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 NbZrTiAl₄ (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 NbZrTiAl₄ 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.