Overview: Experimental studies of crystal nucleation: Metals and colloids

Abstract

Crystallization is one of the most important phase transformations of first order. In the case of metals and alloys, the liquid phase is the parent phase of materials production. The conditions of the crystallization process control the as-solidified material in its chemical and physical properties. Nucleation initiates the crystallization of a liquid. It selects the crystallographic phase, stable or meta-stable. Its detailed knowledge is therefore mandatory for the design of materials. We present techniques of containerless processing for nucleation studies of metals and alloys. Experimental results demonstrate the power of these methods not only for crystal nucleation of stable solids but in particular also for investigations of crystal nucleation of metastable solids at extreme undercooling. This concerns the physical nature of heterogeneous versus homogeneous nucleation and nucleation of phases nucleated under non-equilibrium conditions. The results are analyzed within classical nucleation theory that defines the activation energy of homogeneous nucleation in terms of the interfacial energy and the difference of Gibbs free energies of solid and liquid. The interfacial energy acts as barrier for the nucleation process. Its experimental determination is difficult in the case of metals. In the second part of this work we therefore explore the potential of colloidal suspensions as model systems for the crystallization process. The nucleation process of colloids is observed in situ by optical observation and ultra-small angle X-ray diffraction using high intensity synchrotron radiation. It allows an unambiguous discrimination of homogeneous and heterogeneous nucleation as well as the determination of the interfacial free energy of the solid-liquid interface. Our results are used to construct Turnbull plots of colloids, which are discussed in relation to Turnbull plots of metals and support the hypothesis that colloids are useful model systems to investigate crystal nucleation.

The effect of hydrodynamics on the crystal nucleation of nearly hard spheres

“…3,4 The considerable body of work performed so far indicates the relevance of the topic both in fundamental physics and other disciplines. [5][6][7][8][9][10] Despite this, the mechanism by which a supersaturated fluid of colloidal hard spheres transforms into a crystal is still unclear. Several scenarios such as a one-step, two-step, devitrification, or spinodal-like process have been proposed, [11][12][13][14][15] but consensus has not yet been reached.…”

Free-energy barriers for crystal nucleation from fluid phases

“…microstructure of metallic alloys obtained by cooling their melts), for instance [1][2][3][4][5][6]. The classical theory assumes that nanoscopic nuclei of the crystalline solid form spontaneously by thermal fluctuations when one suddenly has brought the system from the region where the fluid phase is the thermodynamically stable phase beyond the fluid-solid coexistence curve, so that the fluid phase is only metastable ("homogeneous nucleation").…”

The droplet formation-dissolution transition in different ensembles: Finite-size scaling from two perspectives

“…9 shows our attained data points along with the analytic prediction (T c (V ) T g + a T V −1/3 ), where a T ≈ −0.467 was again calculated from the grand canonical reference. The plotted free fit is of similar form as before (T c (V ) = Tg + ãT V −1/3 + bT V −2/3 ) and fitting the complete range of data gives Tg = 0.40018 (6) with ãT = −0.430(3) at good χ 2 ≈ 1.6.…”

“…However, the additive corrections to σ will influence only higher-order corrections of ∆ when plugged into Eq. (6). Hence, we stick to convention [14] and use the infinite-size estimate τ = 2 √ πσ ∞ .…”

“…In principle, the formation of a droplet in an equilibrium vapour can be induced by a pressure or temperature quench, corresponding to a supersaturated or supercooled gas. Due to experimental constraints on controlling the temperature homogeneously, most experimental approaches follow a setup at fixed temperature with increasing oversaturation [4], whereas undercooling is typically used for crystal nucleation in metals and colloids [5,6].…”