Isothermal Transformation Diagrams(Time-Temperature-Transformation (TTT) Diagrams)

• Plot temperature on the y-axis

• Plot time on the x-axis (typically logarithmic scale)

• Maps of phase creation as a function time at temperature

• These are ONLY valid for isothermal (constant temperature) transformation

• Each diagram is ONLY valid for a specific composition

Consider Eutectoid Transformation … Eutectoid transformation (Fe-C): + Fe3C

0.76 wt% C0.022 wt% C

6.7 wt% C

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

ferrite727°C

Eutectoid:

Equil. Cooling: Ttransf. = 727ºCT

Undercooling by Ttransf. < 727C

0.7

6

0.0

22

Isothermal Transformation Diagrams

• Fe-C system, Co = 0.76 wt% C• Transformation at T = 675°C.

100

50

01 102 104

T = 675°C

y,

% tr

ansf

orm

ed

time (s)

400

500

600

700

1 10 102 103 104 105

0%pearlite

100%

50%

Austenite (stable) TE (727C)Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.

Effect of Cooling History in Fe-C System

400

500

600

700

0%pearlite

100%

50%

Austenite (stable)TE (727C)

Austenite (unstable)

Pearlite

T(°C)

1 10 102 103 104 105

time (s)

Hypoeutectoid TTT Diagram

Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Fe 3

C

0 1 2 3 4 5 6 6.7

L

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

727°C

Isothermal transformation diagram for 0.35% C, 0.37% Mn

• Af represents highest temperature ferrite can form

• As is the eutectoid temperature

• MS is the martensite start temperature

Hypereutectoid TTT Diagram

Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Fe 3

C

0 1 2 3 4 5 6 6.7

L

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

727°C

Isothermal transformation diagram for 1.13%C, 0.30% Mn

• Af represents highest temperature ferrite can form

• As is the eutectoid temperature

• MS is the martensite start temperature

Metastable Phase Transformations

• Where on this diagram is martensite shown?

• How about bainite?• How about spheroidite?

This is the EQUILIBRIUM Phase Diagram for Fe-C system

Metastable phases are temporary phase which are intermediate between the initial and equilibrium states

Spheroidite: -- grains with spherical Fe3C

Spheroidite

60 m

(ferrite)

(cementite)

Fe3C

Fe 3

C

0 1 2 3 4 5 6 6.7

L

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

727°C

• Equilibrium phase diagram tells us that the stable phase distribution in the two phase field is:

+ Fe3C – typically as lamellar microstructural constituent pearlite

• Is pearlite the lowest energy state – NO! With lamellar structure pearlite has a lot of interfacial energy

• If we anneal a pearlite microstructure we will get a transformation to a new phase distribution that minimizes the energy of the system when atomic mobility is activated

Martensite Transformation• Formed by rapid quenching of an alloy

• Occurs in several alloy systems (indium-thallium, titanium, nickel-iron, gold-cadmium, and Steel)

• Shear driven atomic realignment – similar to deformation twinning only more complex

• Militaristic transformation – diffusionless transformation

• Only driven by changes in temperature -- G

• New lattice is formed around a habit plane (plane shared between parent and daughter phases)

• Form lens-shaped shear plates during transformation – speed of transformation can approach speed of sound in the material

• Congruent phase change

Martensitic Transformation

Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Bain distortion: conversion of one lattice into another by expansion or contraction along crystallographic axes

Lattice parameters change as we increase the amount of carbon in solution

Martensite Effects

• Change in volume associated with formation of martensite

• For a 1% carbon steel we see a volume increase of 4%

• The shear transformation and the volume change combine to create a high density of dislocations

• Lath martensite has internal dislocation density on the order of 1015 – 1016 /m2

• Very fine microstruture of cell boundaries and laths

Mechanical Properties

Source: H. K. D. H. Bhadeshia, Bainite in Steels, 2nd Edition, Cambridge Press, 2001.

Bainite• Bainite: -- lathes (strips) with long

rods of Fe3C --diffusion controlled.• Isothermal Transf. Diagram

Fe3C

(cementite)

5 m

(ferrite)

10 103 105

time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

pearlite/bainite boundary

A

A

100% bainite

100% pearlite

Tempering Martensite• reduces brittleness of martensite,• reduces internal stress caused by quenching.

Adapted from Fig. 10.33, Callister 7e. (Fig. 10.33 copyright by United States Steel Corporation, 1971.)

• decreases TS, YS but increases %RA• produces extremely small Fe3C particles surrounded by

9 m

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

30

40

50

60

200 400 600Tempering T (°C)

%RA

TS

YS

%RA

Alloying Additions

Effect of adding other elementsChange transition temp.

Cr, Ni, Mo, Si, Mn

retard + Fe3C

transformation

4340 steel (alloyed steel)

Cooling Curveplot temp vs. time