Nucleation in bulk undercooled nickel-base alloys

Abstract

The undercooling behaviour of binary Cu 1− x Ni x bulk melts was investigated over the entire concentration range 0 ⩽ × ⩽ 1 using an electromagnetic levitation technique in an ultraclean environment. The elimination of container-wall induced nucleation has led to undercooling levels of bulk melts up to 30 K larger than the highest values obtained so far by droplet dispersion. The analysis of the results within the framework of a nucleation theory for binary systems indicates that, despite the large undercoolings, the limit of homogeneous nucleation is not achieved. Microstructural investigations of the samples reveal heterogeneous surface nucleation as the prevailing nucleation mechanism. The role of surface oxidation as the responsible catalyst for nucleation is discussed with respect to the different undercooling behaviour of the samples in helium and HeH 2 atmospheres.