Atomic Ordering at the Liquid-Al/MgAl 2 O 4 Interfaces from Ab Initio Molecular Dynamics Simulations
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I.
INTRODUCTION
MGAL2O4 spinel particles form inevitably during melting and liquid handling of Al-Mg-based alloy melts.[1,2] The native spinel particles have an octahedral morphology with {1 1 1} facets. Moreover, MgAl2O4 particles form in steelmaking processes as Al and Mg are added to remove excess oxygen, can thus exist in the steel products as inclusions.[3] These particles have non-trivial impacts on the materials performances.[1,3,4] They may also act as potential nucleation sites during solidification processing.[1,5] Recent study showed that early stage of solidification process contains several steps.[6] At temperature above the nucleation temperature, liquid metal adjacent to a solid substrate exhibits atomic ordering. This phenomenon is referred to as prenucleation.[6–8] The epitaxial nucleation model[9] suggested that heterogeneous nucleation occurs in a layer-by-layer growth mechanism. The substrate surface provides a structural template to induce atomic ordering in the liquid, i.e., prenucleation. Prenucleation provides a precursor at the nucleation temperature for heterogeneous nucleation of the solid phase. Therefore, knowledge about prenucleation at the liquid-Al/MgA2O4{1 1 1} interfaces is crucial to gain insight into the role of the spinel particles in heterogeneous nucleation during solidification of Al-Mg alloys.
C.M. FANG and Z. FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, U.K. Contact e-mail: [email protected] Manuscript submitted February 26, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
MgAl2O4 belongs to the spinel family with chemical formula AB2X4, where A, B are cations, X is an anion. Spinel has a rich variety of crystal chemistry with 56 atoms in the conventional face-centered cubic (FCC) cell with space group Fd3m (No. 227).[10] The structural frame consists of a distorted FCC oxygen sub-lattice (32 O atoms at the Wyckoff sites 32e) which provides 96 interstices. Al atoms occupy half of the 32 octahedral sites (16c) and Mg occupy one-eighth of the 64 tetragonal sites (8a) in MgAl2O4. Each O is coordinated by three Al and one Mg. Along its [1 1 1] axis (Figure 1(a)), the structure of MgAl2O4 is composed of alternative O layers which have a two-dimensional (2D) hexagonal close-packed atomic arrangement (Figures 1(b) and (c)), an Al layer (Figure 1(d)), and a mixed metal layer (MgAlMg tri-sublayers) (Figure 1(g)). Chemically, MgAl2O4 is an ionic compound with Mg2+, Al3+, and O2 in the ionic model due to the large differences of electronegativity values of the metals (1.61 for Al, 1.31 for Mg in Pauling scale) and the oxygen (3.44). This implies that the smooth surfaces, e.g., the MgAl2O4{1 1 1} surfaces with an O termination (Figures 1(c) and (d)) contain net charges, being polar and are not stable at ambient conditions.[11,12] However, the situation is different for polar surfaces in liquid metal as the free electrons of the metal atoms compensate the charges.[13,14] There have been experimental and theoretical efforts to understand the struct
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