Multiscale hierarchical heterostructure yields combined high strength and excellent ductility in a Co-Cr-Fe-Ni-Al negative enthalpy alloy

Developing high-performance metallic materials with high yield strength and excellent ductility is important for various applications, such as automobiles, power plants, and aerospace industries. However, conventional alloys typically exhibit a trade-off between strength and ductility, making it difficult to develop materials that are both strong and ductile. In this study, we report that a cast Co-Cr-Fe-Ni-Al alloy can achieve a high room temperature yield strength of up to 500 MPa, which is twice that of conventional high-entropy alloys (HEAs) with face- centered cubic structures, with a tensile strain of 29 %. After thermomechanical treatment, the alloy exhibits even better synergy of strength and ductility, with a yield strength of 900 MPa and 30 % elongation. These exceptional mechanical properties are conferred by a multiscale hierarchical heterostructure, which was introduced through careful control of the alloy composition using a negative enthalpy alloy design strategy. The heterostructure ranges from the micrometer to sub-micrometer and nanometer scales. This multiscale hierarchical structure acts as a continuous impediment to dislocation motion, greatly increasing strength, and facilitating hetero-deformation induced hardening via strain partitioning, resulting in sustained ultrahigh strain hardening. Importantly, multiscale hierarchical heterostructures facilitate coordinated plastic deformation and multiple plastic deformation mechanisms, and stress concentration relieving, and play important roles in improving ductility. This work reveals the effect of different types (and scales) of heterogeneities on the deformation mechanism of HEAs and opens new perspectives for constructing heterostructures, which serve as a new design approach for high strength and excellent ductility by using a negative enthalpy alloy design strategy.