Ni기 초내열 합금에서 응고속도 및 온도구배가 응고조직에 미치는 영향
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Morphological evolution and growth mechanism of solidification interface during solidification were investigated in the Ni-base superalloy[IN738LC, GTD111M] by directional soldification and quenching(DSQ) technique. The experiments were conducted with changing solidification rate(V) and thermal gradient(G) which are major solidification process. High thermal gradient condition could be obtained not only increasing the furnace temperature but also closely attaching the heating and cooling zones in Bridgeman type furnace. Three types of the solidified structure were identified as solidified part(directional solidification), mushyzone part(dendrite) and liquid part(quenched equiax). The width of a mushyzone was narrowed when solidification rate increased from 5㎛/sec to 100㎛/sec. The dendritic/equiaxed transition was found in the condition of G/V value lower than 0.05×10³℃s/mm², and the planar interface of the MC-γ eutectic was found under 17×10³℃s/mm². It was confirmed that the dendrite spacing depending on the cooling rate(GV), and the primary spacing were affected by the thermal gradient more than solidification rate. The dendrite lengths were decreased as increasing the thermal gradient, and the dendrite tip temperature was close to the liquidus temperature at 50㎛/s. The carbide morphologies were affected by solidification rate and thermal gradient, they formed into blocky-type, rod-type[GTD111M] in the planar interface and do script-type, spotty-type in the dendrite interface.