Microstructure on remelting interface of Ni-W heterogeneous seed in preparing Ni-based single crystal superalloys

Ni-W heterogeneous seed was used to prepare the Ni-based single crystal superalloys and the microstructure at remelting interface under different conditions was analyzed. The results showed that there were lots of fine cellular grains with the basically consistent orientations and compositions at the remelting interface under high rate solidification condition. However, these grains disappeared under the liquid metal cooling condition, indicating that the formation of fine cellular grains might be controlled by constitutional undercooling related with the temperature gradient in front of the remelting interface. Therefore, a relationship between constitutional undercooling and temperature gradient was calculated to clarify the solidification mechanism of fine cellular grains. It was found that the high rate solidification method with a smaller temperature gradient could produce an enough constitutional undercooling at the front of remelting interface to form fine cellular grains, and then they competed with each other to adjust the dendrite spacing, finally obtained the single crystal microstructure with uniform dendrite spacing, while that microstructure could only be obtained through the flat-cell-branch transformation under the LMC condition.