First-principles Study Of Heterogeneous Nucleation In Magnesium Alloy

Magnesium alloy is currently the lightest and commonly used non-ferrous metal materials in practical applications.By refining the alloy structure,the strength and ductility of the magnesium alloy can be improved,and the plastic deformation capacity of the alloy can be increased.The addition of an effective nucleating agent is an effective grain refinement technique for improving the mechanical properties of magnesium alloys.Zirconium has been the most successful and effective grain refiner for the currently known aluminumfreemagnesium alloys.However,when zirconium exist in the aluminum-containing magnesium alloy,Zr will lose the effect of refining the crystal grains since an Al₃Zr intermetallic compounds are formed with Al.Experimental studies have shown that the Al-Ti-B master alloy can refine Mg-Al alloy,in which the high-melting compounds AlB₂ and TiB₂ with a dense hexagonal structure both can be used as heterogeneous cores ofα-Mg,and the existence of a large number of heterogeneous nucleation cores is the main reason leading to the grain refinement ofα-Mg.The researchers observed AlB₂ particles and TiB₂ particles in the Al-Ti-B master alloy and figure out the low mismatch between Mg and AlB₂,Mg and TiB₂ from the crystallographic point of view,whereas,the atomic interaction across the interface was not involved to study the nucleation efficiency of thetwo kinds of refined particles.The first-principle methods based on density functional theory（DFT）can be utilized to investigate the properties of the interface betweenα-Mg and heterogeneous nucleus from the atomic scale,analyze the interfacial energy and atomic scale interface structure ofα-Mg/nucleus core phase interface,and discuss the nucleation efficiency of nucleation particles.It is a feasible way to reveal the intrinsic mechanism of heterogeneous nucleation and significant to promote the development of magnesium alloy grain refinement theory.In the present paper,first-principles calculation method was utilized to investigate and discuss the refining effect of different refiner in aluminum-free magnesium alloy and aluminum-magnesium alloy.The atomic structure,bonding properties,adhesion work and interfacial energies of Mg/Zr,Mg/AlB₂ and Mg/TiB₂ interface were calculated.The main research findings are as follows:1.For the Mg/Zr system,the ionic bonds between Mg and Zr enhance the interaction between them,and the interface energy of Mg/Zr is much smaller than that of solid-liquid interface between?-Mg and Mg melts,about 0.1 J/m²,in the range of chemical potential,which reasonably explains the experimental phenomenon of Zr can act as an excellent heterogeneous core refine Mg grains from the energy point of view.The interfacial calculation results of Al₃Zr and Mg show that the system constructed by Al₃Zr and Mg is unstable or nonexistent,Al₃Zr can not promote grain refinement as anα-Mg nucleation substrate since the large interfacial energy of Mg/Al₃Zr.2.The metallic bonds of Al-terminated"HCP"stacking interface are formed,and the interface interaction of the B-terminated"HCP"stacking is polar covalent bonds and the ionic bonds.A further comparison of adhesion work shows that the B-terminated"HCP"stacking interfaces have the strongest and most stable interface binding.The Ti-terminated"HCP"stacking interface forms metallic bonds,mainly contributed by Mg-3p and Ti-3d orbital electrons.The interface interaction of B-terminated"HCP"stacking interface is mainly ionic bonds.3.Calculated Mg/AlB₂ interfacial energies vary with the chemical composition of the system.The interface energy of B-terminated"HCP"stacking interface can be less than that of solid-liquid interface betweenα-Mg/Mg melts,about 0.1J/m² in the whole range of chemical potential,and the energy of the stacking interface is the lowest and the most stable.Therefore,in the magnesium melt,α-Mg grains can nucleate on the surface of AlB₂（0001）by this stacking sequence,which provides a theoretical explanation for the experimental phenomena of grain refinement in the presence of AlB₂ particles in magnesium melt.The interfacial energies of Mg/TiB₂ interface also vary with the chemical composition of the system.Unlike the Mg/AlB₂ interface,the interface energy of Mg/TiB₂interface is much larger than that ofα-Mg/Mg melt interface,indicating that TiB₂ particles possess poor nucleation potency forα-Mg grains from thermodynamic considerations.The Ti solute in the melts thermodynamically has the potential to segregate on the surface of the TiB₂ particles,making free Ti to segregate to the TiB₂ melt interface.The content of free Ti in the alloy melt increases,and the growth inhibition factor of the system increases significantly.It will lead to the formation of supercooled regions in the dendritic growth of solid/liquid,and hinder the growth of dendritic crystals.Thus,the grains of magnesium alloy are excellently refined.This is consistent with the observations in the experiment,indicating that the free Ti and TiB₂ phases in the Mg melts from Al-5Ti-B master alloys play a dominant role in the refinement of magnesium alloys.