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 which are located 5.5 cm and 12.5 cm from the chill end. (a) and (c) is from the DWDS process. (b) and (d) is from the Bridgman process.
The measured average primary dendrite arm spacings (PDAS) corresponding to the locations and processes are listed in Table 2. From the experimental results, one can observe that the primary dendrites are obviously refined in the DWDS process in comparison to the Bridgman process. The primary dendrite arm spacings are 2.1–2.5 times smaller than those in the Bridgman process. Besides this, the variation in the primary dendrite arm spacing along the length of the rods is 1.
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35 ?m/mm for the DWDS process, while this value is 1.92 ?m/mm for the Bridgman process. This indicates that the DWDS process has great ability to retain the stability of the cooling rates as the distance from the chill end increases.