Abstract
The precipitation of matrix (i.e. non-dislocation nucleated) niobium carbide in a 20/25 austenitic stainless steel has been studied using thin-foil electron microscopy. It is found that the nucleation of particles depends more on the vacancy concentration than on the solute or vacancy supersaturation. It is suggested that excess vacancies relieve the strain associated with the nucleation of NbC particles by becoming incorporated within the nucleus, and a model is proposed which attempts to interpret this in terms of classical nucleation theory. The nucleation of the particles is considerably enhanced by small additions of phosphorus to the steel. The role of phosphorus is not clear but it probably enhances the clustering of vacancies and prolongs the excess vacancy life time, thereby promoting the aggregation of solute atoms and vacancies forming the nucleus.
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