The formation of grain core is called nucleation. There are two different ways of nucleation in liquid metal crystallization
(1) Autogenic nucleus
When crystallizing, it is not affected by external factors. With the decrease of temperature, metal atoms or molecules aggregate to form solid particles as crystal nucleus, which is usually called homogeneous nucleation.
(2) Non autogenic nucleus
In the process of crystallization, if there are fine non-metallic particles in the liquid metal which are not suitable for the metal lattice, the metal atoms or molecules can agglomerate and nucleate on its surface, which is usually called heterogeneous nucleation.