Materials Science Chapter 10 1

Chapter 10Phase Transformations in Metals

Materials Science Chapter 10 2

The Kinetics of Solid-State Reactions• Obstacles in solid-state

transformation– Diffusion

– Energy increase

• Phase transformation stages– Nucleation: at grain boundaries

– growth

• Kinetics (time-dependent)

y = 1 – exp(-ktn)

T = constant

• Rate of transformation

• Characterization of transformation

S-Curve

5.0

1t

r =

Materials Science Chapter 10 3

Temperature Dependence RTQAer /−=

Percent recrystallization for pure copper

Materials Science Chapter 10 4

Iron-Carbon Isothermal Transformation• Pearlite

)%7.6()%022.0()%76.0( 3 CwtCFeCwtCwt +⇔αγ

Materials Science Chapter 10 5

Iron-Carbon Isothermal Transformation• T-T-T diagram (isothermal

transformation diagram)• Eutectoid temperature• Supercooling• Phase regions

• Transformation rate• Pearlite nucleation

• This curve only valid for eutectoid composition

Materials Science Chapter 10 6

Iron-Carbon Isothermal Transformation

• Thermal treatment curve

Materials Science Chapter 10 7

Iron-Carbon Isothermal Transformation

• Coarse pearlite • Fine pearlite

Materials Science Chapter 10 8

Iron-Carbon Isothermal Transformation

Materials Science Chapter 10 9

Iron-Carbon Isothermal Transformation

• Coarse pearlite • Fine pearlite

Materials Science Chapter 10 10

Iron-Carbon Isothermal Transformation• Bainite Upper bainite

(300 ~ 540 oC)Pearlite vs. Bainite

•Competitive

Lower bainite

(200 ~ 300 oC)

Materials Science Chapter 10 11

Iron-Carbon Isothermal Transformation

• Spheroidite– From pearlite or bainite

– Heated about 700 oC for 18 ~24 hours

– Driving force• Reduction of α - Fe3C

phase boundary area

Materials Science Chapter 10 12

Iron-Carbon Isothermal Transformation

• Martensite– Nonequilibrium single-

phase– From diffusionless

transformation of austenite– Rapid quenching– Occurs instantaneously– Time-independent– Competitive with pearlite

and bainite– Body-centered tetragonal

(BCT)– Supersaturated solid

solution

Lenticular

(more than 0.6 wt% C)

Lath

(less than 0.6 wt% C)

Materials Science Chapter 10 13

Iron-Carbon Isothermal Transformation

Materials Science Chapter 10 14

Iron-Carbon Isothermal Transformation

1. Rapidly cool to 350 oC, hold for 104 s, and quench to room temperature.

2. Rapidly coo to 250 oC, hold for 100 s, and quench to room temperature.

3. Rapidly cool to 650 oC, hold for 20 s, rapidly cool to 400 oC, hold for 103 s, and quench to room temperature.

Materials Science Chapter 10 15

Iron-Carbon Isothermal Transformation

Materials Science Chapter 10 16

Continuous Cooling Transformation Diagram

Shift to longer time and lower temperature

Materials Science Chapter 10 17

Continuous Cooling Transformation Diagram

Materials Science Chapter 10 18

Critical Quenching Rate

• The minimum rate of quenching to generate 100% martensite.

Materials Science Chapter 10 19

Mechanical Behavior of Pearlite

• Cementite is very hard and brittle.

• Fine pearlite is harder and stronger than coarse pearlite.

Materials Science Chapter 10 20

Mechanical Behavior of Spheroidite• Less boundary area per unit

volume in spheroidite• Spheroidized steels are

extremely ductile.

Materials Science Chapter 10 21

Mechanical Behavior of Bainite• Bainite is stronger and harder than

pearlite.Upper bainite

(300 ~ 540 oC)Pearlite vs. Bainite

•Competitive

Lower bainite

(200 ~ 300 oC)

Materials Science Chapter 10 22

Mechanical Behavior of Martensite

• Martensite is the hardest and strongest and most brittle (negligible ductility).

• Related to interstitial carbon atoms.

• Volume increase during phase transformation.

Materials Science Chapter 10 23

Tempered Martensite• Heat treatment martensite

between 250 ~ 650 oC.• Internal stress is relieved.• Ductility and toughness are

enhanced.martensite (BCT, single phase)

tempered martensite (ferrite + cementite)

9300 X

Materials Science Chapter 10 24

Temperature and Time Influence on Tempering

Materials Science Chapter 10 25

Iron-Carbon Transformation

Materials Science Chapter 10 26

Iron-Carbon Transformation

Materials Science Chapter 10 27

Mechanical Behavior

Materials Science Chapter 10 28

Mechanical Behavior

Materials Science Chapter 10 29

Mechanical Behavior

Materials Science Chapter 10 30

Mechanical Behavior