High Temperature Oxidation Behavior And Mechanism Of Mg-Ca Alloys

Magnesium and its alloys have the advantage of low density,and are the lightest metal structural materials in the industry,which have broad application prospects in the fields of automobiles,aerospace and 3C products.However,Mg shows high affinity with oxygen.Notably at high temperature,Mg and its alloys are easily oxidized,and even burn,releasing a large amount of heat,which has become one of the bottlenecks restricting the widespread application of Mg alloys.Therefore,improving the high temperature oxidation resistance of Mg alloys is of great significance to further expand the application of Mg alloys.The research object of this paper is Mg alloys for automobile,aerospace and other industries,which puts forward higher requirements for the oxidation resistance of Mg alloys.In addition to requiring Mg alloys to maintain excellent oxidation resistance in air,requirements for oxidation resistance of Mg alloys under flame conditions are also required.Reports showed that the oxidation resistance of Mg and its alloys can be significantly improved by alloying.Ca element is favored by researchers because of its low price and good properties of Mg-Ca alloys.Ca can significantly improve the high temperature oxidation resistance and ignition temperature of Mg and its alloys,which is related to the oxide film containing Ca O formed on the surface.Previous researches showed that only Ca dissolved inα-Mg participates in the formation of Ca O oxide film,while the oxidation mechanism of the second phase Mg₂Ca is controversial.Meanwhile,the oxidation behavior of Mg-Ca alloy under flame is in blank state,and the influence and mechanism ofα-Mg and Mg₂Ca in Mg-Ca alloy on the oxidation resistance of Mg-Ca alloy under flame are not clear.In this paper,combined with first-principles calculations and experimental analysis methods,the oxidation resistance at air and under flame of Mg-Ca alloys were systematically studied from both theoretical and experimental perspectives.The main research results and conclusions are as follows:Based on first-principles calculations,the oxidation models ofα-Mg and Mg₂Ca were established from the atomic point of view:1)O₂ is chemically adsorbed on Mg,α-Mg and Mg₂Ca and forms stable structures,and O₂ can react preferentially with Ca inα-Mg and Mg₂Ca,and then the Mg-Ca-O oxide film is formed on the surface.The Mg-Ca-O structure onα-Mg is more stable than Mg O;the Mg-Ca-O structure formed on Mg₂Ca is less stable than the Mg-Ca-O structure formed onα-Mg.2)During the O₂adsorption process ofα-Mg,the Ca atoms will move outward for a certain distance to react with O,while the outward movement of Mg atoms is small.That is,during the oxidation process,the Ca atoms will diffuse outward and chemically react with O₂;During the O₂ adsorption process of Mg₂Ca,the Mg and Ca atoms still maintain their original positions.That is,during the oxidation process,the oxidation of Mg₂Ca is in-situ oxidation,and there is no outward diffusion of Mg and Ca atoms.The effect of different Ca content on the oxidation resistance at high temperature of Mg-Ca alloys was studied,and the roles ofα-Mg and Mg₂Ca in the oxidation process of Mg-Ca alloys were proved.Besides,the oxidation models onα-Mg and Mg₂Ca were proposed:1)the oxidation resistance of Mg is significantly improved by adding different contents of Ca,because the Mg-Ca-O oxide film with good protective effect is formed on the surface;the oxidation resistance increases first and then decreases,and the oxidation resistance of Mg-1.0Ca shows the best.2)In Mg-Ca alloys,α-Mg and Mg₂Ca play different roles in the oxidation process:the oxide film formed onα-Mg shows an effective protective effect on the substrate;the oxide film formed on Mg₂Ca is not stable and will form protrude as the oxidation process,then peel off from the substrate,and finally form a concave structure,leading to further oxidation.The effect of Ca on the oxidation resistance of Mg-Ca alloys under flame was studied,and the mechanism ofα-Mg and Mg₂Ca in the flame resistance of Mg-Ca alloys were proved.Besides,the oxidation models ofα-Mg and Mg₂Ca under flame were proposed:1)The oxidation resistance under flame of Mg is significantly improved by adding different content of Ca,among which the Mg-Ca alloys with low Ca content shows poor oxidation resistance,while Mg-Ca alloys with high Ca content show good oxidation resistance,and Mg-3.6Ca shows the best.For the Mg-0.3Ca and Mg-0.5Ca alloys,the Mg vapor is high;and the Mg-Ca-O oxide films formed on the surface has no Ca accumulation,hence the oxide films show a limited protective effect,resulting in poor oxidation resistance.For Mg-1.0Ca,Mg-2.0Ca and Mg-3.6Ca alloys,the Mg vapor is relatively low;and the Mg-Ca-O oxide films formed on the surface contains an aggregated layer of Ca,which show an excellent protective effect for the substrate,thereby the alloys show better oxidation resistance.2)White worm-like distribution of Ca O are formed on the grain boundaries of Mg-0.3Ca and Mg-0.5Ca alloys and Mg₂Ca in Mg-1.0Ca,Mg-2.0Ca and Mg-3.6Ca alloys under flame.As time goes on,Ca O furhter aggregates and grows.That is,under the flame of high temperature,Ca in grain boundaries and Mg₂Ca will aggregate and react with to form Ca O.3)The oxidation behavior of Mg-Ca alloys in air and under flame is compared,and it is found that when oxidized at 515℃in air,the formation of Mg₂Ca is unfavorable to the oxidation resistance of Mg-Ca alloys;while oxidation under flame,the formation of Mg₂Ca is beneficial to the oxidation resistance of Mg-Ca alloys.By applying an intumescent coating on the surface of the alloy,the oxidation resistance under flame of the alloy is further improved,and the application range of Mg-Ca alloys is expanded:1)An intumescent coating with flame retardant effect is applied on the surface of the Mg-3.6Ca alloy,which remained under the flame for 10minutes without burning,and its oxidation resistance under flame is greatly improved.2)The protective coating can significantly reduce the surface temperature of the alloy,thereby effectively slowing down the oxidation reaction;at the same time,the surface layer containing Ca,P,Si,C and O formed on the surface ultimately improves the oxidation resistance under flame.