Heterogeneous Nucletation During Rapid Solidification by Laser Surface Melting

Crstal nucleation during rapid solidification generally occurs rather slowly except at active heterogeneities or unless very large undercoolings are achieved. Many typical microstructures are then essentially of columnar morphology, for example from the bottom surface of a melt-spun ribbon or from a heterogeneity on the surface of a powder particle. In such cases the microstructure may be considerably refined by the presence of many active nucleants for heterogeneous nucleation distributed throughout the melt. Such microstructural refinement is analysed here during rapid solidification of a laser-melted surface containing fine TiB2 particles. Simulation of the cooling and solidification conditions confirms these particles to be highly effective nucleating substrates, capable of greatly increasing nucleating rates. As a result it is possible to obtain materials possessing greatly refined grain sizes.