Effects of annealing treatment on phase composition and microstructure of CoCrFeNiTiAlx high-entropy alloys

CoCrFeNiTiAlx (x: molar ratio) high-entropy alloys with different Al content were prepared by vacuum arc-melt casting and the as-synthesized alloys were subsequently heat treated at 1000 °C for 2 h. Effects of Al content and annealing treatment on phase composition and microstructure of these alloys were investigated by intensive characterization and analysis of the crystal structure, morphology and elemental segregation. Al addition promotes the formation of BCC structured phases in both as-cast and as-annealed alloys. The alloy without Al addition is mainly composed of face-centered cubic (FCC) solid-solution while the main phase transforms to stabilized (α-Fe,Cr)-based body-centered cubic (BCC) solid-solution as Al is introduced. Annealing demonstrates no obvious influence on the main phase and elemental segregation of CoCrFeNiTiAlx alloys except for alloys with low Al content (x = 0, 0.5 and 1.0) contain more intermetallic compounds after annealing. The phase formation rules of high-entropy alloys is explored in this study as the empirical formation rules of simple solid-solutions are proposed as: mixing entropy (∆Smix) ≥ 13.38 J/K·mol, −10 kJ/mol ≤ mixing enthalpy (∆Hmix) ≤ 5 kJ/mol and atom-size difference (δ) ≤ 4.