The term "plastic deformation" refers to which of the following?

Plastic deformation refers to the permanent change in shape that occurs when a material is subjected to stress beyond its elastic limit. Unlike elastic deformation, where a material will return to its original shape upon the removal of stress, plastic deformation results in a new shape that is retained even after the load is no longer applied.

In the context of metals, when they are heated or subjected to stress, they can undergo changes in their internal structure, allowing them to flow and take on new shapes without fracturing. This characteristic is crucial in metalworking processes such as forging, rolling, and bending, where the ability to permanently deform the metal is exploited to create desired shapes.

The other options do not accurately describe plastic deformation. Alteration of a metal's color under stress is typically related to surface phenomena, rather than a change in shape. Melting and solidifying involves phase changes, which do not pertain to the concept of deformation. Lastly, recovery of a metal's shape after stress removal relates to elastic deformation, not plastic deformation, as this implies that the material has not exceeded its elastic limit.