Long Period Stacking Ordered Structures In Mg Alloys Studied By Electron Microscopies

Long period stacking ordered(LPSO)structures in Mg alloys have attracted great research interest for their considerable strengthening and toughing effect,as well as the novel structural characteristics.In this dissertation,to explore new LPSO structures,study the defects and interfaces and understand the formation process of LPSO structures in Mg alloys,the IPSO and related structures of Mg88Co5Y7,Mg92Co2Y6,Mg977Zn1Y2 and Mg97Ga1Y2(at.%)alloys were investigated by(scanning)transmission electron microscopy.Meanwhile,we proposed the way to determine unknown crystal structures via(S)TEM.The main results are listed as follow:1.The method and detail processes to determine crystal structureWe proposed a new way to determine Bravias cell of 3-D space lattice based on its symmetry,and further to clarify Bravias lattices,space groups and 3-D atomic coordinates of unknown phases via TEM.Meanwhile,the Bravias lattice and the structure of Mg3(Co,Y)phase are explored.Based on the structural characteristics of LPSO structures in Mg-M-RE alloys,we introduce a new notation to characterize their structure.Rules are as follow:AB'C'A(and/or AB'C)building blocks are defined as F-block(T-block)and denoted as F(T);Bar sign presents opposite shear direction of the block;The integer number n means the number of Mg layers;The subscript number presents the number of sub unit cells.Besides,we proposed a new method to determine their structure based on new notation.2.New polytypes of long period stacking ordered structuresWe find the co-existence of AB'C'A and AB'C building blocks in a single LPSO structure of the Mg92Co2YC6 alloy,leading to the formation of six new polytypes of LPSO structures determined as 29H,51R,60H,72R,102R and 192R.A random distribution of Co/Y elements in basal planes of AB'C'A and AB'C blocks is observed and discussed.Eight new polytypes of LPSO structures in a near-equilibrium Mg97Zn1Y2 alloy are analyzed and determined as 60R,78R,261-H,96R,38H,40H,108H and 246R.All of them feature AB'C'A building blocks with two and three Mg layers sandwiched between them.Their Bravias lattices and space groups were easily determined via our newly introduced method.A structural relationship between LPSO structures is proposed.3.Defects in LPSO structures and their formation mechanismsWe found an ordered intergrowth of 15R and 12H in an Mg88Co5Y7 as-cast alloy,leading to the formation of 654R LPSO phase.Based on the experimental results,the formation mechanism of intergrowth structure is proposed.We characterized stacking faults and growth twins in 12H,15R and 18R structures in Mg88Co5Y7 and Mg92Co2Y6 as-cast alloys.The stacking faults or growth twins in LPSO structures are denoted as faulted arrangement of n-Mg components and T/F-blocks.Some stacking faults of same components distribute heterogeneously in LPSO structures.The structure,symmetry and distribution of growth twins and stacking faults in the 18R and 14H structure in a near-equilibrium Mg97Zr1Y2 alloy are investigated systematically.Superstructures formed via statistically ordered arrangement of stacking faults in 18R and 14H structure were also observed,including 62L,86L,152L and 235L.Based on their structure details,their formation processes are proposed.4.LPSO related stacking disordered structuresStacking disordered structures in Mg-Co-Y alloys have been observed.All these structures mainly consist of T-blocks,F-blocks,2-Mg,3-Mg and 4-Mg,which arrange disordered or locally ordered.These component can further form complex structure-related structures.Periodicity and symmetry of locally ordered structures was illustrated.5.18R-Mg incoherent interfacesComplex boundaries with dissociated 54R(or 54R')LPSO structures in the incoherent 18R/Mg(0110)interfaces,have been observed and investigated.The periodical spacing of b1 partial dislocations and double-core dislocations consisting of b2 and b3 are expected to produce two pure tilt boundaries in the 54R/Mg and 54R/18R interfaces by a tilt angel of 2.26°.The elastic forces between dislocations were calculated to estimate the stability of the complex boundaries and interpret their structural feature.