Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions.
As a casting having a cored structure is reheated, grain boundary regions will melt first in as much as they are richer in the low-melting component.
This produces a sudden loss in mechanical integrity due to the thin liquid film that separates the grains.
Coring may be eliminated by a homogenization heat treatment carried out at a temperature below the solidus point for the particular alloy composition.
Coring is predominantly observed in alloys having a marked difference between liquidus and solidus temperatures.