Understanding the solidification and heat treatment characteristics in the CoCrNiSix medium-entropy alloy by experimentally verifiable multiscale thermodynamic and kinetic computational techniques

Abstract

CoCrNi medium-entropy alloy (MEA) possesses an FCC crystal structure with multiple slip systems and low stacking fault energy [1]; a substantial amount of nanoscale deformation twins can be generated under low-temperature and high-speed deformation. Adding a proper amount of Si can not only reduce the manufacturing cost and mass density but also enhance ballistic resistance by further lowering the stacking fault energy. Previous studies [2] utilized small-scale vacuum arc remelting techniques to investigate the solid solution or secondary phase strengthening of CoCrNi-based MEAs with Al or Si additions. However, to extend the application of lightweight, high-entropy alloys to industrial-grade impact-resistant plate manufacturing, especially for low-temperature environments, it is necessary to study the solidification and heat treatment characteristics of CoCrNiSix castings. This study employs finite element analysis at the macroscopic scale to investigate the solidification phase transformation and heat transfer characteristics of CoCrNiSix under precision-cast conditions. Additionally, at the mesoscopic scale, the phase-field method [3] is used to simulate the dendritic solidification microstructure and element segregation of CoCrNiSix. Thermodynamic parameters required for simulations are calculated using Thermo-Calc high-entropy alloy databases TCHEA6 and MOBHEA2. This research also utilizes electron microscopy to analyze the microstructures of chemically complex CoCrNiSix ingots, focusing on measuring the secondary dendrite arm spacing and elemental segregation profiles. Collecting these microstructure-related features allows us to reasonably infer the cooling rate corresponding to the investment casting process of CoCrNiSix and design rational parameter combinations for homogenization heat treatment of the cast ingots in terms of temperature and isothermal holding time. By validating macroscopic and mesoscopic simulation results through CoCrNiSix microstructure analysis experiments, the multiscale kinetic computational techniques included in this study can be further applied to cost-saving and process optimization practices in the manufacturing of various lightweight high-entropy alloys