First-principles Study On The Structure And Stability Of GP Zones And β" Phases In Al-Mg-Si(-Cu) Alloys

Al-Mg-Si（-Cu）alloys have been widely used due to their high specific strength and good corrosion resistance.Age strengthening is commonly used in such alloys to form strengthening precipitates that increase the strength of the alloy.The exact composition,crystal structure,and energetic information of the key strengthening phases GP zones andβ"in the alloy are very important for rationally deciphering the evolution law of phase structures during heat treatment and alloy design.To this end,the present work uses density functional theory-based first-principles calculations to elucidate the crystal structure and stability of bulk GP zones andβ"phases in aluminum alloys,thereby explaining the formation of the precipitates observed in the experiments.The main research results are as follows:（1）The GP zones is composed of{Mg Si}atomic layers and there is no Al atom on the（100）crystal plane,the size of GP zones tends to grow to 2 nm.Along the[100]growth direction,the even-numbered{Mg Si}atomic layers are energetically more favorable,and there are some Al atomic layers to relieve the coherent strain.{Mg Si}₂Al₄ is the most energetically favorable GP zones configuration,and the substitution sequence of Cu atoms in the{Mg Si}₂Al₄ GP zones during the aging process is Al→Si→Mg.The growth of the GP zones is accompanied by the increase of Mg and Si solute atoms and the reduction of Al atoms.Cu atoms tend to segregate within the Al layer near the GP zones,while vacancies tend to be trapped in the GP zones.（2）The substitution order of Cu atoms in the β" phases can be summarized as one Si3/Al site→two Si3/Al sites→two Si3/Al sites and one Mg1 site.Cu atoms strongly tend to occupy one of each pair of Si3/Al sites,and the thermodynamically most favorable Cu-containingβ"phases have a stoichiometric of Mg_4+xAl_2-xCu Si₄（x=0,1）.All calculatedβ"phases are ductile and mechanically stable,and the addition of Cu atoms increases the bulk modulus（B）,shear modulus（G）and Young’s modulus（E）of theβ"phase,respectively.Meanwhile,Mg₄₊x Al_2-xCu Si₄（x=0,1）are also kinetically stable.The electronic structure analysis shows that the bonds between the Si atom and the Cu atom are covalent-like.The incorporation of the Cu atom enhances the electron interaction between the Mg2 and the Si3 atom so that the Mg2 atom also joins the“Si network”,which may be one of the reasons why Cu atoms increase the structure stability of the β" phases.