First-principles Study On Influence Mechanism Of Ca On ?-Mg Heterogeneous Nucleation In The Inoculated As-cast Magnesium Alloy

With the advantages of light weight and high specific strength,magnesium alloys are widely used in aerospace industry,automotive industry and 3C products.However,magnesium alloys have low strength,poor plastic toughness,and are prone to corrosion.These shortcomings limit the development and large-scale application of magnesium alloys.Grain refinement can significantly improve the strength and ductility of magnesium alloys.At present,casting is the main method of Mg alloy forming,it is very important to control its microstructure in the casting stage.Carbon-inoculation is one of the most concerned refining methods to refine the casting Mg alloy grain.For decades,the most commonly accepted refinement mechanism of carbon-inoculation is that Al₄C₃ and Al₂MgC₂ heterogeneous nucleation.In recent years,the oxide inoculation refinement of magnesium alloy such as ZnO and MgAl₂O₄ have attracted much attention.Based on the synchronous action between heterogeneous nucleation and alloying elements that can increase the grain refining efficiency,previous studies by the present group have found that Ca can significantly improve the carbon-inoculation refinement effect However,the mechanism of how Ca is synchronously refined,especially its influence mechanism on the adsorption growth process,has not been revealed in depthIn the present study,the influence mechanism of Ca on the heterogeneous nucleation of ?-Mg phase of magnesium alloys was studied by first-principles calculation.Four typical heterogeneous nuclei,Al₄C₃,Al₂MgC₂,ZnO and MgAlO4,were selected to study the effect of Ca on the surface properties,interface properties and atomic adsorption of the crystal nuclei The calculation results reveal the influence mechanism of Ca on the process of ?-Mg heterogeneous nucleation refinement.The main conclusions are drawn as follows,The finest grains of Mg-3%Al alloy have achieved by carbon-inoculation combining with Ca addition.The grain refinement effect of carbon-inoculation is well,followed by the addition of MgAlO4,and the addition of ZnO is weaker than the former two.Ca has a more significant effect on the promotion of carbon-inoculation refinement,but has less effect on the refinement of ZnO and MgAlO4.Ca adsorbs on the surface of carbon-inoculation nuclei of Al₄C₃ and Al₂MgC₂,and segregates at the interfaces of Mg/Al₄C₃ and Mg/Al₂MgC₂For Al₄C₃ nuclei and Al₂MgC₂ nuclei,when Mg/Ca atoms adsorb on the first layer and the second layer surface,Ca is more easily adsorbed than Mg,and Ca will promote the adsorption of Mg atoms.When the second layer is adsorbed on the surface,Ca and Mg atoms are more likely to be adsorbed on the surface of the adsorbed 3Mg+1Ca particles under low concentration adsorption.When Ca is present in the second layer,the surface that adsorbed 4Mg is easier to grow by adsorbing Mg atoms than the surface that adsorbed 3Mg+1Ca.The analysis results of Ca-doped Mg/Al₄C₃ interface and Mg/Al₂MgC₂ interface show that Ca replacing Mg-2 site doping defects are the easiest to be formed,and the interface bonding strength is enhanced.In contrast,it is difficult to form doping defects in the replacement of Mg-1 by Ca,and the interface of this doping configuration is unstableCombining the adsorption and doping analysis results,the analysis of the Ca atom diffusion barrier shows that Ca can diffuse from the first layer of Al₄C₃ and Al₂MgC₂ surfaces to the second layer of adsorption.In the initial nucleation stage,Ca will preferentially adsorb to the first layer of Al₄C₃ and Al₂MgC₂ surface,and then promote the adsorption of Mg atoms As the adsorption process progresses,the crystal grains gradually grow up,Ca will migrate from the first adsorption layer to the second adsorption layer.As a result,the adsorption of Mg atoms is continuously promoted after Ca migration.Ca can also diffuse from the first layer of the interface to the second layer of the interface during the initial formation of the Mg/Al₄C₃ interface and the Mg/Al₂MgC₂ interface,and enhance the interface stabilityThe ZnO structure has two stable surface configurations:Zn-terminated ZnO(0001)and O-terminated ZnO(0001)surfaces.The ZnO(0001)/Mg(0001)interface is unstable,while the OT configuration of the ZnO(0001)/Mg(0001)interface is easy to form in the magnesium melt environment.MgAlO4 has three kinds of surfaces,i.e.,MgAl₂O₄(100),MgAl₂O₄(110)and MgAl₂O₄(111).Ca has little effect on the adsorption of Mg on the surface of MgAl₂O₄(100)and MgAl₂O₄(111),but Ca promotes the adsorption of Mg on the surface of MgAl₂O₄(110).In general,Ca has not great promotion effect on the adsorption of Mg atoms on the surface of MgAl₂O₄The synergistic refining mechanism of Ca and heterogeneous nuclei in magnesium alloys is the promotion of the adsorption of Mg atoms to the surface of heterogeneous nuclei induced by Ca.Ca is easier to adsorb to the surface of heterogeneous crystal nuclei and promote the adsorption of Mg.After Ca and Mg are adsorbed,the radius of heterogeneous crystal nucleus particles is increased,and some of the particle radii reach the critical nucleation radius Consequently,the number of potent nuclei increases in the melt,promoting the grain refinement of heterogeneous nucleationCa is easier to adsorb to the surface of heterogeneous crystal nuclei Al₄C₃,Al₂MgC₂,ZnO and MgAl₂O₄ than Mg.Ca significantly promotes the adsorption of Mg atoms on the surface of Al₄C₃ and Al₂MgC₂ crystal nuclei.However,adverse effect is found on the adsorption of Mg atoms on the surface of ZnO.As for MgAl₂O₄ nuclei,Ca has little effect on the adsorption of Mg atoms on its surface.The effect of Ca on the adsorption of Mg atoms to the surface of different heterogeneous crystal nuclei is related to the type of crystal nucleus.The effect of Ca on the adsorption of Mg atoms to the surface of heterogeneous nucleation particles is positively correlated with the effect of Ca on the refinement of heterogeneous nucleation.The greater the promoting effect of Ca on the adsorption of Mg atoms to the surface of nucleating particles,the more Ca can improve the grain refinement effect of heterogeneous nucleationThis study confirmed that the effect of Ca on the heterogeneous nucleation refinement depends on the effect of Ca on the adsorption of Mg atoms to the surface of heterogeneous crystal nuclei.Moreover,the effect of Ca on the adsorption of Mg atoms to the surface of the crystal nucleus is related to the type of crystal nucleus.The presence of Ca makes it easier for heterogeneous crystal nuclei in the melt to adsorb Mg atoms in the melt,increasing the number of potent nuclei in the Mg melt and increasing the nucleation rate.The obtained results of this study have important guiding significance and reference value for the study of grain refinement of Mg alloys,especially for the design of grain refiners and the screening of alloy elements can provide theoretical references.