The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfine dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings λ1 and the cooling rate Vc is λ1=0.013Vc-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2=0.00258Vc-0.31, which gives good agreement with the Feurer/Wunderlin model.
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