Microstructure and mechanical properties of Co-free Ni₂FeCrW_x medium-entropy alloys reinforced by coherent precipitates

In the present work, novel Co-free Ni₂FeCrW_x medium-entropy alloys (MEAs) are designed by equiatomically replacing Co with Ni in the CoCrFeNi high-entropy alloys (HEAs). The effects of W addition coupled with subsequent heat treatment on microstructure evolution and mechanical properties were systematically investigated and analyzed. The W-addition is conducive to the formation of μ phase in Ni₂FeCrW_x MEAs, while the annealing treatment promotes the precipitation of submicron-sized BCC particles and the spheroidization of μ phase. The orientation relationship (OR) of 1¯21¯0_{μ phase} // [011]_FCC and 303¯0_{μ phase} // 111¯_FCC and the coherent interface is formed between μ phase and FCC matrix in as-cast alloys, and the orientation relationship of the interfaces between BCC precipitate and FCC matrix have been clarified as the Kurdjumov-Sachs (KS) interfaces presenting [011]_FCC //1¯11_BCC and 1¯1¯1_FCC // 011¯_BCC. The annealed Ni₂FeCrW_0.5 MEA exhibited outstanding strength-ductility synergy, combining a high ultimate tensile strength of ∼982 MPa and an excellent ductility of ∼29%. Strengthening mechanisms were estimated and compared with experimental measurements, revealing that the coherent BCC/μ precipitates bring out effective precipitation strengthening.