Abstract
Magnesia (MgO) particles inevitably exist in liquid Mg and may be used as potential sites for heterogeneous nucleation to achieve effective grain refinement. Understanding of the atomic configurations on MgO surfaces and in the liquid Mg adjacent to the liquid Mg/MgO interfaces is therefore of both scientific and practical interests. We investigate the surface structures of MgO in liquid Mg and the atomic arrangements of liquid Mg adjacent to liquid/substrate interfaces, using an ab initio molecular dynamics (MD) simulation technique. We find that an atomically rough terminating Mg layer forms on the {1 1 1} terminated MgO substrate (octahedral MgO) in liquid Mg. The simulations also reveal that on the structurally flat {0 0 1} terminated MgO substrate (cubic MgO) a rough Mg layer forms due to the unique chemical interactions between the ions on the substrate and the liquid metals. The surface roughness together with the large lattice misfits with solid Mg makes both octahedral and cubic MgO substrates impotent for heterogeneous nucleation of α-Mg. The present results may shed new light on grain refinement of Mg-alloys.
Highlights
Zr is iso-structural to Mg with a small lattice misfit (0.67 pct), and Zr particles should act as potent nucleation sites for a-Mg during solidification according to the epitaxial nucleation model.[11]
Using a parameter-free ab initio molecular dynamics simulation technique, we investigated the atomic configurations and chemistry of MgO{0 0 1} and MgO{1 1 1} surfaces in liquid Mg
We showed that an atomically rough terminating Mg layer forms on the MgO{1 1 1} substrate in liquid Mg
Summary
GRAIN refinement is usually desirable during metal casting since it facilitates the casting processes, and accomplishes a grain refined microstructure with reduced cast defects, which in turn enhances mechanical performance of as-cast components.[1,2,3,4] 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.[2,4,5,6,7,8,9,10] Zr is iso-structural to Mg with a small lattice misfit (0.67 pct), and Zr particles should act as potent nucleation sites for a-Mg during solidification according to the epitaxial nucleation model.[11].
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