Liquid AlCoCrFeNi and AlCoCrFeNiX (X = Mo, Ta) high-entropy alloys on graphite: Wetting, reactivity and CALPHAD modelling

Abstract

Wettability and interfacial reactivity with graphite of three equimolar High-Entropy Alloys (HEAs), namely AlCoCrFeNi (HEA-base), AlCoCrFeNiMo (HEA-Mo) and AlCoCrFeNiTa (HEA-Ta), were investigated for the first time, aiming to support industrial sectors involving liquid-phase processing routes. Isothermal high-temperature wettability tests were performed at 1400 °C for HEA-base and HEA-Mo, and at 1580 °C for HEA-Ta. The samples were then analysed by SEM-EDS. Based on the in-house-built GHEA thermodynamic database, CALPHAD calculations were used to simulate and discuss liquid-solid interactions occurring at high temperatures as well as transformations taking place in the samples during cooling. HEA-base wetted the graphite well after 5 min and the wetting behaviour significantly improved for HEA-Mo after the same contact time. HEA-Ta, after initial melting and wetting stage, solidified due to high-melting phases formation. All the graphite/HEA samples formed refractory carbides. For HEA-base and HEA-Mo, such carbides were found homogeneously dispersed in the whole drop. In HEA-Ta, TaC formed a compact layer separating two liquids of different compositions, with only one of these being in contact with the graphite substrate. The here proposed combination of experiments and thermodynamic calculations allowed to understand and discuss both high-temperature reactivity and evolution of the systems over cooling. Overall, a remarkable agreement between equilibrium calculations and experimental findings was observed for these complex dynamic systems.