Undercooling and Nucleation Studies on Pure Refractory Metals by Drop Tube Processing: Comments on the Limit to Crystalline Nucleation of Cubic and Compact Metals

A graphic interpretation for homogeneous nucleation is discussed in the framework of the classical theory of nucleation. If the value of two-thirds of the melting temperature may be regarded as an ultimate limit for liquid undercooling, the physical limit is shown to be an intrinsic property of the element under consideration, as verified by undercooling experiments performed on refractory metals under ultrahigh vacuum conditions. The graphic interpretation also suggests elements for new experiments, since the likelihood of heterogeneous nucleation remains. Some of them could be relevant to a nucleation mechanism through metastable polymorphism as proposed for Zr.