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

Abstract

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 α-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 effectively break up the oxide films and disperse the oxide particles. For the first time, orientation relationships (ORs) of OR I: [1 1 1]_MgO∼2° from (0 0 0 1)_α-Mg and (0 1 1)_MgO //[2 1 1 0)_α-Mg; and OR II: (1 1 1)_MgO//(1 1 0 1)_α-Mg and [0 1 1]_MgO//[1 2 1 1]_α-Mg, were observed between the MgO particles and the α-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 α-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 α-Mg grains is discussed in terms of the crystallographic criterion.

Keywords:

• magnesium alloys
• magnesium oxide
• interface
• transmission electron microscopy

Acknowledgements

The authors are grateful to the EPSRC (UK) for financial support. They also thank Mr. T. Hashimoto from the University of Manchester for his help on the TEM work.