Synergistic effect of Nb and Ta alloying on microstructure and mechanical properties of CoCrFeNi high-entropy alloy

Abstract

CoCrFeNi-(Nb, Ta) high-entropy alloys (HEAs) were prepared by a vacuum arc melting process, and the synergistic effect of Nb and Ta concentrations on the microstructure evolution and mechanical properties was investigated in detail. Nb and Ta concentrations in the HEAs affect the microstructure compositions, eutectic phase lamella spacings, Laves phase size topographies, phase volume fractions and compositions. The alloy with an equal atomic ratio of Nb/Ta show an eutectic microstructure of the face centered cubic (FCC) crystal matrix and Laves phase, while those with non-equal atomic ratio of Nb/Ta exhibit the microstructure of the eutectic (FCC+Laves) phase and primary Laves phase. The volume fraction and grain size of the primary Laves phase monotonically increase with the increase of Nb/Ta atomic ratio. The compressive yield strength of HEAs is monotonously improved with the volume fraction of Laves phase, whereas the ultimate strength is hardly affected by the microstructure composition. Compression plasticity is negatively correlated with the volume fraction, type, and size distribution of Laves phase. The strengthening mechanism of CoCrFeNi-(Nb, Ta) HEAs was analyzed with theoretical calculation, and the relationship between the microstructure composition and the alloy strengths was established. The results indicate that the fine-grain strengthening and second phase strengthening of Laves phase are the main factors for the improvement of yield strength.

Keywords

• high-entropy alloys
• synergistic alloying
• microstructure
• laves phase
• yield strength