Coring happens when a heated alloy, such as a Cu-Ni

Cupronickel

Cupronickel or copper-nickel or "cupernickel" is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese. Cupronickel is highly resistant to corrosion in seawater, because its electrode potential is adjusted to be neutral with regard to seawater...

 system, cools in non-equilibrium conditions. This causes the exterior of the material to harden faster than the interior. Coring causes the exterior layers to retain more of the higher melting temperature element. In this case, the dendrite

Dendrite (crystal)

A crystal dendrite is a crystal that develops with a typical multi-branching tree-like form. Dendritic crystal growth is very common and illustrated by snowflake formation and frost patterns on a window. Dendritic crystallization forms a natural fractal pattern...

arms formed from the exterior have a different composition than the alloy in the inner regions, resulting in a local compositional difference.