Experimental studies of crystal nucleation: metals and colloids Dieter M. Herlach1), Thomas Palberg2), Ina Klassen1)3), Stefan Klein1), Raphael Kobold 1) 1) Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 51170 Köln, Germany 2) Institut für Physik, Johannes Gutenberg Universität Mainz, 55099 Mainz, Germany 3) Projektträger Jülich, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany 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 con- ditions 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 crystal- lographic 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 nucle- ated under non-equilibrium conditions. The results are analyzed within classical nucleation theory that defines the activation energy of homogeneous nucleation in terms of the inter- facial 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 pro- cess of colloids is observed in situ by optical observation and ultra-small angle X-ray dif- fraction using high intensity synchrotron radiation. It allows an unambiguous discrimina- tion of homogeneous and heterogeneous nucleation as well as the determination of the in- terfacial 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. 1 1 Introduction ........................................................................................................................ 3 2 Experimental set ups........................................................................................................... 5 2.1 Metals .......................................................................................................................... 5 2.1.1 Electromagnetic levitation.................................................................................... 5 2.1.2 Electromagnetic levitation in reduced gravity...................................................... 8 2.1.3 Electrostatic levitation........................................................................................ 10 2.2 Colloids...................................................................................................................... 14 2.2.1 Preparation of charge stabilized colloidal suspensions ...................................... 14 2.2.2 Laser light scattering ......................................................................................... 17 2.2.2 Ultra small Angle X-ray scattering..................................................................... 19 3. Experimental results and discussions – a representative selection.................................. 23 3.1 Metals and alloys....................................................................................................... 23 3.1.1 Short range order in the undercooled liquid state of pure metals and alloys .... 23 3.1.2 Phase selection by crystal nucleation ................................................................. 26 3.1.3. Crystal nucleation in the undercooled melt of pure Zr and binary Zr-based compound .................................................................................................................... 28 3.2 Colloidal suspensions ................................................................................................ 34 3.2.1 Short range order in the liquid state ................................................................... 34 3.2.2 Determination of the metastability of colloidal suspensions in liquid state....... 38 3.2.2 Nucleation studies in colloidal suspensions ....................................................... 41 3.2.3 Interfacial energy, entropy and enthalpy of fusion............................................. 47 3.2.4 Turnbull plots for colloids, metals and simulations: a comparison.................... 50 4. Conclusion....................................................................................................................... 51 2 1 Introduction Metallic materials are prepared from the liquid state as their parent phase. The condi- tions under which the liquid solidifies determine the physical and chemical properties of the as-solidified material. In most cases, time and energy consuming post-solidification treatment of the ingot material is mandatory to obtain the final product with its desired properties and design performance. Therefore, efforts are directed towards virtual material design with computer assisted modelling. This can shorten the entire production chain - ranging from casting the shaped solid from the melt to the final tuning of the product in order to save costs during the production process. The goal is to fabricate novel materials with improved properties for specific applications. Computational materials science performed from the liquid state requires detailed knowledge of the physical mechanisms involved in the solidification process. In particular, crystal nucleation is the decisive process that initiates solidification and pre-selects the crystallographic phase either stable or metastable. The short-range order in the liquid as precursor often influences nucleation and the phase transformations of first order. Nuclea- tion is a thermally activated process that requires an undercooling of the liquid below its equilibrium melting temperature to create a driving force for the formation of supercritical nuclei. In particular, at large undercooling, nucleation pathways for different crystallo- graphic phases can compete with each other. This gives access to non-equilibrium solidifi- cation, which can form meta-stable solids, which may differ in their physical and chemical properties from their stable counterparts. Detailed modelling of solidification, far away from thermodynamic equilibrium, requires that the solidification process must be investi- gated in every detail [1]. In order to achieve the state of an undercooled melt, it is advantageous to remove het- erogeneous nucleation sites which otherwise limit the amount of achievable undercooling. One efficient way to realize such conditions is containerless processing of melts [2]. In such, the most dominant heterogeneous nucleation process, involving interaction with con- tainer walls, is completely avoided. Nowadays, electromagnetic [3] and electrostatic levi- tation techniques [4, 5] have been developed for containerless undercooling and solidifica- tion of molten metals and alloys. A freely suspended drop gives the extra benefit to di- rectly observe the solidification process by combining the levitation technique with proper diagnostic means [6, 7]. Short range ordering in undercooled metallic melts as precursor of crystal nucleation has been investigated by using neutron diffraction [8] and synchro- tron radiation [9] on containerless undercooled melts [10]. Additionally, primary phase selection processes for rapid solidification of meta-stable phases has been observed in situ by energy dispersive X-ray radiation using synchrotron radiation of high intensity [11]. Nucleation is governed by heat and mass transport. Thus, any transport process stimu- lated externally by natural convection and/or forced convection due to stirring effects of alternating electromagnetic fields in electromagnetic levitation experiments may cause serious influence on the solidification process. To understand this effect and to develop a 3 quantitative description of crystallization in the presence of forced convection comparative experiments on Earth and in reduced gravity are of great help. Under the special conditions of reduced gravity for instance in Space, the forces needed to compensate disturbing accel- erations are about three orders of magnitude smaller than the forces needed to compensate the gravitational force for levitation experiments on Earth. In a cooperative effort by the European Space Agency (ESA) and the German Space Center – Space Management (DLR), a facility for Electro-Magnetic Levitation (EML) was constructed and accommo- dated on board the Columbus European module of the International Space Station (ISS). Many experiments are in progress by international investigator teams to apply this multi- user facility for investigations on undercooled melts under the specific environment condi- tions of reduced gravity in Space [12]. Even though a great variety of models ranging from physical atomistic to pure phe- nomenological models have been developed to understand