Evolution of microstructure homogeneity and mechanical properties in nano-/ultrafine eutectic CoCrFeNiNbx (0.45 ≤ x ≤ 0.65) high entropy alloy ingots and cast rods

Abstract

We report the evolution of microstructure and hardness homogeneity in arc melted ingots (AMIs) and suction cast rods (SCRs) of CoCrFeNiNbx (0.45 ≤ x ≤ 0.65) eutectic high entropy alloys (EHEAs) solidified at cooling rates of ~10–103 K/s. The effect of cooling rate on the evolution of phase and microstructure, has been investigated thoroughly by using scanning electron microscopy, x-ray diffraction. Whereas the mechanical properties have been investigated by compression test and Vickers microhardness. The microstructure of the EHEAs comprised of FCC and Fe2Nb type Laves phase, which remained the same under different processing conditions. The AMIs exhibited a high yield strength of 1.4–2.0 GPa with high compressive fracture strain up to 17.4% at room temperature. The SCRs showed improved yield strength of 1.5–2.3 GPa, due to the higher cooling rate and nano-/ultrafine lamellae thickness (λw). The λw values and hardness vary between 130 and 220 nm and 581–620 HV, respectively, in fully eutectic x = 0.5 AMI pointing to the evolution of a homogeneous microstructure and homogeneous distribution of the alloying elements in the phases. The hardness in 3 mmØ SCRs is 10% higher than that of AMI due to similar microstructure homogeneity and refinement of λw. The proeutectic FCC shows low Nb solubility with a partitioning coefficient of kNb <<1, and Laves phase with kNb>>1 shows higher Nb solubility and preferential dissolution due to the high melting temperature, large atomic radii and large negative enthalpy with other constituent elements.