidite

Hypo Hyper

0 30 60 90

wt%C

D u c tilit y (%R A)

fine pearlite coarse pearlite spheroidite

Hypo Hyper

0 0.5 1

38

Mechanical Props:

c. Fine Pearlite vs. Martensite

• Hardness: fine pearlite << martensite.

Fig. 12.32, Callister & Rethwisch 9e.

(Adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, 1939; and R. A.

Grange, C. R. Hribal, and L. F. Porter, Metall.

Trans. A, Vol. 8A. Reproduced by permission of ASM International, Materials Park, OH.)

0 200

wt% C

0 0.5 1

400 600

B ri n e ll h ar d n es s

martensite

fine pearlite Hypo Hyper

39

Tempered Martensite

• tempered martensite less brittle than martensite

• tempering reduces internal stresses caused by quenching

Figure 12.33, Callister &

Rethwisch 9e.

(Copyright 1971 by United States Steel Corporation.)

• tempering decreases TS, YS but increases %RA

• tempering produces extremely small Fe

C particles surrounded by α .

Fig. 12.34, Callister &

Rethwisch 9e.

(Adapted from Edgar C. Bain, Functions of the Alloying

Elements in Steel, 1939. Reproduced by permission of ASM International, Materials Park, OH.)

9 μ m

YS(MPa) TS(MPa)

800 1000 1200 1400 1600 1800

30 40 50 60

200 400 600

Tempering T(°C)

%RA TS

YS

%RA

Heat treat martensite to form tempered martensite

40

그림 12.32

순 탄소 마텐자이트강, 템퍼링된 마텐자이트강 [371°C 템버링], 펄라이트 강의 탄소농도에 따른 상온 경도값

41

Summary of Possible Transformations

in Fe-C binary phase diagram

Adapted from Fig. 12.36, Callister &

Rethwisch 9e.

Austenite ( γ)

Pearlite

( α + Fe

C layers + a proeutectoid phase)

slow cool

Bainite

( α + elong. Fe

C particles)

moderate cool

Martensite

(BCT phase diffusionless transformation)

rapid quench

Tempered Martensite

( α + very fine Fe

C particles)

reheat

S tr eng th Du c tilit y

Martensite T Martensite

bainite fine pearlite coarse pearlite

spheroidite General Trends

42

eTL 공지사항 참고

Homework : 9장, 10장 예제문제 (10th edition 기준/ 11장,12장 9th edition)

Incentive Homework 4: 관심있는 Advanced Engineering Materials 하나 정해서 5 pages 이내 정리

Incentive Homework 5 : 서울대학교 재료공학부 진학이유와

앞으로의 포부- ”자기자신에게 보내는 편지”

Introduction to Materials Science and Engineering

Eun Soo Park

Office: 33-313

Telephone: 880-7221 Email: espark@snu.ac.kr

Office hours: by appointment

2020 Fall

11. 12. 2020

1

• Atomic

• Crystal

• Microstructure

• Macrostructure

• Mechanical

• Electrical

• Magnetic

• Thermal

• Optical

Materials Science and Engineering

Structure

Properties

Processing

Theory &

Design

• Sintering

• Heat treatment

• Thin Film

• Melt process

• Mechanical

2

Chapter 12

Transformations & Undercooling

• For transf. to occur, must cool to below 727°C (i.e., must “undercool”)

• ^Eutectoid transf. (Fe-Fe

C system): γ ⇒ α + Fe

C

0.76 wt% C

0.022 wt% C

6.7 wt% C

Fe

C ( c em ent it e)

1600 1400 1200 1000 800

600

400 0 1 2 3 4 5 6 6.7

L γ

(austenite) γ+L

γ+Fe

C

α+Fe

C

L+Fe

C

δ

(Fe) C, wt%C

1148°C

T(°C)

α ferrite

727°C

Eutectoid:

Equil. Cooling: T _transf. = 727°C ΔT

Undercooling by T _transf. < 727°C

0. 76

0. 022

Fig. 11.23, Callister &

Rethwisch 9e.

[Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 1, T. B.

Massalski (Editor-in-Chief), 1990.

Reprinted by permission of ASM International, Materials Park, OH.]

* Contents for previous class: 철-탄소 합금에서 미세조직과 특성의 변화

3

• Eutectoid composition, C

= 0.76 wt% C

• Begin at T > 727°C

• Rapidly cool to 625°C

• Hold T (625°C) constant (isothermal treatment)

Austenite-to-Pearlite Isothermal Transformation

Fig. 12.14, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977.

