Improving the mechanical properties of high-entropy alloys via germanium addition: A focused review

Recent research efforts have focused on developing novel high-entropy alloys (HEAs) with exceptional mechanical properties through strategic metalloid alloying. Among metalloids, germanium exhibits a significant difference in atomic size and electronic structure with transition metals, which, in conjunction with the effect of Ge on the stacking fault energy (SFE) and phase formation, make it a promising candidate for enhancing the strength-ductility synergy in HEAs. This review aims to highlight the latest advancements in optimizing the mechanical properties and strengthening mechanisms of HEAs and medium-entropy alloys (MEAs) through Ge addition. Accordingly, following a discussion on strengthening mechanisms in HEAs, the specific contributions of Ge to solid-solution hardening, secondary phase evolution, and additional plasticity mechanisms such as the twinning-induced plasticity (TWIP) effect are addressed. Subsequently, the research gaps and future directions in the field of Ge-containing HEAs, including strategies for overcoming the strength-ductility trade-off via transformation-induced plasticity (TRIP) effect by formation of martensite during deformation, the development of heterogeneous structures, additive manufacturing techniques, cryogenic behavior, and superplasticity are outlined.