Superior strength-ductility synergy and strain hardenability of Al/Ta co-doped NiCoCr twinned medium entropy alloy for cryogenic applications

Metallic materials with outstanding cryogenic mechanical properties are highly demanded for cryogenic engineering applications. Here we developed a novel (NiCoCr)₉₂Al₆Ta₂ medium-entropy alloy (MEA) towards superior cryogenic mechanical properties via elaborating the chemical composition designing and the thermo-mechanical processing. It appears that the twinned (NiCoCr)₉₂Al₆Ta₂ MEA shows a strongly temperature-dependent mechanical behavior, i.e., when the testing temperature from 298 down to 77 K, the yield strength, ultimate strength and tensile ductility are increased from ∼600 to ∼800 MPa, from ∼1.0 to ∼1.35 GPa and from ∼52% to ∼90%, respectively. An excellent strength-ductility synergy and extraordinary strain hardening capacity were realized in this alloy, in particular the product of tensile strength and elongation is more superior to their reported counterparts. There is a strongly temperature-dependent deformation mechanism transition from the ordinary planar-slip at 298 K to the cooperative plastic mechanisms at 77 K, including stacking faults (SFs) and deformation twins. The excellent cryogenic mechanical properties of this designed MEA stems from the synergic effects of nanotwins, hierarchical SFs and Lomer-Cottrell locks, as well as their extensive interactions, rarely observed in their siblings deformed at room temperature. Furthermore, the deformation twinning acting as an additional/significant mechanism for plasticity and favoring strain hardening, along with the effect of temperature-dependent dislocation-twin interactions, collaboratively delays the onset of necking for enhanced ductility at 77 K.