The Effect of Withdrawal Rate on the Microstructural Defects of a New Cobalt-nickel Based Directionally Solidified Superalloy

The microstructure of new cobalt-nickel-based superalloys contains γ´ precipitates. Withdrawal rate and temperature gradient are two important parameters affecting the formation of defects such as segregation, microporosity, and eutectic zones. This research discusses the impacts of the withdrawal rate on these defects. To this end, Bridgman furnace was used for directional growth of the samples under a vacuum at the withdrawal rates of 1.5, 3, and 6 mm/min. Upon increasing the speed of the withdrawal rate mold, segregation of the elements is reduced due to the shorter diffusion time. Co and W elements tend to segregate in the dendritic core while Al, Ti, and Ta tend to segregate in the inter-dendritic regions. Throughout the study, the size of microporosity decreased from 14.3 to 9.5 μm while increasing the withdrawal rate from 1.5 up to 6 mm/min. This decrease in the size of micropores resulted from the lack of melt feeding in the inter-dendritic areas. Moreover, the size of the eutectic zones decreased from 14.6 to 8.9 μm upon increasing the withdrawal rate from 1.5 up to 6 mm/min, mainly due to the smaller melt pools in the final stage of solidification.