Determination of the A2/B2 BCC phase fraction in AlxCoCrFeNi (1 ≤ x ≤ 4) at elevated temperatures

The AlxCoCrFeNi high-entropy alloy system exhibits complex evolution between disordered (A2) and ordered (B2) BCC structures, which strongly affects mechanical and corrosion properties. For x ≳ 0.3, A2 and B2 often coexist, yet the corresponding high-temperature phase boundaries remain controversial across published phase diagrams. Here, we quantify the retained A2/B2 fraction for 1 ≤ x ≤ 4 after heat treatment at 1593–1623 K for 30 min, followed by water quenching, using X-ray diffraction (XRD), transmission electron microscopy (TEM), and CALPHAD calculations (Thermo-Calc, TCHEA7). Because the lattice parameters of A2 and B2 are nearly indistinguishable, TEM provided the primary phase-fraction assessment, particularly at lower x. The B2 fraction increases with both Al content and treatment temperature; however, a single B2 phase was not retained experimentally even at x = 4, owing to the persistence of nanoscale A2 precipitates in the B2 matrix. The A2 fraction is consistently higher than the CALPHAD-predicted equilibrium value. These results show that conventional heat treatment plus water quenching is insufficient to retain the high-temperature B2 phase at room temperature and therefore cannot, by itself, validate the equilibrium A2/B2 boundaries.