A Comparative Study of Prenucleation on Zr and MgO Substrates by ab initio MD Simulations

Abstract

The traditional wisdom for grain refiners is by selecting the most potent solid particles (i.e. having the least misfit between the substrate and the metal) [1,2]. A good example is the Mg-Zr grain refiner for Mg and Mg-alloys. Zr is iso-structural to Mg with a small lattice misfit (~0.7 % at room temperature) [3]. The melting temperature of Zr is 1855°C (2128 K), notably higher than that of Mg (649°C). Interestingly, Zr is chemically repulsive to Mg with a heat of mixing +6 kJ/mol, and there forms no Zr-Mg intermetallic compound [3,4]. Zr particles have been considered to be good for heterogeneous nucleation of Mg through structural templating according to the epitaxial nucleation model [5]. Recently, it was also suggested that the most effective grain refinement can be achieved by the least potent particles (i.e. having largest misfit) if no other more potent particles of significance exist in the melt [6]. The role of MgO in heterogeneous nucleation of Mg can be a good demonstration of this theory [6]. [...]