Prenucleation Induced by Crystalline Substrates
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UNDERSTANDING heterogeneous nucleation is of both scientific and practical importance in terms of controlling solidification, which occurs in many industrial processes.[1] The classical nucleation theory (CNT) states that a cap of the solid phase with a critical size forms on the substrate through fluctuation of atomic configuration, chemical composition, and temperature in the undercooled liquid.[1] The potency of a substrate in the CNT is defined by the contact angle between the cap and the substrate, which depends on both structural and chemical characteristics of the substrate for a given liquid. In the CNT, however, the structural and chemical effects of the substrate on heterogeneous nucleation are not taken explicitly into consideration, and the liquid adjacent to the substrate is assumed to be completely disordered prior to heterogeneous nucleation. The epitaxial nucleation model proposes an atomistic mechanism for heterogeneous nucleation, where the substrate lattice provides a template for the formation of the new phase in consideration of the pronounced atomic ordering in the liquid adjacent to the substrate/liquid interface (SuLI).[2] Recent theoretical and experimental findings, as reviewed by Kaplan et al.,[3] suggest that at temperatures above the
H. MEN and Z. FAN are with BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, U.K. Contact e-mail: [email protected] Manuscript submitted November 30, 2017
METALLURGICAL AND MATERIALS TRANSACTIONS A
liquidus, the atoms in the liquid at the interface become layered within a few atomic layers away from the interface (atomic layering) and that the atoms within each individual layer may have a significantly ordered structure (in-plane atomic ordering). It has been speculated that such atomic ordering may have significant influence on the subsequent heterogeneous nucleation process.[4] Both experimental observations and atomistic simulations have provided direct evidence for the existence of atomic ordering at the interface. Experimental observations confirmed that substrates can induce atomic ordering in the liquid at the interface.[5–12] X-ray diffraction experiments revealed that the liquid metal becomes layered within a few atomic layers away from the surface or interface.[5,6,9] This is consistent with the theoretical calculations,[13–17] which showed that the liquid has an oscillatory density profile at the interface with a solid wall, due to the ‘hard wall’ effect.[18,19] Oh et al.[20,21] have provided firm evidence for the atomic layering and in-plane ordering in liquid Al adjacent to an a-Al2O3 substrate, through direct high-resolution transmission electron microscopy (HRTEM) observation. They found that the Al atoms at the interface have a high degree of structural correlation to the crystal structure of the a-Al2O3 substrate. On the other hand, Schu¨lli et al.[22] observed that the Au atoms in AuSi eutectic droplets have a pentagonal atomic arrangement at the interface with a Si (111) substrate, using in situ X-ray scattering. They argued that
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