Effect Of A Small Amount Of Ca Or CaO On Oxidation Resistance Of Mg Under Heat Treatment

Magnesium(Mg),one of the lightest metal structural materials,has wide application prospects owing to its advantages of high specific rigidity and specific strength,excellent electromagnetic shielding and damping properties,and easy recycling.However,compared with other alloys,the high-temperature oxidation resistance of magnesium alloys is poor,which limits its wider application.Therefore,in order to expand the application range of magnesium alloys,it is of great significance to improve the oxidation resistance of magnesium alloys at high temperatures.The existing methods for improving the oxidation resistance of metal Mg usually include micro-arc oxidation,electroplating,electroless plating,and so on.Whereas,these methods have disadvantages such as complex process,environmental pollution or short service period.At present,it is proved that heat treatment is an effective method to improve the oxidation resistance of alloys.Nontheless,most of the researches of the effect of heat-treatment process on the magnesium alloys mainly focus on the improvement of mechanical properties.There are studies showing that heat treatment can reamarkably improve the oxidation resistance of copper alloys.However,the research of effect of heat treatment on the oxidation resistance of magnesium alloys is still absent.Calcium(Ca),as an abundant resource on the earth,enables magnesium alloys to obtain better mechanical properties by refining the grain size.Thus Ca has become a frequantly-used alloying element added to magnesium alloys.For this reason,this work takes Mg-x Ca(x:1,3,5 wt.%)alloys as the research object which is heat-treated at 400°C for 8 h in an argon-hydrogen mixed protective atmosphere to investigate the oxidation resistance.The scanning electron microscopy(SEM),thermogravimetric analysis(TGA)and other methods have been used to explore the oxidation behavior of the heat-treated alloy samples.Compared with Ca element,CaO has the advantages of low cost,convenient transportation and storage.Therefore,the effect of heat treatment on the oxidation resistance of Mg-x CaO(x:1?3?5 wt.%)alloys and the mechanism are investigated.The experimental results show that after heat treatment in protective atmosphere,a compact Mg O-CaO oxide film is formed on the surface of Mg alloys added with small amount Ca element.Compared with pure Mg and Mg-Ca alloys without heat treatment,Mg-Ca alloys with heat treatment are hardly oxidized in 1 atm O₂atmosphere and at 400°C.The improvement of oxidation resistance can be attributed to the formation of dense Mg O-CaO protective film during heat treatment.Nevertheless,because of the formation of reticular compounds Mg₂Ca on the surface of Mg-Ca alloys,some cracks or shallow pits are produced during heat treatment and oxidation process,which to some extent affects the quality of protective film of Mg-Ca alloys.On the other hand,it is explored that dense Mg O-CaO oxide film is also formed on the surface of Mg-CaO alloys with heat treatment under the same conditions.Similar to Mg-Ca alloys,no oxidation can be observed in the interior of Mg-CaO alloys.Further characterization analysis manifests that a dense,smooth and flat Mg O-CaO protective film is formed on the surface of Mg-CaO alloys during heat treatment.Different from Mg-Ca alloys,no microcracks and reticular texture compound Mg₂Ca are observed on the surface of Mg-CaO alloys before and after heat treatment,which is beneficial to the improvement of alloy quality and oxidation resistance.