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The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus.The experiments were performed under a thermal gradient of approximately 45K·cm-1 and at withdrawal rates ranging from 30 to 200m·s-1.The experimental results show that the primary and secondary dendritic arm spacings (PDAS and SDAS)decrease when the withdrawal rate is increased.Compared with the theoretical models of PDAS,the results are in good agreement with Trivedis model.The relationships of PDAS and SDAS with withdrawal rates can be described as λ1 =649.7V-0.24±0.02 and λ2 =281V-0.32±0.03,respectively.In addition,the size of the γ phase significantly decreases with increasing withdrawal rate.