急冷凝固したAl-11%Si-2.5%Cu合金の共晶Si相形態の変化と機械的性質への影響

抄録

  The cooling rates of cast products made of Al-11%Si-2.5%Cu alloy were measured and their eutectic Si phase morphology was examined. With permanent mold cast and die-cast products (low cooling rates), their eutectic Si phase was plate-like in the mid-central region, and rod-like near the surface. In the case of high-speed twin-roll cast products (high cooling rates), their eutectic Si phase was rod-like both in the mid-central region and the region near the surface. The morphological change of the eutectic Si phase from plate-like to rod-like occurs at the estimated cooling rate of about 1000 K/s. The aspect ratio and average particle diameter of the rod-like Si phase in high-speed twin-roll cast products were close to the modified eutectic Si phase. In order to investigate the effects of the morphological change of eutectic Si phase on mechanical properties, tensile tests, tear toughness tests, and bending tests were performed. Elongation, tear toughness, and bendability were improved by the morphological change of the eutectic Si phase from plate-like to rod-like. Especially, unit crack propagation energy (UE_p) increased by six times, and it was possible to bend the specimens up to 180 degrees. This is considered due to the suppression of crack initiation and propagation by the change in the morphology of the eutectic Si phase. These results show that morphological change of the eutectic Si phase by rapid solidification is effective for improving the elongation, toughness and bendability (formability) of materials that contain large amounts of eutectic Si phase.