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Theoretical and phase field modeling of chemical segregation and microstructure formation in rapid solidification of nickel-based superalloys

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 448319030
 
Final Report Year 2024

Final Report Abstract

The primary solidification and mechanical properties of sample from INCONEL 718 (IN718) alloy have been studied by experimental methods and methods of theoretical modeling. EXPERIMENTAL STUDIES have been made using Electromagnetic Levitation Facility (EML), pyrometric, optical and digital methods of quantifying of solidification kinetics. Atomic and electronic methods (ESRD and TEM) have been used for samples characterizing. Using these methods, the dendrite growth velocity, dendrite arm spacing, interdendritic microsegregation as function of undercooling have been obtained experimentally. The microhardness of samples as function of phase fraction and initial undercooling of samples processed by EML has been measured in connection to microsegregation and dispersity of dendrites. Theoretical modeling has been carried out using atomistic and mesoscopic methods. The latter ones are related to sharp interface and diffuse interface models of phase transformations in application to dendritic growth. ATOMISTIC MODELING has been carried out using molecular dynamic simulations. Thermodynamic and kinetic parameters of Ni-based allos have been found. These parameters have been used as an input into mesoscopic models for mesoscopic modeling. THE SHARP-INTERFACE MDELING: the three different regimes, which are characterized by different values of length scales, are described using the threecomponent ($Ni-Nb-Cr$) approximation for IN718: the transition to the pure thermally controlled growth is predicted by the experimental measurements of the dendrite growth velocity as the sharp transition consistently with the sharp change in the shape of recalescence front that is fixed at ∆T =170 K; drastic change in the crystalline microstructure fixed at ∆T=170; novel exponential behavior of the dendrite arm spacing fixed at ∆T >170 K. THE DIFFUSE-INTERFACE MODELING: The prediction of the equilibrium and metastable morphologies during the solidification of Ni-based superalloys on the mesoscopic scale has been performed using phase-field modeling. With this aim, the phase-field model has been used to simulate the evolution of solidification microstructures depending on undercooling in a quasi-binary approximation. The results of modeling have been compared with experimental data obtained on samples of the alloy IN718 processed using EML technique. The final microstructure, concentration profiles of niobium (interdendritic microsegregation), and the “interface velocity–undercooling” relationship predicted by the phase field modeling are in good agreement with the experimental findings. The simulated microstructures and concentration fields can be used as inputs for the simulation of the precipitation of secondary phases and for prediction of mechanical properties of IN718.

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