Microstructures and Properties of Aluminium-Magnesium Alloys with Additions of Manganese, Zirconium and Scandium
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The present work reports on the effect of Mn-, Zr- and Sc-additions upon hot deformation properties, recrystallization properties and mechanical properties for different temper conditions of Al-Mg alloys. It can be stated that the addition of Mn, Zr and Sc improves the recrystallization properties and the mechanical properties of Al-Mg alloys. It should be emphasised that the precipitation of the metastable cubic Al3Zr and the stable cubic Al3(Sc,Zr) is favourable in an aluminium-magnesium matrix due to a close similarity of the lattice structures. The Al3(Sc,Zr)-phase is similar to the equilibrium Al3Sc-phase and has a high thermal stability and thus the coherency with the aluminium matrix is retained to very high temperatures. The present work has demonstrated the beneficial features of the Al3(Sc,Zr)- phase upon recrystallization and strength. This also results in an increase in the deformation resistance and a reduction in the hot ductility. In particluar, manganese reduces hot ductility. After casting most of the Zr and Sc remained in solid solution. The Mn was partly present in large primary constituent particles and partly in solid solution. Segregations of all three elements were detected. Decomposition of solid solutions of these elements resulted in the formation of dispersoids of the type Al3Mn (orthorombic), Al3Zr (cubic) and Al3(Sc,Zr) (cubic). It was found that the flow stress increased in the presence of the dispersoids. As compared to the alloy without dispersoids, the presence of Al6Mn and Al3Zr or Al3(Sc,Zr) increased the flow stress by 20-100% depending on the temperature and strain rate. The effect of the particles decreases as the Zener- Hollomon parameter increases. Extrusion experiments also confirm these results. In addition, manganese reduces the hot ductility considerably. Furthermore, the present work has demonstrated that the recrystallization properties of Al-Mg alloys may be affected considerably by introducing Mn, Zr and Sc. The recrystallization behaviour after hot deformation may be effectively determined by the Zener drag exhibited by the dispersoids on grain boundaries. Al6Mn showed to be least effective while Al3(Sc,Zr) is extremely effective in retarding recrystallization. After cold deformation, however, the recrystallization behaviour is different due to a higher amount of stored energy. In the alloy without dispersoids, recrystallization occurred by classical nucleation at microstructural heterogeneities, while particle stimulated nucleation operates in the other alloys. Recrystallization of cold rolled material resulted in an extremely finegrained microstructure. Once recrystallized, extensive grain growth occurs in alloys containing Al6Mn and/or Al3Zr. Contrary, alloys containing Al6Mn and Al3(Sc,Zr) are very stable and the fine-grained structure seems to be very stable up to 550°C. This clearly proves that Al3(Sc,Zr) are thermally stable and efficiently pin grain boundaries up to very high temperatures. In the last part of this thesis the mechanical properties of the investigated alloys were mechanically tested in several temper conditions. It was found that the presence of Al6Mn and Al3(Sc,Zr) caused an increase in the flow stress of 36 MPa in the O-temper condition, probably due to the Orowan mechanism. The effect of Al6Mn and Al3Zr alone or in combination was less pronounced. Furthermore, the retained deformation microstructure after extrusion was associated with the Zener drag forces exhibited by the dispersoids and resulted in considerable strengthening. For instance, the combination of Al6Mn and Al3(Sc,Zr) increased the strength by approximately 100 MPa compared to the dispersoid free alloy. Again the effect of Al6Mn and Al3Zr is less pronounced due to the lower capacity in retarding recrystallization. The capability of the dispersoids to retard recrystallization should be an opportunity to increase the strength of the heat-affected zone after fusion welding. This is an important aspect since strain hardened conditions are used commercially. However, it has been demonstrated that a complete utilisation of the strength increase in the base material is not achieved as long as the weld metal is the weakest part in the weldment. However, a yield strength of 160 MPa was achieved for the material containing both Al6Mn and Al3(Sc,Zr), while somewhat lower values were obtained for the alloys with Al6Mn and/or Al3Zr.