Reproduced by permission of ASM International, Materials Park, OH.]

4

Coarse perlite Fine perlite

5

Fig. 11.23, Callister & Rethwisch 9e.

[Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol.

1, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International, Materials Park, OH.]

Transformations Involving noneutectoid compositions

Hypereutectoid composition – proeutectoid cementite Consider C ₀ = 1.13 wt% C

a T _E (727°C)

T(°C)

time (s)

A

A

+ A C

P

1 10 10

10

10

500 700 900

600 800

A + P

Fig. 12.16, Callister & Rethwisch 9e.

[Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977. Reproduced by permission of ASM International, Materials Park, OH.]

Fe

C ( c em ent it e)

1600 1400 1200 1000 800 600

400 0 1 2 3 4 5 6 6.7

L

(austenite) γ

γ+L

γ+Fe

C α+Fe

C

L+Fe

C

δ

(Fe) C, wt%C

T(°C)

727°C

0. 76

0. 022 1.13

6

10 10

10

time (s)

10

400 600 800

T(°C)

Austenite (stable)

200

P

B

T _E

A

A

Bainite: Another Fe-Fe ₃ C Transformation Product

• Bainite:

-- elongated Fe

C particles in α-ferrite matrix

-- diffusion controlled

• Isothermal Transf. Diagram, C

= 0.76 wt% C

Fig. 12.18, Callister & Rethwisch 9e. [Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977. Reproduced by permission of ASM International, Materials Park, OH.]

Fig. 12.17, Callister & Rethwisch 9e.

(From Metals Handbook, Vol. 8, 8th edition, Metallography, Structures and Phase Diagrams, 1973. Reproduced by permission of ASM International, Materials Park, OH.)

Fe

C

(cementite)

5 μm α (ferrite)

100% bainite 100% pearlite

7

Upper Banite in medium-carbon steel Lower Bainite in 0.69wt% C low-alloy steel At high temp. 350 ~ 550

C, ferrite laths, K-S

relationship, similar to Widmanstäten plates At sufficiently low temp. laths → plates

Carbide dispersion becomes much finer, rather like in tempered M.

5.8.2 Bainite Transformation The microstructure of bainite depends mainly on the temperature at which it forms.

(b) Schematic of growth mechanism. Widmanstatten ferrite laths growth into γ₂. Cementite plates

nucleate in carbon-enriched austenite.

(b) A possible growth mechanism. α/γ interface advances as fast as carbides precipitate at interface thereby removing the excess carbon in front of the α.

Surface tilts by bainite trans. like M trans.

Due to Shear mechanism/ordered military manner

8

• Martensite:

-- γ(FCC) to Martensite (BCT)

Fig. 12.21, Callister & Rethwisch 9e.

(Courtesy United States Steel Corporation.)

Adapted from Fig. 12.20, Callister & Rethwisch 9e.

Martensite: A Nonequilibrium Transformation Product

Martensite needles Austenite

60 μm

그그그 그그그 그 그그그그.

그그그 그그그 그 그그그그.

x

x x

x x

x potential C atom sites Fe atom

sites

Adapted from Fig. 12.22, Callister &

Rethwisch 9e.

• Isothermal Transf. Diagram

• γ to martensite (M) transformation.

-- is rapid! (diffusionless)

-- % transformation depends only on T to which rapidly cooled

10 10

10

time (s)

10

400 600 800

T(°C)

Austenite (stable)

200

P

B

T _E

A

A

M + A M + A M + A

0%

50%

90%

9

Tempered Martensite

• tempered martensite less brittle than martensite

• tempering reduces internal stresses caused by quenching

Figure 12.33, Callister &

Rethwisch 9e.

(Copyright 1971 by United States Steel Corporation.)

• tempering decreases TS, YS but increases %RA

• tempering produces extremely small Fe ₃ C particles surrounded by α .

Fig. 12.34, Callister &

Rethwisch 9e.

(Adapted from Edgar C. Bain, Functions of the Alloying

Elements in Steel, 1939. Reproduced by permission of ASM International, Materials Park, OH.)

9 μm

YS(MPa) TS(MPa)

800 1000 1200 1400 1600 1800

30 40 50 60

200 400 600

Tempering T (°C)

%RA TS

YS

%RA

Heat treat martensite to form tempered martensite

10

그림 12.32

순 탄소 마텐자이트강, 템퍼링된 마텐자이트강 [371°C 템버링], 펄라이트 강의 탄소농도에 따른 상온 경도값

11

그림 12.26 공석 조성을 갖는 철-탄소 합금의 연속 냉각 변태도 위에 그려진 적당한 급랭과 서냉온도 곡선

그림 12.27 공석 조성의 철-탄소 합금의 연속 냉각 변태도와 냉각 곡선. 냉각 중에 일어나는 변태에 따른 최종 미세조직 변화를 볼 수 있다.

