Cold work, recovery, recrystallization

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Cold work, recovery, recrystallization
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ExampleSingle crystal
State IAnnealed metal
Fairly strong and ductile
r ? 106/cm2
Low internal energy stable
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Cold worked
State IICold worked metal
r ? 1010-12/cm2
Very strong and brittle
High internal energy not very stable
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Heated at 0.5 Tm
State IIIRecovered metal
r ? 108/cm2 Dislocation alignment Polygonization
(one recovery process)
Strong and less brittle Compared to state II
Internal energy is lowered
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Recall what a grain boundary is
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Heated at 0.5 Tm
State IVRecrystallization begins
r ? 106/cm2 New strain free grains are formed by
nucleation
Stronger than state I and ductile
Internal energy is further lowered
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Heated at 0.5 Tm
State VRecrystallization complete
r ? 106/cm2 Recrystallized
Stronger than state I and ductile
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In summary

• We started with a single crystal and ended with a
  polycrystal grain refinement
• The driving force for this process is decrease in
  internal energy

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For a polycrystals ( in real life)
Cold worked
Initial recrystallization
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Recrystallized
Grain growth
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Some facts

• Without a minimum amount of cold work (5-15),
  there is no recrystallization
• After the metal is completely recrystallized,
  grain growth occurs
• Grain growth law at a constant temperature

Di2 - D02 Kt
Di final grain size
D0 initial grain size
t time