Effect Of Sr And P On Morphology Control And Heterogeneous Nucleation Mechanism Of Mg₂Si In Aluminum Alloys

By control the content of Mg and Si in aluminum alloys,primary Mg₂Si can be obtained in the in situ reaction to improve the mechanical properties of aluminum alloys.However,in normal as-cast hypereutectic Al–Mg₂Si alloys,the primary Mg₂Si shows enormous dendrite,which will result in poor properties to these alloys.Refining and modification is widely used to improve the appearance and size of primary Mg₂Si because of its low cost and convenient operation.At the same time,first principle calculation method plays an important role in materials study in microcosmic scale.In this paper,the morphological evolution of primary Mg₂Si in hypereutectic Al–20%Mg₂Si alloys with the addition of Sr or P element was observed by experimental method.The adsorption models of Mg₂Si surfaces and modification elemens（Sr or P）were established to calculate adsorption energy to study the morphology evolution mechanism.And then the electronic structure and adhesion work of AlP/Mg₂Si interfaces were calculated using the first-principles method.Also stacking sequence of Mg,Si atoms on AlP substrate was discussed,which can provide theoretical basis for AlP as the heterogeneous nucleus of Mg₂Si from the energetic theory.The results show that addition of Al–Sr and Al–P master alloys in Al–20%Mg₂Si alloys can modify the morphology and size of primary Mg₂Si.All coarse dendritic Mg₂Si particles disappear and evolve into regular polygon,and their size also decreases:primary Mg₂Si changed into a cube due to Sr modification;dendritic Mg₂Si converted into octahedron or truncated octahedron due to P modification.First-principle calculations were used to calculate the adsorption energy,it suggested that Sr was adsorbed on the Mg₂Si{100}surface selectively and restrained the growth of<1 0 0>directions,which leads Mg₂Si to transform into cubic morphology.P is more easily adsorbed on the Mg₂Si{100}surface when it is on the top site of Mg,which can obtain truncated octahedron.However,when P is on the top site of Si,it prefers to be adsorbed on Mg₂Si{111}surface,which leads to a octahedral Mg₂Si.Besides,the results suggest that the AlP compound existing in Al–P master alloys can act as effective heterogeneous nucleus of Mg₂Si.AlP（331）/Mg₂Si（110）,Al-terminated AlP（100）/Mg₂Si（211）and P-terminated AlP（100）/Mg₂Si（211）interface were studied in this paper.The results show that the three AlP/Mg2Si interfaces all can form steadily,and P-terminated AlP（100）/Mg₂Si（211）interface with the largest adhesion work is the most stable.Through the calculation of adsorption energy between Mg,Si atoms and AlP substrate,it can be seen that Mg,Si atoms tend to adsorbed on P-terminated AlP（100）preferentially;As for AlP（331）,Mg and Si atoms prefer to be adsorbed on top-site of Al atoms firstly,which would be the basis of AlP（331）/Mg₂Si（110）interface.