Research Of Annealed Microstructure And Deformation Twins Of Polycrystalline Cobalt Subjected To Dynamic Plastic Deformation

This thesis reports detailed investigations of the microstructure of polycrystalline cobalt during annealed process and room temperature dynamic plastic deformation (DPD), respectively. Microstructure observation of annealed cobalt was performed in an X-ray diffraction (XRD), in a scanning electron microscopy (SEM) with an electron backscatter diffracrion (EBSD) detector, and in a transmission electron microscopy (TEM) to investigate the annealing twins generated in polycrystalline cobalt containing face center cubic (FCC) and hexagonal close packed (HCP) phases during the annealed process. TEM and high-resolution TEM (HRTEM) were applied to characterize the multi-types of deformation twins of polycrystalline HCP cobalt during DPD. The relation of the dislocation dissociation and the formation of the deformation twins were discussed.The investigation of annealed microstructure and annealing twins in FCC and HCP phases shows:(1) a small amount of high temperature (above690K) FCC phase is retained at room temperature, and there is a Shoji-Nishiyama orientation relationship between the low temperature HCP and the retained FCC phases of{111}FCC||{0001}HCP and<110>FCC||<1120> HCP-(2)∑3twin boundaries were observed in the retained FCC phase. These twins are produced during dissociation of fast moving random high-angle boundaries into a∑3boundary and another random high-angle boundary.(3) a high fraction of boundaries with twin misorientation of71.4°/<1120> is formed as a result of phase transformation from FCC to HCP following the Shoji-Nishiyama orientation relationship.The investigation of variation of the microstructure of polycrystalline cobalt during DPD shows the results as following:(1) with the increasation of the strain, the<1123>-type dislocations have been observed in the vicinity of the stacking-faults, resulting in the fornation of moire fringes and activation of {1011}<1123> slip system.(2) with the increasation of the strain, the relative fractions of twin boundary length of various twins have been changed. Thereinto, the relative fractions of twin boundary length of tension twins have decreased acutely with the strain increasing. On the contrary, the relative fractions of twin boundary length of compression twins have increased with the strain increasing. The relative fractions of twin boundary length of double twins change unobviously with the strain increasing.The detailed investigation of the characterization of deformation in polycrastalline cobalt during DPD shows the following results:(1) several deformation twinning modes including the tension twins of{1012} and{1121}, and compression twins of{1122},{1013} and{1011}, as well as double twins of{1011}-{1012} and{1013}-{1012} have been confirmed to take place during the deformations.(2) tension and compression twinning can take place in one grain. The c axes of most of the grains containing tension twins are nearly perpendicular to the compression direction, while the c axes of grains containing compounded twins are usually away from the compression direction. A similar twinning mode and microstructure of quasi-static compression have been found.(3) the twin boundaries of{1012} deformation twins can be regard as the step boundaries, which result in the unsuccessive characteristic. The height of the step contains the atomic player from one plater to more than ten players. The twin boundaries of{1121} deformation twins do not present step-like twin boundaries. The step-like twin boundaries are also observed in the {1011} deformation twin boundaries. In addition, a new slip system,{1011}<1123> is also activated, which can also provide strain in the c-axis direction. In other words, both the deformation twinning and the newly activated slip system can facilitate plastic strain in the c-axis direction in polycrystalline cobalt during DPD.The discussion of the relation of the dislocation dissociation and the formation of the deformation twins shows that the dislocation dissociation has played an important role the nucleation of deformation twins of cobalt during deformation. The full dislocation of AB can dissociate different types of sessile partial dislocation, glissile partial dislocation and twinning partial dislocation in {211l} and{101l} panes under the applied stress. The sessile partial dislocations can pin at the intersection of the twinning planes, and the glissile partial dislocations glide to nucleate a twin on the (0002) planes, and the twinning partial dislocations can lead to the shear or the shuffle under the applied stress. The multi-types of deformation twins in polycrystalline cobalt can be generated by the reaction among the sessile partials, the glissile partialsand the twinning partials during deformation.