Simultaneous enhancement of strength and resistance to shear localization in CrCoNi medium entropy alloys by Al alloying under dynamic loading

CrCoNi-based face-centered-cubic(fcc) medium- or high-entropy alloys with low stacking-faults energies exhibit extraordinary fracture toughness ranging from cryogenic temperature to room temperature. Various deformation mechanisms such as dislocation slip and deformation twinning enable them to absorb considerable deformation energy and resist crack propagation. However, their yield strength is not high enough for structural applications. This study systematically designed CrCoNi-based medium-entropy alloys (MEAs) through supplementing aluminum (Al) content (0–12 at. %). Primary objectives were to enhance the yield strength and analyze dynamic deformation mechanisms and adiabatic shear localization for these CrCoNi-based MEAs. At ∼6000 s⁻¹, yield strength increased from 639 MPa for the (CrCoNi)₉₇Al₃ MEA to 1224 MPa for the (CrCoNi)₈₈Al₁₂ MEA, primarily due to the strengthening of secondary B2 phase. However, the B2 phase in (CrCoNi)₈₈Al₁₂ MEA accelerates dynamic recrystallization, thereby promoting the occurrence of adiabatic shear localization. The (CrCoNi)_92.5Al_7.5 MEA exhibited significant resistance to shear localization, which was attributed to the presence of numerous deformation mechanisms, including dislocation glide, stacking-faults, Lomer-Contrell locks, deformation twins, and fcc to hexagonal close-packed (hcp) phase transformation. This gave rise to absorbed strain energy of 714 MJ/m³ in (CrCoNi)_92.5Al_7.5 MEA. This study establishes a reliable theoretical basis for the engineering deployment of CrCoNi MEAs as lightweight protective materials.