What is meant by 'undercooling' in solidification?

Undercooling refers to the phenomenon where a liquid is cooled below its freezing point without it becoming solid. This process typically occurs during the solidification of materials, particularly in metals and other crystalline solids. When a liquid is undercooled, it remains in a liquid state even though its temperature has fallen below the temperature at which it would normally solidify. This can allow for the formation of a more uniform structure upon solidification once nucleation occurs.

When considering the concept of undercooling, it's important to understand that the process enables the liquid to achieve conditions that could lead to a finer grain structure upon transitioning to solid, as the delayed nucleation typically leads to a more stable final product. In practical terms, undercooling is crucial in processes such as casting and metallurgy.

The other choices do not accurately capture the essence of undercooling. For example, cooling with no changes in solidification suggests that the material remains in a fixed state without any transformation, which contradicts the very nature of undercooling. Heating above the solidification point describes a scenario that does not relate to the cooling process at all, and cooling gradually without causing solidification implies a controlled process but does not highlight the specific condition of being below the solidification temperature,