Study On Microstructure Of Mg-Al-Li-Zn-Y Alloy

As an ultra-light metal structural material,magnesium alloy has high specific strength,specific rigidity,excellent electromagnetic shielding characteristics,excellent damping and thermal conductivity characteristics.It has been widely used in a wide range of application,such as automobiles,3 C,transportation and so on.Magnesium aluminum alloy has won more due to its advantages in castability,mechanical strength and ductility.By adding Li,Zn,Y and other elements,the strength of the magnesium aluminum alloy can be improved.At the same time,the large plastic deformation method such as multidirectional forging can refine the grains and improve the performance of the material In this paper,a variety of testing methods are used to study the microstructure,crystallographic characteristics,orientation relationships,and interface bonding modes of precipitated phases in Mg-Al-Li-Zn-Y alloys,providing a basis theoretical and experimental for the design of lightweight magnesium alloys.The metallographic microscope(OM)and X-ray diffraction(XRD)were used to test and analyze the Mg-7.05Al-2.65Li-2.08Zn-0.15Y alloy.The results show that the Mg-Al-Li-Zn-Y alloy mainly includes matrix ?-Mg,a long strip-shaped,granular second phase ?-Mg17Al12 which precipitated along grain boundaries,and a small amount of Al2Y,AlLi,MgLi2Al.Among them,Zn element was completely dissolved in the Mg matrix due to its low content,and no other precipitated phase was formed.After six passes of multidirectional forging,the dendrites are broken,the grain size is reduced,a large number of dislocations are generated in the alloy,and the dislocations are packed at the grain boundaries.The main precipitated phase ?-Mg17Al12 in the alloy was characterized by electron diffraction(ED)and transmission electron microscope(TEM).The main morphologys of?-Mg17Al12 phase in the alloy are rod-shaped,short rod-shaped,and rhombohedral.The difference in morphology is related to the orientation relationship with the matrix.Four orientation relationships between ?-Mg17Al12 and ?-Mg matrix were observed in the alloy,respectively:[010]?//[2110]?,(101)?//(0001)?;[111]?//[1210]?,(101)?//(0001)?differs by 2°;[111]?//[0001]?,(0110)?//(111)?;[151]?//[0001]?,(0110)?//(101).The polar diagrams of eight orientation relationships in Mg-Al-based alloys have been drawn for comparison,and it is found that they correspond to the Pitsch-Schrader OR,Potter OR,Crawley OR,and Porter OR orientations.Among them,in the two orientation relationships of Pitsch-Schrader OR and Potter OR,the interface between the ?-Mg17Al12 phase and the ?-Mg phase is a semi-coherent interface.Because the solid solubility of the rare earth element Y in the Mg matrix is very low,most of the added Y elements are combined with Al to form Al2Y phases.These Al2Y phases have a variety of shapes such as irregular cube,rod and sphere,and the size is about 100 nm-1 ?m.Al2Y phase contains SRO,twin and a long period structure(LPSO).These twins is a typical composite twins with the face of(111)Al2Y The observed LPSO structure is a new long-period structure.The stacking surface is the {111} plane of Al2Y,and the stacking cycle is three times that of the Al2Y {111} plane.The MgLi2AI precipitation phase in the alloy coexists with other structures(such as incommensurate phases and more complex phases)in one grain.Since the MgLi2Al phase is a metastable phase,there may be a structural transition between these phases.A new ?' phase was also observed in Mg-7.05Al-2.65Li-2.08Zn-0.15Y alloy,and[0001]?//[001]?',(020)?//(1100)?'-existed between this phase and the matrix ?' orientation relationship.This ?' phase is different from the ?' phase commonly found in rare earth Mg alloys,and its habitual plane and growth orientation are different.The growth mechanism needs to be further studied.