Decoupling tensile ductility and fracture toughness in additively manufactured AlCoCrFeNi2.1 eutectic high entropy alloy

Eutectic high-entropy alloys (EHEAs) have attracted significant attention due to their excellent strength-ductility synergy under quasi-static tensile loading. This favorable mechanical performance has intuitively led to the assumption that EHEAs also possess high fracture toughness, given their capacity for substantial plastic energy absorption during deformation. In this study, we decouple tensile ductility and fracture toughness in a dual-phase AlCoCrFeNi 2.1 EHEA produced via laser powder bed fusion. Experimental results demonstrate that this EHEA exhibits excellent tensile behavior with a yield strength of 1320 MPa, a tensile strength of 1590 MPa and a uniform elongation of 10.5 %. Paradoxically, this EHEA exhibits a relatively low fracture toughness of 39 MPa m 1/2 . Fractographic analysis reveals a largely brittle fracture process, wherein the fatigue pre-crack propagates nearly straight with limited evidence of dislocation-mediated plasticity at the crack tip. Crack propagation is facilitated by microcracks that form at the phase boundaries and coalesce with the main crack along the lamellar direction. This embrittled behavior contrasts sharply with the coordinated deformation observed in both face-centered cubic (FCC) and body-centered cubic (BCC) phases during uniaxial tensile loading. Crystal plasticity simulations under varying stress states reveal that the local stresses within the FCC and BCC phases increase substantially under high stress triaxiality (i.e. with a pre-crack), reaching values approximately four times higher than those under low stress triaxiality condition (i.e. uniaxial tension). Such elevated local triaxial stress impedes dislocation slip across phase boundaries, promoting rapid crack propagation with minimal plastic deformation.