5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Grain refinement of aluminium alloys has always been desirable in foundries. It is a common practice to add grain refiners into the alloy melt to promote heterogeneous nucleation and hence achieve effective grain refinement. However, the efficiency of commercial grain refiners is extremely low as less than approximately 1% of the added particles initiate aluminium grains during solidification while the rest remain inactive and stay in the final castings. These particles become inclusions when the castings are recycled and the effect accumulates with repeated recycling. Recently, it has been demonstrated that the native oxide particles can be utilised for effective grain refinement in aluminium alloys. This holds great promise to achieve grain refinement without the addition of grain refiners and provides a solution to produce closed-loop recyclable aluminium alloys. Nevertheless, further investigation is still needed for commercial aluminium alloys, which normally contain certain levels of alloying and impurity elements. One of the major reasons is that it is unclear how these solute elements affect heterogeneous nucleation, particularly the potency of nucleation substrates. Most recently, it has been found that the adsorption of solute elements onto the surface of nucleant particles plays a significant role in affecting the effectiveness of these particles acting as nucleation sites for aluminium grains. Therefore, it is intriguing to study the potential elemental adsorption on the surface of native oxide particles in aluminium alloys. In the present work, the potential element adsorption onto the surface of native oxide particles has been investigated by analytical transmission electron microscopy on samples prepared using a melt filtration technique. Using selected area diffraction and high-resolution imaging, the crystal structure and the habit plane of the native oxide particles has been firstly determined. Furthermore, the potential elemental adsorption at the interface between oxide particles and Al grain matrix has been studied by using super scanning transmission electron microscopy combined with electron energy loss spectroscopy. Based on the experimental results, the nature of the elemental adsorption in terms of composition and structure has been analysed using a simple lattice matching model. Finally, the effect of elemental adsorption on the nucleation potency of oxide particles and hence on their role in solidification will be discussed.