Cold work, recovery, recrystallization

Example:Single crystalr = 106/cm2State I:Annealed metalFairly strong and ductileLow internal energy: stable

Cold workedr = 1010-12/cm2Very strong and brittleState II:Cold worked metalHigh internal energy: not very stable

Heated at ~0.5 Tmr = 108/cm2Dislocation alignment:Polygonization (one recovery process)Strong and less brittleCompared to state IIState III:Recovered metal Internal energy is lowered

Recall what a grain boundary is

Heated at ~0.5 Tmr = 106/cm2New strain free grains are formed by nucleationStronger than state I and ductileState IV:Recrystallization begins Internal energy is further lowered

Heated at ~0.5 Tmr = 106/cm2RecrystallizedStronger than state I and ductileState V:Recrystallization complete

In summaryWe started with a single crystal and ended with a polycrystal : grain refinementThe driving force for this process is decrease in internal energy

For a polycrystals ( in real life)Cold workedInitial recrystallization

RecrystallizedGrain growth

Some factsWithout a minimum amount of cold work (5-15%), there is no recrystallizationAfter the metal is completely recrystallized, grain growth occursGrain growth law at a constant temperatureDi2 - D02 = KtDi = final grain sizeD0 = initial grain sizet = time

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