Microstructural characterization of oxide layers in CaO added AZ31 Mg alloy

Using transmission electron microscopy (TEM) we studied the oxidation behaviors of CaO added Mg-3.5Al-1.4Zn (AZ31) alloys as a function of the CaO content and oxidation time, at 500 °C in air. It was shown that the AZ31 alloys with CaO exhibit substantially enhanced oxidation resistance. We identified the mechanism for this behavior: with added CaO the AZ31 alloys form protective oxide layers consisting of inner columnar and outer Ca/Mg rich structures. The thicknesses of the protective layers were observed and found to be correlated with the CaO content. The inner columnar layer grows along the [100]MgO due to the small energy barrier to the diffusion of interstitial oxygen ions in MgO. We observed three distinctive steps during the oxidation reaction: the formation of oxide islands, the dissipation of scale and the growth of a columnar oxide layer, and the formation of a double oxide layer due to Ca reduced from CaO. We clearly captured the microstructural evolution of the three different steps, leading to a detailed description of the formation of the protective oxide layers.