1. 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

2. Consider Eutectoid Transformation …
transformation (Fe-C): g Þ a +
Fe3C
0.76 wt% C
0.022 wt% C
6.7 wt% C
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
Co , wt%C
1148°C
T(°C)
ferrite
727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC DT
Undercooling by DTtransf. < 727C
0.76
0.022

3. Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% C
• Transformation at T = 675°C.
100
T = 675°C y,
% transformed 50 2 4 1 10 10
time (s)
400
500
600
700 1 10 2 3 4 5
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C

4. Effect of Cooling History in Fe-C System
• Eutectoid composition, Co = 0.76 wt% C
• Begin at T > 727°C
• Rapidly cool to 625°C and hold isothermally.
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C) 1 10 2 3 4 5
time (s) g g

5. Hypoeutectoid TTT Diagram
T(°C) d L
g +L g
1148°C
L+Fe3C
g +Fe3C a
727°C
Fe3C
a+Fe3C 1 2 3 4 5 6
6.7
(Fe)
Co , wt%C
Af represents highest temperature ferrite can form
As is the eutectoid temperature
MS is the martensite start temperature
Isothermal transformation diagram for 0.35% C, 0.37% Mn
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

6. Hypereutectoid TTT Diagram
Fe3C 1 2 3 4 5 6
6.7 L g
g +L
g +Fe3C
a+Fe3C
L+Fe3C d
(Fe)
Co , wt%C
1148°C
T(°C) a
727°C
Af represents highest temperature ferrite can form
As is the eutectoid temperature
MS is the martensite start temperature
Isothermal transformation diagram for 1.13%C, 0.30% Mn
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

7. 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

8. Spheroidite Spheroidite: -- a grains with spherical Fe3C d L g a a1 2 3 4 5 6
6.7 L g
g +L
g +Fe3C
a+Fe3C
L+Fe3C d
(Fe)
Co , wt%C
1148°C
T(°C) a
727°C
60 m a
(ferrite)
(cementite)
Fe3C
Equilibrium phase diagram tells us that the stable phase distribution in the two phase field is:
a + 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
Spheroidite: -- a grains with spherical Fe3C

9. 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 -- DG
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

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

11. 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

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

13. Bainite • Bainite: • Isothermal Transf. Diagram TE T(°C) time (s)
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram
Fe3C
(cementite) a
(ferrite) 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50%
pearlite/bainite boundary A
100% bainite
100% pearlite
5 mm

14. Tempering Martensite • • • reduces brittleness of martensite,
• reduces internal stress caused by quenching.
YS(MPa)
TS(MPa)
800
1000
1200
1400
1600
1800 30 40 50 60
200
400
600
Tempering T (°C)
%RA TS YS
Adapted from Fig , Callister 7e. (Fig copyright by United States Steel Corporation, 1971.)
9 mm
produces extremely small Fe3C particles surrounded by a. • •
decreases TS, YS but increases %RA

15. Alloying Additions Effect of adding other elements Cr, Ni, Mo, Si, Mn
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard    + Fe3C
transformation
4340 steel (alloyed steel)

16. Cooling Curve
plot temp vs. time