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I. INTRODUCTION RECYCLABLE design for sustainable development has been proposed in the design of ecomaterials (environmentally conscious materials),[1,2] and has been discussed in terms of design factors, e.g., recyclability and ecology for raw materials, simplification of recycling and scrapping, and adequately balanced properties.[2,3] Al-Si systems, which have plural phases with low mutual solid solubility, are effective as in situ metal-metal composites which possess simplified recycle models and adequately balanced properties such as strength and elongation.[3] Although Al-Si alloys are a major cast material, it is extremely important, from an aluminum recycling standpoint, to improve their ductility. A look at the stress-strain curves of polycrystalline aluminum (Figure 1(a)) demonstrates that cold-working operation results in an increase in the flow stress and almost no uniform elongation at 293 K. Those are undesirable in balancing higher strength and ductility. To control balanced properties, a fine microstructure with plural phases is one of the candidates for alloy design.[1] Reinforcing aluminum alloys with SiC particles of approximately equiaxed shapes, on the order of several microns in size, strengthened the alloys at low strains due to the increases in the elastic modulus and strainhardening rate.[4,5] Humphreys[6] reported deformation and recrystallization of two-phase aluminum alloys containing nondeformable large particles a few microns in size. A deformation zone containing a high dislocation density and large lattice misorientations was observed in the vicinity of particles on deformation, indicating that geometrically necessary dislocations[7] are stored in the deformation zone to accommodate deformation gradients. These geometrically necessary dislocations enhance strain hardening, particularly in a low-strain regime. Heavy cold working cannot, however, be applied to AlSi cast alloys, because it causes severe cracking in coarse OSAMU UMEZAWA, Senior Scientist, and KOTOBU NAGAI, Group Leader, are with the Frontier Research Center for Structural Materials, National Research Institute for Metals, Ibaraki 305-0047, Japan. Manuscript submitted September 24, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A

primary and/or eutectic silicon crystals. A eutectic Al-Si cast material posesses increased strain hardening at low temperatures accompanied by increased flow stress and decreased strain to failure, although an increase in the strainhardening rate results in greater strain to failure with decreasing temperature for polycrystalline aluminum. For the unmodified Al-Si alloy, the silicon phase must be refined to avoid sample fracture due to cracking. To improve the mechanical properties of Al-Si cast materials, microstructural modifications have been made by adding elements such as Sr and P to the melt, or by hot forging and long-term solution heat treatment. The ductility of treated materials remains insufficient, however, and they cannot be applied to wrought materials. The most promising refining