Abstract
The low density of magnesium alloys makes them an ideal material for reducing the weight of structural components. The use of magnesium flat products is difficult due to the limited formability at room temperature and the anisotropy of the mechanical properties. The reason for the limited formability is the hexagonal crystal structure of magnesium and the formation of a strong crystallographic texture resulting from the massive forming. Therefore, to improve the mechanical and forming properties of flat products, it is necessary to know precisely how the microstructure and texture are influenced by alloying elements as a function of the manufacturing process.
In the present thesis, flat products have been produced by varying the process temperature, on the one hand, via the conventional rolling process and, on the other hand, by direct extrusion. The supporting mechanisms for microstructure development are static recrystallization (SRX) during rolling and dynamic recrystallization (DRX) during extrusion. The influence of alloying was analyzed on the basis of a magnesium alloy containing yttrium and alloyed in small amounts with Zn, Mn, Ca (W1, WZ10, WZM100, WZX100 and WZMX1000). AZ31 has been chosen as a comparative alloy to exclude the influence of RE on the RX mechanisms. A systematic analysis of all conditions was carried out by metallographic microstructural analysis and XRD texture measurements. EBSD measurements on selected conditions were used to distinguish between relevant microstructural fractions and their textures. The microstructural fractions and the textures were correlated with the recrystallization mechanisms. The mechanical properties at room temperature of the semi-finished products were determined by uniaxial tensile tests in the longitudinal and transverse directions and by biaxial Erichsen cupping tests.
This work highlights the fundamental importance of understanding both the influences of the process and the alloying elements on the deformation and recrystallization mechanisms. A clear correlation can be shown between process parameters, microstructure or texture development and the resulting forming properties for the alloys used in this work. It can be seen that alloying elements can have different effects on microstructure and texture development depending on the process. This is due to the prevailing dynamic or static recrystallization mechanisms depending on the manufacturing route. The comprehensive study of the alloy influence shows that Ca is not suitable as an alloying element for the WZ10 alloy in terms of improving the semi-finished product properties at room temperature. This is valid for flat products which were produced by both rolling process and direct extrusion. And it could be demonstrated for the first time on an accelerated static and dynamic recrystallization behavior by Ca and the resulting changed texture developments. Furthermore, this work clearly distinguishes for the first time between the static (double peak in TD) and dynamic RE texture components (double peak in ED/RD) and discusses the development of the quadruple texture in detail.
