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Control of the structural parameters in the (Zn) - Zn16Ti single crystal growth

Wołczyński W., Boczkal G., Musielak P., Sibiga G., Mikułowski B.

Archives of Foundry Engineering

2011

Vol. 11, iss. 4

149-156

The (Zn) - single crystal was obtained by means of the Bridgman system. Several growth rates were applied during the experiment. The graphite crucible was used in order to perform the solidification process. The unidirectional solidification occurred with the presence of the moving temperature field. The thermal gradient was positive so that the constrained growth of the single crystal was ensured. The (Zn) single crystal was doped with small addition of titanium and copper. The titanium formed an intermetallic compound Zn16-Ti. The copper was solved in the solid solution (Zn). The precipitates of (Zn) and Zn16-Ti formed a stripes localized cyclically along the single crystal length. The intermetallic compound Zn16-Ti strengthened the (Zn) single crystal. The structural transitions were observed in the stripes with the increasing solidification rate. Within the first range of the solidification rates ( 0 -v1) the irregular L-shape rod-like intermetallic compound was revealed. At the 1 v - threshold growth rate branches disappear continuously till the growth rate equal to [...]. At the same range of growth rates the regular lamellar eutectic structure (Zn) - Zn16-Ti appeared continuously and it existed exclusively till the second threshold growth rate equal to v2 . Above the second threshold growth rate the regular rod-like eutectic structure was formed, only. The general theory for the stationary eutectic solidification was developed. According to this theory the eutectic structure localized within the stripes is formed under stationary state. Therefore, the criterion of the minimum entropy production defines well the stationary solidification. The entropy production was calculated for the regular rod-like eutectic structure formation and for the regular lamellar eutectic structure formation. It was postulated that the observed structure are subjected to the competition. That is why the structural transition were observed at the revealed threshold growth rates. Moreover, it was postulated that this structure is winner in the competition which manifests a lower minimum entropy production within a studied range of growth rates. Exceptionally, the phenomenon of branching was formed under the marginal stability. Therefore, the spacing of the regular structure selected by the criterion of minimum entropy production depends on the growth rate only. In the case of the irregular eutectic structure the average spacing depends on both growth rate and thermal gradient. The irregular eutectic structure localized within the stripes contains some areas with regular rods and some areas with maximum destabilization of solid / liquid interface of the (Zn) - non-faceted phase. The length of the destabilized non-faceted phase is treated as equal to the wavelength of the perturbation which appears at the solid / liquid interface of the non-faceted phase, (Zn). In spite of the interface destabilization the selection of a given structure depends on the localization of the minimum entropy production, only.