Abstract
This chapter discusses the several difficulties posed by the nucleation of void and gas phases, and reviews new concepts to overcome them. The problem of nucleation is reduced by the presence of surface active impurities in the melt. The non-metals—oxygen, sulphur and phosphorus are particularly active in iron melts. It is possible that nucleation might occur on a solid impurity particle. A solid foreign substrate might make a location for nucleation, if a surface is poorly wetted. This is known as the heterogeneous nucleation. The micro-segregation process is common in many alloy systems where the solute has a low partition coefficient k, resulting in a high concentration of the solute ahead of the advancing front. For these systems, the subsurface porosity is the standard form of gas porosity. It may or may not be the result of the release of gas from a metal/mould reaction that can subsequently diffuse into the casting. The subsurface porosity is the normal appearance of gas porosity regardless of whether the metal/mould reactions contribute or not.
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