What is the term for the porosity that develops when solid grains and gas bubbles nucleate simultaneously at the initial stage of metal solidification against the mold walls?

Explanation

Subsurface porosity refers to the voids formed beneath the surface due to the concurrent nucleation of solid grains and gas bubbles right as the metal begins to freeze at the mold interface. This distinguishes it from other types such as pinhole or gas inclusion porosity, which form under different conditions.