Kinetics of heterogeneous nucleation on intrinsic nucleants in pure fcc transition metals

Abstract

Nucleation during solidification is heterogeneous in nature in an overwhelmingly large fraction of all solidification events. Yet, most often the identity of the heterogeneous nucleants that initiate nucleation remains a matter of speculation. In fact, a series of dedicated experiments needs to be designed in order to verify if nucleation of the material under study is based on one type of heterogeneous nucleant and if the potency of that nucleant is constant, e.g. for a population of individual droplets, or stays constant over time, e.g. throughout repeated melting/solidification cycles. In this work it is demonstrated that one way to circumvent ambiguities and analyze nucleation kinetics under well-defined conditions experimentally is given by performing statistically significant numbers of repeated single-droplet experiments. The application of proper statistics analyses based upon a non-homogeneous Poisson process is shown to yield nucleation rates that are independent of a specific nucleation model. Based upon this approach nucleation undercooling measurements on pure Au, Cu and Ni as model materials have confirmed that the experimental strategy and analysis method are valid. The results are comparable to those obtained by classical nucleation theory applied to experimental data that has been verified to comply with the assertions that are necessary for applying this model framework. However, the results reveal also other complex nucleant-sample interactions such as an initial transient undercooling behavior and impurity removal during repeated cycling treatments. The transient undercooling behavior has been analyzed by a nucleant refining model to provide new insight on the operation of melt fluxing treatments.

Scanning AC Nanocalorimetry and Its Applications

“…The stochastic nature can be used to obtain information on the underlying nucleation kinetics by measuring the statistical fluctuations in the undercooling. When a melt is cooled down below its melting point, the undercooling at which solidification occurs typically follows a Poisson distribution and the distribution of undercooling values, typically over a 10-20 K range [60,[63][64][65], can be used to derive the nucleation rate as a function of temperature [66,67]. In the present case, the Bi and In solidification peaks are highly reproducible, indicating a more deterministic solidification process.…”

“…The very large interface-to-volume ratio of a thin film provides ample opportunity for heterogeneous nucleation as confirmed by the polycrystalline structure of the samples after solidification. By contrast, many studies of metal undercooling go to great lengths to minimize nucleation through use of small spherical samples suspended in vacuum or surrounded by a flux [60,[63][64][65]. Because so many nucleation events occur during solidification of a thin-film sample, the calorimetry trace can be regarded as a cumulative superposition of traces obtained for many individual nucleation events, and analysis of the shape of the solidification peak should provide information on nucleation statistics.…”

“…The undercooling of liquid metals and alloys below their thermodynamic freezing temperature has been a topic of interest for many years [63,64,[68][69][70][71][72][73]. The degree of undercooling and the nucleation process that initiates the liquid-to-solid phase transformation greatly influence the ensuing microstructure and properties of a material.…”

“…Instead, the kinetics of solidification is studied by measuring a large number of independent nucleation events in dispersions of similar individual droplets [70,71,[74][75][76][77][78], or by measuring nucleation in a single droplet multiple times using the same temperature trace [60,63,64,79,80]. While the single droplet method provides a direct observation of individual nucleation events, the * Reprinted with permission from K. Xiao and J. J. Vlassak, Thermochimica Acta 603 (0), 29-38 (2015).…”

“…The nucleation rate can then be determined from the distribution of undercooling values obtained for a fixed cooling rate [63,64,66], or by measuring the undercooling as a function of cooling rate [70,78].…”