Electron Microscopy Study Of The Interaction Structure Involving Twins And Precipitates In Deformed Mg-Zn Alloy

With the research progress for having a deep insight into the room-temperature deformation process of aged magnesium alloys,a key issue that one has to face is how to understand the essence of interaction between various deformation twins and aging precipitates.Due to the facts that the present cognition concerning the precipitation dynamics in various magnesium alloys is not universal,and that twinning mode is diversified as well as the related twinning mechanisms are complex,the interaction mechanism for the deformation twinning to shear the aging precipitates is still not clear.Actually this has been an important issue being of great concern but difficult to tackle.Therefore,revealing the twinning behavior under the influence of precipitates,twin/precipitate interaction feature and the related structural variation can provide valuable information for exploring a new twin+precipitate composite strengthening mode so as to design and develop high-performance magnesium alloys.In this thesis,the aged Mg-8Zn(wt.%)with dense distribution of various precipitates was rolled at room temperature.The morphologies and structures of {10(?)2} deformation twins and aging precipitates in the rolled alloy were characterized by a combination of TEM,HRTEM and HAADF-STEM techniques.In particular,the structural investigation on the interaction between deformation twinning and aging precipitates was conducted.Furthermore the interfacial structure analysis and related structural modeling for the precipitates subjected to the twinning shear were also carried out,through which the interaction feature was discussed.The main results obtained are summarized as follows:1.A large number of rod-like ?1'precipitates were observed to be formed in aged Mg-Zn alloy by electron microscopy.The crystal structure and lattice constant of the?1'precipitate were examined by atomic resolution scale observation,and the existence of three structurally equivalent variants with the same morphology but different orientation was inferred with the aid of the structural simulation.Also it is clearly revealed that these exists a transition region involving lattice distortion near the semi-coherent interface between the ?1' and the magnesium matrix,and that growth steps can be formed along the interface.In view of the lattice difference between the ?1' precipitate phase and the magnesium matrix,the differently-orientated?1'/matrix interfaces with different mismatching degrees were also observed,which allowed to explain why each ?1' precipitate phase tended to grow into a rod along the c-axis of the magnesium matrix.2.The structural variation of the ?1' precipitate crossed by the {10(?)2} twinning interface in the Mg-Zn alloy rolled at room temperature was observed at atomic scale for the first time.The observation results show that the ?1' precipitate has a deflection of 3.6° in the morphology after subjected to the {10(?)2} twinning shear.The presence of this morphology deflection is the result of strain accommodation by forming regular steps in the boundary between the ?1' precipitate and {10(?)2} twinned region in order to maintain their semi coherent relationship.3.The twinning tip and boundary structures of the {10(?)2} twin formed in the aged Mg-Zn alloy subjected to room-temperature rolling were characterized at atomic scale.The results show that the existence of ?1' precipitates is conductive to increasing the nucleation rate of {10(?)2} twins,but is not to the migration of twinning boundaries.As a result of the twin-?1' interaction,the delamination-like twin boundaries which correspond to an unevenly-inclined interface can be formed.The pinning strengthening effect of ?1' precipitates on the matrix could allow its lattice parameters to vary under high stress,such that the unstable {10(?)2} twinned structure can be formed by rotating 90° relative to the matrix,giving rise to a non-standard twin boundary composed entirely of BP planes.These observations can provide valuable information for clarifying the mechanism of the interaction between the {10(?)2} twin and ?1' precipitates.