What does "Work Hardening" of metals refer to?

Work hardening, also known as strain hardening, is a phenomenon that occurs when a metal is subjected to mechanical deformation, such as bending, stretching, or compressing. During this process, the metal's internal structure is altered due to the dislocation movement within the material.

As the metal is worked on, dislocations multiply and interact leading to an increase in the number of obstacles that impede further dislocation movement. This results in an increase in hardness and strength, primarily because the material becomes more resistant to further deformation. Therefore, the correct definition of work hardening is indeed the increase in hardness of a metal due to mechanical work.

The other options describe processes or effects that do not accurately reflect the concept of work hardening. For instance, increasing ductility of a metal would typically occur through annealing or other heat treatments rather than work hardening. Reducing the weight of a metal through removal is a definition more aligned with machining processes rather than a change in the material itself. Spontaneous cooling of metals does not relate to changes in hardness or material properties post-deformation, as this refers more to thermal processes rather than mechanical work. Thus, the definition that highlights the increase in hardness resulting from mechanical work accurately describes work hardening