| Magnesium alloys are the most widely used metallic structure materials,however,their poor plastic deformation ability greatly limits their applications.Therefore,how to improve the plasticity of magnesium alloys is a scientific problem that must be solved.Due to the scarcity of slip systems in HCP structure and being unadaptable to the Von-Mises criterion to coordinate plastic deformation,consequently twinning has become the dominant mechanism at room temperature.However,the twin mechanism in the magnesium alloys is still debated because many factors affect the twinning process.Therefore,the study of the twinning behavior about a popular AZ31 magnesium alloy has important practical significance for guiding the plastic process of magnesium alloys.In this thesis,the deformation structure of the AZ31 magnesium alloy during rolling process at room temperature was investigated in detail by using electron microscopy characterization technique,in order to reveal the process of nucleation,growth and dynamic recrystallization of the twins in the AZ31 magnesium alloy.The main research results are summarized as follows:(1)Twinning behavior in deformed kink bands of the magnesium alloy during rolling process at room temperature has been studied,and the formation mechanism of the twins in kink bands has also been discussed.The deformed kink band is important to the plastic deformation ability of magnesium alloy as an additional deformation pattern except for dislocation slip and twinning.In the initial stage of deformation,the deformed kink bands are formed due to the inhomogeneous deformation caused by basal dislocation pile-up.With the continuous stressing,the {101?2} twins nucleate and grow in the deformed kink band.As the degree of deformation increases,the twins gradually swallow the deformed kink band,and the dislocation wall will be formed in the twins to absorb the basal dislocation and coordinate further plastic deformation.In addition,during the process of deformation,a {101?3} twin was found to be formed by the mechanism of “deformed kink band ? {101?2} twinning ? dislocation slipping”.(2)Investigations on dynamic recrystallization(DRX)behavior of {101?1} twins in the AZ31 magnesium alloy during rolling process at room temperature were carried out systematically.It has been revealed that the distortion energy accumulated by {101?1} twins with uneven deformation is the reason for the nucleation of the DRX,therefore,{101?1} twins are effective points for the DRX.Meanwhile,the DRX can efficiently weaken the basal texture of the matrix materials.We found that the misorientation angles is closed to {101?1} twins.However,only a few of the DRX grains shows the same c-axis orientation of {101?1} twins,and the majority deviate more or less in their c-axis orientation,because non-basal <a+c> dislocations might be activated or the grains might suffer further deformation after their formation.Therefore,there is a scattered orientation distribution compared with the typical twin orientations for the DRX.(3)High-index twins were found to be formed abundantly in {101?1} twins.Such special twins are formed by the process of “{101?1} twins ? lattice deflection ?{101?2} double twins”,rather than the traditional twinning modes.It is just lattice orientation in {101?1} twins altered and twin boundaries remain the characteristics of {101?1} twins,that is to say,actual high index twin boundaries are approximately parallel to(101?1)crystal plane.We can speculate more high index twins along <21?1?0> axis such as {303?2},{303?4},and {101?6} by the complex double twinning mode.However,there is no obvious regularity about lattice deflection angles as the amount of dislocation pile-up is uncertain in the process of plastic deformation.So,it is not always easy to form the perfect high index twins. |
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