Evolution of phase constitution with mechanical alloying and spark plasma sintering of nanocrystalline AlxCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high-entropy alloys

Nanocrystalline AlxCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high-entropy alloys were synthesized by mechanical alloying (MA) and consolidated by spark plasma sintering (SPS). Single-phase face-centered cubic (FCC) structure was achieved with x = 0 and 0.3. Dual-phase body-centered cubic (BCC)/B2 + FCC structure was obtained in x = 0.6 and 1. Complete solid solution was achieved with a significantly reduced duration of MA (10 h) followed by a brief SPS leading to crystallite size of < 10 nm for MA powders and < 60 nm post-SPS. Incremental changes in Al content translate into increase of BCC + B2 phase fraction and alter the co-existing FCC phase composition consequently. Ni dominates FCC phase of CoCrFeNi and Al0.3CoCrFeNi, whereas Fe dominates FCC phase of Al0.6CoCrFeNi and AlCoCrFeNi. Carbon and Oxygen addition during milling process resulted in nanoscale dispersion of Cr7C3 carbide and Cr2O3 /Al2O3 post-SPS. The hardness of AlCoCrFeNi (730HV0.3) was the highest among all MA-SPS alloys.Graphical abstract