Fine-scale precipitation in the high-entropy alloy Al0.5CrFeCoNiCu

The high-entropy alloy Al0.5CrFeCoNiCu has been shown to consist of two stable, face-centred cubic solid solutions at temperatures approaching its solidus: one rich in Cr, Fe, Co & Ni (dendritic) and the other rich in Cu (interdendritic). While some studies have suggested that the high-temperature microstructure may be metastably retained to room temperature through rapid cooling, evidence of phase decomposition has also been reported. In this study, fine-scale precipitation has been observed in samples of Al0.5CrFeCoNiCu that have been rapidly cooled after casting, and water quenched following ageing for 1000 h at 1000 °C. Contrary to previous reports, in the as-cast state, the two face-centred cubic phases, as well as an L12 phase, were found in both dendritic and interdendritic areas, with the dendritic areas having undergone a spinodal decomposition. After ageing and quenching, L12 precipitates were found in both dendritic and interdendritic areas, and precipitates of the Cr-, Fe-, Co- and Ni-enriched face-centred cubic phase were found in the Cu-rich interdendritic regions. Given the nature of the heat treatments applied, the results suggest that precipitation in the alloy is rapid and cannot be avoided, even when the material is cooled quickly to room temperature.