Effect of cooling rate on the solidification behavior and elemental segregation in a cast NiCoCr-based superalloy

Abstract

Understanding solidification behavior and elemental segregation in Ni-based superalloys is crucial for predicting the microstructural evolution in complex-structured castings, optimizing heat-treatment protocols, and enhancing product quality. In this work, a systematic study is conducted to elucidate the influence of cooling rate on the solidification microstructure and elemental segregation behavior in a newly NiCoCr-based superalloy, K439B. Differential scanning calorimetry and Thermo-Calc calculations are employed to obtain the alloy's liquidus (1349 ± 3 °C), solidus (1277 ± 1 °C), and primary solidification sequence (γ → γ + MC → γ + MC + η). In-situ high-temperature confocal scanning laser microscopy observations reveal a decrease in nucleation temperature (1316.7–1304.3 °C) and a Boltzmann-type evolution of solid fraction at different cooling rates. The segregation coefficients for Ti, Nb, and Ta exhibit strong positive segregation, while Ni, Co, and Cr show negative segregation. Dendrite morphology analysis shows that the secondary dendrite arm spacing (λ₂) refines from 44.4 μm to 19.4 μm as the cooling rate increases. Precipitate analyses demonstrate that the γ′ phase refines from blocky (74 nm) to spherical (11 nm) with nearly constant volume fraction (15 %), and MC carbides evolve from equiaxed to rods (aspect ratio from 1.35 to 13.9) with a “rise-then-fall” in volume fraction (1.31 → 1.78 →1.57 %). In addition, a Ti-rich η-Ni₃Ti phase precipitates in the interdendritic regions at a cooling rate of 200 °C·min⁻¹, due to strong Ti segregation. These findings provide relationships between cooling rate, microstructure, segregation, and phase formation in K439B alloy, allowing for more precise process optimization and more consistent casting quality.