Prenucleation at the Interface Between MgO and Liquid Magnesium: An Ab Initio Molecular Dynamics Study
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GRAIN refinement is usually desirable during metal casting since it not only facilitates the casting processes, but also accomplishes a grain refined microstructure with reduced cast defects, which in turn enhances mechanical performance of as-cast components.[1–4] A well-established approach to grain refinement enhances heterogeneous nucleation by addition of grain refiners which contain potent solid particles as nucleation sites.[1,2] A typical example is the grain refinement of Al-free Mg-alloys by addition of Mg-Zr master alloys.[2,4–10] Zr is iso-structural to Mg with a small lattice misfit (0.67 pct), and therefore Zr particles should act as potent nucleation sites for a-Mg during solidification according to the epitaxial nucleation model.[11] Recently, a new concept of grain refinement was introduced based on the concept of explosive grain initiation, in which the most effective grain refinement can be achieved by the least potent particles if there exist no other more potent particles of significance in the melt.[12] This new approach to grain refinement can be best demonstrated by grain refinement of Mg-alloys by
C.M. FANG and Z. FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, United Kingdom. Contact e-mail: [email protected] Manuscript submitted 3 December, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the native MgO particles.[13] Without addition of any grain refiner, high pressure die casting of commercial purity Mg resulted in an average grain size of 6 lm.[12] To understand better such experimental results and to obtain new insight into the heterogeneous nucleation process, it is essential to have detailed knowledge about the surface structures of MgO particles in contact with liquid Mg and the atomic arrangement in the liquid adjacent to the liquid Mg/MgO interfaces (denoted as L-Mg/MgO interfaces hereafter). Magnesia (MgO) particles always exist in Mg melts due to the high affinity between O and Mg. MgO has a NaCl-type structure. It is a typical ionic crystal and belongs to the family of MX (M represents a metallic element, X an element of high electronegativity). The ionic MX crystals under ambient conditions have a stable {0 0 1} surface termination (denoted as MX{0 0 1} hereafter), which contains equal numbers of Mn+ and Xn ions and therefore are non-polar.[14] A cleavage along the MX[1 1 1] orientation produces two smooth surfaces: one with the M surface termination, the other with X surface termination, with both surfaces being polar. Such polar surfaces are unstable under ambient conditions, but can be stabilized by defects, e.g., M or X domains.[15] However, the situation may become different when an ionic crystal is in a liquid metal environment. The free electrons of the liquid metal can eliminate the polar effect and stabilize the polar surfaces, such as in the case of MgO{1 1 1} in liquid Mg. MgO{1 1 1} surfaces have a two dimensional hexagonal lattice,
the same as that of the close packed Mg{0 0 0 1} plane. However, there exists a large lattice misfit betwe
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