Effect of the cooling rates on the microstructure and segregation characteristics in directionally solidified GH4151 superalloy

Due to the varying cooling conditions within different internal areas, there is a notable disparity in the cooling rates. This discrepancy has resulted in severe elemental segregation and the formation of coarse MC phases in the Φ508 mm GH4151 superalloy ingot, which has undergone triple smelting (VIM + ESR + VAR). These factors have had detrimental effects on the stability of the alloy structure. The solidification segregation, microstructure evolution, and secondary phase precipitation behaviors of GH4151 alloy under a wide cooling rate range were studied through Differential Scanning Calorimetry (DSC), in situ observations with High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM), Directional Solidification (DS), and Computational Fluid Dynamics (CFD). The results show that the solidification path of GH4151 can be summarized as (1)L → γ (1334.6 °C); (2) L → γ + MC (1281 °C and 1258 °C); (3)L → γ + MC + γ/γ' + Laves (1152 °C); (4)L → γ + MC + γ/γ' + Laves+M2B (1084 °C), where MC, γ/γ', Laves, and M2B are the main precipitate types during the solidification process. Mo, Nb, and Ti are the main dendritic segregation elements. Due to the reverse diffusion of solute elements under low cooling rates, the element segregation presents an increasing and then decreasing trend with the increase in cooling rate. The maximum segregation rate occurs at 24 °C/min. The cooling rate dramatically influences the morphology of the solid-liquid interface. The prediction formulas for PDAS and SDAS of GH4151 alloy under slow cooling conditions are λ1 = 1137.1·V-0.25·G-0.5 and λ2 = 153.6·Vc-0.32 respectively. Similarly, with the increase of solidification speed, the size of γ’ phase, Laves phase, γ/γ’ phase, and MC gradually decreases. When the cooling rate exceeds 12 °C/min, the large blocky MC phase evolves into elongated bars. The cooling rate from the center to the surface of the Φ508 mm VAR ingot ranges from 5 to 20 °C/min, with the standout being that the fastest cooling rate at the edge is about 48 °C/min.