Achieving sustainable strain hardening in a carbon-doped CuFeMnNi high-entropy alloy via dual-level heterogeneous microstructures

Abstract

Phase decomposition is commonly observed in Fe-Cu alloy and Cu-HEAs that has been proved to harmful for the mechanical properties. Here we found that the phase decomposition based on the immiscibility between Cu and Fe plays an active role in the mechanical properties of a carbon-doped face-centered-cubic (FCC) CuFeMnNi high-entropy alloy (HEA) while maintaining low raw-material costs, by employing only hot-forging and annealing. Upon annealing at 900 °C for 120 min, the annealed HEA exhibits dual-level heterogeneous microstructures, i.e., (i) the heterogeneous grain distribution composed of fine grains (≤ 10 µm) within the Fe-rich FCC1 phase and coarse grains (>10 µm) in the Cu-rich FCC2 phase, and (ii) special duplex phases characterized by the transitional region (∼ 1 µm in thickness) at the junction of the above dual phases. Moreover, metastable FeMn-carbides in the FCC1 phase, chemical medium-range order, nano-twins, and 9R-martensite in the transitional region, as well as nanosized Cu-precipitates in the whole microstructure are detected. Compared with C-free CuFeMnNi HEAs, the present alloy exhibits satisfactory ultimate tensile strength around 800 ± 10 MPa and excellent ductility of 56 ± 1.14%. Sustainable strain hardening is achieved by sequentially triggering multistage strain-hardening mechanisms including hetero-deformation-induced hardening, precipitation hardening, and hardening jointly contributed by microbands, twins and dislocation cells. These findings open a new insight for regulating phase decomposition in Fe-Cu alloy and Cu-HEAs.