Effect of CaO/MgO bilayer film formation on nonflammability and oxidation resistance of Mg–Al–Ca alloys determined by high-temperature in situ residual stress measurement

In this study, the relationship between the oxide film structure and the protective properties of the oxide film of Mg–Al–Ca–Mn alloys was investigated by TEM observation and high-temperature in situ residual stress measurements. At 773 K, a CaO monolayer film formed on the surface. Then, a CaO/MgO bilayer film formed above 823 K. On the CaO monolayer film, tensile stress acted owing to the thermal expansion of the alloy substrate. Nevertheless, once the inner MgO layer formed, the tensile stress was relaxed, and compressive stress acted on the outer CaO layer. Additionally, the inner MgO layer acted as a scaffold for the outer CaO layer after the alloy melted, and strong compressive stress continued acting on the outer CaO layer. This suggests that the CaO/MgO bilayer can easily maintain its soundness and protectivity after melting because of the strong compressive stress. • A CaO/MgO bilayer film is formed on the surface of Mg–4.5Al–2.5Ca–0.02Mn alloy above 823 K. • Stresses acting on the CaO film at high temperatures can be successfully measured in situ. • A stress gradient exists in the CaO film because CaO is formed at the air/CaO interface. • A strong compressive stress acts on the CaO film owing to the formation of inner MgO film at 873 K.