Mg-Y alloys show significantly enhanced room temperature ductility compared to pure Mg and other classical Mg wrought alloys. The presented study focuses on understanding the mechanisms for this ductility improvement by microstructure analysis, texture analysis and slip trace analysis based on electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). As expected pure Mg mainly deforms by basal slip and tensile twinning. In contrast, MgY shows a high activity of compression twinning, secondary twinning and pyramidal slip. These additional deformation modes cause a homogeneous deformation with weaker basal texture and more balanced work hardening and enhanced ductility. Additionally, in Mg-Y shear bands are much more frequent and carry less strain than those in pure Mg. As a consequence failure in shear bands occurs at significantly higher strain. The experimental results are discussed focusing on the mechanisms effecting the observed high activation of pyramidal deformation modes in Mg-Y. 1 Introduction Magnesium is a preferable material for lightweight automotive applications and shows a great potential for material substitution due to its high density-strength ratio. Wide industrial application of magnesium is hindered because of its poor room temperature formability, which is caused by pronounced basal slip and a strong basal type texture.

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The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth.