Additive Manufacturing of High-Entropy Alloys: Microstructural Metastability and Mechanical Behavior

High-entropy alloys (HEAs) have received considerable interest over the past decade due to their intriguing structural, chemical, and physical properties. Additive manufacturing (AM), also termed three-dimensional (3D) printing, generates parts with complex geometries and internal features layer-upon-layer from a computer-aided design (CAD) 3D file. In recent years, explosive research on AM of HEAs has been inspired. This paper performs a comprehensive and critical review on the recent progress in additively manufactured (AM-ed) HEAs, with a special focus placed on the similarities and differences in the microstructure and mechanical behavior between the AM-ed HEAs and the as-cast or thermo-mechanically processed (TMP-ed) counterparts. To gain a better understanding of the formation of the AM microstructure, the working principles, rapid solidification effects, and subsequent thermal cycling effects of various metal AM techniques, e.g., directed energy deposition (DED), selective laser melting (SLM), and electron beam melting (EBM), are also introduced. In the end, several future research directions are suggested towards the design of advanced HEAs with the superior strength-ductility synergy via 3D printing.