Developing novel ultra‐thin refractory medium‐entropy foils with excellent strength‐ductility synergy

The equimolar NbZrTi medium‐entropy alloy (MEA) has attracted attention due to its excellent comprehensive mechanical properties. In this study, the designed body‐centered cubic NbZrTiAl 4 (atomic percent, at%) MEA by Al addition, having a superplastic extensibility of ~ 5000% under cold rolling, enables directly fabricated ultrathin foils with a thickness down to ~ 0.2 mm without any treatments. Particularly, the annealed NbZrTiAl 4 MEA foils, containing a coherent nanoscale B2, exhibit an ultrahigh yield strength of up to ~ 1130 MPa, which even surpasses the bulk counterpart, while maintaining a good fracture elongation of up to ~ 14%. The Al addition induced a stronger solid solution strengthening and fine‐grain strengthening in the foils. Complex dislocation interactions and dislocation–B2 interactions promoted a dynamical formation of dislocation bands, which yielded work‐hardening ability and tensile ductility. These findings provide a novel strategy for the design of ultrathin refractory medium‐entropy foils to break through their performance limits at ultrahigh temperatures and guide the design of high‐performance lightweight foils for structural applications.