Microstructural Investigations of Ni-Based Superalloys by Directional Solidification Quenching Technique

The improvement of the mechanical properties of Ni-based superalloys is achieved in most cases by modifying the chemical composition. Besides that, the processing can be modified to optimize the as-cast microstructure with regard to the mechanical properties. In this context, the present study highlights the solidification mechanism of several Nibased superalloys by conducting experiments using a modified, laboratory-scale BridgmanStockbarger furnace. In that context, the single-crystal rods are partially melted, directionally solidified and quenched sequentially. Several characterization methods are applied to further analyze the influence of the alloying elements and the variation of the withdrawal rate on the ascast microstructure. Four stages of solidification are distinguished whereby the morphology observed in the different stages mainly depends on the cooling rate and the local concentration of the carbide forming elements. The effect of carbide precipitation and the effect on the as-cast microstructure is investigated by employing energy dispersive Xray spectrometry (EDX) and electron backscatter diffraction (EBSD) analysis techniques. A local polycrystalline structure is observed in the single-crystal system as consequence of the influence of the carbon content and the cooling rate. The present work aims to develop strategies to suppress the formation of the polycrystalline structure to maintain the single-crystal microstructure.