In situ synchrotron X-ray diffraction revealing competition between A1 and B2 phases in AlCoCrFeNi high-entropy alloys

Phase competition in AlCoCrFeNi x high-entropy alloys ( x = 2.0, 2.1, 2.2, and 2.4) was investigated using synchrotron X-ray diffraction combined with electromagnetic levitation. In undercooled melts with x = 2.0, 2.1, and 2.2, the ordered B2 phase forms first via the transformation L → B2, marking the initial stage of solidification. This observation aligns well with the double recalescence phenomenon, where B2 crystallizes directly from the liquid, followed by the formation of a B2/A1 eutectic structure from the mushy zone, as captured in high-speed video recordings of the AlCoCrFeNi 2.1 sample. In contrast, the AlCoCrFeNi 2.4 alloy solidifies initially with the primary A1 phase via L → A1. The delay time (Δ t ) between nucleation of the primary phase and onset of eutectic transformation is strongly influenced by the nickel concentration. Moreover, the dissolution temperature of the ordered L1 2 phase during solid-state heating increases steadily with rising nickel content. These experimental findings show strong agreement with phase evolution predictions obtained through thermodynamic modeling of the AlCoCrFeNi system using Thermo-Calc software. Overall, this study provides a comprehensive picture of the phase formation and stability, solidification kinetics, and microstructural development of AlCoCrFeNi x high-entropy alloys.