Microstructure, segregation and precipitate evolution in directionally solidified GH4742 superalloy

Abstract

The evolution of microstructure, elemental segregation, and precipitation in GH4742 superalloy under a wide range of cooling rates was investigated using zonal melting liquid metal cooling (ZMLMC) experiments. Comparing various nickel-based superalloys, the primary dendrite spacing is significantly linearly correlated with G ^−1/2 V ^−1/4 at high cooling rates, where G and V are temperature gradient and drawing rate, respectively. As the cooling rate decreases, the primary dendrite spacing increases in a dispersive manner. The secondary dendrite arm spacing is significantly correlated with (GV)^−0.4 for all cooling rate ranges. The degree of elemental segregation increases and then decreases as the cooling rate increases, which is due to the competition between solute counter-diffusion and dendrite tip subcooling. With increasing the so-lidification rate, the size of γ′, carbides, and non-metallic inclusions gradually decreases. The morphology of the γ′ precipitate changes from plume-like to cubic to spherical. The morphology of carbide changes from block to fine-strip then to Chinese-script. The morphology of carbide is controlled by both dendrite interstitial shape and element diffusion. The inclusions are mainly composite inclusions, which usually show the growth of Ti(C,N) with oxide as the heterogeneous nucleation center and carbide on the outer surface of the carbonitride. As the cooling rate increases, the number density of composite inclusions first increases and then decreases, which is closely related to the elemental segregation behavior.

Keywords

superalloys / microstructure / segregation / precipitation / inclusions

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