12

13

그림 12.27

4030 합금강의 연속 냉각 변태도와

일어나는 변태에 따른 최종 미세조직의 변화를 볼 수 있다.

Summary of Possible Transformations

in Fe-C binary phase diagram

Adapted from Fig. 12.36, Callister &

Rethwisch 9e.

Austenite ( γ)

Pearlite

( α + Fe

C layers + a proeutectoid phase)

slow cool

Bainite

( α + elong. Fe

C particles)

moderate cool

Martensite

(BCT phase diffusionless transformation)

rapid quench

Tempered Martensite

( α + very fine Fe

C particles)

reheat

S tr ength D uc ti lity

Martensite T Martensite

bainite fine pearlite coarse pearlite

spheroidite General Trends

14

15

ISSUES TO ADDRESS...

Chapter 11:

Applications and Processing of Metal Alloys

• How are metal alloys classified and what are their common applications?

• What are some of the common fabrication techniques for metals?

• What heat treatment procedures are used to improve the

mechanical properties of both ferrous and nonferrous alloys?

Materials Design-for-Properties : “Alloyed Pleasure”

Materials Design-for-Properties : “Alloyed Pleasure”

Rising trend of alloy chemical complexity versus time. Note that ‘‘IMs’’ stands

for intermetallics or metallic compounds and ‘‘HEA’’ for high-entropy alloy.

19 Adapted from Fig.

13.1, Callister &

Rethwisch 9e.

Classification of Metal Alloys

Metal Alloys

Steels

Ferrous Nonferrous

Cast Irons

<1.4wt%C 3-4.5 wt%C

Steels

<1.4 wt% C Cast Irons 3-4.5 wt% C

Fe3C cementite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L γ

austenite

γ + L

γ + Fe

C α

ferrite

α + Fe

C

L+ Fe

C δ

(Fe)

C

, wt% C

Eutectic:

Eutectoid:

0.76

4.30

727ºC

1148ºC

T(ºC) microstructure: ferrite,

graphite/cementite

Fig. 11.23, Callister & Rethwisch 9e.

[Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 1, T. B.

Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International,

Materials Park, OH.]

20

1. Ferrous Alloy

Merits: (1) iron-containing compounds exist in abundant quantities within the Earth’s crust

지구 전체를 구성하는 원소의 중량 비율로는 철 (Fe)이 35%로 제일 많이 있는 원소이다.

철은 특히 지구 내부 코어 중량의 90.8%를 차지하는 것으로 추정되고 있다. 그 다음으로는 산소 (O)가 30%, 실리콘(Si) 15%, 마그네슘(Mg) 13% 순이다.

하지만 , 지각을 구성하는 원소를 기준으로 중량 비율을 살펴보면 산소가 제일 많다. 철은 산소 46.60%, 실리콘 27.70%, 알루미늄 8.13%에 이어 지각 중량의 5.0%를 차지하며 금속

중에서는 알루미늄에 이어 두 번째로 많다 .

재미있는 것은 지각을 구성하는 물질 가운데 비중이 큰 4개 원소(산소, 실리콘, 알루미늄, 철 )의 누적 중량 비율이 87.4%이고 상위 9개 원소(상기 4개 원소 외 칼슘, 나트륨, 칼륨,

마그네슘 및 티타늄 )의 총 중량이 지각 전체의 99.0%를 차지한다는 점이다. 이 외 다른 원소의 중량의 합은 지각 전체의 1.0%에도 미치지 못하는 것이다. 예를 들어 구리, 니켈 및 납 등

금속들은 지각에서 존재하는 비중이 겨우 각각 75ppm, 55ppm, 13ppm 밖에 되지 않는다.

지각에 55ppm 수준으로 존재하는 구리 같은 자원이 지각 속에서 균등하게 분포한다면 이를

채취하여 자원으로 만드는 것은 거의 불가능 → 비교적 농집된 형태로 지역에 존재 (광산)

21

1. Ferrous Alloy

Merits: (1) iron-containing compounds exist in abundant quantities within the Earth’s crust.

(2) metallic iron and steel alloys may be produced

using relatively economical extraction, refining, alloying,

and fabrication techniques.

