Microstructure evolution and mechanical properties of directionally solidified Ni36Co30Fe11Cr11Al8Nb4 high entropy alloy

In order to investigate the microstructure evolution, tensile deformation mechanism and strengthening mecha-nism of Ni36Co30Fe11Cr11Al8Nb4 high entropy alloy (HEA), the alloy samples were prepared by directional so-lidification at different solidification rates (5, 10, 20, 50 and 100 mu m/s). Results indicate that the microstructure of the directionally solidified Ni36Co30Fe11Cr11Al8Nb4 HEA is composed of primary bulky dendrites (FCC1 phase) and inter-dendritic lamellar structures which consist of the Laves phase and the Nb-enriched FCC2 phase. With the increase of solidification rate, the dendrites are refined, the volume fraction of inter-dendritic lamellar structure decreases, and Laves phase accumulates at the grain boundaries. Since FCC2 phase has lower nucleation barriers, the nucleation and growth of FCC2 phase are preferential to those of Laves phase in the inter-dendrite region. Tensile results indicate that the directionally solidified Ni36Co30Fe11Cr11Al8Nb4 HEA possesses the highest ultimate tensile strength and fracture strain at 5 mu m/s, which are 740.15 MPa and 35.23 %, respectively. The microscopic deformation behavior shows that FCC1 phase yields first, followed by FCC2 and Laves phases in sequence during tensile deformation. The moderately strong FCC2 phase coordinates the large inherent strength gap between FCC1 and Laves phases to avoid stress concentration and maintain ductility. FCC2- Laves interface plays a significant role in hindering dislocation movement and increasing the strength, the overall interfacial strengthening stress is estimated to be 9.1 GPa.

Automated summary

The microstructure evolution, tensile deformation mechanism and strengthening mechanism of directionally solidified Ni36Co30Fe11Cr11Al8Nb4 high entropy alloy were investigated. Results showed that the microstructure consisted of bulky dendrites (FCC1 phase) and inter-dendritic lamellar structures of Laves phase and Nb-enriched FCC2 phase. The increase of solidification rate led to refined dendrites, decreased lamellar structure, and accumulation of Laves phase at grain boundaries. Tensile tests revealed that the alloy exhibited the highest ultimate tensile strength and fracture strain at 5μm/s.