High Temperature Oxidation Behavior of Mg-9Al-xCa Alloys
The work aims to study the effect of Ca content on the isothermal oxidation behavior of magnesium alloys at high temperature,and analyze the mechanism of Ca in the oxidation process,so as to provide a reference for the application of mag-nesium alloys at high temperature.The as-cast alloy structure was analyzed with the Mg-9Al-xCa alloy system as the object.At the same time,isothermal oxidation experiments were carried out for magnesium alloys with different Ca contents at 480 ℃ for different durations using self-designed experimental equipment.The oxidation weight gain was measured,the oxide layer of the alloy was characterized and analyzed,and the mechanism of action was revealed by thermodynamic calculations.The main phases in the Mg-9Al alloy were the matrix α-Mg phase and the Mg17Al12 phase at the grain boundary.When Ca was added,the alloy was mainly composed of α-Mg,Mg17Al12 phase at the grain boundary and Al2Ca phase,in which the Al2Ca phase formed(α+Al2Ca)eutectic with α-Mg.With the increase of Ca content,the Mg17Al12 phase formed when the alloy melt solidified gradually decreased,and the proportion of(α+Al2Ca)eutectic in the microstructure increased.After oxidation for 200 min,the oxidative weight gain of Mg-9Al alloy reached 32.781 mg/cm2,a convex and concave structure formed on the oxide layer,the oxidative weight gain of Mg-9Al-containing magnesium alloy was greatly reduced,and the oxidative weight gain of Mg-9Al-5.0Ca was 0.046 mg/cm2,and the morphology of magnesium oxide layer of magnesium alloy with different Ca content showed differences.Thermodynamic calculations showed that when the Ca content was greater than 0.4wt.％,Ca could easily undergo oxidation reaction to form an oxide layer dominated by CaO.In conclusion,Ca content is the key factor affecting the oxidation behavior of magnesium alloys,and the oxidation resistance of magnesium alloys gradually improves with the increase of Ca content.
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