Origin Of Two-dimensional Growth Of γ" Phase In Mg-RE-Zn(Ag) Series Alloys:Studied By First-Principle Calculations

As a typical light-weight structural metal material,magnesium alloys have a series of advantages,such as low density,high specific strength,high specific stiffness,etc.It has a wide application prospect in aerospace,automobile manufacturing,3C products,medical devices and other fields.However,the development of magnesium alloy is greatly limited by its low mechanical properties,poor creep-resistance,poor plasticity and corrosion resistance.Precipitation strengthening mechanism is an effective approach to improve the strength or creep resistance of the alloy by isothermal aging.At present,there are many kinds of high strength aluminum alloys have been developed by precipitation strengthening in the field of aluminum alloy.In recent years,precipitation hardening magnesium alloys have been widely concerned because of its simple preparation,low cost and excellent aging strengthening effect.Among them,Mg-RE-Zn(Ag)ternary alloys have became research hotspot in recent years owing to its lower RE content,higher strength and excellent creep resistance.The reason lies in the γ" phase on the basal plane of Mg-RE-Zn(Ag)ternary alloys during isothermal aging.Interestingly,the single-unit-cell height,i.e.two-dimensional growth,is maintained throughout the isothermal aging process of the γ" phase,and there is no thickening in the direction perpendicular to the basal plane.The γ" phase remains high thermal stability while maintaining nanometer scale,which has not been reported in other alloy systems.In view of this phenomenon,the fine crystal structure and mechanical properties of γ" phase in Mg-RE-Zn(Ag)series alloys have been systematically studied by the first-principle calculations.On the basis of fine structure,the geometric and electronic structures of γ" phase and γ"/α-Mg heterogeneous interface were studied.The nucleation mechanism of γ" phase,growth with clusters and remaining two-dimensional was explained.In this paper,the fine structure and mechanical properties of γ" phase w ere studied systematically.It is found that the γ" phase contain quasi-five atomic layers,and the atoms in the middle layer are actually remain a partially ordered stacked hexagonal structure.The structure of γ"phase is not only one,but there are many variants.With the change of Re:Zn(Ag)atomic ratio in the alloy,it is different from our general cognition of the strengthening phase in the alloy.The stacking ordered extent of the middle layer of γ" phase is related to the RE:Zn(Ag)value and RE/Zn(Ag)content in the alloy.When the RE:Zn(Ag)atomic ratio in the alloy decreases,the stacking ordered extent of the middle-layer increases,and vice versa.In this paper,the mechanical properties of γ"phase are studied for the first time by using the combination rule of two-phase materials.In the study of the mechanical anisotropy of γ" phase,it is found that when Ag and Zn dissolve into the middle layer of γ" phase,almost opposite mechanical anisotropy will be produced.Moreover,under the same structure,the effect of Ag on the mechanical properties of γ" phase is much larger than that of Zn.On the basis of fine structure of γ" phase,the geometric and electronic structure of γ" phase andγ"/α-Mg heterogeneous interface were further investigated.The results show that the precipitation ofγ" phase in Mg matrix produce compressive strain for Mg lattice around it,and strongly changes the charge density distribution(electronic structure)of Mg lattice near the γ"/α-Mg heterogeneous interface.This change causes the formation energies of vacancy of Mg atom near the γ"/α-Mg heterogeneous interface and the migration energy barrier value of solute atoms to be lower than that in the bulk Mg,which provides sufficient kinetic conditions for the rapid diffusion of solute atom near the heterogeneous interface.It is considered that the precipitation of γ" phase in the matrix is similar to that of ’catalyst’.Once the precipitation of γ" phase in the matrix changes the geometric and chemical environment of Mg lattice nearby.The solute atoms can diffuse rapidly around the matrix and promote the precipitation of other γ" phases.In such a continuous reaction,the γ" phase generally grow and precipitate in pairs or clusters in the matrix.Through the characterization of bonding energy and volume strain,in order to reduce the formation barrier of nucleation in Mg matrix,γ" phase precipitated in a highly symmetrical sandwich structure,thus γ" phase precipitated with single-unit-cell height.According to the common ledge-thickening mechanism model,the diffusion thermodynamics and kinetics of solute atoms near the γ"/α-Mg heterogeneous interface show that solute atoms do not tend to migrate to the heterogeneous interface in the real aging process(～200℃),and few solute atoms that can diffuse to the hetero-interface.In addition,the lower aging temperature and almost completely coherent γ"/α-Mg heterogeneous interface lead to the lack of stable nucleation sites of anchored solute atoms,and the ledges are very difficult to nucleate at the hetero-interface.Therefore,the two-dimensional growth of γ" phase were maintained throughout the isothermal aging process.