The document discusses TTT diagrams and phase transformations in steel. It explains that TTT diagrams plot transformation over time and temperature, showing the different phases formed. The C-shaped curve represents the austenitic, pearlitic, bainitic, and martensitic regions. Ms and Mf lines mark the start and end of martensite formation. Martensite has a body-centered tetragonal crystal structure formed by atomic realignment without diffusion. The document also discusses order and disorder in alloy solutions above and below the critical temperature.

2. Muhammad Umair
Bukhari
Engr.umair.bukhari@gmail.com
http://onlinemetallurgy.com

3. T-T-T diagram is plotted for different length
of times and temperature
These curves gives us the information of
transformation for different phases
With the help of T-T-T diagram we can
control the phase transformation by
changing the conditions

4.  The left side of C-shape
curve is Austenitic
region
 The right side of C-
shape curve contain
three regions
• Pearlite
• Pearlite+Bainite
• Bainite

5.  The left line of C-shape
curve shows the start of
phase transformation
 The right line shows the
end of transformation
 The dotted line is
theoretically determined
line shows 50%
transformation

6. The region below Ms
is martensitic region
Ms shows start and
Mf shows end of
Martensite
transformation

7. Martensitic crystal structure is Body
centered tetragonal (BCT)
It is the intermediate structure of Fcc and
Bcc
It forms due to the realignment of atoms
from austenitic structure to martensitic
structure

8. Atoms arrange themselves to move
towards stability from higher temperature
to lower temperature
Only realignment is occur but there is no
diffusion
There is no concentration change in parent
and product phase

9.  Due to the realignment of atoms the
lattice smmetry is different from the
parent phase

10.  At 300oc the bainite and
martensite are formed
along curve 3
 As temperature
decreases the bainite
formation rate is lowers
 At 160oC for 20min the
50% martensite and
austenite are formed

11.  Mf line which shows
the end of martensite
transformation
 But 100% martensite
can’t be formed even
theoretically at
absolute zero
 At room temperature
there is 3% retaine
austenite

12.  The reason is the retained
autenite
 As the little amount of
austanite is remain it is
difficult to transform
 That’s why Mf lline is
showed dotted

13. •Regular arrangements of atoms in a
crystal is called order.
•Order of atoms in crystals are mostly at
room temperature

14. Irregular arrangements of atoms in a
crystal is called disorder .
Disorder mostly occurs at elevated
temperature.

15.  In most substitutional solid solutions two kinds of
atoms A and B are arranged more or less
random on the atomic sites of the lattice.
 In solution of this type of a change in
temperature is to Increase or decrease of a
thermal vibrations.
 If the ordered solution is heated above Tc the
atomic arrangement becomes more or less
random again and the solution is said to be
disordered.

16.  The gold and copper atoms of AuCu3 above a
critical temperature of about 390’C are arranged
more or less random on the atomistic sites of
FCC lattice as shown in following figure.
 Below the critical temperature the gold atoms in
a perfectly ordered alloy occupy only the corner
position of the unit cube and the copper atoms
on the face centered cubic as shown in following
figure.

18. When the periodic arrangement of A and B
atoms persists over longer range distance
in a crystal than it is called a long range
order.