Ballistic impact responses and mechanisms of medium-entropy CrCoNi alloy plates: Experiments and numerical modeling

Compared to other medium- and high-entropy alloys (MEAs and HEAs), CrCoNi MEA demonstrates the highest dynamic tensile strength (including spall strength) at elevated strain rates, which renders it a promising candidate material for protective structures. Nevertheless, its ballistic responses and associated deformation/damage mechanisms have not been fully investigated. In this work, ballistic impact tests are, for the first time, performed on a 5-mm-thick CrCoNi MEA using a 5-mm-diameter steel projectile. The dynamic process is captured with in situ high-speed optical imaging. Impact-recovered samples are comprehensively characterized through a series of techniques. As the impact velocity increases, the normalized crater diameter increases slightly, while the normalized crater depth and crater volume increase significantly. The three crater parameters, including the normalized diameter ($d_c^{*}$), depth ($D_c^{*}$), and volume ($V_c^{*}$), as a function of impact velocity (v_i) follow power laws with different exponents, i.e., $d_c^{*}\propto v_i^{0.25}$, $D_c^{*}\propto v_i^{4∕3}$ and $V_c^{*}\propto v_i^2$. Furthermore, the CrCoNi MEA exhibits superior penetration resistance compared to conventional alloys with similar densities (e.g., copper and steel), as well as the CrMnFeCoNi HEA. This superior ballistic impact resistance is mainly attributed to the highest dynamic ultimate tensile strength of the CrCoNi MEA. Microstructural analyses reveal a range of deformation features within the CrCoNi target, including dislocations, kink bands, stacking faults, Lomer-Cottrell locks, and nano-twins. These microstructural characteristics play a crucial role in accommodating the high strain rate compression and shear deformation of the CrCoNi MEA. Additionally, numerical simulations utilizing the Johnson-Cook constitutive model and damage criterion effectively reproduce the experimental observations, confirming the robustness of the model in predicting the ballistic impact behavior of CrCoNi MEA.