Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of...

43
1 Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties

Transcript of Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of...

Page 1: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

1

Phase Transformations

in Metals Development of Microstructure and

Alteration of Mechanical Properties

Page 2: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

2

Page 3: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

3

Introduction

The development of microstructure in

alloys involves phase transformation:

Phase transformation need time, and cooling

rate greatly modify the phase transformation.

Page 4: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

4

Phase Transformation

Basic Concepts

Phase transformation as well as the

transformation rate are important to develop a

specific microstructure which in turn affect the

mechanical properties of materials.

For example, the tensile strength of Fe-C

eutectoid alloy can be 700 MPa to 2000 MPa

depending on heat treatment condition.

Page 5: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

5

Basic Concepts

Three classifications of phase transformations:

No change in composition, simple diffusion-

dependent transformation

Composition change, diffusion-dependent

transformation

Diffusionless transformation

Page 6: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

6

The 1st kind of phase transformation

No change in composition, simple

diffusion-dependent transformation

Solidification

Allotropic transformation (αγ)

Recrystallization and grain growth

No change in composition of phases.

Page 7: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

7

Composition change, diffusion-dependent

transformation

Eutectoid reaction (γ pearlite)

The 2nd kind of phase transformation

Page 8: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

8

The 3rd kind of phase transformation

Diffusionless transformation

Metastable phase

Martensitic transformation

Page 9: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

9

The Kinetics of Phase

Transformation

The progress of a phase transformation can be broken down into two distinct stages:

Nucleation

Growth

There are two types of nucleation :

Homogeneous

Hetergeneous

The growth begins once an embryo has exceeded the critical size, r*

Page 10: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

10

Solidification: Nucleation Processes

Homogeneous nucleation

nuclei form in the bulk of liquid metal

requires supercooling (typically 80-300°C max)

Heterogeneous nucleation much easier since stable “nucleus” is already present

Could be wall of mold or impurities in the liquid phase

allows solidification with only 0.1-10ºC supercooling

Page 11: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

11

The Kinetics of Phase

Transformation

Homogeneous nucleation

Nuclei of the new phase form uniformly throughout

the parent phase.

Will occur spontaneously only when free energy

change G is negative.

Fig. 10.1 solidification of a pure material from liquid

phase.

Page 12: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

12

The Kinetics of Phase

Transformation

Page 13: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

13

The Kinetics of Phase

Transformation

The are two contributions to the total free

energy change that accompany a solidification

transformation:

Free energy difference between the solid and

liquid phase: Volume free energy : Gv < 0

Formation of the solid-liquid phase boundary:

surface free energy : >0

23 43

4rGrG v

Page 14: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

14

The Kinetics of Phase

Transformation

Page 15: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

15

r* = critical nucleus: nuclei < r* shrink; nuclei>r* 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 16: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

16

The Kinetics of Phase

Transformation

The subcritical particle is an embryo, whereas

the particle of radius greater than r* is termed a

nucleus.

Since r* and G* appear at the maximum on the

free energy-versus-radius curve of Fig. 10.2b,

derivation of expressions for these two

parameters is a simple matter. equation 10.3

and 10.4

Page 17: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation

17

vGr

2*

2

3

)(3

16*

vGG

m

mf

vT

TTHG

)(

)1

)(2

(*TTH

Tr

mf

m

22

23

)(

1)

3

16(*

TTH

TG

mf

m

Page 18: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

18

The Kinetics of Phase

Transformation

T1>T2

Page 19: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation The number of stable nuclei n* (radii greater than r*) is

a function of temperature:

Another temperature-dependent parameter is also

important for nucleation: rate of diffusion, the

frequency atoms from the liquid attach to the solid

nucleus.

19

)*

exp(* 1kT

GKn

)exp(2kT

QKv d

d

Page 20: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

20

The Kinetics of Phase

Transformation

)exp()

*exp(* 3213

kT

Q

kT

GKKKvnKN d

d

Page 21: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

21

The Kinetics of Phase

Transformation

Page 22: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

22

The Kinetics of Phase

Transformation

Example Problem

Computation of critical nucleus radius and

activation free energy

Computing the number of atoms in a critical

nucleus.

