Studies Of Segregation On Twin Boundaries In Mg-Y-Nd Alloy And Deformaton Behaviours Of Long Period Stacking Ordered Phase In Mg-Y-Zn Alloy

In this thesis,several material characterization techniques,including optical microscopy?OM?,atomforcemicroscopy?AFM?,scanningelectron microscopy?SEM?,transmission electron microscopy?TEM?,high resolution transmission electron microscopy?HRTEM?and high angle annular dark field-scanning transmission electron microscopy?HAADF-STEM?,have been used to study the microstructure of{10???1}twin boundary in Mg-Y-Nd alloy which was aged after cold rolling?It provides some experimental support to some scientific problems including the study of twin boundary and the segregation of solute atoms.Meanwhile,the deformation microstructures of long period stacking ordered?LPSO?phases in Mg-Zn-Y alloys subjected to deformation at room temperature have also been examined and analysed,in which a pivotal reason why magnesium alloys strengthened by LPSO phase possess excellent plasticity has been uncovered.The detailed researches are as followed:The {10???1} twin boundary in Mg_98.3Y_1.16Nd_0.54?at.%?alloy which was aged at423 K for 5 h after cold rolling has been investigated by HRTEM.It was found that some of these SFs formed in twining side are closely related with S_4/4?S is the abbreviation of step.The first 4 indicates there are four layer{10???1}planes in the matrix region and the second 4 indicates there are four layer{10???1}planes in the twin region?twin interface steps.The follow-up HAADF-STEM study has indicated that soluted Nd atoms are preferable to soluted Y atoms for segregating at the{10???1}twin boundary.This result strongly indicates that the Nd atom in the Mg matrix has a higher diffusion rate and a higher driving force for segregation to the{10???1}twin boundary in comparison with the Y atom.it was also revealed that the twin steps with misfit dislocation at the{10???1}twin boundary involve more pronounced lattice distortion in contrast to the coherent twin boundary,and this leads to preferential segregation of rare earth solute atoms to the twin steps.The deformation behaviour of nanoindentation in Mg-Zn-Y 18R-LPSO phase at room temperature has been studied by TEM.It indicated that the deformation microstructures vary with strain gradient.At the position which was located at a long distance from the nanoindentation surface,basal slip is the predominant deformation mechanism and kink sometimes coordinates the deformation,while kink and<c+a>pyramidal II slip are activated more frequently with the decrease of the distance from the nanoindentation surface and the increase of strain.The activation of<c+a>pyramidal II slip lets the deformation of 18R-LPSO phase satisfy the condition of five independent slip systems which is sufficient and necessary condition of uniform plastic deformation.A comparative analysis of the observed deformation microstructures around the nanoindentation and the mechanical response curve of the nanoindentation indicated that pop-ins on the mechanical curve are probably due to the activation of<c+a>pyramidal II slip and kink.The TEM specimen was characterized by HAADF-STEM after 4 month natural ageing,which showed that Zn/Y elements segregate at some positions of<c+a>pyramidal II slip plane.The reason for this phenomenon is that Cottrell atmospheres formed at dislocations by the segregation of Zn/Y elements.Additionally,<c+a>pyramidal II slip can shuffle the18R-LPSO structure,which reduces the energy barrier of atom diffusion on one hand;on the other hand,<c+a>pyramidal II slip can lead the mismatch between heavy elements layers and Mg layers in 18R-LPSO phase,which produces mismatch stress at the pyramidal II slip plane.After the follow-up natural ageing process,atomic interdiffusion was driven by local mismatch stress,and finally the transition region around 4nm was formed at the pyramidal II slip plane both sides.The deformation microstructure of Mg-Zn-Y 18R-LPSO homogeneous alloy which was rolling at room temperature has been observed by TEM.It showed that besides basal<a>dislocation and kink band,there are non-basal dislocations in this deformed alloy.The Burgers vector of these non-basal dislocations are confirmed by weak-beam dark field images,b=?[11?00].Both non-basal slip and basal slip can be activated in the same grain.Deformation microstructures of 14H-LPSO in the Mg_96.77Y_2.14Zn_1.08?at.%?alloy which was rolling at room temperature after recrystallization annealing have also been studied by TEM.It indicated that activations of both basal<a>slip and kink band are restricted when the compressive stress is applied along the normal direction of basal plane.Under these condition,the<c+a>pyramidal II slip is activated to coordinate the deformation.