Abstract: Precipitation-hardened medium/high-entropy alloys (M/HEAs) normally possess compromised strain hardening behavior concomitant with reduced ductility. Here, an exceptional strength-ductility synergy can be achieved in a novel laser-powder bed fusion (L-PBF) processed γ′-hardened lamellar sandwich structure, in which out-layer and inner-filling are ((CoCrNi)₉₄Ti₃Al₃)₉₈W₂ and CoCrNi, respectively. After ageing at 700 °C for 1 h, the lamellar sandwich sample possesses a high ultimate tensile strength of 1293 MPa and a decent fractured strain of 33.2%. Electron microscopy characterizations show that γ′ precipitates within thermally stable cellular structures are detected in the outer-layer, and the broken-up cellular structures are observed within the inner-filling. Significantly, the strain concentration in the outer layer can be transferred to the inner filling with increasing tensile strain. An obvious strain disparity delays the critical strain concentration associated with failure, enabling the hard outer-layer to develop an enhanced dislocation multiplication and accommodation capacity by introducing dislocation pile-ups, stacking faults (SFs), Lomer-Cottrell locks (L-C locks), jog formation and deformation twins (DTs). The coordinated regulation of strain distribution via compositional and structural design thus provides a promising approach for preparing high-performance precipitation-strengthening metallic materials.

Key words: γ′-hardened CoCrNi alloy, Laser-powder bed fusion (L-PBF), Lamellar sandwich structure, Strain distribution, Strength-ductility synergy