Magnesium alloys are considered to be the material that has most importantly influenced our life in the 21st century. They are light, recyclable and moderate in physical and mechanical properties, so they are growing in the applications for automobiles, bicycles, hand tools and 3C products. Most such products are made by die-casting and semisolid thixomolding because Mg alloys with hcp structures do not have good ductility and workability. Moreover, they are not sufficiently strong to justify their being used in place of highstrength Al alloys. New magnesium alloys with superior properties must be developed to enable solid-state forming processes, such as extrusion, forging, stamping and press forming, to be used to make parts by mass production, and thus expand the application of wrought products.According to the Hall-Petch relation, [1,2] the flow stress of a metal depends on the grain size:where r is the flow stress of a metal; r 0 is the flow stress in the absence of resistance to slip across grain boundaries; K is a constant and d is the grain size. The constant K in magnesium alloys is well known to exceed those of fcc metals, such as aluminum alloys. [3,4] That is, the flow stress of magnesium alloys rises strongly as grain size decreases. Furthermore, reducing the grain size also improves ductility and superplasticity of magnesium alloys. [3,5±7] Therefore, controlling the grain size is crucial not only in augmenting the strength and ductility but also in obtaining high-strain-rate superplasticity of magnesium alloys.Recently, several studies of modified magnesium alloys, such as Mg-1Zn-2Y [8] and Mg-Al-Ga, [9] have been conducted with rapid solidification, revealing favorable tensile properties and high-strain-rate superplasticity. However, magnesium powder is extremely dangerous and must be processed with many extra steps in an environment of a protective gas. The addition of elements, Ga and Y, is also expensive, narrowing their range of applications.The Mg-Al-Zn magnesium alloy system is the most popular to be used in casting, thixomolding and wrought processes. Previous studies have shown that Mg-Al-Zn alloys exhibit good superplasticity at optimum strain rates and temperatures. [7,8,10,11] The microstructure of Mg-Al-Zn alloys essentially consists of a main a phase of Mg solid solution and a minor b phase of intermetallic Mg 17 Al 12 , whose volume fraction increases with the aluminum content. Although high aluminum and zinc contents improves the strength and corrosion resistance with a slight increase in density from 1.78 to 1.90 g/cm 3 , [12] the b phase is inherently brittle, rendering the alloy brittle. Daloz et al. [13] proposed that the aluminum and zinc contents should be less than 15 and 3 at%, or 14.5 and 2.46 wt%, respectively for feasible applications. Attractive COMMUNICATIONS 948
