Characterisation of magnesium oxide and its interface with α-Mg in Mg-Al-based alloys

Magnesium oxide (MgO) films and particles have been collected by pressurised filtration of a Mg-8.6wt% Al-0.67wt% Zn (AZ91D) alloy melt. The morphology of the oxides and their interfaces with the alpha-Mg phase were investigated by high-resolution transmission electron microscopy. It was found that the oxide films consisted of large numbers of sub-micrometre- sized MgO particles, and that melt shearing can effectivelybreak up the oxide films and disperse the oxide particles. For the first time, orientation relationships (ORs) of OR I: [1 (1) over bar 11(Mg)O similar to 2 degrees from (0001)(alpha-Mg) and (011)(MgO)//[2 (1) over bar(1) over bar0)(alpha-Mg); and OR II: ((1) over bar(1) over bar1)(MgO)//(1 (1) over bar 01)(alpha-Mg) and inverted right perpendicular011inverted left perpendicular(alpha-Mg), were observed between the MgO particles and the alpha-Mg matrix. The calculated Bramfitt planar disregistries were 5.5% and 2.5% for the two ORs, respectively, indicating good lattice matching between MgO and alpha-Mg at the interface. With the evidence of grain refinement effect observed in the sheared AZ91D magnesium alloy, the possibility of MgO particles to act as potent nucleants for heterogeneous nucleation of alpha-Mg grains is discussed in terms of the crystallographic criterion.