Hierarchical Eutectic Structure Enabling Superior Fracture Toughness and Superb Strength in CoCrFeNiNb0.5 Eutectic High Entropy Alloy at Room Temperature

Laves phase is hard but notorious for its brittleness at a low temperature. Over the past decades, a great deal of efforts is paid to toughen Laves phase by forming Lave phase-based metal–metal composites. In this work, the authors demonstrate that, through the mixing of carefully selected elements, one can obtain the CoCrFeNiNb0.5 eutectic high entropy alloy (EHEA), which exhibits a hierarchical eutectic lamellar structure consisting of Laves and face centered cubic (FCC) phases mixed in a nearly equal volume fraction. Compared to the conventional Laves phase matrix composites, their EHEAs possess a unique combination of superb hardness (≈9.2 GPa) and superior fracture toughness (≈15 MPa · m^0.5) at room temperature. Through the combined experiments and modeling, the authors are able to show quantitatively this high fracture toughness is derived from the hierarchical eutectic structure and the interplay of a series of extrinsic and intrinsic toughening mechanisms, which are strongly dependent on local lamellar size and orientation. At the fundamental level, their current research demonstrates that the tradeoff between strength and fracture toughness, as commonly perceived for conventional alloys, may be evaded in chemically complex alloys through the formation of hierarchical eutectic structures.