Effects Of Doping And Adsorption Of Trace Modifying Elements On The Morphology Of Mg₂Si:A First-principles Study

Aluminum and magnesium alloys,especially the Al-Mg-Si alloys,are attractive for applications in various fields,such as automobile,aerospace,communications and national defense.Owing to the excellent combination of physical and mechanical properties,intermetallic compound Mg₂ Si,can be used as a reinforced phase to enhance the room and elevated temperature properties of light alloys.However,in the condition of gravity cast,primary Mg₂ Si is prone to forming an undesirable coarse dendrite phase which would deteriorate the strength and ductility of light alloys and thus limit their development and applications.Therefore,it’s necessary to regulate the morphology and size of Mg₂ Si.Unfortunately,how to predict the modification effect of modifiers so as to regulate the Mg₂ Si into a certain shape is still a key problem in the field of solidification.So it is urgent to explore the effects and mechanism of modifying elements on the morphology of Mg₂ Si,and further predict the modification effect of modifiers.Note that the surface energy and the adsorption energy,connected with the equilibrium morphology of Mg₂ Si,are two important physical parameters which can be used to predicte the modification effect and illustrate the mechanism of modifiers on the morphology of Mg₂ Si.Specifically,the surface energy can reflect the stability of a certain crystal facet,and the ratio of surface energies depicts the competition and anisotropy between crystal facets,which further dominates the evolution trend in crystal morphology.The adsorption energy can evaluate the modifiers’ capability to adsorb on a certain facet,which can affect the progress of crystal growth.In this work,the effects of doping and adsorption of trace modifying elements on the morphology of Mg₂ Si are investigated by calculating the surface energy and adsorption energy using first-principles method.This work here are beneficial to the realization of artificial regulation of the morphology of Mg₂ Si,and can provide a reference for the development of a new light alloys.Following is the main conclusions drawn in this work:（1）Effects of doping atoms（Sb,Te,Sn,P and Bi）on the surface energies of Mg₂Si（100）and（111）facets in regard to the doping coverage are studied.In the condition of Te,Sn,P and Bi dopings,the surface energies of（100）and（111）planes increase with the doping coverages（1/8-1/4 monolayer）,meaning that the stability of（100）and（111）facets decreases.In the condition of Sb doping,the surface energies of both（100）and（111）facets decrease with the doping coverages（1/8-1/4 monolayer）,meaning that the stability of（100）and（111）facets increases.（2）Based on the results of doping surface energies,effects of doping elements（Sb,Te,Sn,P and Bi）on the equilibrium morphology of Mg₂ Si are predicted from the surface anisotropy.According to the anisotropy between pure（100）and（110）facets,the equilibrium shape for pure Mg₂ Si is predicted to be an octahedron surrounded by the {111} planes.In the condition of Te,Sn,P and Bi dopings,the surface energy anisotropy between（100）and（111）planes drops compared with that of pure Mg₂ Si,on this occasion Mg₂ Si crystal tends to form a truncated octahedron composed of {111} and {100} planes.Hereby,Te and P are more effective modifiers to promote the appearance frequency of（100）facets on Mg₂ Si crystal,and can be used as candidate modifiers to realize the artificial manipulation of the morphology of Mg₂ Si into truncated octahedron.The findings here are useful to optimize the kinds of modifying element.（3）The adsorption behavior of modifying elements（Li,Be,Ca,Sr and P）on Mg₂Si（100）and（111）facets is investigated by calculating the adsorption energy,and the correlation between the adsorption behavior of modifiers and the equilibrium morphology of Mg₂ Si is inllustrated.The most stable adsorption sites for modifying atoms on（100）and（111）surface are confirmed,and the five modifying elements are more capable to adsorb on（100）surface than（111）facet.As the total adsorption energy increasing,the adsorption capability for the five modifying elements on Mg₂ Si crystal enhances.Moreover,the five modifiers are all preferred to adsorb on（100）facets,promoting the stability and extending of（100）facets on Mg₂ Si grains.In this condition,Mg₂ Si crystal tends to form a truncated octahedron composed of {100} and {111} surfaces even a cube entirely surrounded by {100} facets.Overall,the effect of doping and adsorption of modifying elements on the morphology of Mg₂ Si are studied.According to the surface anisotropy between（100）and（111）facets,the equilibrium morphologies Mg₂ Si doped with and without modifying elements are predicted.This find provides a criterion to classify the kinds of modifiers.Combining the weighted total adsorption energy and modifiers’ preferred adsorption on（100）facets,the adsorption and poisonin effect of modifiers on Mg₂ Si is illustrated.This work provide a basis for further understanding the effect and mechanism of modifying elements on the morphology of Mg₂ Si,and will facilitate the development and fabrication of new light alloys.