Heterogeneous nucleation at inoculant particles in a glass forming alloy: An ab initio molecular dynamics investigation of interfacial properties and local chemical bonding

• Heterogeneous nucleation in a glass forming alloy simulated by ab initio MD.
• Nucleated and nucleant structures studied, orientations compared with experiments.
• Lattice misfit (chem. bonding) at Mg/Cu similar (stronger than) Mg/yttria.
• Work of separation (interfacial energy) at Mg/Cu > (<) that of Mg/yttria.
• Nucleation elaborated via interfacial properties (lattice mismatch and chem. bonding).

The formation of a crystalline phase from the liquid is a nucleation-induced first order phase transformation, but one that cannot be monitored easily nor has it been comprehensively explained at the atomic level. In the context of heterogeneous nucleation, we have found that, during casting of Mg-base glass forming alloys inoculated with micron-sized yttria particles, crystalline Mg nucleates preferentially on a Cu buffer layer that initially forms on the surface of yttria particles. This partial transformation generated a unique bulk metallic glass/Mg flake composite structure at room temperature exhibiting an excellent combination of high strength and ductility. To understand this transformation at the atomic level, ab initio molecular dynamics simulations have been performed, the structures of the nucleated and nucleant particles have been analyzed and their orientation relationships have been compared with the experimental observations. Furthermore, the local electronic structure at the interface has been examined and the nature of chemical bonding assessed. It is revealed that while the strain generated in Mg structures formed onto the yttria substrate and the Cu buffer layer are comparable, the nucleation is promoted by the formation of chemical bonding between atomic constituents of the nucleus and nucleant and frustrated if such bonding fails to form. In addition, the works of separation for ordered Mg/Cu and ordered Mg/yttria interfaces obtained by applying density functional theory, are found to be consistent to the results of the local chemical bonding analysis. The outcomes of the simulations provide insight into the preferential nucleation of phases at inoculant particles in terms of the interfacial properties such as interfacial lattice mismatch and chemical bonding.

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