EFFECT OF SHARP CONCENTRATION GRADIENTS ON THE NUCLEATION OF INTERMETALLICS IN DISORDERED SOLIDS - INFLUENCE OF THE EMBRYO SHAPE

This work, which concerns the effect of unidirectional sharp concentra tion gradients on the nucleation of intermetallics in amorphous layers or in supersaturated solid solutions, is focused on the embryo shape. The study leads to a new analysis of the critical concentration gradi ent beyond which the nucleation driving force: is suppressed. Owing to the fluctuations in the embryo shape, it is shown that the minimizati on of the thermodynamic potential leading to the equilibrium aspect ra tio of a parallelepiped-shaped embryo is only possible above a certain value of the concentration gradient. Beyond such a concentration grad ient, the embryo aspect ratio is found to deviate significantly from t hat given by the Wulff conditions. Application is presented to the nuc leation of the compounds Al3Ni and Ni10Zr7 in a supersaturated Al-Ni s olid solution and in a Ni-Zr amorphous layer respectively.