The effects of withdrawal and melt overheating histories on the microstructure of a nicked-based single crystal superalloy

The solidification interface morphologies of a single crystal superalloy DD3 have been studied over a range of cooling rate with large variations in withdrawal speeds in high-thermal gradient directional solidification. The effect of withdrawal and thermal histories on the dendrite morphologies and spacing is also studied. The results show that increasing either thermal gradient or withdrawal rate will refine the dendrite. Especially, super-fine dendrite structure with suppressed side branches was found under the cooling rate as high as 10K/s during high thermal gradient directional solidification. The initial withdrawal histories and melt superheat treatment on dendrite morphologies are significant: with the increase in the initial acceleration withdrawal speed, the primary dendrite arm spacing in stable growth zone decreases. By growth velocity transition from higher one to lower one abruptly, the primary dendrite spacing, and constitutional segregation in the stable growth zone can be reduced and vice versa. Melt superheating treatment prior to solidification can stabilize liquid/solid interface and refine primary dendrite and γ′ phase obviously. Whereas, the secondary dendrite arms seem not sensitive neither to melt superheating treatment, nor initial acceleration withdrawing.