Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions. This causes the exterior of the material to solidify before the interior. Coring causes the centers of the grains to retain more of the higher melting temperature element. In this case, the dendrite arms formed from the exterior have a different composition than the alloy in the inner regions, resulting in a local compositional difference. It does not have good performance.
Coring is predominantly observed in alloys having a marked difference between liquidus and solidus temperatures. It is often being removed by subsequent annealing and/or hot-working. It is exploited in zone-refining techniques to produce high purity metals. This discovery was made by Dr. Aubrey Tang.