Enhance the mechanical properties of CoCrFeNi high entropy alloy: Rare-earth element Gd was employed as a "cladding structure" to refine grains

The CoCrFeNi high entropy alloy (HEA) with face-centered cubic structure exhibits unique mechanical properties including excellent ductility and low strength. Therefore, the problem of low strength is exposed in the application of structural materials. In order to compensate for the deficiency in strength, the approach of alloying RE elements to enhance the mechanical properties of metal materials was applied to high entropy alloys (HEAs).The (CoCrFeNi)100-xGdx (x=0,0.5,1,2,3) HEAs (denoted as Gd0,Gd05,Gd1,Gd2,Gd3, respectively) were prepared by vacuum arc melting furnace. The microstructure of the alloy system was characterized by XRD, SEM and TEM. The mechanical properties of the alloy were tested by nano-indentation and compression tests. Finally, the correlation between microstructure evolution and mechanical properties was established. The addition of Gd alters the microstructure of CoCrFeNi high entropy alloy, resulting in the formation of a novel "cladding structure". Microstructure analysis reveals that in the novel system, the emergence of the "cladding structure" is accompanied by grain refinement, and a transition from coarse columnar crystal morphology to homogeneous dendritic structure is achieved in CoCrFeNi high entropy alloy. Apparently, the mechanical properties of (CoCrFeNi)100-xGdx alloys are enhanced due to grain refinement, as subsequent compression tests conducted at room temperature showed that the yield strength of Gd3 alloy reached 650 MPa, which is about three times higher than that of CoCrFeNi high entropy alloy (235 MPa). However, the enhancement of mechanical properties not only attributed to grain refinement, and this paper further reveals the strengthening mechanism of solid solution strengthening induced by Gd incorporation and the second phase strengthening for enhancing the mechanical properties of alloys in this paper.