Which of the following occurs when a mechanical force is repeatedly applied to most metals at room temperature, such as rolling, hammering, or bending?

• A. The metals become cold worked, strain or work hardened
• B. The metals become soft
• C. The metals crack and fail
• D. The metals become more ductile

Answer: (A)

Repeated mechanical work at room temperature causes cold working, also known as strain or work hardening. When you roll, hammer, or bend metal, you plastically deform it and create many new dislocations. These dislocations start to interact and tangle with each other, forming obstacles that make it harder for other dislocations to move. That increased difficulty to move dislocations raises the metal’s yield strength and hardness, so the material becomes stiffer and less ductile. This is why cold-working metals often feels stronger but more brittle than the same metal in its annealed state. If you heat the metal, recovery or recrystallization can occur and restore ductility by reducing dislocation density. The other outcomes don’t fit because softening at room temperature doesn’t happen from ordinary deformation, and while severe deformation can lead to cracking, the common result of repeated cold work is hardening rather than universal failure or increased ductility.