3. MKK_18M-304C_SM_Engg_Mat 3
Crystallization of Metals
Crystallization is a process of growth of crystal during
conversion of a metal from liquid to solid state
During solidification as the metal cools, the atoms loose
their speed and arrange themselves in Crystallization form
When solidification starts, it will start at number of centers
simultaneously
3
4. MKK_18M-304C_SM_Engg_Mat 4
Crystallisation of Metals
Crystallisation or solidification occur in two stages
a) nuclei formation (Initial stage of formation of crystals)
b) crystal growth
• Atoms are very active in the liquid state and loose
speed as the liquid cools .
5. MKK_18M-304C_SM_Engg_Mat 5
When solidification starts in several places crystal
centers are formed
There are as many centers of solidification as there are
crystals
Crystal growth occurs in a dendritic pattern
Structure that has a tree like appearance is called
dendrites
5
Crystallisation of Metals
6. MKK_18M-304C_SM_Engg_Mat 6
• The crystal grows out wards until contact is made
with neighboring crystals
• This contact surface is the boundary of the crystal
• Slow cooling makes the dendrites to grow long
• Fast cooling causes short dendrite growth
• As dendrite becomes grain, slow cooling results in
large grain structure and fast cooling in small grain
structure
6
Crystallisation of Metals
8. MKK_18M-304C_SM_Engg_Mat 8
The rate of crystallization is determined by the rate of
nucleation (N) and rate of crystal growth (G)
Rate of nucleation is the number of crystals nucleating
per unit time per unit volume
The rate of crystal growth is the rate at which linear size of
crystal increases .
8
Crystallisation of Metals
9. MKK_18M-304C_SM_Engg_Mat 9
Grain Size
The grain size is determined by rate of crystal growth and
rate of nucleation
Grain size also depends on other factors
a) Rate of cooling
b)Temperature of liquid metal
c) Chemical composition
d)Impurities present in the metal
9
10. MKK_18M-304C_SM_Engg_Mat 10
Coarse grains are formed at higher value of ‘G’ and lower
value of N
Fine grains are formed at lower value of ‘G’ and higher
value of N
When the grain size is small it is known as fine grain
When the grain size is large it is known as coarse grain
structure
10
Grain Size
17. MKK_18M-304C_SM_Engg_Mat 17
Effects of Grain Size on Properties
The grain size of the metal strongly affects the mechanical
properties
Metal with fine grain structure posses high strength, and
hardness, and also they posses high fatigue strength, and
toughness
Metals with coarse grains structure are less toughness and
less hard, but exhibit better creep resistance, course grain
structure gives high ductility, good machinability, and
formability, but decrease the hardenability
18. MKK_18M-304C_SM_Engg_Mat 18
Recrystallisation
The temperature at which the new grains are formed is
called as Recrystallisation temperature
Recrystallisation has a marked effect on properties and
microstructure
Recrystallisation results in the formation new crystal and
grain growth
Recrystallisation follows recovery
18
19. MKK_18M-304C_SM_Engg_Mat 19
Recovery is the initial stage in which internal stresses
are eliminated
Recovery will not effect the structure .
There is no appreciable reduction in strength and
hardness .
It establishes a pattern of stress free equi-axed grains.
19
Recrystallisation
20. MKK_18M-304C_SM_Engg_Mat 20
Recrystallisation
Recrystallisation is the temperature level at which the first
tiny new grain appears.
It is lowest for pure metals and generally raises/lowers
by the presence of other elements (depends on type of
impurity/Alloy)
20
23. MKK_18M-304C_SM_Engg_Mat 23
Hot Working Vs Cold Working
Working of material above its recrystallisation temperature
is known as Hot working .
Working of material below its recrystallisation temperature
is known as Cold working.
23
24. MKK_18M-304C_SM_Engg_Mat 24
Large grains have lower free energy than small grains .
This is associated with the reduction of the amount of grain
boundary .
Under ideal condition the lowest energy state for a metal
would be as a single crystal.
24
Hot Working Vs Cold Working
25. MKK_18M-304C_SM_Engg_Mat 25
As the temperature increases the rigidity of the lattice
decreases and the rate of growth is more rapid.
It is possible to grow very large grains by holding a
specimen for a long time high in the grain growth region .
The specimen should be held at a temperature just below
the melting point of the alloy.
25
Hot Working Vs Cold Working
26. MKK_18M-304C_SM_Engg_Mat 26
Large grain size metals are highly ductile but low strength
and hardness .
Therefore coarse grains are not desirable in the final
structure .
26
Hot Working Vs Cold Working
28. MKK_18M-304C_SM_Engg_Mat 28
Recrystallisation temperature of metals
Metal Min Recrystallisation temperature Melting
Point
Lead Below00c 3200c
Tin Below room temperature 2320c
Zinc 200c To 1200c 41900c
Copper 1200c To 2400c 10830c
Aliminium 1600c To2800c 6600c
Silver 2800c 9600c
Gold 2800c 10600c
Iron 4500c 15400c
31. Grain Growth
If a recrystallized material is further annealed at the
same temperature or at a higher temperature grain
growth usually occurs. Boundaries between annealed
grains migrate and larger grains grow by an increase in
the average grain size (or a decrease in the ASTM grain
size number, n).
MKK_18M-304C_SM_Engg_Mat 31
32. Grain Growth
Grain growth depends on the fact that the grain
boundary energy of the material is reduced due to the
decrease in grain boundary area for a given volume of
material. The effect of recovery, recrystallization and
grain growth on grain size, internal stress and strength
(or hardness) of a plastically deformed material
MKK_18M-304C_SM_Engg_Mat 32
