Electron Microscopic Characterization On Deformation Structure Of Long Period Stacking Ordered Phase In Mg-Zn-Y Alloys

As a type of special strengthening phase in magnesium alloys,various Long Period Stacking Ordered phases(LPSO phase)have aroused wide attention because of their remarkable strengthening effect on the mechanical properties of magnesium alloys.However,due to the fact that a LPSO phase commonly has complex layered structure,its deformation mechanism is still not fully understood so far.Since the deformation structures of the LPSO-strengthened Mg alloys can essentially affects their mechanical properties,it is necessary to reveal the relevant deformation mechanism involved with LPSO phases in order to provide important theoretical and experimental guidances for designing and deformation-processing Mg alloys strengthened by LPSO phase.In this thesis,the deformation microstructures involved with the Mg-Zn-Y LPSO phase existing in various states when subjected to the deformation in different modes have been investigated,including the solution-treated Mg₈₅Zn₆Y₉(at%)LPSO single-phase rolled at 500?,the as-cast Mg₈₅Zn₆Y₉ LPSO single-phase alloy indented at room temperature,the tensile-deformation LPSO phase in the solution-treated Mg₉₇Zn₁Y₂(at%)alloy rolled at 500?.Various advanced electron microscopic techniques have been applied to characterize the deformation structures,such as scanning electron microscopy(SEM),transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HREM),high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM)and focused ion beam(FIB).The main results obtained are summarized as follows:The deformation structure in the solution-treated Mg₈₅Zn₆Y₉LPSO single-phase alloy subjected to multi-pass hot rolling at 500?has been examined by TEM.It is showed that the primary kinking and secondary kinking bands are formed by different types of dislocations.Non-basal dislocation slipping can be activated through which the secondary kinking can occur in addition to the deformation involving basal<a>dislocation activation and the related kinking band formation.Kinking is main mode for the LPSO single-phase alloy to accommodate deformation during the process of multi-pass hot rolling.It is supposed that the primary kinking band could turn to produce some local stress concentration regions during the multi-pass hot rolling process,where prism dislocations can be activated to forme secondary kinking bands.This deformation mode enables the LPSO phase to carry a larger amount of deformation,and to refine grains of the LPSO phase as well during the multi-pass hot rolling.For the as-cast Mg₈₅Zn₆Y₉LPSO single-phase alloy indented at room temperature,its deformation structure is characterized by dislocation slip.Due to the deformation is confined in a small region with extremely high local stress concentration,and the kinking in the deformed LPSO phase is suppressed.Instead,non-basal dislocation slip with c component has been observed to be activated in the indentation deformation region.The deformation microstructure in the LPSO-strengthened Mg₉₇Zn₁Y₂ alloy subjected to the solution treatment followed by rolling at 500?has been examined by TEM observations.It is revealed that the deformation in the magnesium matrix is dominated mainly by dislocation slip with the twinning and kinking being restrained considerably.Hot rolling deformation can change the orientation of the 14H-type LPSO plate in the magnesium matrix,which allows a large orientational difference with respect to the basal planes of the matrix at two side of the LPSO plate.The FIB technique was applied to precisely slice the LPSO phase deformed with the magnesium matrix in the Mg₉₇Zn₁Y₂(at%)alloy subjected to solution treatment and subsequent tensile deformation at room temperature.The deformation structure in 14H-type LPSO phase and the surrounding magnesium matrix had been characterized by a combination of HRTEM and HAADF-STEM techniques.The results clearly illustrate that a LPSO plate phase in the matrix can accommodate matrix deformation through both basal sliding and kinking under a tensile stress condition.The basal sliding was observed to occur preferentially in nano-thin basal magnesium layers retained in LPSO plate,and more specifically to occur along the LPSO/Mg phase boundaries through which surface deformation steps could be formed.For the kinking deformation in the LPSO plate,it has been confirmed that,in addition to those usual basal dislocations,the unusual prismatic dislocations could also be activated to involve in the kinking due to local stress concentration.