Jia-Jun ChenPo-Han ChiuChu-Jen ChenYu-An ChenChien-Yu TsengTzu-Ching TsaoShih-Yuan LuCheng-Ling TaiLe-Min WangTE-CHENG SUYo-Lun YangChih-Yuan ChenJien-Wei YehJER-REN YANG2025-04-152025-04-152025-04https://www.scopus.com/record/display.uri?eid=2-s2.0-85217657405&origin=recordpagehttps://scholars.lib.ntu.edu.tw/handle/123456789/728101CrCoNiSix (x = 0, 0.15, 0.3) medium entropy alloys (hot rolled to a 70 % thickness reduction at 1000 °C and annealed at 1000 °C) were subjected to ballistic impact by ultra-high speed bullets (900 m/s, 9.8 g). Notably, the CrCoNiSi0.3 alloy demonstrated an outstanding energy absorption capacity of 82 % upon ballistic impact, surpassing the 76.7 % of CrCoNiSi0.15 and 68.9 % of CrCoNi. The strategic addition of silicon (Si) to a CoCrNi alloy significantly enhances its strength and imparts exceptional energy absorption capability, as demonstrated by the CrCoNiSi0.3 alloy subjected to ballistic impact. A detailed microstructural analysis of the CrCoNiSi0.3 armor plate post-impact was therefore conducted, focusing on three distinct regions (regions I, II, and III) from lightly deformed to heavily deformed regions. In Region I, 25 mm from the bullet hole, deformation twins are absent due to the low strain in this region. In Region II, 20 mm from the bullet hole, the intersection of two variants of deformation twins forms submicron-blocks with sizes ranging from ∼100 nm to ∼200 nm. In Region III, 10 mm from the bullet hole, two variants of deformation bundles further partition the submicron-block into nano-blocks with sizes of the nano-blocks ranging from ∼10 nm to ∼50 nm. These intersecting deformation bundles contribute to dynamic grain refinement due to the continuous segmentation that occurs during high-speed deformation. Atomic-resolution HAADF-STEM images of the deformation bundle reveal the alternating arrangement of HCP nano-lamellae and deformation nanotwins, suggesting that the combination of high strain and high strain rate during ballistic deformation activates TWIP and TRIP effects within highly localized nano-block regions, thereby achieving further work-hardening.Ballistic impact deformationDeformation twinDynamic Hall–Petch effectFCC-type medium entropy alloy (MEA)HCP phaseHierarchical deformation structureHigh resolution scanning transmission electron microscopy (HR-STEM)Strain induced phase transformationjournal article10.1016/j.matchar.2025.114814