High-entropy alloys (HEAs) and related concept of complex concentrated alloys (CCAs) expand the diversity of the materials world and inspire new ideas and approaches for the design of materials with an attractive combination of properties. Here, we present a critical review of the field with the intent of summarizing the principles underlying their birth and growth. We highlight the major accomplishments and progresses over the last 14 years, especially in the discovery of new microstructures and mechanical properties. Finally, we outline the main challenges and provide guidance for future works.

A new bcc Ti-rich high-entropy alloy (HEA) of composition Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅ was designed using the ‘d-electron alloy design’ approach. The tensile behavior displays a marked transformation-induced plasticity effect resulting in a high normalized work-hardening rate of 0.103 without loss of ductility when compared to the reference composition Ti₂₀Zr₂₀Hf₂₀Nb₂₀Ta₂₀. In this paper, a detailed microstructural analysis was performed to understand the deformation process, revealing architectural-type microstructures and a high volume fraction (65%) of internally twinned stress-induced martensite α^″ after mechanical testing. This study opens the way to mechanical properties optimization and enhancement of titanium-based HEAs by combining multiple alloying designs. IMPACT STATEMENT For the first time, proof is given that transformation-induced plasticity was triggered in a bcc refractory high-entropy alloy, leading to a twofold increase in the normalized work-hardening rate. GRAPHICAL ABSTRACT