22

철강의 생산과정 : Iron making → Steelmaking → Rolling

23

철강의 생산과정 : Iron making → Steelmaking → Rolling

24

a. Refinement of Steel from Ore

Iron Ore

Coke

Limestone

3CO + Fe ₂ O ₃ →2Fe +3CO

C + O ₂ →CO

CO

+ C→ 2CO

CaCO ₃ → CaO+CO

CaO + SiO

+ Al

O

→ slag purification

reduction of iron ore to metal heat generation

Molten iron

BLAST FURNACE

slag air layers of coke and iron ore

gas refractory vessel

처음 생산된 Pig Iron = 선철(銑鐵) = 용선(溶銑) = 쇳물 = 물쇠 = 무쇠

25

Cast Irons ^(주철)

• Ferrous alloys with > 2.1 wt% C – more commonly 3 - 4.5 wt% C

• Low melting – relatively easy to cast

• Generally brittle

• Cementite decomposes to ferrite + graphite Fe ₃ C  3 Fe ( α) + C (graphite)

– generally a slow process

회주철 연성(구상)주철 백주철 가단주철 조밀흑연주철

26 Based on data provided in Tables 13.1(b), 14.4(b), 13.3, and 13.4, Callister & Rethwisch 9e.

Steels

Low Alloy High Alloy

low carbon

<0.25 wt% C

Med carbon 0.25-0.6 wt% C

high carbon 0.6-1.4 wt% C

Uses auto struc.

sheet

bridges towers press.

vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic.

turbines furnaces Very corros.

resistant

Example 1010 4310 1040 4340 1095 4190 304, 409 Additions none Cr,V

Ni, Mo none Cr, Ni

Mo none Cr, V,

Mo, W Cr, Ni, Mo plain HSLA plain heat

treatable plain tool stainless Name

Hardenability 0 + + ++ ++ +++ varies

TS - 0 + ++ + ++ varies

EL + + 0 - - -- ++

increasing strength, cost, decreasing ductility

27

철강의 생산과정 : Iron making → Steelmaking → Rolling

28

I. Ferrous Alloy

Merits: (1) iron-containing compounds exist in abundant quantities within the Earth’s crust.

(2) metallic iron and steel alloys may be produced

using relatively economical extraction, refining , alloying, and fabrication techniques.

(3) ferrous alloys are extremely versatile, in that they may be tailored to have a wide range of mechanical and physical properties.

Limitations: (1) Relatively high densities

(2) Relatively low electrical conductivities

(3) Generally poor corrosion resistance

29

Ferrous Alloys

Iron-based alloys

Nomenclature for steels (AISI/SAE) 10xx Plain Carbon Steels

11xx Plain Carbon Steels (resulfurized for machinability) 15xx Mn (1.00 - 1.65%)

40xx Mo (0.20 ~ 0.30%)

43xx Ni (1.65 - 2.00%), Cr (0.40 - 0.90%), Mo (0.20 - 0.30%) 44xx Mo (0.5%)

where xx is wt% C x 100

example: 1060 steel – plain carbon steel with 0.60 wt% C Stainless Steel >11% Cr

• Steels

• Cast Irons

30

Ferrous Alloys

Figure 13.1

Classification scheme for the

various ferrous alloys

Stable

Metastable

* Cast Iron: Fe-C alloy (1.7 ≤ c ≤ 4.5%)

Cementite

Eutectic

Eutectoid Peritectic

Ledeburite

Perlite

32 Fe-Fe₃C eutectic temp. < Fe-graphite eutectic temp.

graphite

Fe

C

6°C

* If solidification proceeds at

interface temperature above the cementite eutectic temperature,

Graphite eutectic formation

→ Gray cast Iron

* If the solidification proceed below Cementite eutectic temperature due to lower the liquidus temperature through fast quenching and a

suitable nucleation agent to form an over-solute layer,

→ White cast Iron

* Two eutectic system: Fe-graphite & Fe-Fe ₃ C (cementite)

: If there is no other additive element, the Fe-graphite system is stable

& Fe-Fe

C eutectic is formed by rapid cooling of liquid phase

Fig. 6.35. Eutectic region of the iron carbon system.

① Carbon → graphite

회주철

백주철

② Carbon → Fe

C

Chapter 13 - 33

Types of Cast Iron

Gray iron

• graphite flakes

• weak & brittle in tension

• stronger in compression

• excellent vibrational dampening

• wear resistant

Ductile iron

• add Mg and/or Ce

• graphite as nodules not flakes

• matrix often pearlite – stronger but less ductile

Figs. 13.3(a) & (b), Callister &

Rethwisch 9e.

