The mechanical properties of Ni–based superalloy are mainly determined by quantity, shape, size and distribution of γ' precipitates. For a given alloy, the quantity, shape, size and distribution of γ' precipitates are mostly affected by solidification parameters during solidification process. In present, the influence of solidification parameters on γ' precipitates has been extensively reported. But unfortunately, these researches on γ' precipitates under as cast condition are mostly intuitive descriptionof solidification phenomena and the in–depth research on the influence of solidification parameters on the morphology, distribution and size of γ' precipitates is little, especially under high thermal gradient directional solidification. In this paper, the influence of cooling rates on the morphology, size and distribution of γ' precipitates in dendrite core and interdendritic areas of directionally solidified nickel–based superalloy DZ4125 was investigated under high thermal gradient about 500 K/cm. The cooling rates used in this experiment were 2.525, 5.15, 13.17, 25.875 and 36.4 K/s, respectively. The relative mechanisms of changes in the morphology, distribution and size of γ' precipitates were discussed. These results show that, with cooling rate increasing, the dendrite microstructure of DZ4125 alloy becomes fine. At the solidification rate of 36.4 K/s, the dendrite morphology changes to a superfine dendrite solidification mode. In this process, the morphology of γ' precipitates changes from cubic to spheric shape and the sphericized speed of γ' precipitates in dendrite core quicker than in interdendritic. Furthermore, the average size of γ' precipitates decreases gradually so that they become well–distributed in both dendrite core and interdendritic. This average size of γ' precipitates in dendrite core is smaller than in interdendritic, however the difference in size decreases with cooling rate increasing. These changes in the morphology, distribution and size of γ' precipitates result mainly from the degree of supersaturation of solute in γ solid solution ΔX, degree of undercooling in γ solid solution ΔT, critical precipitation nucleation work of γ' precipitate ΔG* and solution diffusion coefficient in γ solid solution
D caused by altering cooling rate.