First-principles Study Of Grain Refinement Mechanism Of Magnesium Alloy

In recent years,considerable research has been done to seek and develop new effective grain refiners for Mg-Al based alloy.Adding grain refiners can reduce the grain size of cast Mg alloys,but neither is as effective as Zr in Mg alloys or Al₃Ti and TiB₂ in Al alloys.This is partly due to the lack of a comprehensive understanding of the grain refinement mechanism.Most of the new grain refiners developed is tentative to some extent.In this paper,the first-principles calculation method was used to explore the refinement mechanism of potential grain refiners in magnesium aluminum alloys.The atomic structure,electronic properties,work of adhesion,and interfacial energy between Mg/NbB2,Mg/AlN,and Mg/Al2Y interface were studied by first-principles calculations to investgate the heterogeneous nucleation potential of NbB₂,AlN,and Al₂Y particles as nucleation substrate for?-Mg grains.With the same terminated interface structure,compared with different stacking sequences,the"HCP"stacking sequence was the most stable due to its larger adhesion work and smaller interface energy.Comparing the interface properties of the most stable interface structures of Mg/NbB₂,Mg/Al N,and Mg/Al2Y,it is found that the interface adhesion work of the Mg/AlN interface of the N-terminated"TL"stack was 9.91 J/m²,and the main bonding type of the interface was mixed ionic bond/weak covalent bond.Overall,the bonding strength and bonding characteristics of the Mg/Al N interface were stronger,followed by the Mg/NbB₂ interface and the Mg/Al₂Y interface.The interfacial energy of several interface structure will change with the corresponding chemical potential of the system.The interface energy of the B-terminated"HCP"stacking interface in the B-rich environment and the Nb-terminated"HCP"stacking interface in the Nb-rich environment was less than the interface energy between the primary?-Mg/Mg melt?0.10 J/m²?.Therefore,under certain conditions,NbB₂particles are an effective heterogeneous nucleation substrate for?-Mg.The interface energy of?-Mg/AlN interface of the N-terminated"TL"stack was less than the solid-liquid interface energy of the?-Mg/Mg melt?0.10 J/m²?.It has been thermodynamically proven that Al N particles can be used as an effective nucleation substrate for?-Mg.The interface energy of the?-Mg/Al₂Y interface structure with Y and Al terminated was much larger than 0.1 J/m²,so the nucleation efficiency of Al₂Y grains was poor.From the thermodynamics point of view,it shows that single Al₂Y grains can not act as the heterogeneous nucleation substrate of?-Mg during solidification.Among the three interface models of Mg/NbB2,Mg/AlN,and Mg/Al2Y,the interface energy of the Mg/AlN interface was the lowest,and?-Mg can be more easily nucleated on the N-terminated AlN surface.