Applied Calphad to Cast and Wrought Successors to IN718: A Physics-Based Approach with Implications for Phase Stabilities, Precipitation, and Microstructural Modeling

Abstract

This comparative computational study of Applied Calphad aims on the simulative analysis of mechanical properties and microstructures of recent C&W IN718 successors with similar application at elevated temperatures. Predictions are presented for candidates AD-730, and M647. Moreover, Rene88DT and derivatives GH4096, as well as Co-free EXP-G27, which, due to a new C&W technology involving electro-slag remelting continuous directionally solidification [1], enter the list of promising C&W candidates, are researched. The different alloying concepts are presented, and their influence on microstructures during thermal processing is shown. Using the assessed MatCalc [2] multi-component Calphad thermodynamics and diffusion mobilities databases „mc_ni“ (thermodynamic databases for fcc γ and γ‘, as well as the diffusion mobilities database used here are added at the end of this paper) for Ni-base superalloys within the system Ni-Fe-Al-Ti-Co-Cr-Mo-Nb-W, phase stabilities of Ni3Al-based strengthening precipitates, affected by different alloying contributions are presented. Evaluated solubility behaviour of elements, and the thermodynamic dissolution behaviour of second phases in the Ni-fcc matrix are discussed. By Scheil-Gulliver simulations of as-cast microstructures, segregation trends during casting are described. The kinetic phase evolutions during supersolvus heat treatment conditions are evaluated with nucleation and growth modeling by using Matcalc (version 6.04 rel0.127), predicting particle distributions, sizes and densities. We show that fully predictive, computational strengthening trends, employing simple Applied Calphad and computational thermokinetics of precipitation, combined with strengthening models for solid solution strengthening and order strengthening for γ‘, are a valuable tool for decisions on appropriate thermal operation conditions of IN718 successors.