Eutectic Formation During Solidification of Ni-Based Single-Crystal Superalloys with Additional Carbon

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ARBON is conventionally added to Ni-based equiaxed (EQ) and directional solidified (DS) superalloys to strengthen grain boundaries (GB) through the formation of carbides.[1] Due to the disappearance of GB in Ni-based single-crystal (SC) superalloy components, this element was eliminated in earlier-developed SC superalloys to increase incipient melting temperatures during the heat treatment process.[2] With the development of Ni-based SC superalloys, carbon has been reintroduced into Ni-based SC superalloys because it purifies the alloy melt,[3] reduces the extent of segregation of refractory elements,[4,5] impedes convection in the interdendritic regions and further alleviates the occurrence of freckle defects,[6] reduces the formation of surface scale on Ni-based SC castings[7] as well as strengthens any low-angle grain boundaries (LAGBs), which are often present in large SC components.[8] Despite these advantages, a small amount of MC-type carbides formed during solidification negates the local monocrystallinity of Ni-based SC components and is detrimental to the mechanical properties by acting as initiation sites of cracks thus leading to the component’s final fracture.[1] Due to the different orientations of carbides than that of the c matrix, macroscopically defined SC castings of Ni-based superalloy with additional carbon can be microscopically considered as pseudo-SC components.

During their solidification, multiple-phase transformations occur.[9–11] When the temperature of the superalloy melt decreases below its melting point, the c phase having dendritic morphology primarily crystalizes from the melt. Following this, primary MC-type carbides precipitate from the melt. As the solidification proceeds, the c dendrites coarsen and the MC-type carbides grow larger. During the final stage of the solidification, c/c¢ eutectics form from the remaining liquid in the interdendritic regions. Meanwhile, fine c¢ particles precipitate from the c matrix through solid-state phase transformation. As one of the most important phases in as-cast Ni-based SC superalloys, the formation of the c/c¢ eutectics are significantly affected by the alloy’s composition and the casting process. During the previous several decades, a large number of studies were conducted to investigate the morphological evolution, the size, and the volume fraction of the c/c¢ eutectics under different solidification conditions[12,13] as well as the internal microstructural formation of the c/c¢ eutectic.[14–19] However, the relationship of the nucleation behavior of the c/c¢ eutectics with c dendrites, MC-type carbides, and the residual, interdendritic liquid has not been investigated. The purpose of this study is to explore the effect of the existing c dendrites and MC-type carbides as well as the residual liquid on the c/c¢ eutectic formation in the Ni-based SC superalloy with additional carbon.

II. FU WANG, DEXIN MA, and ANDREAS BU¨HRIG-POLACZEK are with the Foundry Institute, RWTH Aachen University, Intzestrasse 5, 52072 Aachen, Germany. Contact e-mail: darrel011203

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