The formation of sigma phase in the CoCrFeNi high-entropy alloys

The as-cast CoCrFeNi high-entropy alloys show simple FCC structure and outstanding deformation ability at room temperature. Respectively, after 50% of deforming at room temperature and annealing at intermediate temperature range of 600 °C–800 °C for 2 h, CoCrFeNi alloys still show simple-phase FCC structure. It is noteworthy that CoCrFeNi high-entropy alloys can’t be strengthened by means of annealing, while deformation is an effective way to strengthen the alloys. It is investigated that the formation of sigma ( σ ) phase in high-entropy alloys is closely related to the difference of atomic radius ( δ ), the concentration of valence electron ( VEC ) and paired sigma-forming element ( PSFE ) content, and PSFE plays an crucial role. When PSFE  > 40 at.%, 6.75 ≤  VEC  ≤ 7.86, 4.0 ≤  δ  ≤ 7.2, σ phase is prone to form in high-entropy alloys. In addition, alloying and processing route are two important factors that affect the sigma-formation in CoCrFeNi high-entropy alloys.