| 其他摘要 | Magnesium alloys are the lightest structure materials for the practical engineering applications. They have many outstanding physical and chemical properties, and have been widely used in electronics, electrical, automobile, aerial and astronautical industries. But now most of Mg alloy components are formed by die-casting technology. For the low formability of Mg alloys at room temperature, the plasticity forming technology is still in development stage and needs high costs to form good shape of the components, which limits the applications of Mg alloys. Research on the deformation mechanisms for Mg alloys sheets at room temperature is one of the important tasks to improve the applications of Mg alloys. If the research achievement can be used for the industrial production of the sheets and components of Mg alloys, the applications will be greatly promoted for each field.
The commercial sheets of AZ31B Mg alloys are studied in this dissertation. Three specimens of rolled sheets for compression, and one specimen of rolled sheets and one specimen of extruded sheets for tension, are subjected to uniaxial compression or uniaxial tension test from different deformation directions at one or several successive strains. The experiments of tracking evolution of crystal orientation in individual grains during the course of plastic deformation were conducted successfully by using SEM/EBSD technique firstly. So the EBSD data of the initial grain orientation and the changed orientations for the same grain can be related together to study the slip, twinning mechanisms of Mg alloy sheets directly and quantitatively at room temperature.
The processing methods and programme for the EBSD orientation data of Mg alloys are established, and they have the following functions: identifying the parent of grains, extensive twins variants, compressive twins variants and the secondary twins; calculating the area fraction for each type of twins; the boundary can be summarized according to the misorientation relationship between neighboring points, and identifying the type of twin boundary; calculating the mean orientation of grain parent, strain attributed by extensive twinning, and the Schmid factor value for twinning; slip trace analysis; the grain orientation rotation calculation based on Sachs crystal deformation model.
According to the experimental data and the analysis methods, the orientation information and deformation mechanism are related to study the deformation behaviors for each individual grain, and analyze the orientation changes, active slip and twining systems, boundary characteristic, and misorientation distributions for each specimen. The deformation mechanisms associated for the grains of each specimen are also explained.
The quantification relationship between Schmid factor value, angle between c-axis and deformation direction, and the area fraction of extensive twins, are analyzed in this dissertation. The results show that the law of Schmid can also be applied to predict the activation of extensive twinning system; the grains which have larger value of Schmid factors can generate more area fraction of twins. The uncertainty identification of extensive twin variants and the special boundary between variants of extensive twin are also discussed.
The distribution maps of rotation angle/axis are established to describe the change of orientation rotation in the local area of grain, and those maps are associated with the predicted results by Sachs model to express the physical meaning of the rotation features. According to the rotation features, the grain can be divided into several domains to analyze the slip mechanisms for the local area of grain, which can express visually the orientation rotation and the corresponding active slip systems at the same domain, and definitely exhibit the slip mechanisms for each domain in the grain. Three typical grains of the compression specimens have been analyzed in detail by using this method to study the slip systems, and the similarities and differences between these grains are also discussed, which describe the slip activation for different orientation of grains. The rotation differences of different deformation surfaces are also analyzed and compared by using the same method.
In this dissertation, the microcracks on the surface and mid-section and the local orientations near the microcracks at the final strain of the compression specimen are studied, and the twin features of other specimen at the final strain are also analyzed. The results show that there is a close correlation between microcracks and double twins.
According the experimental and analytical results, the activation rules of slip and twinning systems, and the rules of orientation changes at uniaxial compression and uniaxial tension deformed state for the grain with a certain orientation have been determined. The grains with different orientations show the different deformation behaviors, and the grains with similar orientations show the similar deformation features. The specimens with the orientation in favor of active extensive twinning at the initial deformation state show the better plasticity compared with others. So the formability of Mg alloys sheets at room temperature can be improved by controlling the starting orientation. |
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