Theoretical And Experimental Study On The Effect Mechanism Of Rare Earth Elements Additions On The Growth Of Mg₂Si Phase In Magnesium Alloys

As the lightest metal structural material at present,magnesium alloy is widely used in aerospace,automotive,3C and other fields.However,the high temperature performance of magnesium alloys is poor,which seriously hinders the further development of magnesium alloys.The Mg₂Si phase has high melting point,high hardness,low expansion coefficient and similar density to the magnesium matrix,and can be used as a reinforcing phase to improve the thermal stability and high temperature mechanical properties of the magnesium alloy.However,the Mg₂Si phase distribution in the magnesium alloy under conventional casting is uneven and the shape is irregular,which reduces the mechanical properties of the alloy material.Therefore,it is necessary to improve the microstructure of the Mg₂Si phase in the magnesium alloy.At present,a common method is to modify the Mg₂Si by adding modificator.In this paper,the eutectic Mg₂Si in Mg-1Si alloy was modified by adding different rare earth elements.The effect of different rare earth elements on the morphology of eutectic Mg₂Si phase and its mechanism were studied from the experimental point of view.At the same time,the first-principles calculation method was used to calculate the effect of rare earth atom doping and adsorption on the Mg₂Si?100?and?111?surface.The mechanism of the influence of rare earth elements on the morphology of Mg₂Si phase is discussed through the combination of experimental and theoretical calculations.By adding 0.3wt.%La,Ce,Nd or Sm to the eutectic Mg₂Si in the Mg-1Si alloy,we can find that the rare earth La,Ce,Nd or Sm can refine the eutectic Mg₂Si phase in the Mg-1Si alloy,in which La has the best metamorphism effect,most of the large strips are transformed into small dots,followed by Nd,Ce and Sm,some of the thick strips of Mg₂Si become small dots,and the other part become fibrous.Through SEM analysis,we can know that during the solidification process,the rare earth atoms will be enriched in the frontier of the solid-liquid interface of Mg₂Si growth,affecting the growth of Mg₂Si crystals by adsorption or solid solution,thus changing the morphology of Mg₂Si crystals.The surface energy of the rare earths La,Ce,Nd and Sm doped on the surface of Mg₂Si?100?and?111?was calculated by the method of first-principles calculation,and using the value of surface energy to judge the effect of rare earth doping on the surface of Mg₂Si on the final morphology of Mg₂Si.The surface energy of the rare earth atoms doping on Mg₂Si?100?and?111?surfaces becomes smaller than that of the clean surface,and the surface energy of the Mg₂Si?100?and?111?surfaces varies to a different extent,resulting in the degree of anisotropy of crystal growth changes,which affects the final morphology of the crystal.In addition,we can roughly judge that the doping effect of Nd is best,and then Sm,Ce and La.The first-principles calculation method was used to study the adsorption sites,structural changes and electronic structures of rare earth atoms La,Ce,Nd and Sm adsorbed on the surface of Mg₂Si?100?and?111?.In the surface of Mg₂Si?100?,the rare earth atoms La,Ce,Nd and Sm are preferentially adsorbed to the vacancies,and the adsorption energy at the vacancies is La<Ce<Nd<Sm;in the surface of Mg₂Si?111?,rare earth atoms La,Ce,Nd and Sm are preferentially adsorbed to the H₂position,and the adsorption energy at the H₂ position is also La<Ce<Nd<Sm.When the rare earth atoms La,Ce,Nd and Sm are adsorbed on the surface of Mg₂Si?100?and?111?,the structure changes,and the structure changes of?100?surface more than?111?surface.Moreover,the variation of interlayer spacing changes after the rare earth atoms are adsorbed on the surface of Mg₂Si?100?is La>Ce>Nd>Sm,and the change of Mg₂Si?111?surface is Nd>Ce>La>Sm.Further analysis of the charge density and differential charge density map shows that after the rare earth atoms La,Ce,Nd and Sm are adsorbed on the surface of Mg₂Si?100?,the rare earth atoms and the Si atoms on first layer are mainly bonded by ionic bonding;and the rare earth atoms La,Ce,Nd and Sm are adsorbed on the surface of Mg₂Si?111?are mainly bonded by means of ionic bonds and covalent bonds.Through experimental and theoretical calculations,the mechanism of the effect of rare earth elements on eutectic Mg₂Si may be that some rare earth atoms replace Mg atoms in Mg₂Si,causing changes in surface energy,causing changes in the degree of anisotropy of crystal growth,thereby changing the crystal morphology of Mg₂Si;another part of rare earth atoms adsorbed on the growth surface of Mg₂Si,changing the surface structure,thereby suppressing the stacking of Mg atoms and Si atoms on the crystal faces,and finally changing the crystal morphology of Mg₂Si.The combined effect of these two aspects determines the deterioration effect of rare earth elements.