Short-range ordering heredity in eutectic high entropy alloys: A new model based on pseudo-ternary eutectics

Abstract

Eutectic high entropy alloys (EHEAs) have garnered wide attention in the metal community due to their excellent castability and unique microstructures. However, the formation mechanisms of these eutectic microstructures and their compositional design remain puzzling. In this paper, we have developed a simple yet unified pseudo-ternary model to unravel the puzzle of multiple eutectics in Ni-Al-containing high entropy alloys (HEAs) deploying a combination of the chemical similarity/dissimilarity coefficient concept, CALPHAD technique and first principle molecular dynamics simulation. Using the chemical similarity/dissimilarity parameters derived from CALPHAD model parameters and the eutectic valleys of Ni-Al-Cr, we reproduce the multiple eutectic patterns in the Ni-Al-containing HEAs and present a general approach for pinpointing the composition of eutectic HEA (EHEA) in the vast temperature-composition space. Aided by the first principle molecular dynamics calculation, we have shown that although the eutectic structures of Ni-Al containing EHEAs are different, the formation mechanism of the eutectic structure is similar, which stems from the short-range-ordering (SRO) heredity of EHEA liquid from the eutectic valleys of the pseudo Ni-Al-Cr ternary. Two EHEAs, EHEA 1 (Al16.95Cr16.95Fe19.1Ni47) with L12+B2 eutectic structures and EHEA 2 (Al33.5Cr33Co4Ni29.5) with BCC+B2 eutectic structures were designed using the model and experimentally verified. This unified Ni-Al-Cr ternary model, validated by experimental data in both literature and our fabricated EHEAs, presents a compelling solution to the long-debated grouping strategy in EHEA design and offers a new paradigm to map the complicated but intriguing eutectic microstructures in EHEAs and conventional alloys.