Effect of multimodal microstructure evolution on mechanical properties of Mg-Zn-Y extruded alloy

A high strength Mg-Zn-Y alloy featuring increased ductility and a multimodal microstructure is developed. The microstructure of the extruded Mg-Zn-Y alloy consists of three regions: a dynamically recrystallized alpha-Mg fine-grain region with random orientation; a hot-worked alpha-Mg coarse-grain region with strong basal texture; and a long-period stacking ordered (LPSO) phase grain region. Having found that bimodal microstructure evolution in the alpha-Mg matrix is influenced by the morphology of the LPSO phase in the as-cast state, the authors investigate the effect of secondary dendrite arm spacing (SDAS) in the cast state on the microstructure evolution and mechanical properties of the extruded Mg-Zn-Y alloy. Mg-Zn-Y alloy ingots with various SDAS are obtained by temperature-controlled solidification techniques at various cooling rates. Mg-Zn-Y ingots are extruded at 623 K and an extrusion ratio of 10. A decrease in SDAS is associated with dynamic recrystallization of the alpha-Mg phase region and a high dispersion of fiber-shaped LPSO phase during extrusion. An increase in dynamically recrystallized alpha-Mg grains with very weak texture improves ductility; the effective dispersion of the hot-worked alpha-Mg grains with a strong basal texture and the fiber-shaped LPSO phase grains conspire to strengthen the alloy. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.