Misfit paradox on nucleation potency of MgO and MgAl2O4 for Al

MgO and MgAl2O4 are believed to be effective heterogeneous nuclei for Al based alloys due to their small lattice misfits with Al. However, there is a strong evidence to suggest that liquid Al reacts with MgO and MgAl2O4 phases but the heterogeneous nucleation behavior of such phases is rarely discussed. In order to identify the nucleation mechanism of Al, under the interference of the chemical reaction, the heterogeneous nucleation process is systematically investigated through thermal analysis and high resolution transmission electron microscopy (HRTEM). The observed multi-nucleation interfaces (Al/MgO, Al/MgAl2O4 and Al/Al2O3) and scattered experimental undercooling data indicate an independent multi-phase nucleation process in these systems.