Heterostructuring an equiatomic CoNiFe medium-entropy alloy for enhanced yield strength and ductility synergy

High-/medium-entropy alloys (H/MEAs) of face-centered-cubic-structured single phase usually suffer from a glaring drawback of low yield strength. Even worse, the trade-off emerges frustratingly between strength and ductility as strength increases. Here, the lamellar heterostructure (HS) is designed in an equiatomic ternary CoNiFe MEA by means of cold rolling followed by an incomplete recrystallization annealing. The lamellar HS consists of the soft recrystallized grains as well as severely deformed structures which are partly reserved. By comparison to the coarse-grained counterpart, the lamellar HS, shows a well enhanced yield strength-ductility synergy, together with an increased yield strength. This is ascribed to the hetero-deformation-induced (HDI) stress in HS during tensile deformation. Accordingly, the HDI strain hardening is induced, serving as an important addition to the conventional forest hardening. The HDI hardening is evidenced experimentally to account for a large proportion of global strain hardening. Furthermore, a fully recrystallized microstructure is obtained to show a simultaneous increase in both yield strength and ductility. The microstructures are evaluated in detail prior to and after tensile deformation by using the electron backscattered diffraction and transmission electron microscope observations. The mechanism for HDI strain hardening in various microstructures is analyzed to correlate to the evolution of microstructures in terms of the kernel average misorientation values, Schmid factor, and dislocation behaviors in response to plastic deformation. Graphical abstract