What process occurs before solidification begins?

The process that occurs before solidification begins is undercooling the metal below its solidification temperature. Undercooling, or supercooling, involves cooling a liquid metal below its freezing point without it solidifying. This is a critical aspect of phase change because it allows for the development of the necessary conditions that promote nucleation, which is the initial step in the solidification process. Once the metal is sufficiently undercooled, impurities or imperfections can help in the nucleation process, leading to the formation of solid crystals as the metal transitions from liquid to solid.

In the context of the other options, heating the metal to its melting point is part of the process of making a metal melt, not a precursor to solidification. Forming an alloy with better properties typically refers to a design choice or improvement after solidification has already occurred. Creating thermal equilibrium after melting does not directly lead to solidification; rather, it suggests a uniform temperature throughout the molten metal, which is essential but happens at a different stage in the process. Hence, undercooling directly correlates with the onset of solidification.