[Courtesy of C. H.

Brady and L. C. Smith, National Bureau of Standards, Washington, DC (now the National Institute of Standards and Technology, Gaithersburg, MD]

회주철

연성(구상)주철

Chapter 13 - 34

Types of Cast Iron (cont.)

White iron

• < 1 wt% Si

• pearlite + cementite

• very hard and brittle

Malleable iron

• heat treat white iron at 800-900°C

• graphite in rosettes

• reasonably strong and ductile

Figs. 13.3(c) & (d), Callister &

Rethwisch 9e.

Courtesy of Amcast Industrial Corporation Reprinted with permission of the Iron Castings Society, Des Plaines, IL

백주철

가단주철

(장미상 조직)

조밀흑연주철

35

Types of Cast Iron (cont.)

Compacted graphite iron

• relatively high thermal conductivity

• good resistance to thermal shock

• lower oxidation at elevated temperatures

Fig. 13.3(e), Callister & Rethwisch 9e.

Courtesy of Sinter-Cast, Ltd.

조밀흑연주철

Chapter 13 - 36

Production of Cast Irons

Fig.13.5, Callister & Rethwisch 9e.

(Adapted from W. G. Moffatt, G. W.

Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, Structure, p.

195. Copyright © 1964 by John Wiley &

Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.)

36

Limitations of Ferrous Alloys

1) Relatively high densities

2) Relatively low electrical conductivities 3) Generally poor corrosion resistance

37

38

II. Nonferrous Alloys

Based on discussion and data provided in Chapter 13, Callister & Rethwisch 9e. 39

II. Nonferrous Alloys

NonFerrous Alloys

• Al Alloys

-low ρ: 2.7 g/cm ³

-Cu, Mg, Si, Mn, Zn additions -solid sol. or precip.

strengthened (struct.

aircraft parts

& packaging)

• Mg Alloys

-very low ρ: 1.7g/cm

-ignites easily

-aircraft, missiles

• Refractory metals

-high melting T’s -Nb, Mo, W, Ta

• Noble metals

-Ag, Au, Pt

-oxid./corr. resistant

• Ti Alloys

-relatively low ρ : 4.5 g/cm

vs 7.9 for steel

-reactive at high T’s -space applic.

• Cu Alloys

Brass : Zn is subst. impurity (costume jewelry, coins, corrosion resistant)

Bronze : Sn, Al, Si, Ni are subst. impurities

(bushings, landing gear)

Cu-Be :

precip. hardened

for strength

1_“Superalloy” (~1940)

III. Advanced Engineering Alloys .

The term “superalloy” was first used shortly after World War II (~1945) to describe a group of alloys developed for use in turbosuper-chargers and aircraft turbine engines that required high performance at elevated temperatures ( γ’ precipi- tation hardening/ Creep property).

Around 1950, vacuum melting became comer-

cialized, which allowed metallurgists to create

higher purity alloys with more precise composition.

1940 1960 1980 2000 2020 800

1000 1200 1400

M ax temp erat ure ( o C) (1 00 0 h r. a t 1 37 M Pa )

Year

750

Nimonic105 Nimonic90

Nimonic115 IN100

Inconel713

DS Mar-M002 CMSX10

CMSX4 SRR99

TMS-162

EPM-102 TMS-196 Rene N5

TMS-238

70% T_m (Theoretical limit) : Superalloy structure

TBC coating

80% Tm : Single crystal (microstructure control) 85% Tm (Theoretical limit) : Ni-base superalloy

기존 한계 극복을 위한 新기술 개발 : 패러다임 전환

Ni 계 superalloy 발전 과정

42

Ni기 초내열 구조소재의 한계

43

Long time

(> 1000 h)

“국내 운영되는 약 130기의 가스복합발전소

유지보수비용은 연간 5,000억원

정도로 추산될 정도로 발전사업자들에게 심각한 이슈임.”

석탄화력과 가스복합화력 정책방향 설정을 위한 해외사례 조사 및 발전원가 산정에 관한 연구, 노재형 외, 2013

장시간 고온 극한 환경에 의한 재료 열화 → Ni 의 낮은 녹는점 때문

“고융점 초고온 구조 소재 개발에 대한 필요성 대두”

2_“Metallic Glass” (~1960)

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of

crystal growth. Because of this lack of crystal structure, sharp obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points and cutting and piercing tools, and its modern use as surgical scalpel blades.

Glassmaking by humans can be traced back to 3500 BCE in Mesopotamia (current Iraq).

http://en.wikipedia.org/wiki/Obsidian