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 …
g  a + Fe3C
Eutectoid transformation (Fe-C):
0.76 wt% C
6.7 wt% C
0.022 wt% C
1600
d
L
1400
1200
g +Fe3C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC
800
DT
727°C
a +Fe3C
Undercooling by DTtransf. < 727C
600
400
0
(Fe)
0.76
ferrite
L+Fe3C
1148°C
1000
0.022
a
g +L
g
(austenite)
Fe3C (cementite)
T(°C)
1
2
3
4
5
6
6.7
Co , wt%C
Isothermal Transformation Diagrams
y,
% transformed
• Fe-C system, Co = 0.76 wt% C
• Transformation at T = 675°C.
100
T = 675°C
50
0
10 2
1
T(°C)
Austenite (stable)
10 4
time (s)
TE (727C)
700
Austenite
(unstable)
600
Pearlite
isothermal transformation at 675°C
500
400
1
10
10 2 10 3 10 4 10 5
time (s)
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.
T(°C)
Austenite (stable)
700
Austenite
(unstable)
600
g
g
500
TE (727C)
Pearlite
g
g
g
g
400
1
10
10 2
10 3
time (s)
10 4
10 5
Hypoeutectoid TTT Diagram
T(°C)
d
g
L
g +L
L+Fe3C
1148°C
a
0 1
(Fe)
Fe3C
g +Fe3C
727°C
a+Fe3C
2
3
4
5
6 6.7
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.
Hypereutectoid TTT Diagram
T(°C)
d
g
L
g +L
L+Fe3C
1148°C
a
0 1
(Fe)
Fe3C
g +Fe3C
727°C
a+Fe3C
2
3
4
5
6 6.7
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 1.13%C, 0.30% Mn
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
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
T(°C)
d
L
g +L
g
a
(ferrite)
L+Fe3C
1148°C
Fe3C
(cementite)
a
727°C
a+Fe3C
0
1
(Fe)
2
3
4
5
Fe3C
g +Fe3C
6 6.7
Co , wt%C
60 m
• 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
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
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.
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:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram
800
Austenite (stable)
T(°C)
A
pearlite/bainite boundary
100% bainite
400
B
A
200
10-1
10
103
a (ferrite)
TE
P pearlite
100%
600
Fe3C
(cementite)
105
time (s)
5 m
Tempering Martensite
• reduces brittleness of martensite,
• reduces internal stress caused by quenching.
TS(MPa)
YS(MPa)
1800
1400
TS
YS
1200
1000
60
50
%RA
40
30
%RA
800
200
400
9 m
1600
Adapted from
Fig. 10.33,
Callister 7e.
(Fig. 10.33
copyright by
United States
Steel
Corporation,
1971.)
600
Tempering T (°C)
• produces extremely small Fe3C particles surrounded by a.
• decreases TS, YS but increases %RA
Alloying Additions
Effect of adding other
elements
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard g  a + Fe3C
transformation
4340 steel (alloyed steel)
Cooling Curve
plot temp vs. time