2015 年 79 巻 5 号 p. 280-286
The volume fraction of primary β-Sn (fβ-Sn) is larger than the equilibrium value when hypoeutectic Sn-X alloys solidify in a crucible. However, fβ-Sn decreases with time when the alloy sample is held in the mushy state after observing recalescence. This study investigated the effect of the holding temperature on the change in fβ-Sn, using Sn-25.7 mass% Pb alloy. The volume fraction of primary β-Sn was characterized as a function of holding temperature and time. Primary β-Sn formed after recalescence (primary I) ripened during isothermal holding, and its volume fraction decreased. In addition, dendritic primary β-Sn (primary II) formed during cooling after isothermal holding. The volume fraction of primary I decreased rapidly as the holding temperature increased. Reaction rate analysis revealed that the rate-determining step may be diffusion in the solid. The grains of primary I increased in size in accordance with the Ostwald ripening effect even though the volume fraction decreased, and the grains floated upward in the crucible. Since a large quantity of primary II solidified, the total volume fraction of β-Sn was larger than the equilibrium value.
