Discontinuous Precipitation of a Liquid in Cu-base Alloys – Nucleation and Growth Mechanisms

The morphology of discontinuous precipitation of a liquid has been studied in Cu-9.5 at.% In and Cu-7.5 at.% Sn alloys. The transformation of a supersaturated solid solution (α o ) was carried out in the two-phase (α + liquid) field. A lamellar structure, consisting of alternate solute-depleted (a) and liquid phases, has been observed to grow from grain boundaries of the supersaturated α o . The nucleation of the liquid droplets occurred at grain boundaries as two hemispherical caps abutted at the original grain boundary positions. A thin liquid film replaced the grain boundary and started migrating as the transformation progressed. The lamellae multiplication occurred by nucleation of new lamellae at the migrating front. The lamellae multiplication also occurred by nucleation of new lamellae at midlines between the liquid lamellae where the interlamellar spacing was large. A cylindrical loop of liquid formed behind a migrating liquid film, resulting into double-seam morphology and an irregular lamellar structure on one side of the original grain boundary. The liquid film also migrated in the shape of sine wave when the liquid film between neighbouring droplets migrated in opposite directions near the original grain boundary. This has resulted in double-seam morphology with a regular lamellar structure.