Phase formation in mechanically alloyed Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al2.5CoCrCuFeNi and Al5CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al5CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions