Relation Between Strength and Hardness of High-Entropy Alloys

High-entropy alloys (HEAs) are composed of multiple principal elements and exhibit not only remarkable mechanical properties, but also promising potentials for developing numerous new compositions. To fully realize such potentials, high-throughput preparation and characterization technologies are especially useful; thereby, the fast evaluations of mechanical properties will be urgently required. Revealing the relation between strength and hardness is of significance for quickly predicting the strength of materials through simple hardness testing. However, up to now the strength-hardness relation for HEAs is still a puzzle. In this work, the relations between tensile or compressive strength and Vickers hardness of various HEAs with hundreds of compositions at room temperature are investigated, and finally, the solution for estimating the strengths of HEAs from their hardness values is achieved. Data for hundreds of different HEAs were extracted from studies reported in the period from 2010 to 2020. The results suggested that the well-known three-time relation (i.e., hardness equals to three times the magnitude of strength) works for nearly all HEAs, except for a few brittle HEAs which show quite high hardness but low strength due to early fracture. However, for HEAs with different phase structures, different strengths should be applied in using the 3-time relation, i.e., yield strength for low ductility body-centered cubic (BCC) HEAs and ultimate strength for highly plastic and work-hardenable face-centered cubic (FCC) HEAs. As for dual-phase or multi-phase HEAs, similar 3-time relations can be also found. The present approach sheds light on the mechanisms of hardness and also provides useful guidelines for quick estimation of strength from hardness for various HEAs.