Abstract
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 1000h 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.
Highlights
High-entropy alloys (HEAs) are commonly defined as materials that contain five or more elements in near-equiatomic ratios [1]
A region containing an interface between dendritic and interdendritic materials is shown in the bright-field image in Fig. 1a, in which the dendritic and interdendritic regions are labelled 1 and 2, respectively
The solubility of Cr, Co & Fe is reduced in the interdendritic fcc2 at low temperatures, which leads to the precipitation of fcc1 as precipitates with rectangular cross-sections
Summary
High-entropy alloys (HEAs) are commonly defined as materials that contain five or more elements in near-equiatomic ratios [1]. Such alloys have been referred to as ‘high-entropy’ because, in contrast to conventional alloys, their multi-principal component compositions theoretically deliver significantly higher entropies of mixing. The original investigation of Al0.5CrFeCoNiCu reported the presence of two face-centred cubic (fcc, A1 in Strukturbericht notation) phases below the solidus temperature in the as-cast condition: one constituting the dendritic material and the other the interdendritic material. One of the most recent Al0.5CrFeCoNiCu studies identified only one fcc phase alongside an L12 phase in melt-spun material [18], in agreement with the full fcc → L12 ordering proposed originally. Reports have identified a Ni–Al-based B2 phase and a Cr–Co-Fe-based s phase in the alloy, following prolonged exposures at temperatures in the range 700–1000 °C [14,16]
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