Page 23: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation

Heterogeneous Nucleation

The activation energy (energy barrier) for nucleation

(△G* ) is lowered when nuclei form on preexisting

surfaces or interfaces.

The surface free energy is reduced.

23

Page 24: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

24

The Kinetics of Phase

Transformation

Hetergeneous Nucleation

cosSLSIIL

Page 25: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation

25

v

SL

Gr

2*

)()()(3

16hom

*

2

3

*

SGSG

Gv

SLhet

=30-90o, S()=0.01~0.5

Page 26: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

26

The Kinetics of Phase

Transformation

Page 27: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

27

The Kinetics of Phase

Transformation

The nucleation

rate is shifted to

higher temperature

for heterogeneous.

Page 28: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation

The growth rate is determined by the rate of

diffusion and its temperature dependence is the

same.

The overall phase transformation rate is equal to

some product of N and G

28

)exp(kT

QCG

Page 29: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

29

The Kinetics of Phase

Transformation

Page 30: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

The Kinetics of Phase

Transformation

The rate of transformation and the time required

for the transformation (say 50%) is inversely

proportional to one another.

Rate of transformation : r = 1/ t0.5

t 0.5 = time required for halfway to completion.

30

Page 31: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

31

10.3 The Kinetics of Phase

Transformation

Page 32: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

32

Rate of transformation

Page 33: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

33

The Kinetics of Phase Transformation-

Kinetic considerations of solid-state transformation

Rate of transformation : r = 1/ t0.5

t 0.5 = time required for halfway to completion.

Temperature will affect the rate of transformation:

Q = activation energy

Phase transformation is a thermal activated process.

)(RT

Q

eAr

Page 34: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

34

The Kinetics of Phase Transformation-

Kinetic considerations of solid-state

transformation

Solid-state transformation did not occur instantly.

Need composition redistribution or atomic

rearrangement

Energy increase for nucleation or new phase

boundaries.

Time-dependent process (kinetics): Avrami equation.

k, n = time-independent constant ; k relates to r.

)exp(1 nkty

Page 35: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

35

Rate of transformation

Page 36: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

36

Metastable versus Equilibrium

states

Phase transformation is a time-dependent

process, long time is required to reach

equilibrium stage.

Speed or rate of cooling is designed to produce

specific microstructure which the phase may not the

equilibrium one. metastable phase.

Supercooling or superheating happens very often in

practical purpose.

Page 37: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

37

Microstructure and Property Changes in Fe-C

Alloys

10.5 Isothermal Transformation Diagrams

Eutectoid reaction, at 727oC

γ pearlite (α+ Fe3C)

See Fig. 10.12

The lower the temperature, the fast the phase

transforms.

Page 38: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

38

Isothermal Transformation Diagrams

Page 39: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

39

Isothermal Transformation Diagrams

T~< 727oC (slightly lower):

Phase transformation needs longer time

Microstructure becomes coarse pearlite.

T << 727oC (much lower):

Phase transformation is quick.

Microstructure is fine pearlite.

See Fig. 10.13, which is called isothermal transformation diagram. (Time-temperature-transformation, T-T-T plot)

Page 40: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

40

T-T-T plot

Page 41: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

41

Cooling curve ABCD in T-T-T plot for

eutectoid reaction

Page 42: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

42

(a) Coarse pearlite and (b) fine pearlite

Page 43: Chapter 10 Phase Transformations in Metals...Phase Transformations in Metals Development of Microstructure and Alteration of Mechanical Properties 2 3 Introduction The development

43

End of section (1)

You have learned:

What is eutectoid phase transformation

Phase transformation needs time

Cooling rate affect phase transformation, and

eventually microstructure.

T-T-T plot to show how can the coarse pearlite and

fine pearlite be formed.