Kinetics of heterogeneous nucleation on intrinsic nucleants in pure fcc transitionmetals

Abstract

Nucleation during solidification is heterogeneous in nature in an overwhelmingly largefraction of all solidification events. Yet, most often the identity of the heterogeneousnucleants that initiate nucleation remains a matter of speculation. In fact, a series ofdedicated experiments needs to be designed in order to verify if nucleation of the materialunder study is based on one type of heterogeneous nucleant and if the potency of thatnucleant is constant, e.g. for a population of individual droplets, or stays constantover time, e.g. throughout repeated melting/solidification cycles. In this work it isdemonstrated that one way to circumvent ambiguities and analyze nucleation kineticsunder well-defined conditions experimentally is given by performing statisticallysignificant numbers of repeated single-droplet experiments. The application of properstatistics analyses based upon a non-homogeneous Poisson process is shown to yieldnucleation rates that are independent of a specific nucleation model. Based upon thisapproach nucleation undercooling measurements on pure Au, Cu and Ni as modelmaterials have confirmed that the experimental strategy and analysis methodare valid. The results are comparable to those obtained by classical nucleationtheory applied to experimental data that has been verified to comply with theassertions that are necessary for applying this model framework. However, theresults reveal also other complex nucleant–sample interactions such as an initialtransient undercooling behavior and impurity removal during repeated cyclingtreatments. The transient undercooling behavior has been analyzed by a nucleantrefining model to provide new insight on the operation of melt fluxing treatments.