Study On The Interfical Structure And Segregation In Mg-Ag Alloys

As light-weight structural materials,magnesium?Mg?alloys have been widely applied in aerospaces,transportations and electronic communications.For Mg alloys,the grain boundary is very stable at room temperature,while it exhibits softening at elevated temperature,leading to a drastic drop of the strength.In order to improve the strength of Mg alloys at both room and elevated temperatures,it is very important to develop the Mg alloys with excellent mechanical properties and high-temperature stability.It is known both the strength of Mg alloys can be enhanced effectively and the thermal stability of grain boundary can be improved by the solute atoms.Due to the highly disorder orientation and complicated structure of grain boundary,it is very difficult to characterize the real structures of grain boundary.Previous researches on grain boundary were mainly focused on the ordered interfacial structures such as twin boundary and symmetric grain boundary.However,the real structures of general asymmetric grain boundary are still unclear.In this study,asymmetric grain boundary structures,grain boundaries segregation and interfacial phases of Mg-Ag and Mg-Gd-Y-Ag-Zr alloys are systematically investigated by using the high-resolution transmission electron microscopy and high angle annular dark filed-scanning transmission electron microscopy?HAADF-STEM?.The main results are summarized as follows:1.The formation mechanism of coaxial grain boundaries?CGBs?in Mg-Ag alloys was studied.Multiple twinning was produced by repeated plastic deformation,which results in the formation of the asymmetrical CGBs with a same<11???0>zone axis.These coaxial asymmetrical CGBs with different misorientation angles are formed by secondary or tertiary twinning with different types.The asymmetrical CGBs with misorientation angles with a range from 138°to 149°are the most common grain boundaries,which generated by{10???2}-{10???1}secondary twins.When the secondary or tertiary twinning are proceeding,other types of CGBs with misorientation angles of109°,172°or 102°,115°,164°are generated,respectively.2.Grain boundary segregation,twin boundary segregation and interfacial defects structures were investigated in Mg-Ag alloys.Periodical segregations on{10???1},{10???2},{10???3}twin boundary and 78°symmetric coaxial grain boundaries were observed at atomic-scale by HAADF-STEM imaging.Segregation on asymmetric grain boundaries is not periodic very well for the large mismatch on grain boundaries.Defect structures of{10???1}twinning steps were investigated.It is clearly revealed that the nature of the defect structrues of different{10???1}twinning steps can be described as two types.The first one is h>nd_?101??1with b=1/3[10???0]?n includes 1,2 and 4?and another one is h=4d_?101??1.3.Segregated structure of twin boundary in Mg-Ag alloy and its formation mechanism were investigated.The atomic radius and lattice structure of Ag,grain boundary structure and interfacial energy are the main factors affecting the Ag segregation.Periodic segregation on{10???1}and{10???2}twin boundary was observed,both which formed by subsituted Ag with Mg atom alternatively.Moreover,Ag atoms were located at the compressive sites of{10???2}twin boundary.A peculiar structure with three Ag atoms was formed along the{10???3}twin boundary,which is different from{10???1}and{10???2}.Carefully analysis reveals that another Ag atom is loated in twin boundary space,which is not the Mg occupions.An intersitite type of segregation on grain boundary was observed in metals for the first time.4.The thermal stability of 142°CGBs in the Mg-Gd-Y-Ag-Zr alloy was studied and the structure of interfacial phase was investigated.An unreported interfacial phase which is stable up to 400°C was observed in the annealed Mg-Gd-Y-Ag-Zr alloys.The interfacial phase has a monoclinic structure?C2/m?with the stoichiometry of Mg₄GdAg₃,and its lattice parameters are a=1.20 nm,b=1.04 nm,c=1.59 nm and?=139.1°.The orientation relationship between the new interfical phase and?-Mg is:[101]//[112?0]_?,[302]//[10???0]_?and?010?//?0001?_?.5.The formation mechanism of solute-clusters assisted nanocrystallization in a Mg alloy was studied.Mg-Ag alloy with an average grain size of 100 nm was obtained by cold rolling and subsequent annealing treatment.High density of nano-scale Ag clusters precipitating from matrix interact with dislocations,which become the heterogeneous nucleation sites for recrystallizaiton.Ag clusters have pinning effects on recrystallize grain boundaries,and the inhibited the grain growth quickly.