Exceptional high-strain-rate tensile mechanical properties in a CrCoNi medium-entropy alloy

Alloys with combined outstanding strength and excellent ductility are highly desirable for many structural applications. However, alloys subjected to deformation at very high strain rates and/or cryogenic temperatures usually suffer from very limited ductility. Here, we demonstrate that a bulk CrCoNi medium-entropy alloy presents exceptional combination of high strength and excellent ductility during deformation at high strain rates over a wide temperature range. Full tensile stress-strain curves at a high strain rate of 2000 s⁻¹ and temperatures down to 77 K were successfully obtained using an electromagnetic Hopkinson tension bar system attached with a cooling device, revealing high true ultimate tensile strength (σ_UTS,T) of 1.8 GPa and true strain of ∼54% at σ_UTS,T. These outstanding mechanical properties were mainly attributed to profuse deformation twinning. Both high strain rate and cryogenic temperature promoted deformation twinning. Grain refinement caused by deformation twinning, dislocation slip and dynamic recrystallisation added to work hardening and the excellent tensile strain.[Figure not available: see fulltext.]