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

Abstract In this study, we develop a non-equiatomic Co2CrNi1.5Al0.25Ti0.25 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 ( e te ) up to 16.0% at 293 K, as well as ultra-high σ y up to 2.02 GPa, σ u up to 2.31 GPa and excellent e 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 σ y 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.