Accelerated atomic-scale exploration of phase evolution in high-entropy alloys
Experimental and simulation results show that many high-entropy alloys (HEAs) are metastable and may decompose into multiple phases upon long-term annealing at elevated temperatures, which consequently changes their superior mechanical properties obtained in the initial solid solution single phase. Unfortunately, investigation of phase stability of HEAs can be extremely time consuming, when the traditional approach, mainly consisting of melting, casting, homogenization, long-term annealing and preparation of samples for microscopy, is applied, which frequently requires months or even years. To assess the phase stability of HEAs in an accelerated way, we combine all processes by a fast combinatorial processing platform (CPP) approach, which enables (1) simultaneous synthesis of 36 identical atomic-scale-mixed films by co-deposition on an microtip array of 10 nm-diameter Si tips; (2) rapid phase evolution in the formed nanocrystalline HEAs on the tips; (3) direct atomic-scale analysis of phase evolution after each processing step by atom probe tomography, complemented by transmission electron microscopy. As a proof of concept, an equiatomic CrMnFeCoNi alloy was deposited on a CPP and subjected to isochronal annealing. It is observed that (1) phase decomposition occurs in several hours; (2) the decomposed phase products are the same as those observed after hundreds of days annealing time for conventional, coarse-grained bulk HEAs; (3) phase decomposition can occur at different temperatures of different annealing time. This approach has been extended to other HEAs and medium-entropy alloys for studying oxidation and corrosion. Influences of grain size and grain boundary elemental segregation on phase stability and oxidation behavior of HEAs will be discussed.