Use of diffusion multiples to explore the Co-Cr-Fe-Mn-Ni high entropy system, The

Abstract

High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. The system chosen for this experiment is the Co-Cr-Fe-Mn-Ni system. The methodology developed for creating these diffusion multiples involved a two-step process. In the first step two binary alloys (50at-% Fe-Mn and 50 at%- Ni-Co ) were diffusion bonded together. In the second step, under uniaxial compression, was used to bond Cr to diffusion couple prepared in Step I. Successful diffusion multiples were created by this method. An auxiliary method named differential melting liquid impingement (DMLI) was developed that created diffusion multiples using liquid processing methods that will be described. After creation of these multiples, the ternary and quinary interface regions were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and nanoindentation. The Cr/NiCo region experienced interdiffusion but no intermediate phase formation retaining the FCC / BCC interface at the hot-pressing temperature (1200 °C). However, upon cooling from 1200 °C, the BCC region adjacent to the interface decomposed into BCC + σ. In contrast, the Cr/FeMn interface region developed a layered structure of FCC/σ/BCC suggesting that σ is stable at 1200 °C in contradiction to the published 1200 °C ternary phase diagram. Upon cooling, the σ present at 1200 °C decomposed into FCC + σ, except in samples that were contaminated with C; in those cases, FCC + M23C6 was observed as the decomposition product. The quinary regions were evaluated using the various HEA parameters, namely, ΔSmix, ΔHmix, Ω, Δχ, and δ. No strong correlations with phase stability were found using these parameters in contrast to expectations based on the literature. It was found that Cr solubility in the quinary disordered FCC varied linearly between the two ternary system endpoints (Co-Cr-Ni and Cr-Fe-Mn) Additionally, while nano-hardness maps did not support the severe lattice distortion hypothesis proposed for HEAs, a comparison of different solid solution strengthening mechanisms suggests that elastic modulus mismatch and a change in the lattice friction stress were the most likely contributors to strengthening.