Sub-angstrom-resolution Electron Microscopy Studies On Complex Phases And Related Defects In Mg-Zn Based Alloys

Mg-Zn based alloys represent an important series of precipitation hardening Mg alloys.Formation of C14 MgZn2]Laves phase,Mg4Zn7,?1',-MgZn and Mg21,Zn25 binary intermetallic second phases,and Mg3Zn6Y ternary quasicrystals plays an important role in improving the mechanical properties of Mg-Zn based alloys.These intermetallic compounds have complex crystal structures,which may be a critical obstacle for atomic scale investigations on the atomic structure,lattice defects in these nanometer-sized intermetallic compounds.Until now various issues and questions about these precipitates need to be solved.Development of aberration-corrected electron microscopy techniques with sub-angstrom resolution makes it possible to investigate atomic structures,fine structure of crystal defects and the transformation of nanometer-sized complex precipitates in Mg-Zn based alloys.In the present study,aberration-corrected Z-contrast technique was used to investigate the atomic structure,crystal defects and related phase transformations in several kinds of complex intermetallic compounds in Mg-3Zn(at.%)and Mg-3.6Zn-0.6Y(at.%)alloys.Our findings may be helpful for further understanding the relationship between microstructure and mechanical properties of Mg-Zn based alloys,and for optimizing the heat-treatment processes of Mg-Zn based alloys.There are three kinds of {1120} domain boundaries(DBs)in Mg21Zn25,which were formed in the eutectoid reaction.These DBs divided Mg21 Zn25 into nanometer-sized domains,with the same crystal orientation.Three types,designated as DB-?,DB-? and DB-?,were formed by sharing quadrangular prisms,thin hexagonal prisms and prismatic faces of the Laves-type columns on both sides of different DBs,respectively.The DBs in Mg2iZn25 are different from previously known DBs with displacing vectors along the interfaces.A new concept of separation vector is thus introduced to characterize the DBs and their junctions in Mg21Zn25.The separation vectors of three kinds of DBs are<1010>ac14,1/3(1120)ac14 and 2/3(1120)ac14.The angle between DBs and separation vector is 90°,600°and 600°for DB-?,DB-? and DB-?,respectively.Various DB junctions were formed,and the sum of separation vectors at each junction is zero for all kinds of junctions,which keeps Mg2,Zn25 atomically compact.Local Zn enrichment in front of growing Mg2,Zn25 domains activated overgrowth of Laves-type columns,playing a critical role in the formation of DBs in Mg2,Zn25 during eutectoid transformation.Besides formation of coarse Mg3Zn6Y quasicrystals at grain boundaries,lots of nanometer thick MgYZn4 plates,were formed in Mg matrix.Atomic structure of MgYZn4 plates were determined by three-dimensional atomic reconstruction.MgYZn4 has an orthorhombic structure with a space group of Pmnn,and lattice parameters a=5.30 A,b=9.49 A,c=8.5966 A,?=?=?-90°.Mg and Y atoms in MgYZn4 orderly occupied the 4g lattice sites.Importantly,both size and structure of MgYZn4 are stable upon treatment at 625 K for 5h,or 595K for 100h,showing thermostability much better than Laves MgZn2,which should be important for applications at elevated temperatures.First-principle calculations indicated that the enhanced thermostability of MgYZn4 is mainly attributed to the lower formation energy and bonding enhancement due to Y substitution.MgYZn4 plates were partially transformed into icosahedral Mg3Zn6Y quasicrystals after annealing at 625K for 10h.The orientation relationship among Mg matrix,MgYZn4 and Mg3Zn6Y quasicrystals is[2110]a//[010]MgYZn4//2-FoldIQc and[01 10]?//[100]MgYZn4//2-FoldIQC.The atomic structure of Mg3Zn6Y icosahedral quasicrystals in Mg-3.6Zn-0.6Y alloys was investigated by means of atomic-resolution Z contrast technique,in combinations with the n-dimensional embedding method.Atoms in each shell of the Bergman cluster correspond to a lattice site of the periodic structure in the six-dimensional space.The outmost shell of the Bergman cluster is composed of a pyritohedron(24 Y atoms)and an icosahedron(12 Mg atoms).Interestingly,lots of twin existed in solidified coarse Mg3Zn6Y quasicrystals and nanometer-sized Mg3Zn6Y quasicrystal precipitates.The twin boundary in Mg3Zn6Y quasicrystals is a 5-Fold plane which passes one pentagon face of the pyritohedron composed of Y from the outmost shell of the Bergman cluster.Each pair of Bergman clusters on both sides of the twin boundary shares one Y pentagon.