Abstract
Undercooling experiments have been carried out on seven refractory metals (W, Re, Ta, Mo, Nb, Ir, and Zr) using the 48 m high Grenoble drop tube. A statistical analysis of the nucleation events allows a comparison of the experimental and theoretical pre-exponential terms Kv of the nucleation rate, giving confidence in a homogeneous process for these metals, except Zr. In the framework of the classical theory of nucleation, the relevant parameters are shown to be both the entropy of fusion and the solid-liquid interfacial energy. A new scaling is then proposed which relates the relative amount of undercooling to a dimensionless number, characteristic of the self-resistance of the metal to nucleation. Each experimental value of the undercooling, from 0.10 of the melting temperature for Mo, to 0.25 for Re, is well interpreted through a verification of the semi-empirical Skapski's correlation based on the hypothesis of perfect wetting of a solid by its own melt. This treatment also supports the maximum undercoolings obtained so far on other cubic or compact metals (Cu, Ni, Co, …). Finally, high-drop tube processing ensures a favourable chemical context to attempt homogeneous nucleation on refractory metals.