Design of ultrastrong but ductile medium-entropy alloy with controlled precipitations and heterogeneous grain structures

In this study, we develop a non-equiatomic Co₂CrNi_1.5Al_0.25Ti_0.25 quinary medium-entropy alloy (MEA). The heterogeneous grain structure (HGS) (comprising residual deformed grains, micro-sized recrystallized grains and nano-sized recrystallized grains) and heterogeneous precipitation (HP) of the L1₂ phase corresponding to different kinds of grain structures are simultaneously introduced into the alloy by the well-designed thermomechanical processing. The alloy exhibits an ultra-high yield stress (σ_y) of 1.73 GPa, ultimate tensile stress (σ_u) of 1.89 GPa and remarkable total elongation (ε_te) up to 16.0% at σ_y 293 K, as well as ultra-high up to 2.02 GPa, up to 2.31 GPa and excellent ε_te up to 21.2% at 77 K. The synergy of multiple hardening mechanisms, mainly including back-stress hardening, precipitation hardening and pre-existed dislocation hardening contributes to the ultra-high at 293 K and 77 K. The HGS promotes the continuously increased geometrically necessary dislocations (GNDs) density, which, combined with the nano-spaced stacking fault (SF) networks, leads to the outstanding three-stage work-hardening behaviors during tensile tests at 293 K and 77 K. The simultaneous introduction of HGS and HP into this newly-developed MEA without brittle intermetallic phase, is a new effective method in designing high-performance alloys applied at both ambient and cryogenic temperatures.