Ostwald Ripening and Solidification Growth of Primary Silicon Crystals during Solidification of an Al-19%Si-0.02%P Alloy

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  • Kaneko Junichi
    Department of Mechanical Engineering, College of Industrial Technology, Nihon University
  • Sugamata Makoto
    Department of Mechanical Engineering, College of Industrial Technology, Nihon University
  • Aoki Ken-ichiro
    Department of Mechanical Engineering, College of Industrial Technology, Nihon University

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Isothermal holding of an Al-19%Si-0.02%P alloy has been carried out at 893 K to determine the rate of Ostwald ripening of primary silicon crystals in aluminum melt. The average radius of these crystals during isothermal holding is expressed by an equation; R3−R03=kt, where a constant k is determined as 1.6×10−12 cm3/s. When a specimen is solidified at a constant cooling rate of ΔT⁄Δt, solidification growth and Ostwald ripening take place simultaneously. Such a solidification process can be approximated as repetitions of quenching by ΔT and isothermal holding for the period Δt. The change of the average radius of primary silicon crystals during solidification has been numerically analyzed by separating Ostwald ripening and solidification growth from each other. It is shown that the contribution of Ostwald ripening increases with decreasing cooling rate and with increasing number of primary silicon crystals formed at the onset of solidification. By comparing the analytical and experimental results, it is concluded that the number of primary silicon crystals after complete solidification is primarily dependent on their number at the onset of solidification and the effect of Ostwald ripening is negligibly small. Growth of primary silicon crystals during solidification is almost entirely due to solidification growth. These results on primary silicon crystals are also discussed in comparison with those reported on primary dendrites.

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