First Principle Calculation And Experimental Study On Interface Mechanism Of Heterogeneous Nucleation For Mg-Al Alloy

As one of the lightest structural material with high strength, magnesium alloysare widely used in the automobile, aerospace, and portable electronic products.However, the worse plastic deformation of magnesium alloys restricts theirapplication and development. Controlling the grain size can effectively reduce themicrostructural defects, improve the mechanical properties of the alloy. In addition,trace elements in the alloy such as Mn, and other elements have a significant effect ongrain size of magnesium alloy. Therefore, how to obtain a fine grain, uniformcomposition of solidified structure, is a hotspot in the research of the magnesiumalloy. Among various kinds of grain refinement method, the carbon refining methodbecome the most widely used in industrial grain refinement technology. The refiningmechanism widely accepted is that Al4C3and Al2CO particles are the most effectivenucleation core. However, there is still a lot of controversy in the refining mechanism.In order to clarify the controversy, the process of heterogeneous nucleation forMg-Al-Mn alloy was studied in this paper，and the interface characteristic between-Mg and heterogeneous core was also carried out by the theoretical simulation andexperiment research to reveal underlying refining mechanism of-Mg in Mg-Alalloy.Firstly, the microstructure of Mg-6Al alloy with addition of Mn-25Al masteralloy and carbon-containing refiner were studied by analyzing the characterstic andmorphology of heterogeneous phase. It was found that both the Mn-25Al master alloyand carbon-containing compound have the excellent refinement effect for Mg-Alalloy. XRD and SEM results show that the circular granular ε-AlMn phases exist inthe as-cast Mg-6Al alloy and act as the nucleation core for-Mg grain. And in thecarbonaceous refinement experiments, the excess Mn in the Mg-Al alloy is bad forthe carbonaceous refining, which owing to the negative effect of Al-C-O-Mn, whichmay be formed by adsorbing Mn elements on the Al4C3surface, on the grain refining.Secondly, the surface relaxation, atomic structure, surface energy and surfaceelectronic state density of the different termination structure model for ε-AlMn(001), Al2CO(001) and AlCMn3(111) surface slab were caculated by First-principle methodbased on DFT. And the relatively stable surface slabs will be used to build theinterface model and analyze the stability of the interfaces. The results ofMg(002)/ε-AlMn(001) interface show that the “Over Mn” stacking structure is morestable. However, the theoretical calculation of mixture atom phase of ε-AlMn, whichadopted the Virtual Crystal Approximation method, can not obtain the properties ofthe different atoms. Thus, we shuld consider how to change the mixture atomstructure. Unfortunately, the reconstitution system become unstable and results in thehigher interface energy. In view of this, the nucleation potential of ε-AlMn will not bediscussed.The calculation results of the Mg(002)/Al2CO(001) interface indicate its strongionic/metallic bond formation. Therefore, the “Over O” stacking structure is relativelystable. The size of the interface energy is related to the chemical potential of Al and C.Under certain conditions, the Al2CO particles can be used as a effective nucleationbase of-Mg and stable exist in Mg-Al alloy. Therefore, it provides strong theoreticalsupport to the Al2CO nucles hypothesis from interfacial atomic structure and atomicbonding energy considerations.In addition, by analyzing the characterstic of AlCMn3lattice, it is found that theclose-packed hexagonal surface of AlCMn3(111) consists of two main classes of (111)terminations, the directly cleaved AlMn3and C terminations, and the AlMn2and CMnterminations which can provide the polarity compensation with surface structurechanges. By analyzing the surface stability of four terminations, it is found that theAlMn2and C terminations are relatively stable, which the change of the stability isrelated to the chemical potential of Al and Mn atoms. By comparing the surface slabsstructure of AlCMn3(111) and Al4C3(001), it discovered that the upmost four atomiclayers of these two surface have the similar stacking sequence. Therefore, it isconcluded that the Mn element adsorb on the Al4C3(001) surface and form two unitsof ACMn3phase, which result in the lattice distortion and reduce the heterogeneousnucleation effect of Al4C3particles.