http://onlinemetallurgy.com/

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.