Microstructures and mechanical properties of CoCrFeNiHfx high-entropy alloys

CoCrFeNiHfx (x = 0.1–0.6) high-entropy alloys with different Hf additions are investigated in this paper. As the Hf content increases, the alloy microstructure gradually transforms from the hypoeutectic type to the hypereutectic type, with primary phases of either FCC dendrites or Laves phases. The eutectic composition of the alloy is identified to be close to CoCrFeNiHf_0.42. At room temperature, CoCrFeNiHfx alloys exhibit increased yield stresses accompanied by the degradation of ductility as raising the Hf content. CoCrFeNiHf_0.1 and CoCrFeNiHf_0.2 show strain hardening rates >2500 MPa at strain <0.3 and high strengths of ~3000 MPa at the strain of 0.5. At the high temperature of 800 °C, the alloy featuring a typical eutectic microstructure exhibits the maximum yield strength of 536 MPa compared to hypoeutectic and hypereutectic compositions. Nevertheless, as-cast eutectic and hypereutectic alloys show severe microstructural instability at 800 °C, as evidenced by the transformation of the eutectic microstructure from lamella to irregular morphology with significant coarsening.