Effects of rare-earth element, Y, additions on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

The effects of rare-earth element, Y, additions on the microstructures and mechanical properties of CoCrFeNi alloy have been investigated. The new series of CoCrFeNiYx (x = 0, 0.05, 0.1, 0.2, and 0.3) high entropy alloys were synthesized by vacuum arc-melting method. Microstructure characterizations were performed by the means of X-ray diffraction, scanning electron microscope, and transmission electron microscope. It was found that alloying Y element could lead to the formation of a simple hexagonal structure phase (CaCu5 type). And another Ni3Y-type hexagonal structure phase was observed in the alloy with higher Y contents (0.3 at%). The phase evolution of the present alloy system was evaluated using the previous criteria (ΔHmix−δ,Ω−δ,Δχ, VEC, and Λ). The results of the nanoindentation measurements on different phases indicated that the hexagonal structure phases (~ 10.5 GPa) had a higher nanohardness than the face-centered cubic phase (~ 3 GPa). Furthermore, the maximum shear stress of the incipient plasticity was calculated to be 3.2 GPa for the face-centered cubic phase and 5.2 GPa for the hexagonal structure phase. The dislocation nucleation under the indenter in different phases was also discussed. The Vickers hardness and yield strength increased with increasing the Y content, while the fracture strength and plastic strain decreased. The strengthening mechanisms of the present alloys included solid-solution strengthening and the second phase strengthening. Fracture surface observations suggested a ductile fracture in the face-centered cubic phase and a cleavage fracture in the hexagonal structure phase for the alloys containing Y element. The results of this investigation can provide a guide for the design of new high entropy alloy system with excellent properties.