Competition between damage initiation versus crack propagation: Uncovering fracture mechanisms in eutectic high entropy alloys

The AlCoCrFeNi₂.₁ eutectic high entropy alloy (EHEA) is a model system for achieving high strength-ductility balance via its duplex lamellar FCC/B2 microstructure. However, its defect sensitivity and underlying toughening mechanisms remain underexplored, particularly following thermomechanical processing. In this study, fracture toughness and crack propagation behavior are evaluated for both as-cast (AC) and hot-rolled (HR) EHEA variants. The HR microstructure exhibits enhanced tensile properties due to refined lamellae which delay damage initiation. Fracture toughness is reduced in the HR variant however, due to diminished crack blunting and suppression of extrinsic toughening mechanisms such as grain bridging. Post-fracture analysis reveals mixed-mode propagation in the AC condition to predominantly BCC cleavage in the HR variant, coinciding with reduced FCC continuity at the crack tip. These results demonstrate the competition between damage initiation resistance versus crack propagation resistance to provide toughness and gives insights into optimization strategies for duplex EHEAs.