Grain-Size Dependence of Microstructural Evolution and Mechanical Behavior during High-Temperature Deformation of Mg and Mg Alloy
 
H. Miura, X. Yang, J. Xing and T. Sakai
Department of Mechanical Engineering and Intelligent Systems,
UEC Tokyo (University of Electro-Communications), Chofu, Tokyo 182-8585, Japan
 
Grain size dependence of high-temperature deformation behavior and microstructural evolution in Mg and AZ31 Mg alloy were investigated. For that purpose, Mg and AZ31 Mg alloy with different grain sizes was deformed in tension and compression at elevated temperatures between 423 K and 673 K. The microstructural evolution and deformation behavior depended strongly on the grain size. Ultra fine-grained (UFGed) AZ31 alloy (d = 0.36 m), which was prepared by multi-directional forging, exhibited superplastic behavior of ductility over 300% in nominal strain (Fig. 1). Contrary to that, medium (d = 22 m) and coarse (d = 90 m) grained AZ31 alloys showed different dynamic recrystallization (DRX) behavior; the former classical continuous DRX accelerated by kinking and the latter discontinuous or in situ DRX by twinning (Fig. 2). The ductility of Mg single crystal depended strongly on crystal orientation, the lower fracture strains being associated with multiple slips. Single crystals in which single slip was activated exhibited extended ductilities corresponding to more than 400% (Fig. 3). Both Mg single crystal and the ultra fine-grained AZ 31 Mg alloy, therefore, exhibited extraordinary large ductility, although the mechanisms were different. It is also revealed that Mg grain itself possesses potential of high ductility.

[Published in Proceedings of 1st Asian symposium on Magnesium Alloys, Jeju, Korea, 2005, pp. 77-80 (Invited).]

 
Fig. 1 True stress-nominal strain curves for ultra fine-grained AZ31 Mg alloy produced by MDF method.


Fig. 2 Evolved microstructures after high-temperature deformation at 573 K in medium grained, (a) and (b), and coarse grained, (c) and (d), AZ31 Mg alloys. An arrow mark indicates compression axis.


Fig. 3 A typical series of flow curves of Mg single crystals; (a) deformation accompanied by multiple slip and DRX and (b) single slip without evolution of heterogeneous structures.