First-Principles Investigation Of The Effects Of Microalloying Elements On The Precipitates In Al-Mg-Si-(Cu) Alloys

As age-strengthable aluminum alloys,Al-Mg-Si-(Cu)alloys will form a large number of nano-sized precipitates during the aging process.These precipitates can effectively hinder the movement of dislocations and improve the mechanical properties of the alloys.It has been shown that certain microalloying elements can effectively modulate the microstructure of the important strengthening phases β",C and Q’ phases in Al-Mg-Si-(Cu)alloys,thus affecting their mechanical properties.In this thesis,first-principles calculations combined with advanced high-angle annular dark-field scanning transmission microscopy(HAADF-STEM)characterization were used to systematically study the effects of microalloying elements on the β"-phase,β"/Al interface,C-phase and Q’-phase in Al-Mg-Si-(Cu)alloys.The purpose of this thesis is to screen promising microalloying elements for better properties of Al-Mg-Si-(Cu)alloys.Firstly,the behaviours of microalloying elements at the β "/Al interface and in theβ" phase bulk is investigated.The results of substitution energy of elements show that all the microalloying elements studied tend to segregate at the β "/Al interface.Zn,Cu,Ge,Er,Tm,Yb,Na and Ca also tend to enter the β" phase bulk to occupy certain atomic sites.Among them,the behaviors of Zn,Cu,and Ge have been confirmed by atomic-resolution transmission electron microscopy.The effects of all considered microalloying elements on the structure of the β "/Al interface or the β" phase bulk indicate that the stable occupancy of solute atoms is related to their own radius and the radius of Mg,Si,and Al.The interface adhesion work calculations show that segregation of Pt,Zr,Ti,V,Mn,Fe,Co,Nb,and Cu at Al sit es at the β "/Al interface and Ge occupying Si sites in the β" bulk or at the β "/Al interface can increase the interfacial bonding.As for the eight elements that can enter the β" phase,Zn,Cu,Er,Tm,Yb,Na and Ca can improve the stiffness of β " phase,while Ge can improve the ductility of β" phase instead.Secondly,the occupancy and stability of microalloying elements in the C phase is investigated.The results of the formation enthalpy indicate that Ag,Na,Ca,In,Sn,Pb,Ge and Sb and rare earth elements are inclined to enter the C phase to occupy certain atomic sites.However,all elements that are strongly bound to Mg or Si have no preference for entering the C phase.Using the most stable structures doped with microalloying elements,the lattice constants and charge density results indicate that the lattice constants and electronic structure of the C phase structure change little when doped with microalloying elements,thus allowing the structure to remain stable.Finally,the occupancy and stability of microalloying elements in the Q’ phase is investigated.The results of the formation enthalpy imply that Cd,Na,Ca,In,Sn,Pb,Ge and Sb have a strong tendency to enter the Q’ phase.As with the C phase,all the rare earth elements can enter the Q’ phase,while all elements that are strongly bound to Mg or Si have no preference for entering the Q’ phase.When the microalloying elements occupy their most preferential Q’ phase sites,the lattice constants and electronic structure of the Q’ phase structure are nearly unchanged,which also allows the structure to remain stable.The calculations in this thesis predict that Zn and Cu can promote the formation of β " phase,Ag can promote the formation of C phase,Cd can promote the formation of Q’ phase,In,Sn,Pb,Sb,Sc,La,Ce,Pr,Nd,Pm,Sm,Eu,Gd,Tb,Dy and Ho can promote the formation of both C and Q’ phases,and Na,Ca,Ge,Er,Tm and Yb can even promote the formation of the three strengthing phases of β",C and Q’ phases simultaneously.These findings will provide some theoretical basis for future microalloying strategies of Al-Mg-Si-(Cu)alloys.