Exploring multiphasic multi principal element alloys: an approach using experimental and computational tools

Abstract

High entropy alloys (HEAs) are a class of metals that do not have a single principal element in their composition. Initially great effort was made to search for single phase HEAs but with the development of more recent frameworks, great interest emerged around multiphasic microstructures. In this study, two different routes aim to search phase stability and structural properties involving multiphasic HEAs. The first study investigates phase stability of the Cr51.5Co15Ni33.5 duplex HEA. This alloy was designed using CALPHAD simulations to achieve a dual-phase microstructure of Face-Centered Cubic (FCC) and Body-Centered Cubic (BCC) phases, while suppressing the formation of the detrimental sigma phase. The HEA was characterized using SEM, TEM, XRD and synchrotron diffraction. Experimental results confirmed the presence of the FCC and BCC phases, although the formation of the sigma phase was also observed, consistent with TSAI´s criteria and warranting further analysis of the CALPHAD predictions. The second Chapter shows a combination of simulation techniques and experimental analysis to estimate interfacial energies of a HEA based on Ni superalloys. The composition of the HEA was designed by high throughput calculations to have thermal stability at high temperature and a microstructure composed of FCC matrix and L12 Ni3Al-type precipitates. The main simulation technique employed was Density Functional Theory – DFT trough the code Quantum Espresso using the supercell method. The results obtained were compared to experimental analysis of synchrotron x-ray diffraction data and precipitation simulations showing that the combination of techniques can lead to reasonable values of interfacial energy. That combination of methods serves as a starting point to develop reliable methodologies for estimating interfacial energies for metallic alloys, which is important since that energies between FCC and L12 in superalloys and HEAs are difficult to measure experimentally and play a key role in phase stability and final properties of the material.