The Study On The Liquid-solid Structural Correlation Of LPSO Mg-Y-X Alloys

As a high strength lightweight metallic material,Mg alloy shows excellent performance advantages in the application of structural lightweight due to its high specific strength and stiffness.But the low absolute strength and toughness of Mg alloy restrict its large-scale application.In recent years,it was found that the long period stacking ordered(LPSO)phase is an effective strengthening-toughening phase,which can significantly improve the comprehensive mechanical properties of Mg alloy.Therefore,it is of great significance to design as many LPSO Mg alloys as possible for promoting the industrial application of Mg alloy.In this thesis,the ab-initio molecular dynamics method was adopted.Firstly,the liquid structures of Mg-Y-Zn system were systematically studied,and the liquid-solid structural correlation of Mg-Y-Zn alloy was explored in terms of the structural characteristics of LPSO phase.Furthermore,a prediction strategy for as-cast LPSO MgY-X alloys was tentatively proposed by exploring the common characteristics in liquid structures of the known LPSO Mg-Y-X alloys(X=Co,Ni,Cu,Zn).Finally,several critical as-cast alloys were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),and transmission electron microscopy(TEM),and the reliability of the prediction strategy was verified.The chemical environment and structural ordering of liquid Mg-rich Mg-Y-Zn system were investigated using ab-initio molecular dynamics simulation.In the liquid Mg-rich Mg-Y binary alloys,the strong Mg-Y interaction promotes the formation of fivefold symmetric local structures.In the liquid Mg-rich Mg-Zn binary alloys,the weak Mg-Zn interaction leads to the weakening of the fivefold local symmetry.Due to the coexistence of Y and Zn,strong attractive Y-Zn interaction is introduced in liquid Mgrich Mg-Y-Zn alloy,and contributes to the clustering of Mg,Y,Zn launched from Zn.Compared with Mg-Y and Mg-Zn binary alloys,the distribution of local structural symmetry in Mg-Y-Zn ternary alloy becomes closer to that in pure Mg.These results should be related to the Y/Zn segregation zone and close-packed arrangement in LPSO structure.Through the ab-initio molecular dynamics simulation of liquid Mg-Y-X alloys(X covers metallic elements of period II to VI in periodic table),the common characteristics in liquid structures of the known LPSO Mg-Y-X alloys(X=Co,Ni,Cu,Zn)were extracted from three aspects of cohesive energy and equilibrium volume,chemical short-range order,and local structural symmetry.The common characteristics were used as prediction criteria to form a prediction strategy.According to this strategy,Mg-Y-Co,-Ni,-Cu,-Zn,-Ga,-Ru,-Rh,-Pd,-Ir,-Pt,and-Au alloys were predicted to be as-cast LPSO Mg-Y-X alloys because they followed all common characteristics.The as-cast Mg-Y-Ga,-Au,-Ag,-Cd and-In alloys were chosen as the experimental subjects,among them,Mg-Y-Ga and-Au alloys have been predicted to be as-cast LPSO Mg-Y-X alloys,and Mg-Y-Ag,-Cd and-In alloys have been predicted to be as-cast nonLPSO Mg-Y-X alloys.Above alloys were observed by SEM,XRD and TEM,the experimental results confirm for the first time that the LPSO phases exist in Mg-Y-Ga and-Au alloys and their structural types are 14 H and 18 R respectively.Besides,no LPSO phase is observed in Mg-Y-Ag,-Cd and-In alloys.The successful prediction of the above results confirms the effectiveness of using the liquid structural characteristics as the prediction strategy,which further supports the liquid-solid structural correlation of LPSO Mg-Y-X alloys in the casting process,and provides a theoretical foundation for the design of LPSO Mg alloy.