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The microstructure scales of dendrites,such as primary and secondary dendrite arm spacings,control the segregation profiles and the formation of secondary phases within interdendritic regions,which determine the properties of solidified structures.Investigations on primary and secondary dendrite arm spacings of primary α-phase during directionally solidified Pb-26wt％Bi hypo-peritectic alloy were carried out in this research,and systematic studies were conducted using cylindrical samples with different diameters (φ =1.8 and 7.0 mm) in order to analyze the effects of sample diameter on the primary and secondary dendrite arm spacings.In this work,the dependence of dendrite arm spacings on growth velocity was established.In addition,the experimental data concing the primary and secondary dendrite arm spacings were compared with the main predictive dendritic models from the literatures.A comparison between experimental results for dendrite arm spacings of the 1.8-mm-diameter sample and 7.0-mm-diameter sample was also conducted.