The document discusses the concepts of cold work, recovery, recrystallization, and grain growth in metals. Cold work increases strength and hardness but reduces ductility, while recovery involves the rearrangement of defects at low temperatures. Recrystallization replaces deformed grains with strain-free grains, and grain growth occurs when recovery and recrystallization are completed at high temperatures.

1. Notes
On
“Recovery, Recrystallisation and
Grain Growth”Grain Growth”
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2. Introduction
We now introduce a ‘technical term’ called Cold
Work. We will arrive at a formal definition of the
term at the end of this topic.
For now we use a working definition of cold work
as:as:
Plastic deformation in the temperature range
(0.3 – 0.5) Tm → COLD WORK
During cold work the point defect density
(vacancies, self interstitials…) and dislocation
density increase.
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3. Typical cold working techniques are rolling, forging, extrusion etc.
Cold working is typically done on ductile metals (e.g. Al, Cu, Ni)
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Cold working is typically done on ductile metals (e.g. Al, Cu, Ni)

4. The cold worked material is stronger (harder), but
is brittle (as noted before).
 Heating the material (typically below 0.5 Tm) is
and holding for sufficient time is a heat treatment
process called annealing.
Depending on the temperature of annealing
processes like Recovery (at lower temperatures)
or Recrystallisation (at higher temperatures) mayor Recrystallisation (at higher temperatures) may
take place. During these processes the material
tends to go from a microstructurally metastable
state to a lower energy state (towards a stable
state).
Further ‘annealing’ of the recrystallized material
can lead to grain growth.
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7. Recovery is a process by which
deformed grains can reduce their stored
energy by the removal or rearrangement of
defects in their crystal structure.
These defects, primarily dislocations, are
introduced by plastic deformation of theintroduced by plastic deformation of the
material and act to increase the yield strength
of a material.
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8. recovery
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9. Recovery is a process that kicks in at low
temperatures; in this stage, the excess defects
are annealed out, dislocations of opposite signs
annihilate each other, and dislocations align to
form low energy configurations, namely tiltform low energy configurations, namely tilt
and twist boundaries
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10. Basically we can say,
Basically we can say,
In Cold working
• Strength, Electrical Resistance and hardness
Increases.
• Ductility Decreases
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11. Summary
 Recovery takes place at low temperatures of annealing
“Apparently no change in microstructure”
 Excess point defects created during Cold work are absorbed:
 at surface or grain boundaries
 by dislocation climb
 Random dislocations of opposite sign come together and annihilate
each othereach other
 Dislocations of same sign arrange into low energy configurations:
 Edge → Tilt boundaries
 Screw → Twist boundaries
 POLYGONIZATION
 Overall reduction in dislocation density is small
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12. Recrystallisation
Recrystallisation is the process in which
deformed grains are replaced by strain-free
grains.
It is generally observed that higher cold workIt is generally observed that higher cold work
and smaller initial grain size lead to finer
recrystallised grains.
The driving force for recrystallisation is the
stored strain energy in the material.
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13. Recrystallisation is defined as the process in
which grains of a crystal structure come in a
new structure or new crystal shape.
A precise definition of recrystallisation isA precise definition of recrystallisation is
difficult to state as the process is strongly
related to several other processes, most
notably recovery and grain growth. In some
cases it is difficult to precisely define the point
at which one process begins and another ends.
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14. Process of Recrystallisation
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Recrystallisation of a metallic material (a → b)
and crystal grains growth (b → c → d).

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17. For Recrystallisation to take place, a minimum
of cold working is needed; if the deformation
is very low, recrystallisation does not occur.
During recrystallisation, the stress-free grains
nucleate and grow.
In a recrystallised system (after more thanIn a recrystallised system (after more than
about 90% deformation) the resultant
microstructure is textured; that is, most of the
grains have similar orientation.
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Changes in properties with % cold work and annealing temperature; increasing
annealing temperatures leads to recovery and recrystallisation. Note that still higher
annealing temperatures leads to grain growth.

19. The temperature at which a given material
completely recrystallises in an hour is defined as
the recrystallisation temperature.
The recrystallisation temperature is low if the
degree of deformation is high and/or if the
temperature of deformation is low and/or if the
initial grain size is small.initial grain size is small.
The recrystallisation temperature is also very
sensitive to the purity of the material; an addition
of 0.01 at.% of Te to pure copper, for example,
can increase the recrystallisation temperature by
about 240◦C.
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21. Grain Growth
• Grain growth is the increase in size of grains
(crystallites) in a material at high temperature.
• This occurs when recovery an recrystallisation
are complete and further reduction in the
internal energy can only be achieved byinternal energy can only be achieved by
reducing the total area of grain boundary.
• The term is commonly used in metallurgy but
is also used in reference to ceramics and
minerals.
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22. Grain Growth
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23. Grain Growth
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24. a) Grain growth occurs by the movement of grain
boundaries and not by coalescence (i.e. like water
droplets)
b) Boundary movement is discontinuous and the
direction of motion may change suddenly.
c) One grain may grow into another grain whilst being
consumed from the other side
d) The rate of consumption often increases when the
grain is nearly consumedgrain is nearly consumed
e) A curved boundary typically migrates towards its
centre of curvature
f) When grain boundaries in a single phase meet at
angles other than 120 degrees, the grain included by
the more acute angle will be consumed so that the
angles approach 120 degrees.
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27. ThanksThanks
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