A comprehensive study of phase transitions in Al_0.5CoCrFeNi high-entropy alloy at intermediate temperatures (400 ≤ T ≤ 900 °C)

High-entropy alloys (HEAs) have been extensively investigated primarily because of their wide range of properties (mechanical, thermal, corrosion, etc.) that enable their application in countless applications. One of the most promising families is the Al_xCoCrFeNi based HEA, which tends to form simple phases and exhibit simple phase evolution with temperature. In this system, the complexity of the phase evolution tends to increase with decreasing temperatures, which complicates the collection and analysis of experimental thermodynamic data. Therefore, most of the work done in this system regarding phase transitions has been at high temperatures (T > 1000 °C). Al_0.5CoCrFeNi has promising mechanical properties due to its duplex nature. It has an FCC dendrite core (DC) region at low temperatures, which occupies about 90 vol%: the reminder is a B2/BCC mixture inter-dendritic (ID) region. Each region has a different chemical composition, which leads to different phase evolution at intermediate temperatures (400 ≤ T ≤ 900 °C). The phase composition and evolution were studied using high-sensitivity calorimetry followed by electron microscopy and XRD characterization. The DC region exhibits a simpler phase evolution with ordered FCC (L1₂) nano-precipitation at 508 °C that transitions to B2 precipitation at 778 °C. The L1₂ → B2 transition is associated with a large exothermic event caused by the release of ‘symmetry-breaking’ strain and significant shrinkage. The ID region exhibits a more complicated phase evolution that starts with precipitation of a Co-Cr rich HCP phase at 626 °C and continues with the precipitation of Cr-Fe rich sigma (σ) phase along the boundaries between the regions at 699 °C, followed by the transition of the BCC precipitation into FCC at 733 °C. All these transitions in the ID region are associated with non-linear expansion. The experimental findings were compared with the thermodynamic evolution made using the ThermoCalc software and two thermodynamic databases.