Kosaku Nakano1, Yasunobu Kobayashi2, Akihiko Kamio3 and Shigeo Oya4
1Graduate School, Waseda University, Tokyo.
The 99.99% pure copper and its alloys were unidirectionally solidified and decanted to examine the morphology of the solid-liquid interface. The interfacial morphology shows transitions from plane to depressions, irregular cells, elongated cells, hexagonal cells, broken cells and dendritic cells, successively, with the decrease of freezing conditions G/RC0 and are governed strongly by the distribution coefficient of solutes in alloys (where G is the temperature gradient in the liquid, R is the growth rate and C0 is the initial solute concentration). Each morphological transition occurs under constant G/RC0. These critical conditions in copper-base alloys are much smaller than those in other alloys, such as tin, lead and aluminum alloys.
The entrapping of solutes into the solid at the interface during freezing forms segregating regions. According to the degree of constitutional undercooling, these regions increase their areas in forming depressions, cell nodes or cell walls. The solute content at cell nodes in the hexagonal cell interface is about 8 times as much as C0 in Cu-Mn alloys, about 12 times C0 in Cu-Ti alloys, about 7 times C0 in Cu-Sn alloys and about 30 times C0 in Cu-Cr alloys.
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