Question

Explain how cold work stops the movement of dislocations and strengthens a material.

Answer 1

Cold working is a process which is usually performed below recrystallisation temperature or generally at room temperature for most metals. The elastic recovery of the deformation is negligible thus we can say that the material leads to be permanently deformed which is also called plastic deformation. Due to plastic deformation, dislocations breaks and which leads to the generation of additional dislocations. Thus with the process of multiplication, number of dislocations increases. Because of higher dislocation density, the space for the movement of dislocation becomes limited and at the stage these dislocations become pinned or tangled.

As we discussed, cold working reduces the mobility of the dislocations and increases the dislocation density. Due to higher density, the interaction between the dislocations becomes higher under the applied load which leads to increase in the strength of the material. This phenomenon is also known as strain hardening. Also the following formula represents how strength increases with dislocation density.

Shear stress (t) = to + A√(dislocation density)