Tailoring formation and proportion of strengthening phase in non-equiatomic CoCrFeNi high entropy alloy by alloying Si element

Abstract

To adjust the formation and proportion of the strengthening phase in high entropy alloys (HEAs), the non-metallic Si element was added in non-equiatomic CoCrFeNi HEA. Co30Cr30(FeNi)40-xSix HEAs (x = 2, 4, 6, and 8 at.%, hereinafter, simplified as HSi2, HSi4, HSi6, and HSi8, respectively) were prepared. The phase evolution, tensile properties, and strengthening mechanisms were investigated. The results show that the phase structures are FCC, rich Cr–Si, and rich Ni–Si phases in HSi2 and HSi4. The rich Cr–Si phase disappears and the rich Ni–Si phase grows gradually with the increase of Si content. HSi6 presents an appropriate proportion of FCC and discontinuous rod-like rich Ni–Si phases. The mixing enthalpy (ΔHmix) is more suitable for the phase prediction in Co30Cr30(FeNi)40-xSix HEAs. With the increase of Si content, Ni atoms diffuse into the primary rich Cr–Si phase and replace Cr atoms to form a rich Ni–Si phase. The rich Ni–Si phase has a stronger binding force due to the Ni and Si atoms with more negative ΔHmix. The yield strength gradually increases from 215 MPa to 330 MPa with the increase of Si content. HSi6 has excellent tensile strength-ductility synergy with a tensile strength of 694 MPa and a fracture strain of 67.8%, which is attributed to high strain hardening ability. The discontinuous and rich Ni–Si phase plays an important role as a strengthening phase in the deformation process, reducing the free path of dislocation movement and delaying the occurrence of necking. The FCC phase with large plastic deformation ability leads to crack passivation and maintains a certain ductility. More rich Ni–Si phases will deteriorate the ductility of HSi8.