Strengthening CoCrFeNi High Entropy Alloy by In-Situ Phases of Laves and ZrC

To attain in-situ ceramic-reinforced phases by adding C powder, Zr pure metal in face-centered cubic (FCC) type CoCrFeNi high entropy alloy (HEA), a series of (CoCrFeNi)(100-x)(ZrC)(x) (x=0, 2, 4, 6, 8 atomic percent, at%) high entropy alloys were synthesized by vacuum arc melting. The phase composition, microstructure, tensile properties were studied in this work. It is found that the alloys possess microstructure of FCC phase, (Zr, Ni)-rich Laves and ZrC phases. The in-situ synthesis ZrC phase is randomly precipitated in the dendrite. Eutectic-like microstructure is formed by the Laves and Ni-poor FCC, which is generated in the dendritic region. With the addition of Zr, C elements, the volume fraction of ZrC and Laves phases increases gradually from 0 to 7% and 18% respectively. The ultimate tensile strength of the alloy is up to 600 MPa with the increasing of Zr, C content. The fracture mechanism of (CoCrFeNi)(100-x)(ZrC)(x) (x=2, 4, 6, 8 at%) is mixed fracture with multiple fracture modes. The increase of strength of (CoCrFeNi)(100-x)(ZrC)(x) HEAs mainly comes from the second phase strengthening fabricated by ZrC and Laves phases. The strain of the alloy maintains a high value due to the eutectic structure formed in the interdendritic region. The effect of mixing enthalpy (Delta H-mix) and large atomic radius difference (delta) promotes the formation of in-situ ZrC phase in (CoCrFeNi)(100-x)(ZrC)(x) and the coordination effect of reduced VEC and increased electronegativity difference (Delta(chi)) promotes the formation of Laves.