The cooling rates at the positions 5

The cooling rates at the positions 5.5 cm and 12.5 cm from the chill end in the rods were measured and recorded into the table below. The changing rates of both positions are also recorded in the table. In the samples solidified by using the downward directional solidification (DWDS) process, the cooling rates at the two locations were 8 times (for 5.5 cm) and 13.5 times (for 12.5 cm), respectively, greater than those in the Bridgman samples.

The morphologies of the primary dendrite at the two locations of the rods solidified by using the DWDS and the Bridgman processes are shown in figure above. Morphologies of ? phase on the transversal sections are located 5.5 cm and 12.5 cm from the chill end. (a) and (c) are the DWDS process. (b) and (d) are the Bridgman process.

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In the table above, the measured average primary dendrite arm spacings (PDAS) corresponding to the locations and processes are recorded. From the experimental results, one can observe that the primary dendrites are refined better in the DWDS process in than comapred to the Bridgman process. The primary dendrite arm spacings for DWDS process are around 2.3 times smaller than those in the Bridgman process. In addition, the variation in the primary dendrite arm spacing is 1.35 ?m/mm for the DWDS process. On the other hand, this value is 1.92 ?m/mm for the Bridgman process. As a result, it can be concluded that the DWDS process has greater ability to retain the stability of the cooling rates as the distance from the chill end increases when compared to the Bridgeman process.

This figure above shows the morphologies of ? phase in the longitudinal sections which are located 5.5 cm and 12.5 cm from the chill end. (a) and (c) are the DWDS process. (b) and (d) are the Bridgman process. It shows the typical micrographs of the secondary dendrite at various distances from the chill end.

The graph above is a typical secondary dendrite arm spacings at the distances of 5.5 cm and 12.5 cm from the chill end.