Effect of Withdrawal Rate of Directional Solidification on the Microstructure of a New Cobalt-Nickel Based Superalloy

The new cobalt-nickel superalloys containing γ´ precipitates‏ ‏have been noticed due to their ‎strength properties and higher corrosion resistance than nickel-base superalloys. The ‎research investigates the effect of the withdrawal rate in the directional solidification process on the ‎structure and hardness of a new cobalt-nickel base superalloy. For this ‎purpose, Bridgman's directional solidification process was used for the directional growth of ‎samples under vacuum at 1.5, 3, and 6 mm/min withdrawal rate. The results show that a ‎directional structure was obtained at all speeds, and with the increase of the withdrawal ‎rate, the distance between the primary dendritic arms increased from 176 to 147 μm, the ‎distance between the secondary dendritic arms increased from 90 to 55 μm, and The size of ‎γ´ precipitates‏ ‏decreases from 6.4 to 4.5 µm. These changes are due to the change in the ‎heat transfer mechanism near the surface of the refrigerant and at a height above the ‎surface of the refrigerant, as well as the increase in the supercooling temperature and the ‎decrease in the freezing time with the increase in the extrusion speed, which changes the ‎microstructure from rough dendritic to fine dendritic. The hardness results of the samples ‎increased from 371 to 375 and (HV) 393 by increasing the withdrawal rate from 1.5 to 3 and ‎‎6 mm/min. The increase in hardness can be due to the regularization of the structure and ‎the decrease in the size of γ´ precipitates.‎