Significant transitions of microstructure and mechanical properties in additively manufactured Al–Co–Cr–Fe–Ni high-entropy alloy under heat treatment

The non-equimolar Al–Co–Cr–Fe–Ni high-entropy alloy was fabricated by combined cable wire arc additive manufacturing (CCW-AAM). Microstructure evolution and mechanical properties of this dual-phase (FCC + ordered BCC (B2)) alloy under 600 °C, 800 °C and 1000 °C heat treatment for 8 h were investigated. The as-deposited alloy was composed of FeCr-rich FCC phase and AlNi-rich B2 phase. Under heat treatment at 600 °C, a large number of Cr-rich σ phases precipitated in the B2 matrix and nano-sized ordered FCC (L12) precipitated in the FCC matrix, which improved the hardness (from 338 HV to 420 HV), yield strength (from 654 MPa to 810 MPa) and ultimate tensile strength (from 976 MPa to 1115 MPa), but declined the elongation (from 3.11% to 2.46%). When the heat treatment temperature rose to 800 °C, the size of σ phase increased. In addition, the L12 phase transformed into the rod-like AlNi-rich B2 phase precipitated in the FCC matrix. The yield strength and ultimate tensile strength were similar to those of the as-deposited sample, but elongation increased by 176%. For heat treatment at 1000 °C case, σ phase dissolved in B2 matrix and the rod-like B2 precipitations significantly coarsened, which softened the alloy. The hardness (308 HV) and yield strength (542 MPa) declined markedly, but the elongation (14.19%) greatly improved. This work shines new insights on the fabrication of Al–Co–Cr–Fe–Ni HEA with controllable microstructure and excellent mechanical properties via a combined process of CCW-AAM and subsequent heat treatment.