Cold work, recovery, Cold work, recovery, recrystallizationrecrystallization

Example:Single crystalExample:Single crystal

= 106/cm2

State I:Annealed metal

Fairly strong and ductile

Low internal energy: stable

Cold workedCold worked

= 1010-12/cm2

Very strong and brittle

State II:Cold worked metal

High internal energy: not very stable

Heated at ~0.5 THeated at ~0.5 Tmm

= 108/cm2

Dislocation alignment:Polygonization (one recovery process)

Strong and less brittleCompared to state II

State III:Recovered metal

Internal energy is lowered

Recall what a grain Recall what a grain boundary isboundary is

Heated at ~0.5 THeated at ~0.5 Tmm

= 106/cm2

New strain free grains are formed by nucleation

Stronger than state I and ductile

State IV:Recrystallization begins

Internal energy is further lowered

Heated at ~0.5 THeated at ~0.5 Tmm

= 106/cm2

RecrystallizedStronger than state I and ductile

State V:Recrystallization complete

In summaryIn summary

We started with a single crystal We started with a single crystal and ended with a polycrystal : and ended with a polycrystal : grain refinementgrain refinement

The driving force for this process is The driving force for this process is decrease in internal energydecrease in internal energy

For a polycrystals ( in real For a polycrystals ( in real life)life)

Cold worked Initial recrystallization

Recrystallized Grain growth

Some factsSome facts

Without a minimum amount of cold Without a minimum amount of cold work (5-15%), there is no work (5-15%), there is no recrystallizationrecrystallization

After the metal is completely After the metal is completely recrystallized, grain growth occursrecrystallized, grain growth occurs

Grain growth law at a constant Grain growth law at a constant temperaturetemperatureDi

2 - D02 = Kt

Di = final grain size D0 = initial grain sizet = time