Work hardening

This is also called strain hardening. When an external force exceeding the yield point is applied to a metal crystal, it undergoes plastic deformation that does not recover even when the external force is removed. To advance this plastic deformation further, a larger external force must be applied. The increase in deformation resistance that accompanies the progression of this plastic deformation is called work hardening. In the case of a single crystal, slip occurs only in one slip system at first, so work hardening is weak. However, as the crystal axis rotates due to deformation, other slip systems begin to function, dislocations cut each other, and the obstacles to dislocations on the slip plane increase rapidly, entering the second stage of hardening. Eventually, the dislocations cross-slip to avoid the obstacles and harden to a level where they can proceed (third stage). Hardening after this stress is not very noticeable. In polycrystals, only the second and third stages of hardening are observed. Work hardening is an obstacle in plastic processing, but on the other hand, it increases the yield point of the product, improving its mechanical properties. Work hardening can be restored by annealing.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

ひずみ硬化ともいう．金属結晶に降伏点を超えた外力が加えられると，外力を除いても回復しない塑性変形を起こす．この塑性変形をさらに進めるためには，さらに大きな外力を加えなければならない．この塑性変形の進行に伴う変形抵抗の増加を加工硬化という．単結晶の場合，最初に一つのすべり系のみですべりが起こるので，加工硬化は微弱である．しかし，変形により結晶軸が回転を起こすため，ほかのすべり系がはたらきはじめ，転位の切合いが起こり，すべり面上に転位の障害が急激に増加し，硬化する第二の段階に入る．ついには転位は交差すべりにより障害物を避けて，進行できる応力にまで硬化する(第三段階)．この応力以後の硬化はあまり顕著ではない．多結晶では第二および第三段階の硬化のみ観察される．加工硬化は塑性加工において障害となるが，一方，製品の降伏点を高めるので，機械的性質は改善される．加工硬化は焼なましにより回復される．

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