Characterisation of Native MgO and Its Roles in Solidification of Mg Alloys

Abstract

Magnesium (Mg) and its alloys are becoming one of the most important structural materials in modern society. Since a refining microstructure is beneficial to nearly all aspects of properties of cast and wrought Mg alloys, grain refinement has been a major task to be tackled. Although zirconium (Zr) is currently considered as the most effective grain refiner for Al-free Mg alloys, there is not a universal grain refiner for Mg alloys yet. Recently, the native MgO particles present in Mg alloy melts have shown their ability for grain refinement in commercial purity (CP) Mg and AZ series alloys. However, the limited knowledge of the nature of native MgO and the interaction between MgO and solute inhibits the understanding of solidification process. The widespread application of native MgO particles for grain refinement of Mg alloys is therefore prevented. In this thesis, extensive research was carried out to investigate the native MgO particles and their effect on solidification of commercial purity (CP) Mg and binary Mg alloys including Mg-Y (yttrium), Mg-Ca (calcium), Mg-Sn (tin) and Mg-Al (aluminium). The solidified structure and grain size of CP-Mg and the binary Mg alloys were assessed by TP-1 casting method, and the native MgO particles in these alloy melts were comprehensively characterised through SEM, TEM and the state-of-the-art aberration corrected STEM plus EELS. Firstly, the nature of native MgO present in the CP-Mg and the Mg alloys was well understood by comprehensive characterisation from micron-meter scale to the atomic scale. The native MgO was found to appear as liquid films that are composed of discrete MgO particles. Owing to different formation mechanisms during oxidation, two types of MgO particles differed in morphologies form. One is octahedral MgO with {111} facets and the other is cubic MgO with {100} facets. Secondly, the native MgO particles were demonstrated to act as substrates for heterogeneous nucleation of Mg, leading to significant grain refinement of CP-Mg, although they were impotent substrates in terms of the lattice misfit calculated according to the experimentally observed orientation relationships. Thirdly, elemental segregation on the surface of native MgO particles was experimentally identified in the binary Mg alloys containing Y, Ca and Sn, respectively. It is found that the structural and chemical configuration of the segregation layer formed on the MgO surface were affected by the atomic configuration of the faceting crystal planes of MgO and the specific solutes inside the melt. The potency of the native MgO particles for heterogeneous nucleation was therefore modified accordingly. Finally, with the knowledge of native MgO and its interaction with solute, the mechanism for grain refinement by the native MgO particles in CP-Mg and the binary Mg alloys were understood. Due to the modifiable potency of native MgO particles by the interfacial segregation, the corresponding grain initiation behaviour can be dominated by either progressive or explosive manner. The application of native MgO particles can potentially be adopted as a universal technique for grain refinement of Mg alloys.